U.S. Department of Labor

Mine Safety and Health Administration

Mine Safety and Health Administration

M SH A

ENGINEERING AND DESIGN

MANUAL

COAL REFUSE DISPOSAL FACILITIES

Second Edition

May 2009

Prepared by:

 

D’APPOLONIA ENGINEERING

NOTE: Instructions for using this DVD version of the Manual are

 provided on the next page
ii

The engineering guidance presented in this document has been compiled or developed by D’Appolonia 

Engineering under contract with the Mine Safety and Health Administration (MSHA) from referenced 

sources as well as input from MSHA and the coal industry. However, the guidance, recommendations, and conclusions presented herein may not necessarily represent the official policies of MSHA 

or the U.S. government.

INSTRUCTIONS FOR USAGE OF THIS DVD MANUAL

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On Mac computers hold down the Command key (⌘) and hit the left arrow key. The more you move 

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If you load the Manual PDF to your computer, the references should be loaded to the same folder. 

Suggestions for changes, corrections, or updates to the Manual should be directed to:

 Mine Safety and Health Administration

 Pittsburgh Technical Support Center

 Mine Waste and Geotechnical Engineering Division

 P.O. Box 18233

 Pittsburgh, PA 15236

Attention: Dam Safety Officer
Preface

MAY 2009

This Manual presents guidance on procedures for use in the engineering design, construction monitoring, operation and inspection of coal refuse impoundments and embankments in the United States. 

It is an update of the original 1975 edition and reflects advances in engineering, construction, and facility monitoring and operations practices. The primary intent of the Manual is to serve as a uniform 

guide to safe refuse disposal practices for those concerned with coal mining and preparation. The 

Manual serves this purpose in several ways by: (1) providing experienced embankment dam design 

engineers with the characteristics of coal refuse and its disposal so that their experience can be appropriately applied; (2) providing specialized technical knowledge concerning embankment design 

in a form that can be used by engineers who do not specialize in this field; (3) updating geotechnical, 

structural, hydrologic and hydraulic design criteria for a range of embankment and impoundment 

conditions, and spillway and drainage structures; (4) providing guidance on disposal requirements 

and limitations for mine operators to include refuse disposal in the overall coal production operation; 

and (5) providing guidance on construction, operation, inspection, monitoring and instrumentation, 

and emergency action planning associated with the implementation of safe and reliable designs. 

The 1975 edition of the Manual was prepared following the failure of a coal waste dam at Buffalo 

Creek, West Virginia, that resulted in 125 fatalities. This Manual update was prompted by the recognition that significant advances have been made in the fields of coal waste disposal and dam safety 

in the 30-plus years since the original Manual was published. Another impetus was an incident that 

occurred in Martin County, Kentucky, in 2000 in which over 300 million gallons of water and fine coal 

refuse from a slurry impoundment broke into an underground mine. Slurry subsequently flowed out 

of two mine openings and impacted streams in two separate watersheds. This incident prompted the 

U.S. Congress to provide funding to the National Research Council (NRC) to examine ways to reduce 

the potential for similar accidents. The NRC’s report, “Coal Waste Impoundments: Risks, Responses, 

and Alternatives,” which was released in 2002, included a number of recommendations for MSHA. 

One recommendation was that MSHA “continue to adopt and promote the best available technology 

and practices with regard to site evaluation, design, construction, and operation of impoundments.” 

MSHA reported to Congress that one measure to address the NRC recommendations would be this 

updating of the original Coal Refuse Design Manual.

The guidance presented in this Manual represents information, methods and procedures that are recommended for consideration by designers, coal operators, and regulators. The guidance presented in 

this Manual is not regulation and cannot be enforced as such. It is not intended to preclude the application of other credible methods and procedures or the use of other and new information that will 

result in a safe and reliable coal refuse disposal facility. It is the responsibility of the designer to investigate the requirements of the project, recognize the unique and critical aspects of the site conditions, 

and prepare designs that reflect actual site conditions, features, loadings, and constraints.

iii
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Preface

MAY 2009

In this update of the Manual, new chapters have been developed on seismic design and on site mining and foundation issues, two topics that can have an impact on the type of disposal facility designed. The long operating life of coal refuse facilities makes monitoring of embankment behavior 

and facility maintenance particularly important. The sections on operation, monitoring, and instrumentation summarize procedures and devices to aid the designer and operator for defining and 

implementing an appropriate field observation program. In addition, general guidance is provided 

for the preparation of emergency action plans. These plans are recommended for certain dams and 

impoundments,and are required by some state regulatory agencies and encouraged by MSHA as 

part of addressing hazardous conditions under 30 CFR § 77.216-3.

In addition to concern for safety, the Manual addresses environmental considerations and controls 

that may influence the design of embankments and impoundments. Executive Order 11514 – Protection and Enhancement of Environmental Quality, dated March 5, 1970, requires all Federal agencies 

to “Monitor, evaluate and control on a continuing basis their agencies’ activities so as to protect and 

enhance the quality of the environment. Such activities shall include those directed to controlling 

pollution and enhancing the environment and those designed to accomplish other program objectives which may affect the quality of the environment.” The Manual does not present guidance in establishing criteria for environmental controls (e.g., hydraulic conductivity of liner systems), because 

such guidance is more appropriately left to other references and regulatory agencies.

This Manual is intended to provide the designer with an important source of information. However, 

the text and accompanying figures, tables and references should not be applied without proper engineering knowledge and judgment. Responsibility for actual design lies with the Professional Engineer 

in responsible charge of the work. The use or application of the methods and information contained 

herein is strictly the responsibility of the person utilizing the material. Designs should be based on 

sound engineering principles and judgment and reflect actual site conditions, and they should not 

merely be patterned after a successful design used at another location or possibly portrayed in the 

Manual. The designer should be diligent and recognize that advances in approaches, criteria, and 

methods will occur that may affect the applicability of portions of any reference or design guide. 

This Manual was prepared by engineers and scientists with background and experience in the 

subject matter, with input from MSHA personnel who review design plans and conduct investigations at disposal sites. Overall direction and technical content were provided by Mr. Robert E. 

Snow of D’Appolonia Engineering. Manual coordination was performed by Dr. James L. Withiam, 

D’Appolonia, and final editing of the text was provided by Dr. J. Timothy Onstott. Proper recognition 

to the entire Project Team and D’Appolonia staff for their devoted efforts is not possible here. The 

main contributors to and reviewers of technical chapters are noted in Table 1.

Special recognition is also given to Mr. John W. Fredland, Dam Safety Officer for MSHA, as the contracting officer’s technical representative, Mr. Harold L. Owens, Mr. George H. Gardner, and to other MSHA 

personnel who provided many valuable comments in suggesting content and reviewing the text for 

publication. Finally, grateful appreciation is given to those in industry who provided input and review 

comments during the process of preparation of this document. This includes input from the National 

Mining Association and its consultants, as well as federal and state agencies and universities.

This Manual is available in hard copy and DVD. The DVD format includes hyperlinks and search capabilities using Adobe Acrobat Reader software. Hyperlinks allow the display of the highlighted citation 

of a figure, table, appendix, or selected reference in the text. Selected references (in PDF format) that 

are available in the public domain are included on the DVD version. For references not in the public 

domain, reasonable efforts were made to obtain copyright permission. No hyperlink is provided for 

references of substantial size, lack of availability in the public domain, or where permission for reprint 

could not be obtained. The complete citation for all references is provided in the References section.
MAY 2009 v

Engineering and Design Manual Coal Refuse Disposal Facilities

Suggestions for changes, corrections, or updates to this Manual should be directed to:

 Mine Safety and Health Administration

Pittsburgh Technical Support Center

Mine Waste and Geotechnical Engineering Division

P.O. Box 18233

Pittsburgh, PA 15236

Attention: Dam Safety Officer

For any suggested changes, corrections or updates, commenters should identify specific pages, paragraphs, tables, or figures within the Manual, along with proposed replacement or new material. 

Sources of proposed material should be completely cited, along with permission for use in a future 

revised edition of this document.

This document is intended solely for noncommercial and educational purposes. The material presented in this Manual has been prepared in accordance with recognized engineering practices as cited 

herein. The guidance should not be used without first securing competent advice with respect to its 

suitability for any application. The publication of the material contained herein is not intended as any 

representation or warranty, either expressed or implied, on the part of the individuals, firms or agencies involved, or any other person or organization named herein, that this information is suitable for 

TABLE 1 MAJOR AND EXPERT CONTRIBUTORS AND REVIEWERS

Chapters 1 to 5

 Introduction, Background,

 Planning and Design Components

Mr. Robert E. Snow, D’Appolonia Mr. Richard G. Almes

 Chapter 6

 Geotechnical Exploration,

 Materials Testing, Engineering 

 Analysis and Design

Dr. James L. Withiam, D’Appolonia

Mr. William J. Johnson, D’Appolonia

Mr. Joseph W. Premozic, D’Appolonia

Mr. Christopher J. Lewis, D’Appolonia

Mr. Richard G. Almes

Dr. Umesh Dayal

 Chapter 7

 Seismic Design: Stability and

 Deformation Analyses

Mr. Michael Paster, GEI

Mr. William J. Johnson, D’Appolonia

Mr. Christopher J. Lewis, D’Appolonia

Mr. Richard F. Tobin, GEI

Dr. Gonzalo Castro, GEI

Dr. William F. Marcuson

Dr. Peter K. Robertson

Mr. Blaise E. Genes, CEC

Mr. Richard G. Almes

 Chapter 8

 Site Mining and Foundation

 Issues

Dr. James L. Withiam, D’Appolonia

Mr. Robert E. Snow, D’Appolonia

Mr. Christopher J. Lewis, D’Appolonia

Dr. Vincent A. Scovazzo, J.T. Boyd

Mr. Frederick R. Vass, Alliance

Dr. Kelvin K. Wu

Mr. Richard G. Almes

Chapters 9 and 10 

 Hydrology and Hydraulics

 Environmental Considerations

Ms. Colleen M. Campbell, D’Appolonia

Mr. Robert E. Snow, D’Appolonia

Dr. J. Timothy Onstott, D’Appolonia

Mr. Richard G. Almes

Chapters 11 and 12

 Construction and Disposal

 Operations

 Monitoring, Inspection and

 Facility Maintenance

Mr. Robert M. Shusko, D’Appolonia

Mr. Robert E. Snow, D’Appolonia

Mr. Frederick R. Vass, Alliance

Mr. Richard G. Almes

Chapter 13

 Instrumentation

Dr. James L. Withiam, D’Appolonia Mr. Richard G. Almes

Chapter 14

 Emergency Action Planning

Dr. J. Timothy Onstott, D’Appolonia Mr. Richard G. Almes
vi

Preface

MAY 2009

any general or specific use or promises freedom from infringement of any patent or patents. Anyone 

making use of this information assumes all liability resulting from such use. Reference to commercial 

products or firms and use of trade names and trademarks in this document is for descriptive purposes 

only, does not constitute endorsement, and may not be used for advertising or promotion purposes.
Table of Contents

May 2009 vii

Preface..........................................................................................................................................................................................iii

List of Tables ...................................................................................................................................................................xxvii

List of Figures ...............................................................................................................................................................xxxiii

Acronyms ................................................................................................................................................................................xli

Chapter 1 Introduction

Chapter 2 Background and Characterization

of Coal Refuse

2.1 GEOLOGICAL NATURE OF COAL REFUSE............................................................................................................2-2

2.1.1 Origin of Coal ..............................................................................................................................................................2-2

2.1.2 Coal-Related Rocks....................................................................................................................................................2-3

2.2 COAL MINING AND COAL PREPARATION (CLEANING)..................................................................................2-4

2.3 REFUSE TRANSPORT AND DISPOSAL PLACEMENT ........................................................................................2-6

2.3.1 Coarse Refuse..............................................................................................................................................................2-6

2.3.2 Fine Refuse and Combined Refuse .....................................................................................................................2-6

2.4 COAL REFUSE CHARACTERIZATION......................................................................................................................2-7

2.4.1 General Geochemical Characteristics................................................................................................................2-7

2.4.2 General Geotechnical Characteristics................................................................................................................2-7

2.4.2.1 Coarse Coal Refuse....................................................................................................................................................2-8

2.4.2.2 Fine Coal Refuse Slurry ............................................................................................................................................2-8

2.4.2.3 Dewatered Fine Coal Refuse..................................................................................................................................2-9

2.4.2.4 Combined Coal Refuse ............................................................................................................................................2-9

2.4.2.5 Fine Coal Refuse Paste ..........................................................................................................................................2-10

2.5 DISPOSAL PRACTICES..............................................................................................................................................2-10

2.5.1 Disposal Practices Prior to the Buffalo Creek Failure .................................................................................2-10

2.5.2 Disposal Practices Subsequent to the Buffalo Creek Failure...................................................................2-11
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Table of Contents

Chapter 3 Coal Refuse Disposal Facilities and

Other Impounding Structures

3.1 Hazard Potential for Disposal Facilities and Other Impounding Structures................................................................3-1

3.2 Coal Refuse Impoundment Facilities..................................................................................................................3-3

3.2.1 Size and Hazard Considerations...........................................................................................................................3-4

3.2.1.1 Impoundment Definition ...................................................................................................................................3-4

3.2.1.2 Impoundments in Series.....................................................................................................................................3-5

3.2.1.3 Incised Impoundments.......................................................................................................................................3-5

3.2.2 Disposal Facility Configuration.............................................................................................................................3-6

3.2.3 Hazard Potential Rating...........................................................................................................................................3-9

3.3 COAL REFUSE NON-IMPOUNDING FACILITIES (REFUSE PILES) ..............................................................3-10

3.3.1 Coarse Refuse Embankments............................................................................................................................. 3-10

3.3.2 Combined Refuse Embankments..................................................................................................................... 3-11

3.3.3 Segregated Refuse Embankments................................................................................................................... 3-11

3.3.4 Configuration and Development Staging..................................................................................................... 3-11

3.4 COAL REFUSE SLURRY CELL FACILITIES............................................................................................................3-12

3.4.1 Size and Hazard Classification............................................................................................................................ 3-13

3.4.2 Disposal Facility Configuration and Development .................................................................................... 3-16

3.5 UNDERGROUND INJECTION SITES.....................................................................................................................3-17

3.5.1 Siting........................................................................................................................................................................... 3-17

3.5.2 Injection System Design ...................................................................................................................................... 3-18

3.5.3 Risk Assessment and Response Plan............................................................................................................... 3-19

3.5.4 Contingency Plan ................................................................................................................................................... 3-20

3.5.5 Monitoring Plan ...................................................................................................................................................... 3-20

3.6 RECOVERY (REMINING) OF COAL REFUSE DISPOSAL FACILITIES ..........................................................3-21

3.7 OTHER IMPOUNDING STRUCTURES ..................................................................................................................3-22

3.8 SMALL PONDS AND SIMILAR STRUCTURES ...................................................................................................3-23

Chapter 4 Project Planning

4.1 UNIQUE ASPECTS OF REFUSE DISPOSAL ...........................................................................................................4-1

4.2 OPERATIONS AND SITE-RELATED FACTORS ......................................................................................................4-2

4.3 SUSTAINABLE MINING PRACTICES........................................................................................................................4-4

4.4 ECONOMIC CONSIDERATIONS................................................................................................................................4-4

4.5 ENVIRONMENTAL AND REGULATORY CONSIDERATIONS ...........................................................................4-6

4.6 PLANNING SEQUENCE................................................................................................................................................4-7

4.7 DESIGN SEQUENCE......................................................................................................................................................4-7
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Engineering and Design Manual Coal Refuse Disposal Facilities

Chapter 5 Coal Refuse Disposal Facility

Design Components

5.1 DISPOSAL CAPACITY REQUIREMENTS AND SCHEDULING..........................................................................5-1

5.1.1 Refuse Generation Rates and Design Capacity/Life......................................................................................5-1

5.1.2 Slurry Impoundment Staging and Scheduling...............................................................................................5-2

5.1.3 Slurry Cell Staging.....................................................................................................................................................5-3

5.1.4 Non-impounding Refuse Embankment Staging ...........................................................................................5-5

5.1.5 Amendments and Co-disposal of Combustion Waste.................................................................................5-7

5.1.5.1 Amendments for Neutralization and Stabilization of Refuse................................................................5-7

5.1.5.2 Co-disposal of Combustion Waste ..................................................................................................................5-7

5.2 EROSION AND SEDIMENTATION CONTROL AND DESIGN STORM RUNOFF

MANAGEMENT...............................................................................................................................................................5-8

5.2.1 Erosion and Sedimentation Control ...................................................................................................................5-8

5.2.2 Slurry Impoundment Inflow Design Storm......................................................................................................5-9

5.2.3 Slurry Cell Facilities...................................................................................................................................................5-9

5.2.4 Refuse Embankment Design Storm................................................................................................................. 5-11

5.3 FACILITY CONFIGURATION AND GEOMETRY................................................................................................. 5-11

5.3.1 Regulatory Criteria ................................................................................................................................................. 5-11

5.3.2 Embankment Design Considerations............................................................................................................. 5-11

5.3.2.1 Stability under Normal Operating Conditions......................................................................................... 5-11

5.3.2.2 Stability under Extreme Events..................................................................................................................... 5-12

5.3.2.3 Settlement and Subsidence............................................................................................................................ 5-12

5.3.2.4 Environmental Issues, Impoundment Elimination and Site Reclamation...................................... 5-13

5.3.3 Impoundment Design Considerations........................................................................................................... 5-13

5.3.3.1 Settling and Clarification ................................................................................................................................. 5-13

5.3.3.2 Seepage Control and Containment............................................................................................................. 5-13

5.3.3.3 Spillway and Decant Systems......................................................................................................................... 5-13

5.3.3.4 Subsidence and Breakthrough Potential ................................................................................................... 5-13

5.4 FOUNDATION AND MINE SUBSIDENCE CONSIDERATIONS ..................................................................... 5-13

5.4.1 Foundation Materials............................................................................................................................................ 5-13

5.4.2 Abutment and Foundation Geometry............................................................................................................ 5-14

5.4.3 Mine Subsidence Potential ................................................................................................................................. 5-14

5.4.4 Impoundment Breakthrough Potential into Mine Workings.................................................................. 5-15

5.4.5 Mine Entry Barriers and Bulkheads.................................................................................................................. 5-16

5.5 INTERNAL DRAINAGE AND EMBANKMENT SEEPAGE CONTROL............................................................ 5-16

5.6 DECANT AND EMERGENCY SPILLWAY SYSTEMS.......................................................................................... 5-17

5.6.1 Decant Performance Considerations.............................................................................................................. 5-17

5.6.2 Open-Channel Spillway Performance Considerations.............................................................................. 5-18

5.7 SURFACE DRAINAGE CONTROLS......................................................................................................................... 5-18
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5.7.1 Permanent Drainage Controls........................................................................................................................... 5-18

5.7.2 Temporary Drainage Controls............................................................................................................................ 5-19

5.8 INSTRUMENTATION AND PERFORMANCE MONITORING .........................................................................5-19

5.8.1 Seepage .....................................................................................................................................................................5-19

5.8.2 Piezometric Levels..................................................................................................................................................5-19

5.8.3 Pool Levels................................................................................................................................................................5-19

5.8.4 Rainfall Data .............................................................................................................................................................5-20

5.8.5 Deformation or Movement.................................................................................................................................5-20

5.9 RECLAMATION, ABANDONMENT AND POST-MINING LAND USE.........................................................5-20

Chapter 6 Geotechnical Exploration, Material Testing,

Engineering Analysis and Design

6.1 GEOTECHNICAL DESIGN PROCEDURES ..............................................................................................................6-2

6.2 GENERAL CONSIDERATIONS....................................................................................................................................6-2

6.2.1 Unique Characteristics of Refuse Disposal Facilities.....................................................................................6-2

6.2.2 Site Conditions...........................................................................................................................................................6-4

6.2.2.1 Topography..............................................................................................................................................................6-4

6.2.2.2 Climate/Weather....................................................................................................................................................6-6

6.2.2.3 Geology and Surficial Soils.................................................................................................................................6-6

6.2.2.4 Miscellaneous Site Considerations..................................................................................................................6-7

6.2.3 Embankment Materials...........................................................................................................................................6-8

6.2.3.1 Coarse Coal Refuse................................................................................................................................................6-8

6.2.3.2 Fine Coal Refuse.................................................................................................................................................. 6-10

6.2.3.3 Combined Refuse ............................................................................................................................................... 6-10

6.2.3.4 Borrow Materials................................................................................................................................................. 6-11

6.2.3.5 Coal Combustion Products from Power Plants........................................................................................ 6-13

6.2.4 Scheduling................................................................................................................................................................ 6-18

6.3 DESIGN CONSIDERATIONS ....................................................................................................................................6-19

6.3.1 Impounding Embankments............................................................................................................................... 6-19

6.3.1.1 Homogeneous Embankments........................................................................................................................... 6-21

6.3.1.2 Zoned Embankments........................................................................................................................................ 6-21

6.3.1.3 Foundation Seepage Control......................................................................................................................... 6-26

6.3.2 Non-impounding Embankments..................................................................................................................... 6-30

6.3.3 Slurry Cell Embankments.................................................................................................................................... 6-34

6.3.4 Embankment Construction Staging................................................................................................................ 6-35

6.3.4.1 Upstream Method.............................................................................................................................................. 6-35

6.3.4.2 Downstream Method........................................................................................................................................ 6-37

6.3.4.3 Centerline Method............................................................................................................................................. 6-38

6.3.4.4 Embankments Supplemented with Borrow Material............................................................................ 6-39

6.3.5 Special Considerations for Existing Embankments.................................................................................... 6-39

6.3.6 Other Impounding Embankments................................................................................................................... 6-41
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Engineering and Design Manual Coal Refuse Disposal Facilities

6.4 SITE GEOTECHNICAL/GEOLOGICAL EXPLORATION ....................................................................................6-43

6.4.1 Background Data Sources................................................................................................................................... 6-44

6.4.1.1 Topographic Maps.............................................................................................................................................. 6-44

6.4.1.2 Geologic Maps and Publications................................................................................................................... 6-46

6.4.1.3 Agricultural Soil Surveys.................................................................................................................................. 6-47

6.4.1.4 Satellite Imagery, Aerial Photographs and Other Imagery.................................................................. 6-47

6.4.1.5 Past Investigations and Area Mining Plans............................................................................................... 6-48

6.4.1.6 Individual Site Mapping................................................................................................................................... 6-49

6.4.2 Surficial Reconnaissance and Geologic Mapping....................................................................................... 6-49

6.4.3 Subsurface Exploration and In-Situ Test Planning...................................................................................... 6-50

6.4.3.1 Program Planning............................................................................................................................................... 6-52

6.4.3.2 Subsurface Exploration and In-situ Test Methods and Applicability............................................... 6-54

6.4.3.3 Test Pits................................................................................................................................................................... 6-55

6.4.3.4 Boring Methods and the Standard Penetration Test (SPT).................................................................. 6-57

6.4.3.5 Undisturbed Soil Sampling............................................................................................................................. 6-59

6.4.3.6 Rock Coring........................................................................................................................................................... 6-59

6.4.3.7 Cone Penetrometer Test (CPT) and Piezocone Penetrometer Test (CPTu or PCPT).................... 6-61

6.4.3.8 Field Vane Shear Test (FVST) ........................................................................................................................... 6-63

6.4.3.9 Directional (Longhole) Drilling...................................................................................................................... 6-66

6.4.3.10 Field Hydraulic Conductivity Tests............................................................................................................... 6-67

6.4.3.11 Groundwater-Level Measurements............................................................................................................. 6-68

6.4.3.12 Water Flow and Quality Tests......................................................................................................................... 6-69

6.4.3.13 Backfilling of Boreholes.................................................................................................................................... 6-69

6.4.4 Geophysical Methods........................................................................................................................................... 6-70

6.4.4.1 Surficial Geophysical Techniques.................................................................................................................. 6-71

6.4.4.1.1 Seismic Refraction.......................................................................................................................................... 6-71

6.4.4.1.2 Seismic Reflection .......................................................................................................................................... 6-74

6.4.4.1.3 Electrical Resistivity........................................................................................................................................ 6-76

6.4.4.1.4 Spontaneous Potential (SP) ........................................................................................................................ 6-78

6.4.4.1.5 Electromagnetics (EM).................................................................................................................................. 6-79

6.4.1.1.6 Ground Penetrating Radar (GPR) .............................................................................................................. 6-80

6.4.4.1.7 Gravity................................................................................................................................................................. 6-82

6.4.4.1.8 Magnetics.......................................................................................................................................................... 6-83

6.4.4.2 Borehole Geophysical Techniques............................................................................................................... 6-83

6.4.4.2.1 Crosshole and Downhole/Uphole Seismic Surveys........................................................................... 6-85

6.4.4.2.2 Video and Laser/Sonar Imaging................................................................................................................ 6-89

6.5 MATERIAL PROPERTY DETERMINATION THROUGH TESTING..................................................................6-90

6.5.1 Selection of Samples for Testing....................................................................................................................... 6-91

6.5.1.1 Bulk and Disturbed Samples.......................................................................................................................... 6-91

6.5.1.2 Undisturbed Samples....................................................................................................................................... 6-95

6.5.1.3 Reconstituted Samples..................................................................................................................................... 6-96
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6.5.2 Classification and Index Property Tests.......................................................................................................... 6-96

6.5.2.1 Moisture Content................................................................................................................................................ 6-97

6.5.2.2 Specific Gravity..................................................................................................................................................6-100

6.5.2.3 Atterberg Limits................................................................................................................................................6-101

6.5.2.4 Particle-size Distribution................................................................................................................................6-102

6.5.2.5 Chemical Characterization............................................................................................................................6-104

6.5.3 Compaction and Density Tests........................................................................................................................6-105

6.5.3.1 Fine-grained Soils and Coal Refuse............................................................................................................6-105

6.5.3.2 Coarse-Grained Soils and Coal Refuse......................................................................................................6-108

6.5.4 Hydraulic Conductivity Tests............................................................................................................................6-109

6.5.5 Geosynthetic Materials Tests............................................................................................................................6-111

6.5.6 Consolidation Tests..............................................................................................................................................6-112

6.5.7 Shear Strength and Related Tests...................................................................................................................6-116

6.5.7.1 Vane-Shear Test.................................................................................................................................................6-120

6.5.7.2 Direct-Shear Test...............................................................................................................................................6-120

6.5.7.3 Unconfined-Compression Test ....................................................................................................................6-122

6.5.7.4 Triaxial-Compression Test..............................................................................................................................6-124

6.5.8 Seismic Property Characterization.................................................................................................................6-128

6.5.8.1 Cyclic-Triaxial Test.............................................................................................................................................6-128

6.5.8.2 Cyclic Loading Followed by Monotonic Loading..................................................................................6-128

6.5.8.3 Resonant-Column Test....................................................................................................................................6-129

6.5.9 Rock Property Tests..............................................................................................................................................6-129

6.5.9.1 Point-Load Index Test......................................................................................................................................6-129

6.5.9.2 Unconfined Compressive Strength Test...................................................................................................6-132

6.5.9.3 Indirect Tensile Strength Test.......................................................................................................................6-133

6.5.9.4 Rock Durability ..................................................................................................................................................6-133

6.5.10 Structural Material Tests.....................................................................................................................................6-135

6.5.10.1 Concrete...............................................................................................................................................................6-135

6.5.10.2 Controlled Low-Strength Materials (CLSM).............................................................................................6-135

6.5.11 Verification of Test Results.................................................................................................................................6-137

6.6 GEOTECHNICAL ANALYSES .................................................................................................................................6-138

6.6.1 Analytical and Numerical Methods in Geotechnical Engineering......................................................6-138

6.6.2 Seepage Analysis..................................................................................................................................................6-139

6.6.2.1 Basic Assumptions, Conditions Requiring Seepage Analysis...........................................................6-139

6.6.2.2 Seepage Analysis Methods...........................................................................................................................6-142

6.6.2.2.1 Graphical (Flow Net) Approach ...............................................................................................................6-143

6.6.2.2.2 Analytical Solutions.....................................................................................................................................6-144

6.6.2.2.3 Numerical Modeling Approach...............................................................................................................6-145

6.6.2.3 Seepage Control Measures...............................................................................................................................6-146

6.6.2.3.1 Granular Drain and Filter Requirements...............................................................................................6-146

6.6.2.3.2 Geotextile-Wrapped Drains......................................................................................................................6-149

6.6.2.3.3 Seepage Control along Conduits............................................................................................................6-156
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6.6.2.3.4 Relief Wells......................................................................................................................................................6-160

6.6.2.3.5 Horizontal Drains..........................................................................................................................................6-161

6.6.2.3.6 Impoundment Liners..................................................................................................................................6-163

6.6.2.3.7 Foundation Seepage Cutoffs....................................................................................................................6-164

6.6.2.3.8 Impoundment Water and Slurry Deposition Management..........................................................6-164

6.6.2.4 Seepage Measurements.................................................................................................................................6-164

6.6.3 Settlement Analysis.............................................................................................................................................6-164

6.6.3.1 Conditions Requiring Deformation Analysis..........................................................................................6-164

6.6.3.2 Settlement and Deformation Analysis......................................................................................................6-167

6.6.3.3 Deformation Control Measures...................................................................................................................6-170

6.6.3.4 Deformation Measurements........................................................................................................................6-170

6.6.4 Slope Stability Analysis.......................................................................................................................................6-171

6.6.4.1 Conditions Requiring Stability Analysis....................................................................................................6-171

6.6.4.2 Methods of Stability Analysis.......................................................................................................................6-173

6.6.4.2.1 Limit Equilibrium Stability Analysis........................................................................................................6-173

6.6.4.2.2 Stability/Deformation Analysis Using Finite Element Methods...................................................6-181

6.6.4.3 Acceptable Factors of Safety ........................................................................................................................6-181

6.6.4.4 Stability Control Measures............................................................................................................................6-184

6.6.4.5 Stability Measurements..................................................................................................................................6-185

6.6.5 Rock Excavations..................................................................................................................................................6-185

6.6.5.1 Conditions Requiring Stability Analysis....................................................................................................6-185

6.6.5.1.1 Existing Ground Surface Slope ................................................................................................................6-185

6.6.5.1.2 Overburden Soil Thickness, Type and Quality....................................................................................6-185

6.6.5.1.3 Bedrock Surface Conditions.....................................................................................................................6-186

6.6.5.1.4 Rock Type, Quality and Configuration...................................................................................................6-187

6.6.5.1.5 Groundwater Conditions...........................................................................................................................6-189

6.6.5.1.6 Other Factors..................................................................................................................................................6-189

6.6.5.2 Slope Stability Analysis...................................................................................................................................6-189

6.6.5.3 Stability Control Measures............................................................................................................................6-191

6.6.5.4 Stability Measurements..................................................................................................................................6-193

6.6.6 Conduit Structural Design for Earthen Fill Loads......................................................................................6-193

6.6.6.1 Conduit Types....................................................................................................................................................6-194

6.6.6.1.1 Concrete...........................................................................................................................................................6-194

6.6.6.1.2 Thermoplastic ................................................................................................................................................6-195

6.6.6.1.3 Metal .................................................................................................................................................................6-195

6.6.6.2 Soil-Structure Design ......................................................................................................................................6-196

6.6.6.3 Conduit Installation and Design Details...................................................................................................6-201

6.6.6.3.1 Installation ......................................................................................................................................................6-201

6.6.6.3.2 Rigid Conduit Support................................................................................................................................6-201

6.6.6.3.3 Flexible Conduit Support...........................................................................................................................6-202

6.6.6.3.4 Watertightness..............................................................................................................................................6-204

6.6.7 Blasting Impacts....................................................................................................................................................6-206
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Chapter 7 Seismic Design: Stability and

Deformation Analyses

7.1 GENERAL ..........................................................................................................................................................................7-1

7.1.1 Design Approach.......................................................................................................................................................7-1

7.1.1.1 Seismic Instability..................................................................................................................................................7-2

7.1.1.2 Excessive Deformations......................................................................................................................................7-3

7.1.2 Seismic Design Considerations and Flow Chart.............................................................................................7-3

7.1.3 Sand-Like Versus Clay-Like Material....................................................................................................................7-9

7.1.4 Susceptibility to Strength Loss.......................................................................................................................... 7-11

7.1.5 Simplified Steps for Seismic Stability .............................................................................................................. 7-12

7.1.5.1 Step 1 ...................................................................................................................................................................... 7-12

7.1.5.2 Step 2 ...................................................................................................................................................................... 7-12

7.1.5.3 Step 3 ...................................................................................................................................................................... 7-12

7.1.5.4 Step 4 ...................................................................................................................................................................... 7-12

7.1.5.5 Step 5 ...................................................................................................................................................................... 7-13

7.1.5.6 Step 6 ...................................................................................................................................................................... 7-13

7.2 TERMINOLOGY............................................................................................................................................................7-14

7.3 CHARACTERIZATION OF SUBSURFACE CONDITIONS AND MATERIAL PROPERTIES......................7-18

7.4 SEISMIC STABILITY ANALYSES..............................................................................................................................7-21

7.4.1 General Discussion................................................................................................................................................. 7-21

7.4.1.1 Triggering Analyses........................................................................................................................................... 7-21

7.4.1.2 Evaluation of Post-Earthquake Strength.................................................................................................... 7-22

7.4.1.3 Stability Analysis Using Post-Earthquake Strengths.............................................................................. 7-23

7.4.2 Triggering Analyses............................................................................................................................................... 7-23

7.4.2.1 Pore-Pressure-Based Method for Triggering of Strength Loss in Clay-Like Material.................. 7-23

7.4.2.2 Pore-Pressure-Based Method for Triggering of Strength Loss in Sand-Like Material................ 7-23

7.4.2.2.1 Evaluation of Cyclic Stress Ratio (CSR)..................................................................................................... 7-25

7.4.2.2.2 Evaluation of Cyclic Resistance Ratio (CRR)........................................................................................... 7-28

7.4.2.2.3 Correction Factors for Earthquake Magnitude, High Overburden, Sloping Ground 

and Age of Deposit........................................................................................................................................ 7-31

7.4.2.3 Strain-Based and Stress-Based Methods to Evaluate Triggering of Strength Loss......................... 7-32

7.4.2.3.1 Clay-Like Material: Strain-Based Method for Triggering....................................................................... 7-32

7.4.2.3.2 Sand-Like Material: Strain-Based Method for Triggering..................................................................... 7-33

7.4.2.3.3 Sand-Like Material: Stress-Based Method for Triggering..................................................................... 7-35

7.4.3 Evaluation of Post-Earthquake Strength........................................................................................................ 7-36

7.4.3.1 Correlations of SPT and CPT with Sus of Sand-Like Material ................................................................ 7-36

7.4.3.2 Laboratory Testing for Measuring Post-Earthquake Strength for Clay-Like Material 

and Sus for Sand-Like Material......................................................................................................................... 7-41

7.4.3.2.1 Laboratory Testing Issues............................................................................................................................ 7-41

7.4.3.2.2 Laboratory Testing of Soft Clay-like Material to Measure Post-Earthquake Strength............ 7-41

7.4.3.2.3 Laboratory Testing of Sand-like Material to Measure Sus..................................................................7-44
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7.4.3.3 Field testing to Measure Sus of Soft Clay-Like Material.......................................................................... 7-51

7.4.4 Analysis Steps.......................................................................................................................................................... 7-51

7.4.4.1 Step 1 - Define Embankment Geometry.................................................................................................... 7-51

7.4.4.2 Step 2 - Screen for Potential Strength Loss............................................................................................... 7-51

7.4.4.2.1 Sand-Like Material.......................................................................................................................................... 7-51

7.4.4.2.2 Clay-Like Material ........................................................................................................................................... 7-52

7.4.4.2.3 Screening Results........................................................................................................................................... 7-54

7.4.4.3 Step 3 - Define Post-Earthquake Strengths for Limit Equilibrium Stability Analyses................. 7-54

7.4.4.4 Step 4 - Perform Initial Stability Analysis.................................................................................................... 7-56

7.4.4.5 Optional Step 5 - Perform More Detailed Evaluation of Sus for Sand-Like Material .................... 7-57

7.4.4.6 Optional Step 6 - Perform Triggering Analysis......................................................................................... 7-57

7.4.5 Comments on Methods for Evaluating Triggering and Strength.......................................................... 7-58

7.5 SEISMIC DEFORMATION ANALYSES...................................................................................................................7-59

7.5.1 General Discussion................................................................................................................................................. 7-59

7.5.2 Preliminary Screening........................................................................................................................................... 7-61

7.5.3 Pseudo-Static Procedure for Screening.......................................................................................................... 7-61

7.5.4 Newmark-Type Analysis (No Cyclic Mobility) ............................................................................................... 7-62

7.5.5 Numerical Modeling with No Cyclic Mobility............................................................................................... 7-65

7.5.6 Numerical Modeling Considering Cyclic Mobility ...................................................................................... 7-65

7.5.7 Acceptable Deformations.................................................................................................................................... 7-66

7.6 EMBANKMENT MODIFICATIONS FOR IMPROVING STABILITY OR RESISTANCE TO 

DEFORMATIONS.........................................................................................................................................................7-66

7.7 SEISMIC HAZARD ASSESSMENT (SEISMICITY)...............................................................................................7-66

7.7.1 Analytical Procedures............................................................................................................................................ 7-71

7.7.1.1 Deterministic Seismic Hazard Analysis (DSHA)........................................................................................ 7-71

7.7.1.2 Probabilistic Seismic Hazard Analysis (PSHA)........................................................................................... 7-73

7.7.1.3 DSHA versus PSHA............................................................................................................................................. 7-74

7.7.2 Seismotectonic Modeling ................................................................................................................................... 7-76

7.7.2.1 Regional Tectonic Framework........................................................................................................................ 7-77

7.7.2.2 Historical Seismicity........................................................................................................................................... 7-78

7.7.2.3 Selection of Seismic Sources.......................................................................................................................... 7-78

7.7.2.4 Maximum Magnitude ....................................................................................................................................... 7-79

7.7.2.5 Earthquake Recurrence.................................................................................................................................... 7-81

7.7.3 Design Ground Motion......................................................................................................................................... 7-81

7.7.3.1 Selection of Design Earthquakes.................................................................................................................. 7-82

7.7.3.2 Ground Motion Attenuation........................................................................................................................... 7-83

7.7.3.3 Selection of Peak Ground Motion Parameters......................................................................................... 7-84

7.7.3.4 Selection of Acceleration Time Histories.................................................................................................... 7-84

7.7.3.5 Applicability of Design Ground Motion ..................................................................................................... 7-86

7.7.3.6 Application of Seismic Parameters in Design Process .......................................................................... 7-86

7.7.3.7 Simplified Design Ground Motion for Areas of Low Seismic Hazard............................................... 7-86

7.8 SEISMIC DESIGN OVERVIEW..................................................................................................................................7-88
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APPENDIX 7A DERIVATIONS OF BASIC EQUATIONS FOR STEADY-STATE LABORATORY TESTING

APPENDIX 7B VOID-RATIO MEASUREMENTS DURING UNDISTRUBED TUBE SAMPLING AND

LABORATORY TESTING 

APPENDIX 7C PROCEDURE FOR UNDISTURBED, FIXED-PISTON SAMPLING OF COHESIONLESS SOIL

APPENDIX 7D COMBINING NEWMARK-TYPE DISPLACEMENTS COMPUTED USING 1D SITERESPONSE ANALYSIS

Chapter 8 Site Mining and Foundation Issues

8.1 GENERAL CONSIDERATIONS....................................................................................................................................8-2

8.2 Available Sources of Site Information................................................................................................................8-3

8.2.1 Mine Maps....................................................................................................................................................................8-3

8.2.2 Coal Contour Maps and Outcrops.......................................................................................................................8-4

8.2.3 Interpretation and Accuracy of Mine Maps.....................................................................................................8-5

8.3 ON-SITE RECONNAISSANCE AND EXPLORATION............................................................................................8-6

8.3.1 Surficial Reconnaissance.........................................................................................................................................8-6

8.3.2 Geophysical Methods..............................................................................................................................................8-7

8.3.3 Long-Hole Directional Drilling..............................................................................................................................8-7

8.3.4 Conventional Drilling and Sampling..................................................................................................................8-7

8.4 EVALUATION OF MINE SUBSIDENCE AND BREAKTHROUGH......................................................................8-9

8.4.1 Mine Subsidence Considerations.....................................................................................................................8-10

8.4.2 Potential Subsidence and Failure Mechanisms...........................................................................................8-14

8.4.2.1 Sinkholes...............................................................................................................................................................8-15

8.4.2.2 Pillar Failure...........................................................................................................................................................8-18

8.4.2.3 Pillar Punching (Floor Failure) ........................................................................................................................8-19

8.4.2.4 Outcrop Barrier Failure by Shear or Punching..........................................................................................8-20

8.4.2.5 Internal Erosion ...................................................................................................................................................8-20

8.4.2.6 Bulkhead Failure..................................................................................................................................................8-22

8.4.2.7 Trough Subsidence and Subsidence Cracks.............................................................................................8-22

8.4.2.8 Failures Related to Auger and Highwall Mining......................................................................................8-23

8.4.2.9 Hillside Movement or Disturbance ..............................................................................................................8-24

8.4.2.10 Mine Blowout.......................................................................................................................................................8-24

8.4.2.11 Seismic and Blasting Events...........................................................................................................................8-24

8.4.3 Mine Subsidence Potential and Analysis.......................................................................................................8-25

8.4.3.1 Pillar Evaluation and Analysis.........................................................................................................................8-25

8.4.3.2 Subsidence Evaluation and Analysis...........................................................................................................8-28

8.4.3.3 Subsidence Damage Criteria..........................................................................................................................8-29

8.4.4 Mine Breakthrough Potential and Analysis...................................................................................................8-30

8.4.4.1 Outcrop Barriers..................................................................................................................................................8-30

8.4.4.2 Overburden Barriers..........................................................................................................................................8-33

8.5 SUBSIDENCE AND BREAKTHROUGH MITIGATION METHODS ................................................................8-34

8.5.1 Mine Subsidence and Breakthrough Mitigation.........................................................................................8-35
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8.5.1.1 Use of Safety Zones............................................................................................................................................8-35

8.5.1.2 Mine Backfilling...................................................................................................................................................8-35

8.5.1.3 Stowing of Mine Openings and Associated Barrier Construction ....................................................8-36

8.5.1.4 Bulkhead Construction.....................................................................................................................................8-38

8.5.1.5 Construction of Compacted Earthen Barriers on the Surface............................................................8-39

8.5.1.6 Conversion to Slurry Cells................................................................................................................................8-39

8.5.1.7 Sealing Sources of Leakage ............................................................................................................................8-40

8.5.1.8 Stabilization of Fines.........................................................................................................................................8-40

8.5.1.9 Overexcavation and Induced Subsidence.................................................................................................8-41

8.5.1.10 Monitoring Provisions.......................................................................................................................................8-42

8.5.2 Mine Entry Bulkhead Seal Design.....................................................................................................................8-42

8.5.2.1 Site Preparation...................................................................................................................................................8-42

8.5.2.2 Bulkhead Types....................................................................................................................................................8-43

8.5.2.3 Design Loads........................................................................................................................................................8-47

8.5.2.4 Design of Solid Concrete Block Bulkheads................................................................................................8-47

8.5.2.5 Design of Concrete Bulkheads.......................................................................................................................8-48

8.5.2.5.1 Design of Reinforced-Concrete Bulkheads for Flexure.....................................................................8-48

8.5.2.5.2 Design of Unreinforced-Concrete Plug Bulkheads for Shear..........................................................8-49

8.5.2.5.3 Design of Reinforced-Concrete Bulkheads for Shear ........................................................................8-50

8.5.2.6 Monitoring of Bulkheads.................................................................................................................................8-51

8.6 FOUNDATION-RELATED CONSTRUCTION AND OPERATIONS MONITORING...................................8-52

8.7 MINE BACKFILLING DESIGN..................................................................................................................................8-53

8.8 SURFACE MINE SPOIL ISSUES ...............................................................................................................................8-55

8.8.1 Surface Mine Spoil Characteristics...............................................................................................................8-55

8.8.2 Design and Construction Considerations..................................................................................................8-56

8.9 SURFACE MINE HIGHWALL ISSUES.....................................................................................................................8-59

Chapter 9 Hydrology and Hydraulics

9.1 GENERAL CONSIDERATIONS....................................................................................................................................9-2

9.2 HYDROLOGY AND HYDRAULICS PRINCIPLES...................................................................................................9-2

9.2.1 Basic Design Principles............................................................................................................................................9-5

9.2.2 Definition and Discussion of Key Runoff Elements.......................................................................................9-7

9.2.2.1 Watershed Boundary and Area.........................................................................................................................9-7

9.2.2.2 Precipitation ............................................................................................................................................................9-7

9.2.2.2.1 Rainfall Curves....................................................................................................................................................9-9

9.2.2.2.2 Rainfall Intensity.................................................................................................................................................9-9

9.2.2.3 Watershed Characteristics............................................................................................................................... 9-10

9.2.3 Key Storage and Outflow Elements................................................................................................................. 9-11

9.2.3.1 Impoundment Capacity................................................................................................................................... 9-11

9.2.3.2 Decants, Principal Spillways and Auxiliary Spillways............................................................................. 9-11

9.3 GENERAL CONSIDERATIONS FOR COAL REFUSE DISPOSAL FACILITIES .............................................9-15
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9.3.1 Special Characteristics.......................................................................................................................................... 9-15

9.3.2 Site Conditions........................................................................................................................................................ 9-15

9.3.2.1 Topography........................................................................................................................................................... 9-16

9.3.2.1.1 Steep Terrain..................................................................................................................................................... 9-16

9.3.2.1.2 Gently Sloping Terrain................................................................................................................................... 9-16

9.3.2.1.3 Effects of Slope on Facility Staging .......................................................................................................... 9-17

9.3.2.2 Weather and Climate......................................................................................................................................... 9-19

9.3.2.3 Geology.................................................................................................................................................................. 9-19

9.3.3 Construction Materials......................................................................................................................................... 9-20

9.4 DESIGN CONSIDERATIONS FOR DISPOSAL FACILITY EMBANKMENT TYPES ....................................9-20

9.4.1 Non-Impounding Embankments..................................................................................................................... 9-20

9.4.1.1 Valley-Fill Embankments.................................................................................................................................. 9-21

9.4.1.2 Side-Hill, Ridge and Heaped Embankments............................................................................................. 9-22

9.4.2 Slurry Cell Embankments.................................................................................................................................... 9-22

9.4.3 Slurry Impoundments........................................................................................................................................... 9-24

9.4.3.1 Cross-Valley Impoundment............................................................................................................................ 9-25

9.4.3.2 Diked Impoundment......................................................................................................................................... 9-26

9.4.3.3 Incised Impoundment...................................................................................................................................... 9-26

9.4.4 Other Impounding Structures........................................................................................................................... 9-26

9.4.4.1 Sedimentation and Treatment Ponds......................................................................................................... 9-28

9.4.4.2 Fresh Water Impoundments........................................................................................................................... 9-29

9.5 DESIGN STORM CRITERIA.......................................................................................................................................9-29

9.5.1 Design Storms for Impoundments................................................................................................................... 9-29

9.5.1.1 General Considerations.................................................................................................................................... 9-29

9.5.1.2 Recommended Design Storm Criteria........................................................................................................ 9-32

9.5.1.3 Size and Hazard-Potential Classification.................................................................................................... 9-32

9.5.1.3.1 Impoundment-Size Classification............................................................................................................. 9-33

9.5.1.3.2 Hazard-Potential Classification.................................................................................................................. 9-33

9.5.1.4 Determination of Design Storm Precipitation ............................................................................................. 9-34

9.5.1.4.1 Prediction of the PMP and PMTS............................................................................................................... 9-34

9.5.1.4.2 Prediction of the 100-year and Lesser Design Storms....................................................................... 9-35

9.5.2 Special Considerations for Short-Term Conditions.................................................................................... 9-36

9.5.3 Hydraulic Design Criteria for Drainage Conveyance Installations........................................................ 9-37

9.6 DETERMINATION OF RUNOFF QUANTITIES....................................................................................................9-38

9.6.1 Basic Hydrology Parameters............................................................................................................................... 9-38

9.6.1.1 Precipitation Intensity-Duration and Distribution.................................................................................. 9-38

9.6.1.2 Unit Hydrographs and Time of Concentration......................................................................................... 9-40

9.6.1.2.1 Snyder Unit Hydrograph.............................................................................................................................. 9-41

9.6.1.2.2 Clark Unit Hydrograph.................................................................................................................................. 9-42

9.6.1.2.3 SCS Dimensionless Unit Hydrograph...................................................................................................... 9-42

9.6.1.3 Precipitation-Runoff Relationship ................................................................................................................ 9-46

9.6.1.3.1 General Rainfall Conditions........................................................................................................................ 9-46
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9.6.1.3.2 Thunderstorm Rainfall Conditions........................................................................................................... 9-49

9.6.1.4 Channel and Impoundment Storage Characteristics............................................................................ 9-52

9.6.2 Runoff Determination: Hydrograph Method............................................................................................... 9-54

9.6.3 Peak Runoff Determination – Key Parameters Method............................................................................ 9-55

9.6.4 Runoff Determination – Rational Method..................................................................................................... 9-58

9.7 DESIGN OF OUTFLOW SYSTEMS .........................................................................................................................9-60

9.7.1 Basic Considerations............................................................................................................................................. 9-61

9.7.2 Hydraulic System Components......................................................................................................................... 9-63

9.7.2.1 Inlets........................................................................................................................................................................ 9-63

9.7.2.2 Transport Sections............................................................................................................................................. 9-67

9.7.2.2.1 Open-Channel Flow....................................................................................................................................... 9-67

9.7.2.2.2 Pressure Flow ................................................................................................................................................... 9-72

9.7.2.3 Outlets.................................................................................................................................................................... 9-81

9.7.3 Special Design Considerations.......................................................................................................................... 9-83

9.7.3.1 Channel Freeboard ............................................................................................................................................ 9-83

9.7.3.2 Erosion Protection.............................................................................................................................................. 9-83

9.7.3.2.1 Grass-Lined Channels.................................................................................................................................... 9-86

9.7.3.2.2 Riprap-Lined Channels................................................................................................................................. 9-86

9.7.3.2.3 Grouted-Riprap Channels............................................................................................................................ 9-90

9.7.3.2.4 Concrete-Lined Channels............................................................................................................................ 9-92

9.7.3.2.5 Commercially Available Composite Erosion Control Products...................................................... 9-93

9.7.3.3 Cavitation .............................................................................................................................................................. 9-93

9.7.3.4 Directional Changes in Open-Channel Flow ............................................................................................ 9-94

9.7.3.5 Directional Changes in Conduit Flow.......................................................................................................... 9-94

9.7.3.6 Materials Selection............................................................................................................................................. 9-95

9.7.3.6.1 Open-Channel Lining Systems.................................................................................................................. 9-95

9.7.3.6.2 Conduit Materials........................................................................................................................................... 9-95

9.7.4 Types of Hydraulic Systems................................................................................................................................. 9-95

9.7.4.1 Decant Systems................................................................................................................................................... 9-96

9.7.4.1.1 Inlet...................................................................................................................................................................... 9-96

9.7.4.1.2 Transport Section ........................................................................................................................................... 9-98

9.7.4.2 Overflow Spillway Systems............................................................................................................................. 9-99

9.7.4.3 Diversion and Collection Ditches................................................................................................................9-110

9.7.4.4 Culverts................................................................................................................................................................9-110

9.7.4.5 Natural Streams.................................................................................................................................................9-110

9.8 RESERVOIR ROUTING.............................................................................................................................................9-111

9.8.1 Basic Routing Methodology .............................................................................................................................9-111

9.8.2 Basic Routing Parameters..................................................................................................................................9-112

9.9 DAM BREACH ANALYSIS AND INUNDATION MAPPING...........................................................................9-113

9.9.1 Background ............................................................................................................................................................9-113

9.9.2 Failure Scenarios...................................................................................................................................................9-113

9.9.3 Analytical Methods..............................................................................................................................................9-115
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9.9.4 Input Data ...............................................................................................................................................................9-116

9.9.4.1 Breach Parameters...........................................................................................................................................9-116

9.9.4.2 Initial Conditions...............................................................................................................................................9-118

9.9.4.3 Flow Channel Geometry and Roughness................................................................................................9-118

9.9.5 Results of Analysis/Inundation Mapping.....................................................................................................9-118

9.9.6 Sensitivity Analyses..............................................................................................................................................9-119

9.9.7 Hazard Classification ...........................................................................................................................................9-119

Chapter 10 Environmental Considerations

10.1 STREAMS AND WETLANDS....................................................................................................................................10-1

10.2 AIR QUALITY ................................................................................................................................................................10-3

10.2.1 Dust Control ............................................................................................................................................................. 10-3

10.2.2 Combustion Control.............................................................................................................................................. 10-4

10.2.3 Refuse Fire Extinguishment................................................................................................................................ 10-5

10.2.3.1 Excavation and Removal.................................................................................................................................. 10-6

10.2.3.2 Surface Treatment.............................................................................................................................................. 10-8

10.2.3.3 Water and Slurry Injection............................................................................................................................... 10-9

10.3 WATER QUALITY.......................................................................................................................................................10-10

10.3.1 Erosion and Sedimentation ..............................................................................................................................10-10

10.3.1.1 Prevention...........................................................................................................................................................10-10

10.3.1.2 Control..................................................................................................................................................................10-11

10.3.1.2.1 Sedimentation Ponds..................................................................................................................................10-11

10.3.1.2.2 Sediment Traps and Check Dams...........................................................................................................10-12

10.3.1.2.3 Silt Fences........................................................................................................................................................10-12

10.3.1.2.4 Erosion Control Blankets and Reinforcement Mats..........................................................................10-12

10.3.1.2.5 Vegetation.......................................................................................................................................................10-12

10.3.2 Acid Generation and Control ...........................................................................................................................10-12

10.3.2.1 Background.........................................................................................................................................................10-13

10.3.2.2 Grading, Compaction and Sealing .............................................................................................................10-13

10.3.2.3 Amendments.....................................................................................................................................................10-13

10.3.2.4 Reclamation and Vegetative Cover............................................................................................................10-15

10.3.3 Water Quality Control .........................................................................................................................................10-15

10.3.3.1 Diversion of Runoff ..........................................................................................................................................10-15

10.3.3.2 Treatment............................................................................................................................................................10-15

10.3.4 Water Quality Impacts on Construction Materials...................................................................................10-16

10.3.5 Hydrogeology........................................................................................................................................................10-17

10.4 LINER SYSTEMS.........................................................................................................................................................10-19

10.4.1 Design Requirements..........................................................................................................................................10-20

10.4.2 Stability ....................................................................................................................................................................10-23

10.4.3 Performance Considerations............................................................................................................................10-24

10.5 RECLAMATION..........................................................................................................................................................10-26
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10.5.1 Design Considerations........................................................................................................................................10-26

10.5.2 Grading.....................................................................................................................................................................10-27

10.5.3 Impoundment Elimination ...............................................................................................................................10-28

10.5.4 Soil and Topsoil Cover.........................................................................................................................................10-29

10.5.5 Vegetation...............................................................................................................................................................10-30

Chapter 11 Construction and Disposal Operations

11.1 OPERATIONAL PLANS AND CONSTRUCTION DOCUMENTS....................................................................11-1

11.1.1 Design Report.......................................................................................................................................................... 11-3

11.1.2 Construction Drawings......................................................................................................................................... 11-4

11.1.2.1 Title Sheet and Existing Conditions Plans.................................................................................................. 11-4

11.1.2.2 Initial Site Development Plans....................................................................................................................... 11-5

11.1.2.3 Refuse Embankment Construction Plan Views........................................................................................ 11-6

11.1.2.4 Cross Sections and Profiles.............................................................................................................................. 11-6

11.1.2.5 Detailed Plan Views and Cross Sections..................................................................................................... 11-8

11.1.2.6 Miscellaneous Details and Information...................................................................................................... 11-8

11.1.3 Technical Specifications....................................................................................................................................... 11-9

11.1.4 Operation and Maintenance Plan...................................................................................................................11-11

11.1.5 Calculation Brief....................................................................................................................................................11-13

11.2 PLANNING AND SCHEDULING...........................................................................................................................11-14

11.2.1 Planning of Coal Refuse Disposal Operations............................................................................................11-14

11.2.1.1 Long-Term Planning ........................................................................................................................................11-15

11.2.1.2 Short-Term Planning........................................................................................................................................11-16

11.2.2 Scheduling Methods and Application..........................................................................................................11-17

11.2.2.1 Scheduling Data................................................................................................................................................11-17

11.2.2.2 Applicability of Scheduling Techniques...................................................................................................11-18

11.3 REFUSE TRANSPORT...............................................................................................................................................11-19

11.3.1 Trucks and Scrapers.............................................................................................................................................11-20

11.3.2 Conveyor Belt Systems.......................................................................................................................................11-20

11.4 HANDLING EQUIPMENT........................................................................................................................................11-21

11.4.1 Loading and Hauling Equipment ...................................................................................................................11-21

11.4.2 Spreading Equipment.........................................................................................................................................11-22

11.4.3 Compaction Equipment.....................................................................................................................................11-23

11.4.4 Use of Handling Equipment for Related Activities...................................................................................11-26

11.5 CONSTRUCTION AND PLACEMENT OF EMBANKMENT MATERIALS...................................................11-26

11.5.1 Embankment Fill...................................................................................................................................................11-26

11.5.2 Upstream Construction Implementation Procedure...............................................................................11-33

11.5.3 Excess Coarse Refuse/Inclement Weather Disposal.................................................................................11-35

11.5.4 Disposal of Fine Refuse Slurry..........................................................................................................................11-36

11.5.5 Disposal of Combined Refuse..........................................................................................................................11-38

11.5.6 Use of Amendments............................................................................................................................................11-38
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11.6 RELATED ACTIVITIES...............................................................................................................................................11-38

11.6.1 Haul and Access Road Construction and Maintenance..........................................................................11-39

11.6.2 Liner Systems.........................................................................................................................................................11-39

11.6.3 Embankment Foundation Preparation.........................................................................................................11-40

11.6.3.1 Clearing and Topsoil Stockpiling ................................................................................................................11-40

11.6.3.2 Soft Soil Foundations......................................................................................................................................11-40

11.6.3.3 Competent Soil Foundations.......................................................................................................................11-40

11.6.3.4 Rock Foundations.............................................................................................................................................11-40

11.6.3.5 Mine Spoil............................................................................................................................................................11-41

11.6.4 Water Control.........................................................................................................................................................11-41

11.6.5 General Excavation ..............................................................................................................................................11-43

11.6.6 Embankment Fill Constructed from On-Site Borrow Materials...........................................................11-43

11.6.6.1 Clayey and Silty Borrow Materials..............................................................................................................11-43

11.6.6.2 Rock and Granular Borrow Materials.........................................................................................................11-44

11.6.6.3 Use of Mine Spoil..............................................................................................................................................11-45

11.7 FACILITY APPURTENANT STRUCTURES..........................................................................................................11-46

11.7.1 Material Requirements.......................................................................................................................................11-46

11.7.2 Filters and Drains..................................................................................................................................................11-46

11.7.2.1 Granular and Geotextile Filters....................................................................................................................11-50

11.7.2.2 Granular Underdrains......................................................................................................................................11-51

11.7.2.3 Drainage Pipe.....................................................................................................................................................11-53

11.7.3 Culverts and Decants..........................................................................................................................................11-53

11.7.4 Concrete Structures.............................................................................................................................................11-54

11.7.5 Gates, Valves, Pipelines and Other Metal Work..........................................................................................11-55

11.7.6 Support Structures...............................................................................................................................................11-55

11.7.7 Mine-Opening and Auger-Hole Seals and Drains.....................................................................................11-56

11.8 QUALITY CONTROL AND FIELD TESTING.......................................................................................................11-56

11.8.1 Compaction Control............................................................................................................................................11-56

11.8.2 Material Testing.....................................................................................................................................................11-58

Chapter 12 Monitoring, Inspections and

Facility Maintenance

12.1 MONITORING AND INSPECTIONS....................................................................................................................... 12-2

12.1.1 Monitoring and Inspection Objectives........................................................................................................... 12-3

12.1.2 Visual Observations............................................................................................................................................... 12-4

12.1.2.1 Schedule and Checklist for Visual (and Other) Observations............................................................. 12-7

12.1.2.2 Facility Development........................................................................................................................................ 12-8

12.1.2.2.1 Site Preparation............................................................................................................................................... 12-8

12.1.2.2.2 Starter Embankment and Internal Drains.............................................................................................. 12-8

12.1.2.2.3 Decant Pipes...................................................................................................................................................12-12

12.1.2.2.4 Channel Construction.................................................................................................................................12-12
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12.1.2.2.5 Instrumentation............................................................................................................................................12-12

12.1.2.2.6 Reclamation....................................................................................................................................................12-12

12.1.2.3 Adjoining Area Conditions................................................................................................................................12-13

12.1.2.3.1 Sloughing or Sliding....................................................................................................................................12-13

12.1.2.3.2 Signs of Subsidence.....................................................................................................................................12-13

12.1.2.3.3 Evidence of Erosion......................................................................................................................................12-13

12.1.2.3.4 Springs or Seepage Areas..........................................................................................................................12-13

12.1.2.3.5 Changes in Vegetation................................................................................................................................12-14

12.1.2.4 Other Development Work .................................................................................................................................12-14

12.1.2.4.1 Upstream Development.............................................................................................................................12-14

12.1.2.4.2 Downstream Development......................................................................................................................12-14

12.1.2.4.3 Disposal of Materials Other than Coal Refuse....................................................................................12-14

12.1.2.5 Refuse Disposal Operations and Maintenance..........................................................................................12-14

12.1.2.5.1 Refuse Placement and Compaction.......................................................................................................12-14

12.1.2.5.2 Working Surface Maintenance ................................................................................................................12-15

12.1.2.5.3 Embankment Crest Conditions...............................................................................................................12-15

12.1.2.5.4 Embankment Slope Conditions..............................................................................................................12-15

12.1.2.5.5 Liner, Foundation and Abutment Conditions....................................................................................12-17

12.1.2.5.6 Internal Drains...............................................................................................................................................12-17

12.1.2.5.7 Decant Pipe.....................................................................................................................................................12-17

12.1.2.5.8 Channel Conditions.....................................................................................................................................12-18

12.1.2.5.9 Instrumentation............................................................................................................................................12-21

12.1.2.5.10 Reclaimed Slopes..........................................................................................................................................12-21

12.1.3 Quality Control Sampling and Testing..........................................................................................................12-21

12.2 MONITORING INSTRUMENTATION...................................................................................................................12-22

12.3 FACILITY AND DAM SAFETY INSPECTIONS...................................................................................................12-24

12.3.1 Routine Inspections.............................................................................................................................................12-24

12.3.1.1 Impoundments and Dams............................................................................................................................12-24

12.3.1.2 Non-Impounding Facilities............................................................................................................................12-27

12.3.2 Event-Related Inspections.................................................................................................................................12-27

12.3.3 Annual Dam Safety Inspections......................................................................................................................12-27

12.4 CERTIFICATION AND CONFORMANCE REPORTING...................................................................................12-28

12.5 MAINTENANCE .........................................................................................................................................................12-29

12.5.1 Routine Maintenance..........................................................................................................................................12-29

12.5.2 Maintenance after Unusual Events................................................................................................................12-30

12.5.3 Long-Term Maintenance ...................................................................................................................................12-30

Chapter 13 Instrumentation and

Performance Monitoring

13.1 INSTRUMENTATION PROGRAM PLANNING....................................................................................................13-1

13.2 MEASUREMENT TECHNIQUES ..............................................................................................................................13-2
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13.2.1 Movements and Displacements....................................................................................................................... 13-3

13.2.1.1 General Observations....................................................................................................................................... 13-3

13.2.1.2 Movement Measurement Techniques........................................................................................................ 13-8

13.2.1.2.1 Surveying Methods........................................................................................................................................ 13-8

13.2.1.2.2 Surface Extensometers...............................................................................................................................13-16

13.2.1.2.3 Tiltmeters.........................................................................................................................................................13-16

13.2.1.2.4 Probe Extensometers..................................................................................................................................13-18

13.2.1.2.5 Fixed Embankment Extensometers.......................................................................................................13-19

13.2.1.2.6 Fixed Borehole Extensometers................................................................................................................13-20

13.2.1.2.7 Inclinometers.................................................................................................................................................13-21

13.2.2 Piezometric Levels and Pore-Water Pressures............................................................................................13-23

13.2.2.1 Piezometer Types..............................................................................................................................................13-23

13.2.2.2 Observation Wells.............................................................................................................................................13-24

13.2.2.3 Open-Standpipe Piezometers......................................................................................................................13-25

13.2.2.4 Closed-System Piezometers.........................................................................................................................13-29

13.2.3 Surface Water Flows and Hydrologic Parameters.....................................................................................13-31

13.2.3.1 Measurement of Impoundment Water Level .........................................................................................13-31

13.2.3.2 Measurement of Seepage Flows and Other Surface Water Flows..................................................13-32

13.2.3.2.1 Importance of Seepage Flows.................................................................................................................13-32

13.2.3.2.2 Measurement of Seepage Flow Rates...................................................................................................13-34

13.2.3.3 Measurement of Rainfall and Snowfall.....................................................................................................13-36

13.2.3.4 Observation of Temperature and General Weather Conditions......................................................13-36

13.2.4 Miscellaneous Instrumentation and Monitoring......................................................................................13-36

13.2.4.1 Measurement of Soil Pressure......................................................................................................................13-36

13.2.4.2 Measurement of Blast Vibrations................................................................................................................13-39

13.2.4.3 Measurement of Temperature .....................................................................................................................13-41

13.3 INSTRUMENTATION MAINTENANCE................................................................................................................13-41

13.3.1 Importance of Maintenance and Recalibration.........................................................................................13-41

13.3.2 Recalibration and Maintenance during Service Life................................................................................13-42

13.4 AUTOMATED DATA ACQUISITION.....................................................................................................................13-42

13.5 INSTRUMENTATION COSTS..................................................................................................................................13-42

APPENDIX 13A MEASUREMENTS, TRANDUCERS AND DATA-ACQUISITION SYSTEMS

Chapter 14 Emergency Action Planning

14.1 PURPOSE .......................................................................................................................................................................14-1

14.2 EMERGENCY ACTION PLAN PREPARATION.....................................................................................................14-1

14.2.1 Background .............................................................................................................................................................14-1

14.2.2 Federal Guidelines for Dam Safety..................................................................................................................14-3

14.2.3 Basic Elements of an EAP....................................................................................................................................14-3

14.2.4 Dam Inspection and Inundation Area Reconnaissance ..........................................................................14-4

14.2.4.1 Dam Inspection..................................................................................................................................................14-4
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14.2.4.2 Downstream Channel Reconnaissance.....................................................................................................14-5

14.2.4.3 Physical Description .........................................................................................................................................14-5

14.2.5 Inundation Map Preparation.............................................................................................................................14-5

14.2.5.1 Causes of Dam Failures....................................................................................................................................14-5

14.2.5.2 Dam Breach Analysis........................................................................................................................................14-6

14.2.5.3 Inundation Map .................................................................................................................................................14-6

14.2.6 Participant Responsibilities................................................................................................................................14-6

14.2.7 Implementation Triggers and Emergency Responses.............................................................................14-7

14.2.8 Notification Requirements.................................................................................................................................14-9

14.2.9 EAP Public Copies..................................................................................................................................................14-9

14.2.10 Exercises/Training....................................................................................................................................................14-9

14.2.11 Plan Maintenance..................................................................................................................................................14-10

14.2.12 EAP Appendices.....................................................................................................................................................14-11

14.3 PREPARATION FOR POTENTIAL EMERGENCY REMEDIAL ACTIONS....................................................14-11

14.3.1 Potential Overtopping During a Storm.......................................................................................................14-12

14.3.2 Movement or Sliding of the Embankment.................................................................................................14-12

14.3.3 Internal Erosion or Piping through the Dam, Foundation or Abutments.......................................14-12

14.3.4 Excessive Seepage and a High Level of Saturation in the Embankment ........................................14-13

14.3.5 Spillway Failure or Erosion that Could Cause a Dam Breach................................................................14-13

14.3.6 Excessive Settlement or Subsidence of the Embankment ..................................................................14-13

14.3.7 Formation of a Sinkhole on the Crest or Face of the Dam....................................................................14-14

14.3.8 Earthquake.............................................................................................................................................................14-14
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TABLE 

NO. TITLE PAGE NO.

2.1 Classification of Coals by Rank 2-3

2.2 Coal Refuse Disposal Facility Failures and Incidents 2-13

3.1 Guidance for Flood Warning Systems at Surface Mine Pits 3-24

4.1 Coal Refuse Disposal Facility Cost Components 4-5

4.2 Planning Steps for Coal Refuse Facility Design 4-8

4.3 Typical Design Sequence 4-9

6.1 Typical Geotechnical Design Procedure for Coal Refuse Disposal Facilities 6-3

6.2 Basic Characteristics of Coal Refuse Disposal and Design Significance 6-5

6.3 Coarse Coal Refuse Characterization Summary of Average/Range of Values 6-9

6.4 Fine Coal Refuse Characterization Summary of Average/Range of Values 6-11

6.5 Combined Coal Refuse Characterization 6-12

6.6 Correlation Between USCS Classification and Properties of Compacted Soils 6-14

6.7 Correlation between USCS Classification and Relative Desirability of Soils

as Compacted Fill

6-15

6.8 Approximate Correlation Between Engineering Properties and Soil Classification 

Groups

6-16

6.9 Class F Fly Ash 6-17

6.10 Class C Fly Ash 6-17

6.11 Fly Ash/Coal Refuse Mixture Properties 6-18

6.12 Advantages and Disadvantages of Internal Drain Types 6-23

6.13 Relative Advantages of Vertical and Sloping Embankment Cores 6-25

6.14 Clay Dispersion Potential 6-28

6.15 Guideline Subsurface Exploration Program for a New Impounding Embankment 6-54

6.16 Guideline Subsurface Exploration Program for a New Embankment without an 

Impoundment

6-55

6.17 Guideline Subsurface Exploration Program for An Existing Embankment 6-56
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TABLE 

NO. TITLE PAGE NO.

6.18 Common Sampling Methods 6-57

6.19 Advantages and Limitations of the Standard Penetration Test 6-59

6.20 Advantages and Limitations of the Cone Penetrometer Test 6-62

6.21 Advantages and Limitations of the Field Vane Shear Test 6-65

6.22 Field Methods for Measurement of Hydraulic Conductivity 6-68

6.23 Advantages and Limitations of Surface Geophysical Testing 6-71

6.24 Surface Geophysical Methods in Relation to Typical Investigation Objectives 6-72

6.25 Physical Properties and Typical Ground Penetrating Radar Velocities for Common 

Earth Materials

6-82

6.26 Applicability of Common Borehole Geophysical Methods 6-87

6.27 Typical Soil and Rock Laboratory Tests for Coal Refuse Disposal Site 

Characterization

6-92

6.28 Typical Laboratory Soil Tests for Various Materials 6-94

6.29 Guidelines for Minimum Sample Weights 6-96

6.30 Soil Classification Chart (Laboratory Method) 6-98

6.31 In-Place Unit Weight and Specific Gravity of Coarse Coal Refuse 6-100

6.32 Unit Weight and Specific Gravity for Fine Coal Refuse 6-101

6.33 Statistical Properties for Atterberg Limits and Moisture Contents of

Fine Coal Refuse at Northern Appalachian Sites

6-103

6.34 Particle-Size Distribution for Fine Coal Refuse 6-105

6.35 Test Methods for Chemical Characterization of Soil, Rock and Refuse Materials 6-106

6.36 Estimated Horizontal Hydraulic Conductivity of Fine Coal Refuse

Based on Piezocone Dissipation Tests

6-111

6.37 Methods for Quality Control Testing of Geotextiles 6-113

6.38 Methods for Quality Control Testing of Geomembranes 6-114

6.39 Methods for Quality Control Testing of Geosynthetic Clay Liners 6-115

6.40 Laboratory Tests for Determining Soil Shear Strength 6-121

6.41 Summary of Fine Coal Refuse Shear Strength Parameters Based on

Triaxial Test Results

6-127

6.42 Standards and Procedures for Laboratory Testing of Intact Rock 6-131

6.43 Slake Durability Classification 6-136

6.44 Quality Control Standards for Portland-Cement Concrete 6-136

6.45 Quality Control Standards for CLSM 6-136

6.46 CLSM Mix Design Guide 6-137
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6.47 Filter Criteria As A Function of Base Soil and Percent Passing No. 200 Sieve 6-148

6.48 Restrictions on Filter Particle Size to Limit Segregation 6-148

6.49 Guidelines for Evaluating Critical Nature of Severity for Drainage Applications 6-153

6.50 Geotextile Strength Property Requirements for Drainage Geotextiles 6-155

6.51 Recommended Minimum Properties for General Geomembrane Installation 

Survivability

6-163

6.52 Stress-Strain Relationships Used for Finite Element Deformation Analyses

of Embankments and Foundations

6-169

6.53 Characteristics of Equilibrium Procedures for Slope Stability Analysis 6-174

6.54 Shear Strengths, Water Pressures, and Unit Weights for Slope-Stability Analyses 6-177

6.55 Guideline Friction Values and Efficiencies for Various Geosynthetic and Soil 

Combinations

6-178

6.56 Recommended Minimum Factors of Safety for Design of Coal Refuse 

Embankments

6-183

6.57 Common Residential Velocity Criteria and Effects 6-207

7.1 Comparison of Basic Criteria for Sand-Like, Clay-Like and Borderline Materials 7-14

7.2 References for Correlations of SUS with SPT and CPT Data 7-36

7.3 Correlations of SUS/σV versus SPT and CPT Data Back-Calculated from Case Histories 7-38

7.4 Upper-Bound Values of SUS for Corresponding Values of N1,60 7-45

7.5 Error in Void Ratio and Corresponding Percent Error in SUS 7-49

7.6 MDE and Seismic Hazard Assessment Recommendations 7-72

7.7 Safe Shutdown Earthquake (SSE) Peak Horizontal Ground Accelerations

for U.S. Nuclear Power Plants

7-85

8.1 Sources of Information for Evaluation of Breakthrough Potential 8-4

8.2 Sources for Mine Maps 8-5

8.3 Exploration Considerations for Assessing Impoundment Breakthrough Potential To 

Underground Mines

8-8

8.4 Outcrop Barrier Exploration Considerations 8-9

8.5 Summary of Guidelines for Mining Under Or Near Bodies of Water 8-11

8.6 Guidelines for Cement Grout Programs 8-63

9.1 Hydrologic and Hydraulic Design Procedures for Coal Refuse Disposal Facilities 9-3

9.2 Hydrologic and Hydraulic Design Considerations for Coal Refuse Disposal Facilities 9-6

9.3 Facility Characteristics Influencing Hydraulic System Design 9-15

9.4 Recommended Minimum Design Storm Criteria for Coal Refuse Disposal 

Impoundments

9-30
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TABLE 

NO. TITLE PAGE NO.

9.5 NWS Precipitation Frequency Publications 9-37

9.6 Typical Design Criteria for Minor Hydraulic Structures 9-38

9.7 Methods for Determining Runoff Rate and Volume 9-39

9.8 Recommended Correction Factors for Tc for Watersheds West of the 105th Meridian 9-43

9.9 Roughness Coefficients (Manning’s n) for Sheet Flow 9-44

9.10 Runoff Curve Numbers for Watershed Complexes and AMC-II 9-50

9.11 Runoff Curve Numbers (CN) for Antecedent Moisture Conditions 9-52

9.12 Runoff Coefficient s for Use in the Rational Method 9-60

9.13 Manning’s Coefficients of Channel Roughness 9-72

9.14 Head Loss Coefficients for Conduits 9-77

9.15 References for Secondary Hydraulic Design Issues 9-81

9.16 Permissible Velocity and Tractive Force for Channel Linings 9-85

9.17 Vegetal Retardance Classes 9-87

9.18 Riprap Size Designation 9-89

9.19 Recommended Grading of Grouted Rock Riprap Lining 9-91

9.20 Inundation Analysis Software 9-115

9.21 Hazard Potential Classifications 9-119

10.1 Fire Extinguishment Techniques 10-7

10.2 Summary of Suggested Criteria for Interpreting Acid-Base Accounting 10-14

10.3 Chemical Compounds Used in AMD Treatment 10-16

10.4 Design Considerations for AMD Passive Treatment Systems 10-17

10.5 Overview of the Hydrogeologic Process 10-18

10.6 Site Specific Factors to Consider in Liner System Design 10-19

10.7 Available Materials or Procedures for Liner System Components 10-21

10.8 Stability Considerations in Liner Design 10-24

10.9 Liner System Performance Factors and Considerations 10-25

10.10 Potential Final Land Uses 10-26

10.11 Design Considerations for Reclamation Soils 10-27

10.12 Summary of Reclamation Guidelines 10-30

10.13 Characteristics of Common Species Potentially Suitable for Reclamation 10-31

11.1 Refuse Disposal Operations 11-2

11.2 Transport Systems and Associated Handling Equipment at the Point of Disposal 11-22
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11.3 Compaction Capabilities of Transport and Spreading Equipment 11-24

11.4 Compaction Guide for Material Types 11-28

11.5 Suggested Compaction Equipment and Methods 11-30

11.6 Effects of Corrosion and Countermeasures 11-47

12.1 Typical Observation and Inspection Checklist 12-9

12.2 Field Quality Control Testing Summary 12-25

13.1 Instrumentation Planning and Design Steps 13-3

13.2 Performance Questions for Slurry Impoundments 13-4

13.3 Summary of General Methods for Detecting Embankment Movements

and Displacements

13-6

13.4 Instrument Categories for Measuring Movements 13-9

13.5 Surveying Methods 13-12

13.6 Instruments for Measuring Ground-Water Pressure 13-24

13.7 Cement-Bentonite Grout Mix 13-26

13.8 Guidance for Accurate Weir Flow Measurement 13-39

14.1 Historical U.S. Dam Failures 14-2

14.2 Guidance for EAP Events and Situations 14-8
xxxii MAY 2009

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List of Figures

MAY 2009 xxxiii

FIGURE 

NO. TITLE PAGE NO.

2.1 Coarse Coal Refuse Embankment and Sample Gradation 2-8

2.2 Fine Coal Refuse Slurry Characteristics 2-9

2.3 Combined Coal Refuse Materials 2-10

3.1 Cross-Valley Impounding Embankment 3-5

3.2 Incised Impounding Facility 3-6

3.3 Side-Hill Impounding Embankment 3-7

3.4 Diked-Pond (Impounding) Embankment 3-8

3.5 Upstream Staging Method 3-8

3.6 Downstream Staging Method 3-9

3.7 Centerline Staging Method 3-9

3.8 Valley-Fill, Non-Impounding Embankment 3-12

3.9 Cross-Valley, Non-Impounding Embankment 3-12

3.10 Side-Hill, Non-Impounding Embankment 3-13

3.11 Ridge (Non-Impounding) Embankment 3-14

3.12 Heaped (Non-Impounding) Embankment 3-15

3.13 Slurry Cell in Valley Fill, Non-Impounding Embankment 3-16

3.14 Slurry Cell, Side-Hill, Non-Impounding Embankment 3-17

3.15 Slurry Cells in Cross-Valley Fill, Impounding Embankment 3-19

5.1 Typical Slurry Impoundment Components 5-4

5.2 Typical Slurry Cell Facility (Impounding Embankment) 5-5

5.3 Typical Refuse Embankment Components 5-6

5.4 Illustration of Impoundment Design Storm Control 5-10

5.5 Underground Mining at Slurry Impoundment 5-16

6.1 Drain Configurations for Homogeneous Embankments with Impoundments 6-22
xxxiv MAY 2009

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FIGURE 

NO. TITLE PAGE NO.

6.2 Zoned Embankments with Impoundments 6-24

6.3 Resistance of Core Materials to Piping and Cracking 6-27

6.4 Seepage Cutoffs for Pervious Foundations 6-29

6.5 Impoundment Internal Drain Concept 6-30

6.6 Non-Impounding Embankments 6-32

6.7 Example Spring Collection Zones and Rock Underdrain 6-33

6.8 Upstream Construction Following Initial Development 6-35

6.9 Downstream Embankment Construction Followed by Expansion Using the 

Downstream and Upstream Methods

6-38

6.10 Internal Drainage Systems for Existing Impounding Embankments 6-42

6.11 Flow Chart for Site Exploration, Material Property Testing, and Facility Design 6-45

6.12 Piezocone Penetrometer 6-62

6.13 Field Vane Shear Test Setup 6-64

6.14 Vane Correction Factor 6-66

6.15 Typical Seismic Refraction Data with Interpretation 6-73

6.16 Correlation of Rippability with P-Wave Velocity for Caterpillar D-9 6-74

6.17 Seismic Reflection Principle and Schematic of Reflection Data Record 6-75

6.18 Seismic Reflection Survey Profile Over Abandoned Coal Mine Workings 6-76

6.19 Commonly Used Electrode Configurations for Ground Electrical Measurements 6-77

6.20 Resistivity Profile Across Fine Coal Refuse Impoundment 6-77

6.21 Electrical Survey of Flooded Mine Workings 6-78

6.22 Illustration of Use of SP Method to Identify Area of Seepage 6-79

6.23 Delineation of Flooded Mine Workings with TDEM Method 6-81

6.24 Resistivity of Coal Refuse from EM-31 Measurements 6-81

6.25 GPR Record of Shallow Coal Mine Workings 6-83

6.26 Theoretical Response of Gravity Geophysical Method Over 20-Foot-Diameter, Air-Filled 

Tunnel

6-84

6.27 Magnetic Intensity Recorded at Various Heights Above Well Casing 6-86

6.28 Field Setup for Crosshole Seismic Survey 6-88

6.29 Field Setup for Downhole Seismic Survey 6-89

6.30 Image of Mine Entry From Borehole Laser Scanner 6-90

6.31 Laser Image of Mine Workings 6-91

6.32 Plasticity Chart 6-99
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FIGURE 

NO. TITLE PAGE NO.

6.33 Effect of Carbon Content on Specific Gravity of Coal Refuse Material 6-102

6.34 Particle Size Distribution 6-103

6.35 Typical Moisture-Density Relationships for Dynamic Compaction 6-107

6.36 Particle-Size Distribution of Coarse Coal Refuse After Compaction 6-108

6.37 Results of Consolidation Test on Fine Coal Refuse 6-116

6.38 Effective Stress Mohr’s Circle for CU Triaxial Test 6-118

6.39 Effective Stress Paths and Failure Envelope for CU Triaxial Test 6-119

6.40 Direct Shear Testing Arrangement 6-122

6.41 Results of Drained Direct Shear Test on Clay 6-123

6.42 Unconfined Compression Test Results 6-124

6.43 Variation of Strength of Fine Coal Refuse with Fines Content 6-127

6.44 Typical Results for Resonant Column Test on Fine Coal Refuse 6-130

6.45 Point-Load Test Apparatus 6-132

6.46 Rotating Drum Assembly and Setup of Slake Durability Equipment 6-134

6.47 Approximation for Estimated Reduction in Hydraulic Conductivity Under

Turbulent Flow

6-142

6.48 Flow Net Analysis 6-144

6.49 Flow Net Analysis for Anisotropic Conditions 6-145

6.50 Filter Diaphragm Design for Conduit 6-158

6.51 Example Design Configuration for Filter Diaphragm and Decant Pipe 6-159

6.52 Typical Relief Well Installation 6-161

6.53 Example Horizontal Drain Installation 6-162

6.54 Cracking Patterns Observed for Dams in Narrow Valleys 6-167

6.55 Histogram of Factor of Safety Distribution 6-180

6.56 Horizontal and Vertical Extent of Cuts for Base Width of 10 Feet 6-186

6.57 Schematic Geological Profile of A Bedrock Slope 6-187

6.58 Unfavorable Orientation of Bedding Planes 6-188

6.59 Favorable Orientation of Bedding Planes 6-190

6.60 Stepped Slope Example for Cuts in Weathered Rock 6-191

6.61 Examples of Rock-Cut Control 6-192

6.62 Deformations Associated With Conduits In Embankments 6-198

6.63 Reinforced Concrete Cylinder Pipe (RCCP) Details 6-204

6.64 Prestressed Concrete Cylinder Pipe (PCCP) Details (Lined Cylinder) 6-205
xxxvi MAY 2009

List of Figures

FIGURE 

NO. TITLE PAGE NO.

6.65 Comparison of Predicted and Observed Conduit Settlements 6-206

6.66 Acceptable Limits of Vibration for Houses 6-208

7.1a Seismic Stability Screening 7-6

7.1b Triggering and Seismic-Stability Analysis 7-7

7.1c Deformation Screening and Analysis 7-8

7.2 Typical Undrained Stress-Strain Curve 7-11

7.3 Comparison of Peak Base and Crest Transverse Accelerations Measured at Earth Dams 7-27

7.4 Variation of Maximum Acceleration Ratio with Depth of Sliding Mass 7-28

7.5 SPT Clean-Sand-Based Curve for Magnitude 7.5 Earthquakes with Data from 

Liquefaction Case Histories

7-30

7.6 Calculation of CRR from CPT Data Along with Empirical Liquefaction Data from 

Compiled Case Histories

7-31

7.7 Sus versus N1,60(CS) 7-37

7.8 Sus versus N1,60 7-38

7.9 Sus / σ ́v

 versus qt1 7-39

7.10 Steady-State Line 7-46

7.11 Method for Correcting Laboratory SUS to In-Situ SUS 7-48

7.12 Interpretation of Laboratory SUS Testing 7-49

7.13 Consolidation Curve and Steady-State Line 7-50

7.14 Use of CPT Data to Screen Clay-Like Material 7-53

7.15 Cyclic Stress-Strain Curve Demonstrating Racheting Mechanism 7-59

7.16 Double Integration to Obtain Displacement 7-64

7.17 Location of U.S. Coal Resources 7-67

7.18 Seismic Hazard Map of the U.S. with Overlay of Main Bituminous Coal Fields 7-68

7.19 Illustration of Moderate- to High-Hazard Region 7-69

7.20 Quaternary Faults and Significant Seismic Sources with Overlay of Main Bituminous 

Coal Fields

7-70

7.21 Return Period for M > 5.5 Event at a Distance Less than 50 km 7-80

7.22 Location of Nuclear Power Plants with Respect to U.S. Coal Fields 7-83

7.23 Seismic Hazard Assessment Flow Diagram 7-87

7.24 Example 1: Time History for Low-Seismic-Hazard Area 7-89

7.25 Example 2: Time History for Low-Seismic-Hazard Area 7-90

7.26 Example 3: Time History for Low-Seismic-Hazard Area 7-91
MAY 2009 xxxvii

Engineering and Design Manual Coal Refuse Disposal Facilities

FIGURE 

NO. TITLE PAGE NO.

8.1 Strata Disturbance and Subsidence Caused By Mining 8-12

8.2 Influence of Extraction Width on Subsidence 8-12

8.3 Displacement and Strain for Various Extraction Widths 8-13

8.4 Development of Subsidence Through and Strains with Face Advance 8-14

8.5 Ground Movements Caused By Subsidence 8-15

8.6 Safety Zone Guidelines for Mining in the Vicinity of Dams and Impoundments 8-17

8.7 Hillside Disturbance at Coal Outcrop 8-21

8.8 Comparison of Pillar Strength Formulas with Respect to Width-to-Height Ratio 8-26

8.9 Outcrop Barrier Breakthrough Analysis for Shear Failure 8-32

8.10 Illustration of Overburden Barrier 8-34

8.11 Stowed Mine Openings and Barrier Construction 8-37

8.12 Barrier Against Outcrop with Low-Permeability Fill and Drain 8-38

8.13 Basic Bulkhead Design Concepts 8-44

8.14 Drift Opening Bulkhead Design Concepts 8-45

8.15 Bulkhead Configuration Showing Concrete-Block Retaining Walls and Concrete Center 8-46

8.16 Classification and Interpretation of Spoil Durability 8-58

8.17 Excavation Slopes for Preparation of Dam Abutments in Rock 8-60

8.18 Slope Modification to Reduce Differential Settlement and Potential for Cracking 8-61

8.19 Slope Modification and Seam Treatment for Sedimentary Rock Strata 8-62

9.1 Runoff and Impoundment Inflow Sources 9-8

9.2 Watershed Boundary Delineated on USGS Topographic Map 9-9

9.3 Typical Impoundment Inflow and Outflow Hydrographs 9-10

9.4 Typical Impoundment Area and Storage Volume Curves 9-12

9.5 Typical Decant Systems 9-13

9.6 Typical Decant and Spillway Systems 9-14

9.7 Channel Construction in Moderately and Steeply Sloping Terrain 9-17

9.8 Channel Construction in Gently Sloping Terrain 9-18

9.9 Drainage Control for Valley-Fill, Non-Impounding Embankment 9-21

9.10 Drainage Control for Side-Hill and Heaped Embankments 9-23

9.11 Drainage Control for Slurry-Cell Facility 9-25

9.12 Drainage Control for Cross-Valley Impoundment 9-27

9.13 Drainage Control for Diked Impoundment 9-28
xxxviii MAY 2009

List of Figures

FIGURE 

NO. TITLE PAGE NO.

9.14 U.S. Regions Covered by Generalized PMP Studies 9-35

9.15 Dimensionless Design Storm Distribution 9-40

9.16 Average Velocities for Estimating Travel Time for Shallow, Concentrated Flow 9-45

9.17 Direct Runoff for Rainfall Less than or Equal to 8 Inches 9-47

9.18 Direct Runoff for Rainfall Greater than or Equal to 8 Inches 9-48

9.19 Estimated Total Runoff Volume for CN = 70 9-56

9.20 Estimated Total Runoff Volume for CN = 80 9-57

9.21 Estimated Peak Runoff Rate for 6-Hour Design Storm and CN = 70 9-58

9.22 Estimated Peak Runoff Rate for 6-Hour Design Storm and CN = 80 9-59

9.23 Example of Rational Method of Inflow Calculation 9-62

9.24 Typical Head-Discharge Curves for Hydraulic Systems 9-64

9.25 Spillway Approach Channels 9-65

9.26 Drop-Inlet Decant Systems 9-66

9.27 Open-Channel Cross Sections 9-68

9.28 Channel Flow Conditions 9-70

9.29 Critical Depth for Trapezoidal and Circular Sections 9-71

9.30 Closed-Conduit Hydraulic Systems 9-73

9.31 Typical Flow Conditions – Culvert Conduits on Mild Slopes 9-74

9.32 Typical Flow Conditions – Culvert Conduits on Steep Slopes 9-75

9.33 Culvert Inlet Configurations 9-78

9.34 Flow and Discharge Characteristics of Decant-Inlet, Closed-Conduit System 9-80

9.35 Examples of Channel and Conduit Outlets 9-82

9.36 Manning’s n Versus VR for Various Retardance Classes 9-88

9.37 Grouted Riprap Lining Thickness as a Function of Flow Velocity 9-91

9.38 Examples of Decants 9-100

9.39 Example Decant System at Diked Impoundment 9-101

9.40 Example Decant System at Cross-Valley Impoundment 9-102

9.41 Example Spillway System – A 9-104

9.42 Example Spillway System – B 9-105

9.43 Example Spillway System – C 9-106

9.44 Spillway Inlet Control 9-107

9.45 Spillway Outlet Control 9-108
MAY 2009 xxxix

Engineering and Design Manual Coal Refuse Disposal Facilities

FIGURE 

NO. TITLE PAGE NO.

9.46 Sectional View Through Impounding Facility 9-112

9.47 Breach Parameters 9-117

10.1 Liner Systems Used at Coal Refuse Disposal Facilities 10-23

11.1 Compaction by Hauling Units Using Successive Offset Passes 11-25

11.2 Procedures for Controlled Placement of Refuse 11-27

11.3 Benching of Embankment Fill Into Natural Slope 11-33

11.4 Slurry Disposal 11-37

11.5 Stockpiling and Handling of Granular Materials 11-52

12.1 Failure Modes, Warning Signs and Surveillance Measures 12-5

12.2 Example Data Collection Form 12-6

13.1 Location of Monuments Using Triangulation 13-14

13.2 Example of Benchmark in Rock 13-15

13.3 Typical Monument Installations 13-17

13.4 Probe Extensometer 13-19

13.5 Settlement Platform 13-20

13.6 Fixed Borehole Extensometer 13-21

13.7 Inclinometer Operation 13-22

13.8 Approximate Response Time for Various Types of Piezometers 13-25

13.9 Open-Standpipe Piezometer/Observation Well 13-27

13.10 Normally-Closed Pneumatic Piezometer 13-30

13.11 Vibrating-Wire Piezometer 13-31

13.12 Bonded Electrical-Resistance Piezometer 13-33

13.13 Measurement of Flow from Pipes 13-37

13.14 Typical V-Notch Weir 13-38

13.15 Flow Depth-Discharge Relationship for 90-Degree, V-Notch Weir 13-40

13.16 Automated Data Acquisition System at Coal Refuse Disposal Facility 13-43

14.1 Inundation Map for Coal Refuse Impoundment 14-7
xl MAY 2009

List of Figures
Acronyms

MAY 2009

 ACRONYM REPRESENTS

AASHTO American Association of State Highway and Transportation Officials

ABS Acrylonitrile-Butadiene-Styrene

ACI American Concrete Institute

AC alternating current

ALD anoxic limestone drain

AMC antecedent moisture condition

AMD acid mine drainage

AMRL AASHTO Materials Reference Laboratory

AOS apparent opening size

ArcGIS GIS software developed by Environmental Systems Research Institute (ESRI)

ARMPS Analysis of Retreat Mining Pillar Stability (software)

ARMPS-HWM ARMPS program for highwall mine pillars (software)

ASCE American Society of Civil Engineers

ASTM American Society for Testing and Materials

AWWA American Water Works Association

BOD biochemical oxygen demand

BREACH breach parameter computation software

CADD computer-aided design and drafting

CANDE Culvert Analysis and Design (software)

CD consolidated drained

CGS Canadian Geotechnical Society

CIDC consolidated isotropic drained compression

CISPM Comprehensive and Integrated Subsidence Prediction Model (software)

CIUC consolidated isotropic undrained compression

CLSM controlled low-strength material

CMP corrugated metal pipe

CMRR Coal Mine Roof Rating (software)

xli
xlii

Acronyms

MAY 2009

 ACRONYM REPRESENTS

CN curve number

CO carbon monoxide

COSMOS Consortium of Organizations for Strong-Motion Observation Systems

COV coefficient of variability

CPE chlorinated polyethylene

CPM Critical Path Method (software)

CPP corrugated plastic pipe

CPT cone penetrometer test

CPTu piezocone penetrometer test

CRR cyclic resistance ratio

CFR Code of Federal Regulations

CSI Construction Specifications Institute

CSPE chlorosulfonated polyethylene

CSR cyclic stress ratio

CU consolidated undrained

CU consolidated undrained with pore pressure measurements

CWA Clean Water Act

DAMBRK Dam Break (software)

DC direct current

DCDT direct current differential transformer

DEM digital elevation model

DEP Department of Environmental Protection (state environmental agencies)

DI degradation index

DMLR Virginia Department of Mines, Minerals, and Energy, Division of Mined Land 

Reclamation

DOD Department of Defense

DOQ digital orthophoto quadrangle

DOQQ digital orthophoto quarter quadrangle

DRG digital raster grid

DSHA deterministic seismic hazard analysis

DWOPER Dynamic Wave Routing Model (software)

EAP Emergency Action Plan

EDM electronic distance measuring
xliii

Engineering and Design Manual Coal Refuse Disposal Facilities

MAY 2009

 ACRONYM REPRESENTS

EEGS Environmental and Engineering Geophysical Society

EERT Eastern Energy Resources Team

EM electromagnetic method

EMA Emergency Management Agency

EMC Emergency Management Coordinator

EMS Emergency Medical Service

EPA U.S. Environmental Protection Agency

EPRI Electric Power Research Institute

EROS Earth Resources Observation and Science

ESRI Environmental Systems Research Institute

FBC fluidized bed combustion

FE finite element

FEMA Federal Emergency Management Agency

FERC Federal Energy Regulatory Commission

FGD flue gas desulfurization

FHWA Federal Highway Administration

FLAC Fast Lagrangian Analysis of Continua (software)

FLDWAV Flood Wave (software)

FLUSH seismic soil-structure interaction software

FR friction ratio

FS factor of safety

FVST field vane shear test

GAI-LAP Geosynthetic Accreditation Institute - Laboratory Accreditation Program

GCL geosynthetic clay liner

GEI Geotechnical Engineers, Inc.

GIS geographic information system

GPR ground penetrating radar

GPS global positioning system

GRM generalized reciprocal method

HDPE high-density polyethylene

HEC USACE Hydrologic Engineering Center

HEC-1 open channel flow analysis software

HEC-GeoHMS GIS -based version of HEC-HMS software
xliv

Acronyms

MAY 2009

 ACRONYM REPRESENTS

HEC-HMS Hydraulic Engineering Center Hydrologic Modeling System (software)

HEC-RAS Hydraulic Engineering Center River Analysis System (software)

HMR Hydrometeorological Report

HSG hydrologic soil group

ICODS Interagency Committee on Dam Safety

IDF inflow design flood

LAMODEL software for computing stresses and displacements in mines

LI liquidity index

LIDAR light detection and ranging

LL liquid limit

LLNL Lawrence Livermore National Laboratory

LVDT linear variable differential transformer

MAE Mid-America Earthquake

MARV minimum average roll value

MCE maximum credible earthquake

MDE maximum design earthquake

MIBC methylisobutyl carbinol

MMI Modified Mercali Intensity

MPBX multiple-point borehole extensometer

MSF magnitude scaling factor

MSHA Mine Safety and Health Administration

NAS National Academy of Sciences

NASA National Aeronautics and Space Administration

NCB National Coal Board (Britain)

NEH National Engineering Handbook

NEHRP National Earthquake Hazard Reduction Program

NIOSH National Institute for Occupational Safety and Health

NMO normal moveout

NMSZ New Madrid Seismic Zone

NOAA National Oceanic and Atmospheric Administration

NR not reported

NRC National Research Council

NRCS Natural Resources Conservation Service

NSF National Science Foundation
xlv

Engineering and Design Manual Coal Refuse Disposal Facilities

MAY 2009

 ACRONYM REPRESENTS

NSSGA National Stone, Sand and Gravel Association

NWS National Weather Service

OBE operating basis earthquake

OLC open limestone channel

OSHA Occupational Safety and Health Administration

OSM Office of Surface Mining

PCCP prestressed concrete cylinder pipe

PCPT piezocone penetrometer test

PE polyethylene

PERT scheduling software

PGA peak ground acceleration

PI plastic index

PL plastic limit

PMF Probable Maximum Flood

PMP Probable Maximum Precipitation

PMTS Probable Maximum Thunderstorm

POA percent open area

PSHA probabilistic seismic hazard analysis

PV prefabricated vertical

PVC polyvinyl chloride

Q equivalent to UU

QA quality assurance

QC quality control

QUAD4 software for finite element seismic analysis of earth structures

R equivalent to CU

RCCP reinforced concrete cylinder pipe

RCP reinforced concrete pipe

RMR rock mass rating

RQD rock quality designation

RVSP reverse vertical seismic profile

S equivalent to CD

SAGEEP Symposium on the Application of Geophysics to Engineering and Environmental 

Problems
xlvi

Acronyms

MAY 2009

 ACRONYM REPRESENTS

SAPS successive alkalinity producing system

SCPTu seismic piezocone penetrometer test

SCS Soil Conservation Service (now NRCS)

SDI slake durability index

SDPS Surface Deformation Prediction System (software)

SEE safety evaluation earthquake

SF safety factor (for channel linings)

SHAKE software for seismic analysis of subsurface layers

SHANSEP stress history and normalized soil engineering parameters

SL shrinkage limit

SMCRA Surface Mining and Reclamation Act

SMPDBK Simplified Dam Break (software)

SP spontaneous or self potential

SPBX single-point borehole extensometer

SPECTEXT specification software

SPT standard penetration test

SSE safe shutdown earthquake

SSHAC Senior Seismic Hazard Analysis Committee

TDEM time-domain electromagnetic method

TDR time-domain reflectometry

TR technical release

USACE U.S. Army Corps of Engineers

USBM U.S. Bureau of Mines

USBR U.S. Bureau of Reclamation

USCS Unified Soil Classification System

USDA U.S. Department of Agriculture

USDOE U.S. Department of Energy

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

USNRC U.S. Nuclear Regulatory Commission

UU unconsolidated undrained

VLDPE very low density polyethylene

VLF very low frequency
xlvii

Engineering and Design Manual Coal Refuse Disposal Facilities

MAY 2009

 ACRONYM REPRESENTS

VST vane shear test

VSP vertical seismic profile

WMS Watershed Modeling System (software)

WP wetted perimeter

WVDOT West Virginia Department of Transportation
xlviii

Acronyms

MAY 2009
Chapter 1

INTRODUCTION

MAY 2009 1–1

This Engineering and Design Manual for Coal Refuse Disposal Facilities (Manual), originally published by the Mining Enforcement and Safety Administration (MESA, 1975), has been updated for 

the Mine Safety and Health Administration (MSHA) of the United States Department of Labor to 

present current guidelines and procedures for design, construction and operation of coal refuse disposal facilities. Guidance related to dam safety issues for other impounding structures at coal mine 

sites, including fresh water impoundments and sedimentation and treatment ponds, is also provided 

in individual sections or through discussion of the associated design features. Emphasis has been 

placed on facility planning and materials handling and placement techniques. The input provided 

by MSHA, other regulatory agencies, the coal mining industry, consulting engineers and the general 

public has been beneficial to the preparation of this document. The guidance presented in this Manual is advisory and not intended to discourage new and innovative methods that may be applicable 

in specific situations.

The focus of this Manual is on design, construction and operational practices for achieving stable embankment and impoundment conditions based upon performance criteria, established regulations, 

and accepted engineering standards. Typical performance criteria are associated with long-term 

stability, flood and seismic conditions, and abandonment. Where the application to disposal practices requires interpretation that may not have been envisioned by the developers of design criteria, 

or where such criteria may not be available (as with some elements of seismic stability analyses), 

guidance is provided along with commentary describing the basis for development of the guidance. 

Criteria for environmental control of impacts from coal refuse disposal facilities vary from state to 

state. The Manual discusses features currently incorporated into disposal facilities for environmental 

control and provides guidance for their design and construction, as related to dam and embankment 

safety concerns of MSHA. Guidance for evaluating potential environmental impacts or for establishing environmental design criteria is left to other references and regulatory agencies. For example, the 

Manual provides information on the design and construction of liner systems for disposal facilities, 

but guidance relative to establishing the need for or the required hydraulic conductivity of a liner is 

available in other references.

Before the Buffalo Creek coal refuse facility failure in February 1972, coal refuse disposal was often 

subject to only cursory engineering, planning and design. Through the attention of the industry, ensuing regulatory programs, and guidance provided in the original 1975 Manual, improvements were 

made in the design and construction of disposal facilities. Since 1975, mining and coal processing 
1–2

Chapter 1

MAY 2009

have undergone changes that affect the design of disposal facilities; advances in engineering practice 

have led to modification of design criteria; and other events such as the breakthrough of impoundment basins into abandoned underground mine workings have disclosed potential considerations 

that must be addressed. Thus, the original Manual has been updated to reflect these changes. While 

the structure and sequence of the previous Manual have been maintained, additional chapters have 

been added to address design components such as seismic stability, site mining and foundation issues, instrumentation, and emergency action planning.

This Manual is intended as a reference document for use by coal industry personnel and engineering 

staff, design consultants, and dam safety regulators involved with the planning, design, construction 

and regulation of refuse disposal facilities and other mine-related impoundments. It may be particularly valuable to industry personnel who perform design coordination, as a guidance document for 

exploration, testing, engineering analysis, design and construction document preparation, and construction monitoring and inspection. For design engineers, the Manual provides background information on coal refuse, along with methods and procedures for detailed design of coal refuse disposal 

facilities. Construction personnel will find descriptions of critical engineering issues that should be 

addressed during initial development and throughout the operational life of disposal facilities.

Chapter 2 presents an introduction to coal refuse including its origin, characteristics of the various 

materials making up coal refuse, and disposal practices. Current challenges in the disposal of coal 

refuse are also discussed.

Chapter 3 describes various types of coal refuse disposal facilities (both impounding and non-impounding) and other site impoundments and the operations and hazard potential associated with 

each. The classification and terminology used for embankments and impoundments are presented, 

consistent with conventions adopted in the 1975 Manual and MSHA publications. Additionally, underground injection of coal refuse and recovery (remining) of coal from refuse disposal facilities are 

also discussed.

Chapter 4 presents updated planning and technical considerations associated with disposal site selection and facility design and emphasizes the importance of close coordination between the designer 

and the coal operator’s production staff. Past experiences with improving and expanding existing facilities and the planning of new facilities demonstrate the importance of integrating planning, design, 

construction and operations. The continually changing structure of refuse disposal facilities and the 

need to eventually reach an acceptable abandonment condition are important factors in the planning 

process.

Chapter 5 presents a new discussion of the design components of coal refuse disposal facilities, with 

emphasis on the objective of meeting the generation rates of coal preparation plants. The chapter provides the design coordinator and the engineer with an overview of the objectives of disposal facility 

design. The interrelationship between accommodating coal refuse generation rates, storm water and 

environmental control requirements, stable embankment development, and post-mining land use 

objectives are discussed.

Chapter 6 presents updated and new procedures for geotechnical exploration, engineering analysis, 

and design of coal refuse disposal facilities. Initially, the discussion addresses the unique characteristics of refuse disposal that affect facility design, as compared to the design of more commonly 

encountered geotechnical structures. Design concepts for embankment zoning, internal drainage 

control, and foundations are discussed with reference to related site exploration, testing, and geotechnical engineering analyses. The site exploration section provides new and updated guidance 

on cone penetration testing and geophysical methods applied to siting and design of impounding 

and non-impounding refuse embankments. The laboratory testing section presents descriptions and 
1–3

Introduction

MAY 2009

references to standards for commonly used tests for refuse disposal facility design, along with summaries of coal refuse material properties. Geotechnical analysis topics discussed include seepage, 

settlement, stability, rock excavation, and conduit design. Chapter 6 is intended to provide an overview of geotechnical engineering requirements for disposal facilities for design coordinators who 

are familiar with coal mining operations, but may not routinely practice civil engineering as related 

to embankments and dams. It should also be a useful source to the practicing engineer for guidance 

on approaches, methods, and design elements of geotechnical engineering applied to coal refuse 

disposal facilities.

Chapter 7 presents new guidance for analysis of seismic stability and deformations applicable to 

dams and coal refuse embankments. Many slurry impoundments are constructed using the upstream 

construction method. Such sites are susceptible to instability during an earthquake because a portion of the dam is founded on relatively loose and saturated fine material. Major advances in the 

analysis of seismic stability of embankments and impoundments have occurred since the original 

1975 Manual was published. Chapter 7 presents recommended design methods and flow charts to 

assist in seismic stability analyses. Also discussed is the importance of variation in material gradation 

(clay-like versus sand-like) that affects the selected approach and methods employed. Seismic hazard 

assessment is discussed, and guidance is provided for determining ground motion parameters considering the potential source zones and conditions found in coal regions. Deformational analysis is 

discussed along with criteria for tolerable deformations. Chapter 7 provides a condensed discussion 

of a complex subject that should allow design coordinators to gain an appreciation for the engineering required for addressing seismic stability. It will also help practicing engineers with this portion of 

the design by providing simplifying approaches in low-seismic-hazard regions. In regions of greater 

seismic hazard and for significant- and high-hazard-potential structures, more complex approaches 

for use by geotechnical engineers with experience in earthquake engineering are presented.

Chapter 8 has been added to the Manual to address site mining conditions, embankment foundation 

conditions, and impoundment stability at coal refuse facilities and the potential for subsidence and 

breakthrough into underground mine workings. Mitigation measures are also discussed. Additionally, unique foundation issues that may be encountered at mine sites are addressed, including evaluation of the effect of mine spoil materials and surface mine benches and highwalls on embankment 

and foundation design.

Chapter 9 summarizes hydrologic and hydraulic engineering methods applicable to the design of 

coal refuse disposal facilities. The Manual has been updated to reflect current design storm criteria. 

Examples of spillways, decants and channels with methods for performing design analyses and routing storms through disposal facilities are presented.

Chapter 10 addresses environmental considerations for coal refuse disposal facility design, including 

streams and wetlands, air quality, water quality, and reclamation. Updated guidance is provided as to 

methods for mitigating water quality concerns, including amendments and liner design. References 

are provided for evaluation of water quality impacts and treatment methods. Reclamation issues are 

also discussed. Design coordinators and engineers will find these sections useful when dealing with 

environmental issues and mitigation methods, but they will also need to be familiar with applicable 

state regulations and guidance.

Chapter 11 addresses disposal facility construction and operation, and Chapter 12 discusses construction monitoring, inspection and maintenance. These chapters reflect updates in construction 

and disposal practices since the 1975 Manual was prepared and discuss some of the issues associated with large production mines such as material handling, placement, and compaction requirements. Guidance has been added relative to the content of construction documents and quality 

control programs.
1–4

Chapter 1

MAY 2009

Chapter 13 provides a discussion of instrumentation for monitoring the performance of coal refuse 

disposal embankments and impoundments.

Chapter 14 provides a discussion of emergency action planning. It is recommended practice in dam 

safety that emergency action plans (EAPs) be developed and maintained for all dams that will have a 

significant downstream impact in the event of their failure.
Chapter 2

BACKGROUND AND

CHARACTERIZATION

OF COAL REFUSE

MAY 2009 2–1

Safe, economical and environmentally acceptable disposal of coal refuse has been made possible 

through the application of technology from the disciplines of geology, soil and rock mechanics, 

hydrology, hydraulics, geochemistry, soil science, agronomy and environmental sciences. These 

engineering and scientific disciplines have traditionally supported the design and construction 

of earthfill and rockfill embankments and dams, as well as a wide variety of waste disposal structures used in other industries. Among the various types of coal refuse disposal facilities, impoundments are one of the most critical types of structures, and they entail many of the same 

features and safety concerns as typical water-impounding dams. However, there are distinct differences between coal refuse disposal impoundments and conventional dams that must be considered in the transfer of dam engineering technology to coal refuse disposal, as discussed in the 

following:

• The primary purpose of a coal refuse disposal facility is to dispose of unusable materials from mining, not to construct an embankment to satisfy a secondary function. 

However, at some sites coal refuse impoundments do serve secondary purposes such 

as providing water storage capacity for coal processing and flood attenuation.

• Coal refuse is composed of rock fragments such as friable shale materials, and it 

typically includes varying amounts of coal that can have an effect on material behavior. For example, the low specific gravity of coal results in the refuse having 

lower specific gravities and densities than the materials encountered in typical earth 

embankments.

• Metals and sulfur present in some coal refuse materials may result in environmentally undesirable leachate water with serious corrosive characteristics that require 

special measures such as liner and collection systems or even amendments to neutralize acid production of the refuse.

• The potential for variations in coal refuse production as a result of mining or processing, plus the long-term phased method of refuse placement, should be accounted 

for in the disposal facility design at inception. The potential for changes in generation rates and engineering properties of the coal refuse during the life of the facility 

should also be recognized and provisions to accommodate these changes should be 

included in the design.
2–2

Chapter 2

MAY 2009

• The configuration of a disposal facility changes continually throughout its operational period, resulting in complications relating to control, but providing increased 

flexibility for planning, design and construction.

• Coal refuse disposal occurs year round, through prolonged inclement weather periods, such that the design and maintenance of the facility has to accommodate construction under adverse weather conditions.

• The construction of a disposal facility over an operational period of many years or 

even decades introduces the potential for discontinuity in construction oversight, 

quality control, monitoring, and recognition of performance factors that can affect 

operation and safety. The design plans should include specifications for oversight, 

monitoring, and quality control sufficient so that facility construction and operation 

meets the design intent.

• A primary planning and construction goal for a disposal facility is achievement of 

a safe and environmentally acceptable condition during use and upon closure and 

abandonment. Elimination of the impounding capability of a coal refuse disposal facility is an important design issue associated with abandonment unless maintenance 

of the facility is part of the planned post-mining operation of the site.

2.1 GEOLOGICAL NATURE OF COAL REFUSE

Coal refuse is composed of rock fragments unavoidably removed from the earth during the coal mining process and small amounts of coal not separated during processing. The primary source of these 

rocks and minerals is normally the formations immediately above and below the coal seam and the 

sediments within the seam. Surface mine overburden and rock removed to provide shafts, haulageways and other working space for the mining operation may constitute an important percentage of 

the total refuse material.

The proportion of rock, coal and associated minerals in any disposal facility will depend upon: (1) 

the geologic formation of the coal stratum, (2) the geochemical properties of the coal and adjacent 

materials, (3) the geometry of the coal seam, (4) the method used for mining, and (5) the process used 

to separate the coal from the refuse. A preparation plant may process coal from multiple seams and 

mining operations, resulting in some variability of the refuse product. Also, a disposal facility may 

receive refuse from multiple preparation plants.

2.1.1 Origin of Coal

Coal is a sedimentary rock resulting from past accumulation of plant materials in swampy conditions 

and subsequent metamorphism by pressure and heat to form a hard, rock-like organic material. The 

diversity of the original plant materials and the degree of metamorphism (coalification) that has affected these materials cause coal to be a non-uniform, combustible substance varying in both physical 

and chemical composition.

The predominant elements in coal are carbon, hydrogen and oxygen. Coals are rich in carbon and are 

classified based primarily as to the content of volatile matter. Table 2.1 presents the American Society 

for Testing and Materials (ASTM) system for classification of coals by rank. For the anthracite and 

low- and medium-volatile bituminous coals, the carbon content exceeds about 70 percent. When the 

volatile matter exceeds about 30 percent, it becomes difficult to classify coals on the basis of volatile 

matter alone, and caloric value is used to distinguish high-volatile-bituminous, subbituminous, and 

lignite coals.

Impurities present in coal include nitrogen, sulfur, iron and various other inorganic materials (ash). 

These impurities can be divided into the following classes: (1) impurities that are chemically or struc-
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turally a part of the coal and (2) impurities that can be mechanically removed from the coal. The 

amount and type of impurities that cannot be economically separated from a particular coal vary 

greatly and affect value. The impurities that are separable affect the refuse characteristics from a geochemical and physical standpoint. These characteristics contribute to the engineering properties of 

coal refuse and are thus critical to the engineering and design of disposal facilities.

TABLE 2.1 CLASSIFICATION OF COALS BY RANK

Group

Fixed Carbon

Limits(1)

(%)

Volatile Matter

Limits(1)

(%)

Calorific Value 

Limits(2)

(BTU/lb)

Anthracite

Meta-anthracite ≥ 98 < 2 −

Anthracite 92 to 98 2 to 8 −

Semianthracite 86 to 92 8 to 14 −

Bituminous

Low-volatility bituminous coal 78 to 86 14 to 22 −

Medium-volatility bituminous coal 69 to 78 22 to 31 −

High-volatility-A bituminous coal < 69 ≥ 31 ≥ 14,000

High-volatility-B bituminous coal − − 13,000 to 14,000

High-volatility-C bituminous coal − − 10,500 to 13,000

Subbituminous

Subbituminous-A coal − − 10,500 to 11,500

Subbituminous-B coal − − 9,500 to 10,500

Subbituminous-C coal − − 8,300 to 9,500

Lignite

Lignite A − − 6,300 to 8,300

Lignite B − − < 6,300

Note: 1. Dry mineral-matter-free basis.

2. Moist mineral-matter-free basis.

 (ADAPTED FROM ASTM, 2008a)

Sulfur, an impurity of major importance, occurs in three principal forms: (1) organic sulfur, (2) sulfate sulfur, and (3) pyritic sulfur. Generally, pyritic sulfur (FeS2) is the predominant form. Pyrites are 

of significance in the design of coal refuse disposal facilities because, when exposed to air and water, they oxidize to create acidic conditions that adversely affect the weathering resistance of certain 

rocks, cause negative environmental impacts, and cause corrosion of some construction materials.

2.1.2 Coal-Related Rocks

The rock formations immediately above and below a coal seam represent a large portion of the material in coal refuse. The character of this rock varies from seam to seam and with the geographic location of an individual coal seam, depending upon the geologic conditions preceding and following the 

deposition of the coal-forming materials.

Inorganic sedimentary rocks, such as claystone, siltstone, shale, sandstone and occasionally limestone, compose the bulk of the strata where coal is found. These sediments are frequently found in 
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the coal as inorganic debris deposited during the formation of the organic layer. The most abundant 

minerals contained in these inorganic rocks are quartz, feldspar, mica (usually muscovite with minor 

chlorite) and calcite. However, numerous minor components may also be present, and these can vary 

greatly from site to site. The following sedimentary rocks are listed in an order of decreasing resistance to weathering:

 Limestone A relatively hard, dense rock consisting largely of calcite (CaCO3). This rock 

is very durable in the weathering cycle, except in the presence of acid (including acid conditions that result from pyritic sulfur), which will dissolve it.

 Sandstone A rock composed of relatively coarse particles cemented together by calcite, 

silica (SiO2) argillaceous material or, less commonly, iron. Its strength and 

resistance to weathering depend upon the cementing agent.

 Siltstone A rock quite similar to sandstone, except for the generally smaller grain size 

and substantial amount of clay present. The clay portion is subject to rapid 

weathering, and high clay content can cause rapid deterioration.

 Claystone A rock composed predominantly of clay-sized particles. It may be massive 

(mudstone) or fissile (shale) in appearance, but in either case, it has a low 

resistance to weathering

Iron and sulfur impurities are also common in many coal-bearing sedimentary rocks. As with the 

coal itself, the most significant of these impurities are the pyrites.

2.2 COAL MINING AND COAL PREPARATION (CLEANING)

Coal is mined in more than 400 coal fields and small deposits in 38 states, split between the Eastern 

(Appalachian and Interior) and Western regions. In the Eastern region, the quality of raw coal has 

declined as higher quality reserves have been depleted, requiring more cleaning to meet product requirements. The thicker Western region coal seams typically contain fewer in-seam and out-of-seam 

rock, and much of that coal is shipped raw without extensive processing (NRC, 2002). Accordingly, 

coal cleaning and the generation of coal refuse is a significant part of mining projects in the Eastern 

region, which is where most coal refuse impoundments are operated.

In general, the removal of extraneous materials with the coal during the mining process is controlled 

by three primary factors: (1) the general geology of the mining area and to a greater extent the immediate type and quality of the floor, partings and roof materials, (2) the thickness of the seam to be 

mined, and (3) the type of mining equipment selected to remove the coal. For example, in longwall 

mining where a shearer or cutting head is drawn across the face of a coal seam, a constant thickness 

of material is removed and will contain roof or floor rock in areas of decreased coal seam thickness. 

This extraneous material, often referred to as “out of seam dilution,” is a major source of coal refuse, 

particularly in the coarser (> 19-mm) size fractions.

Coal preparation (cleaning) removes refuse from the coal. The preparation technique affects the refuse 

disposal primarily in three ways: (1) the proportion of coal in the refuse (efficiency of the cleaning 

process), (2) the gradation of the refuse particles, and (3) the moisture content of the refuse as it leaves 

the preparation plant. Additionally, the use of flocculants and other chemical additives can also affect 

the composition of refuse materials. Most modern coal preparation plants today recover coal down to 

100-mesh (150-micron) size, and some have incorporated cleaning all of the raw coal. Usually a preparation plant will have either two or three circuits (possibly four if the preparation plant cleans all the 

way to zero impurities) that handle the distinct size fractions discussed in the following paragraphs.

Often raw coal is processed through a scalping screen or rotary breaker ahead of the preparation 

plant. This is done to control the maximum size of the raw coal arriving at the preparation plant. 
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Historically, screening has been done at the 4- to 6-inch size range (100- to 150-mm). Recently, however, the trend has been to reduce the separation size down to as small as 11⁄2 inch (37 mm). This has 

been done to accommodate the use of the simpler two-circuit coal preparation plant. Depending on 

the mining method and coal characteristics, the rejected material that must be handled at the refuse 

disposal site can represent 5 to 15 percent of the raw coal.

Typically, coal preparation plants size raw coal into discrete fractions that are processed separately 

using various types of process technologies. Currently, heavy media technology is typically used for 

processing raw coal down to 1 mm. Heavy media vessels (down to 9 mm) or heavy media cyclones 

(down to 1 mm) or a combination of these two technologies are used to process the plus 1-mm raw 

coal. The heavy media process involves adding magnetite to water to create slurry with a specific 

gravity that improves the separation efficiency of the process. The magnetite slurry creates a suspension with specific gravity high enough that the coal floats and the refuse materials with higher 

specific gravity sink. These types of processes are the most efficient and produce a refuse stream with 

minimal coal. After the separation of the coal from the refuse, the materials are drained and rinsed of 

the magnetite media on a vibrating screen. Rinse water is added to the vibrating screen to improve 

the efficiency of the magnetite removal. Typically, the refuse material discharging from the end of 

the vibrating screen after being rinsed will have a surface moisture in the range of 8 to 15 percent 

depending upon the minimum size of the refuse being processed.

The 1-mm by 100-mesh (150-micron) size fraction is typically processed using water-only technologies. Technologies used to process this size range include water-only cyclones, spirals, and hinderedbed settlers. After processing, the refuse materials are typically dewatered using a high frequency 

dewatering screen designed to retain the plus 100-mesh material. The installation of this type of 

screen ahead of the thickener can significantly reduce the amount of plus 200-mesh particles in the 

refuse slurry.

If coal cleaning below 100-mesh size is performed, then typically the differences between the surface 

chemistry of the coal and refuse are exploited to perform a separation (e.g., froth flotation). Froth 

flotation requires the addition of a collector (typically fuel oil) and a frother (typically glycol or methylisobutyl carbinol (MIBC)) to the slurry. The fine coal particles are hydrophobic and collect on 

the surface of the air bubbles created by the flotation machine and rise up through the slurry and 

are removed with the froth. Alternatives to flotation include high gravity concentrators and fine coal 

spirals used to process the 100-mesh by 325-mesh (44-micron) size fraction.

Due to the high volume of water used in the processing of coal, thickeners are typically used to reclaim the wash water for recirculation. Generally, the use of flocculants is sufficient to remove the 

majority of coal fines, but when there are high levels of clay in the mined coal, it may be necessary 

to also add coagulants. The sequential addition of flocculants and coagulants aids in controlling the 

turbidity of recycled process water and settling of solids that are pumped to the impoundment.

Technologies for dewatering of the thickener underflow include belt filter presses, vacuum filters, 

plate and frame filters, solid bowl centrifuges, horizontal belt filters, and paste thickeners. When fines 

are dewatered, the most commonly used technology is the belt filter press. The dewatered product 

from these technologies typically has moisture contents in the range of 35 to 45 percent. The amount 

of minus 325-mesh solids in the feed to these unit operations dictates the final moisture of the product. The higher the amount of minus 325-mesh particles in the feed (which is controlled by the geology and mining method), the more difficult it is for the system to operate and to produce a product 

with manageable moisture.

If fine refuse dewatering is included in the process, refuse streams must be handled either individually or as a combined product. Amendments may be required for pH control or to aid in the handling 
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of the refuse products. Depending on the amount of fine refuse and its moisture content, the disposal 

of the combined product may be difficult.

Prior to 1970, many preparation plants did not attempt to separate coal and impurities in the finewashed material, resulting in a fine refuse with high coal content. This practice resulted in a relatively 

high ratio of fine refuse volume to coarse refuse volume, and these disposal sites are of interest for 

remining and recovery of the coal. In general, modern coal preparation plants are more efficient (i.e., 

lower coal losses to refuse). In addition, the amount of out-of-seam dilution has increased, resulting in higher coarse to fine ratios. This is particularly true in areas where the coal reserve is being 

depleted. As coal preparation techniques have advanced, the amount of fine coal deposited with the 

fine refuse slurry has decreased, thus increasing the amount of coarse refuse disposed in relation to 

the amount of fine refuse.

2.3 REFUSE TRANSPORT AND DISPOSAL PLACEMENT

2.3.1 Coarse Refuse

The method of transporting coarse coal refuse from the preparation plant to the disposal facility is determined by: (1) material gradation, (2) production rate, (3) the distance and topography of the route 

to the disposal facility and (4) the size, type and configuration of the disposal facility. Although refuse 

transport methods vary widely, the most common method for transporting coarse refuse is by trucks 

or scrapers for relatively short distances over moderate terrain, by conveyors for long distances over 

moderate to steep terrain, or by a combination of these methods. The ramifications of these transport 

methods on the disposal facilities are further described in Chapter 11.

The placement of coarse refuse at a disposal facility depends upon the method of transport to the 

disposal facility and the facility configuration. Prior to 1970, coarse refuse transported by truck was 

generally end-dumped from the truck and allowed to form a progressing embankment at the angle 

of repose of the material. Another method from that era was to transfer the coarse refuse from a 

conveyor or continuous bucket tram to an aerial tram that dumped the refuse from a high elevation 

along a fixed axis, allowing the refuse to form a progressing embankment at the angle of repose of the 

material except as modified by dozers or scrapers.

Current practice typically includes the use of conveyor systems and/or trucks. Trucks dump the refuse in piles on the disposal surface with subsequent spreading by dozers. If the refuse is transported 

by conveyor, it is normally dumped into a hopper bin and transferred to trucks or scrapers for placement, as described above. There has been some use of mobile conveyors to deposit coarse refuse directly on the embankment surface for spreading by dozers. The common configurations of coal refuse 

embankments are described in Chapter 3.

2.3.2 Fine Refuse and Combined Refuse

A common method of transporting fine refuse material has been as slurry pumped to a disposal 

pond for settling of the solids and clarification of the remaining water. Use of thickeners with 

the application of flocculants, and in some cases coagulants, improves clarification and settling 

of solids and facilitates deposition within a slurry impoundment. These additives typically overcome the use of reagents used in the separation and concentration processes of coal preparation.

Another method for transporting fine refuse involves partially dewatering the slurry at the preparation plant. The resulting material is then disposed separately or mixed and placed with the coarse 

refuse material as combined refuse. There have been many problems associated with transporting 

and placing the combined material due to the relatively high moisture content of the dewatered fine 

refuse. Consideration has recently been given to transporting the fine refuse as a paste (thickened 
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tailings) that can be pumped to a disposal location. The objective in using thickened tailings is to 

minimize the use of water, generally because of limited availability.

2.4 COAL REFUSE CHARACTERIZATION

Coal refuse is composed of rock fragments, sand, silt and clay particles and contains small amounts 

of coal not separated during processing.

2.4.1 General Geochemical Characteristics

Coal refuse geochemical characteristics result from the depositional environment of the coal and 

adjacent strata, the mining and coal preparation process, and the weathering process when the refuse is exposed to air. Coal refuse shares many properties with the coal seam where it originated, 

with sulfur usually representing the most significant environmental concern. Mining processes may 

take mine roof and floor materials that contain significant pyrite materials, further affecting sulfur 

content. Processing of the coal includes crushing, separation, and in some plants recombining coarse 

and fine refuse. The efficiency of the processing plant at removing sulfur and the degree to which the 

sulfide fragments are fractured and reduced in size influences the reactivity of the final refuse product. Reagents and additives such as surfactants, oils, and strong bases are used in various separation 

processes.

Most of the environmental problems associated with coal refuse result from the oxidation of pyrite and 

subsequent production of acidity. The exposure of coal refuse to weather (air and moisture) enhances 

the production of acid, leading to a condition of low pH. The pH of a refuse deposit may depend not 

only on the pyrite content, but also on the length of exposure time and the acid-neutralizing capacity 

of the refuse. The majority of the coal refuse in the Appalachian region of the U.S. contains an excess of 

oxidizable sulfur compared to neutralizing carbonates and is therefore net acid producing over time. 

During the oxidation process, metals are released into soluble forms, resulting in impacts to water quality from drainage through or off the surface of refuse deposits and accumulation of phytotoxic concentrations of acid-soluble metal ions and sulfate salts on refuse surfaces that inhibit the establishment of 

vegetation during reclamation of the disposal site (Daniels and Stewart, 1993). Western coal regions in 

the U.S. tend to be less acid producing, but may still pose environmental concerns.

Variability may be found in coal refuse within the same disposal area because: (1) the coal preparation plant may process coal from multiple seams, (2) the process may change over time, and (3) the 

rate of processing (and thus accumulation and successive covering of earlier refuse deposits) may 

change due to production and mining factors. The management of refuse at the disposal site can 

affect the weathering process, with the result that geochemical characteristics can change over relatively short periods of time.

The geochemical characteristics of coal refuse are important planning considerations for coal refuse 

disposal facilities and can be addressed in a variety of ways. Amendments (additives with neutralization characteristics such as lime products) may be added to provide additional acid-neutralizing 

capacity that can affect the physical properties of the deposit. Liners (natural soil or synthetic) may 

be employed for control of seepage from the refuse disposal area and thus will affect foundation requirements and facility configuration.

2.4.2 General Geotechnical Characteristics

Each type of coal refuse has characteristic geotechnical properties. Coarse refuse from the preparation 

process represents a substantial portion of and sometimes the entire waste stream. Where mechanical 

crushers are included in the coal preparation process, large-diameter rock (on the order of 6 inches in 

size and referred to as “breaker rock”) may also be generated, although it typically represents a small 

fraction of the refuse materials.
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Fine refuse is a separate waste stream resulting from the wet processing of coal. It may be: (1) disposed as a slurry (fine coal refuse slurry) separate from the coarse refuse, (2) dewatered and disposed 

with the coarse refuse as combined refuse, or (3) dewatered and disposed separately from the coarse 

refuse (dewatered fine coal refuse or filter cake). An additional form of refuse, although not common, 

is the slurrying of both coarse and fine refuse (coarse and fine refuse slurry).

Most slurry disposal operations use slurry pumps to transport fine coal refuse at between 5 and 20 

percent solids. Thickened tailings are increasingly being considered for fine coal refuse disposal. This 

practice involves processing the fine coal refuse into a non-settling and non-segregating suspension 

of solids taking the form of a paste with about 50 percent solids.

2.4.2.1 Coarse Coal Refuse

Coarse coal refuse is typically a well-graded material with particle sizes ranging up to 3 inches and a 

fines content (as measured by the amount passing the No. 200 sieve or 0.075 mm) ranging from less 

than 10 percent to more than 20 percent. It is generally classified as a silty, clayey sand with gravel to 

a clayey, silty gravel with sand. Coarse coal refuse typically has a specific gravity ranging from 1.8 to 

2.3, which is lower than many natural soils and aggregates due to the carbon content. Run-of-plant 

coarse coal refuse generally has a water content of between 8 and 15 percent and loses moisture during transport to the disposal site. As a result, it can readily be compacted to form dense fill in embankments. Figure 2.1 provides the range of grain-size distributions usually encountered for coarse coal 

refuse at disposal sites and a photograph of a coarse refuse embankment.

Coarse coal refuse typically exhibits weathering and some physical degradation during placement 

and compaction. Thus, the resulting percentage of fines in compacted, weathered coarse coal refuse 

can be greater than in fresh or recently placed material. Sieve analyses performed on fresh samples 

before and after compaction have exhibited an average increase in fines content of approximately 4 

percent in one northern Appalachian mine. Coarse coal refuse generally has strength, settlement and 

hydraulic conductivity properties reflective of a well-graded soil and rock fill. Further characterization of coarse refuse is discussed in Chapter 6.

2.4.2.2 Fine Coal Refuse Slurry

Fine coal refuse slurry, when discharged into an impoundment, meanders across previous deposits 

and the sand-size material commences settling in shallow water. As the slurry discharge velocity 

FIGURE 2.1 COARSE COAL REFUSE EMBANKMENT AND SAMPLE GRADATION

 100 10 1 0.1 0.01 0.001 

(HEGAZY ET AL., 2004)

10

20

30

40

50

60

70

80

90

100

0

SIEVE SIZE (mm)

PERCENT PASSING

SAMPLE GRAIN-SIZE

RANGE FOR COARSE

COAL REFUSE

AVERAGE

FIGURE 2.1 COARSE COAL REFUSE EMBANKMENT AND SAMPLE GRADATION
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Background and Characterization of Coal Refuse

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slows and deeper water conditions are encountered, settling of the finer materials occurs, resulting in 

greater variability in grain size than encountered with other forms of coal refuse placement. Samples 

collected from or close to the slurry discharge point are predominantly sand- and silt-size material, 

whereas samples collected away from the discharge point are predominantly silt- and clay-size material. Because the discharge location is typically shifted periodically, there is variability in the grain 

size of fine refuse materials with elevation as well as location. Figure 2.2 illustrates typical grain-size 

distributions encountered with discharge of fine coal refuse slurry at impoundments.

Fine coal refuse consists of particles that are mostly less than 1 millimeter in size and frequently has 

a fines content (as measured by the amount passing the No. 200 sieve) of typically between 30 and 80 

percent. The specific gravity of fine coal refuse tends to be lower than that of coarse refuse, typically 

in the range of 1.4 to 2.0, depending on the percentage of coal fines. Water content will vary significantly dependent upon the grain-size distribution and location within the impoundment. Settled fine 

coal refuse slurry has strength, settlement and hydraulic conductivity properties reflective of hydraulically-placed deposits of sand, silt and clay, as further discussed in Chapter 6.

2.4.2.3 Dewatered Fine Coal Refuse

Dewatering of fine coal refuse results in a filter cake material with less variability in grain size than 

observed in hydraulically-placed fine refuse slurry. While the grain-size distribution is relatively consistent for a given coal seam and processing plant, there is typically variation between samples from 

different coals and processing systems. Filter cake may have a retained water content in excess of 30 

percent, and considering the significant amount of silt and clay fraction in the material, frequently 

cannot be placed and compacted into an embankment without further drying and/or the addition 

of other materials such as coarse refuse or amendments for moisture control and stabilization. Consequently, dewatered fine coal refuse is generally disposed with coarse refuse either intermixed as 

combined refuse or segregated to form containment cells.

2.4.2.4 Combined Coal Refuse

Combined coal refuse is a mixture of coarse and fine coal refuse, and thus has a greater fines content 

and water content than coarse coal refuse. While still well graded, combined refuse may have fines 

FIGURE 2.2 FINE COAL REFUSE SLURRY CHARACTERISTICS

SIEVE SIZE (mm)

PERCENT PASSING

(HEGAZY ET AL., 2004)

100 10 1 0.1 0.01 0.001 

10

20

30

40

50

60

70

80

90

100

0

SAMPLE GRAIN-SIZE

RANGE FOR FINE

COAL SLURRY

AVERAGE

FIGURE 2.2 FINE COAL REFUSE SLURRY CHARACTERISTICS
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content in the range of 20 to 40 percent, and water content between 12 and 30 percent. When the fines 

and/or water content are relatively high, combined refuse may be difficult to compact into a dense 

embankment and may also be sensitive to inclement weather impacts. Figure 2.3 presents a photograph of combined coal refuse and an illustration of a typical grain-size distribution for combined 

refuse with a moisture content of 30 percent.

2.4.2.5 Fine Coal Refuse Paste

To reduce the quantity of water associated with fine refuse disposal, thickened tailings technology is currently under study. It is anticipated that the resulting paste will have properties similar 

to dewatered fine coal refuse from filter presses, but with greater water content. The attraction to 

this technology lies in certain perceived advantages over filter press operating requirements and 

the ability to transport the resulting paste by pumping, using less water than required for slurry 

impoundments. Placement and containment issues are anticipated to be similar to those encountered with disposal of dewatered filter cake. The use of thickened tailings has been implemented 

at two existing impoundments in the United States (Henry, 2007; Gupta et al., 2008), achieving a 

paste solids content of 45 to 55 percent that reportedly builds or “stacks” without separation as the 

paste is deposited.

2.5 DISPOSAL PRACTICES

2.5.1 Disposal Practices Prior to the Buffalo Creek Failure

In February 1972, a coal refuse disposal facility located on a tributary to Buffalo Creek in southern 

West Virginia failed after several days of rainfall, sending flood water and refuse material through 

the narrow downstream valley. A number of coal mining communities were devastated, and 125 

people were killed. This monumental failure, known as the Buffalo Creek failure, brought the potential danger of similar disposal facilities to the attention of the nation. A summary description 

of this failure along with other incidents reported at coal refuse disposal facilities is provided in 

Table 2.2.

Prior to the Buffalo Creek failure, minimal technical effort was devoted to the planning and design of 

coal refuse disposal facilities in the U.S. Geotechnical and hydraulic characteristics of refuse materials 

and disposal facilities were seldom considered. Instead, disposal facilities were often developed at 

FIGURE 2.3 COMBINED COAL REFUSE MATERIALS

100 10 1 0.1 0.01 0.001 

10

20

30

40

50

60

70

80

90

100

0

SIEVE SIZE (mm)

% PASSING

SAMPLE GRAIN-SIZE

RANGE FOR COMBINED

COAL REFUSE

NOTE: DATA WERE OBTAINED FROM UNPUBLISHED STUDIES.

FIGURE 2.3 COMBINED COAL REFUSE MATERIALS
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Background and Characterization of Coal Refuse

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sites chosen for expediency and were often operated with equipment selected by availability rather 

than suitability for site materials. Most research devoted to coal refuse disposal in the U.S. before 

1972 was related to: (1) the prevention of refuse embankment burning, (2) the extinguishment of 

embankment burning, and (3) the development of vegetation on refuse materials through refuse 

conditioning and seeding.

Prior to 1972, little or no planning or design was devoted to the control of water entering an 

impoundment from a preparation plant or from watershed runoff upstream of the disposal 

facility. Experience had suggested that excess preparation plant water could be clarified naturally by settlement at the impoundment and filtration through the refuse embankment. In most 

coal refuse impoundments, the water level above the settled slurry reached an equilibrium 

condition at a depth of less than 10 to 15 feet, at which time the outflow of water from seepage 

equaled the inflow to the impoundment. This effect, plus the large volume of coarse refuse 

often placed in the embankment, generally resulted in the maintenance of a large freeboard 

between the normal water surface and the embankment crest that provided storage for runoff 

from moderate storms.

The potential risk of coal refuse disposal facilities is related to the storage of water and waste that may 

be flowable and that, if released, may result in downstream inundation with substantial public safety, 

economic and environmental consequences. A major reason for emphasizing the potential hazard of 

disposal facilities with impoundments, as compared to disposal facilities without impoundments, 

is the large geographical area that may be affected by the rapid release of large volumes of water 

or waste following embankment failure. Unless a non-impounding embankment is located directly 

above a populated area such as the disastrous failure in Aberfan, Wales in 1966 (Bignell et al., 1977), 

or adjacent to a stream where materials movement could block flow, the slippage of such an embankment will cause only localized problems related more to environmental and aesthetic considerations 

than to public safety. However, embankment failure allowing the rapid release of large volumes of 

water may cause flooding that is costly not only in terms of miner and public safety, and property 

and environmental damage, but also in terms of extensive cleanup of the dispersed wet refuse for 

many miles downstream. In mountainous mining areas complementary industrial, commercial and 

residential developments are usually located in the downstream valley bottom.

2.5.2 Disposal Practices Subsequent to the Buffalo Creek Failure

Following the Buffalo Creek failure in February 1972, regulations were promulgated for the design 

of embankment structures, and programs and resources were developed, including the 1975 Manual 

to assist in design, inspection and construction of coal refuse disposal facilities. Hundreds of existing and planned refuse disposal facilities were upgraded to new and safer standards, and for slurry 

impoundments, significant emphasis was placed on dam safety.

The various programs and resources have produced the following major, positive results:

• It has been recognized that coal refuse disposal facilities can represent a threat to 

public safety and the environment, and positive steps toward eliminating these 

threats in future disposal operations have been taken.

• The principles of geotechnical engineering, hydrology, hydraulics, and dam-safety 

engineering have been applied to the design and construction of coal industry impoundments to enhance the safety of these structures.

• There is industry-wide recognition of the importance of coal refuse disposal to 

preparation plant operation, and disposal practices are being incorporated into the 

feasibility, planning, and management of mine operations.
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• Data that contribute to the application of geology, soil and rock mechanics, hydrology, hydraulics, dam engineering and geochemistry technologies to the evaluation 

and design of disposal facilities have been collected and analyzed.

• Research has yielded important advances in the prediction of the performance of 

disposal facilities, and areas have been identified where new research is needed for 

the design and operation of disposal facilities.

• The potential economic benefit of integrating the disposal facility design into the 

total plan for coal production operations has been demonstrated.

While effective engineering and design measures have been implemented to minimize the potential 

for failure of coal refuse disposal facilities, incidents have occurred that may be instructive. A summary of these incidents is presented in Table 2.2.

Challenges remain for the coal industry, including the following:

• Advances in mining technology, including the more widespread application of longwall mining systems, increases the potential for subsidence and associated impacts 

to the overburden strata that must be addressed at disposal facility sites, either as 

part of siting and new facility design, or to permit mining beneath or adjacent to existing disposal facilities. Additionally, the threat of subsidence from old mine workings at some sites may require evaluation and remedial action.

• Production capacity at mines has continued to increase as a result of larger longwall 

mining systems and operations with continuous miners, and in some cases more 

reject material (non-coal rock) is being mined, with a corresponding increase in the 

generation of coal refuse. This has resulted in larger embankments and impoundments that must accommodate refuse placement at a greater rate.

• Longwall systems and larger mines with high production rates may exhibit variability in the refuse generation rate. Disposal facilities should be planned and designed 

considering this potential variability.

• Integration of surface mine operations with coal refuse disposal poses opportunities 

for more efficient construction, provided design issues associated with mine spoil/refuse embankments are adequately addressed.

• Amendments and admixtures have become common, whether as part of operations 

(transport of combustion ash from power plants to mine sites) or to address environmental requirements (neutralization of acid generation from refuse). These material 

additions may affect the performance of the refuse embankments and should be 

considered during the design process.

• Re-mining of existing coal refuse facilities to recover coal has developed into a niche 

industry. Development plans must be prepared for the safe excavation and removal 

of coal refuse from disposal facilities while maintaining the original geotechnical and 

hydraulic design criteria and considering the potential for shutdowns experienced 

with such operations.

• Slurry impoundments should be designed considering the potential impact of 

seismic events. While seismic failures of such facilities have not been reported in the 

U.S., the potential exists at some refuse disposal facilities.

• Multiple regulatory criteria from federal and state agencies with differing perspectives must be accommodated in site selection and design.
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Background and Characterization of Coal Refuse

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TABLE 2.2 COAL REFUSE DISPOSAL FACILITY FAILURES AND INCIDENTS

February 26, 1972: Buffalo Mining Company,

Buffalo Creek, West Virginia

On February 26, 1972, the most destructive flood in West Virginia’s history occurred when a coal waste impounding 

structure collapsed on the Buffalo Creek tributary of Middle Fork. Shortly before 8:00 a.m., the impounding structure 

collapsed, releasing approximately 132 million gallons of water. The water passed through two more piles of coal waste 

blocking the Middle Fork. At that time, there were no federal standards requiring either impoundments or hazardous 

refuse piles to be constructed and maintained in an approved manner.

Around 1957, as part of its surface mining operations, the Buffalo Mining Company (a subsidiary of the Pittston Coal 

Company) had begun depositing mine waste consisting of rock and coal in Middle Fork. Buffalo Mining constructed 

its first impounding structure, near the mouth of Middle Fork in 1960. Six years later, it added a second impounding 

structure, 600 feet upstream. By 1968, the company was depositing more waste another 600 feet upstream. By 1972, 

the height of this third impounding structure ranged from 45 to 60 feet.

Between February 24 and 26, 1972, the National Weather Service measured 3.7 inches of rain in the area of 

Logan County and Buffalo Creek. The impounding structure probably failed because foundation deficiencies led 

to sliding and slumping of the front face of the refuse bank. The waterlogged refuse bank accelerated the failure. 

The slumping lowered the top of the refuse bank and allowed the impounded water to breach and then rapidly 

erode the crest of the bank. Upon failure of the refuse bank, the floodwater moved into pockets of burning coal 

waste.

As result of the flood, 125 people were killed, 1,100 were injured, and more than 4,000 were left homeless. In addition, 

the flood completely demolished 1,000 cars and trucks, 502 houses, 44 mobile homes, and damaged 943 houses and 

mobile homes to varying extents. Property damage was estimated at $50 million.

Source: Davies et al., 1972

August 14, 1977: Island Creek Coal Company,

Boone County, West Virginia

An embankment under construction failed at Island Creek Coal Company’s impoundment in Boone County, West 

Virginia, on August 14, 1977. Heavy rainfall overflowed a temporary diversion ditch, causing the water level in the 

impoundment to rise. Because the embankment was still under construction, storage capacity had not yet reached the 

required minimum, and the sudden influx of additional water overtopped the embankment. Meanwhile, the water eroded 

the embankment, reducing its height 23 feet during a two-day period. During this time, 6.8 acre-feet were released, 

clogging a drainage pipe downstream.

Source: Owens, 1977

December 18, 1981: Eastover Mining Company,

Harlan County, Kentucky

On December 18, 1981, Eastover Mining Company’s Hollow No. 3 combined refuse disposal site failed, releasing about 

25 million gallons of saturated coal refuse. The operation, which had been permitted for disposal of coarse coal refuse 

and dewatered slurry “filter cake” that contained approximately 30 percent moisture behind an embankment 192 feet 

high, had reached 90 percent of its planned capacity. Several factors contributed to the increased pore water pressure 

in the dewatered fine refuse zone, including: (1) the filter cake layers had not been allowed sufficient time to dry before 

additional material was added; (2) layers of filter cake were not completely covered with coarse coal refuse; (3) a stream 

flowed into the impoundment material, increasing saturation; and (4) material used in construction of the embankment 

did not allow water to seep out. The failure released a mudflow approximately 5 feet deep that traveled 4,400 feet downstream (500 feet in vertical distance) into the community of Ages, Kentucky. One resident was killed, three houses were 

destroyed and 30 homes were damaged.

Source: Cannon, 1981

April 8, 1987: Peabody Coal Company,

Raleigh County, West Virginia

On April 8, 1987, a breach developed in the principal spillway pipe in the Lower Big Branch impoundment at Peabody’s 

Montcoal No. 7 complex in Raleigh County, West Virginia. The 36-inch-diameter pipe ran through the impoundment and 

under part of the embankment at a depth of 55 feet. The rupture released nearly 23 million gallons of water, slurry, and 

fine coal refuse.
2–14

Chapter 2

MAY 2009

The exact cause of the accident was not identified but was probably the result of a combination of factors: (1) Heavy 

snowfall (16 inches of snow with a rainfall equivalent to 1.9 inches), followed by rapid temperature increases and 

snowmelt, sent excessive amounts of water through the pipe. (2) Two landslides occurred in the slope above the rupture. 

Although the relative timing of the landslides and the breach is not known, the slides could have caused the pipe to 

collapse or separate. (3) Erosion of particles near the pipe connections could have reduced the bearing strength of the 

pipe. (4) The strength of an “elbow” in the piping may have been exceeded by massive and rapid fluid flow. In addition, 

a sinkhole that developed from the rupture threatened the stability of the embankment. The sinkhole came within 100 

feet of several upstream-constructed additions to the cross-valley embankment before stability was maintained through 

mitigation of the breach.

The impoundment, upstream from several communities, was rated at the time as high hazard. A 50-mile stretch of Coal 

River from Montcoal to its mouth at St. Albans was visibly affected, and five water plants were shut down. Although 1,700 

customers’ water supply was disrupted in the Racine Public Service District, no human injuries or fatalities occurred as 

a result of this incident.

Source: Owens, 1987

January 28, 1994: Consolidation Coal Company,

Morgantown, West Virginia

On January 28, 1994, a 5-foot earthen berm failed at a slurry refuse impoundment at the Arkwright Mine in Granville, 

West Virginia. Heavy rain and melting snow resulted in 30 inches of water collecting behind the berm; it was determined 

that the 4-inch discharge pipe and rock underdrain at the site were insufficient to prevent water accumulation. The 

incident released 375,000 gallons of water into the town of Granville. Although no one was injured, three residences 

directly downstream were damaged.

Source: Betoney, 1994

May 22, 1994: Martin County Coal Corporation,

Davella, Kentucky

On May 22, 1994, a breakthrough occurred at Martin County Coal Corporation’s Big Hollow slurry impoundment in 

Davella, Kentucky. Nearly 32 million gallons of black water inundated an abandoned and sealed-off portion of the 

mine. The breakthrough resulted either from collapse or water penetration of the Coalburg coal seam bordering the 

impoundment. Slurry had been impounded 32 feet higher than the coal seam’s elevation. The mine’s 16-inch, concreteblock seals held the black water inundating the mine, but water broke through portal seals and a coal seam outcrop 

barrier. Although the slurry level dropped by 6 feet, the embankment structure was not damaged, and no injuries or 

fatalities occurred.

Source: Stewart and Robinson, 1994

August 9, 1996: Lone Mountain Processing Incorporated,

St. Charles, Virginia

On August 9, 1996, there was a breakthrough at Lone Mountain Processing’s Miller Cove slurry impoundment. The 

evening before the failure, approximately 2.75 inches of rain had fallen, and most of it within an hour and a half. 

Approximately 1 million gallons of black water were released into Gin Creek through an abandoned mine. (Underground 

mines had operated in areas adjacent to the impoundment from the 1920s to the 1980s.)

Excavation of the breach showed that the leak occurred in an area where available mine maps indicated a barrier of at 

least 25 feet of solid coal between the outcrop and the underground mine workings. Further exploration revealed that the 

barrier was in fact less than 2 feet thick. It is believed that hydrostatic pressure from the slurry opened cracks in the coal 

seam and began a piping-type failure. The thin coal barrier was progressively eroded, allowing slurry to flow uncontrolled 

into the abandoned mine.

Source: Michalek et al., 1996

October 24, 1996: Lone Mountain Processing Incorporated,

St. Charles, Virginia

On October 24, 1996, a second breakthrough occurred at Lone Mountain Processing’s Miller Cove impoundment, but in 

another area of the abandoned mine. This release was more serious than the event in August 1996 because the water 

contained more solids. Approximately 6 million gallons of water and slurry exited the abandoned mine into Gin Creek 

and flowed 11 miles, where it entered the Powell River’s North Fork. Reportedly, the river was discolored for more than 

40 miles.

TABLE 2.2 COAL REFUSE DISPOSAL FACILITY FAILURES AND INCIDENTS

(Continued)
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Background and Characterization of Coal Refuse

MAY 2009

The failure resulted from two large sinkholes that had developed on the northwestern end of the impoundment. When 

the site was excavated to locate the breach, it was determined that the slurry had entered through a fracture in the mine 

roof that coincided with these sinkholes.

Source: Michalek et al., 1996

November 26, 1996: Consolidation Coal Company,

Oakwood, Virginia

On November 26 1996, the Buchanan No. 1 impoundment in Buchanan County, Virginia, failed. In the 1960s, the 

Kennedy coal seam at the site had been excavated by both surface area mining and underground auger mining. After the 

impoundment was constructed (1984), another company mining underground in the adjacent drainage area apparently 

intersected the historic auger mine workings, providing a conduit for the slurry.

Coal refuse and slurry from the impoundment broke into an abandoned underground mine and discharged about 1,000 

gallons per minute at its peak through two mine portals into the adjacent North Branch Hollow of the Levisa Fork of the 

Big Sandy River. There was no detrimental impact on the embankment, and no one was killed or injured.

Source: Michalek el al., 1996

October 11, 2000: Martin County Coal Corporation,

Inez, Kentucky

On October 11, 2000, a coal waste impoundment of the Martin County Coal’s preparation plant near Inez, Kentucky, 

released slurry containing an estimated 250 million gallons of water and 31 million gallons of coal waste into local 

streams. Reportedly, the failure was caused by the collapse of the slurry pond into underground coal mine workings next 

to the impoundment. The slurry broke through an underground mine seal and discharged from mine entrances 2 miles 

apart into two different watersheds (Wolf Creek and Coldwater Fork).

Although no human life was lost, the release killed aquatic life along the Tug Fork of the Big Sandy River and its tributaries. 

Public water supplies were disrupted when communities along the rivers in both Kentucky and West Virginia shut down 

water plants to prevent contamination with black water. 

Source: Various issues of the Herald Leader, the Courier-Journal, and the Charleston

 Gazette from 2000 and 2001; National Research Council (2002)

TABLE 2.2 COAL REFUSE DISPOSAL FACILITY FAILURES AND INCIDENTS

(Continued)

 (NATIONAL RESEARCH COUNCIL, 2002) 
Chapter 3

COAL REFUSE DISPOSAL

FACILITIES AND OTHER

IMPOUNDING STRUCTURES

MAY 2009 3-1

This chapter presents an overview of the types of coal refuse disposal facilities and other impounding structures that are employed at mine sites and provides a discussion of terminology applied to 

these embankments and impoundments. Refuse disposal facilities are generally classified in the following terms: (1) refuse piles (non-impounding facilities), (2) impounding facilities, (3) slurry cell 

facilities, and (4) underground injection facilities. Non-impounding refuse piles typically consist of 

an embankment fill where drainage is directed away from the site without retention. These facilities 

are employed for disposal of coarse coal refuse and dewatered fine coal refuse.

Impounding facilities have the capacity to retain water, sediment and/or fine coal refuse slurry. Under 

MSHA regulations, a facility or structure requires an approved impoundment plan when it exceeds 

a threshold height or has the potential for impounding a threshold volume of water and/or fine coal 

refuse slurry and/or represents a potential hazard to miners. In practice, MSHA also considers any 

potential hazard to the public downstream. While not always true, slurry disposal facilities typically exceed impoundment threshold parameters, resulting in their being subject to MSHA impoundment plan regulations. Slurry disposal may also occur in slurry cell facilities that do not exceed the 

impoundment threshold height or volume. In addition to disposal in embankments, impoundments 

and cells, fine coal refuse slurry may also be disposed in underground mine workings at underground injection sites.

3.1 HAZARD POTENTIAL FOR DISPOSAL FACILITIES AND OTHER IMPOUNDING STRUCTURES

Coal refuse disposal facilities and other mining impoundments are evaluated relative to their hazardpotential classification, which dictates the design criteria that must be incorporated in their planning, 

development, and construction. While hazard-potential classification primarily relates to impoundments, it can also be applied to non-impounding facilities where dewatered fine coal refuse may 

exhibit flowable characteristics after disposal. Consistent with the hazard-potential-classification 

system and criteria for dams in use by federal agencies (FEMA, 2004a), the three hazard-potential 

classifications for disposal facilities are as follows:

• Low hazard potential – Facilities where failure would result in no probable loss of 

human life and low economic and/or environmental losses. Such facilities are usually located in rural or agricultural areas where losses are limited principally to the 

owner’s property or where failure would cause only slight damage to farm buildings, forest and agricultural land, and minor roads.
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• Significant hazard potential – Facilities where failure would likely not result in loss 

of human life, but can cause economic loss, environmental damage, or disruption of 

lifeline facilities. Such facilities are generally located in predominantly rural areas, 

but could be in populated areas with significant infrastructure and where failure 

could damage isolated homes, main highways, and minor railroads or disrupt the 

use of service of public utilities.

• High hazard potential – Facilities where failure will probably cause loss of human 

life. Such facilities are generally located in populated areas or where dwellings are 

found in the flood plain and failure can reasonably be expected to cause loss of life; 

serious damage to homes, industrial and commercial buildings; and damage to 

important utilities, highways or railroads.

The purpose of hazard-potential classification is not to determine the likelihood of a failure occurring, but rather to assess the potential impacts should a failure occur and to establish appropriate 

criteria for use in the design and operation of the facility. Thus, more conservative design and 

operations criteria apply as the potential for loss of life or property damage from failure increases. 

For example, more subsurface exploration and material property testing is normally performed 

for a facility with high hazard potential than for one with low hazard potential. An impoundment dam with high hazard potential would be designed to accommodate the probable maximum 

flood (PMF), while a dam with low hazard potential would be designed for a smaller storm event. 

While this chapter introduces hazard-potential classification, specific design criteria that are associated with the hazard-potential classification are discussed in later chapters. The application of 

the hazard-potential classification to storm water management is discussed in Chapter 5; stability 

considerations are discussed in Chapters 6 and 7; and hydrology and hydraulic engineering issues 

are addressed in Chapter 9.

Determination of possible damage due to failure of an impounding refuse disposal facility must be 

based upon an evaluation of conditions for an appropriate downstream distance. This distance is 

normally determined by performing a breach analysis that defines an inundation area resulting from 

a breach of the impounding embankment. This analysis is particularly important in mountainous 

mining areas where complementary industrial, commercial, and residential developments are usually located in valley bottom. Conditions downstream from the disposal facility may also be important even if the facility embankment is incapable of impounding water. An example is failure of an 

embankment that blocks a stream, temporarily forming a dam that impounds water that suddenly 

fails, releasing a flood wave of water and coal refuse.

Many mine impoundments currently under MSHA jurisdiction have underground mine works either 

beneath or near the dam or reservoir. This creates a situation where a failure of the impoundment 

may release flowable fine coal refuse and water into the underground mine workings. A release of 

flowable fine coal refuse and water can also occur due to failure of natural ground or man-made barriers, resulting in breakthrough into the mine workings. In the event of breakthrough, not only will 

mine personnel potentially be endangered, but the water or slurry may subsequently discharge from 

the mine and potentially affect an area different from that affected by a dam failure. MSHA addresses 

this possibility as follows:

• The official hazard-potential classification for an impoundment facility is based on 

the three classifications discussed previously and is assigned regardless of whether 

the potential hazard is from a failure of the dam or from a breakthrough into the 

mine workings and subsequent discharge.

• For the purpose of selecting appropriate design criteria for a coal refuse dam, the 

hazard classification is based on the appropriate rating for the type of failure con-
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Coal Refuse Disposal Facilities and Other Impounding Structures

MAY 2009

sidered. For example, an impoundment could have a high-hazard-potential rating 

based solely on the potential for breakthrough into the underground mine workings, 

with low consequences associated with a failure of the dam itself. In such a case, the 

dam embankment can be designed based on the low-hazard-potential classification 

(e.g., the 100-year design storm), while the breakthrough evaluation and prevention 

measures, with respect to the extent of exploration, testing, monitoring, etc., would 

need to be appropriate for a high-hazard-potential facility.

3.2 COAL REFUSE IMPOUNDMENT FACILITIES

Coal refuse dams are typically designed using coarse refuse and soil or rock fill materials for construction with placement of fine coal refuse slurry in the associated impoundment. The development 

frequently initiates with a starter dam constructed of earthen materials, coarse refuse or a combination of both for the period of initial slurry disposal. Coarse refuse generated by the coal preparation 

plant is then used to expand and raise the starter dam, thus providing additional disposal capacity 

for the fine coal refuse slurry. Typical features and construction practices associated with a slurry 

impoundment facility include:

• Temporary surface drainage diversion away from areas of embankment dam 

construction.

• Removal and stockpiling of topsoil and soils for future reclamation.

• Removal of unsuitable foundation materials that would adversely affect the construction of the embankment dam.

• Collection and conveyance of groundwater springs.

• Construction of impoundment and embankment liners, as required by state 

regulations.

• Construction of the dam and emergency spillway to design grades to provide ample 

freeboard for design storm runoff storage and discharge capacity.

• Construction of a foundation cutoff, underdrains and internal drainage structures within the embankment dam to control seepage and phreatic surface 

development.

• Construction of decant pipe structures for removal of accumulated water (surface 

runoff and/or clarified water) from the impoundment.

• Construction and maintenance of haul roads on the embankment or adjacent areas to 

transport refuse to the disposal area.

• Grading of the refuse embankment surface to maintain drainage toward collection 

and/or diversion ditches.

• Grading and sealing of the refuse embankment for drainage control and minimizing 

water retention on the embankment.

• Collection of surface drainage on the embankment and delivery to sediment control 

structures.

• Reclamation of completed surfaces such as the embankment downstream face and, 

at completion of slurry disposal operations, covering of the settled fine coal refuse 

by evacuating accumulated water, covering the fine coal refuse with coarse refuse or 

earthen materials, and placement of soil and topsoil, and vegetation.

A significant issue in the development of a slurry impoundment is the design and construction of 

the starter dam. The size, materials, and hydraulic appurtenances associated with the starter dam 

impact future operations of the disposal facility, require substantial engineering oversight during 

construction, and can represent a significant part of the development cost for the facility. The sched-
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ule for construction of a starter dam for a new mine is normally critical, as the coal preparation 

plant will not usually be able to operate at full capacity until there is a functioning slurry disposal 

area that meets initial short-term design storm criteria. The starter dam must also be designed so 

that it can be transitioned to meet long-term criteria. For existing preparation plants, the designer 

should coordinate the closure of an existing impoundment with the activation of a new impoundment to avoid lapses in available refuse disposal capacity and to facilitate reclamation of completed 

areas.

Coarse refuse embankment dams are usually large in cross section in order to accommodate the 

production of refuse from the preparation plant. They may be constructed either as a homogeneous 

or zoned embankment. If an embankment dam is constructed as a zoned embankment, it may be 

built with earthen materials of finer gradation in upstream areas of the cross section and with coarse 

refuse in downstream zones. While the term homogeneous is often applied to coarse refuse embankment dams, the coarse refuse may exhibit variability in grain size and specific gravity, but still have 

adequate shear strength to meet design assumptions and be sufficiently competent to support construction equipment.

3.2.1 Size and Hazard Considerations

3.2.1.1 Impoundment Definition

MSHA currently requires that plans be prepared and approved for the design, construction, operation and maintenance of structures that impound water, sediment or slurry if such structures:

• Impound water, sediment, or slurry to an elevation of 5 feet or more above the 

upstream toe of the structure and have a storage volume of 20 acre-feet or more; or

• Impound water, sediment, or slurry to an elevation of 20 feet or more above the 

upstream toe of the structure; or

• Are determined by the MSHA District Manager to present a hazard to coal miners.

For purposes of determining inclusion under MSHA regulation, the height of an embankment dam 

is measured from the upstream toe of the structure to the lowest point on the crest of the dam. Other 

regulatory agencies may define height differently. Figure 3.1 illustrates this measurement for a crossvalley impounding embankment. If the lowest point on the crest of the structure is the invert of a 

properly designed open-channel spillway capable of conveying the maximum water flow from the 

design storm with sufficient freeboard and erosion protection, then that point is the proper location 

for the upper measurement. Where decant pipes and pipe/box spillways are used, the elevation must 

still be measured to the lowest point on the crest of the structure or embankment, not to the invert of 

the decant riser or spillway pipe.

The storage volume of a dam is calculated using either the invert elevation of the spillway or the 

lowest point on the crest of the structure. However, the storage volume based on a measurement to 

the invert of the spillway may only be used if two conditions are satisfied. First, the spillway must be 

an open-channel configuration. Second, the open-channel spillway must be designed to convey the 

flow from the design storm with adequate freeboard and must have appropriate erosion protection. 

If either of these requirements is not met, the capacity of a dam must be determined based on a measurement made to the lowest point on the crest of the structure without reference to the spillway. The 

design of the open-channel spillway can be verified using a flood routing analysis of the design storm 

(selected based on the purpose of the structure and projected hazard potential classification) through 

the impoundment’s open-channel spillway and other outlet works (if applicable).

Small ponds are sometimes used for disposal of fine refuse slurry and are referred to as slurry cells. 

These cells may be located within an embankment and sized and sequenced to preclude classification 
3-5

Coal Refuse Disposal Facilities and Other Impounding Structures

MAY 2009

of the embankment as an impoundment. However, there are also situations where the slurry cells are 

of sufficient size and are operated such that the facility may be classified as an impoundment. Section 

3.4 addresses slurry cell facilities. If a pond or impoundment meets any of the preceding criteria, 

the facility is subject to regulation by MSHA as an impoundment. It is important to note that state 

regulatory agencies may define structures that are regulated as dams using different dimensions or 

definitions.

3.2.1.2 Impoundments in Series

In the case of multiple impounding structures in series that individually do not meet the size criteria cited in Section 3.2.1.1 and where a failure of one of these structures can result in the failure 

of another, the cumulative storage capacity should be used when applying the impoundment size 

criterion.

In the case of multiple slurry cells (addressed separately in Section 3.4) that individually do not meet 

the size criteria cited in Section 3.2.1.1 and where failure of one slurry cell can lead to the failure of 

others, or where a slope failure can result in the release of water, sediment or slurry from multiple 

cells, the cumulative capacity of the potentially affected cells should be used when applying the 

impoundment size criterion.

3.2.1.3 Incised Impoundments

An incised impoundment is one created by excavating below the natural ground surface, as illustrated 

in Figure 3.2. An impoundment may be totally below natural ground, or an embankment may be constructed so that only a portion of the impoundment is below natural ground. For purposes of determining the height or storage volume of an impoundment with respect to the criteria presented in Section 

3.2.1.1, the portion contained below natural ground is not included in the height or storage volume 

calculation. However, even for an incised impoundment, if it is determined that the impoundment 

could become a hazard, appropriate design and construction requirements should be implemented. In 

situations where mining is planned or occurs near an incised impoundment, mine operators should be 

sure that there is a sufficient thickness of undisturbed ground left in place to preclude failure.

FIGURE 3.1 CROSS-VALLEY IMPOUNDING EMBANKMENT

SECTION A - A

A A

PLAN

HEIGHT

NOTE: HEIGHT DIMENSION SHOWN

 SOLELY FOR DETERMINING

 COVERAGE BY MSHA REG-

 ULATIONS.

FIGURE 3.1 CROSS-VALLEY IMPOUNDING EMBANKMENT
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Cases may arise where a surface mining pit is used as an impoundment. If any portion of the ground 

creating the impoundment is mine spoil or fill, then that portion should not be considered to be 

incised (MSHA, 2007).

3.2.2 Disposal Facility Configuration

Impounding coal refuse disposal facilities are classified based on their configuration and development/construction staging. The disposal facility configuration is dependent upon the terrain and 

size requirements, and the development staging reflects the general sequence or direction of disposal activity. The facility configuration categories are: (1) cross-valley impounding embankment 

(Figure 3.1), (2) incised impoundment (Figure 3.2), (3) side-hill impounding embankment (Figure 

3.3), and (4) diked impounding embankment (Figure 3.4).

Planning and design of an impounding embankment generally involves distinct development/construction stages that are associated with intermediate points in the facility construction and the general timing and directional development of the disposal operations. These stages typically reflect a 

few months to a few years of disposal operation and usually reflect the direction in which development occurs. The direction of construction normally falls into two categories: upstream and downstream. Upstream construction, as shown in Figure 3.5, involves initial construction and placement 

of coarse refuse in downstream areas to form the impoundment with sequential placement during 

subsequent stages in upstream locations, typically at higher elevations. Downstream construction, 

as shown in Figure 3.6, involves initial construction and placement of coarse refuse in upstream 

areas with placement during subsequent stages in downstream locations. It is common to have both 

upstream and downstream construction stages as part of a disposal facility design.

An intermediate development condition is centerline construction (which is essentially the same 

as alternating upstream and downstream construction), where refuse stages are constructed both 

upstream and downstream of the previous stage, with the crest of the two stages generally in 

alignment, but separated by the elevation increment of the stage (Figure 3.7). This terminology for 

SECTION A - A

FIGURE 3.2 INCISED-POND (IMPOUNDING) FACILITY

HEIGHT

A A

PLAN

NOTE: HEIGHT DIMENSION SHOWN

 SOLELY FOR DETERMINING

 COVERAGE BY MSHA REG-

 ULATIONS.

FIGURE 3.2 INCISED IMPOUNDING FACILITY
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Coal Refuse Disposal Facilities and Other Impounding Structures

MAY 2009

impounding embankments is relevant primarily for identifying whether portions of the coarse refuse 

embankment will be constructed on settled fine coal refuse slurry and the timing for reclamation of 

completed surfaces of the embankment.

Upstream construction, and to a lesser degree centerline construction, with placement of coarse 

refuse embankments on settled fine coal refuse, introduces stability concerns due to the potentially 

low strength of the fine coal refuse during initial covering and the potential for seismically-induced 

strength degradation. The field exploration, testing and analysis, and design methodology and criteria used for upstream construction plans are typically more extensive than for downstream or centerline construction. On the other hand, upstream construction, when the final toe and lower elevations 

of the embankment face are established early in the disposal facility development, allows concurrent 

reclamation of the completed face and thus improved erosion and sediment control.

Downstream construction mitigates issues related to the stability of the embankment due to the soft 

foundation conditions associated with fine refuse, but the reclamation of the embankment face is 

delayed until much later in the facility life, thus requiring more comprehensive erosion and sediment 

control measures.

At some sites, both upstream and downstream stages are constructed in order to: (1) fit site terrain conditions, (2) meet disposal capacity requirements, and (3) provide multiple coarse refuse dis

FIGURE 3.3 SIDE-HILL IMPOUNDING EMBANKMENT

SECTION A - A

A A

PLAN

FIGURE 3.3 SIDE-HILL IMPOUNDING EMBANKMENT
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posal areas. This type of construction is sometimes referred to as the modified upstream construction 

method. Thacker (1997) recommends initiating upstream construction early in the project life, with 

subsequent placement of coarse refuse in a downstream buttress zone to allow pore pressure to dissipate in the previously loaded fine refuse.

The decision whether to construct a refuse facility utilizing upstream, downstream and centerline 

construction sequencing is also dependent upon the geometry of the valley and the refuse disposal 

capacity requirements. For instance, downstream construction generally requires use of greater 

quantities of coarse refuse to gain embankment elevation than upstream construction. Centerline 

construction normally requires a greater amount of coarse refuse during the initial stages of the facility with lesser amounts required as construction progresses. Additional discussion of the advantages 

and disadvantages of various types of embankment construction is provided in Chapter 5.

STAGE I

SLURRY DISCHARGED AT EMBANKMENT

STAGE II

STAGE III

FIGURE 3.4 DIKED-POND (IMPOUNDING) EMBANKMENT

SECTION A - A

A A

PLAN

FIGURE 3.4 DIKED-POND (IMPOUNDING) EMBANKMENT

FIGURE 3.5 UPSTREAM STAGING METHOD
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3.2.3 Hazard Potential Rating

The determination of the hazard-potential classification for impoundments discussed in Section 3.1

is based on the probable loss of life in the downstream area and the presence of structures and infrastructure that could be affected if the dam were to fail. It may be apparent from the presence of 

nearby, downstream mining facilities or off-site occupied structures that a dam failure could result 

in loss of life and that the classification should be high hazard. Since this is the most severe hazard 

classification, no further analyses may be necessary for hazard potential determination, although the 

evaluation of potential downstream impacts would still be required for preparation of an emergency 

action plan. If the hazard potential classification is not apparent, dam breach analyses should be performed, with evaluation of the resulting flood inundation levels and flow velocities at downstream 

structures to determine the potential for loss of life and severity of impact to structures and infrastructure. Dam breach analyses are discussed in Section 9.9.

The assignment of high hazard potential principally hinges on the determination of probable loss of 

human life. In situations where there is no probable loss of human life due to the dam failure, other 

site-specific factors are applied to determine the hazard potential. Estimates of economic loss and 

damage to infrastructure can be generated to provide guidance in differentiating between low and 

significant hazard, but assessment of environmental damage is more controversial.

Where there are mine workings beneath or near a refuse embankment or impoundment, a release 

of water and slurry into the mine could result due to failure of the natural ground or a man-made 

barrier between the impoundment and mine workings. Such a potential failure is referred to as an 

SLURRY DISCHARGED AT EMBANKMENT

STAGE I

STAGE II

STAGE III

STAGE IV

FIGURE 3.6 DOWNSTREAM STAGING METHOD

SLURRY DISCHARGED AT EMBANKMENT

STAGE I

STAGE II

STAGE III

STAGE IV

FIGURE 3.6 DOWNSTREAM STAGING METHOD

FIGURE 3.7 CENTERLINE STAGING METHOD
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impoundment breakthrough and not only poses a threat to mine workers, but the water and slurry 

could discharge from the mine and potentially affect an area different from the one that would be 

affected by a dam failure. Chapter 8 addresses the design requirements for this situation, and MSHA 

(2007) provides the following guidance on hazard potential classification:

• The official hazard-potential classification for an impoundment is based on the three 

classifications indicated in Section 3.1 and is assigned regardless of whether the 

potential hazard is from a failure of the dam or a failure into mine workings.

• For the purpose of selecting the appropriate design criteria for the impounding 

embankment, the hazard-potential classification is based on the appropriate rating in 

the event of a failure of the embankment itself. For example, an impoundment could 

have a high-hazard-potential rating based solely on the potential for a failure into 

underground mine workings, but have low consequences due to a failure of the dam 

itself. In such case, the dam can be designed based on low hazard potential (e.g., the 

100-year design storm), while the breakthrough evaluation and prevention measures 

(with respect to the extent of exploration, testing, monitoring, etc.) must be appropriate for a high-hazard-potential site.

3.3 COAL REFUSE NON-IMPOUNDING FACILITIES (REFUSE PILES)

Coal refuse disposal facilities that do not retain water or slurry are considered to be non-impounding 

structures and are also referred to as refuse piles. As indicated in the MSHA Coal Mine Impoundment 

Inspection and Plan Review Handbook (MSHA, 2007), a refuse pile may have small isolated sediment 

control facilities and cells for the disposal of filter cake, sediments, etc. provided that the size of these 

cells would not result in the classification of the structure as an impoundment, their location would 

not affect structural stability, and the configuration does not impede drainage (e.g., block a drainage 

course). Where this material is not compacted in two-foot lifts, the disposal should be approved by 

the MSHA district manager. In addition to filter cake, fine coal refuse slurry can also be disposed in 

appropriately designed cells, as discussed in Section 3.4.

3.3.1 Coarse Refuse Embankments

If not part of an impoundment plan, a coarse refuse embankment is typically designed for separate 

disposal of coarse coal refuse and fine coal refuse. Typical features and construction practices associated with a coarse refuse disposal facility include the following:

• Surface drainage diversion away from the limits of the disposal embankment.

• Removal and stockpiling of topsoil and soils for future reclamation.

• Removal of unsuitable foundation materials that would adversely affect the construction or stability of the embankment.

• Collection of discharges from springs under the foot print of the embankment that 

may adversely affect stability.

• Placement and compaction of run-of-plant coarse refuse in lifts to the designed lines 

and grades of the embankment.

• Construction and maintenance of haul roads on the embankment for transporting 

the refuse to the disposal area.

• Grading and sealing of the refuse embankment to maintain drainage control and to 

prevent water retention behind the embankment.

• Collection of surface drainage on the embankment and delivery to sediment control 

structures.

• Reclamation of completed surfaces, consisting of placement of soil and topsoil and 

re-vegetating.
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Coarse refuse embankments are usually developed as homogeneous embankments, without multiple zones. While the term homogeneous is applied to such embankments, the coarse refuse materials 

may exhibit some variability in grain size and specific gravity. Typically, however, coarse refuse has 

the strength to support construction equipment and to meet the embankment design assumptions.

3.3.2 Combined Refuse Embankments

Combined refuse embankments are designed for disposal of coarse refuse and dewatered fine coal 

refuse as a mixed material. While combined refuse embankments may be homogeneous, they can 

also be constructed as zoned embankments where more workable refuse materials such as coarse 

refuse are used (either periodically, or in sufficient continuous quantity) to construct a well-compacted downstream shell of sufficient width (sometimes referred to as a structural zone) to contain 

the combined refuse and to construct haul roads within the disposal area. Typical features of a combined refuse disposal facility are similar to those for a coarse refuse embankment. Because of the 

presence of dewatered fine coal refuse and overall wetter material conditions, large disposal areas 

are generally needed so that the dumped materials can drain before spreading and compaction, and 

more extensive internal drainage systems may also be required. In some cases amendments may be 

needed to stabilize the wet materials.

3.3.3 Segregated Refuse Embankments

Dewatered fine coal refuse can be disposed in segregated areas of a coarse refuse embankment. 

This approach results in a zoned embankment in which coarse refuse forms a downstream zone 

or shell that supports the overall stability of the embankment, and isolated areas are provided 

within the upstream or interior of the embankment for depositing the dewatered fine coal refuse. 

Typically, these segregated refuse embankments are designed with provisions for haulage routes 

within the embankment and for containment of the dewatered fine refuse without creating depressions that could be classified as impoundments. Such containment structures require measures to 

control surface drainage that collects in the dewatered fine refuse disposal area. Typical features 

of segregated refuse embankments are similar to those for coarse refuse embankments, but with 

more extensive internal drainage systems and control of drainage within the dewatered fine refuse 

containment area.

3.3.4 Configuration and Development Staging

Non-impounding facilities are constructed with a range of configurations and development staging. 

The disposal facility configuration depends upon the terrain, and the development staging reflects 

the general sequence or direction of disposal activity. Non-impounding facility configuration categories are: valley-fill (Figure 3.8), cross-valley (Figure 3.9), side-hill (Figure 3.10), ridge-dump (Figure 

3.11), and heaped (Figure 3.12).

Planning and design of a non-impounding embankment generally involves development stages 

related to intermediate points in the facility construction and the general development direction of 

the disposal operations. The development staging is characterized by the direction in which development occurs. Upstream construction involves the construction and placement of refuse in downstream areas initially, with sequential placement during subsequent stages in upstream locations, 

typically at higher elevations. Downstream construction involves the construction and placement 

of refuse in upstream areas initially, with sequential placement during subsequent stages in downstream locations. This terminology for non-impounding embankments is more relevant for valley-fill 

configurations, where upstream and downstream directions reflect the shape of the valley.

Cross-valley non-impounding embankments (Figure 3.9) can potentially impound water, and 

they may be classified as impoundments by MSHA if they can impound water for such a period 
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of time that they can create a hazard. MSHA (2007) presents factors considered for such classification. Abandonment of cross-valley non-impounding embankments can require substantial regrading 

in order to satisfy long-term drainage concerns. Therefore, this type of embankment is generally 

avoided.

3.4 COAL REFUSE SLURRY CELL FACILITIES

Small cells or ponds in a coarse refuse facility that receive fine refuse are commonly referred to as 

slurry cells. Disposal of fine coal refuse using slurry cells has been implemented at sites where conPERMANENT DRAINAGE PIPE

 OR CULVERT

A A

PLAN

SECTION A - A

CULVERT

FIGURE 3.8 VALLEY-FILL, NON-IMPOUNDING EMBANKMENT

SECTION A - A

A A

PLAN

STAGE III

STAGE II

STAGE I

UPSTREAM

STAGING

METHOD

FIGURE 3.9 CROSS-VALLEY, NON-IMPOUNDING EMBANKMENT

FIGURE 3.8 VALLEY-FILL, NON-IMPOUNDING EMBANKMENT
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straints may deter or preclude the construction of a large slurry impoundment, and/or the quantity 

of fine coal refuse slurry is relatively small and can be reasonably accommodated with a small cell 

structure. Small cells are created, usually within the upstream zone of a coarse refuse embankment, 

and are filled with slurry. The upstream zone and cells are contained by a well-compacted downstream shell (sometimes referred to as a structural zone). Once an individual cell is filled, it is allowed 

to drain, and the fine coal refuse is covered with coarse coal refuse while another cell is operated. 

Because of the typical small cell size, operation of multiple cells intermittently will facilitate settling 

and draining of fine refuse and optimize the capacity of each cell.

Slurry cells may also be used to dewater fine refuse prior to final disposal. Instead of encapsulating 

the slurry cell in a coarse refuse embankment, the fine refuse is excavated from the cell after substantial dewatering has taken place and then is typically mixed with coarse refuse prior to being incorporated into an embankment for final disposal.

3.4.1 Size and Hazard Classification

The slurry cell concept is typically focused on: (1) sequencing the construction of cells to match the slurry 

generation rate, (2) accommodating drainage and covering of each cell, and (3) maintaining a total capacity of all open cells at a level that does not meet impoundment classification or, if classified as an impoundment, has a low hazard potential. In the case of multiple slurry cells where they individually do not meet 

the size criteria requirements of 30 CFR § 77.216(a), if the failure of one cell can result in the failure of 

another or if a slope failure can result in the release of water, sediment, or slurry from multiple cells, then 

the cumulative storage capacity of the affected cells is used for application of the impoundment size criteria. This includes cases where the initial slurry cells are covered with coarse refuse and additional cells 

are to be placed above the initial set of cells (MSHA, 2007).

The operation of multiple cells, particularly if subsequent cells are built on previously covered 

cells similar to the upstream construction method for an impoundment, can lead to classification 

as an impoundment. If multiple cells are arranged such that they affect structural stability and 

there is a possibility of sequential failure and a large release with significant downstream impacts, 

A

A

PLAN

SECTION A - A

FIGURE 3.10 SIDE-HILL, NON-IMPOUNDING EMBANKMENT

FIGURE 3.10 SIDE-HILL, NON-IMPOUNDING EMBANKMENT
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the impounding facility could receive a significant- or high-hazard-potential classification, as discussed in Section 3.1. In such a case, the design storm requirement will increase, necessitating 

greater diversion and freeboard, thus limiting the feasibility of the concept for some configuration 

categories (e.g., valley fills).

The advantages of the slurry cell concept are the following:

• As a low-hazard-potential facility with limited cell size, managing the design storm 

runoff can generally be accommodated with minimal storage, enabling more capacity for disposal of slurry and clarification of process water. Control of surface drainage from the surrounding embankment area is critical, along with effective diversion 

of runoff from watershed areas around the facility.

• Slurry cells have limited capacity, comprising generally a thin deposit of slurry 

with coarse refuse containment and covering layers that allow the fine refuse to 

dewater and consolidate, making the total mass less flowable. This limited capacity 

is viewed as an advantage at sites that are undermined and have the potential for 

breakthrough.

• With the fines compartmentalized in cells, a problem at one location is less likely 

to affect the entire facility, which also helps to mitigate concerns for breakthrough 

potential.

A A

PLAN

SECTION A - A

FIGURE 3.11 RIDGE (NON-IMPOUNDING) EMBANKMENT

FIGURE 3.11 RIDGE (NON-IMPOUNDING) EMBANKMENT
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• As a low-hazard-potential facility, fewer and less extensive hydraulic appurtenances 

are required, with lower associated construction costs.

The features of a slurry cell facility are similar to those discussed for a non-impounding facility. It 

has become common practice to design a well-compacted shell (sometimes referred to as a structural 

zone) at the face of the disposal facility embankment as the downstream containment structure for 

cells. The design of the well-compacted shell is based upon evaluation of factors such as width, slope, 

benches, internal drainage, and access or haul roads. Similar to non-impounding refuse embankments, diversion of runoff from upstream and hillside areas is accomplished by locating ditches and 

channels, which are separate structures from the slurry cells, either above or on the embankment surface. To keep runoff from entering the disposal area, the construction and maintenance of the diversion ditches requires sequential planning and periodic construction activities apart from disposal 

operations. Additionally, control and discharge of process water from the slurry operation is required 

at each cell. Typically, this will require small-diameter decant systems or stabilized channels. These 

structures usually have limited service requirements because of the low capacity of the cells.

In addition to the critical sequencing of cells to match the fine refuse generation rate and maintain 

low-hazard-potential classification, a slurry cell facility has many of the same design considerations 

as impoundments: (1) geotechnical investigation to determine embankment and foundation characteristics, (2) testing for relevant material properties including shear strength and hydraulic conductivity, (3) static and seismic slope stability analyses, (4) underdrains to control the phreatic level, (5) a 

dam breach analysis, unless the hazard potential is otherwise apparent, to determine the appropriate 

hazard potential rating and design storm, (6) instrumentation to confirm design assumptions and 

performance, (7) preparation of construction specifications, and (8) construction monitoring.

Some disadvantages of slurry cell facilities include requirements for: (1) frequent construction of 

diversion ditches, new cells, and cell spillways (decant structures) as the site elevation increases, (2) a 

SECTION A - A

A A

PLAN

FIGURE 3.12 HEAPED (NON-IMPOUNDING) EMBANKMENT
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relatively large ratio of coarse refuse to fine refuse, and (3) more detailed planning and supervision of 

the site so that the construction, filling, and covering of cells is accomplished in the proper sequence. 

Also, a slurry cell facility can potentially be reclassified as having high hazard potential, increasing 

the diversion and cell spillway requirements and impacting the long-term feasibility of the concept.

While one of the attractions of the slurry cell concept is disposal of fine refuse slurry in a structure 

with low-hazard-potential classification and thus less stringent design storm requirements, it may 

be advantageous to employ the concept at an impoundment site to mitigate potential impacts from 

underground mining and breakthrough potential. In such a case, the embankment design would be 

in accordance with the appropriate impoundment criteria, and additional operating plans would be 

required for disposal of slurry within cells constructed in the impoundment area.

3.4.2 Disposal Facility Configuration and Development

Non-impounding facility configurations established by MSHA may be developed using the slurry 

cell concept, although the quantity of available coarse refuse typically requires a valley-fill or side-hill 

configuration. The valley-fill configuration is generally developed in the upstream direction after a 

sufficient embankment height and top surface is reached to enable individual cells to be constructed. 

Consequently, beginning a slurry cell system may require operation of another disposal facility (e.g., 

underground injection), an existing disposal facility (e.g., old impoundment), or available fill material (e.g., mine spoil from other site development work) in order to achieve a sufficient working 

surface and embankment configuration to initiate slurry cell operation. As the disposal embankment 

is raised in height and additional cells are constructed over covered cells, hazard classification may 

become an issue. Therefore, the use of a valley fill for a slurry cell facility will likely have limitations. 

Figure 3.13 shows a slurry cell facility developed in a valley-fill configuration.

Use of slurry cells with side-hill and heaped configurations is less common, unless backup disposal 

capacity is required in conjunction with the underground injection of fine coal refuse. These configurations require a larger quantity of coarse refuse for development of the structural shell, but this 

SECTION A - A

PLAN

A A

ACTIVE SLURRY CELL

FILLED AND COVERED

 SLURRY CELL

FIGURE 3.13 SLURRY CELL IN VALLEY FILL, NON-IMPOUNDING EMBANKMENT
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3.5 UNDERGROUND INJECTION SITES

Fine coal refuse slurry can be disposed within underground mines by injection. Background information for the technical feasibility of underground disposal can be found in a publication by the National 

Academy of Sciences (NAS, 1975). There are state and federal regulatory programs for these operations, 

and the Virginia Department of Mines, Minerals and Energy, Division of Mined Land Reclamation has 

developed guidance for planning, design, operation and monitoring (VA DMLR, 2006). Planning and 

design for underground slurry disposal is discussed in the following subsections.

3.5.1 Siting

Background information useful for planning and design includes property ownership, geologic strata, 

hydrogeologic conditions, and data describing the mine workings to be used for disposal, including mapping, extraction information, mine dewatering and discharge conditions, proximity to active mine areas 

FIGURE 3.14 SLURRY CELL SIDE-HILL FILL, NON-IMPOUNDING EMBANKMENT

SECTION A - A

FILLED AND COVERED

 SLURRY CELL

ACTIVE SLURRY CELL

A

A

PLAN

material may not be available considering the need for coarse refuse to construct individual small 

cells. Figure 3.14 illustrates a slurry cell facility developed with a side-hill configuration.

Slurry cells may also be incorporated into an impoundment configuration, if issues of underground 

mining and breakthrough potential cause concern over the quantity of flowable impounded material. 

Figure 3.15 illustrates a slurry cell facility developed at an impounding embankment.

FIGURE 3.14 SLURRY-CELL, SIDE-HILL, NON-IMPOUNDING EMBANKMENT
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and associated barrier information (coal barrier and bulkheads). The capacity of the underground workings should be determined, and the potential duration for slurry disposal should be estimated based upon 

settling and the ultimate solids content of deposits. Consideration should be given to the following:

• Lineaments, faults, and fractures as possible conduits of slurry.

• Recharge and discharge of groundwater into the mine workings as an indication of 

the hydraulic connection of the mine works with the fracture flow system.

• Surface openings that could or will become a discharge point, and coal barrier 

requirements and stability, with an assessment of the amount of weathering of outcrop, jointing, and previous or planned surface or auger mining.

• Presence of groundwater users, radius of influence of withdrawal wells, and connectivity with mine voids.

• Mine works overlying and underlying the mine work receiving injected slurry, 

including inventory and assessment of vertical dewatering holes, subsidence fracturing, and associated surface openings.

• Proximity of and impacts on active mines.

• Slurry injection and movement through mine workings, including water balance 

analysis of injection flows and surface and groundwater conditions.

• Leaching of contaminants from emplaced slurry, including potential impacts from 

process chemicals, and sorption characteristics of processing chemicals on coal and 

sand/clay particles.

• Hydrologic conditions that could develop after cessation of injection, including 

development of equilibrium groundwater conditions.

• Potential impacts to near-surface groundwater resources

3.5.2 Injection System Design

The following issues related to injection system design should be addressed:

• Drilling methods for penetrating the abandoned mine to reduce the potential of creating an ignition source.

• Design of bulkheads and seals that may be required to control the deposition of 

slurry or direct drainage toward acceptable discharge locations.

• Injection site and slurry line/alignment, including construction and service access, 

drainage control, and secondary containment.

• Injection well design including maximum pressure (below level for hydraulic fracturing of 

overburden or mine barriers, and system components such as casing), complete casing of 

overburden (with double casing provisions when penetrating upper mine voids), casing 

grouting methods through overburden, and wellhead completion with air gap to prevent 

pressurization of the well, if applicable, or pressure control and monitoring. Where pressure injection is required, a control system to limit pressures to the maximum design value 

must be employed, along with backflow prevention in the event of slurry line rupture.

• Secondary containment for piping and injection site, including monitoring and controls that shut down the slurry pumps in the event of piping failure.

• Operating procedures and flow rates, including hours of slurry disposal/process 

water pumping, and measures to prevent the introduction of oxygen into the mine.
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Coal Refuse Disposal Facilities and Other Impounding Structures

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3.5.3 Risk Assessment and Response Plan

The following are issues that should be considered in the evaluation of risks associated with release 

and potential response actions:

• Avenues of release (both surface and underground) and human exposure pathways. 

A concern is ingestion through drinking water wells with capture zones that may 

draw water from underground mine works or be connected to mine works through FIGURE 3.15 SLURR fracture systems. Y CELLS IN CROSS-VALLEY FILL IMPOUNDING EMBANKMENT

PLAN

FILLED AND COVERED

 SLURRY CELLS

A A

ACTIVE SLURRY CELLS

ABANDONED MINE WORKINGS

 IN COAL SEAM

SECTION A - A

FIGURE 3.15 SLURRY CELLS IN CROSS-VALLEY-FILL, IMPOUNDING EMBANKMENT
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• Blowout potential associated with the injection pressures, slurry and water accumulation, and barrier stability. The barrier stability evaluation should reflect subsequent 

surface, auger, or highwall mining that may have thinned or penetrated the barriers. 

Future restrictions on mining in barriers should be considered if such potential exists.

• Environmental receptor identification relative to groundwater and surface water, 

considering contamination potential due to acid forming materials and petroleum 

solvents or other chemicals used in the coal preparation plant and solids deposition 

within streams.

• Response to release, including: (1) steps to determine the extent of release, (2) identification of emergency containment and cleanup resources (in-house and contracted) 

and associated steps to initiate action, (3) disposal of wastes generated during 

cleanup, and (4) implementation of operational contingency plan for ongoing slurry 

disposal.

3.5.4 Contingency Plan

Contingency disposal options should be established for fine coal refuse slurry if injection is suspended for emergency or performance reasons. The plan should include steps to be taken to shut 

down the current injection operation; facilities and designs for interim slurry disposal such as 

alternate injection locations (in the event of performance based shutdown) or emergency ponds 

and drying cells, dewatering equipment, etc.; and longer-term options for disposal such as slurry 

disposal cells.

3.5.5 Monitoring Plan

Monitoring for slurry injection, emplacement, mine water levels and mine discharge rates/quality for 

the injection target and adjacent mines, as applicable, should be developed, along with groundwater and surface water monitoring. The parameters, methods, and frequency for monitoring should 

be established based on site conditions, sound engineering judgment, and applicable regulatory 

programs.

• Slurry injection monitoring for organic and inorganic parameters should be performed to assess potential groundwater and surface water impacts, as required by 

regulatory programs.

• Slurry emplacement monitoring should include monitoring wells and discharge 

points within the mine works for detecting the presence of slurry solids for comparison with predicted slurry and water accumulation and movement, and planning 

subsequent injection sites.

• Mine pool/discharge water monitoring should be performed and the data should be 

compared to the maximum predicted mine pool level and discharge rates, and water 

balance analysis.

• Groundwater monitoring should include wells located in hydrogeologic units most 

likely to be influenced by the injection operation, such as the fracture flow system, 

the subsidence fracture zone overlying workings, and the coal seam adjacent to mine 

workings impounding the slurry. Lineaments should also be considered when selecting locations to be monitored. Potable wells within the expected zone of influence of 

the injection operation should be monitored, but these wells should not be considered as groundwater monitoring unless well construction details are known.

With the use of an underground injection site for slurry, the coal preparation plant will also require a 

surface disposal facility for coarse coal refuse. As indicated above, contingency disposal options should 
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Coal Refuse Disposal Facilities and Other Impounding Structures

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be developed if underground injection proves to be infeasible. Such options could include use of slurry 

cells or conversion to combined refuse if the preparation plant is equipped for fine refuse dewatering.

3.6 RECOVERY (REMINING) OF COAL REFUSE DISPOSAL FACILITIES

Recovery (sometimes referred to as remining) of coal refuse from existing active or previously 

abandoned embankments and impoundments is performed at some sites and involves the use of 

advanced processing methods to obtain additional coal for fuel or power generation. In such situations, a ground control plan addressing safety issues associated with remining operations should be 

submitted to MSHA. The remining process involves the excavation and removal of coal refuse and, 

where processing is performed on site, disposal of waste from the coal processing plant. Thus, a 

modification of the existing site refuse disposal plan will generally be necessary.

Recovery or remining may involve exploration, excavation and handling, final grading, and reclamation on soft or loose materials including operation in wet, saturated or submerged conditions. 

Exploration will typically include initial as well as periodic operational borings and test pits for 

evaluation of geotechnical and groundwater conditions as well as marketability of the excavated coal 

refuse. Low-ground-pressure equipment, upstream pushout of a coarse refuse pad, or barge operation may be needed depending on impoundment conditions, and procedures and safety precautions 

appropriate to the method used should be developed.

Under relatively dry conditions, dozers, excavators, end-loaders, scrapers, or a clamshell can be used 

for excavation of refuse and fines from an impoundment. Procedures and safety precautions that are 

suitable for the conditions encountered should be developed, and when soft or loose fine refuse is 

present, stability analyses to determine if the excavation slopes will be stable should be conducted. 

For impoundments built by the upstream construction method, a buffer zone or distance should be 

maintained between the area of excavation and the upstream slope of the impounding embankment 

or other fill that may be present.

Surface runoff should be diverted around the operation area to the extent possible, and the active 

work area should be graded so that water drains toward ditches and sumps. Lowering the phreatic 

level in the fine coal refuse to the extent possible by surface runoff control and removal of groundwater will improve ground conditions for equipment operation, allowing maximum recovery of 

coal. Consideration should be given to grading, deep sumps, and other dewatering measures for 

lowering the phreatic level, and the phreatic levels should be continually monitored during operation. At sites where control and removal of surface and groundwater is not practical at all times 

of the year, safe and economic removal of fines may only be possible during dry periods. In such 

instances, evaluation of the degree and depth of saturation of the fines should be made prior to 

resumption of operations.

When wet and submerged conditions are present, dredges and hydraulic sluicing (water cannons) are 

sometimes employed to remove fines from an impoundment. Accidents have occurred when steepened slopes of fines have collapsed, sending a wave of material across the impoundment. Besides the 

operating concerns discussed above, additional safety concerns must be addressed, and it is important to establish guidance for equipment operation. Land anchors used to position dredges should 

be located on solid ground and kept well back from areas that might be susceptible to slope failure 

as the dredging progresses. The safe distance that any operator or equipment not on solid natural 

ground (including anchor equipment) approaches the working face should be evaluated and should 

generally be limited to no closer than twice the vertical height of the working face. When dikes built 

across the impoundment to divide it into sections are present, personnel and equipment should not 

be allowed on dikes adjacent to active dredging operations until dredging has ceased and the stability of the area has been assessed. Personnel and operating equipment must have the proper safety 

equipment for working at a dredging operation.
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Upon completion of the recovery operation, final grading and reclamation should provide long-term 

stability of slopes and drainage channels. This work is typically performed with conventional earthmoving equipment, and many of the operating procedures and safety precautions for this phase of 

recovery are similar to typical reclamation requirements.

For embankments or impoundments that are to be remined, engineering plans addressing the following should be prepared for review and approval:

• Excavation of coal refuse, including the method of removal, temporary slopes, 

sequence, and resulting configuration of the facility, with associated provisions for 

controlling drainage and maintaining stability during operations and following 

completion.

• Slope stability analyses (for both permanent and temporary slopes) demonstrating 

acceptable factors of safety where miners are subject to potential slope failure hazards. The slope stability analyses should include consideration of rapid drawdown 

conditions associated with the removal of fine refuse. If excavated slopes that are 

intended to be temporary and are designed for short-term conditions remain in place 

much longer than planned (or become permanent because of the idling of recovery 

operations), they should be re-designed with long-term factors of safety.

• Changes in seepage resulting from pooling of water as fine coal refuse is removed.

• Operating procedures and precautions specific to the excavation and recovery methods used (e.g., conventional construction equipment, barge and dredge equipment, 

water cannon operation) to provide for safe access and mining.

• Monitoring of compliance with the excavation plan and inspection of slopes and 

drainage control structures.

• Disposal plans for waste from the reprocessing of the coal refuse.

• Reclamation and abandonment of the disposal facility.

If fine coal refuse is to be removed from a slurry impoundment, the potential impacts on the safety 

of the impoundment should be addressed in the plans. This should include potential impacts on the 

outlet structures and facility operation during the design storm and limitations on the extent and 

slope of the excavation so that other parts of the facility are not compromised. For instance, if there 

is upstream construction or barriers related to mine workings are built over slurry, removal of fines 

could remove support for the embankment crest or barrier structure. In such cases, an analysis for 

determining the necessary buffer and excavation slope such that fines recovery can take place without compromising the stability of the embankment or barrier structure is required.

3.7 OTHER IMPOUNDING STRUCTURES

In addition to coal refuse disposal facilities, mine sites may have other impounding structures such 

as fresh-water reservoirs to provide make-up water for the processing of coal and sedimentation and 

treatment ponds to handle runoff and drainage from refuse embankments, surface mined areas and 

other disturbed surfaces. In some limited surface mining situations primarily in the western U.S., 

dams may be used for flood control during the temporary period when the mine pit advances near a 

water course. These structures are generally traditional dams and reservoirs that, while having many 

of the same features as a slurry impoundment, may also have additional features associated with 

influent and effluent controls. Additionally, these impoundments are generally constructed during a 

limited time period that may or may not be related to mining or coal processing rates.

Fresh water impoundments are typically located close to the coal preparation plant and are sized and 

located within a watershed to provide an adequate quantity of process water. These structures are 
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Coal Refuse Disposal Facilities and Other Impounding Structures

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typically dams, generally built of soil and rock fill, with primary and emergency spillways and piping 

tied into the preparation plant. They are differentiated from slurry impoundments by their design to 

maximize water storage and the presence of gated spillways and distribution pipelines. Additionally, 

they may have a significant depth of water and thus be subject to greater hydraulic head than would 

be expected for a slurry impoundment.

Sedimentation ponds are required as part of erosion and sediment control measures for runoff from 

disturbed mining areas, including coal refuse disposal facilities. These ponds are smaller structures 

than slurry impoundments and fresh water reservoirs, and they are sized to meet state regulatory 

criteria based on the contributing disturbed area and other hydrologic factors. They are frequently 

designed so as not to be classified as an impoundment regulated by MSHA. They typically have primary and emergency spillways.

Treatment ponds are similar to sedimentation ponds and are used to treat drainage from disturbed 

mining areas, including drainage from coal refuse disposal facilities and water pumped from underground mine workings to meet suspended solids and water quality effluent requirements. In many 

cases, chemicals are added to these ponds to neutralize acidic conditions and to precipitate metals 

such as iron and manganese. Because they are used for treatment of drainage to improve water quality, the associated drainage area is limited and the surface runoff entering the ponds is minimal. These 

ponds are generally small structures, frequently below the MSHA size classification for impoundments. They typically have primary and emergency spillways, and gated controls are often part of 

the primary spillway.

Flood-control dams are sometimes constructed in a water course to prevent or mitigate flooding 

of a surface mine pit. The water course may have significant flow, particularly during thunderstorms or periods of snowmelt. These dams may be small or large temporary structures, generally 

located in the western U.S., and are required only during the period when the mine pit could be 

affected by flooding from the water course. In situations where there is no threat to the public off 

of mine property, it may be possible to design the dam using low- or significant-hazard-potential 

hydrologic criteria, provided that a warning system and plan is developed and maintained for 

notifying and evacuating personnel involved with the mining operation when the water behind 

the dam reaches a specified level. Table 3.1 presents guidance that should be considered or evaluated as part of the MSHA impoundment plan if a warning system is being used to support the 

selection of low- or significant-hazard-potential hydrologic criteria for a flood-control dam at a 

surface mine pit.

The geotechnical and hydrology/hydraulic engineering requirements for fresh water impoundments, 

flood control dams, sedimentation ponds, and treatment ponds are substantially the same as for 

slurry impoundments, and design criteria are typically identical. Aspects of engineering analyses 

and design that may be different for fresh water impoundments than for slurry impoundments are 

addressed in other chapters of this Manual.

3.8 SMALL PONDS AND SIMILAR STRUCTURES

Some structures that are capable of impounding water or temporarily storing slurry on mine sites 

may not exceed the threshold size that would require an approved MSHA plan, but they should be 

designed in a manner consistent with the engineering guidance provided herein. Many sedimentation ponds and emergency slurry holding ponds at preparation plants are deliberately sized below 

impoundment threshold criteria and generally do not have significant hazard potential. These structures are typically regulated by states as ponds or small dams and are designed in accordance with 

applicable state criteria.
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MAY 2009

TABLE 3.1 GUIDANCE FOR FLOOD WARNING SYSTEMS AT SURFACE MINE PITS(1)

1. A warning system should typically consist of power supply equipment (primary and emergency back up), 

water level monitors, and automated communications equipment. The entire system should be designed by 

a qualified engineer.

2. The evacuation warning level should be established based upon the potential time required for evacuation 

of the surface mine pit and the potential rate of inflow. Ideally, the warning should be triggered at a level 

that allows for evacuation before overtopping of the dam by the PMF and resulting inundation within the pit 

to a critical level. The flood control structure should typically be maintained either in a dry condition of with 

a limited water level and storage volume that would not present a hazard to downstream personnel. When 

determining the water level for the warning system, any water stored behing the dam must be taken into 

account.

3. Multiple warning levels should be considered. For example, in addition to the evacuation warning level, an 

alert should also be issued at a lower level that would result in mine personnel coming to the dam to verify 

that the system is working correctly and to monitor the situation.

4. While the mining operation is immediately downstream of the dam, the warning system should be tested on 

a frequent basis and, if practicable, before expected large meteorologic events.

5. The warning plan should clearly define the procedures to be followed when the warning system is activated, 

and mine personnel should be trained on the appropriate response to a warning.

6. The warning plan should address the status of the dam after the mine pit has advanced beyond the influence of the water course. The plan should state whether the dam will be removed or will remain in place. If 

the dam is to remain in place, it should be evaluated, as necessary, to verify that the hazard potential classification is appropriate for the downstream area potentially affected by a dam failure. The warning system 

approach should only be in effect for the potential hazard posed to the mining operation for the temporary 

period when the pit could be affected.

Note: 1. Use of a warning system must not be a substitute for appropriate dam design and construction. MSHA has 

indicated that a warning system may be acceptable on a case-by-case basis for support of the use of low- or

significant-hazard-potential design criteria at flood-control structures to prevent or mitigate flooding of a surface

mine pit. The guidance presented herein reflects conditions that should be considered or evaluated as part of 

an MSHA impoundment plan.

 (FREDLAND, 2008)
Chapter 4

PROJECT PLANNING

MAY 2009 4–1

Planning for a coal refuse disposal facility should be integrated with overall mine development and 

operation plans. Procedures for the planning and design of safe, environmentally acceptable and 

economical refuse disposal facilities are stressed throughout this and subsequent chapters of this 

Manual. This chapter presents important planning and design sequences and procedures that can be 

employed to optimize refuse disposal operations. Topics covered include:

• Unique aspects of refuse disposal

• Operations and site-related considerations

• Sustainable mining practices

• Economic considerations

• Environmental and regulatory considerations

• Planning sequence

• Design sequence

Understanding these topics is essential to the design and construction of a refuse disposal facility that 

imposes minimal restraints on the production of coal, assures the safety of miners and the surrounding public, and protects the environment. A refuse facility that meets these goals ultimately optimizes 

control of the overall cost of mining operation.

4.1 UNIQUE ASPECTS OF REFUSE DISPOSAL

The disposal of coal refuse materials from preparation to transport and placement imposes problems 

that are similar in many ways to more routine types of civil or mining construction projects. However, some aspects of refuse disposal may provide opportunities for optimization as follows:

• Dissimilar refuse materials are continually generated. These include: (1) coarse 

refuse in solid form, (2) fine refuse in slurry or dewatered sludge form, and (3) for 

some sites, combined coarse and fine refuse. The characteristics and quantities of 

coal refuse may change over time due to changes in geology, production, mining 

methods, and coal preparation. Anticipating these changes in advance provides an 

opportunity to develop a cost-effective disposal plan.
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MAY 2009

• Refuse materials can be disposed in a manner that allows for post-mining land use 

that can enhance the future value of the property.

• Coarse refuse, at times supplemented with amounts of borrowed soil and rock 

materials, may be used to develop the structural capability to retain less carefully 

placed or slurried refuse. In some instances, amendments such as industrial byproducts containing lime to address acid generation within the refuse are necessary and these amendments can make up a significant percentage of the material 

placed.

• Co-disposal of coal combustion waste (fly ash and bottom ash) with coal refuse may 

be an effective contracting strategy or necessity for the mine, and this waste can be 

used as an amendment to benefit the refuse disposal facility. Combustion waste can 

be a significant percentage of the material placed and, like amendments, can alter 

embankment material properties.

• A disposal facility may be adapted to serve other operating requirements, such as the 

recovery of clarified water for return to the preparation plant.

• Short- or long-term water impoundments may be created behind properly constructed refuse embankments.

• Some structural or hydraulic features must be adaptable to modification as the facility grows in size and shape over its service life.

• Transport methods and equipment may change as the facility grows horizontally 

and vertically.

• Technological advancements, market demands and regulation changes may occur 

within the service life of the facility.

• Final portions of coarse refuse from normal operations, rather than borrow material, 

may be used to make the completed facility structurally safe and environmentally 

acceptable for abandonment.

• Portions of refuse, properly mixed with appropriate amendments, may be used to 

facilitate planting on a completed portion of the facility when reclamation soils are 

scarce.

A major factor in many of the above items is the abundance of refuse material available for construction, at little or no added cost. Proper planning must be employed to take advantage of the favorable 

characteristics of refuse and to minimize the need for using more costly borrow soil and rock and 

specially purchased construction materials.

4.2 OPERATIONS AND SITE-RELATED FACTORS

Each mine has characteristics that uniquely affect overall coal production and processing operations. Proper planning on a site-by-site basis will assure that processing and refuse disposal 

procedures that best suit the mining operation are established. It is possible, however, to develop 

a list of considerations for the planning of practically all refuse disposal facilities, corresponding 

to elements of federal and state permitting requirements. The length of this list and its subject 

breadth illustrates the importance of meaningful coordination between management, operating 

personnel and the design/permitting team. These factors can be grouped into two categories, as 

follows:

Factors Primarily Related to Disposal

• Capacity, number and location of potential disposal areas at the site.

• General topographical, soils, geologic, seismological, hydrologic and cultural char-
4–3

Project Planning

MAY 2009

acteristics of the site. Cultural characteristics may include buildings, infrastructure 

(such as roadways, pipelines, power lines, etc.), and cemeteries.

• Environmental and ecological conditions at the site, including surface water, groundwater, wetlands, wildlife and plant species.

• Mining (past, current, or potential future), oil and gas development, and other natural resources at the site. Analyzing the potential impacts from active or abandoned 

underground mines that may underlie a refuse site is of utmost importance. Previous site development and refuse disposal activities should be identified.

• Capital requirements for initiating disposal at each disposal area.

• Operating and maintenance costs at each disposal area.

• The relationship between the capacity of each disposal area and its distance from the 

preparation plant.

• Proximity of population centers and residences.

• Current property boundaries and land availability.

• Land-use practices within and in adjacent areas and post-mining land-use considerations.

• Contingencies for unexpected conditions (i.e., changes in mine life resulting in 

changes to the final embankment configuration and abandonment provisions).

Factors Primarily Related to Overall Operations

• Public relations.

• Source of coal and type of mining.

• Extent of preparation of coarse and fine refuse.

• Production rates of coarse and fine refuse and their reliability.

• Changes in geologic conditions as the mine advances.

• Anticipated life of the mine.

• Geochemical characteristics of coarse and fine refuse.

• Water requirements for the preparation process.

• Modifications needed to meet future market demands and regulation requirements.

• Available and preferred materials handling equipment.

• Flexibility in location of preparation plant.

• Required infrastructure (roads, portals, slopes, shafts, conveyor beltlines, railroads, 

etc.) during the life of the mine.

• Additional materials generated at the mine by other operations or by nearby construction projects (site developments, power plants, etc.) that might provide economic value and be beneficial when combined with the disposed refuse.

• Water treatment facilities required for operation or environmental control.

• Corporate policy on capital expenditures versus operating costs.

Data relative to each of the above factors can be obtained through preliminary investigations and 

discussions with appropriate mine personnel. This information can then be used to formulate specific 

studies leading to refinement of the project plan, performance of site-selection studies, and initial 

design. As discussed in Sections 4.6 and 4.7, the designer must coordinate closely with the operating 

staff during the early project stages and must incorporate environmental, permitting and regulatory 

requirements into the facility planning.
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MAY 2009

4.3 SUSTAINABLE MINING PRACTICES

Sustainable mining practices have evolved into management of natural resource development in a 

manner compatible with the environment and the needs of the community in which mining plays a 

significant role. By practicing sustainable development, mining companies can improve their access 

to land and markets, as well as enhance their reputations and value, and thus promote the coal industry. Yearly (2003) cites the following three tenets for sustainable mining:

• Integrated approaches to decision making on a full, life-cycle basis that satisfy obligations to shareholders and that are balanced and supported by sound science and social, 

environmental and economic analysis within a framework of good governance.

• Consideration of the needs of current and future generations.

• Establishment of meaningful relationships with key constituencies based on mutual 

trust and a desire for mutually beneficial outcomes, including those inevitable situations that require informed trade-offs.

Inherent in sustainable development is: (1) the commitment to implement effective planning and design practices and (2) the need for research and development leading to improved methods for harnessing natural resources with minimal impacts on the environment. Government regulation creates 

the framework for protecting the environment from impacts, although it is the mining professionals 

who are responsible for establishing the plans and designs that accomplish the mining operations in 

a sustainable manner. This may require the involvement of a diverse professional community. Sustainable mining goals established for engineers and leaders in the mining industry and detailed in 

the Milos Statement from the International Conference on Sustainable Development Indicators in the 

Mineral Industries held in Milos, Greece are discussed in Karmis (2003).

Project planning for coal refuse disposal should be integrated with overall development and operations planning for mining activities. In the following paragraphs, economic, environmental and regulatory considerations are addressed, along with planning and design sequence.

4.4 ECONOMIC CONSIDERATIONS

To achieve the desired optimum solution, economic factors must be considered during all phases of 

disposal facility planning, design and implementation. Major economic decisions are required for: (1) 

selecting the disposal facility site, (2) selecting the materials handling systems, and (3) for planning 

the entire sequence of disposal operations from initiation through abandonment. The expenditures 

required are usually classified as:

• Capital costs

• Operation and maintenance costs

• Reclamation (abandonment) costs

• Potential long-term liability costs

The determination of these costs is part of the planning process and influences the design, permitting 

and site operations. Periodic updates of estimated costs should be part of the planning and design 

process. Disposal facility planning and design should be keyed to the financial requirements of the 

mine, so that cash projections cover the costs associated with operating and maintaining the disposal 

facility. Carefully planned investment during initial phases of site development and selection of disposal facility configurations can have a major beneficial effect on subsequent operating and reclamation (abandonment) costs.

Table 4.1 presents a general breakdown of various cost components associated with planning, design, 

and implementation of a coal refuse disposal facility. These components are provided as a general 
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Project Planning

MAY 2009

TABLE 4.1 COAL REFUSE DISPOSAL FACILITY COST COMPONENTS

I. Capital Cost

Property Acquisition

Permitting and Planning

Engineering and Design

Site Development

 Access Roads •	 Erosion and Sedimentation Control

•	 Wetlands and Stream Mitigation •	 Amendment Transport and Handling System

•	 Site Preparation •	 Starter Embankment

•	 Perimeter and Diversion Ditches •	 Internal Drainage System

•	 Liner and Underdrain System Decant Installation •	 Decant Installation

•	 Mine/Entry Stabilization or Protection •	 Spillway Construction

•	 Refuse Transport and Handling System

II. Operating Cost

Equipment Operations

•	 Refuse Transport (haulage & pumping systems)

•	 Refuse Placement and Compaction

Facility Construction

•	 Site Preparation for Facility Expansion •	 Haul/Access Roads and Surfacing

•	 Perimeter and Diversion Ditch Extension •	 Internal Drains Installed in Stages

•	 Mine/Entry Stabilization or Protection •	 Decant Pipe Extension and Capping

•	 Liner and Underdrain System Extension •	 Spillway Extension

•	 Amendment Materials •	 Survey Control

Maintenance

•	 Erosion and Sediment Control

•	 Haul/Access Roads and Surfacing

•	 Liner and Ditch Repairs

Regulatory Compliance and Reporting

•	 Site Inspections and Testing

•	 Monthly, Quarterly, and Annual Reporting

III. Reclamation Costs

Engineering/Permitting Oversight

Facility Construction

•	 Impoundment Backfilling and Stabilization

•	 Site Grading

•	 Cap/Soil and Topsoil Placement

•	 Revegetation

IV. Potential Long-Term Liability Costs

Acid Mine Drainage Treatment

Mine Discharge

Erosion and Sedimentation Control

Revegetation

guideline for developing costs. Specific site conditions and operational requirements may introduce 

other elements not reflected in the table.
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MAY 2009

4.5 ENVIRONMENTAL AND REGULATORY CONSIDERATIONS

Environmental and regulatory considerations should be incorporated into the planning process, 

and can be defined steps performed as part of, or in advance of, the permitting of the coal refuse disposal facility with state and federal agencies. These considerations are addressed jointly, 

because frequently the method for addressing environmental issues may be directed by regulatory guidance or law. Primary federal statutes and regulations include the Federal Mine Safety 

and Health Act (1977) and the Surface Mining and Reclamation Act of 1977 (SMCRA), as well as 

more widely applicable statutes and regulations such as the Clean Air Act and Clean Water Act. 

SMCRA established the Office of Surface Mining (OSM) with authority to implement a national 

program to regulate surface effects resulting from coal mining. State regulatory programs and 

agencies may take on this responsibility, if approved by the OSM, which will remain in an oversight role. For the states that take on this responsibility, most of the regulatory requirements of 

their programs are similar to the OSM regulations found in 30 CFR Chapter VII. In response to 

local concerns and conditions, some state programs have requirements that are more extensive 

than the federal rules. Additionally, some state agencies responsible for other programs, such 

as dam safety, impose requirements for coal refuse disposal facilities. Designers should contact 

state regulatory agencies to obtain current information on their regulatory programs and requirements.

Environmental issues at some sites can require significant time and effort to address, affecting the 

entire regulatory approval process. This section discusses environmental and regulatory considerations in the planning process, and the engineering and design of specific containment structures at 

disposal facility sites are discussed in subsequent chapters of this Manual. However, the state and 

federal permitting process, including environmental impact studies and mitigation requirements, is 

left for other publications. Because state regulatory input shapes many of the decisions associated 

with planning and design of a refuse disposal facility, contact with these agencies and review of their 

publications is essential.

Some specific environmental and regulatory factors that influence the planning and design of coal 

refuse disposal facilities include the following:

• Site selection process and permitting submittals prior to facility design. Some states 

require a rigorous process for site selection as part of permitting new coal refuse 

disposal facilities or the expansion of existing facilities.

• Facility configuration (slopes, benches, crest width, etc.). Federal and state regulations include specific requirements for some facility parameters, either as part of 

mining, coal refuse disposal, or dam permitting guidance.

• Erosion and sedimentation and stormwater control structures. State regulations and 

some local (municipal) entities provide specific guidance for meeting erosion and 

sedimentation control and stormwater requirements.

• Liners for containment of refuse materials. Some state regulations provide guidance 

for liners for refuse disposal facilities.

• Amendments for neutralization of refuse materials. State regulations provide guidance for acid neutralization of refuse materials.

• Wetlands and stream encroachments. Federal and state regulations provide guidance 

for addressing wetlands and stream encroachments, including mitigation requirements where necessary.

• Prime farmlands. State regulations provide guidance for addressing the presence 

of prime farmlands near proposed refuse facilities and include mitigation measures where necessary.
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4.6 PLANNING SEQUENCE

Table 4.2 shows a typical sequence of events involved in the planning, design, operation and abandonment of coal refuse disposal facilities. Site-specific factors and the specific objectives of persons 

collectively involved in the process may preclude direct application of the indicated sequence. However, proceeding generally in the manner shown should aid in the design, permitting and construction of a coal refuse disposal facility. Particular notation is made of the following items:

• Continuing interaction between operations/mine personnel and the designer is 

shown. The important relationship between coal production and refuse disposal 

dictates this cooperation, if mine operation and facility construction are to proceed 

optimally.

• Interaction between the designers and the regulatory agencies relative to site selection and design elements is shown. This interaction will allow identification of special regional or site-specific concerns, methods to address concerns, and will generally facilitate the permitting process.

• Engineering should be an integral part of disposal operations, as well as construction 

monitoring and inspections, particularly on complex, long-term projects. Involvement of the designer, or engineering personnel thoroughly familiar with the design 

requirements, allows review of performance information relative to design parameters and identification of needed adjustments.

• The next to the last step (XI) in Table 4.2 is periodic review of mine operations and 

the disposal facility development. Regardless of the accuracy of initial planning, the 

typically long active period of use of a disposal facility makes it likely that unanticipated changes to the original design will occur. Periodic review of the effects of these 

changes must be performed so that continued safe, economical and environmentally 

acceptable refuse disposal can be achieved.

4.7 DESIGN SEQUENCE

The design sequence presented in Table 4.3 details the technical aspects of the total planning-implementation process for disposal facility development in a safe and environmentally acceptable manner. 

The primary design emphasis occurs during the third to seventh steps of the planning sequence in 

Table 4.2. The elements of coal refuse disposal facility design and their general interrelationship are 

presented in Chapter 5. Subsequent chapters of this Manual present detailed technical information 

and procedures related to completion of facility design.

Table 4.3 presents a typical sequence of design steps and provides a checklist of the most important 

items requiring consideration for a typical refuse disposal project. Reference is made in the table to 

sections of the Manual where additional detailed discussion is presented.

The items in Table 4.3 may not be totally applicable in all instances. In the case of small facilities, a 

very detailed study may not be appropriate, and portions of the investigation and analyses can possibly be eliminated by simply using conservative design assumptions. On the other hand, conditions 

may be present at other sites that require studies beyond those identified in Table 4.3. The existence 

of such special conditions can only be determined by an experienced designer through careful study 

of site conditions.

Item VIII in Table 4.3 summarizes the deliverable products of the design sequence, including the 

design report and preparation of facility plans and specifications. General guidance on the content of 

these documents is presented in Chapter 11; however, site-specific issues may require elements not 

identified in the table.
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MAY 2009

Preparation of a design report is suggested for all facilities, regardless of the required extent of investigation and analyses. This report: (1) provides the opportunity for all parties (designer, operator and regulatory groups) to understand design assumptions and limitations, (2) provides the opportunity for the designer to clearly state validating design assumptions during the construction or operations phases, and (3) 

helps to avoid confusion or misunderstanding between the designer, operator and regulatory groups.

Clear and accurate plans and specifications are essential for operations personnel to properly construct 

and develop a disposal facility and for the operator’s quality control representatives to verify that all 

details are completed correctly. Accurate plans and specifications minimize the potential for misunderstandings that could cause schedule delays during the review period and during construction. General 

guidance for the content of refuse facility plans and specifications is presented in Chapter 11 of this 

Manual; however, there may be site-specific elements that are not addressed in Chapter 11.

TABLE 4.2 PLANNING STEPS FOR COAL REFUSE DISPOSAL FACILITY DESIGN

Planning Steps Participants Regulatory 

Involvement

I Gather and Evaluate Mine Development and Disposal 

Information

Management

Operations

Engineering

No

II

Perform Initial Siting Studies, Develop Disposal 

Concepts and Conduct Alternatives Analyses for 

Potential Sites

Operations

Engineering Yes

III Perform Detailed Investigations and Prepare 

Preliminary Design Engineering No

IV Modify to Best Suit Operations and Evaluate 

Development Costs

Operations

Engineering No

V Confirm Preliminary Design Assumptions and 

Regulatory Criteria

Management

Operations

Engineering

Yes

VI Finalize Plans, Specifications and Permit Documents; 

Refine Development Costs Engineering No

VII Final Approval

Management

Operations

Engineering

Yes

VIII Implement Site Development Operations

Engineering Yes

IX Conduct Disposal Operations Operations

Engineering No

X Construction Monitoring, Maintenance Operations

Engineering No

XI Periodic Inspections and Review of Operations

Management

Operations

Engineering

Yes

XII Abandonment and Reclamation Operations

Engineering Yes
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TABLE 4.3 TYPICAL DESIGN SEQUENCE(1)

Subject Manual Section

I. Site Contour Survey Data

A. USGS Topographic Maps 6.4.1.1

Hazard potential

Watershed area

B. Aerial Photography 6.4.1.4

Embankment and pond area: five-foot minimum contour interval (two-foot interval is 

preferable)

Spillway and outlet works: five-foot minimum contour interval (two-foot interval is 

preferable)

II. Review of Available Publications and Data

A. Soils and Geology Chapter 6

Government agencies:

•	 United States Geological Survey (USGS)

•	 Natural Resource Conservation Service (NRCS, formerly SCS)

•	 State geologic survey(s)

Other Sources:

•	 Universities

•	 Studies from nearby sites

•	 Coal mine exploration borings

•	 Aerial photographs

B. Hydrology Data Chapter 9

Government agencies:

•	 National Weather Service

•	 USGS gaging station data

•	 NRCS runoff data

C. Mining Status Chapter 8

Coal operator’s records

Government agencies:

•	 Office of Surface Mining, U.S. Department of the Interior

•	 State agencies

D. Seismicity 7.7

U.S. Geological Survey (USGS)

Published records of recorded earthquake epicenters

III. Site Reconnaissance

A. Area Upstream and/or Upgradient and Downgradient from the Facility 6.4.2, 14.4.2

B. Reconnaissance of Natural Features 6.4.2

Topography and morphology

Soil conditions, rock outcrops, sinkholes

Vegetation cover

Drainage patterns, springs and streams

Erosion

Stability (sliding and sloughing)

Wetlands

C. Reconnaissance of Man-Made Features 6.4.2

Roads and railroads
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TABLE 4.3 TYPICAL DESIGN SEQUENCE

(Continued)

Subject Manual Section

Buildings and other structures

Bridges

Stream modifications and channels

Mine entrances and features (shafts, boreholes, highwalls, auger holes, spoil/refuse, 

AMD discharges, mine subsidence features, etc.)

8.3

Other infrastructure (gas wells, pipelines, power lines, etc.)

Water treatment facilities

IV. Site and Facility Configuration Selection

A. General Considerations Chapters 3, 5

B. Geotechnical Considerations Chapters 6, 7, 8

C. Drainage Considerations Chapter 9

D. Environmental Considerations Chapter 10

E. Equipment and Construction Considerations Chapter 11

V. Field Investigations

A. New Facilities

Borings – Locations, depths and types of sampling should be selected by an 

experienced designer.

6.4.3

Geophysical surveys – Supplemental to borings for exploration and testing. 6.4.4, 8.3.2

Piezometers – Normally piezometers are required only in selected borings for 

monitoring water levels in abutments, etc. or to determine if water conditions will 

create construction difficulties.

6.4, 13.4.2

Test pits – Test pits can be used to economically gain additional data in area 

planned for facilities and adjacent areas where borrow material may be obtained.

6.4.3.3

Water samples – Normally from surface water streams, major springs, and ground 

water wells.

10.3

Field mapping – Notation should be made of major spring locations, rock outcrop 

zones, mine openings, evidence of subsidence, existing landslides and any other 

conditions that might affect construction of refuse facility structures.

6.4.2, 8.2

B. Existing Facilities

Borings – Locations, depths and types of sampling should be selected by a qualified 

engineer.

6.4.3

Geophysical surveys – Supplemental to borings for exploration and testing. 6.4.4, 8.3.2

Piezometers – Since phreatic conditions in an existing facility cannot be accurately 

estimated, piezometers should be installed in borings critical to stability analyses.

6.4, 13.4.2

Test pits – Test pits can be used for the same purpose as for new facilities, plus to 

evaluate the nature of weathering with depth, to obtain large samples of existing 

embankment materials, and to conduct in-situ density tests.

6.4.3.3

Water samples – From stream above and below facility, from the impoundment 

(if any) and from all major downstream seeps or springs. Also, from groundwater 

wells.

10.3

Field mapping – Same as for new facilities, plus observation should be made 

to locate any seepage zones on downstream face and to note any evidence of 

cracking or movement on any portion of the existing facility.

6.4.2, 8.2

Flow measuring weirs – Weirs or calibrated pipes should be installed to monitor the 

flow from any major seepage zone noted on the downstream face of the facility.

13.4.3

Survey monitoring – If any evidence of movement of the existing facility is noted in 

a critical zone, survey monuments can be installed to determine if the movement is 

active and the rate of movement.

13.4.1
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TABLE 4.3 TYPICAL DESIGN SEQUENCE

(Continued)

Subject Manual Section

VI. Laboratory Investigations

A. Soils/Rock and Refuse Materials 6.5

General and index properties:

•	 Water contents of all samples

•	 Grain-size analysis of representative samples

•	 Liquid and plastic limits of fine-grained materials

•	 Specific gravity of refuse materials

Materials behavior:

•	 In-situ properties

On Shelby tube samples or prepared samples for approximating in-situ 

conditions

Strength tests with representative pore-pressure conditions

Consolidation of fine-grained materials

Hydraulic conductivity tests

•	 Materials to be used for construction

Compaction tests

Strength tests with representative pore-pressure conditions

Consolidation of fine-grained materials

Hydraulic conductivity tests

Special tests for refuse materials:

Ash content and ignition tests

B. Water Quality Testing Chapter 10

The water quality testing program will be a function of the potential source of 

water, environmental conditions, and specific facility design features (may require 

determination of pH, temperature, specific conductance, suspended and dissolved 

solids, sulfates and metals).

VII. Analyses and Design

A. General Considerations

Hazard classification Chapter 3

Site development and startup Chapters 4, 5

Erosion and sedimentation control Chapters 5, 6, 9

Staging Chapters 5, 6

Reclamation and abandonment Chapters 5, 6, 10

B. Geotechnical

Parameters – Establish material properties from field/laboratory data and/or other 

sources. For new facilities the properties of refuse may have to be estimated 

based on coal seam and preparation procedures because the material may not be 

available.

6.4, 6,5

Geometry – Establish basic embankment cross section configuration for each stage, 

including methods for controlling seepage. Chapters 5, 6

Static stability – Estimate pore pressure conditions for critical stages or conditions. 

Modify geometry if required to achieve satisfactory static stability conditions. 6.6.4, 6.6.5

Dynamic stability – Perform seismic hazard assessment, liquefaction, stability, and 

deformation analysis, as required, considering hazard classification of embankment 

and seismic risk zone of site. Modify geometry if required to achieve satisfactory 

dynamic stability.

Chapter 7

Settlement – Evaluate if settlement of soft layers could cause loss of freeboard, 

embankment cracking or damage to drainage facilities. 6.6.3
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MAY 2009

TABLE 4.3 TYPICAL DESIGN SEQUENCE

(Continued)

Subject Manual Section

Subsidence – Evaluate if underground mining could affect stability. Chapter 8

Buried pipe design – Analyze pipe stresses and strain, select materials, and design 

installation and backfill requirements for conduits and decant pipes. 6.6.6

Special considerations:

•	 Determine special foundation preparation requirements, including treatment of 

existing mine spoil or refuse.

Chapters 6, 8

•	 Design special material requirements such as starter embankments, drainage 

materials, filters and soil cover or intermediate layers, if any.

Chapter 11

•	 Establish special construction requirements for diversion ditches, spillway cuts, 

structure and pipe foundations, etc.

11.7

•	 Specify borrow areas for required soils and rock materials. 6.2.3.4

•	 Specify construction procedures to satisfy design assumptions. 11.1

C. Hydrology and Hydraulics Chapter 9

Determination of design storms 9.5

•	 Establish requirements for critical stages such as facility startup or 

abandonment.

•	 Establish basic storage-decant-spillway scheme

Sedimentation Control 9.4.4

•	 Establish sedimentation pond requirements.

Decant and spillway systems

•	 Perform hydrology analyses for storm served only by storage and decant 9.6

•	 Design decant system 9.7.4

•	 Perform hydrology analyses for maximum design storm

•	 Design spillway structures (or cuts) for various stages 9.7

•	 Design erosion protection system and/or stilling mechanisms.

Diversion Systems

•	 Design size of diversion ditches for various stages, including abandonment.

Chapters 9, 10

D. Special Considerations

Environmental protection – Evaluate potential acid generation and seepage quality, 

and determine containment or neutralization requirements. Chapter 10

Corrosion – Evaluate probable seepage quality and related limitations on 

construction materials.

6.5.2.5, 6.6.6.1,

11.7

Vegetation – Determine requirements for vegetation of completed surface, including 

the need for soil cover or surface preparation and treatment. Chapter 10

Monitoring – Design monitoring and inspection program commensurate with the total 

design. Chapter 12

VIII. Preparation of Design Documents

A. Designer’s Report Contents Chapter 11

Introduction

History (for existing sites)

Discussion of site conditions and previous mining/site development/refuse disposal

Field investigations

Laboratory testing

Geotechnical analyses

Hydrology and hydraulics analyses

Special considerations

Facility staging
4–13

Project Planning

MAY 2009

TABLE 4.3 TYPICAL DESIGN SEQUENCE

(Continued)

Subject Manual Section

Recommended design

Abandonment requirements

Monitoring and inspection

B. Plans Chapter 11

Location map

Location plan on USGS quadrangle base

Plan of borings and field investigation

Boring and test pit logs

Laboratory data

Hydrology data

Results of stability analyses

Plans of facility at critical stages

Plan and cross sections of hydraulic structures

Details of hydraulic structure components

Capacity curves for control sections of hydraulic structures

Details of monitoring installations

C. Specifications Chapter 11

Site preparation

Foundation preparation

Embankment construction and internal drainage facilities

Surface drainage facilities

Decant system

Emergency spillway construction

Instrumentation

Reclamation and abandonment

D. Calculation Brief (should include input files for computer runs) Chapter 11

Coal refuse production rates (by weight and volume)

Starter embankment and staging

Hydrology and hydraulics analyses (sedimentation control, design storm routing, 

drainage channel design, dam breach analysis)

Stability analyses (including seismic hazard assessment and stability analysis)

Settlement analyses

Seepage analyses and internal drain design

Surface drainage channel lining design

Buried pipe analyses

Environmental analyses

E. Operation and Maintenance Plan Chapter 11

 Note: 1. The design procedure should be modified appropriately to suit the size, arrangement and specific

 characteristics of the facility being designed.
Chapter 5

COAL REFUSE DISPOSAL

FACILITY DESIGN

COMPONENTS

MAY 2009 5-1

Coal refuse disposal facility design involves the evaluation of a number of interrelated components 

for development of an embankment that can accommodate refuse generation rates, storm water and 

environmental control requirements and provide a stable structure consistent with the operational 

demands of the mine and post-mining, land-use intentions. The detail and extent of analysis required 

for evaluation of each of these components depends upon the type of refuse facility being considered. 

The process of designing a disposal facility normally starts with determination of the refuse disposal 

volume requirements and, for slurry impoundments, includes consideration of the refuse generation 

rates and the design storm runoff volume that may need to be temporarily retained. Foundation conditions and possible required treatment, either resulting from existing soil conditions or past mining 

practices (underground or surface), may influence the locations of disposal facility structures and the 

overall configuration of the site, resulting in a need for special structures and instrumentation. The 

following components are normally evaluated as part of the design of a refuse disposal facility:

• Disposal capacity requirements, facility configuration/staging and scheduling

• Design storm management and erosion and sediment control

• Facility configuration, geometry and stability

• Foundation and mine subsidence considerations

• Stability of all embankment, channel and other site slopes affected by construction

• Internal drainage for embankment/foundation seepage control

• Decant and emergency spillway requirements

• Surface drainage controls

• Instrumentation and monitoring requirements

• Reclamation, abandonment and post-mining land use

5.1 DISPOSAL CAPACITY REQUIREMENTS AND SCHEDULING

5.1.1 Refuse Generation Rates and Design Capacity/Life

Refuse generation rates are the basis for determining disposal facility construction scheduling and 

the life of a disposal site. The refuse generation rate can be estimated based upon process flow and 

coal preparation studies for the new mine development and sampling, testing and volume data from 

existing disposal sites. Process flow data typically provide refuse generation rates for coarse refuse 
5-2

Chapter 5

MAY 2009

and fine refuse separately on a dry-weight basis, along with estimates of the moisture content (either 

as percent moisture or percent solids for slurry). To estimate the disposal capacity and life of a facility, a conversion of the process data on a weight basis to in-place disposal data on a volume basis is 

required. Where sampling and testing data are not available, estimates of the in-place refuse properties must be developed, typically by using data from similar processing plants, coal strata, and refuse 

disposal sites. The in-place refuse properties required for initial estimation of the life and scheduling 

of a facility normally include unit weight/density and moisture content of both coarse and fine refuse. 

Ultimately, engineering properties of the refuse will also be required for designing the facility. These 

properties should be verified during the initial phases of the operation of the facility and at periodic 

intervals during the facility life. Available test data should be scrutinized to verify that they are representative, and the reliability and potential for variability of the refuse material should be evaluated. 

Provisions for verification of material properties and a schedule for doing so should be indicated in 

the design plans.

The design capacity and construction scheduling for a disposal site is also dependent on the planned 

configuration of the facility, as well as the type of handling of the fine refuse (slurry, filter cake, or 

mixed with coarse refuse). Based on an assumed site configuration, the capacity and life can then be 

estimated based upon the computed volume and refuse generation rates.

While the type of handling and processing used for the fine refuse is typically an operational decision, physical characteristics of fine refuse when disposed may impose certain limitations at the disposal site. For instance, past undermining may preclude the option of a slurry impoundment or may 

entail extensive foundation treatment that makes that option infeasible. Therefore, knowledge of the 

planned type of handling proposed for the fine refuse prior to selecting a site configuration for the 

refuse facility is important.

5.1.2 Slurry Impoundment Staging and Scheduling

The staging and scheduling of the construction of a slurry impoundment must be carefully analyzed 

so that the embankment elevation is sufficient to provide storage to contain the slurried fine refuse 

generated as well as the runoff from a design storm. Additional embankment height to account for 

material consolidation or permanent seismic deformations may also be needed. In general, additional embankment height and width can be added to provide design conservatism and enhance 

safety. Although the overall capacity of a slurry impoundment disposal site configuration can be estimated from the refuse generation rates, the rate of incremental construction of embankments or dikes 

must be sufficient to accommodate the generation and deposition of fine refuse slurry (referred to as 

staging). Additionally, storm runoff entering the impoundment will need to be stored temporarily, 

increasing capacity requirements. Short-term design storm criteria are applicable during initial site 

construction with transition to long-term criteria during subsequent development. Other factors that 

may enter into the staging of a slurry impoundment include the following:

• Initial construction of the beginning embankment or dike for impounding slurry 

(termed “starter dam,” “starter embankment” or “starter dike”) typically requires 

borrow material. The size of this embankment is optimized based upon the available borrow material, topography of the disposal site (which affects the initial slurry 

capacity), and the ability to schedule subsequent embankment or dike construction 

using solely coarse refuse to impound slurry at the expected refuse generation rates. 

Therefore, the reliability and consistency of refuse generation rates is important to 

the construction of embankment stages and operation of the impoundment.

• Availability and capacity of existing disposal facilities at the mine that could be 

used for temporary disposal during site development activities and while the starter 

embankment is being constructed.
5-3

Coal Refuse Disposal Facility Design Components

MAY 2009

• Amendments to the refuse or co-disposal of combustion waste with refuse, particularly when such additional material is a significant fraction of the refuse.

• The type of construction staging (upstream, downstream and centerline) will affect the 

capacity and schedule for the facility. Upstream construction utilizes less coarse refuse 

to achieve a comparable impoundment capacity because the stages are constructed 

partially on the settled fine refuse in the impoundment. Downstream, and to a lesser 

extent, centerline construction staging require greater quantities of coarse refuse to 

achieve equivalent impoundment capacity in comparison to upstream staging. At 

some sites a combination of upstream and downstream staging is employed to meet 

site configuration or specific refuse disposal requirements and to control excess pore 

pressures by initiating loading and consolidation of the fines early in the project life.

• In areas subject to seismic loadings, where deformation of the embankment may 

occur, additional freeboard between the impoundment surface and embankment 

crest may be necessary.

• Other special considerations such as impoundment breakthrough potential into 

underground mines, mine subsidence, reclamation, and post-mining land use can 

also influence the staging and configuration of the disposal facility.

When determining the type of construction staging (downstream, upstream, centerline, or a combination thereof), the ratio of coarse refuse to fine refuse generated by the preparation plant is a 

major factor. When the coarse refuse to fine refuse ratio is relatively high, downstream and centerline 

staging is normally preferred. As the coarse to fine ratio decreases, upstream staging is normally 

employed so that sufficient embankment height and containment is provided. As indicated above, 

both upstream and downstream staging may be employed at a site. Figure 5.1 illustrates a construction sequence with upstream and downstream staging through Stage II. In this example, an early 

stage of upstream construction (Stage II-A) is incorporated in order to initiate consolidation of fine 

refuse, while disposal simultaneously occurs in a downstream stage (Stage II-B). The figure also 

shows several of the key components in facility design.

The amount of refuse generated and the coarse to fine ratio must be confirmed from actual production measurements and must be rechecked periodically during operation. These parameters should 

be periodically evaluated so that the staging of the refuse facility can be modified, if needed. In 

general, downstream construction results in more predictable embankment performance and may 

require substantially less testing and engineering to address seismic loading, but this type of staging 

is more sensitive to changes in the coarse to fine ratio.

5.1.3 Slurry Cell Staging

If slurry cells are designed and operated as small ponds with size and hazard potential classification 

that do not require impoundment plans under 30 CFR § 77.216, or are arranged such that a lowhazard-potential impoundment classification results, their staging and construction may be more 

complex than construction of an impounding refuse facility. A slurry cell facility typically has surface 

diversion to prevent runoff from collecting in the cells. Thus, the design storm plays a significantly 

smaller role in slurry cell staging than would be the case with a significant- or high-hazard-potential slurry impoundment. Additionally, when the slurry cells are covered sequentially with layers 

of coarse refuse, the fine refuse may be densified, but this effect typically does not greatly affect the 

capacity of a slurry cell facility.

Figure 5.2 illustrates the components of a slurry cell facility. The small size of individual slurry cells 

necessitates operation of multiple cells, and in some instances, continuous cell construction, putting greater operational demands on the disposal facility. Thus, staging can be critical with a slurry 
5-4

Chapter 5

MAY 2009

cell facility because the need to divert runoff around the cells and to periodically cover the cells can 

require as much or more coarse refuse than with slurry impoundment staging. Thus, the consistency 

of the refuse generation rate is critical to successful operation of slurry cells. Slurry cells are an attractive option only when there are limitations to developing an impoundment at a disposal site, or when 

the ratio of coarse refuse to fine refuse is quite large. Slurry cells are sometimes used as a contingency 

alternative when underground injection is employed.

When slurry cells are operated only to dewater fine refuse slurry, operation and staging difficulties 

are significantly reduced. For this type of operation, slurry is placed in cells for dewatering and subsequent removal and mixing with coarse refuse prior to final disposal. Typically, these slurry cells 

are utilized for refuse processing plants with lower production rates that are operated over a longer 

period of time, thus reducing the burden of continually constructing storm water diversion structures for new cells.

5.1a PLAN

700

675

675

700

725

775

750

800

825

850

875

900

850

875

900

925

925

900

875

850

825

800

750

775

800

825

850

875

900

EMBANKMENT

CREST

EMERGENCY

SPILLWAY

SADDLEBACK

DAM CREST

DOWNSTREAM

STAGE

SEDIMENTATION

POND

IMPOUNDMENT POOL

PERMANENT (GROIN)

DRAINAGE DITCH

UPSTREAM STAGE

SETTLED FINE

REFUSE DELTA

DECANT PIPE INLETS

DECANT PIPE

OUTLET

5.1b LONGITUDINAL CROSS SECTION

STARTER

 DAM STAGE II-B

STAGE II-A

CREST

SEDIMENTATION

POND

INITIAL FINE REFUSE DEPOSITS

SETTLED FINE

REFUSE DELTA

STAGE I

PIEZOMETER

INTERNAL

 DRAIN

UNDERDRAIN

IMPOUNDMENT

 POOL

CUTOFF KEYWAY

NOTE: 1. ROADS AND BENCHES ARE NOT SHOWN.

 2. SLOPES ARE EXAGGERATED FOR ILLUSTRATION PURPOSES.

825

800

725

750

775

650

650

FIGURE 5.1 TYPICAL SLURRY IMPOUNDMENT COMPONENTS
5-5

Coal Refuse Disposal Facility Design Components

MAY 2009

5.1.4 Non-impounding Refuse Embankment Staging

Staging for non-impounding embankments involves surface runoff diversion, drainage provisions for 

significant springs or groundwater flow, erosion and sedimentation control, construction sequencing 

(upstream or downstream construction), and reclamation. When dewatered fine refuse is disposed 

within upstream zones of a non-impounding embankment, staging is a function of: (1) the required 

quantity of coarse refuse and the rate of construction of the well-compacted downstream shell (sometimes referred to as the structural zone), (2) haul road requirements, and (3) surface runoff diversion 

requirements. The construction sequence and reclamation planning will also affect the staging.

Figure 5.3 illustrates the components of a coarse refuse disposal facility. Surface runoff diversion and 

erosion and sedimentation controls influence staging. Because the working surface and some por

5.2b LONGITUDINAL CROSS SECTION UPON COMPLETION

FIGURE 5.2 TYPICAL SLURRY CELL FACILITY (NON-IMPOUNDING EMBANKMENT)

5.2a PLAN

REFUSE

CONVEYOR

1500

1450

1300

1350

1400

1550

1500

1250

1200

1200

1250

1350

1300

1400

1500

1550

1600

1650

1550

1600

1600

1700

1750

1650

1450

CELL DRAINAGE

SYSTEM

TEMPORARY

PERIMETER DITCH

PERMANENT (GROIN)

DRAINAGE DITCH

SEDIMENTATION

POND

STRUCTURAL

ZONE

INTERNAL

DRAIN

UNDERDRAIN

EXTENSION

FUTURE CELLS

UNDER CONSTRUCTION

ACTIVE CELL

COVERED

CELL

BENCH

STARTER EMBANKMENT

CREST

FINE COAL

REFUSE CELL

UNDERDRAIN

INTERNAL DRAIN (TYP.)

NOTE: ROADS AND BENCHES NOT SHOWN

 ON CROSS SECTION.

FIGURE 5.2 TYPICAL SLURRY CELL FACILITY (IMPOUNDING EMBANKMENT)
5-6

Chapter 5

MAY 2009

tions of the embankment slopes will be exposed refuse, surface runoff from the embankment must 

be controlled separately from runoff from non-disturbed areas and must be directed to erosion and 

sedimentation control structures. This requires that flows from the contributing natural watershed be 

diverted around the disposal facility, which may require ditches or channels constructed upstream 

from the embankment or integrated into the embankment perimeter. 

5.3a PLAN

5.3b LONGITUDINAL CROSS SECTION

1800

1675

1650

1625

1575

1800

1775

1750

1725

1700

1675

1775

1500

1750

1725

1525

1700

1550

1600

SEDIMENTATION

POND

TEMPORARY

PERIMETER DITCH

PERMANENT (GROIN)

DRAINAGE DITCH

TEMPORARY

PERIMETER DITCH

BENCH

BENCH

PERMANENT (GROIN)

DRAINAGE DITCH

SEDIMENTATION

POND

UNDERDRAIN

NOTE: 1. ACCESS ROAD NOT SHOWN IN CROSS-SECTION VIEW.

 2. SLOPES ARE EXAGGERATED FOR ILLUSTRATION PURPOSES.

FIGURE 5.3 TYPICAL REFUSE EMBANKMENT COMPONENTS
5-7

Coal Refuse Disposal Facility Design Components

MAY 2009

5.1.5 Amendments and Co-disposal of Combustion Waste

The use of amendments applied to coal refuse and co-disposal of combustion waste and coal refuse 

will influence staging dependent upon the quantity of the materials disposed. Other aspects of 

embankment design may be affected because the engineering properties of the amended/combined 

materials may be different from those of the coal refuse.

5.1.5.1 Amendments for Neutralization and Stabilization of Refuse

Amendments applied to coal refuse have included a variety of materials typically used to neutralize 

the potential acidity of coal refuse or to absorb moisture for stabilization of the refuse during placement and compaction. Amendments have included lime and lime products, kiln dust, and combustion waste.

The inclusion of amendments for neutralization or stabilization of refuse may also have the following 

influences on design of the disposal facility:

• Neutralizing agents if applied in quantity can affect the strengths of the fill (either 

increase or decrease), and may alter handling and placement equipment requirements and embankment slopes. The thoroughness of mixing the amendments with 

the refuse is important to their application as neutralizing or stabilizing agents and 

affects the properties of the combined material.

• Runoff from neutralizing agents can have a high pH and thus may need to be 

retained within treatment ponds prior to discharge into a receiving water body.

• The hydraulic conductivity of the amended embankment materials may be lower 

than the refuse materials, necessitating additional internal drainage measures and 

resulting in precipitates that clog filters and internal drainage structures unless they 

are appropriately designed.

• Final reclamation of completed surfaces of amended embankment materials may be 

facilitated, allowing a thinner soil and topsoil cap.

To assess the behavior of mixed refuse and amendments, physical properties, including strength and 

hydraulic conductivity should be evaluated.

5.1.5.2 Co-disposal of Combustion Waste

Several types of combustion waste have been co-disposed with coal refuse at disposal sites, including: fly ash and bottom ash, fluidized bed combustion (FBC) ash, and flue gas desulfurization (FGD) 

sludge (dewatered). Depending on the characteristics of the waste, it has typically been disposed 

with coal refuse as: (1) part of contracting opportunities, (2) for structural or filter zones, (3) for stabilization of refuse due to wet conditions, (4) for neutralization of potential acidity of refuse, and (5) 

for reclamation improvements.

Co-disposal of combustion waste may have the following effects on the design of a disposal facility:

• The extent of mixing of combustion waste with coal refuse will affect the homogeneity 

of the embankment, particularly with respect to strength and hydraulic conductivity 

because of the influence of the combustion waste on these characteristics. The presence 

of mixed and unmixed layers may result in stratified conditions within the embankment.

• Combustion waste if incorporated within a refuse embankment in quantity may 

result in different (higher or lower) strength characteristics of the fill, requiring different handling and placement equipment and modified embankment slopes. FBC 

ash, if pozzolanic, will exhibit high strength at low strain similar to soil cement.
5-8

Chapter 5

MAY 2009

• The hydraulic conductivity of the embankment materials mixed with combustion 

waste may be different (typically lower, but possibly higher) than the refuse materials alone. This may result in precipitates that clog filters and internal drainage structures necessitating additional internal drainage measures. Combustion waste may 

also increase the anisotropy of embankment materials.

• FBC may have expansive characteristics that can affect instrumentation performance and monitoring, influencing the position and elevation of survey 

monuments and the verticality and integrity of casings for piezometers and 

inclinometers.

• Use of combustion waste may increase dust control requirements at the embankment 

surface.

• Runoff from combustion waste can have high pH and thus may need to be retained 

within treatment ponds prior to discharge to the receiving water body.

• Final reclamation of completed surfaces of embankment materials with co-disposed 

combustion waste may be facilitated, allowing a thinner soil and topsoil cap.

• Combustion waste may contain metals that can result in leachate and groundwater 

impacts.

Research on the beneficial use and co-disposal of combustion waste with coarse coal refuse is reported 

in Daniels et al. (2002). Research on co-disposal of combustion waste with fine coal refuse is presented 

in Kumar et al. (2001).

5.2 EROSION AND SEDIMENTATION CONTROL AND DESIGN STORM RUNOFF

MANAGEMENT

Erosion and sedimentation control features are required for all disposal facilities, and their 

design must be integrated into the overall drainage control plan. Management of runoff from 

design storms is a function of the type of refuse disposal facility and associated design storm. 

Impounding facilities are typically designed to retain a portion of design storm flows within the 

impoundment, thus attenuating the peak flow. Non-impounding embankments utilize diversion channels and ditches that are designed to handle the peak discharge. Embankments for 

both impounding and non-impounding facilities typically have associated drainage channels 

and ditches that are designed to handle storm flows. For larger refuse facilities, erosion and 

sedimentation controls can represent a significant component in the overall design of the facility and can occupy a relatively large portion of the site that should be accounted for during the 

initial phases of design.

5.2.1 Erosion and Sedimentation Control

Erosion and sedimentation control requirements are generally based on state regulatory criteria. 

Their function is to divert runoff from undisturbed areas of a site and to collect and treat runoff 

from disturbed areas. Diversion systems are normally required for all refuse embankments; however, impoundments can serve to provide sediment control for their contributing watershed. The 

design of the erosion and sedimentation control system is based upon the disturbed area at the 

disposal facility during operation and as reclamation is performed.

Sediment control is typically provided by ponds that receive drainage from the embankment and 

disturbed areas, and these are typically located near the downstream toe of the disposal facility. 

As discussed in Chapter 9, the ponds are designed with decant systems and inlet structures that 

provide adequate capacity and settling time for removal of sediment and control of storm events 

without activating the emergency spillway. The design of the emergency spillway is based on the 

pond’s hazard-potential classification and applicable state criteria. 
5-9

Coal Refuse Disposal Facility Design Components

MAY 2009

5.2.2 Slurry Impoundment Inflow Design Storm

Selection of the appropriate design storm (short- and long-term) is a function of the hazard potential of the planned slurry impoundment. Based on the design storm and site conditions, the runoff 

rate and volume can be determined. The facility can then be configured to manage the design storm 

through a combination of storage and routing. The runoff volume is important because frequently 

the size and configuration of the outlet system (decant pipe or spillway) for impoundments are 

determined by the volume of water that must be evacuated from the impoundment within a certain 

specified time period after the storm event (Chapter 9). Similarly, the staging of the facility must be 

adequate to retain the runoff from the design storm with adequate freeboard.

High-hazard-potential facilities must be able to handle the most severe design storm, while a lesser 

magnitude storm will be suitable for a significant- or low-hazard-potential classification facility. The 

determination of the hazard-potential classification is discussed in Section 3.1. Dam breach analyses 

may be performed as part of hazard-potential determination, as discussed in Chapter 9.

Short- and long-term embankment design criteria must be evaluated in the determination of the 

design storm, typically to address conditions during the first two years of construction and operation, 

as the impoundment is being developed, and during the last two years of the facility’s life, when it is 

being reclaimed. Short- and long-term design criteria are discussed in Chapter 9, as part of the hydrologic and hydraulic engineering analyses, and are primarily a function of the hazard potential and 

the size of the planned impoundment. For example, during the initial year of construction and operation of a large, high-hazard-potential impoundment, the capacity to handle the 100-year storm may 

be needed; during the second year the 1⁄2 Probable Maximum Flood (1⁄2 PMF) event; and within two 

years the full PMF event. Short-term criteria are intended for unavoidable construction conditions 

during initial start-up and abandonment and should only be utilized when absolutely necessary.

Impoundments are typically designed to: (1) store the runoff from the design storm with slow release 

of the runoff through a decant pipe system, (2) release the design storm runoff without storage 

through an open-channel spillway, or (3) store a portion of the design storm runoff and release the 

balance through an open-channel spillway. Figure 5.4 illustrates the concepts of 100-percent storage of the design storm runoff and the use of an open-channel spillway to route the design storm 

runoff. An impoundment that is designed to store the design storm runoff with release through a 

decant (Figure 5.4a) requires greater surcharge storage capacity than an impoundment that employs 

an open-channel spillway (Figure 5.4b). The determination of the magnitude of the design storm, 

including the short- and long-term criteria upon which the design storm is based and associated 

runoff rates and volumes, is discussed in Chapter 9.

Drainage channels at other locations at impoundment facilities, such as the downstream embankment face, are typically designed for the 100-year storm, and these structures do not affect the overall 

configuration, geometry and staging of embankment construction. However, if impoundment decant 

systems discharge to embankment drainage channels, the discharge should be added to the surface 

flow collected by the drainage channels. If the drainage channels intercept a substantial amount of 

runoff, the design storm criteria for the impoundment may need to be applied to the channel design 

in order to protect the embankment from erosion during a severe flood.

5.2.3 Slurry Cell Facilities

Slurry cell systems are attractive in situations where they can be kept under the size and hazard 

thresholds of 30 CFR § 77.216, or where they can be classified as having low hazard potential, 

so that design storm management can be based on the 100-year event and not the PMF. A lowhazard-potential determination requires information to substantiate that, in the event of a failure, 

the facility will not release water, sediment, or flowable fine refuse that would cause loss of life 

or significant property damage, as discussed in Chapter 3. Thus, the sequence of active and open 
5-10

Chapter 5

MAY 2009

cell operation must be designed to minimize impounding capacity. Also, the closed and covered 

slurry cells must be oriented and dewatered such that they do not collectively represent a potential 

release of flowable material that would severely impact downstream development.

By achieving a low-hazard-potential classification or avoiding classification as a regulated impoundment, slurry cell systems can be designed for lesser design storm criteria (i.e., 100-year storm). Because 

the watershed area contributing to the slurry cells is minimal, only a small quantity of runoff has to be 

managed as part of the slurry disposal operation. While this reduces the influence of the design storm 

runoff on the overall configuration and staging of the slurry cell system, runoff from adjacent areas, particularly upstream watershed areas, must be diverted, which requires construction and maintenance of 

diversion channels that are incorporated into the embankment or disposal site configuration.

Drainage structures for slurry cells must have the capacity to control runoff from the design storm 

collecting within the cell, along with clarified water produced as the slurry is deposited. This drainage may be achieved by using a decant pipe with an appropriately sized inlet or riser to facilitate 

settling of the fine refuse. An open-channel spillway may also be needed depending on the slurry cell 

arrangement and contributing drainage area.

DECANT

INLET DESIGN STORM FREEBOARD

NORMAL POOL

ELEVATION

NORMAL

FREEBOARD

SURCHARGE STORAGE

SETTLED FINE

REFUSE

EARTH OR

COARSE REFUSE

MINIMUM POOL LEVEL

CONTROLLED BY PUMPING

REFUSE

EMBANKMENT

5.4a IMPOUNDMENT DESIGNED FOR 100-PERCENT STORAGE OF DESIGN STORM

DECANT PIPE

(DIAPHRAGM NOT SHOWN)

5.4b IMPOUNDMENT DESIGNED WITH OPEN-CHANNEL SPILLWAY

DECANT

INLET DESIGN STORM FREEBOARD

SURCHARGE STORAGE

EARTH OR

COARSE REFUSE

MINIMUM POOL LEVEL

CONTROLLED BY PUMPING

REFUSE

EMBANKMENT

PROJECTED INVERT OF

OPEN-CHANNEL SPILLWAY

DECANT PIPE

(DIAPHRAGM NOT SHOWN)

NOTE: SLOPES ARE EXAGGERATED FOR ILLUSTRATION PURPOSES.

SETTLED FINE

REFUSE

FIGURE 5.4 ILLUSTRATION OF IMPOUNDMENT DESIGN STORM CONTROL
5-11

Coal Refuse Disposal Facility Design Components

MAY 2009

As with other site drainage not related to impoundments, the channels and ditches on the downstream face of a slurry cell embankment must be designed for the 100-year storm and do not affect the 

overall configuration or staging. If a slurry cell facility is designed as a significant- or high-hazardpotential impoundment, the associated design storm criteria may need to be used also for the design 

of embankment drainage channels that receive decant discharge.

5.2.4 Refuse Embankment Design Storm

The design storm for non-impounding refuse embankments is primarily used for sizing channels and 

ditches that divert or collect and convey drainage from the embankment, as part of the erosion and 

sedimentation control for the site. Diversion ditches are typically constructed around the perimeter 

of refuse embankments so that channel runoff from undisturbed portions of the site is conducted 

around the disturbed areas to sedimentation ponds. If a diversion ditch must be relocated to a higher 

elevation as an embankment is expanded, a plan of ditch relocation that provides for drainage diversion from disturbed areas during the associated construction must be prepared.

Permanent channels and diversion ditches should be designed for the peak discharge from the 100-

year storm. Temporary channels may be designed for a lesser magnitude event, as discussed in 

Chapter 9. Long-term drainage control required during reclamation and abandonment should be 

incorporated into the drainage channel design.

5.3 FACILITY CONFIGURATION AND GEOMETRY

The configuration and geometry of a refuse disposal facility is a function of staging/construction 

methods, capacity requirements, engineering properties of the refuse, initial site topography, storm 

water management, regulatory criteria and overall facility stability requirements. Operational factors, such as refuse transport and handling equipment also influence the design and overall configuration of the facility.

5.3.1 Regulatory Criteria

Criteria have been established by federal (Office of Surface Mining) and state agencies that address 

some parameters for design of coal refuse disposal facilities. These parameters include embankment 

slope, which should not be steeper than 2:1 (horizontal to vertical) between benches. Benches are 

used to control erosion, retain soil moisture, or facilitate reclamation and are typically integrated into 

embankment slopes.

For slurry impoundments that are classified as dams by state agencies, regulatory criteria for the 

width of the crest and maximum slopes should be reviewed. Other regulatory criteria may affect the 

design configuration and geometry, as a result of required right-of-ways, buffer zones, and related 

off-sets from natural features, structures, utility lines and property lines. State and federal criteria 

may change with time.

Federal and state regulatory agencies require engineering analyses for demonstrating that certain 

design criteria are satisfied, such as the ability of the facility to safely route the design storm or to provide a specified factor of safety relative to stability. State criteria are sometimes different from federal 

criteria. Both should be considered, and the most stringent criteria should be applied.

5.3.2 Embankment Design Considerations

The following subsections provide a discussion of issues related to embankment design.

5.3.2.1 Stability under Normal Operating Conditions

Stability analyses are performed to determine the embankment configuration and slope. The parameters associated with these analyses include the following:
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• Embankment material strengths – These strengths are dependent upon the types of 

refuse, borrow materials, and amended/co-disposed materials used in embankment 

construction, as well as the method of placement and compaction. Sufficient exploration and testing must be performed such that all applicable material properties can 

be accurately characterized.

• Foundation material strengths (as determined by exploration and testing) – These 

may be improved if necessary by removal and replacement or by ground improvement measures. Sufficient exploration and testing must be performed such that insitu material properties can be adequately characterized.

• Seepage and phreatic level – These are a function of normal pool level, internal 

drainage systems and liner systems.

Non-impounding coal refuse disposal facilities are normally developed over several years, and, as 

with impounding facilities, the configuration may vary considerably during various stages of the 

facility’s life due to upstream and downstream development. Stability analyses should be conducted 

at various points or critical stages in the embankment development. A detailed evaluation of the 

facility staging and construction should be performed to identify critical configurations that require 

stability analysis. Chapter 6 presents guidance related to stability analyses and other geotechnical 

engineering issues.

5.3.2.2 Stability under Extreme Events

Embankment stability under design storm and earthquake loading conditions (assumed to not occur 

simultaneously) must be evaluated. For design storm conditions, the embankment stability and configuration may be affected by the following:

• Flood level (peak pool level resulting from the design storm event) at an impoundment, which may affect the embankment phreatic surface and seepage conditions, 

depending on the depth and duration of the elevated reservoir level and the effectiveness of internal drainage measures.

• The location and elevation of decant and spillway structures.

Seismic analyses are performed on impounding embankments and sensitive embankments such 

as slurry cell facilities. Guidance for these analyses is presented in Chapter 7 and includes evaluation of strength loss or degradation of embankment or foundation materials as a result of seismic loading and evaluation of seismic deformations that may affect embankment performance. 

Design features that may be considered for mitigating the adverse effects of earthquake loading 

and associated embankment response include providing extra freeboard, enhancing internal 

drainage and related consolidation of the fine refuse, widening the embankment, and adding a 

buttress.

5.3.2.3 Settlement and Subsidence

Settlement of foundation soils or refuse materials, particularly if upstream construction is employed 

for an impounding embankment, may require placement of additional material to maintain crest 

elevations and grades of drainage structures. Large differential settlements may represent a risk 

of cracking of strain-sensitive materials (e.g., clay zones) or barriers, thus requiring measures to 

limit differential settlement. Chapter 6 provides guidance for settlement and other geotechnical 

analyses.

Subsidence associated with underground mines may affect the design configuration of the embankment and is discussed further in Section 5.4.
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5.3.2.4 Environmental Issues, Impoundment Elimination and Site Reclamation

Amendments to refuse or the presence of a liner system beneath an embankment or zoned embankment (for an impoundment facility) can affect the stability of the embankment, requiring modifications to configuration or slope. Reclamation, post-mining land use, and abandonment practices can 

also affect the embankment configuration and slope.

5.3.3 Impoundment Design Considerations

The impoundment configuration can also be affected by a range of design considerations, as described 

in the following paragraphs.

5.3.3.1 Settling and Clarification

Facilities such as polishing ponds, baffles, clear water cells, and similar structures may be required in 

order to produce water quality that is acceptable for discharge or preparation plant use. Clarification 

may also be accomplished in ponds downstream of the impoundment facility.

5.3.3.2 Seepage Control and Containment

To mitigate environmental impacts, seepage control and liner systems may be necessary for containment of the fine coal refuse.

5.3.3.3 Spillway and Decant Systems

Decant systems often serve as primary spillways and, if the watershed and impoundment sizes are 

compatible, may be sufficient to manage the design storm without an open-channel spillway. This 

will require that significant freeboard be maintained under normal operating conditions. In some 

instances, including during the final years of operation of a facility, open-channel spillways sized 

to handle the design storm are constructed, thus allowing more of the impoundment to be used for 

refuse disposal.

5.3.3.4 Subsidence and Breakthrough Potential

Subsidence analyses may identify potential impacts to an impoundment area, requiring changes in 

layout or other design features that affect the site configuration. Where mine workings are in proximity to an impoundment, evaluation of breakthrough potential may indicate that measures such as 

constructed barriers (embankment fills) are needed to buffer the zone of impact and maintain internal stability. These and other measures are addressed in Chapter 8.

5.4 FOUNDATION AND MINE SUBSIDENCE CONSIDERATIONS

5.4.1 Foundation Materials

Foundation materials must be evaluated relative to their strength, compressibility and hydraulic 

conductivity. Foundation materials without adequate strength, or with high compressibility, may not 

provide adequate support for the disposal facility, leading to detrimental movement or actual failure. 

Hydraulic conductivity is an issue primarily with impounding embankments, where high-hydraulicconductivity foundation materials can lead to elevated pore pressures affecting local stability and 

creating the potential for internal erosion or piping of fine soil particles that can threaten the overall 

stability of the disposal facility. High-hydraulic-conductivity foundation materials can also lead to 

unacceptable loss of clarification water or, in extreme situations, release of impounded materials.

Strength and compressibility are typically concerns for foundation soils, while hydraulic conductivity may be a concern for both soil and bedrock conditions. Bedrock strength conditions can 

be an issue where weak layers, such as claystones, underlie an embankment and behave similarly 

to a soil (primarily where shallow or surface outcrop conditions occur). Weak or fractured bed-
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rock conditions may also become an issue where underground mine workings are in proximity 

to an embankment or impoundment such that subsidence could cause differential movement and 

impose strain on structural components or drainage features of the disposal facility. An additional concern, as discussed in Section 5.4.3, can be actual breakthrough of the disposal facility 

into mine workings.

A variety of measures to address foundation material concerns are available. These may include 

accommodations in the disposal embankment and impoundment design or in-situ foundation 

treatments ranging from removal and replacement of shallow, weak soils to grouting of fractured 

bedrock conditions. Seepage control within foundation materials typically is accomplished by constructing cutoff trenches through high-hydraulic-conductivity natural soils and weathered bedrock and backfilling with low-hydraulic-conductivity soils. Cutoff trenches may disclose geologic 

features such as alluvial deposits, sand dikes, and relief fractures that require treatment and that 

would not be revealed by exploration borings.

5.4.2 Abutment and Foundation Geometry

The geometry of the abutments and foundation (steep slopes and bedrock exposures) of an impounding embankment can lead to differential settlements resulting in cracking of embankment materials, 

disruption of drainage control features, and seepage concerns. This situation may be exacerbated at 

former surface mine sites where embankment construction is planned in areas of former highwalls. 

For non-impounding embankments, such concerns may also need to be addressed in the design of 

drainage control systems.

Measures to address steep foundation geometry may include benching and cutting back of slopes, 

zoning with select structural materials, and planned compensation fills in the embankment design to 

offset cumulative settlements. These issues are addressed in Chapter 8.

5.4.3 Mine Subsidence Potential

The presence of underground mines or the potential for future underground mining in the vicinity of 

a coal refuse disposal facility may influence the location and design of facility features. Additionally, 

auger or highwall mining can lead to subsidence and seepage concerns. Location of an impoundment 

facility in the vicinity of mine workings is of concern because of the potential impacts of subsidence 

on embankment stability and the related potential for release of impounded materials, as well as the 

risk of a breakthrough into the mine, which is discussed in Section 5.4.4. Non-impounding facility 

design may also be influenced by potential mine subsidence, particularly related to impacts on structural elements and drainage and liner systems.

Depending on the depth to mine workings and proximity to the disposal facility, subsidence analyses may be needed for determination of the potential magnitude, tilt, curvature, and strain associated with ground movement related to subsidence. These analyses provide a basis for evaluation 

of the subsidence impact on the disposal facility and can help to identify potential mitigation measures. The engineering methodology for performing these analyses is presented in Chapter 8, and 

the potential impact of differential movement on coal refuse materials and structures is discussed 

in Chapters 6 and 8.

Similar analyses should be performed for proposed mining in the vicinity of disposal facilities. 

Only under very favorable conditions and where development is essential for haulage or ventilation 

should mining be performed under impounding embankments. The extent of mining that may be 

conducted under an impoundment is dependent upon the overburden and mining conditions. It is 

current practice that subsidence analyses are performed for longwall mines in order to establish a 

“safety zone” where extraction is precluded for protection of an impounding embankment.
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Subsidence impacts associated with drainage control systems (liners, internal drains, and surface 

drainage systems) may be addressed through evaluation of potential differential movement and 

strain during design. When mine workings are present beneath or in the vicinity of an impoundment, the following design measures may be employed to mitigate potential subsidence effects on 

embankments:

• Maintenance of ample freeboard to compensate for the anticipated subsidence.

• Broad embankment configuration design that is less sensitive to subsidence impact 

or enhanced zoning of embankments with self-healing characteristics (e.g., cohesionless sand) to mitigate potential cracking and internal erosion.

• Allowance for potential subsidence effects in the design of internal drainage structures or implementation of enhanced seepage control measures (barriers and chimney drains) to minimize the effects of potential subsidence.

• Backfilling or grouting of mine entries in critical support areas to minimize the 

amount of movement that can occur.

• Installation of monitoring systems for detecting subsidence and associated impacts 

such as increased seepage and preparation of contingency plans for mitigating such 

impacts.

To address the potential for subsidence and seepage in areas of auger or highwall mining, backfilling 

and/or protection berms or embankments may be employed, as discussed in Chapter 8.

5.4.4 Impoundment Breakthrough Potential into Mine Workings

In addition to subsidence concerns, if the overburden separating an impoundment from existing 

mine workings is relatively thin, there is a potential for sinkholes or even catastrophic breakthrough 

of the impoundment into the mine workings. In general, areas where the cover over a mine entry 

comprises less than 100 feet of intact rock strata are a concern for sinkhole development. Even greater 

amounts of cover are cause for concern in some situations. In addition to possible sinkhole development, shearing or punching of a coal outcrop is another potential failure mechanism that must 

be prevented. Without proper design, failure can occur through the coal seam itself, through strata 

above the coal, or along material interfaces or discontinuities. Internal erosion along these interfaces 

and discontinuities must also be prevented.

The presence of auger or highwall mining workings adjacent to an impoundment can also be cause 

for concern. The potential for subsidence should be evaluated in design, and measures to control 

seepage and to address the potential for breakthrough into neighboring underground mine workings 

should be developed, as needed. Figure 5.5 illustrates the presence of mine workings immediately 

adjacent to and beneath an impoundment and the use of a berm or barrier and internal drainage 

system to mitigate potential seepage and breakthrough potential.

Chapter 8 presents guidance for analyzing breakthrough potential and preventing breakthroughs. 

Some basic design measures that may be useful for mitigation of the potential for breakthroughs 

include:

• Development of a safety zone (based on verifiable data related to mining and outcrop or overburden conditions) that provides sufficient undisturbed ground between 

the mine and the impoundment so that the ground movement induced by mining 

will not affect the impoundment.

• Development of measures to provide adequate support for the impoundment considering potential failure mechanisms such as some combination of: (1) an engi-
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neered barrier (soil or synthetic barrier) or berm (coarse refuse) to mitigate the 

failure mechanisms, (2) an internal drainage system to control seepage and to reduce 

pressures in the areas of potential breakthrough, or (3) stabilization of the fine coal 

refuse to increase strength and reduce flowability. Any mitigation measure employed 

must also be resistant to subsidence.

• Development of containment or diversion structures, such as bulkheads, within the 

affected mine workings.

• Stabilization of the mine workings that may affect the impoundment by mine filling 

and/or grouting to improve strength and resistance to failure mechanisms.

As an alternative to the berm illustrated in Figure 5.5, slurry cell configurations have been employed 

at some impoundment sites with breakthrough potential, or the operation has been converted to fine 

coal refuse paste in order to restrict the potential release of flowable material.

5.5 UNDERGROUND MINING AT SLURRY IMPOUNDMENT

HIGHWALL

PERIMETER BERM

OR BARRIER

COAL SEAM

DRAIN AND

FILTER SYSTEM

AUGER OR

MINE WORKINGS

MINE WORKINGS

IMPOUNDMENT

POOL

SETTLED

FINE REFUSE

NOTE: SLOPES ARE EXAGGERATED FOR ILLUSTRATION PURPOSES.

5.4.5 Mine Entry Barriers and Bulkheads

Where mining is occurring in the vicinity of a coal refuse disposal facility, barriers and bulkheads 

may need to be constructed for the purpose of controlling drainage and mitigating the potential 

for internal erosion and piping in the embankment. Mine entries may include mine openings, 

ventilation boreholes, auger holes, or highwall mining areas. The design requirements for barriers and bulkheads are dependent on the type of disposal facility, location of the entry, and presence of drainage entering into or discharging from the mine. For support of non-impounding 

embankments, only a barrier may be required, particularly if drainage conditions are not an 

issue. Entries beneath impounding embankments or beneath impounded water will probably 

require installation of a bulkhead and/or barrier along with additional measures for addressing 

drainage at the entry location or within the mine. As an additional line of defense, bulkheads 

should be considered at critical discharge locations. Chapter 8 provides guidance for the design 

of barriers and bulkheads.

5.5 INTERNAL DRAINAGE AND EMBANKMENT SEEPAGE CONTROL

Internal drainage and seepage control features are designed to accomplish: (1) collection of springs 

and hillside subsurface drainage and conveyance to a point downstream of the refuse embankment 

(and sedimentation ponds), (2) environmental containment using liner systems to control seepage to 

subsurface groundwater beneath the site, (3) collection of seepage and conveyance downstream to 

FIGURE 5.5 UNDERGROUND MINING AT SLURRY IMPOUNDMENT
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treatment or sedimentation ponds, (4) control of the phreatic surface or foundation pore pressures 

to maintain embankment stability, and (5) prevention of seepage along penetrations (decants, etc.) 

through the embankment. Reclamation may also involve construction of a low-hydraulic-conductivity 

cap to limit infiltration and reduce internal drainage requirements.

Technical guidance for internal drainage and embankment seepage control is provided in Chapter 6 

and covers the following:

• Seepage analyses for embankments (homogeneous and zoned), including analysis of 

the effects of foundation cutoffs, liners, and internal drains.

• Filter criteria for zoned embankments.

• Design of internal drains, including filters.

The environmental aspects of reclamation, internal drainage and seepage control are addressed in 

Chapter 10.

5.6 DECANT AND EMERGENCY SPILLWAY SYSTEMS

Hydraulic structures for controlling impoundment levels and providing storm routing are designed 

based on design storm runoff management, as discussed in Section 5.2, with specific features influenced by the type of embankment staging employed (upstream versus downstream) and configuration (height and length of the disposal facility), site terrain, and foundation issues.

5.6.1 Decant Performance Considerations

Decant systems provide for discharge of clarified water from an impoundment. While a decant system 

may be a component of the design-storm water management system, in many cases the impoundment level may actually be controlled by pumping when the water surface level is below the decant 

invert. In cases where a decant system is part of the design storm management system, it is typically 

designed to provide sufficient capacity to evacuate at least 90 percent of the design storm runoff 

retained within the impoundment within 10 days, assuming that no design-storm runoff storage is 

available below the lowest ungated decant inlet. The 10-day period is measured from the time of peak 

impoundment pool during the design storm.

The design of decant systems involves the following considerations:

• Hydraulic capacity evaluation (head versus discharge analysis).

• Material selection, as affected by the length and depth of cover, foundation conditions, installation method, and other factors such as strength, flexibility, durability, 

corrosion protection, and joint type.

• Inlet design features, including riser pipe structural stability and support, trash 

guards, anti-vortex devices, and sealing requirements upon discontinuation of use.

• Alignment and grade, including variation in grade due to terrain, potential foundation 

settlement, the need for reaction blocks, resistance to flotation, and venting systems.

• Outfall design, energy dissipation requirements, and erosion protection.

• Pipe backfill, drainage diaphragms and seepage control.

• Monitoring/testing requirements.

If decant systems are designed to discharge to a groin ditch adjacent to the downstream embankment slope, additional design considerations may apply. In such instances, the peak decant discharge 

should be added to the surface flow collected by the groin ditch. If the ditch intercepts a substantial 
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amount of runoff, the design storm criteria for the impoundment may need to be applied to the ditch 

design in order to protect the embankment from erosion during a severe flood.

5.6.2 Open-Channel Spillway Performance Considerations

Many impoundments are designed such that the reservoir storage capacity and decant system discharge capacity provide for design storm runoff management, but sometimes a separate open-channel spillway is also required. These should be designed with a capacity that will allow routing of the 

design storm flow through the impoundment while maintaining adequate freeboard. Open-channel 

spillways are less commonly employed at refuse disposal embankments because of staged development. The following should be considered in the design of open-channel spillways:

• Hydraulic capacity (head versus discharge capacity) and minimum freeboard for the 

approach channel, control section, and discharge channel or conduit so that sufficient capacity is available for releases up to the design peak outflow. Measures to control floating debris 

or hillside trees that could cause obstructions may be required at the approach channel.

• Channel geometry, including considerations for transition sections (e.g., contraction 

section), changes in alignment (e.g., super-elevation at bends), and grade (e.g., sufficient capacity for hydraulic jumps).

• Stable channel conditions, considering excavation slopes as well as potential for erosion due to water flow associated with velocity/tractive force/duration on the channel lining. Channel linings require suitable foundation and drainage systems and 

should be designed for tractive and uplift forces.

• Energy dissipation structures at the spillway outlet.

If large-diameter conduits are used in conjunction with open channels or in place of an open channel, 

material selection, inlet design, alignment and grade, backfill, seepage control, and outlet design issues 

are similar to those for decant systems.

5.7 SURFACE DRAINAGE CONTROLS

Surface drainage controls at coal refuse disposal embankments typically consist of drainage ditches 

and channels, bench and haul road gutters, and culverts that collect and convey runoff to downstream structures.

5.7.1 Permanent Drainage Controls

Permanent drainage controls are structures that will be in service during operation of the disposal 

facility and following reclamation and abandonment of the facility. These structures should be 

designed for the 100-year-recurrence-interval storm. Typically, the 100-year, 24-hour-duration storm 

is used for design, consistent with state regulatory criteria.

The design of permanent drainage controls should be based upon the following considerations:

• Hydraulic capacity considering the peak discharge rate from the design storm for the 

contributing drainage area.

• Channel geometry, including accommodation for transition sections and changes 

in alignment (e.g., super-elevation at bends) and grade (e.g., additional capacity for 

hydraulic jumps in subcritical flow sections).

• Stable channel conditions, considering flow velocity/tractive force/uplift/duration for 

the channel lining, if the channel is not excavated in competent rock.

• Energy dissipation structures at channel outfalls.
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5.7.2 Temporary Drainage Controls

Temporary drainage channels function periodically during operation, but are not part of the longterm surface water controls related to reclamation and abandonment. These may include temporary 

ditches for diversion or runoff collection within the disposal facility, bench and haul road gutters 

(before topsoil placement and reclamation), and most culverts. Temporary ditches and culverts may 

be designed with a hydraulic capacity lower than for permanent drainage controls, because of shorter 

service life and other criteria. For instance, state regulations or recommendations for some drainage structures may specify a 10- or 25-year-recurrence-interval storm. Temporary ditches may be 

designed without linings provided that: (1) their service life is relatively short and potential erosion 

damage can be readily repaired and (2) lack of linings will not cause adverse impacts to the safety of 

the disposal facility or compromise downstream sedimentation controls.

5.8 INSTRUMENTATION AND PERFORMANCE MONITORING

Instrumentation is typically a component of coal refuse disposal plans and is recommended for all 

high or significant-hazard-potential sites. Instrumentation records should be reviewed as they are 

accumulated. The data are best evaluated by maintaining a continuous plot of the readings versus 

time. The data should be reviewed as part of annual inspections and certifications and should be 

maintained for the life of the facility. The data review provides a basis for: (1) assessment of facility 

performance relative to design intentions, (2) detection of trends and problems that may develop, 

and (3) operational plan modification or facility expansion. Design plans should indicate maximum 

acceptable levels for instrument readings or percentage changes that trigger further investigation or 

actions. Facility performance will typically be enhanced by monitoring the parameters discussed in 

the following paragraphs with appropriate instrumentation:

5.8.1 Seepage

Seepage from the impoundment should be monitored for flow rate and changes in appearance 

(discoloration or appearance of fine particulates and precipitates). This monitoring should include 

seepage through the embankment, through internal drainage structures, and through underground 

mines that receive seepage from the impoundment. Weirs should be installed, preferably with a 

staff gauge in the weir approach pool, so that flow rates can be easily and accurately measured. To 

evaluate changes in seepage rates, it is important to know the impoundment pool level and to have 

data pertaining to rainfall and groundwater levels, so that possible correlations can be evaluated.

5.8.2 Piezometric Levels

Saturation levels and water pressures within an impounding embankment or embankment foundation, as well as within any earthen barriers, should be monitored and recorded to determine 

whether hydrostatic pressures are within design limits and whether changes or trends are reasonable. Selection of the type of piezometer and installation location is based on site-specific requirements and the potential for rapid or sudden changes in pore water pressure (as may occur upstream 

of construction areas). Open standpipe piezometers provide direct measurement of groundwater 

levels, while vibrating-wire and pneumatic piezometers allow for monitoring of phreatic conditions 

in fine-grained deposits where rapid changes in pore pressure may be important. A table indicating 

maximum allowable readings for piezometers should be included with the design report.

5.8.3 Pool Levels

Records for the pool level in the impoundment should be maintained for freeboard monitoring and 

for determining correlations between piezometric levels and seepage quantities. Pool level can be 

monitored by installation and reading of staff gauges. When water in nearby mine workings has the 

potential to affect an impoundment or may indicate the performance of a barrier, the mine pool level 

should be monitored.
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5.8.4 Rainfall Data

A rain gauge installed near the disposal site can provide site-specific precipitation data for correlations with piezometric levels and seepage quantities, and these data may be essential in situations 

where there is breakthrough potential and where discharges from a mine are related to seepage from 

an impoundment. Rainfall data should be routinely collected and recorded so that changes in seepage, mine discharge, or water level data can be correlated to rainfall infiltration/runoff.

5.8.5 Deformation or Movement

Where significant embankment settlement can occur or there is a potential for subsidence in the vicinity of an impoundment, movements should be monitored and recorded. Monitoring of the slopes and 

crest of an impounding embankment should be conducted at regular intervals during both operating and dormant phases for evaluating performance and for demonstrating conformance with flood 

routing assumptions. When subsidence is a concern, both horizontal and vertical movements should 

be measured. Movements should also be monitored if evidence of slope displacement is detected. In 

situations where deformation is occurring, the rate of movement, and especially any acceleration of 

the rate, provides valuable information for assessing its significance. Methods for monitoring surface 

deformation include survey monuments and extensometers. Movements below the ground surface 

can be monitored with inclinometers and extensometers. These types of instruments are discussed in 

more detail in Chapter 13.

5.9 RECLAMATION, ABANDONMENT AND POST-MINING LAND USE

A coal refuse disposal plan should address reclamation, abandonment and post-mining land use 

requirements.

General provisions for and plans related to abandonment of a coal refuse embankment are part of 

the final operational stage of the disposal facility and should address elimination of the impoundment, unless the impoundment is a component of planned post-mining land use. Reclamation should 

incorporate the following provisions:

• Stockpiles of soil and topsoil for reclamation should be located near the facility on 

stable ground and within sedimentation controls.

• Reclamation materials should meet the growth medium and nutrient requirements 

of the vegetation plan. Topsoil amendments and alternatives may be necessary.

• To protect against erosion and sedimentation and potentially negative environmental impacts, surface drainage and infiltration should be controlled.

Elimination of impoundments should address the following:

• Regrading of the impounding embankment and backfilling of the impoundment 

should be performed in a manner such that proper surface drainage is established 

and the fine coal refuse is stabilized. Final backfill elevations should facilitate drainage and accommodate settlement of the fine refuse that will occur over time.

• During the final periods of disposal and progressive elimination of the impoundment capacity, the outlet works such as the decant structure or spillway should 

remain operational until impoundment regrading is complete.

• Decant systems should generally be sealed by grouting.

Post-mining land use will affect the reclamation plan, particularly if existing structures such as the 

impoundment or ponds are to be retained.
Chapter 6

GEOTECHNICAL EXPLORATION,

MATERIAL TESTING, ENGINEERING

ANALYSIS AND DESIGN

MAY 2009 6-1

This chapter addresses important aspects of geotechnical exploration, material testing, engineering 

analysis and design for coal refuse disposal embankments and impoundments with consideration 

of past, current and future mining practices; characteristics of foundation, coal refuse, and soil and 

rock borrow materials; and procedures for material placement and facility construction. Development of a subsurface exploration program, implementation of a field and laboratory testing program, and selection of geotechnical parameters are key elements in the design of safe facilities for 

coal refuse disposal. The basic design considerations that must be evaluated for both embankments 

and foundations are seepage, slope stability and settlement. Another important geotechnical consideration is the analysis of soil-structure interaction for buried pipes (conduits or decant pipes) 

that are installed within an embankment. Specification, field control and verification of geotechnical properties are essential to the construction of coal refuse embankments that are consistent with 

design assumptions.

From a geotechnical perspective, the following steps are normally followed in the design and construction of a new or modification of an existing refuse embankment:

• Review of available information

• Site selection and optimization

• Field exploration and in-situ sampling and testing

• Laboratory testing

• Development of geotechnical design parameters

• Analyses and design

• Preparation of plans and specifications

• Construction monitoring (quality control and verification of field conditions)

• Instrumentation

• Embankment and system component performance monitoring

• Maintenance

The level of effort and technical scrutiny required during the above steps will vary depending upon 

the refuse facility intended use, size and hazard-potential classification.
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This chapter describes the scope of geotechnical investigations recommended for support of analyses 

and design of a refuse disposal facility. References for additional information are provided herein. 

Based upon the technical guidance provided in this chapter and supplemental information available 

in the cited references, an experienced geotechnical engineer familiar with the refuse disposal process 

and the design of water-retention embankment structures should be able to design an economical, 

safe and environmentally acceptable coal refuse disposal facility. Designers should recognize that 

investigation programs and studies for specific projects can not realistically be standardized and will 

vary according to site conditions, material properties, embankment geometry, hazard classification 

and proposed staging scheme.

In addition to refuse disposal embankments, this chapter is also applicable to other types of embankments that are employed at mine sites including fresh water dams and sedimentation or treatment 

ponds. Other applications and available design guidance are presented in Section 6.3.6.

In the text that follows, many references are made to ASTM International (ASTM) standards. All 

references to ASTM standards in this chapter can be found in three volumes of Section Four – Construction of the ASTM standards, which are published annually. The most current versions of the 

standards should be used. The applicable volumes and their citations herein are:

• Volume 05.06 – Gaseous Fuels; Coal and Coke (ASTM, 2008a)

• Volume 04.08 – Soil and Rock (I): D 420 – D 5786 (ASTM, 2008b)

• Volume 04.09 – Soil and Rock (II): D 5877 – latest (ASTM, 2008c)

• Volume 04.13 – Geosynthetics (ASTM, 2008d)

Full citations for these volumes are provided in the References section at the end of this Manual.

6.1 GEOTECHNICAL DESIGN PROCEDURES

This section outlines the steps normally required for designing the geotechnical aspects of a coal 

refuse disposal facility. The geotechnical design of a new refuse disposal facility provides flexibilities 

that designers should recognize in their planning. These include:

• Ability to determine the optimum location for the facility

• Flexibility in the design of the starter dam and embankment staging

• Ability to coordinate ongoing and future mining with staging in the vicinity of 

refuse disposal footprints

• Flexibility in designing the internal drainage system and liner system (if required)

• Flexibility in the selection and design of hydraulic structures

Geotechnical design for an expansion of an existing disposal facility typically imposes constraints 

that designers must recognize in their planning, particularly limited flexibility in planning embankment staging and design of hydraulic structures while maintaining ongoing disposal operations. 

Table 6.1 presents guidance for the geotechnical design of refuse disposal facilities with reference to 

applicable sections of this Manual and to supplemental documents.

6.2 GENERAL CONSIDERATIONS

6.2.1 Unique Characteristics of Refuse Disposal Facilities

A typical coal refuse disposal facility has unique characteristics and objectives compared to most 

other engineered structures. Some of the basic characteristics of coal refuse disposal facilities and 

their related significance in geotechnical design are identified in Table 6.2.
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TABLE 6.1 TYPICAL GEOTECHNICAL DESIGN PROCEDURE FOR 

COAL REFUSE DISPOSAL FACILITIES(1)

 Design Considerations Manual Sections for

Reference

Supplemental 

References

I. Obtain and Review Available Information

Topographic Maps

Geologic Maps

Soils Maps

Aerial Photographs

Local Experience

Individual Site Mapping

Seismicity Maps

Mine Maps

6.4 USBR (1992a)

II. Plan Field Exploration

What is probable configuration?

What type of hydraulic structures are likely?

Where might important embankments and structures

 be located?

What are significant foundation characteristics?

What types of borrow material may be required?

What are probable sources of borrow material?

What types of sampling will be required?

What types of environmental control measures should

 be considered?

Is past or present mining in the area being considered?

What changes in the disposal program are likely during

 the life of the facility?

6.2, Chapter 3

Chapters 3, 5, 9

6.2, Chapter 3

6.2, 6.3, 6.4, 6.6

6.2, 6.3

6.2, 6.3

6.4, 6.5

6.3, Chapters 4, 10

6.3, Chapter 8

Chapters 4, 10

USBR (1992a)

Sherard et al. (1963)

III. Field Exploration

Surficial Reconnaissance

Geophysical Surveys

Borings and Sampling

Test Pits

Visual Classification

Field Testing

Soils and Water Inventory

6.4

Arman et al. (1997)

Hvorslev (1948)

Legget (1962)

USBR (1998, 1992a)

IV. Laboratory Program

Natural Materials

•	 Index Property Tests

• Compaction Tests

•	 Hydraulic Conductivity

•	 Consolidation

•	 Shear Strength

•	 Potential Acidity/Neutralization Potential

Refuse Materials:

•	 Index Property Tests

•	 Compaction

•	 Hydraulic Conductivity and Consolidation

•	 Shear Strength

•	 Leachate Quality

6.5

6.5

ASTM (2008b,c)

Lambe (1951)

Bishop and Henkel 

(1962)

USBR (1992a)

MSHA (2007)
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Chapter 6

MAY 2009

 Design Considerations Manual Sections for

Reference

Supplemental 

References

V. Design Considerations and Analyses

Foundation Preparation

Seepage Control

Static and Seismic Stability

Settlement

Rock Excavation

Chapters 6, 7, 8

USBR (1987a,1989)

Leonards (1962)

USACE (1993)

VI. Construction Operations

Refuse Transport and Placement

Foundation Preparation

Borrow Materials

Appurtenant Facility Construction

Materials Selection

Quality Control and Field Testing

Chapter 11

USBR (1987a,1989)

Church (1981)

USBR (1998)

Fell et al. (2005)

VII. Instrumentation and Monitoring

Visual Observations

Movements and Displacements

Pore-Water Pressures

Hydrology and Hydraulics

General Maintenance

Chapters 12, 13

USBR (1998)

Dunnicliff (1993)

USACE (1995c)

 Note: 1. This table is presented as a guide to qualified geotechnical designers. Each site must be evaluated

 according to conditions at that site. In some cases, studies beyond those identified in this table may

 be needed.

TABLE 6.1 TYPICAL GEOTECHNICAL DESIGN PROCEDURE FOR 

COAL REFUSE DISPOSAL FACILITIES(1)

(Continued)

6.2.2 Site Conditions

In contrast to site selection for a dam that must be built across a valley to form a reservoir of designated size, site selection for a coal refuse disposal facility is generally flexible because of the variety of 

embankment types that can be constructed. The principal considerations in the selection of a disposal 

facility site are typically: (1) the potential disposal capacity of the valley/site, (2) the desired preparation plant processing output, (3) the potential influence of previous mining activities, (4) refuse transportation and placement, (5) construction of site development and drainage structures, and (6) other 

environmental control and safety factors.

6.2.2.1 Topography

Site terrain slopes are very important in disposal facility site selection because of their impact on storage volume, methods and costs of materials handling, methods and costs of drainage control, and 

hazard potential.

In areas of rugged and steep terrain, such as southern West Virginia, southwest Virginia, and eastern 

Kentucky, most disposal facilities will be valley-type embankments. Valley slopes are often too steep 

for side-hill embankments. Ridge tops are generally difficult to access and are limited in area to support large embankments; however, some ridge-top sites can be used in conjunction with mountaintop surface mining operations. Major valley bottoms are too confining for the construction of large 
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heaped or diked embankments. Therefore, the typical disposal facility site is generally a small valley 

selected by considering:

• Potential effects of past or future mining beneath the site

• Proximity to existing preparation plants

• Surface land ownership

• Cost of establishing a materials-handling system suitable for the topography

• Ability to sequence construction to handle the types and volumes of refuse to be 

disposed

• Potential to discharge storm water through or around the site

• Stability of existing slopes when modified by construction or imposed loads

In areas of less severe topography, such as in southwestern Pennsylvania, the range of potential site 

location and embankment types is much greater. However, disposal facilities in these areas are frequently located in small valleys because of their capability to accommodate large volumes of refuse 

without extensive modification of natural topography. Also, the rolling topography often makes construction of ridge, side-hill or heaped embankments practical.

In relatively flat terrain, as found in portions of Illinois and Indiana, refuse disposal in valleys may 

not be practical, and the refuse disposal facility will generally be of the heaped (Figure 3.12) or dikedpond embankment type (Figure 3.13. These types of disposal facilities present unique problems for 

fine refuse disposal in slurry form because volume containment by utilizing the natural topography 

is not possible. When fine refuse slurry is a small percentage of the total refuse, the most economical 

disposal facility is a diked-pond embankment constructed from coarse refuse.

TABLE 6.2 BASIC CHARACTERISTICS OF COAL REFUSE DISPOSAL

AND DESIGN SIGNIFICANCE

Basic Characteristic Design Significance

The purpose is safe and economical disposal of 

refuse.

Greater flexibility in choosing location, configuration and 

sequence of placement.

The total disposal volume of coarse refuse is normally much greater than that required to serve safety 

requirements.

Embankment zones may allow different placement specifications (e.g., structural zone).

The refuse disposal occurs over many years and may 

lead to several unforeseen events and constraints not 

realized at the time of design.

Construction monitoring and quality control may be specified for the critical construction items with periodical monitoring of routine construction.

The geotechnical properties of refuse may not be 

available during the design (particularly for a new facility), and changes in the material properties are probable during the life of the facility.

For new facilities, geotechnical design parameters may 

be estimated based on experience and from facilities with 

similar characteristics (e.g., similar seam properties, mining 

technique, and cleaning process). The geotechnical design 

parameters can be verified when actual samples are available and can be re-evaluated if characteristics change.

The refuse being placed can have adverse chemical 

characteristics that may lead to undesirable environmental conditions or deterioration of construction 

materials.

Geotechnical and leachate characteristics of the refuse 

should be evaluated based on experience and/or by laboratory test, and appropriate amendment or containment/protection requirements should be identified.

After completion of disposal operations, the facility will 

need to be abandoned in a safe, economical and environmentally acceptable manner.

Planning and design must allow for an acceptable 

abandonment configuration with materials for effective 

reclamation.
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6.2.2.2 Climate/Weather

Due to the small surface area of coal refuse impoundments, wind, rainfall and temperature conditions are generally not major factors in selecting the most appropriate site for a refuse disposal facility 

for a given mining operation. However, the variation of weather conditions in different regions of the 

country can significantly affect disposal facility configuration and design requirements.

For example, in the Appalachian coal region, rainfall is relatively uniform and abundant throughout 

most of the year. Thus, addition of water to coal refuse for controlled placement in an embankment 

is seldom a major design and cost consideration. A more important criterion in this region may be 

to assure that work in valley bottoms, or in critical structural portions of the embankment, can be 

accomplished during the summer months when conditions are driest.

At the other extreme, such as in the semi-arid Rocky Mountains and Western plains, there is a general lack of precipitation for most of the year. In this situation, the designer must evaluate appropriate control measures and associated costs for adding water to refuse to achieve required placement 

criteria. Also, the precipitation that does occur is often in the form of high intensity thunderstorms, 

increasing the required capacity of flood control and diversion structures.

Clearly, the planning of otherwise similar disposal facilities will vary according to geographic location. Regardless of the site location, the size of the watershed draining into a disposal area should be 

minimized unless other design factors justify the cost of major drainage control systems.

Disposal facilities in regions with extended cold winters and significant snowfalls may require special attention to configuration, materials handling and refuse placement. For example, it may be that 

critical structural portions of an embankment can be constructed most economically during the construction season when drainage and material properties are most easily controlled. Overall efficiency 

is then accomplished by establishing areas for placement of refuse in non-critical areas during the 

remainder of the year.

Blowing dust is generally not considered to be a major design problem with coal refuse disposal. 

However, dust has been a problem with other types of industrial and mining waste disposal, particularly for disposal of fine-particle materials such as combustion waste.

6.2.2.3 Geology and Surficial Soils

Coal refuse disposal facility design is affected by geology and surficial soil conditions as they relate 

to the following:

• Extent and effects of past or future mining

• Necessary foundation treatment

• Available borrow material

• Type and size of the starter dam

• Effects of refuse disposal on groundwater quality and the effects of groundwater 

seepage on embankment design

• Stability and hydraulic conductivity of the existing foundation materials under natural and disturbed conditions

Specific design factors that may be affected by geology and surficial soil conditions include:

• Acceptable embankment slopes as related to the shear strength of the underlying 

foundation materials.
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

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• Limitations on the location and design of the impounding facility posed by active or 

inactive landslides.

• Seepage cutoffs through pervious foundation soils for embankments that impound 

water.

• Embankment construction to account for settlement of underlying soft foundation 

materials.

• Selection of the starter dam configuration and zoning.

• Starter dam and embankment construction rates to keep excess pore-water pressures 

to within acceptable limits.

• Seals between foundation rock and the impervious zone of an impounding 

embankment.

• Situations where badly fractured foundation rock must be grouted to control seepage from an impounding embankment.

• Liner and underdrain systems to address protection of groundwater or seepage into 

the embankment.

• Selection of decanting and other hydraulic structures to control impounded storm 

runoff.

• Erosion control measures for surface runoff and drainage structures

• Potential liquefaction of embankment and foundation materials in regions subject to 

moderate or high seismic loading.

The wide variations of geology and surficial soil conditions that may be present make generalized examples of these situations impossible. However, if mining has occurred beneath a disposal 

facility, it is particularly important to evaluate the effect of existing or potential subsidence on the 

embankment and the potential for the contents of the reservoir to break through into the mine. If it is 

determined that subsidence has occurred or that new or continued subsidence may occur, potential 

detrimental effects on the structural and hydraulic conductivity characteristics of the refuse embankment must be evaluated. The possible effects on groundwater quality due to leachate infiltrations into 

fractured foundation bedrock, or due to discharge of fine coal refuse into underground mine voids if 

the overburden collapses, should also be considered. If mining has not occurred at the planned refuse 

disposal facility site, construction of the disposal facility may limit the extraction of underlying coal 

reserves, and this cost factor should be considered in the economic evaluation.

Procedures for investigating and analyzing geology and surficial soil conditions are discussed in Section 6.4, and construction aspects of foundation preparation are discussed in Chapter 11. Designers 

must understand that an optimum disposal facility can only be achieved if general site conditions are 

well understood prior to the selection of the facility site and configuration. This knowledge will help 

to prevent unnecessarily conservative designs as well as designs that may be susceptible to increased 

maintenance or environmental problems.

6.2.2.4 Miscellaneous Site Considerations

Other considerations that may not be directly related to site conditions, but may influence the design 

and the volume capacity of the facility include:

• Access to public roads

• Availability of utilities

• Future mine developments 

• Mine infrastructure (e.g., conveyor belts, access roads and mine entries)
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Chapter 6

MAY 2009

6.2.3 Embankment Materials

The purpose of a coal refuse disposal impoundment is to provide a means for safe and economical 

disposal of coal refuse. However, due to safety and operational considerations, other materials are 

required for development of the facility. For example, since coarse coal refuse is generally susceptible 

to weathering, crushing and degradation, more resistant granular rock is normally needed for construction of drainage zones and for providing erosion protection. Also, at new facilities, disposal of 

fine coal refuse requires initial construction of a starter embankment or dam for slurry retention and 

settling when sufficient coarse refuse is unavailable during disposal facility startup. Starter dams are 

generally constructed with borrow material, unless suitable coarse coal refuse is available on site. 

The cost of imported borrow materials is much greater than the coarse refuse, so their use should be 

limited to addressing specific design requirements. Borrow materials can be obtained directly from 

mine spoils, from processing of mine spoils, or from a suitable area at or near the site. Embankment 

materials that must be purchased from a commercial quarry and transported to the site are the most 

costly alternative.

Other examples where site borrow or imported materials may be needed to supplement the refuse 

include: (1) fine-grained soils for creating an embankment impervious zone when the material available will not adequately limit seepage, (2) cover soils suitable for revegetation, and (3) soil and rock 

for supplementing embankment construction when coarse refuse is unavailable or inadequate for 

slurry retention and storm routing.

The most convenient and economical source for borrow material is typically the area that will eventually be covered by refuse. Use of this material will minimize transportation costs and will increase the 

capacity of the disposal facility, although it can lead to more extensive seepage control and groundwater protection requirements. Another economical source of borrow material is mine spoil with a matrix 

of soil and rock. To avoid unnecessary double handling and stockpiling and to assure that the required 

quantities of materials are excavated from the disposal area, careful planning is essential. This precaution is especially important when the borrow material will be used for final cover.

When selecting borrow materials from the disposal area, it is important that removal of the material 

will not be detrimental to the long-term performance of the facility. For example, if the natural soils 

form a desirable impermeable boundary between the refuse and underlying rock, borrow activities 

should be restricted to areas of thickest soil cover in order to leave a continuous layer of soil.

The required characteristics for materials used in the construction of refuse embankments are discussed in Sections 6.4 and 6.6; transport and placement procedures are discussed in Chapter 11. The 

following brief discussion of materials generally available for embankment construction is presented 

to aid initial planning and design.

6.2.3.1 Coarse Coal Refuse

Non-impounding embankments (“refuse piles”) are commonly constructed entirely with coarse coal 

refuse. Portions of a non-impounding embankment where supplemental borrow material may be 

needed include: (1) granular underdrain zones for collecting and discharging groundwater seepage 

away from the refuse, (2) granular zones for controlling seepage or collecting leachates, (3) cover 

material for promoting vegetation of the embankment surface, and (4) durable, weather-resistant 

rock for erosion protection in swales, ditches and channels.

Coarse refuse is typically the predominant material used to construct embankments for impounding 

fine coal refuse slurry. However, materials for impervious zones, filters, and drainage zones for these 

structures must have specific characteristics and normally must be obtained from suitable borrow 

areas or from commercial sources.
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A summary of published grain-size, specific-gravity, and strength-testing data for coarse coal 

refuse samples, including their geographic source, is presented in Table 6.3. While coarse coal 

refuse is generally a well-graded material, significant variation, particularly with respect to 

the clay-, silt- and fine-sand-size fraction, has been reported depending on geographic location (which generally relates to geologic conditions) or coal mining or preparation processes. 

Advances in coal mining and preparation processes over the last 15 years have resulted in a trend 

toward greater percentages of fines in coarse coal refuse. While recent published data (Hegazy et 

al., 2004) provide evidence of the increase in fines content of coarse refuse in the northern Appalachian area, resulting in classification as silty, clayey sand with gravel to clayey, silty sand with 

gravel, other regions typically exhibit lower fines content with corresponding classification as a 

well-graded to silty gravel. Reported strength data are consistent with soil and rock content and 

gradation.

TABLE 6.3 COARSE COAL REFUSE CHARACTERIZATION – SUMMARY 

OF AVERAGE/RANGE OF VALUES

Reference Location

Grain Size Specific 

Gravity

Gs

(gm/cm3)

Effective Shear Strength

D30

(mm)

D50

(mm)

D60

(mm)

Passing

No. 200 

Sieve 

(%)

φ′

(degrees)

c′

(psf)

Almes and 

Butail (1976)

PA, WV,

KY, VA 0.7 2.5 4.5 10 1.8-2.4 33-39 0

McCutcheon 

(1981) OH 1.9 4.5 7 7 2.0 36 NR

Saxena et al.

(1984) WV 12 16 22 2 2.6 27-40 0-450

Albuquerque

(1994) VA 3.5 7.5 12 1.5 NR(1) 39 0

Hegazy et al. 

(2004) PA 0.35 1.23 2.02 19.8 2.0 34 250

Busch et al. 

(1974) WV 0.2-4 1-10 3-20 2-19 1.7-2.3 NR NR

Backer et al. 

(1977) UT, NM 1-6 3-15 6-20 4-15 1.7-2.3 NR NR

Stewart and 

Atkins (1983)

Eastern PA

(anthracite) 1-8 5-16 7-22 1-7 2.2-2.4 NR NR

Zeng and 

Goble (2008)

Appalachian

region 2 6 9 12 2.5 NR NR

 Note: 1. NR = not reported

Compaction, equipment traffic and weathering cause degradation of coarse coal refuse. Larger pieces 

of shale will generally crumble to small particles after exposure to the atmosphere for only a short 

period of time. Thus, the percentage of fines in “aged” refuse can be noticeably greater than in fresh 

or recently placed coarse coal refuse. Based upon sieve analyses results for fresh and compacted 

samples, Hegazy et al. (2004) have reported an average increase of fines of about 4 percent due to 

compaction alone.
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Chapter 6

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6.2.3.2 Fine Coal Refuse

Fine coal refuse, when very wet or in slurry form, is not generally suitable for construction of the 

structural portion of an embankment. However, fine refuse can be used as the foundation for portions of an embankment when it has had sufficient time to settle and excess pore-water pressures 

have adequately dissipated. Designers are cautioned that embankments that have fine refuse as a 

foundation material require comprehensive evaluations and analyses of the following:

• Construction schedule

• Geotechnical properties and strength

• Settlement and seepage properties

• Placement procedures

• Measures for equipment operator safety

• Seismicity, dynamic properties and potential for associated strength loss

Fine coal refuse is the product of extracting, crushing, and cleaning raw coal. The fine coal refuse 

slurry is typically pumped upstream of an impounding embankment. The coarser material settles 

out more quickly nearer the discharge location (customarily near the upstream slope of the embankment), forming a fines delta or beach. The finer materials migrate throughout the impoundment, 

because they take longer to settle. Thus, samples collected from or near the delta are predominantly 

sand and silt-sized particles, whereas samples collected away from the delta are predominantly silt 

and clay-sized particles. A summary of published geotechnical data (average and range of values) 

for fine coal refuse samples, including their source location, is presented in Table 6.4. This table is 

based on samples collected from slurry impoundments and may reflect the effect of segregation 

that occurs with settling and deposition. Variations in grain size and plasticity occur due to rock 

strata, coal extraction and processing, and impoundment depositional characteristics, and thus 

properties may vary from those reported in Table 6.4. Site-specific testing has characterized fine 

refuse as plastic clay/silt, low plasticity sandy silt or clay, or low to non-plastic silty/clayey sand 

typically exhibiting a lower specific gravity (and dry density) and lower peak shear strength than 

coarse coal refuse.

The grain-size distribution of thickened and dewatered fine coal refuse can be anticipated to be 

similar to the averages provided in Table 6.4, as this material does not significantly segregate with 

placement.

6.2.3.3 Combined Refuse

Some coal preparation plants produce combined refuse that does not require impoundments for 

disposal of fine refuse. At these preparation plants, partially dewatered fine refuse filter cake is produced in addition to coarse refuse. These materials are normally combined, transported and disposed in a non-impounding disposal facility.

Properties of this combined refuse depend on the initial moisture content of the filter cake, the ratio 

of fine to coarse refuse, and the particle size and moisture content of the coarse refuse. Often, these 

properties make it difficult to place combined refuse in a controlled manner, and they may limit its 

potential for use within the structural portion of an embankment. Combined refuse is sometimes 

mixed with combustion ash for construction of a homogeneous embankment or, in some cases, a 

zoned embankment (for construction of the downstream structural shell). Table 6.5 presents published geotechnical test data for combined coal refuse from several locations for a range of fines content. The strength data are based on remolded samples compacted to 95 percent standard Proctor 

maximum dry density, which can be difficult to achieve if the fine portion of the refuse has a high 

water content.
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6.2.3.4 Borrow Materials

Borrow materials are those soil and rock materials used in an embankment to meet specific design 

criteria. Borrow materials are used principally for:

• Starter dam construction

• Filters and drainage zones

• Impervious zones

• Sedimentation pond embankments

• Erosion protection

• Buttresses

• Reclamation cover

For economical designs, most borrow materials are obtained at the site or from suitable nearby 

mine spoil. Materials for filters, drains, and erosion protection are typically obtained from commercial sources.

TABLE 6.4 FINE COAL REFUSE CHARACTERIZATION – SUMMARY 

OF AVERAGE/RANGE OF VALUES

Reference Location

Grain Size Atterberg

Limits Specific 

Gravity

Gs

(gm/cm3)

Effective Shear 

Strength

Passing 

No. 40 

Sieve

(%)

Passing

No. 200 

Sieve

(%)

LL

(%)

PL

(%)

PI

(%)

φ′

(degrees)

c′

(psf)

Almes and 

Butail (1976)

PA, WV, 

KY, VA 64-100 36-47 20-40 NR(1) <10 1.55-1.65 29-34 0

McCutcheon 

(1983)

OH 81 46 29 22 7 1.85 36 0

Qiu and 

Sego (2001)

Western

Canada 90 66 40 24 16 1.94 32 200

Hegazy et al. 

(2004)

PA 65-100 58 31 20 11 1.52 33 230

Genes et al. 

(2000)

WV NR 16-90 NR NR <12 1.44-2.37 23-36 0

Cowherd and 

Corda (1998)

NR NR 24-91 23-39 NR 0-9 1.4-2.1 NR NR

Huang et al. 

(1987)

KY, OH, PA

TN, VA, WV NR 27-95 22-44 NR 0-12 1.52-2.14 NR NR

Busch et al. 

(1974,1975)

WV 50-98 10-60 34-51 NR 0-13 1.45-2.07 NR NR

Backer et al. 

(1977) UT, NM 60-100 16-98 NR NR NR 1.33-2.07 NR NR

Ullrich et al. 

(1991) KY, TN, OH 45-95 25-85 31-44 NR 0-31 1.8-2.5 NR NR

Zeng and 

Goble (2008)

Appalachian

Region 75-85 40-62 27-36 21-26 3-11 2.02-2.16 NR NR

 Note: 1. NR = not reported
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The available mine spoil and borrow materials in most coal mining regions of the U.S. consist either 

of bedrock or soils derived from bedrock. Typical bedrocks include: (1) soft shales, siltstones and claystones that weather rapidly when excavated and break down to a soil when compacted, (2) harder 

limestones that usually resist weathering except when exposed to acidic waters from leachates passing through pyritic coal and coal refuse, and (3) hard sandstones that often are resistant to natural 

weathering and attack from leachates. The soil components are typically present as alluvial deposits 

in valley bottoms, colluvial deposits that have accumulated toward the base of slopes, residual soils 

derived from surface weathering of the rock, or partially decomposed weathered rock.

Soft rock is normally suitable for the downstream portion of starter dams not critical to seepage control. The initial particle size of soft rock may prevent its use for constructing impervious zones, and 

its weathering characteristics usually prevent its use for drainage or erosion protection purposes. 

When soft rock is used, its design strength characteristics should be based on predicted future compacted or weathered condition, often as a soil. The use of limestone should usually be avoided due to 

its susceptibility to deterioration from leachates. Limestone can be used in situations where: (1) acidic 

conditions will not occur, (2) it is arranged within an embankment in a manner that assures separation from acidic leachates, or (3) when it is used in a manner that does not depend upon continued 

integrity as a granular material. In most coal mining regions, hard sandstone is the best material for 

erosion protection, filters and drainage zones.

Mine spoil tends to be highly variable in soil and rock content and particle size, and processing may 

be necessary. This can be accomplished by segregating over-sized fractions or fines, which can be 

used for other project applications. Sometimes mine spoil can be used for construction without processing of the materials (e.g., in a zoned embankment).

Recent alluvial and older river terrace deposits can vary from relatively clean sands and gravels to 

“dirty” soils with high contents of fine-grained soils and organic material. Material from clean sand 

and gravel deposits may be suitable for constructing filter or drainage zones in an embankment, 

but only after field investigation has determined the quantity of clean material available and laboratory testing has verified the suitability of the grain-size distribution and mineral composition for the 

intended purposes. Otherwise, alluvial soils typically are not desirable for use in a coal refuse disposal embankment because of the expense associated with excavation and preparation.

Colluvial soils generally consist of a combination of soils derived from fine- and coarse-grained rocks 

and vary from clays to primarily sandy material. Fine-grained colluvial soils may be suitable for constructing an impervious zone, while all types of colluvium are normally suitable for use as structural 

TABLE 6.5 COMBINED COAL REFUSE CHARACTERIZATION

Location

Grain Size

Specific Gravity

Gs

(gm/cm3)

Total Shear Strength

Passing 

No. 4 Sieve 

(percent)

Passing 

No. 200 Sieve 

(percent)

φ

(degrees)

c

(psf)

Pennsylvania 25-60 7-26 1.9-2.0 26-28 NR

Ohio 25-58 5-11 1.8 36-38 NR

West Virginia 18-58 4-12 2.1 32 NR

Colorado 18-54 3-18 1.8-1.9 33-38 NR

 (STEWART AND ATKINS, 1982)
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components for stability or as cover material to support vegetation. Because they generally have 

a wide grain-size distribution, colluvial soils normally are not suitable for either filter or drainage 

zones in an embankment.

Residual soils derived from soft rocks (e.g., shale) are normally fine-grained and suitable for either 

an impervious zone or the structural portion of an embankment. When available in thick deposits, 

residual soils can generally be excavated economically. Soils derived from sandstone are too coarse 

for use in impervious zones, but may be suitable for structural fill or use in drainage zones.

For any borrow material, the deposit must be explored to verify that it is available in adequate quantities to meet the design requirements of the disposal facility. This must be followed by laboratory testing 

to evaluate the suitability of the borrow materials/mine spoils for embankment construction. Although 

practically any inorganic, insoluble soil can be incorporated into an embankment when modern compaction equipment and control standards are employed, the following problems may arise:

• Fine-grained soils may have insufficient shear strength or excessive compressibility.

• Clays of medium to high plasticity may expand if placed under low confining pressures and/or at low moisture contents.

• Plastic soils with high natural moisture content may be difficult to adjust for proper 

moisture for compaction.

• Dispersive clays are not suitable for use in dam embankments.

• Silts may have insufficient erosion resistance.

• Stratified soils may require extensive mixing of borrow material.

Table 6.6 shows a correlation between soil classification and the engineering and design properties 

of compacted soils (DOD, 2005). This table can be used for evaluation of borrow materials and preliminary design of starter dams. Table 6.7 provides a correlation between soil classification and the 

relative desirability of soil as compacted fill material for various types of starter embankments. Table 

6.8 (Sherard et al., 1963) illustrates a correlation between soil classification and engineering properties 

related to embankment design and constructability.

6.2.3.5 Coal Combustion Products from Power Plants

The combustion of coal at fossil fuel power plants produces fly ash and bottom ash as residual waste 

products. Two other products of coal combustion air pollution control technology are fluidized-bed 

combustion (FBC) waste and flue-gas-desulfurization (FGD) sludge. While no detailed assessment 

of these and other wastes from the power plants is provided in this Manual, embankments at some 

refuse disposal sites have been constructed by integrating power plant waste products with coal 

refuse. The important issues that designers should consider if these waste products are disposed at 

coal refuse facilities are discussed in this section.

Power plant wastes have certain properties that can be very beneficial if these materials are judiciously 

integrated at coal refuse disposal sites. The economic viability of disposing dissimilar refuse materials can be especially beneficial when the mining operation is near the coal burning plant. With this 

in view, a brief description of the engineering properties of power plant waste materials is provided 

in this section. For more detailed discussion and design considerations, the designer should refer 

to the Electric Power Research Institute (EPRI) Coal Ash Disposal Manual (DiGioia et al., 1995) and 

other references such as McLaren and DiGioia (1987), DiGioia and Gray (1979), Gray and Lin (1972). 

Additionally, research has been published (Daniel et al., 2002) on the material properties of mixtures 

of fly ash and coal refuse in southwest Virginia. A thorough review of the overall environmental 

implications of fly ash use is provided in Carlson and Adriano (1993). The USEPA (2000) performed a
6-14

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TABLE 6.6 CORRELATION BETWEEN USCS CLASSIFICATION AND PROPERTIES OF COMPACTED SOILS

Group 

Symbol Soil Type

Range of 

Max. Dry 

Weight 

(pcf)

Range of 

Optimum 

Moisture

(%)

Typical Value of 

Compression Typical Strength Characteristics Typical 

Hyd.

Cond.

(ft/min)

Range 

of CBR 

Value

Range of 

Subgrade 

Modulus k 

(lbs/in3

)

1.4 tsf =

20 psi

3.6 tsf =

50 psi

Compacted 

Cohesion

(psf)

Saturated 

Cohesion 

(psf)

φ

(deg) tan 

φ

(% of original height)

GW Well graded clean gravels, gravel-sand mixtures 125-135 11-8 0.3 0.6 0 0 >38 >0.79 5 × 10-2 40-80 300-500

GP Poorly-graded clean gravels, gravel-sand mix 115-125 14-11 0.4 0.9 0 0 >37 >0.74 10-1 30-60 250-400

GM Silty gravels, poorly- graded gravel-sand mix 120-135 12-8 0.5 1.1 – – >34 >0.67 >10-6 20-60 100-400

GC Clayey gravels, poorly graded gravel-sand-clay 115-130 14-9 0.7 1.6 – – >31 >0.60 >10-7 20-40 100-300

SW Well-graded clean sands, gravelly sands 110-130 16-9 0.6 1.2 0 0 38 0.79 >10-3 20-40 200-300

SP Poorly-graded clean sands, sand-gravel mix 100-120 21-12 0.8 1.4 0 0 37 0.74 >10-3 10-40 200-300

SM Silty sands, poorly-graded sand-silt mix 110-125 16-11 0.8 1.6 1050 420 34 0.67 5 × 10-5 10-40 100-300

SM-SC Sand-silt-clay mix with slightly plastic fines 110-130 15-11 0.8 1.4 1050 300 33 0.66 2 × 10-6 5-30 100-300

SC Clayey sands, poorly graded sand-clay mix 105-125 19-11 1.1 2.2 1550 230 31 0.60 5 × 10-7 5-20 100-300

ML Inorganic silts and clayey silts 95-120 24-12 0.9 1.7 1400 190 32 0.62 >10-5 ≤15 100-200

ML-CL Mixture of inorganic silt and clay 100-120 22-12 1.0 2.2 1350 460 32 0.62 5 × 10-7 – –

CL Inorganic clays of low to medium plasticity 95-120 24-12 1.3 2.5 1800 270 28 0.54 >10-7 ≤ 15 50-200

OL Organic silts and silt-clays, low plasticity 80-100 33-21 – – – – – – – ≤ 5 50-100

MH Inorganic clayey silts, elastic silts 70-95 40-24 2.0 3.8 1500 420 25 0.47 5 × 10-7 ≤ 10 50-100

CH Inorganic clays of high plasticity 75-105 36-19 2.6 3.9 2150 230 19 0.35 >10-7 ≤ 15 50-150

OH Organic clays and silty clays 65-100 45-21 - - - - - - - ≤ 5 25-100

 Note: 1. All properties are for condition of “Standard Proctor” maximum density, except values of k and CBR which are for “Modified Proctor” maximum density.

2. Typical strength characteristics are for effective strength envelopes and are obtained from USBR data.3. Compression values are for vertical loading with complete lateral confinement.4. (-) indicates insufficient data available for an estimate. (DOD, 2005)
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

MAY 2009 TABLE 6.7 CORRELATION BETWEEN USCS CLASSIFICATION AND RELATIVE DESIRABILITY OF SOILS AS COMPACTED FILL Group 

Symbol Soil Type

Relative Desirability for Various Uses (1 is most desirable. 14 is least desirable)

Rolled Earth Fill Dams Lining Foundation

Homogenous 

Embankment Core Shell Erosion Resistance Compacted Earth Lining Roadway Surfacing Seepage Important Seepage Not Important

GW Well graded clean gravels, gravel-sand mixtures - - 1 1 - 3 - 1

GP Poorly-graded clean gravels, gravel-sand mix - - 2 2 - - - 3

GM Silty gravels, poorly-graded gravel-sand mix 2 4 - 4 4 5 1 4

GC Clayey gravels, poorly graded gravel-sand-clay 1 1 - 3 1 1 2 6

SW Well graded clean sands, gravelly sands - - 3, if gravelly 6 - 4 - 2

SP Poorly graded clean sands, sand-gravel mix - - 4, if gravelly 7, if gravelly - - - 5

SM Silty sands, poorly graded sand-silt mix 4 5 - 8, if gravelly 5, erosion critical 6 3 7

SC Clayey sands, poorly graded sand-clay mix 3 2 - 5 2 2 4 8

ML Inorganic silts and clayey silts 6 6 - - 6, erosion critical - 6 9

CL Inorganic clays of low to medium plasticity 5 3 - 9 3 7 5 10

OL Organic silts and silt-clays, low plasticity 8 8 - - 7, erosion critical - 7 11

MH Inorganic clayey silts, elastic silts 9 9 - - - - 8 12

CH Inorganic clays of high plasticity 7 7 - 10 8, vol.-change critical - 9 13

OH Organic clays and silty clays 10 10 - - - - 10 14

(DOD, 2005)
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MAY 2009

detailed review of the use of coal combustion products in mining environments that supported their 

classification as residual waste, although they recommended continued study of disposal in deep 

mines and in mine backfill situations where the materials may contact groundwater.

Fly ash is a fine, silt-sized material usually ranging in diameter from 0.5 to 100 microns and consisting largely of spherical, sometimes hollow, glassy particles. Bottom ash consists of primarily coarser 

material with heavier particles than fly ash. It is generally angular with a porous surface. For fly ash, 

hydraulic conductivities have been reported in the range of 10-7 to 10-4 cm/sec and for bottom ash in 

the range of 10-3 to 10-1 cm/sec (DiGioia et al., 1995). Depending on the actual material, fine-grained 

fly ash and coarse-grained bottom ash can be used as an “impervious liner” and drainage filter, 

respectively, in conjunction with coarse coal refuse and other borrow materials. If fly ash is enriched 

with nitrogen compounds, it can sometimes be used as a supplement for vegetation growth.

There are two general types of fly ash as defined by ASTM. Class F fly ash contains less than 20 percent calcium oxide and is produced by burning bituminous or anthracite coal. Class C fly ash is produced by burning subbituminous coal or lignite. Both have pozzolanic properties, but Class F fly ash 

is not appreciably self-cementing. Because of the geographical distribution of coal types, Class F fly 

ash is principally produced in the eastern U.S., while most Class C fly ash is produced in the western 

U.S. Class C fly ash is self-cementing due to presence of lime and other chemical compounds. However, much of the fly ash returned to Appalachian mines does not meet either Class C or F criteria 

(Daniels et al., 2002).

TABLE 6.8 APPROXIMATE CORRELATION BETWEEN ENGINEERING PROPERTIES

AND SOIL CLASSIFICATION GROUPS

USCS

Group 

Symbol

Relative 

Hydraulic

Conductivity

Probable 

Range of k

(ft/yr)

Relative Piping 

Resistance

Relative Shear 

Strength

Relative 

Workability(1)

GW Pervious 1,000 – 100,000 High Very High Very Good

GP Pervious to Very 

Pervious

5,000 – 

10,000,000 High to Medium High Very Good

GM Semi-pervious 0.1 – 100 High to Medium High Very Good

GC Impervious 0.01 – 10 Very High High Very Good

SW Pervious 500 – 50,000 High to Medium Very High Very Good

SP Pervious to Semipervious 50 – 500,000 Low to Very Low High Good to Fair

SM Semi-pervious to 

Impervious 0.1 – 500 Medium to Low High Good to Fair

SC Impervious 0.01 – 50 High High to Medium Good to Fair

ML Impervious 0.01 – 50 Low to Very Low Medium to Low Fair to Very Poor

CL Impervious 0.01 – 1 High Medium Good to Fair

OL Impervious 0.01 – 10 Medium Low Fair to Poor

MH Very Impervious 0.001 – 0.1 Medium to High Low Poor to Very Poor

CH Very Impervious 0.0001 – 0.01 Very High Low to Medium Very Poor

 Note: 1. Relative workability = ease of moisture-density control. 

 (ADAPTED FROM SHERARD ET AL., 1963)
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Power plants may also generate combustion waste from FBC systems and SO2 scrubbers by utilizing 

limestone. FBC waste may be pozzolanic or cementitious.

The physical and engineering properties of coal ash that could be of importance when it is used in 

combination with coal refuse disposal are grain size, specific gravity, density, optimum moisture content, hydraulic conductivity, shear strength, and compressibility.

Tables 6.9 and 6.10 show summaries of particle-size testing results for Class F and Class C fly ash, 

respectively.

TABLE 6.9 CLASS F FLY ASH

Gradation 

Property

Number of 

Samples

Mean Value

(mm)

Standard Deviation

(mm)

Coefficient of 

Variation

D85 84 0.079 0.063 0.800

D50 84 0.023 0.015 0.669

D15 84 0.0075 0.0048 0.648

(MCLAREN AND DIGIOIA, 1987)

TABLE 6.10 CLASS C FLY ASH

Gradation 

Property

Number of 

Samples

Mean Value

(mm)

Standard Deviation

(mm)

Coefficient of 

Variation

D85 17 0.063 0.020 0.317

D50 17 0.022 0.011 0.500

D15 17 0.0084 0.0082 0.976

(McCLAREN AND DIGIOIA, 1987)

A review of available data (McLaren and DiGioia, 1987) shows that fly ash is a relatively uniform, siltsized material with a specific gravity slightly lower than most natural soils. Compaction of fly ash is 

moisture dependent, but the range of optimum moisture contents is greater than that of natural silts 

and silty clays. The maximum dry and wet densities of compacted fly ash are somewhat less than 

typical values for natural soils, which makes fly ash useful as a light-weight structural fill.

Of importance when fly ash is used in structural zones of a disposal facility is shear strength. The 

effective angle of internal friction of fly ash and FBC combustion waste varies with the degree of 

compaction, but generally ranges from 25 to 40 degrees. Class F fly ash is non-cohesive, and while 

it may appear to be cohesive when partially saturated, this effect is completely lost when the material is either dried or saturated. In contrast, Class C fly ash can develop considerable cohesive shear 

strength due to cementitious reactions. This cohesion is the dominant factor in the shear strength of 

Class C fly ash. Similar to fly ash, the shear strength of bottom ash varies with the degree of compaction. The effective angle of friction for bottom ash in a loose state can vary from 38 to 42.5 degrees, 

with an average of about 41 degrees. The shear strength of SO2 sludge varies significantly with the 

solids content and the amount of stabilizing agent added. The strength of a stabilized sludge is 

comparable to dense sand and gravel, while the strength of unstabilized sludge is similar to that of 

loose sand. The angle of friction for stabilized sludge ranges from 38 to 51 degrees depending on 

the solids content and amount of additives. The angle of friction ranges from 20 to 30 degrees for 

unstabilized sludge.
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Research into the beneficial reuse of fly ash mixed with coarse coal refuse in southwest Virginia 

was performed by Daniels et al. (2002). Table 6.11 shows variations of maximum compacted dry 

density, shear strength and hydraulic conductivity reported by them for mixtures of fly ash and 

coal refuse.

TABLE 6.11 FLY ASH/COAL REFUSE MIXTURE PROPERTIES

Fly Ash Mix Ratio

(%)

Maximum

Dry Density

γdmax

(lb/ft3)

Effective Angle of 

Internal Friction

φ′

(degrees)

Effective

Cohesion

c′

(lb/ft2)

Hydraulic

Conductivity

k

(cm/sec)

0 125 39 0 2.86 x 10-3

8 123 37.7 0 1.01 x 10-3

16 120 37 0 2.56 x 10-4

24 119 37 0 1.71 x 10-4

32 117 37 0 7.88 x 10-5

100 85 37 0 5.78 x 10-5

(DANIELS ET AL., 2002)

Research conducted on coal refuse and Type F fly ash from southern Illinois (Kumar et al., 2001) indicated an increase in strength for mixtures containing up to 15 percent ash. There was a tendency for 

strength to decrease for mixtures with a higher percentage of ash.

Mixing procedures to blend fly ash with refuse material should be developed based on the strength 

or stabilization requirements for the embankment. In many cases, the primary beneficial use is associated with moisture control, and for this usage blending with spreading equipment on the embankment surface is acceptable. Where enhancement of the strength of the refuse material is a requirement, 

or in cases where fly ash amendments are introduced to control acid generation, greater effort to mix 

or blend the materials may be required or beneficial. Additional discussion concerning blending of 

amendments is presented in Section 11.5.6.

6.2.4 Scheduling

The procedure for scheduling construction of a disposal facility embankment differs from that used 

for most other types of constructed embankments for several reasons:

• Refuse disposal occurs continually over many years.

• Disposal must occur on a year-round basis, regardless of weather conditions.

• The rate of refuse disposal is not only determined by the needs of embankment 

construction, but also by the rate of mining, the quality of the seam, market changes, 

and scheduled and unscheduled work interruptions.

The many variables affecting refuse disposal scheduling limit the ability to provide a detailed flow 

chart that accounts for all construction events that could occur during the operational period of a 

disposal facility. However, the following are questions to be answered in developing the design and 

establishing a general construction schedule for existing and new disposal facilities:

• What is the expected operational period of the disposal facility?

• What volumes of coarse, fine and combined refuse are anticipated from the prepara-
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

MAY 2009

tion plant on an annual basis, and are there reasons to believe that the relative proportions may change?

• What types of borrow material are available, and must they be developed prior to 

covering the borrow site with refuse?

• What embankment configurations would enable critical sections to be constructed 

from available materials during periods of the year most conducive to controlled 

construction?

• What measures to address potential environmental impacts and protection of surface 

and groundwater will be required?

• Should hydraulic structures (e.g., spillways, decants, diversion systems) be constructed in stages corresponding to disposal rates?

• Can soil and rock materials excavated for hydraulic structures be used as borrow for 

embankment construction?

• What measures will be required to economically abandon the disposal facility at the 

end of its expected operational period?

• If for some reason the disposal facility must be abandoned earlier than expected, can 

that abandonment be reasonably accomplished?

• Will the disposal rate allow portions of the embankment to be completed in stages so 

that exposed refuse and eventual abandonment costs are minimized?

• If the rate of refuse production or the operational period of the disposal facility is 

extended beyond initial projections, can the facility be reasonably expanded?

• Will the timing of future surface or underground mining in the vicinity of the facility 

impact the design?

The designer must recognize that these schedule-related questions may not address all of the potential issues. However, it is prudent to consider each of the above questions when designing a coal 

refuse disposal facility.

6.3 DESIGN CONSIDERATIONS

This section presents the basic design considerations for coal refuse embankments and earthen dams. 

Embankments with and without impoundments are considered separately for two reasons. First, an 

impounding embankment generally has a greater hazard potential because the impounded water can 

cause damage for a substantial distance downstream from the disposal facility in the event of a failure. Second, impounded water can contribute directly to a failure if the embankment is improperly 

designed and/or constructed.

The examples discussed in this section are based on the assumption that coarse refuse is the major 

structural component of embankments for disposal of coarse and fine coal refuse. Mine spoil, soil 

and rock borrow materials, which are obtainable at most disposal facility sites, could also be used 

for the starter dam and in the structural portions of the embankment. The designer should refer to 

Sherard, et al. (1963) and USBR (1992a; 1998) for additional discussion of design principles for dams 

and embankments composed primarily of soil and rock materials.

6.3.1 Impounding Embankments

A coal refuse impounding embankment is generally designed based upon the same principles as 

earthen dams, with the exception that refuse materials are used to the maximum extent possible. Two 

major aspects of coal refuse impounding embankment construction are: (1) the starter dam for initial 

disposal of fine refuse and (2) the embankment raising methodology used for long-term refuse disposal. The starter dam is typically constructed with borrow materials, while subsequent crest raisings

generally utilize coarse coal refuse.
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The borrow material available at a site may range from fine to coarse soils to coarse coal refuse, or 

rocks and soil from mine spoils and highwall cuttings. For economy, suitable materials at or near the 

disposal site should be used for starter dam construction. Depending on the type of material available at the site, the following types of starter dam may be designed:

• Homogeneous Embankment

• Zoned Embankment

A homogeneous embankment is generally constructed in situations where the borrow materials vary 

little in hydraulic conductivity or soil type. A zoned embankment is constructed where two or more 

types of materials are available for embankment construction. Rockfill, when large quantities of rocks 

are available from the mine spoil or from spillway construction, can also be used as the stability portion of a zoned embankment.

The principal geotechnical considerations in designing an impounding embankment are:

• Seepage control – internal drainage system, impervious zone, and foundation 

treatment

• Slope and foundation stability – static slope stability, end-of-construction stability, 

seismic slope stability and deformation, sloughing and erosion

• Drainage structures – principal and emergency spillways, conduits and surface 

drainage structures

• Underground mine workings – stability, subsidence and breakthrough potential, 

sealing of mine openings and boreholes, and potential infiltration into underground 

mine workings

For earth dams, foundation support, stability and seepage control are important design considerations. Geologic and geotechnical investigations should be performed to identify potentially unstable 

soils that are incapable of sustaining embankment loadings or susceptible to adverse impacts from 

seepage. Slope failure and piping (internal erosion) are the most common types of embankment failure associated with seepage. In addition to embankment slope and material strength and unit weight, 

the stability of an embankment is a function of the depth of the saturation level (seepage phreatic 

surface) below the embankment face, regardless of the volume of seepage through the embankment. 

If the phreatic surface rises above the level assumed in the design, embankment stability can decrease 

to the point where failure occurs. Piping is a process in which particles are carried out of an embankment with seepage, creating voids. This can lead to failure as the voids progressively extend farther 

back into the embankment as more material is removed and more concentrated seepage flow occurs. 

Installation of an internal drainage system within the embankment is specifically intended to address 

seepage-type failures and is fundamental to the selection of the embankment configuration. The subsequent discussions of basic embankment types repeatedly emphasize the importance of seepage 

control. Cedergren (1989) is an important reference for analyzing seepage conditions in embankments and foundations.

In addition to controlling seepage for embankment stabilization, liner systems may also be needed 

for mitigation of potential environmental impacts to the groundwater and surface water. Impervious 

liners composed of fine-grained borrow materials, geomembranes or geosynthetic clay liners can be 

a critical component of disposal facility design.

Uncontrolled seepage into underground mine workings can affect embankment safety. If the water 

pressure in an impoundment has the potential for breaking through the overburden into abandoned 

entries or through zones of soft, weathered rock, the rapid release of water or slurry into a mine could 
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

MAY 2009

trap mine personnel and equipment and lead to undesirable environmental conditions. Such a situation must be prevented. Even in abandoned workings, the resulting water flow may endanger the 

population downstream of uncontrolled mine discharge points and cause undesirable environmental 

conditions. Guidance related to evaluation of breakthrough potential is presented in Section 8.5.

Analysis of seepage, slope stability and structure foundations is generally performed in detail only 

after the embankment configuration has been selected and the subsurface exploration and laboratory testing programs have been completed. These analyses are introduced in this section and are 

discussed in more detail in Section 6.6.

6.3.1.1 Homogeneous Embankments

A homogeneous embankment is constructed of only one material; however, to control seepage and saturation of the embankment, a granular internal drain is typically incorporated into the cross section. 

Homogeneous embankments constructed for coal refuse sites with various types of internal drains are 

illustrated in Figure 6.1. In all cases, the purpose of the drain is: (1) to maintain stability by keeping the 

phreatic surface low and (2) to control seepage as it leaves the embankment to minimize the potential 

for piping. The drainage system may consist of one or more filter zones of intermediate grain-size material to mitigate potential conveyance of embankment material into the collection zone. Selection of the 

relative gradations of adjacent zones of material and the use of geotextiles to prevent finer material from 

piping into a coarser downstream zone is discussed in Section 6.6.2. If possible, drainage material should 

be selected to act as both the filter and the collection zone to avoid the higher costs and more difficult 

construction associated with placing multiple layers. Selection of the type of internal drainage system 

is normally based upon the fine refuse and flood pool levels, embankment configuration, and material 

characteristics, including anisotropy. Table 6.12 summarizes the advantages and disadvantages of the 

internal drains that are illustrated in Figure 6.1.

When layers of coal refuse are placed during refuse embankment construction, the top surface is 

often broken into smaller-grained, less-permeable material by the movement of equipment and the 

effects of weathering. The materials beneath the top surface retain their original grain-size distribution and greater hydraulic conductivity. As a result, anisotropic conditions can develop, leading to 

an embankment hydraulic conductivity that is greater in the horizontal direction than in the vertical direction. Thus, the effectiveness of a horizontal drainage blanket or toe drain is reduced if the 

anisotropy is large and the height of the embankment is significant. A chimney drain, or other types 

of drains that intercept horizontal seepage planes, may be more effective. Section 6.6.2 presents guidance for seepage analyses for the design of internal drains, including the determination of drain 

dimensions.

At many coarse refuse embankment dams, the fine coal refuse deposited in the impoundment, creates a “delta” or “beach” on the upstream slope that typically restricts seepage, provided that the 

normal pool is maintained upstream of the delta. Economies in internal drain construction can be 

achieved by evaluating the hydraulic conductivity characteristics of the fine refuse material, as well 

as available borrow material, and using zoned embankment design. For homogeneous embankment 

dams used for fresh water supply, some savings can also be gained by using outlet drains for discharging the water from the base of a chimney drain, which will eliminate the need for a granular 

blanket extending beneath the entire downstream portion of the embankment. This alternative is 

illustrated in Figure 6.1d.

6.3.1.2 Zoned Embankments

Coarse coal refuse and mine spoil are generally not sufficiently fine grained to keep seepage at a low 

level. If the design of an embankment requires that seepage be minimized or it is desired to lower 

the saturation level in the downstream portion of the embankment, a less pervious zone within the 
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Chapter 6

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COMPACTED COARSE

 COAL REFUSE

DRAINAGE BLANKET

(FILTERS NOT SHOWN)

6.1a VERTICAL OR STEEPLY SLOPING CHIMNEY DRAIN

FIGURE 61 DRAIN CONFIGURATIONS FOR HOMOGENEOUS EMBANKMENTS

SETTLED FINE

COAL REFUSE

6.1b HORIZONTAL BLANKET DRAIN

COMPACTED COARSE

 COAL REFUSE

DRAINAGE BLANKET

(FILTERS NOT SHOWN)

SETTLED FINE

COAL REFUSE

6.1d CHIMNEY DRAIN WITH OUTLET DRAIN EXITS

PERVIOUS DRAIN OUTLET

DISCHARGING TO DOWNSTREAM TOE (FILTERS NOT

SHOWN)

A

A

SECTION A - A

DRAINAGE

MATERIAL

PERFORATED

PIPE

COMPACTED COARSE

 COAL REFUSE

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

SETTLED FINE

COAL REFUSE

FLOOD POOL LEVEL

FLOOD POOL LEVEL

6.1c DISCRETE INTERNAL DRAIN

DISCRETE INTERNAL DRAINS

(FILTERS NOT SHOWN)

COMPACTED COARSE

 COAL REFUSE A

A

SETTLED FINE

COAL REFUSE

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

NORMAL POOL

LEVEL

NORMAL POOL

LEVEL

PHREATIC

SURFACE

PHREATIC

SURFACE

PHREATIC

SURFACE

PHREATIC

SURFACE

FIGURE 6.1 DRAIN CONFIGURATIONS FOR HOMOGENEOUS EMBANKMENTS 

WITH IMPOUNDMENTS
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

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TABLE 6.12 ADVANTAGES AND DISADVANTAGES OF INTERNAL DRAIN TYPES

Internal Drain Type Figure No. Advantage(s) Disadvantage(s)

Steeply Sloping 

Chimney Drain 6.1a Positive seepage interception and collection system

Expensive and difficult to construct; 

requires careful planning and stringent 

construction control to connect with 

future stages.

Horizontal Blanket 

Drain 6.1b Simple construction Ineffective in high anisotropy conditions.

Discrete Internal 

Drain 6.1c

Relatively inexpensive 

and independent of future 

embankment raising

Partial seepage interruption; 

effectiveness depends on anisotropy.

Chimney Drain and 

Outlet Sections 6.1d Positive seepage interception Expensive and difficult to construct.

embankment may be needed. A zoned embankment consists of multiple material zones, generally 

including an impervious (or low-hydraulic-conductivity, fine-grained material) core or upstream 

zone of limited width and additional zones of coarse material that provide strength and erosion resistance. Employing such a zoned embankment concept can reduce material requirements for internal 

drainage structures and structural embankment zones.

At some coal refuse disposal sites, two zones are incorporated into the embankment cross section – a 

relatively impervious soil zone in the upstream section and coarse material such as mine spoil, granular borrow material or coarse coal refuse in the downstream section. If controlling the volume of seepage is not of primary importance, zoned embankments can be designed with the object of lowering 

the phreatic surface in the downstream face of the embankment. The primary design consideration in 

such cases is that the hydraulic conductivity of the downstream zone be sufficiently high to discharge 

the water seeping through the core or upstream zone without an elevated phreatic surface.

Figure 6.2 illustrates configurations of zoned embankments most commonly used in dam construction. Figures 6.2a and 6.2b show a vertical core and a sloping core, respectively. Figure 6.2c illustrates a 

zoned embankment consisting of finer soil in upstream portion of the dam and coarse material in the 

downstream portion. An upstream zone may be preferred for slurry impoundments, provided erosion 

from pool-level fluctuations or runoff is not significant or can be controlled. Alternately, zoned coarse 

refuse dams, in which the upstream zone receives more compactive effort to increase material breakdown and to lower hydraulic conductivity, may be specified. However, many coarse refuse dams are 

not zoned, taking advantage of the fine coal refuse deposited in the impoundment that may be as effective as zoning in limiting seepage, provided that: (1) the impoundment pool is maintained at a low level 

and does not surcharge the upstream embankment face and (2) the response of the phreatic surface to 

increased pool levels as a result of storm runoff will not compromise embankment stability.

A description of important features related to impervious zone and shell design for zoned embankments is provided in the following text.

Impervious Zone Design

Although small amounts of seepage are typically present in the low-hydraulic-conductivity portion 

of an embankment, these embankment components are generally referred to as impervious zones. 

For design of impervious zones, the primary considerations are the type of impervious zone and the 

thickness and material used for their construction. The upstream zone of a zoned embankment may 

serve a similar purpose, particularly for coal refuse impoundments.
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Chapter 6

MAY 2009

Impervious Zone Type

Both vertical and sloping impervious zones have advantages, as described in Table 6.13. The selection of the type of zoning of the material must be determined on an individual site basis. Valuable 

additional discussion of the design of embankments with impervious zones is provided in USBR 

(1992a).

PERVIOUS FOUNDATION ZONE PERVIOUS FOUNDATION ZONE

TRANSITION

ZONE (IF NECESSARY)

6.2a VERTICAL CORE

FILTER ZONES

(IF NECESSARY)

IMPERVIOUS CORE

FIGURE 6.2 ZONED EMBANKMENTS WITH IMPOUNDMENTS

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

IMPERVIOUS FOUNDATION MATERIAL

PERVIOUS FOUNDATION ZONE

6.2b SLOPING CORE

IMPERVIOUS CORE

TRANSITION ZONE

(IF NECESSARY) FLOOD POOL LEVEL

FILTER ZONES

(IF NECESSARY)

NORMAL POOL

LEVEL

IMPERVIOUS FOUNDATION MATERIAL

6.2c UPSTREAM IMPERVIOUS ZONE

FILTER/TRANSITION ZONE

(IF NECESSARY)

UPSTREAM IMPERVIOUS ZONE

PERVIOUS FOUNDATION ZONE

IMPERVIOUS FOUNDATION MATERIAL

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

SETTLED FINE

COAL REFUSE

EMBANKMENT MATERIAL

(INTERNAL DRAINAGE NOT

SHOWN)

EMBANKMENT MATERIAL

(INTERNAL DRAINAGE NOT

SHOWN)

EMBANKMENT MATERIAL

(INTERNAL DRAINAGE NOT

SHOWN)

FIGURE 6.2 ZONED EMBANKMENTS WITH IMPOUNDMENTS
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

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Impervious Zone Thickness

The impervious zone thickness is normally governed by practicalities (Sherard et al, 1963), including: 

(1) tolerable seepage volume, (2) thickness that will permit proper construction, (3) type, quality and 

cost of available low-hydraulic-conductivity material, (4) type, quality and cost of available material 

for any filter layers between the impervious zone and the adjoining downstream soil, and (5) the 

quantity and quality of available soil/rock for the shell. While subsequent research and testing has 

provided refinement in the design of impervious zone thickness, the following criteria developed for 

water retention dams (USBR, 1992a; McCook, 2002) may serve as preliminary guidelines for acceptable impervious zone thickness for other impounding embankments, recognizing that other dimensions/configurations may be suitable pending seepage and stability analyses:

• Cores with a thickness greater than 30 percent of the depth of water head have 

proven satisfactory for many dams under diverse conditions; for a starter dam, a 

core of this thickness will probably be adequate for many types of impervious core 

material and embankment heights.

• Cores with a thickness of 15 to 20 percent of the depth of water head are considered 

thin, but if adequately designed and constructed filter layers are used, they will 

probably be satisfactory under most circumstances.

• Cores with a thickness of less than 10 percent of the depth of water head should be 

considered only in circumstances where a large leak through the core would not lead 

to embankment failure or unacceptable environmental conditions; very carefully 

designed and constructed filter layers should be considered.

Impervious zones of limited width, governed by material properties and availability as well as construction practicalities, have proven advantageous in limiting seepage at starter dams and slurry

TABLE 6.13 RELATIVE ADVANTAGES OF VERTICAL AND SLOPING EMBANKMENT CORES

Vertical Core Sloping Core

1. Higher confinement pressure, more uniform settlement 

with increased embankment load, and shear strength 

is not as critical as with a sloping core.

2. Higher pressure is present between the impervious 

core and the foundation, providing additional protection against leakage along the contact surface.

3. For a given volume of impervious soil material, the 

thickness of the vertical core will be slightly greater 

than that of sloping core.

4. If it becomes necessary to grout the foundation after 

the embankment has been raised to a significant 

height, the grouting can be conducted from the crest 

of the embankment through the core and directly 

into the foundation, as opposed to a sloping core, 

where the tie between the core and the foundation is 

beneath the impoundment.

5. A vertical core is not subject to damage from sloughing 

or erosion of the upstream toe.

1. The main downstream portion of the embankment can 

be constructed first and the impervious core placed 

later without disrupting construction operations. This 

advantage allows the downstream portion of the 

embankment to be constructed year round, even if 

controlled construction of the core can be done only 

during short, good-weather periods.

2. Filter layers between the core and the upstream and 

downstream portions of the embankment can be 

made thinner and constructed more easily.

3. The core construction can be staged easily if the 

embankment is expanded by the downstream staged 

construction method.
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impoundments. As discussed above, the deposition of fine refuse slurry may also be effective at 

limiting seepage provided that: (1) the impoundment pool is maintained at a low level and (2) the 

response of the phreatic surface to increased pool levels as a result of storm runoff will not compromise embankment stability.

Impervious Zone Material

Impervious zone material is usually selected based on specific requirements for controlling seepage and the availability of suitable material at the site. Typically, fine-grained soil is used for such 

construction. The potential for failure resulting from loss of impervious zone material and leakage 

caused by cracking or differential slippage within the impervious zone will influence the design, 

the materials used and the construction procedure. The possibility of excessive leakage due to 

cracking is a particularly important consideration for embankments on soft foundation material, 

in areas susceptible to subsidence, or in regions of high seismic activity. Brittle soil behavior and 

cracking problems often can be minimized by placing the impervious zone material at a higher 

than optimum moisture content. Figure 6.3 provides a classification of materials according to 

resistance to piping and cracking. If foundation settlements are expected to be high, a suitable 

internal drainage layer should be placed immediately downstream of the impervious zone to control seepage resulting from possible cracking.

Dispersive clay soils have a preponderance of sodium cations in their pore water in contrast to most 

clays, which have a preponderance of calcium and magnesium cations in their pore water. A hole 

through a dispersive clay will increase in size as water flows through (due to the breakdown of 

the soil structure), whereas a hole in a non-dispersive clay will remain essentially constant in size. 

Dispersive clays should not be used in dam construction because they are extremely susceptible to 

piping. The crumb test (ASTM D 6572) can be conducted in the field or laboratory and may indicate 

if soils are dispersive. The dispersion potential can most reliably be determined using the pinhole 

test (ASTM D 4647). The dispersion potential of clay for several ranges of measured dispersion is 

provided in Table 6.14 (Sherard et al., 1976).

Design of filters for impervious soils used for the core can be critical for the downstream interface 

with the shell, but is generally less critical for the upstream interface for a coal refuse impoundment 

because reservoir fluctuations are minimal. Sherard et al. (1985) and McCook (2002) address filter 

criteria and hydraulic gradient issues associated with impervious soils in dams; this topic is further 

discussed in Section 6.6.2.

Shell Design

Embankment shell zone material typically is selected from granular material at the site. Coarse coal 

refuse, mine spoil and rock excavated from the construction of water drainage systems or haul roads 

are generally suitable for shell construction. The earthfill portion (upstream zone or core) may be 

zoned or protected by graded filter zones, as discussed in Section 6.6. The use of rockfill is governed 

by economy and structural stability in addition to slope protection. If a plentiful supply of suitable 

rock is available at low cost, it is often possible to steepen the slopes of the adjoining earthfill portion 

by providing additional free-draining rock on the downstream face, resulting in added stability. With 

careful planning and design, the rockfill section of the starter dam can be utilized as a toe drain for 

future expansion of the facility by the upstream method of construction. However, this may pose a 

design constraint if the expansion is by the downstream method.

6.3.1.3 Foundation Seepage Control

A very important consideration in the design of an impounding embankment is the control of seepage through the underlying foundation materials. Unlike a water storage reservoir, there is little need 

to retain water in a slurry disposal facility. Therefore, minimizing seepage through the foundation 
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

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100 8 6 4 3 2 10 8 6 4 3 2 1

10

20

0

30

40

50

60

70

80

90

100

8 6 4 3 2 0.1 8 6 4 3 2 0.01 8 6 4 3 2 0.001

PERCENT FINER BY WEIGHT

4"

3"

2 1/2"

2"

1 1/2"

1"

3/4"

1/2"

3/8"

4

8

10

16

30

40

50

70

100

200

CLEAR SQUARE OPENINGS U.S. STANDARD SIEVE NUMBERS HYDROMETER ANALYSIS

GRAIN SIZE (mm)

COBBLES GRAVEL SAND SILT OR CLAY (PLASTIC OR NON-PLASTIC) COARSE FINE COARSE MEDIUM FINE

A LEAST CHANCE

FOR CRACKING

C

LITTLE CHANCE

FOR CRACKING

VULNERABLE TO

CRACKING

PIPING

RESISTANCE 1

PIPING RESISTANCE 2

3

B

TYPICAL GRADATION ASSOCIATED

WITH PIPING RESISTANCE 

CATEGORY MATERIAL CHARACTERISTICS

PIPING

RESISTANCE

CL AND CH WITH PI>15, WELL

GRADED SC WITH PI>15.

GREATEST RESISTANCE TO PIPING. SMALL AND MEDIUM CONCENTRATED LEAKS WILL HEAL THEMSELVES.

EMBANKMENT MAY FAIL AS A RESULT OF SLOWLY PROGRESSIVE PIPING CAUSED BY LEAK OF ABOUT ONEHALF CFS.

1

PIPING

RESISTANCE

INTERMEDIATE RESISTANCE TO PIPING. SAFELY RESISTS SATURATION OF LOWER PORTION OF DOWNSTREAM SLOPE INDEFINITELY. MAY FAIL EVENTUALLY AS A RESULT OF EROSION CAUSED BY A SMALL

CONCENTRATED LEAK OR BY PROGRESSIVE SLOUGHING. IF A LARGE LEAK DEVELOPS, PIPING CAUSES

FAILURE IN A SHORT TIME.

CL AND ML WITH PI<15, WELL

GRADED SC AND GC WITH 

7<PI<15.

2

PIPING

RESISTANCE 3

SP AND UNIFORM SM AND ML

WITH PI<7.

LEAST RESISTANCE TO PIPING. USUALLY FAILS IN A FEW YEARS AFTER FIRST RESERVOIR FILLING IF

SEEPAGE IS ABLE TO BREAK OUT ON DOWNSTREAM SLOPE. SMALL CONCENTRATED LEAK ON DOWNSTREAM SLOPE CAN CAUSE FAILURE IN A SHORT PERIOD OF TIME. HIGH DENSITY FROM COMPACTION

INCREASES RESISTANCE SIGNIFICANTLY.

CRACKING

RESISTANCE A

CH WITH D50<0.02MM AND

PI>20.

HIGH POST-CONSTRUCTION SETTLEMENT, PARTICULARLY IF COMPACTED DRY. HAS SUFFICIENT DEFORMABILITY TO UNDERGO LARGE SHEAR STRAINS FROM DIFFERENTIAL SETTLEMENT WITHOUT CRACKING.

CRACKING

RESISTANCE B

GC, SC, SM, SP WITH

D50>0.15MM.

SMALL POST-CONSTRUCTION SETTLEMENT. LITTLE CHANCE FOR CRACKING UNLESS POORLY COMPACTED AND LARGE SETTLEMENT IS IMPOSED ON EMBANKMENT BY CONSOLIDATION OF THE 

FOUNDATION.

CRACKING

RESISTANCE C

CL, ML AND SM WITH PI<20,

0.15MM>D50>0.02MM.

MEDIUM TO HIGH POST-CONSTRUCTION SETTLEMENT AND VULNERABLE TO CRACKING. SHOULD BE

COMPACTED AS WET AS POSSIBLE CONSISTENT WITH STRENGTH REQUIREMENTS.

FIGURE 6.3 RESISTANCE OF CORE MATERIALS TO PIPING AND CRACKING

(DOD, 2005)

is not as critical to the design as it is for storage reservoirs. Where seepage occurs, it must be controlled such that it does not adversely affect the safety of the embankment or result in environmental 

impacts. The foundation conditions likely to be encountered beneath the starter dam are: (1) pervious 

foundation, (2) impervious foundation, or (3) impervious stratum at the surface underlain by a pervious stratum. An additional foundation seepage concern at some coal refuse facilities is the potential 

for fracturing due to subsidence of the ground surface above underlying mines.

Pervious foundations may consist of boulders, gravels, sands or mixtures thereof. For such foundations, measures to minimize seepage quantity and to provide controlled seepage discharge are 

FIGURE 6.3 RESISTANCE OF CORE MATERIALS TO PIPING AND CRACKING
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normally required. Control measures may include low-hydraulic conductivity barriers (e.g., cutoff 

trench and backfill) to decrease or virtually stop seepage, or a collection system can be provided 

beneath the downstream portion of the embankment to control the discharge of seepage. If seepage 

control and/or collection systems are not intended to be employed, the safety (including the potential 

for piping of foundation or embankment materials) and environmental ramifications should be carefully evaluated, and suitable measures should be employed to monitor pore pressures, if necessary.

The most common methods used for controlling foundation seepage are construction of a low-hydraulic-conductivity cutoff through the pervious foundation material and construction of an impervious 

blanket extending far enough upstream to sufficiently restrict the flow. In some situations, construction of an impermeable liner beneath the impoundment may also be used to address foundation seepage control. These methods are illustrated in Figure 6.4. Normally, if the pervious foundation material 

is thin and excessive groundwater problems due to excavating in the valley bottom are not anticipated, 

the low-hydraulic-conductivity cutoff is the least expensive method. Important considerations in the 

design and construction of seepage cutoffs are presented by Sherard et al. (1963) and USBR (1987a, 

1992a). Procedures for designing an impervious blanket are presented by Cedergren (1989).

Where seepage through the foundation is allowed to occur, a collection system is almost always provided. The two major negative effects of allowing seepage to occur are: (1) a decrease in the factor 

of safety against instability of the embankment due to high pore pressures in the foundation and 

(2) the potential for piping in the foundation. If the foundation soil is not stratified in the horizontal 

direction, seepage control can be provided by a horizontal blanket drain (Figure 6.1a). This method 

normally requires analysis of the path of the seepage and assurance during placement of the blanket 

that it is directly tied to underlying pervious material. Seepage is collected at the downstream end of 

the blanket and discharged at a predetermined location near the valley bottom.

Other methods of controlling seepage through a pervious foundation include a deep drainage 

trench constructed near the toe of the embankment and a relief well system, as discussed in greater 

detail in Section 6.6.2.3.4. Measures associated with the design and management of the impoundment and clarified water level can also aid in controlling seepage. In some cases, impoundment 

cells for clarified water are developed in the upstream portion of the impoundment. The deposition of fine refuse and resulting longer seepage path aids in restricting seepage beneath the downstream embankment.

Impervious foundations typically consist of massive rock and predominantly clayey soils. When 

an impoundment is to be constructed upon impervious foundation materials, seepage beneath the 

embankment is not a major design consideration if a proper seal is placed between the embankment 

and the foundation. Methods of foundation preparation that effectively create a seal and references to 

supplemental technical publications on grouting and rock preparation are presented in Chapter 11.

TABLE 6.14 CLAY DISPERSION POTENTIAL

Percent Dispersion(1) Dispersive Tendency

Over 40 Highly Dispersive (do not use)

15 to 40 Moderately Dispersive

0 to 15 Resistant to Dispersion

Note: 1. The ratio between the fraction finer than 0.005 mm in a soil-water suspension that

has been subjected to a minimum of mechanical agitation and the total fraction finer

than 0.005 mm determined from a regular hydrometer test times 100.

 (SHERARD ET AL., 1976)
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FIGURE 6.4 SEEPAGE CUTOFFS FOR PERVIOUS FOUNDATIONS

6.4a PERVIOUS FOUNDATION CUTOFF WITH ZONED EMBANKMENT

COMPACTED COARSE

 COAL REFUSE

FILTER/TRANSITION ZONE

(IF NECESSARY)

PERVIOUS FOUNDATION ZONE

UPSTREAM IMPERVIOUS ZONE

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

SETTLED FINE

COAL REFUSE

IMPERVIOUS FOUNDATION MATERIAL

PERVIOUS FOUNDATION ZONE

IMPERVIOUS FOUNDATION MATERIAL

6.4b IMPERVIOUS BLANKET

COMPACTED COARSE

 COAL REFUSE

IMPERVIOUS CORE

FILTER ZONES

(IF NECESSARY)

PARTIAL IMPERVIOUS BLANKET

SETTLED FINE

COAL REFUSE

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

TRANSITION ZONE

(IF NECESSARY)

NOTE: PARTIAL IMPERVIOUS BLANKET EXTENDS UPSTREAM

SUFFICIENTLY FAR TO CONTROL SEEPAGE.

6.4c IMPERVIOUS LINER WITH HOMOGENEOUS EMBANKMENT

COMPACTED COARSE

 COAL REFUSE

PERVIOUS FOUNDATION ZONE

FLOOD POOL LEVEL

NORMAL POOL

LEVEL

SETTLED FINE

COAL REFUSE

LINER SYSTEM

PHREATIC

SURFACE

IMPERVIOUS FOUNDATION MATERIAL

DISCRETE INTERNAL

DRAINS

 FIGURE 6.4 SEEPAGE CUTOFFS FOR PERVIOUS FOUNDATIONS
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Chapter 6

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For disposal facilities where impoundments are constructed upon an impervious foundation, the 

National Coal Board (1970 and 1972) has suggested an effective concept for allowing controlled 

drainage from settled slurry. Granular drainage and filter material is placed beneath the impoundment area prior to filling. The drainage and filter material transition to pipes that pass under the 

embankment to a downstream collection system or to a discharge outlet. In some cases, to assure 

complete drainage, this zone is constructed over the upstream face of the embankment. D’Appolonia 

(1988) conducted OSM-sponsored research and implemented a design for an impoundment internal 

drain structure to improve consolidation of the fine coal refuse and to intercept seepage before it 

enters the coarse refuse embankment, thus mitigating acid mine drainage potential. This concept is 

illustrated in Figure 6.5. Another method to improve drainage and consolidation of fine coal refuse 

is installation of wick drains.

STAGE I

STAGE II

STAGE III

STAGE I

DRAIN PIPE

FINE COAL REFUSE

CONDUIT SEEPAGE

INTERCEPTION

GRANULAR INTERNAL DRAIN

STAGE II

STAGE III

INTERNAL

DRAIN (TYP.)

For the case of an impervious stratum at the surface overlying more pervious strata below, there is 

potential for high pore-water pressure to occur downstream of the dam. This may cause blow outs, 

boiling, piping or instability at or beyond the downstream toe. An upstream cutoff trench, deep 

drainage using relief wells or construction of berms should be considered in such cases.

6.3.2 Non-impounding Embankments

Non-impounding embankments require many of the same design and construction considerations 

as impoundments. Careful geotechnical investigation can: (1) identify potentially unstable soils that 

are incapable of sustaining embankment loadings and (2) decrease the probability of groundwater 

impacts by identifying certain site characteristics that should either be avoided or recognized during 

the design phase. Safety may become a major design factor when:

• The disposal facility is located in areas where failure would have a high possibility of 

taking lives and could seriously damage infrastructure or buildings.

• The facility is located immediately above a stream and significant movement of the 

embankment could block or restrict the flow, possibly creating a temporary impoundment that could release a flood wave upon breaching of the sloughed material.

FIGURE 6.5 IMPOUNDMENT INTERNAL DRAIN CONCEPT
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

MAY 2009

• The facility is constructed across a large valley with a significant watershed or it may 

temporarily impound water during some stage of development, despite the presence 

of drainage systems.

Embankments should not impede drainage, and cross-valley configurations should be avoided 

because they can retain water and be subject to classification by MSHA as an impoundment. MSHA 

(2007) presents factors to be considered in this regard and also cautions that reclamation at abandonment can require significant regrading to address drainage concerns.

The following discussion of important design considerations for non-impounding embankments 

does not specifically address cases where a high safety hazard may exist. The designer must recognize special safety hazards and formulate the investigation program and analyses accordingly. 

To properly plan and design a non-impounding coal refuse embankment, the following questions 

should normally be answered:

• Considering the required volume and the disposal facility geometry, what is the 

smallest quantity of constructed material that could form the critical downstream 

structural portion of the embankment with assured stability?

• If the embankment were to temporarily impound water, could it lead to pore-water 

pressure conditions that are hazardous to the stability of the embankment?

• Is an impervious liner system needed in order to limit seepage or infiltration of the 

groundwater?

• Should an internal drainage system be placed under critical portions or all of the 

disposal facility to control saturation for stability purposes or to collect seepage for 

treatment before discharge?

Non-impounding coal refuse embankments typically accommodate disposal of coarse, combined, 

and dewatered fine coal refuse. Well-graded coarse refuse may be generated without significant 

excess moisture, such that internal drainage provisions are primarily directed at control of natural 

springs or infiltration. Consequently, seepage control and slope stability can readily be addressed 

with minimal internal drainage requirements and normal placement and compaction of materials. Combined refuse and dewatered fine refuse generally contain excess moisture when they are 

generated, and greater measures are required for seepage control and slope stability. Such embankments may be zoned to establish structural downstream zones for stability and may incorporate 

internal drainage control structures, allowing for upstream zones where the material consistency 

and strength are less important. However, applicable regulations for the construction of “refuse 

piles” must be followed.

Several examples of designs for non-impounding embankments, including provisions for seepage 

control, are illustrated in Figure 6.6. Figures 6.6a and 6.6b show drainage system concepts previously 

discussed in Section 6.3.1, where seepage is controlled by internal drainage systems. The previous 

discussion of internal drain materials and their gradation, and of the need for filters, is also applicable 

to this type of construction. The full extent of seepage control and drainage systems must be determined on a project-by-project basis considering site conditions, the economics of various methods of 

providing embankment stability, the cost of leachate treatment, and appropriate regulations.

For some disposal facilities, usually depending on site location and condition, seepage impacts on the 

groundwater regime may need to be mitigated. In such cases, a relatively impervious liner composed 

of fine-grained borrow materials should be constructed beneath the disposal facility. This can be accomplished as the facility is being constructed by rearranging and compacting native soils or by using borrow 

soils, as illustrated in Figure 6.6c. At some sites, a geomembrane or geosynthetic clay liner (GCL) may 
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Chapter 6

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6.6a BOTTOM INTERNAL DRAIN

6.6b BOTTOM AND DISCRETE INTERNAL DRAINS

6.6c BOTTOM INTERNAL DRAIN WITH LINER SYSTEM

FIGURE 6.6 NON-IMPOUNDING EMBANKMENTS

STRUCTURAL ZONE

(COARSE OR COMBINED REFUSE)

INTERNAL DRAIN

PLACED REFUSE ZONE

(COARSE OR COMBINED REFUSE)

STRUCTURAL ZONE

(COARSE OR COMBINED REFUSE)

INTERNAL DRAIN

PLACED REFUSE ZONE

(COMBINED OR DEWATERED

FINE REFUSE)

DISCRETE INTERNAL

DRAINS

STRUCTURAL ZONE

(COARSE OR COMBINED REFUSE)

INTERNAL DRAIN

PLACED REFUSE ZONE

(COARSE OR COMBINED REFUSE)

LINER SYSTEM

(MAY INCLUDE UNDERDRAIN)

be used as an impervious liner if satisfactory borrow materials are not available. Selected portions of the 

impervious zone may be covered by drainage materials to collect and transport leachates to a common 

point for treatment and/or discharge. The introduction of liner materials may affect the stability of the 

embankment and thus potentially impact the design slopes or configuration of a disposal facility.

Typically, groundwater seepage or mine discharge is collected and conveyed in a manner such that it 

cannot enter the refuse. Figure 6.7a shows a seepage collector drain comprising a collection zone and a 

FIGURE 6.6 NON-IMPOUNDING EMBANKMENTS
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

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EXTENT OF

SEEPAGE ZONE

PROTECTIVE COVER

MATERIAL

FILTER SAND GRANULAR DRAIN MATERIAL

EXTENDS TO

UNDERDRAIN

OR DITCH

GEOTEXTILE WRAP

6.7a SPRING COLLECTION ZONE WITHOUT LINER SYSTEM

6.7b SPRING COLLECTION ZONE AS PART OF LINER SYSTEM

EXTENT OF

SEEPAGE ZONE

FILTER SAND

LINER MATERIAL

PROTECTIVE COVER MATERIAL

GEOTEXTILE

WRAP GEOTEXTILE

GRANULAR DRAIN MATERIAL

6.7c ROCK UNDERDRAIN AS PART OF LINER SYSTEM

LINER MATERIAL

PROTECTIVE COVER MATERIAL

FILTER SAND

GEOTEXTILE

GEOTEXTILE

WRAP

ROCK UNDERDRAIN

FIGURE 6.7 EXAMPLE SPRING COLLECTION ZONES AND ROCK UNDERDRAIN
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Chapter 6

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conveyance system. The collection zone consists of granular material covered with a granular filter layer 

and/or geotextile. The collected water is discharged through the granular drain or pipe to the nearest 

internal drainage system or beyond the downstream toe. The granular filter layer or geotextile situated 

between the water collecting zone and the overlying refuse and perforated pipes incorporated into the 

drainage system (if present) should be designed using procedures discussed in Section 6.6. Inclusion 

of an impervious liner over the collection zone, as shown in Figure 6.7b, may be appropriate for preventing embankment seepage from entering the collected groundwater and to minimize the volume of 

poor-quality water that may require treatment. Rock underdrains may be placed in valley bottoms to 

collect outflows from springs. Figures 6.7b and 6.7c show examples of spring collection zones and rock 

underdrains for a lined facility. Spring collection zones and rock underdrains should be designed for 

compatibility with surrounding materials using the filter criteria presented in Section 6.6.2.

Underground mine voids may cause subsidence that can impact an overlying embankment, potentially disrupting liners and internal drainage structures. Measures for evaluating and addressing 

potential subsidence problems are discussed in Chapter 8.

6.3.3 Slurry Cell Embankments

Slurry cell embankments may be classified as impounding embankments or non-impounding 

embankments depending on the configuration and storage capacity of active, uncovered cells and 

the potential for multiple cells to be involved in a failure. For facilities with significant coal refuse 

production rates, it is difficult to keep the active cell capacity below the impoundment classification 

limit, and thus the aim is to design the facility so that it can be classified as having low hazard potential. Such a system requires a design and construction sequence that minimizes the volume of active 

cells and provides for timely drainage, covering, and consolidation of completed cells so that the fine 

coal refuse is not flowable.

For slurry cell systems that are designed and classified as impoundments, the following geotechnical 

considerations are applicable:

• Seepage control – An internal drainage system and foundation cutoff system 

designed to intercept seepage and prevent it from impacting the structural zone or 

toe of the embankment and a foundation treatment and liner system if necessary 

to address leachate migration from the disposal facility. Slurry cells have limited 

water and slurry storage and thus represent less significant sources of seepage than 

conventional impoundments for fine coal refuse slurry disposal.

• Slope stability – Static slope stability is maintained by a structural zone constructed from coarse coal refuse or borrow material with sufficient width to 

effectively contain and isolate the slurry cells from affecting potential failure 

surfaces. Seismic stability and deformations may be of less concern because 

individual cells tend to be shallower deposits of fine coal refuse that are better 

drained and consolidated by layers of coarse refuse or borrow materials. Sloughing and erosion considerations are essentially the same as for other refuse 

embankments. Slurry cell disposal requires coarse refuse or borrow material for 

the cell structures and covering of completed cells and for any structural zones 

that may be part of a valley-fill configuration. Thus, to address slope stability, 

slurry cell systems may require more coarse refuse or borrow material for structural elements and cell construction than more traditional types of impounding 

embankments.

• Drainage structures – Structural foundations and excavation slopes for diversion 

channels, principal and emergency spillways, conduits and other auxiliary structures associated with impoundments.
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Geotechnical Engineering, Material Testing, Engineering Analysis and Design

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• Underground mines – Stability, sealing of mine openings, and infiltration into 

underground mine workings. Slurry cell systems provide a means to mitigate breakthrough impacts of an impoundment into underground mine workings.

Slurry cell embankments that are designed with limited impounding capacity for water, sediment 

or slurry so that they do not meet the impoundment size criterion (as discussed in Section 3.4.1) are 

less sensitive to the impacts of seepage on stability, provided that effective drainage measures are 

incorporated into the cells. Such embankments may not need principal and emergency spillways and 

conduits associated with impoundments, thus reducing geotechnical considerations.

6.3.4 Embankment Construction Staging

Embankments are raised in stages using one or more modifications of three basic approaches: (1) 

upstream method, (2) downstream method, or (3) centerline method. In fact, many disposal plans use 

a combination of upstream and downstream methods for staging. Figures 6.8 and 6.9 show example 

configurations of embankments constructed using a combination of the upstream and downstream 

methods. This section describes common procedures for staging the development of a new embankment or extending the life of an existing embankment. In the previous two sections, general concepts 

for controlling groundwater and seepage are discussed; they are repeated here only as needed for 

explaining staging methods. The inclusion of drainage systems in the overall embankment and its 

staged parts must be designed on a site-by-site basis.

6.3.4.1 Upstream Method

In the upstream method of construction, the crest of the embankment is shifted progressively 

upstream from the starter dam, as shown in Figure 6.8. The upstream method has several advantages and disadvantages from an operational and safety standpoint, but one important drawback 

is the engineering and design requirements necessary to address structural performance under 

earthquake loading conditions. While there has not been a reported failure of a fine coal refuse 

impoundment due to earthquake loadings in the U.S., other similar tailings impoundments and 

hydraulic-fill dams have failed during or following seismic activity. The engineering analyses presented in Section 6.6 and Chapter 7 employ current methods for evaluating material properties and 

loadings for design.

NOTE: TAKEN FROM ACTUAL STAGING FOR VALLEY FILL SITE IN PENNSYLVANIA. THE FIGURE SHOWS UPSTREAM

 CONSTRUCTION METHOD (STAGES IV - VII) FOLLOWING INITIAL DEVELOPMENT (STAGES I - III). 

STAGE I

STAGE II

STAGE III

STAGE IV

STAGE V

STAGE VI

STAGE VII

FOUNDATION SOILS

FINE COAL REFUSE TYPICAL INTERNAL

DRAIN

CONSTRUCTION KEY

STARTER

 EMBANKMENT

FIGURE 6.8 UPSTREAM CONSTRUCTION FOLLOWING INITIAL DEVELOPMENT
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Chapter 6

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The major advantages of the upstream method are:

• The structural fill volume is minimized because part of the new embankment is constructed in stages on top of the existing embankment and part is constructed over the 

deposited fine refuse slurry.

• The downstream face of the constructed embankment is the final face of the completed embankment, and vegetation and other environmental control measures can 

be performed on a permanent basis.

• The operational requirements such as haul and access roads, culverts, diversion and 

perimeter ditches can be constructed to serve the entire useful life of the facility.

• The impoundment watershed area decreases with the progress of embankment construction, requiring less volume for storm runoff handling with time.

• The starter dam, if properly designed, can provide support and become part of the 

internal drainage control for the subsequent embankment development.

When fine coal refuse slurry is deposited in an impoundment, grain-size sorting and layering occurs 

in both the horizontal and vertical planes during the depositional process. Peripheral discharge of 

slurry, either by single-point discharge or by multiple discharges, results in the formation of a “beach” 

around the discharge point. The coarser particles settle close to the discharge point and the finer particles concentrate in the upstream portion of the impoundment. The depositional process may also 

result in the formation of horizontal layers of coarser and finer fractions of the refuse and affect the 

engineering properties of the settled fine refuse.

The fine coal refuse particles settling from the slurry accumulate in a very loose state initially and 

have a high void ratio and moisture content. These loose deposits consolidate with time as water 

is expelled from the voids between the particles. Consolidation is influenced by such factors as the 

effective pressure of the overlying material, the hydraulic conductivity of the deposit and surrounding soil, the distance that water must travel to drain and the time during which vertical pressure is 

applied. Thus, the consolidation of fine coal refuse deposits varies spatially within the impoundment. 

Even after consolidation under self weight for years, such deposits may have high void ratios and 

high moisture contents, affecting the shear strength of the fine refuse. This should be borne in mind 

by the designer, if the upstream method of construction is utilized for embankment raising.

The disadvantage of the upstream method is that construction occurs atop previously deposited, 

unconsolidated tailings. Under static loading conditions, there is a limiting height to which materials can be placed without the risk of a shear failure. In the initial placement of materials for the 

embankment stage, normal spreading and compaction procedures cannot be followed until a firm or 

stabilized base is prepared. Safety of operating personnel during the initial placement of material for 

an upstream construction stage is very important. Additionally, the height and configuration of an 

upstream constructed stage will depend on the strength of the material within the zone of shearing, 

the downstream slope of the embankment, and the location of the phreatic surface within the facility. Under earthquake loading, this type of embankment may be susceptible to failure by strength 

degradation. However, a stable impoundment can be constructed by following the basic principles 

of embankment design and through judicious handling of material. A disposal facility constructed 

using the upstream method must be designed by an engineer experienced in the behavior of soils, 

coal refuse materials and embankments. Important considerations include:

• The width of the embankment stages or structural fill zones must be adequate to 

provide downstream slope stability. Upstream slope stability must be demonstrated 

by adequate factors of safety for failure surfaces that could compromise the crest of 

the embankment.
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• As the embankment is raised, coarse refuse should be placed over settled slurry 

according to an established schedule, considering the potential for pore pressure 

buildup and dissipation in the saturated material resulting from the applied loads. 

To the extent practical, the initial push out for upstream stages should not be placed 

on submerged fine coal refuse.

• Procedures for placement of materials for the embankment stage should address 

equipment and operator activity and safety during the initial push-out onto the 

settled fine coal refuse.

• Reasonable estimates of movement and differential settlements should be used in 

designing the portion of the embankment constructed over settled slurry.

• The constructed structural portion of each stage of the disposal facility should 

include a system for controlling seepage, as discussed in Section 6.3.1.

• If the starter dam is intended to become the toe portion of the overall embankment, the 

downstream zone should be constructed of well-compacted, free-draining material so 

that it will facilitate the dissipation of pore pressures within the overall embankment.

• In disposal facilities retaining fine refuse slurry, the slurry discharge points should be 

located adjacent to the embankment and above the pool level. To the extent practical, 

the discharge point should be periodically moved along the length of the embankment. This will concentrate the coarsest slurry particles adjacent to the embankment, 

offering the advantage of their greater strength and reducing settlement when the 

next embankment stage is constructed.

• Analyses of embankment stability should consider dynamic loads and potential for 

strength degradation. Seismic design and deformation analyses are discussed in 

Chapter 7.

Exploration, testing, engineering analyses and regulatory review associated with addressing the 

above considerations are typically more complex and lengthy as compared to designs that employ 

the downstream method.

6.3.4.2 Downstream Method

In the downstream method of construction, the crest of the embankment is shifted progressively 

downstream from the starter dam, as designed for the early stages shown in Figure 6.9. This 

method can be used for embankments with or without impoundments. The major advantages of 

the method are:

• The embankment is not built on hydraulically-deposited refuse.

• Placement and compaction control can be exercised as required over the entire fill 

operation.

• The embankment can generally be raised above its initial design height without serious limitations and complicated design modifications.

• Internal drainage systems can be installed, as required, as the construction 

progresses.

• The embankment can be designed with minimal concern for strength loss of the fine 

coal refuse under earthquake loadings. This can result in less complex and lengthy 

exploration, testing, engineering analyses, and regulatory review.

The major disadvantage of the downstream method is that it requires relatively large volumes of fill 

for raising the embankment. In the early stages of construction, it may not be possible for the mine to 
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produce a sufficient volume of coarse refuse to maintain the crest of the embankment above the level 

required for disposal of slurry and routing the design storm. Features such as haul and access roads, 

culverts, benches, gutters, diversion ditches and perimeter ditches may have to be reconstructed at 

each stage, resulting in higher cost. Also, the final external face of the embankment is not created 

until the construction of last stage of disposal, which typically ranges from 5 to 25 years after the 

start of construction. Consequently, interim faces are exposed to the weather prior to covering by the 

next stage. These issues may have a considerable financial impact, if the facility has a relatively small 

drainage area and a decant system is the primary spillway. However, when the construction material is readily available, the downstream method normally results in simpler construction than the 

upstream method and is also more easily implemented because of the many special requirements of 

the upstream method.

The arrangement of the drainage and impervious zones within an embankment constructed using 

the downstream method is usually similar to that of the embankments discussed in Section 6.3.1.1. 

The zones can be placed in stages corresponding to the height of the embankment.

6.3.4.3 Centerline Method

The centerline method of embankment construction is essentially a variation of the downstream 

method where the crest of the embankment is not shifted in the downstream direction, but instead 

is raised vertically above the crest of the starter dam. A major advantage of this method is that the 

downstream portion of the embankment is built on a firm foundation, and therefore placement and 

compaction control can be exercised as required over that portion of the embankment. An important 

consideration in using this method is to maintain an adequate width of structural fill in order to 

FIGURE 6.9 DOWNSTREAM EMBANKMENT CONSTRUCTION FOLLOWED BY

 EXPANSION USING DOWNSTREAM AND UPSTREAM METHODS

6.9a EMBANKMENT STAGES I - V CONSTRUCTED USING DOWNSTREAM METHOD 

6.9b EMBANKMENT STAGES VI - XI-B CONSTRUCTED USING DOWNSTREAM

 AND UPSTREAM METHODS

TYPICAL INTERNAL

DRAIN

STAGE V

STAGE IV

STAGE III

STARTER EMBANKMENT

STAGE II

IMPERVIOUS

ZONE

FINE COAL REFUSE

FOUNDATION SOILS

STAGE I

INTERNAL DRAIN

STAGES I - V

STAGE VI-A

STAGE VII-A

STAGE VII-B

STAGE VIII-B STAGE VIII-A

STAGE IX STAGE X

STAGE XI-B

STAGE XI-A

FINE COAL REFUSE

LOWER INTERNAL

DRAIN

FOUNDATION SOILS

STAGE VI-B

FIGURE 6.9 DOWNSTREAM EMBANKMENT CONSTRUCTION FOLLOWED BY EXPANSION 

USING THE DOWNSTREAM AND UPSTREAM METHODS
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achieve stability. Other design considerations, as discussed in Section 6.3.4.1 for upstream construction, are also applicable to the centerline method. The centerline method also has all the disadvantages listed in Section 6.3.4.2 for downstream method of construction.

The arrangement of the drainage and impervious zones within an embankment constructed using 

the centerline method is usually similar to that of the embankments discussed in Section 6.3.1.1. The 

zones can be placed in stages corresponding to the height of the embankment. The centerline method 

poses many design, construction, environmental and operational problems, and thus is not generally 

a preferred method of refuse disposal in the coal industry. This method is primarily used in tailings 

dam construction where cyclones are used for separating the coarser fraction of the tailings. In the 

coal industry, a combination of the upstream and downstream methods is typically employed to 

meet storm water and slope stability criteria and material requirements.

6.3.4.4 Embankments Supplemented with Borrow Material

When the production of coarse refuse is not sufficient to construct the planned height of an 

impounding embankment, or in other situations dictated by mining conditions, borrow material 

or mine spoil may be used to supplement the planned construction. The embankment can be constructed from borrow material or mine spoil using the upstream or downstream methods, or these 

materials can be used in combination with coarse refuse to meet the specified gradation in a zoned 

embankment, provided that specific zones are designated for such materials. The guidance for 

seepage control and drainage systems discussed in this Manual is appropriate for construction of 

embankments of this type.

6.3.5 Special Considerations for Existing Embankments

The engineering principles and procedures for analyzing and designing modifications to existing 

refuse embankments are the same as those for new facilities. If an existing embankment is not performing adequately, is not consistent with current engineering practice, or involves re-activation of 

an older site, the following issues may need to be considered:

• Current or additional loadings that may impact stability (static and/or seismic loading conditions).

• Potential for liquefaction and foundation failure of the embankment if constructed 

over fine refuse.

• Potential for piping and/or excessive uncontrolled seepage discharge from the 

embankment or around the decant discharge pipe.

• Potential safety problems to miners in nearby underground active workings due to 

overburden breakthrough.

• Potential for excessive erosion along sloped surfaces with inadequate drainage control and vegetative cover.

• For impounding embankments, an inadequate combination of runoff storage and 

discharge capacity to accommodate the design storm.

Specific conditions that need to be evaluated relative to the modification of existing embankments 

include:

• Material Placement – The structural portions of embankment should be evaluated 

relative to shear strength and seepage characteristics.

• Foundation Preparation – The foundation preparation work undertaken prior to construction of the embankment and the potential for sliding along the embankmentfoundation interface zone should be evaluated.
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• Foundation Condition – It should be determined whether the embankment overlies 

soft natural soils or settled fine refuse that must be considered in static and seismic 

stability analyses.

• Piping Potential – It should be determined if existing seepage could lead to a piping 

failure.

• Underground Mining – It should be determined if the facility overlies abandoned 

or active mines, particularly ones with openings previously covered by coal refuse 

that could affect the safety of miners or cause environmental or property damage 

downstream.

• Drainage Facility – It should be determined if the existing drainage facilities will be 

structurally safe for expanded operation and will meet design-storm criteria.

• Site Boundary Constraints – It should be determined if physical restrictions (e.g., 

streams, rivers, mines, utilities, roads, railroads) and property boundary restrictions 

will limit the extent of any future modifications.

• Operations Capabilities – Existing equipment and manpower capabilities or engineering services (e.g., surveying or engineering support) should be evaluated for 

adequacy to implement expansion or abandonment plans or facility modifications

If an existing embankment exhibits unacceptable performance or is determined to be inadequate 

against a stability failure, the following modifications are appropriate:

• Sliding or inadequate factor of safety against slope failure – The slope can be stabilized by: (1) constructing a downstream buttress, (2) removing material from the top, 

or (3) flattening the slope. Of these options, constructing a downstream buttress is 

most effective in stabilizing a slope.

Downstream Buttress – A downstream buttress can be constructed using coarse 

refuse from normal disposal operations, and borrowed granular soil can 

be used for internal drainage control. Major limitations in constructing the 

buttress are physical or property limitations, poor access to the base of the 

embankment, inadequate supply of coarse refuse to accomplish buttress construction in a reasonable period, and extremely poor foundation conditions in 

the area of the buttress requiring excavation. Inclusion of drains within the buttress section of the embankment can be very effective in improving the stability 

of existing embankments. As illustrated in Figure 6.10a, a drain can be placed 

as a granular blanket over the downstream face of an existing embankment 

and then covered with coarse refuse to form a buttress.

Removal of Material from Top – Although this alternative improves slope stability, it is often not practical to remove material from the top of the slope 

because of the resulting effect of the overall embankment configuration. Also, 

the cost of removing the material and disposing it at a different location often 

makes this alternative unattractive. This disadvantage can be partially overcome if the excavated material can be used to construct a buttress at the toe of 

the slope, additionally enhancing stability.

Flattening of the Slope – This alternative increases the stability of a nonimpounding facility, but it is often not suitable for an impounding embankment constructed by the upstream method. The removal of material reduces 

the thickness of the structural section of the embankment and may lower the 

factor of safety. Also, the cost of removing material and disposing it at a different location often makes this alternative unattractive. This disadvantage
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can be partially overcome if the excavated material can be used to construct a 

buttress at the toe of slope.

• Phreatic Level Reduction – If high phreatic surface conditions in the embankment 

contribute to an undesirable factor of safety, it is normally possible to lower the phreatic surface by: (1) installing horizontal drains and (2) reducing seepage.

Horizontal Drains – One means for providing internal seepage control in an 

existing embankment is by drilling horizontal drains beyond the phreatic surface to intercept the seepage, as shown in Figure 6.10b. The design requirements and construction techniques for such drains are discussed in Section 

6.6.2.3.5. The installation of horizontal drains is often very effective. However, the unknown and potentially variable nature of coal refuse introduces 

greater risk that the drains will not meet expectations. If horizontal drains are 

employed, concurrent monitoring of their effectiveness should be performed. 

Extra drains should be added, or the existing drains should be supplemented 

with other drainage improvement methods, if conditions warrant. Normally, 

considerable monitoring of pore pressures conditions is required for evaluation of drain system effectiveness.

Reducing Seepage – It is normally possible to reduce the level of seepage by 

sealing the surface of the upstream face of an impounding embankment. 

The major disadvantage of this alternative is that the magnitude and rate 

of improvement is difficult to predict. A “wait-and-see” approach can be 

taken if the existing factor of safety is not critical on a short-term basis. If the 

impoundment configuration permits, shifting of the fine refuse discharge 

point to aid in sealing of the upstream face of the embankment and to force 

the pool toward the back of the impoundment has been found to be effective 

in reducing seepage.

• Potential for Piping – The most common measure for reducing the potential for 

piping of embankment material in a seepage zone is to provide a granular filter 

around the discharge location so that water can escape without carrying additional 

fines. Often this measure is coupled with construction of a buttress with a filter zone 

between the existing embankment and new buttress material, as discussed above 

and illustrated in Figure 6.10a. The way to minimize the potential for piping is to 

prevent any past piping from extending into the new construction. Geophysical 

exploration may be useful for identifying voids. If a void is present, the designer 

must determine whether a filter system will be adequate and, if not, eliminate the 

void by a technique such as grouting.

• Erosion Control – Erosion control can often be achieved through relatively minor 

regrading and planting of vegetation or use of vegetative mats (erosion control 

blankets). If an existing slope is too steep, regrading the surface to provide horizontal 

benches can be effective.

6.3.6 Other Impounding Embankments

Other impounding embankments at mine sites include fresh water impoundments, sedimentation 

ponds, and treatment ponds. The primary distinctions between these structures and slurry impoundments are: (1) the size of the embankment is usually smaller, with the width and height designed to 

make efficient use of borrow material, typical of an earthen dam; (2) earthen borrow materials that 

are not part of the on-going disposal operation are typically used for embankment construction; (3)

a permanent water pool level is maintained for fresh water impoundments and treatment ponds, 
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resulting in steady state seepage conditions; (4) water pool level fluctuation within a sedimentation 

pond is usually over a limited range, with normal levels maintained at a low level; and (5) without 

the presence of fine coal refuse slurry, the pool level is typically in contact with the upstream face of 

the dam and thus is a source of seepage through the embankment. These geotechnical design considerations are important in the application of this Manual to projects with impoundments used for 

other than the disposal of coal refuse slurry.

The following geotechnical considerations apply to fresh water impoundments, sedimentation ponds, 

and treatment ponds:

• Seepage control – Internal drainage systems and foundation cutoff systems to prevent seepage from impacting the structural zone or toe of the embankment may be 

needed. Foundation treatment and a liner system may be needed. Impoundments 

that maintain a significant water pool depth impose greater hydraulic gradients 

on the embankment, potentially requiring more seepage control. Internal drains 

for these structures should have aggregate filters in order to comply with the 

federal dam safety practices discussed in Section 6.6.2.3. Additionally, the initial 

filling of such impoundments should be monitored closely for evidence of seepage 

or other distress, as many dam incidents and failures have occurred under these 

conditions.

• Slope stability – Static slope stability is maintained by a structural embankment 

constructed of borrow material, with significant attention given to foundation conditions. Seismic stability and deformations are most critical where loose foundation 

conditions are present, as the embankment materials are usually designed to be well 

6.10a BLANKET DRAIN ON EXISTING SLOPE

FIGURE 6.10 INTERNAL DRAINAGE SYSTEMS FOR EXISTING

 IMPOUNDING EMBANKMENTS

EXISTING EMBANKMENT

BUTTRESS OF COMPACTED

COARSE COAL REFUSE OR

BORROW MATERIAL

BLANKET DRAIN

(FILTERS NOT SHOWN) PHREATIC SURFACE

DRILLED HORIZONTAL

DRAINS WITH SLOTTED

PIPE

6.10b DRILLED HORIZONTAL DRAINS

EXISTING EMBANKMENT

ORIGINAL PHREATIC

SURFACE

LOWERED PHREATIC

SURFACE

FIGURE 6.10 INTERNAL DRAINAGE SYSTEMS FOR EXISTING IMPOUNDING EMBANKMENTS
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compacted and not subject to strength degradation. Sloughing and erosion considerations may be more critical than encountered for slurry impoundments because 

slopes may be steeper and the water pool level may fluctuate significantly. Fresh 

water impoundments may impose stresses on the impounding embankment due to 

rapid drawdown of the reservoir.

• Drainage and outlet structures – Structural foundations and excavation slopes 

are required for diversion channels, principal and emergency spillways, conduits 

and other auxiliary structures. Most fresh water impoundments incorporate outlet 

structures that allow emergency drawdown of the reservoir. Control of seepage along conduits should be a point of emphasis. Many dam failures have been 

caused by internal erosion due to excessive seepage through poorly compacted 

backfill around conduits.

• Underground mines – Stability, sealing of mine openings, and infiltration into underground mine workings are concerns. Considering the hydraulic pressures imposed 

by a fresh water dam, sites with shallow underground mine workings may not be 

feasible or may require significant remedial measures.

Several design references are available for earthen dams used for fresh water, sedimentation control, and treatment: USBR (1987a); USBR (1992a); Bigatel et al. (1999); NRCS (2005b).

6.4 SITE GEOTECHNICAL/GEOLOGICAL EXPLORATION

Although the characteristics of foundation materials have been discussed in general terms, specific 

properties required for the design of a disposal facility can be determined only by conducting surface and subsurface geological explorations at the site. The type and extent of explorations to be 

conducted will depend upon the size of the planned embankment, the complexity of the site geology, 

the nature of the foundation materials, the specific function of the facility, and, most importantly, 

whether or not the embankment will impound water. In general, a comprehensive site evaluation and 

exploration program for a coal refuse disposal facility comprises the following tasks:

1. Review of available topographic maps, geologic soil survey and mine maps, satellite 

imagery, and aerial photographs of the site and the surrounding area.

2. Surficial geological and geotechnical reconnaissance of the site.

3. Identification of data needs for design, such as topographic maps, mine void locations, and soil, rock and refuse properties.

4. Preparation of a site investigation plan using compiled information to develop a sitespecific strategy addressing exploration methods and locations, sampling and in-situ 

testing requirements, and contingency activities based on anticipated and possible 

unanticipated subsurface conditions.

5. Conducting a subsurface exploration program consisting of a combination of borings, test pits, in-situ testing and geophysical surveys.

6. Installation of monitoring systems such as piezometers, monitoring wells, and surveying monuments, particularly for exploration at existing facilities.

7. Comparison of sample descriptions to anticipated conditions. Laboratory index testing (i.e., particle-size distribution, Atterberg limits and moisture content) should be 

performed on the samples to confirm visual classifications.

8. Preparation of subsurface profiles based upon results from the field exploration and 

laboratory index tests and review of these profiles relative to the initial site investigation objectives and expectations.
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9. Selection of samples for performance testing and development of engineering properties for facility design from test results.

10. Laboratory performance testing and verification of the results using correlations 

with the index test results. If performance test results are inconsistent with the 

index test data, the inconsistency should be resolved (e.g., performance testing of 

reserve samples or additional field exploration to obtain replacement samples for 

testing).

11. Interpretation of performance test results, comparison to anticipated conditions, and 

selection of engineering properties needed for design.

12. Preparation of facility design, considering constructability issues, with identification 

of construction-phase exploration and testing requirements to confirm critical performance parameters.

The sequence and potentially iterative nature of these steps are summarized in the flow chart presented in Figure 6.11.

The preceding 12-step procedure is a suggested guideline for developing a thorough and cost-effective field exploration and testing program. The effort required for each of the listed steps will vary. 

Experience with similar geotechnical conditions will facilitate the development of the site exploration 

program. Each site exploration program must be sufficiently complete to provide the data required 

for a geotechnical evaluation of the site and design of the coal refuse disposal facility.

6.4.1 Background Data Sources

In planning a refuse disposal facility and in the initial site explorations, much useful information can 

be obtained from topographic, geologic and mine maps; agricultural soil surveys; satellite imagery 

and aerial photographs; and publications available from various government agencies. In addition, 

information from past investigations and area mining plans may also be searched to augment the 

information from public agencies. While typically not of sufficient detail for design purposes, these 

resources are readily available and represent an inexpensive source of valuable planning information.

6.4.1.1 Topographic Maps

Topographic maps are a vital source of information for planning a disposal facility and many 

published and on-line sources are available. The most important source is the series of standard 

topographic maps of the U.S. published by the U.S. Geological Survey (USGS). These maps cover 

quadrangles of 7.5 minutes of latitude and longitude at a scale of 1:24,000 (1 inch = 2,000 feet). 

More than 55,000 7.5-minute maps are available covering the 48 contiguous states. Other scales 

and areas of coverage are available from older map series or from other agencies. These topographic maps show the variation of ground surface elevation using contours (i.e., lines of constant 

elevation). The contour interval (elevation difference between adjacent contour lines) depends 

upon the scale of the map and the steepness of the terrain. In the Eastern United States the contour 

interval is usually 10 to 20 feet. In the western mountains, the contour interval is more commonly 

50 feet. USGS topographic maps also show cultural and man-made features (roads, dams, buildings and political boundaries), water features (lakes, rivers and canals), wooded areas and areas 

of past mining activity.

Topographic maps are useful for investigating an existing disposal facility, even though it may have 

been developed subsequent to the topographic mapping. Comparison of the original topography 

with the existing condition can provide an insight to the history and development of the site. Knowledge of the topography of the surrounding area is important in planning the expansion or abandonment of an existing disposal facility.
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Summarize basic soil/rock data and develop subsurface profile

Perform design, consider constructability issues, and identify construction

exploration and testing requirements

Select material properties and finalize subsurface profile

Review laboratory test data

Conduct laboratory testing

Review design objectives and initial results

Select representative soil/rock samples and plan laboratory testing

Review available site data and develop preliminary subsurface profile

Identify material properties needed for design and estimate scope of field program

Plan site exploration and field testing

Conduct site exploration and field testing

Prepare sample descriptions and perform laboratory index tests

PH

A

SE II S

T

U

D

Y, IF NEEDED

NO

YES

ENGINEERING

DESIGN

LABORATORY

TESTING

AND DATA

INTERPRETATION

SITE

EXPLORATION

AND FIELD

TESTING

FIGURE 6.11 FLOW CHART FOR SITE EXPLORATION, MATERIAL

Are results consistent with

preliminary profile?

Are there additional

data needs?

YES

Are results consistent and

representative of site

conditions?

Is a Phase II study

necessary?

NO

NO

YES

YES

NO

(SABATINI ET AL., 2002)

FIGURE 6.11 FLOW CHART FOR SITE EXPLORATION, MATERIAL PROPERTY TESTING

 AND FACILITY DESIGN
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Another resource is USGS’s The National Map web site, a framework for geographic knowledge of 

the U.S. that provides public access to high-quality, geospatial data and information. The site allows 

users to access, integrate, and apply geospatial data at global, national, and local scales. It includes a 

variety of information layers such as boundaries, elevation, geographic names, geology (global seismic networks, and real-time earthquakes), hydrography (real-time gauging stations and wetlands), 

imagery, land use and land cover, natural hazards (climate and hurricanes), and topographic maps 

that may be useful to designers.

6.4.1.2 Geologic Maps and Publications

Geologic maps of a proposed disposal facility site and surrounding area can provide valuable engineering information. These maps are prepared by the USGS or by state geological surveys and generally show ground surface outcrops of various rock units using a color code and letter symbols. 

Geologic maps usually have a column identifying formations and corresponding symbols and one 

or more geologic sections depicting the regional structure of the rock, identifying the rock units and 

providing a description of the units and their characteristics. To be most useful for evaluation of individual disposal facility sites, a geologic map should preferably have a scale no larger than the USGS 

topographic map of the area. Only a small part of the United States has been mapped in this detail, 

however, and it is often necessary to use maps covering up to several hundred square miles. Geologic 

maps can be valuable in the initial investigation and evaluation of a site, but proper interpretation of 

these maps requires knowledge of the fundamentals of geology and an understanding of how geologic information can be used in planning and design.

Specific regional and local information on geologic conditions should also be considered. For example, in steep Appalachian Valleys, joints and fractures from stress relief can affect excavation and 

abutment stability and impoundment seepage control. Reports on physiography published by the 

USGS (e.g., Wyrick and Borchers, 1981) can provide insight for planning exploration programs. Furthermore, weathered joints and fractures encountered in eastern Kentucky and southern West Virginia, sometimes referred to as “hillseams,” can represent critical foundation or abutment features, 

and mining publications may be helpful in planning associated with site preparation (e.g., Sames and 

Moeb, 1989). Additionally, local mining and highway construction experience should be sought, as 

it can disclose information on bedrock structure and fracture conditions that may influence development plans. Perin (2000) presents a case study that demonstrates the use and limitations of geologic 

publications, mapping, and exploration for a refuse disposal site in eastern Kentucky.

The USGS provides access to a wealth of information resources including maps, reports, publications, and links to related web sites. Resources of possible interest for refuse disposal planning and 

design at the time of publication include:

• USGS Library – Access to over 300,000 book, map, and serial records in the USGS 

Library online catalog. 

• USGS Store – Source for USGS maps and books, as well as products from other 

agencies.

• Publications Warehouse – Search engine for 67,000 bibliographic citations.

• Geologic Information – National clearinghouse for geologic maps, datasets, and 

related geoscience information with links to USGS geoscience databases and programs and resources for creating digital geologic maps.

• National Water Data – NWISWeb – Comprehensive gateway to water-resources data 

throughout the U.S.

• National Atlas of the United States® – Comprehensive collection of small-scale geospatial data from federal agencies.
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• geodata.gov – Web-based portal for access to maps, data, and other geospatial services from across all levels of government.

6.4.1.3 Agricultural Soil Surveys

Much more widely available than large-scale geologic maps are soil surveys prepared by the Natural 

Resources Conservation Service (NRCS). A soil survey is a detailed report on the surficial (i.e., upper 

5 to 6 feet) soils of a specific area. Soil surveys typically have maps showing soil-type boundaries and 

photographs, descriptions of soil characteristics, and tables of soil properties and features. The tables 

section of a soil survey report provides information on soil properties including engineering index 

properties, physical and chemical properties, and soil and water features. The tables section also has 

information on soil use, such as crops and pasture, recreation, and engineering. Although data from 

these surveys are generally not suitable for design analyses, the surveys are valuable tools for initial 

site reconnaissance studies and for planning detailed field explorations. Printed soil surveys can be 

obtained from NRCS regional offices or local soil conservation district offices. Surveys are also available from the NRCS web site.

6.4.1.4 Satellite Imagery, Aerial Photographs and Other Imagery

Aerial and satellite photographs and other imagery can be extremely valuable in the investigation 

and evaluation of a proposed or existing disposal facility site because they reveal much natural and 

man-made detail that may not be apparent from the ground, no matter how carefully ground reconnaissance is carried out. Also, these data can be used with Computer-Aided Design and Drafting 

(CADD) and Geographic Information System (GIS) software to provide informative representations 

of site conditions.

Data and imagery from satellites and aerial reconnaissance flights are increasingly being utilized for 

site exploration and characterization purposes. These data are available from commercial vendors 

as well as state and federal government agencies. At the time of publication of this Manual, satellite 

images are available to the public at a resolution of as small as 2 feet and aerial photographs can be 

obtained with a resolution of as small as 6 inches. An important use of satellite imagery and aerial 

photographs is the performance of terrain analysis and lineament studies for identification and 

location of surface features that are expressions of discontinuities in the underlying bedrock. Such 

features may reflect bedrock joint/fracture zones that warrant additional focus during exploration 

programs for impoundments or that require assessment of the potential for breakthrough potential 

to underground mines.

Photographs represent only a portion of the information available. Sophisticated sensors on satellites and sensory equipment that can be mounted on airplanes can provide a spectrum of light band 

information that when analyzed in combinations with sophisticated software allow identification of 

various surface characteristics. Interpretation of these data is referred to as remote sensing, and some 

applications include classification of land usage, classification of surface cover types, identification of 

stressed vegetation or tree canopy, and delineation of surface thermal variations.

Some satellite systems that currently (2009) generate data that might be used in site exploration and 

reconnaissance studies and for preparation of site drawings and figures include:

• Landsat • QuickBird

• IKONOS • ASTER

• SPOT • EO-1

• OrbView-3
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There are many forms of satellite data and many vendors that can provide satellite data packages. 

These data providers can be identified most easily from an Internet search. Also, a wide range of 

information can be gathered from custom-designed aerial surveys. The types and quality of data 

available and the associated costs should be carefully reviewed prior to purchase of data or contracting for such data to be obtained.

Generally aerial and satellite imagery data are available or can be obtained in an orthorectified format, 

which means that the positions of all the data in the photograph or image are accurately located with 

respect to a known coordinate system. Thus the data can be input to a GIS-based system and automatically shown in true relationship to other orthorectified data such as site boundaries, features, 

structure, and infrastructure. The USGS through its Earth Resources Observation and Science (EROS) 

center provides a wide range of such data, some of which can be obtained without charge.

Digital Raster Grids (DRGs) for some parts of the country can be downloaded free from the Internet 

and printed, and these are generally identical to USGS topographic sheets. They are frequently used 

as bases for CADD drawings, but they are not attractive for GIS applications because the various 

types of data shown (e.g., contours, roads, structures, shadings) are combined into a single layer.

Aerial photographs corresponding to USGS quadrangles (referred to as Digital Orthophoto Quadrangles or DOQs) are also frequently used as bases for CADD drawings where proposed construction or property boundaries are shown over a photographic base. They can be used in a similar 

manner in GIS-based software where individual layers representing roads, structures, utilities, etc. 

are displayed over a photographic base. Also available are Digital Orthophoto Quarter Quadrangles 

(DOQQs), which are orthorectified quarter-quadrangle (7.5-minute coverage) photographs.

Digital Elevation Model (DEM) data can be used to generate elevation contour maps of a site. DRG 

and DEM data can be combined in GIS-based software to generate three-dimensional models of USGS 

topographic sheets.

The above data can be obtained from the USGS, and some states provide extensive data free over the 

Internet. Pennsylvania, for example, provides 7.5-minute DRGs (also available with the surrounding 

border cropped off), DOQs, DOQQs and DEMs for the entire state. Other information such as coal 

mine maps and environmental data may be available from state web sites.

Soil-type data are available for some parts of the country in digital, orthorectified form suitable for 

use in GIS-based software.

These sources of aerial and satellite photographic data are increasingly available in an orthorectified 

(also referred to as georeferenced) format for input to GIS-based software where they can be automatically displayed in accurate relationship to other georeferenced site data. The reliability, availability and costs of various types of data should be carefully evaluated when planning site drawings 

and pictorial displays.

6.4.1.5 Past Investigations and Area Mining Plans

Information from past investigations and old mine plans in the vicinity of a planned or existing operation can provide valuable information for planning and preliminary design of a new facility. Particularly valuable are maps of past or planned future mining, as well as geologic information on bedrock 

structure, jointing and fracturing. As discussed in Section 8.2.1, mine maps are available at state 

agencies (e.g., Virginia Department of Mines, Minerals and Energy; Kentucky Department of Mines 

and Minerals; West Virginia Office of Miners’ Health, Safety and Training; Illinois State Geological 

Survey) and from MSHA and the Office of Surface Mining (OSM). MSHA district offices maintain 
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maps until a mine closes and OSM stores a copy of the final map after closure at the OSM National 

Mine Map Repository in Pittsburgh, Pennsylvania. Additional information related to sources and 

availability of mine maps is presented in Chapter 8.

Underground mine maps can be used to locate mine features with respect to the surface or other 

underground mines and to determine the dimensions of pillars and mine openings (Shackleford, 

2000). Information typically provided on underground mine maps includes (NRC, 2002):

• Pillared, worked-out, and abandoned areas, pillar locations, sealed areas, future 

projections, adjacent mine workings within 1,000 feet, surface or auger mines, mined 

areas of the coalbed, and the extent of pooled water.

• Dates of mining, coal seam sections, and survey data and markers.

• Surface features, coal outcrop, and 100-foot-overburden contour or other prescribed 

mining limit; mineral lease boundaries, surface property or mine boundary lines, 

and identification of coal ownership.

The accuracy and completeness of underground mine maps varies due to the age and non-uniform standards followed in their development. For example, there are significant limitations to 

some maps, particularly those for abandoned mines and mines operating before 1969. In addition, the horizontal and vertical (overburden) distances between mined barriers and an impoundment may not be accurately shown. Designers must consider these factors and the resulting 

impacts in using mine maps for refuse disposal planning and design in the vicinity of active or 

abandoned mines. Compounding the problem, some maps and records of older mines have been 

lost or destroyed. Therefore, to confirm map accuracy, site exploration using the geotechnical and 

geophysical exploration methods described in this chapter may be needed. Sections 8.2 and 8.3

present references and information to assist in locating available mapping and confirming its 

accuracy.

6.4.1.6 Individual Site Mapping

For most disposal facility sites, site mapping should be performed before planning reaches the 

design phase. Usually mapping is accomplished using low-altitude, large-scale aerial photography 

to develop detailed, large-scale topographic maps. The topographic maps can be produced at the 

scale and contour interval required for final site planning and design. Aerial topographic maps typically have contour intervals of one or two feet for relatively flat terrain and as much as 5 to 10 feet in 

steeper terrain.

For geographic areas of the size associated with most disposal facility sites, the major cost of obtaining aerial photographs is that of the aircraft. Therefore, it costs very little more to photograph areas 

adjacent to the anticipated site. This allows flexibility in making final plans and provides additional 

data for interpreting site conditions that may affect the facility design.

6.4.2 Surficial Reconnaissance and Geologic Mapping

The available topographic and geologic maps, aerial photography, and other documentation that pertain to the site should be supplemented by a surficial reconnaissance and geologic mapping, which 

consists of walking the disposal facility site and vicinity and observing topography, rock outcrops, mine 

openings, soil types, vegetative cover, spring discharges, perennial and intermittent watercourses, and 

any other features that may be relevant to the planned use of the site. This type of site reconnaissance generally requires a geologist or engineer who is familiar with refuse disposal and embankment 

design and who can recognize the significance of the observed features. If possible, reconnaissance 

should be conducted during times when vegetation is dormant so that site features are more visible. 
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Consideration should also be given to conducting the reconnaissance shortly after rainfall so that 

spring and flow channel conditions that may be relevant to the design can be observed.

The elevation of the site should be compared to known or correlated elevations of mineable coal 

seams in the vicinity. The site reconnaissance should include a search for evidence of past mining, 

including but not limited to mine entries, sinkholes, highwalls, haul roads, spoil piles, discolored 

seepage or watercourses, and areas with no vegetation or distressed vegetation. The presence of any 

oil and gas wells, pipelines, and other underground or overhead utilities should be noted.

Rock outcrops should be observed for lithology, bedding, and structure. Structural observations 

include strike and dip of strata, fracture orientation and spacing, and observable folds and faults. 

Where weathered joints and fractures are encountered, in-fill materials and widths should be 

recorded. Relatively recent fracturing should be noted as it may be indicative of subsidence. Bedding 

observations include thickness, sedimentary structures (e.g., planar bedding, cross bedding), and lateral continuity. Distinctive or known marker beds, such as coal seams or other persistent strata such 

as limestone or dolomite should be noted. Lithologic observations (e.g., color, grain size, mineral 

presence, weathering, and hardness) should also be noted.

Soils should be observed with respect to density, grain size, stiffness, color, mottling, structure, organic 

matter, and depth. Slopes should be observed for evidence of recent or older landslides, including 

slumps, scarps, and bent tree trunks. Mine rock waste (spoil) piles or other conditions related to previous mining such as cliffs, strip pits or ponds should be noted. General vegetation type and density 

should also be observed and documented.

Surficial reconnaissance may be supplemented by test pits or shallow, hand-augered borings to provide samples for basic laboratory tests for soil classification. Test pits excavated to rock may help 

disclose bedrock types, coal seam outcrops, and overburden jointing or fracturing. A careful surficial 

reconnaissance and accompanying tests can produce data sufficient for a preliminary surficial map 

of a refuse disposal facility site. However, as with any exploration program, it should be recognized 

that undetected subsurface conditions are a risk, and reasonable contingencies should be considered 

in the development of designs.

6.4.3 Subsurface Exploration and In-Situ Test Planning

After available information has been collected and evaluated, the designer can begin planning a 

program for subsurface exploration and in-situ testing. The field exploration methods, sampling 

requirements, and types and frequency of field tests to be performed should be determined based 

on project design requirements, geologic conditions, the availability of existing subsurface information, the availability of equipment resources, and local practice. ASTM D420, “Standard Guide to Site 

Characterization for Engineering Design and Construction Purposes,” provides general guidelines 

for site reconnaissance, exploration planning, equipment and procedures, geophysical exploration, 

sampling, material classification, in-situ testing, interpretation and reporting.

An overall field exploration and in-situ testing program for obtaining the data needed to define subsurface conditions and perform engineering analyses and design should be developed. Once the 

field exploration and testing begins, the program may need to be modified in response to site access 

constraints (e.g., steep terrain may not be accessible to the available drilling equipment) or to address 

variations in subsurface conditions not anticipated during exploration planning.

Site exploration programs are often conducted in phases. To obtain an overview of the geological 

issues that can affect a facility, remote sensing, geophysical exploration and widely-spaced geotechnical sampling and testing may be conducted as part of an initial phase. During a second or subsequent 
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phase, localized disturbed and undisturbed sampling and in-situ testing may be conducted to obtain 

more detailed information for defining geologic features and for determining geotechnical engineering properties for design. The types of subsurface exploration and testing activities in the typical 

sequence in which they are conducted are:

• Remote sensing

• Geophysical investigations

• Test pits

• Disturbed sampling

• In-situ testing

• Undisturbed sampling

Remote sensing data can be used to identify terrain conditions, geologic formations, escarpments and 

surface reflection of faults or highly-jointed bedrock zones, buried stream beds, site access conditions 

and general soil and rock formations. Remote sensing data from satellites (e.g., Landsat images from 

NASA), aerial photographs from the USGS or state geologic surveys, and data from commercial 

aerial mapping services may be useful. The designer should be familiar with the use of such data, 

including limitations.

Geophysical methods offer another means for characterizing subsurface conditions. Geophysical 

methods can be used for general site characterization, mapping abandoned mine workings and measuring physical properties in boreholes at coal refuse disposal facilities. For general site characterization, geophysical methods can be used to determine the depth to bedrock, map ground stratigraphy, 

detect sudden changes in subsurface formations, assess the rippability of bedrock, and map variations of physical properties within coal refuse and groundwater. Surface techniques such as electrical 

resistivity, ground penetrating radar (GPR), electromagnetic conductivity (EM) or seismic refraction 

can be applied. These techniques may be useful in defining broad variations in the subsurface, but 

boreholes are needed for verification and interpretation.

Some geophysical methods can be used for detection of abandoned workings or cavities in karst formations from the surface. For this purpose, the most commonly considered geophysical techniques 

(although only recently applied at refuse impoundment sites) include electrical resistivity, seismic 

reflection and gravity. The success of these techniques depends on the depth to the mine workings, 

degree of flooding, and thickness of the coal seam. Downhole geophysical methods can define vertical variations in physical properties. In particular, crosshole and downhole seismic tests induce 

mechanical waves within the ground mass to provide information on the dynamic elastic properties 

including the shear (S) wave velocity required for seismic site amplification studies of ground shaking 

and for soil liquefaction evaluations. The application of surface and borehole geophysical methods 

suitable for siting and engineering evaluations of refuse disposal sites is presented in Section 6.4.4.

Test pits are small excavations dug to a depth of 10 to 15 feet (i.e., to the typical reach of an excavator 

or to refusal in rock). Compared to other exploration methods, test pits are more efficient because 

they can provide information about a relatively large area inexpensively, and they expose a large 

amount of soil for detailed examination by the field geologist or engineer. Because the side walls of 

a test pit can collapse rapidly, field personnel should not climb into a hole deeper than about four 

feet without assessment of soil stability and use of shoring, as appropriate. The locations for test pits 

are typically selected in the field as the site investigation program progresses. Test pits are generally 

used to supplement data between borings or to explore areas where only near-surface conditions are 

important, such as potential source areas for borrow material. The use of test pits for geologic mapping and material sampling is discussed in Section 6.4.3.3.
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Disturbed samples can be used to determine soil type, gradation, classification, water content, consistency, relative density, and stratification. The samples are considered to be disturbed because 

the sampling process modifies their natural structure. Disturbed samples are typically obtained 

using track- or truck-mounted augers and other rotary-drilling techniques. The most common 

disturbed sampling method is the Standard Penetration Test (SPT), which is performed using a 

split-barrel sampler during the drilling of geotechnical borings. Geotechnical borings allow: (1) 

testing as drilling progresses and recovery of samples, (2) measurement of groundwater levels 

and collection of groundwater samples, and (3) installation of instrumentation for monitoring the 

groundwater level or the deformation of the soil and rock at any depth. In planning an investigation, boring locations should be selected so as to optimize the amount of useful data from the 

drilling program. The basis for choosing boring locations and procedures for drilling borings is 

discussed in Section 6.4.3.1.

Other in-situ test and geophysical methods can be used to supplement soil boring data. For instance, 

the cone penetrometer test (CPT), also referred to as the cone penetration test, provides information on subsurface soils without sampling. Stratigraphy and strength characteristics of soils can be 

determined as the cone penetrometer is advanced. In-situ methods are most effective when they 

are used in combination with conventional sampling to reduce the cost and the time required for 

field work. Data from these tests can be correlated with sampling and testing data obtained by 

conventional means.

Undisturbed samples are obtained for laboratory testing when determinations of the in-place 

strength, compressibility (settlement), natural moisture content, unit weight, or hydraulic conductivity are needed. They also allow observation of discontinuities, fractures and fissures associated 

with subsurface formations. Although the sampling equipment is designed to minimize disturbance and these sample types are designated as “undisturbed,” in reality they are disturbed to 

some degree. The degree of disturbance depends on the type of subsurface materials, type and 

condition of the sampling equipment used, the skill of the drillers, and the storage and transportation methods used.

6.4.3.1 Program Planning

The number and depth of borings and locations of in-situ tests required for a particular subsurface 

exploration program will depend on the size of the disposal facility site, the nature and uniformity of 

the site geology, the magnitude of loads to be applied to the natural materials, the groundwater conditions, the presence of past or active underground mining in the vicinity of the embankment, and 

the complexity of the facility design. For example, explorations for new impounding embankments 

normally require significantly more borings than those for non-impounding embankments.

The appropriate depth and spacing of borings is difficult to generalize because they depend upon 

site conditions and disposal facility plans. An exploration program for a specific site should provide 

sufficient information for identifying, delineating and correlating geologic and soil conditions for 

designing a safe and environmentally acceptable disposal facility. Often the final location of borings 

and in-situ test sites must be determined in the field, or additions must be made to the boring program based upon evaluation of the initial data obtained. Excavation of test pits should be considered 

as a means for supplementation of the boring program. Test pits facilitate examination of shallow 

subsurface conditions and the recovery of bulk samples.

The spacing and number of borings beneath a dam depends on the complexity of the geology. Some 

of the more important factors to consider are the character and continuity of the beds, elevation of the 

strata, presence or absence of joints or faults, and proximity to previous underground mining. The 

depth, thickness, sequence, extent, and continuity of the various strata should be determined.
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USDA (1978) guidance on exploratory borings for fresh-water dams includes the following:

• Centerline of Dam – Minimum of one boring on each abutment and at the outlet 

structure transect, plus one boring at any abandoned stream channel, plus additional 

borings for correlation of strata. Boring depths should not be less than the height of 

the dam unless unweathered rock is encountered and is not underlain by compressible strata or mine workings.

• Outlet Conduit – In addition to the boring at the transect with the centerline of the 

dam, borings should be located at the vertical riser intake, downstream toe of the 

dam, outlet of the conduit, and additional locations as needed to define the rock surface. Boring depths should not be less than the height of the backfill over the conduit 

or 12 feet, whichever is greater, unless unweathered rock is encountered. At the riser 

intake, the depth should not be less than the planned height of the riser above natural ground or 12 feet, whichever is greater.

• Emergency Spillway – Geologic cross sections based upon three or more borings 

should be developed at the control section, intake section, and outlet section with 

additional borings at cross sections as needed for correlation and location of strata 

contacts and identification of excavation materials. Borings should extend to a depth 

not less than 2 feet below the bottom of the proposed spillway.

• Foundation Drain – Carefully-logged borings at the centerline of the dam and toe 

may provide sufficient information, although additional borings or test pits may be 

necessary where subsurface conditions are highly variable.

Tables 6.15 and 6.16 present guidelines for typical subsurface exploration and in-situ testing programs for new disposal facilities both with and without impoundments. The exploration program 

to be used for a particular disposal facility and the associated in-situ testing must be developed by 

a qualified geotechnical engineer familiar with the requirements of the proposed facility. If inadequate data are obtained from the initial boring program, additional borings and/or test pits should 

be advanced to supplement the original boring data.

An in-situ testing program typically involves SPTs obtained during boring advancement and unconfined compression tests on recovered split-barrel samples performed by field personnel using pocket 

penetrometers or field torvane equipment. Where soft sediments or fine coal refuse are critical to 

embankment stability, CPTs provide an effective method for characterizing the consistency and 

strength of the material. In-situ testing also generally includes hydraulic conductivity testing performed in soil and rock to assess foundation conditions in valley bottom and abutment areas. Installation of piezometers in completed borings can facilitate field hydraulic conductivity testing as well 

as site groundwater characterization.

Table 6.17 presents guidelines for a typical subsurface exploration and in-situ testing program for an 

existing disposal facility. Similar to a new disposal facility, the in-situ testing program for an existing 

disposal facility and the interpretation of subsurface data must be performed by a qualified geotechnical engineer familiar with the facility design parameters. For an existing disposal facility, borings 

are normally required when it is suspected that the embankment factors of safety are low or when 

expansion of the embankment is planned, as discussed in Section 6.3.4, and some of the information 

indicated in Table 6.17 may already be available from previous plans. The subsurface exploration and 

in-situ testing program is greatly influenced by site conditions, the disposal facility hazard potential, 

the type of disposed refuse and its current condition, the stage of facility development, and the extent 

of records of previous construction and placement of materials. Often, the in-situ testing program 

will be designed to obtain the same information relative to the underlying foundation materials that 

would be required for a new disposal facility. Extensive data relative to the materials and quality of 

construction of the existing embankment must generally be obtained.
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TABLE 6.15 GUIDELINE SUBSURFACE EXPLORATION PROGRAM 

FOR A NEW IMPOUNDING EMBANKMENT

Location Guideline Exploration Program

Abutments

One boring (minimum) should be drilled in each abutment to the maximum extension of the 

embankment, to the depth of the valley bottom, or to a depth where the boring will overlap geologically with adjacent borings. Multiple borings may be required in each abutment 

depending on the length of the dam and complexity of the geology.

Other locations

Additional borings, particularly for large embankments, beneath the structural portion of 

embankment with spacing and depth to provide sufficient overlap between adjacent borings 

to correlate data and to develop a geologic profile along the embankment.

Valley bottom or lowest 

portion of embankments 

not located in valley 

bottom

A least one and generally multiple borings should be planned beneath the critical structural 

portion of embankment. The depth of at least one boring should be approximately equal 

to the planned height of the embankment unless firm bedrock is encountered at shallower 

depth. Even if rock is encountered, deeper borings may be needed to sufficiently reveal 

ground water seepage/flow, to evaluate underlying deep mining, or for other special requirements. Test pits should be provided for the purpose of observing and documenting the 

continuity of shallow soil stratigraphy and for obtaining bulk samples.

Upstream and 

downstream

of crest

At least one boring upstream and one downstream from valley bottom borings for correlating 

data and developing a geologic profile across embankment. The downstream boring should 

be near the toe of the embankment where slope stability is expected to be most critical. Borings should penetrate to at least softest layer to be incorporated in stability analyses and preferably into competent rock. Sufficient test pits to observe and document continuity of shallow 

soil stratigraphy and obtain bulk samples should be provided.

Decant/spillway 

structures

Borings and/or test pits along probable axes of structures to be founded on natural soil or 

soft rock should be planned and should be drilled to a depth at least equal to the width of 

the structures and preferably into very stiff soils or firm bedrock.

Embankment in vicinity

of past mining

Unless available mine maps and other information confirm that underground mining is distant enough to preclude potential impacts to the embankment, borings should be drilled at 

locations and to depths necessary for assessment of the accuracy of the mine mapping 

and for determination of the potential amount of subsidence, the probability of additional 

subsidence, and the potential for mine breakthrough. Typically at least one boring (and to 

assess some subsidence and breakthrough situations, several borings are required) should 

be drilled to mine elevation to obtain a full profile of the overlying rock and to define the 

groundwater level in the mine. This is particularly important if the mine is located so that it 

could adversely affect embankment stability, or if the mine is still in use and water inflow 

from the impoundment could imperil the miners or the mining operation. Appropriate safety 

provisions should be taken, including wet drilling, with respect to the potential for encountering a potentially explosive gas mixture within the mine.

Impoundment area Sufficient borings and/or test pits to observe and document continuity of shallow soil stratigraphy and obtain bulk samples in order to determine the potential for ground water impacts.

Borrow areas Sufficient borings and/or test pits to characterize materials, estimate available volume, and 

obtain bulk samples.

General

Additional borings, as determined by a qualified engineer or geologist, to meet special 

requirements of planned disposal facility or to gain knowledge of special geologic factors 

affecting design.

6.4.3.2 Subsurface Exploration and In-situ Test Methods and Applicability

This section provides information on various subsurface exploration and in-situ test methods that are 

currently used for site characterization, sampling and determination of site-specific soil and rock properties for the design of coal refuse disposal facilities. Conventional subsurface exploration and testing programs typically include test pits, rotary drilling, SPTs, and disturbed and undisturbed sample 

recovery. In-situ testing methods described in this section include SPT, CPT, piezocone penetrometer 

test (CPTu), seismic piezocone penetrometer test (SCPTu), and vane shear test (VST). Additionally, 

borehole testing to measure hydraulic conductivity is typically conducted. Standardized test proce-
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Location/Condition Guideline Exploration Program

Embankment toe

Sufficient borings and/or test pits should be provided near the planned location of toe to 

explore conditions where foundation material may have a lower strength than embankment 

material. Borings must extend past the depth where stability is a consideration and preferably 

should extend to competent bedrock. Where the depth to bedrock is less than 10 feet, test 

pits may be substituted for these borings.

Abutments

Sufficient borings and/or test pits should be provided to observe and document subsurface 

conditions where stability is a consideration and for planned excavations for diversion ditches 

or access roads.

Potential for groundwater impacts

If seepage through refuse may create an undesirable environmental condition, borings should 

be drilled sufficiently deep to identify the groundwater level. These borings will provide data 

for determining need for a drainage collection system and/or an impervious liner between the 

coal refuse and the underlying foundation. Often one or two additional borings and several 

test pits will need to be advanced at the general disposal facility site to determine groundwater conditions throughout the area that will be covered by refuse.

Embankment in vicinity 

of past mining

Unless available mine maps and other information confirm that underground mining is sufficiently distant to preclude potential impacts to the embankment, borings should be drilled to 

evaluate the potential effects of subsidence, the stability of the structural portion of embankment, the potential for breakthrough, or if leachates from facility could adversely affect groundwater quality in the mine.

Borrow areas Sufficient borings and/or test pits should be provided to characterize materials, estimate 

available volume, and obtain bulk samples.

General

Additional borings, as determined by a qualified engineer or geologist, should be provided to 

meet special requirements of the planned disposal facility or to gain knowledge of geologic 

factors that could affect design.

dures for these in-situ testing methods have been developed by ASTM, and these are identified in the 

sections that follow. ASTM D 420, “Standard Guide to Site Characterization for Engineering Design 

and Construction Purposes,” summarizes the various methods for site characterization. For in-situ test 

methods that do not have standardized ASTM procedures, references for additional details are cited.

Conventional subsurface exploration methods typically involve the retrieval of soil samples and rock 

core. Soil samples may be either disturbed (but representative) or undisturbed. Disturbed samples 

are those obtained using equipment that destroys the macro structure of the soil, but does not alter its 

mineralogical composition. Specimens from these samples can be used for determining the general 

lithology of soil deposits; for identifying soil components and general classification purposes; and 

for determining grain size, Atterberg limits, and compaction characteristics of soils. Undisturbed 

samples are obtained in cohesive or fine-grained soil strata for use in laboratory testing to determine 

engineering properties. Undisturbed samples of granular soils can be obtained, but specialized and 

costly procedures are required such as freezing or resin impregnation and block or core type sampling. The term “undisturbed” refers to the relative degree of disturbance to the in-situ properties 

of the soil. Undisturbed samples are obtained with specialized equipment designed to minimize the 

disturbance to the in-situ structure and moisture content of the soils. Specimens obtained by undisturbed sampling methods are used for determining the strength, soil layering, hydraulic conductivity, density, consolidation, dynamic properties, and other engineering properties. Common methods 

used to obtain disturbed and undisturbed soil samples are presented in Table 6.18.

6.4.3.3 Test Pits

To evaluate materials near the ground surface and to supplement the information gained from the borings, test pits or test trenches are frequently employed. Test pits are usually excavated by a backhoe or 

bulldozer and can range from a few cubic feet to a few cubic yards in volume. In addition to providing 

TABLE 6.16 GUIDELINE SUBSURFACE EXPLORATION PROGRAM FOR 

A NEW EMBANKMENT WITHOUT AN IMPOUNDMENT
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Location/Condition Guideline Exploration Program

Embankment

centerline

Typically, three or more borings should be drilled in a line perpendicular to embankment 

axis. Borings should extend to a depth below the phreatic surface in the embankment 

unless a water surface is not encountered for a depth significantly below that which 

could reasonably be expected to affect the stability of the embankment. Piezometers 

should be installed in selected borings to monitor the level of water surface within the 

embankment and foundation. Special efforts should be made to evaluate materials 

and water levels encountered as a function of depth within these borings to determine if horizontal impervious zones are present that could affect seepage through the 

embankment.

Downstream

embankment

face

A line of borings parallel to the axis of the embankment should be drilled to a depth below 

the phreatic surface. Normally these borings should be drilled in the downstream face 

of embankment where the phreatic surface level is most critical to stability. This location 

should be determined based on a profile developed from data from the first line of borings perpendicular to the embankment axis.

No information on 

foundation preparation or 

foundation stability is a 

concern

If no information is available relative to procedures originally used to prepare the embankment foundation or if the designer believes that stability along the existing foundation 

may be critical, at least one and preferably multiple borings should be drilled in the 

embankment axis and downstream face and should extend through the embankment 

and into competent rock.

Downstream valley

bottom

At least one boring should be drilled at the highest section of embankment where stability is likely to be critical. Multiple borings may be required in this area if expansion of the 

disposal facility to a greater elevation is planned.

Facility expansion Additional borings should be drilled at locations that will appropriately allow for analyses 

associated with enlarging the embankment, as determined by designer.

Settled fine

refuse

If the embankment impounds settled fine refuse slurry and an expansion of the 

embankment over slurry is contemplated, borings should be extended into the slurry 

in order to obtain samples for laboratory testing. The exploration program may also 

entail the use of cone penetration testing and geophysical surveys. If the embankment was built using the upstream method, the extent to which fine refuse underlies 

the embankment should be determined by drilling borings and/or use of geophysical 

methods.

Embankment in vicinity

of past mining

If past mining has been completed beneath any portion of the existing disposal facility, 

borings should be drilled at locations and depths necessary to determine the potential 

subsidence, the probability of additional subsidence, and the potential for mine breakthrough. Generally, at least one boring should be drilled to below the mine elevation 

to obtain a full profile of overlying rock and to define the groundwater level at the mine 

elevation. This is particularly important if the mine is located such that it could adversely 

affect the stability of the existing embankment or if the embankment impounds water and 

the mine is still in use (i.e., where water inflow from the impoundment could be unsafe to 

miners or mining operations). Appropriate safety provisions should be taken to prevent 

possible gas release through any boring drilled to an abandoned mine.

Decant/spillway

structures

If changes in the existing disposal facility include plans for decant or spillway structures 

founded on soil, soft rock or refuse materials, borings should be drilled and/or test pits 

advanced along the probable structure axis to provide data for a sufficient depth to properly design foundations or bedding requirements.

General

The required boring program and any modifications should be determined by a qualified 

geotechnical engineer. Flexibility will be required on the part of the engineer because the 

boring program will frequently need to be modified as the field investigation proceeds in 

order to resolve issues arising from data obtained from earlier borings.

TABLE 6.17 GUIDELINE SUBSURFACE EXPLORATION PROGRAM 

FOR AN EXISTING EMBANKMENT
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access to larger samples than possible from borings, test pits permit direct visual examination of the 

soil in place. In-situ density and field shear strength tests also may be conducted in test pits at various 

depths. Test trenches permit observation of lateral variations of soil conditions over the trench length. 

This may be particularly useful in residual soils produced by in-place weathering of rock where several 

degrees of weathering and initial rock quality may be present and in colluvial soils where large variations in gradation may be observed. If compressible cohesive soils are present, test pits can provide 

access for undisturbed sampling of soil blocks, as described in ASTM D 7015, “Standard Practices for 

Obtaining Undisturbed Block (Cubical and Cylindrical) Samples of Soils.”

6.4.3.4 Boring Methods and the Standard Penetration Test (SPT)

A variety of drilling methods can be used for subsurface geotechnical exploration, including displacement, wash, and auger borings, and percussion drilling. Augering in soil and coring in rock are 

preferred because these methods permit recovery of representative samples for classification and 

testing. Depending on the terrain, either truck- or skid-mounted drilling rigs are used. Truck rigs are 

generally more powerful and drill faster, but skid rigs are more maneuverable in rough or heavily 

wooded terrain and are less difficult to mobilize.

For drilling in soil, augers ranging from 6 to as much as 18 inches in diameter are used. Soil samples 

can be obtained from: (1) the auger cuttings, (2) a split-barrel drive sampler (disturbed sample), or (3) 

a thin-walled tube (undisturbed sample). Usually, the boring is advanced by augering a set distance 

(2 to 10 feet) or until there is a change in soil layer, and either a split-barrel or thin-walled tube sample 

is then taken.

Where undisturbed samples are not required, split-barrel samples are obtained in accordance with 

ASTM D 1586, “Standard Test Method for Penetration Test and Split-Barrel Sampling of Soils.” The 

SPT is accomplished by placing a hollow, thick-walled-tube sampler at the bottom of the boring 

TABLE 6.18 COMMON SAMPLING METHODS

Sampler Sample Type Appropriate Soil Types Method of Penetration

Split-barrel

(Split-spoon) Disturbed Sands, silts, clays Hammer driven

Thin-walled

Shelby tube Undisturbed Clays, silts, fine-grained soils, clayey sands Mechanically pushed

Continuous

push

Partially

Undisturbed Sands, silts, and clays Hydraulic push with plastic lining

Piston Undisturbed Silts and clays Hydraulic push

Pitcher Undisturbed

Stiff to hard clay, silt, sand, partially 

weathered rock, and frozen or resin 

impregnated granular soil

Rotation and hydraulic pressure

Dennison Undisturbed Stiff to hard clay, silt, sand and partially 

weathered rock Rotation and hydraulic pressure

Continuous

auger

Disturbed Cohesive soils Drilling with hollow-stem augers

Bulk Disturbed Gravels, sands, silts, clays Hand tools, bucket augering

Block Undisturbed Cohesive soils and frozen or resinimpregnated granular soil Hand tools

(ADAPTED FROM MAYNE ET AL., 2002)
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and driving it 18 inches into the underlying soil by blows from a 140-pound hammer dropping 30 

inches. The number of blows required to drive the sampler each 6-inch interval is recorded. The first 

6-inch interval is regarded as a seating value, and the blows for the second and third increments 

are summed to give the SPT N-value or blow count resistance of the soil. If the sampler cannot be 

driven 18 inches, the number of blows for each 6-inch increment and for each partial increment is 

recorded on the boring log. For partial increments, the depth of penetration is recorded in addition 

to the number of blows. The SPT can be performed in a wide variety of soil types, as well as weak 

rocks, but it is not particularly useful for the characterization of gravel deposits or soft clays. The 

SPT provides a semi-quantitative measure of the stiffness or density of the soil in place. When the 

sampler is removed from the boring, a representative soil sample is recovered for classification and 

for laboratory tests that are applicable to disturbed soil samples, including moisture content, grain 

size analysis and Atterberg limits. The advantages and limitations of the SPT are summarized in 

Table 6.19.

A properly conducted boring program entails close supervision by an experienced engineer or geologist. This supervision includes: (1) careful and detailed classification of the materials recovered from 

the boring, (2) preparation of a detailed log for each boring, noting the classification of the material 

and its condition, and (3) other significant observations such as water levels in the boring.

The boring log is the basic record for geotechnical exploration and provides a detailed record of the 

work performed and the subsurface conditions at the boring location and can be recorded on paper 

or on electronic data loggers. If recorded on paper, field boring logs should be written or printed 

legibly and should be as clean as is practical considering site conditions and weather. All appropriate portions of the boring logs should be completed in the field as the work is being performed.

A wide variety of drilling log forms are in use, but the specific form(s) to be used for a given type of 

boring will depend upon local practice. A boring log should provide a description of exploration procedures and subsurface conditions encountered during drilling, sampling and coring. The following 

information should be provided on a boring log:

• Survey data including boring location in reference to site coordinates, surface elevation, and bench mark location and datum, if available.

• An accurate record of any change in the planned boring locations.

• Identification of the soil and bedrock encountered, including density, consistency, 

color, moisture, structure, and geologic origin.

• The depths of the various generalized soil and rock strata encountered.

• Sampler type, depth, penetration, and recovery.

• Sampling resistance in terms of hydraulic pressure or blows per depth of sampler 

penetration; size and type of hammer; height of drop.

• Soil sampling interval and recovery.

• Rock core run numbers, including depths and lengths, core recovery, and rock quality designation (RQD).

• Drilling method used to advance and stabilize the hole.

• Comparative resistance to drilling.

• Observed loss of drilling fluid.

• Water level observations.

• The date and time that the boring was started, completed, and when water level 

measurements were made.

• Closure of borings.
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TABLE 6.19 ADVANTAGES AND LIMITATIONS OF THE STANDARD PENETRATION TEST

 Advantages Limitations

Obtain both a sample and a number Disturbed sample (index tests only)

Simple, rugged and suitable in many soils Analysis required if varying numerical results are obtained

Can be used in weak rock Not applicable for soft clays and silts

Available throughout the U.S. High variability and uncertainty

(ADAPTED FROM MAYNE ET AL., 2002)

6.4.3.5 Undisturbed Soil Sampling

Undisturbed samples are usually obtained when the structure of the soil (e.g., strength and compressibility) is important to its behavior. Relatively undisturbed samples are commonly obtained by 

pushing a thin-walled tube into the soil at the bottom of the boring and removing the soil sample 

from the boring in the protective tube. The sampler typically has an approximate outside diameter of 

3 inches and inside diameter of 2 7⁄8 inches. Thin-walled samplers vary in outside diameter between 

2 and 3 inches and typically come in lengths from 28 to 36 inches. Larger diameter tubes are used 

where higher quality samples are desired and sampling disturbance must be kept to a minimum. The 

procedure for thin-walled tube sampling is described in ASTM D 1587, “Standard Practice for ThinWalled Tube Sampling of Soils for Geotechnical Purposes.”

Undisturbed sampling is generally practical only for fine-grained, cohesive soils that contain few rock 

particles. Undisturbed tube sampling of coarse gravels and coarse refuse with large particles is not 

practical because of the resistance to pushing the tube and potential damage to the tube. Special types 

of tubes and procedures are sometimes employed to obtain suitable undisturbed samples of very soft 

or sensitive soils, such as very wet fine refuse. These are described in Mayne et al. (2002). If these types 

of samples are desired, the ability of the potential drillers to obtain them should be verified.

The thin-walled tubes used for undisturbed sampling are manufactured using carbon steel, galvanized carbon steel, stainless steel, and brass. Carbon steel tubes are often used, but are unsuitable if 

the samples are to be stored in the tubes for more than a few days because of rusting. In stiff soils, 

galvanized carbon steel tubes are preferred because carbon steel is stronger, less expensive, and the 

galvanizing provides additional resistance to corrosion. Thin-walled tubes are manufactured with 

a beveled front edge to reduce pushing resistance and sample disturbance. Thin-walled tubes can 

be pushed with a fixed head or piston head. ASTM D 4220, “Standard Practices for Preserving and 

Transporting Soil Samples,” provides guidance for field preparation, transport and storage of undisturbed samples prior to laboratory testing.

6.4.3.6 Rock Coring

Where borings must extend into weathered and unweathered rock, rock drilling and sampling are 

required. For disposal sites, defining the top of rock by drilling can be difficult, especially when large 

boulders are present and where the top of rock profile is irregular. In all cases, care must be taken in 

determining the top of rock because improper identification may lead to a miscalculated thickness of 

rock overburden above a mine or inaccurate determination of material quantities.

Destructive rock drilling is a relatively quick and inexpensive means for advancing a boring when 

an intact rock sample is not required. Destructive drilling can be used to determine the elevation 

of the top of rock or the elevation of the top of a mine void. Types of destructive drilling include 

air-track drilling, down-the-hole percussive drilling, rotary tricone (roller bit) drilling, rotary drag 

bit drilling and, in very soft rocks, augering with carbide-tipped bits. When destructive drilling is 
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employed, caution should be exercised in determining the top of soft rock because drilling proceeds rapidly, and weathered and soft rock can be easily penetrated, resulting in an inaccurate 

top-of-rock elevation.

When formations are encountered that are too hard to be sampled by soil sampling methods (typically more than 50 blows per inch with a 2-inch-diameter, split-barrel sampler), core drilling procedures should be employed, as described in ASTM D 2113, “Standard Practice for Rock Core Drilling 

and Sampling of Rock for Site Investigation.” Seismic refraction or other geophysical methods can 

be used to assist in determining the top-of-rock elevation. Seismic-refraction data can also provide 

information between borings. The depth of rock coring will vary depending on site conditions, but as 

a minimum coring should extend to a depth sufficient to account for the presence of pervious or soft 

strata that could affect the stability of the embankment.

Core barrels may be single-, double-, or triple-tube types. A double-tube core barrel is commonly 

used because the inner and outer core tubes better isolate the rock core from the drilling fluid 

stream and the inner tube isolates the core from the rotating outer tube. In triple-tube core barrels, the inner tube may be longitudinally split to allow observation and removal of the core with 

reduced disturbance.

Rock coring can be accomplished with either conventional or wireline equipment. With conventional 

drilling equipment, the entire string of rods and core barrel are raised to the surface after each core 

run for rock core retrieval. Wireline drilling equipment allows the inner tube to be uncoupled from 

the outer tube and raised rapidly to the surface by means of a wire-line hoist. The main advantage 

of wireline drilling is the increased drilling production resulting from the rapid removal of the core 

from the hole. Wireline coring also provides improved quality of recovered core, particularly in soft 

rock, because this method minimizes rough handling of the core barrel during retrieval of the barrel 

from the borehole and when the core barrel is opened. Wireline drilling can be used on any rock 

coring project, but typically is used where boreholes are more than about 75 feet deep and rapid 

removal of the core from the hole has a greater effect on cost.

Although NX (2.154-inch-diameter) core is the size most frequently used for engineering explorations, both larger and smaller sizes are sometimes used. Larger core sizes will usually produce greater 

recovery and less fracturing during drilling.

The length of each core run should be limited to a maximum of 10 feet. Core run lengths should be 

reduced to 5 feet or less just below the rock surface and in highly fractured or weathered rock zones. 

Shorter core runs generally reduce the degree of damage to the core and improve core recovery in 

poor quality rock.

The core bit provides the grinding action at the bottom of the core barrel assembly that cuts the core 

from the rock mass. Diamond, carbide-tipped and sawtooth core bits are the most commonly used. 

Core bits are generally selected by the driller and are often approved by the geotechnical engineer. 

Bit selection should be based on drill bit performance for the expected formations and the proposed 

drilling fluid.

Clear water is most often used as the drilling fluid in rock coring because it is readily available, does 

not react with most rock types and does not require special disposal procedures. If collapsing holes or 

zones where there is loss of drill water are encountered, a drilling mud may be required for stabilizing the borehole. Drilling mud should be used with care because it will clog open joints and fractures 

and can adversely affect hydraulic conductivity measurements and piezometer installations. A settling 

basin should be used to remove drill cuttings and to allow recirculation of the drilling fluid. Unless 

contaminated with oil or other substances, drilling fluids can be discharged onto the ground surface.
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Rock core should be carefully removed from the core barrel, placed in a rock core box appropriately 

sized for the diameter of core drilled, and visually classified. The rock core recovery and RQD should 

be calculated and recorded. ASTM D 6032, “Standard Test Method for Determining Rock Quality 

Designation (RQD) of Rock Core,” should be followed in determining the RQD, which is a normalized measure of the degree of rock fracturing. The rock core should be preserved and transported 

following the guidance in ASTM D 5079, “Standard Practices for Preserving and Transporting Rock 

Core Samples.” Additional guidance for visual classification, core handling and labeling, and other 

field practices is provided in Mayne et al. (2002). The application of RQD in geotechnical design is 

presented in Section 6.6.

6.4.3.7 Cone Penetrometer Test (CPT) and Piezocone Penetrometer Test (CPTu or PCPT)

An alternative or supplement to the SPT is the Cone Penetrometer Test (CPT), also referred to as Cone 

Penetration Test, an in-situ test that is fast, economical, and provides continuous profiling of soil 

strata and soil properties. The test is described in ASTM D 5778, “Standard Test Method for Performing Electronic Friction Cone and Piezocone Penetration Testing of Soils,” and consists of pushing a 

cylindrical steel probe into the ground at a constant rate of 2 centimeters per second and measuring 

the resistance to penetration. The standard cone penetrometer has a conical tip with an apex angle 

of 60 degrees, a 10-cm2 projected area for the cone, and a 150-cm2 surface area for the friction sleeve. 

The ASTM standard also permits a larger diameter unit that has a 15-cm2 tip and 200-cm2 sleeve. The 

measured point or tip resistance is qc and the measured side or sleeve resistance is fs. An illustration 

of a typical cone penetrometer is provided in Figure 6.12.

The CPT can be used in very soft clays to dense sands, but it does not work well in gravels or rocky 

terrain. The advantages and limitations of using the device are summarized in Table 6.20. Because 

the CPT provides more accurate and reliable parameters for analysis, it is an excellent complement 

to traditional soil borings with SPT measurements. The CPT is not practical for coarse refuse where 

larger rock fragments can impede the penetrometer, but the method has been used to characterize the 

consistency and to estimate the engineering properties of settled fine refuse in impoundments.

A piezocone penetrometer test (CPTu or PCPT) is performed by advancing a cone penetrometer with 

transducers for measuring pore-water pressures. In clean sands, the measured pore pressures are 

nearly hydrostatic because the high hydraulic conductivity of the sand permits immediate dissipation of excess pore-water pressures mobilized by advancement of the cone. In clays, the advancement 

of the cone may result in the development of elevated pore-water pressures. If the advancement of 

the penetrometer is halted, the decay of pore-water pressures can be monitored with time and used 

to calculate an in-situ rate of consolidation and soil hydraulic conductivity. Details related to test 

methods, cone types and calibration, data reduction, and cone maintenance are provided in ASTM 

D 5778 and Lunne et al. (1997).

Piezocone penetrometer testing can be a viable technique for determination of gradational variability, strength, hydraulic conductivity and consolidation properties of fine coal refuse at existing 

refuse disposal sites. However, the measured cone resistance qc must be corrected for pore-water 

pressures acting on unequal areas of the cone tip. This correction is most important for soft to stiff 

clays and silts and for very deep soundings where the hydrostatic pressures are high. Usually in 

sands, the correction is minimal because qc is much greater that any mobilized pore pressures.

Because soil samples are not obtained with the CPT, indirect assessment of soil behavior is typically 

inferred from an examination of the test data. The data can be processed for use in empirical chart 

classification systems, or the raw readings can be interpreted by eye to determine soil strata changes. 

For example, clean sands are generally indicated by a total tip resistance qT greater than 50 tsf, while 

for soft to stiff clays and silts, qT is less than 20 tsf. The total tip resistance qT is a function of the pore 
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pressure behind the cone tip qc and some factors related to cone geometry. This value is automatically 

calculated and plotted during the test.

Generally, pore-water pressures associated with penetration in loose sands are approximately equal 

to hydrostatic pressures, in contrast to penetration in dense sands where the pore-water pressure 

is typically less than hydrostatic. In soft to stiff intact clays, pore-water pressures associated with 

advancement of the penetrometer are generally several times the hydrostatic pressure. Notably, negative pore-water pressures are observed in fissured overconsolidated materials. The sleeve friction, 

(ADAPTED FROM FHWA, 1992)

FRICTION SLEEVE

PIEZO-ELEMENT IN

CONE TIP

PIEZO-ELEMENT

BEHIND CONE TIP

PIEZO-ELEMENT

BEHIND FRICTION

SLEEVE

ELECTRONIC HOUSING

60

CONE TIP

TABLE 6.20 ADVANTAGES AND LIMITATIONS OF THE CONE PENETROMETER TEST

Advantages Limitations

Fast and continuous profiling No soil samples are obtained

Economical and productive Unsuitable for gravel or boulder deposits(1)

Results not operator-dependent Requires skilled operator to run

Strong theoretical basis for interpretation Electronic drift, noise, and calibration

Particularly suitable for soft soils High capital investment

 Note: 1. Except where special rigs are provided and/or additional drilling support is available.

 (MAYNE ET AL., 2002)

FIGURE 6.12 PIEZOCONE PENETROMETER
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often expressed in terms of a friction ratio (FR = fs/qT), is also a general indicator of soil type. In sands, 

FR usually falls in the range of 0.5 to 1.5 percent; in clays FR normally falls between 3 and 10 percent. 

A notable exception is that in sensitive and quick clays, a low FR is observed. An approximate estimate of clay sensitivity is 10/FR (Robertson and Campanella, 1983).

6.4.3.8 Field Vane Shear Test (FVST)

The field vane shear test (FVST) is used to evaluate the in-situ undrained shear strength of soft to stiff 

clays and silts, mine tailings and organic muck. The test is conducted in accordance with ASTM D 

2573, “Standard Test Method for Field Vane Shear Test in Cohesive Soil,” by inserting a four-bladed 

vane (Figure 6.13) into cohesive soil at the bottom of a boring and rotating the device about a vertical 

axis. The torque required to turn the device is measured. A variety of vane sizes, shapes, and configurations are available depending upon the consistency and strength characteristics of the soil. Vanes 

can have a blade diameter D ranging from 1.5 to 4.0 inches, a vane height H ranging between 1.0 and 

2.5 D, and a blade thickness ranging from 0.006 to 0.125 inches. The end of the vane is usually rectangular or tapered at 45 degrees.

ASTM D 2573 provides relationships for converting the measured peak torque to a value of peak 

undrained vane shear strength Suv based on the geometry of the vane. For a rectangular vane with 

H/D = 2:

Suv = 6Tmax / (7pD3) (6-1)

where:

Tmax = maximum measured torque corrected for apparatus and rod friction 

(length times force)

Relationships for other vane geometries are presented in ASTM D 2573.

After the test to determine Suv is completed, the undrained steady-state (residual) shear strength Sur

can be determined by quickly rotating the vane another 5 full revolutions to fully remold the soil and 

then repeating the shear test. The ratio of peak to remolded undrained strengths is the sensitivity St. 

Table 6.21 provides a summary of the advantages and limitations of the FVST. Additional guidelines 

related to application of the FVST are presented in Mayne et al. (2002).

ASTM D 2573 recommends a loading rate of no faster than 0.1 degree per second (15 minutes for 90 

degrees of rotation). At this rotation rate, the time required to reach undrained peak strength typically 

ranges from 2 to 5 minutes, but in very soft clays the time to failure may be as much as 10 to 15 minutes.

Chandler (1988) and Morris and Williams (2000) investigated the applied loading rate. Chandler 

applies a theoretical method by Blight (1968), and indicates that for typical vanes and a time to failure 

of 1 minute, the test will be undrained if the coefficient of consolidation (cv) is less than 0.035 cm2/sec 

(3.3 ft2/day). Blight defined time to failure as the time from the beginning of vane rotation. Morris 

and Williams (2000) proposed a revision to Blight’s theoretical method, which accounts for the pore 

pressure increase due to vane insertion as well as vane rotation and defines time to failure as the time 

from vane insertion. Morris and Williams indicate that, for a vane diameter of 63 mm (2.5 in), a time 

to failure of 2 minutes will result in an undrained test for materials with cv as high as 1450 m2/year (42 

ft2/day). This value of cv should encompass coal refuse and natural materials with a plasticity index 

(plastic limit minus liquid limit) of 10 or higher.

To minimize drainage in fine coal refuse during the FVST, the rotational loading should be applied as 

soon as possible after vane insertion, and the loading rate should be increased significantly from the 
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ASTM D 2573 recommendation in order to achieve failure within about 1 minute. For a soft material, 

if only the peak strength is being measured, a loading rate of 2 to 10 degrees per second is reasonable, 

but if the undrained, steady-state (residual) strength is being measured as well as the undrained peak 

strength, then the following procedure is recommended:

1. Initially apply the torque at a rate of about 10 degrees per second.

2. After the peak strength has been reached, increase the rate of rotation to at least 60 

degrees per second (6 seconds per revolution or faster) for at least 5 complete revolutions to remold the material.

3. Avoiding a rest period, slow the rate of rotation to about 10 degrees per second to 

measure the steady-state strength.

The rotation and torque should be measured and recorded as the test is conducted, which can be 

accomplished with a gear box and stylus recording system or other type of data acquisition system. 

The rod and apparatus friction corrections (per ASTM D 2573) should be performed for the rates of 

rotation actually used in steps 1 and 3.

FIGURE 6.13 FIELD VANE SHEAR TEST SETUP

ADAPTER

TORQUE WRENCH

DRILL ROD

BEARING GUIDE

COUPLING

CASING

VANE

THRUST COLLAR

BEARING THRUST BEARING

GUIDE

VANE TO DRILL

ROD ADAPTER

H

D

45

FIGURE 6.13 FIELD VANE SHEAR TEST SETUP
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Studies by several researchers have demonstrated the importance of correcting the measured vane 

strength for use in stability analyses involving embankments on soft ground, bearing capacity analyses, and for analyses associated with excavations in soft clays. The correction to obtain the mobilized 

shear strength is given by:

Su (mobilized) = μR Suv (6-2)

where:

μR = empirical correction factor related to plasticity index (PI) based on 

back calculation from failure case history records of full-scale projects 

(dimensionless)

Bjerrum (1972) recommended values of μR to correct the measured peak field vane strength (with 

a time to failure of a few minutes or less) to a value of Su (mobilized) (during a full-scale slope failure 

corresponding to a time to failure of several weeks to several months) that may be appropriate for 

stability failures.

Chandler (1988) combined Bjerrum’s case history data and other data sets to develop a more specific 

strain-rate correction factor:

μR = 1.05 – b (PI)0.5 (6-3)

where b is a dimensionless rate factor that depends on the time to failure (tf) in minutes (for a fullscale slope stability failure):

b = 0.015 + 0.0075 log tf (10 min < b < 10,000 min) (6-4)

Values of μR as a function of PI and tf are presented in Figure 6.14. Slope stability failures should generally be considered to have tf

 values of 10,000 minutes (7 days) because of the construction methods 

involved.

These strain rate correction factors are for peak undrained strengths and are based on case histories 

for natural clays, not fine coal refuse. However, until more research is available, the same correction 

factors should be applied to fine coal refuse and to remolded undrained (steady-state) strength as 

well as to undrained peak strength.

The FVST is applicable only to soft to medium stiff clay-like materials that are relatively impermeable 

such that they remain undrained during the test. If drainage occurs, the measured torque and resulting calculated strength will exceed the actual value. No published guidance is available on limitation 

TABLE 6.21 ADVANTAGES AND LIMITATIONS OF THE FIELD VANE SHEAR TEST

Advantages Limitations

Assessment of undrained strength (Suv) Limited application to soft to stiff clays

Simple test and equipment Slow and time-consuming

Measurement of in-situ clay sensitivity (St) Raw Suv needs correction (empirical)

Long history of use in practice Can be affected by sand lenses and seams

(MAYNE ET AL., 2002)
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relative to PI; this Manual recommends that the FVST should not be used for materials with a PI of 7 

or less because these materials are likely to drain during the test. FVST should be used with caution 

in materials with higher PI that contain thin layers of sand-like material because the sand-like layers 

may allow drainage. For material with PI between 7 and 10, FVST should only be used if supporting 

data are provided to demonstrate that the test was undrained. Material samples should be recovered 

from each test zone for geotechnical index testing (moisture content, grain size distribution, and 

Atterberg Limits at a minimum). Also, CPT and piezocone measurements performed adjacent to an 

FVST can be used to measure the rate of pore-pressure dissipation, so that it can be confirmed that 

the zone in which the FVST is run is relatively impermeable. The piezocone data may be used to estimate the value of cv in order to confirm that the rotation rate is sufficiently rapid that the test can be 

considered undrained.

The FVST is not intended for stiff clay-like materials because these materials will normally exceed the 

torque capacity of the FVST device. The FVST cannot be used for testing sand-like material or coarse 

refuse because (1) these materials will drain during the test and (2) the shear strength of these materials will exceed the capacity of the FVST device.

6.4.3.9 Directional (Longhole) Drilling

Directional or longhole drilling refers to: (1) in-mine drilling operations used to identify geological 

and mining conditions in advance of mining and (2) surface drilling through an outcrop to determine 

cover and coal barrier thickness. Development in the 1990s of systems with instrumentation to measure drill bit location, high-thrust drilling equipment, powerful downhole motors and high-strength 

drill tubing has allowed the implementation of this technique. In combination with hydraulic fracPLASTICITY INDEX, PI (%)

VANE CORRECTION FACTOR, 

R

FIGURE 6.14 VANE CORRECTION FACTOR

0 20 40 60 80 100 120

0.5

0.6

0.7

0.8

0.9

1.0

tf = 10

tf = 100

tf = 1000

tf = 10,000

CORRECTION FACTOR FOR 

EMBANKMENTS UNDER NORMAL

RATES OF CONSTRUCTION

tf = TIME TO FAILURE (MINUTES)

(MAYNE ET AL., 2002)

FIGURE 6.14 VANE CORRECTION FACTOR
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turing techniques used to increase connectivity between boreholes, directional drilling has been 

employed to reduce in-situ methane gas contents in low-hydraulic-conductivity coal and to fracture 

a massive sandstone roof in advance of longwall mining (Brunner and Schwoebel, 1999). Depending 

on conditions, drilling rates of 300 feet per shift and drilling accuracies of approximately 1 degree in 

azimuth and 0.5 inches in pitch can be achieved. The longest reported in-mine horizontal borehole 

exceeds 5,000 feet in length (Brunner and Schwoebel, 1999). Directional drilling has also been used to 

locate old abandoned workings, drain accumulations of mine water, and degasify gob areas (Kravits 

and Schwoebel, 1994). While conventional coring should not be attempted in a long, directionallydrilled borehole, spot cores can be taken at selected locations (Kravits and Schwoebel, 1994). As such, 

the technique may have applicability for accurately locating and determining the thickness of horizontal in-mine barriers and can be a means of validating geophysical methods for locating and sizing 

barriers in mines.

6.4.3.10 Field Hydraulic Conductivity Tests

The hydraulic conductivity of soil or rock is often measured in place during subsurface exploration 

to determine if seepage through foundation materials will be an important design consideration. The 

hydraulic conductivity of soils near the ground surface can be measured in hand-dug pits or cased 

holes. At greater depths the hydraulic conductivity can be determined in the borings used for sampling, provided that the borehole is cased and the hydraulic conductivity test will not affect sampling 

of an important soil layer immediately below the test level. Table 6.22 summarizes various methods for measurement of hydraulic conductivity in the field. Additional guidance in selecting field 

test methods is provided in ASTM D 4043, “Standard Guide for Selection of Aquifer-Test Method in 

Determining of Hydraulic Properties by Well Techniques.”

The most commonly performed field hydraulic conductivity test involves a sudden change (increase 

or decrease) of water level in a borehole and measurement of the response in terms of water level 

versus time. The test procedure and methods of analysis are presented in ASTM D 4044, “Standard Test Method (Field Procedure) for Instantaneous Change in Head (Slug) Tests for Determining Hydraulic Properties of Aquifers.” Hydraulic conductivity values determined by this procedure 

often fail to correspond well with values predicted from laboratory testing due to: (1) characteristic 

differences (e.g., gradation and density) between the soils tested in the field and in the laboratory, (2) 

clogging of the boring face by soil particles suspended in the water, (3) varying directional hydraulic 

conductivity of layered soil that cannot be duplicated in the laboratory with disturbed soil samples, 

or (4) failure to conduct the field test in saturated soils, resulting in measurement of the rate of saturation rather than hydraulic conductivity. Because field hydraulic conductivity tests inherently account 

for the effects of geologic variations, they are generally more representative of in-situ conditions than 

laboratory tests. However, the evaluation and interpretation of the test data require knowledge of the 

test conditions and of the possible effects of these test conditions on the results.

The field hydraulic conductivity test for rock is similar to that for soil, as described in ASTM D 4630, 

“Standard Test Method for Determining Transmissivity and Storage Coefficient of Low-Permeability 

Rocks by In Situ Measurements Using the Constant Head Injection Test.” The test is performed in a 

rock boring using water pumped under pressure from the ground surface. Two types of tests can be 

performed: a single-packer test or a double-packer test.

In the single-packer test, a pipe is inserted into a boring with a packer at the lower end of the pipe. 

The packer is expanded mechanically or pneumatically from the ground surface to seal the annulus 

between the walls of the boring and the pipe, and water is pumped down the pipe into the boring 

below the packer. Since the depth and diameter of the hole below the packer, the applied water pressure, and the rate of flow of water through the system are known, the average hydraulic conductivity 

of the rock below the packer can be calculated.
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TABLE 6.22 FIELD METHODS FOR MEASUREMENT OF HYDRAULIC CONDUCTIVITY

Test Method Applicable Soils Reference

Various field methods Soil and rock aquifers ASTM D 4044

Pumping tests Drawdown in soils ASTM D 4044

Slug tests Soils at depth ASTM D 4044

Constant head injection Low-hydraulic-conductivity rocks ASTM D 4630

Pressure pulse technique Low-hydraulic-conductivity rocks ASTM D 4630

(ADAPTED FROM MAYNE ET AL., 2002)

In the double-packer test, a selected zone within the boring is tested by placing one packer at the 

bottom and another packer at the top of the test zone. Water is then pumped through the pipe into the 

annular space between the packers. The hydraulic conductivity is computed by the same procedure 

as for the single-packer test.

During hydraulic pressure testing of rock, the water pressure applied to the test zone must not exceed 

the pressure caused by the weight of overburden above the test zone. Excess pressures may force 

water into joints, bedding planes or fractures and cause additional fracturing of the rock by lifting the 

overburden. This “jacking” of the rock can seriously increase the amount of leakage that will occur 

later and may also decrease the stability of the rock mass and its ability to resist the loads applied by 

an embankment or other surface loading.

Hydraulic pressure testing to measure rock hydraulic conductivity is an essential part of subsurface 

exploration where an impoundment is planned and where groundwater leakage could create unsafe 

uplift pressures or piping of the embankment or foundation soils. In addition to posing a threat to 

the safety of the embankment and natural slopes, excessive leakage can increase stream and groundwater pollution down gradient from the refuse disposal facility. The hydraulic conductivity values 

obtained from hydraulic pressure testing allow the prediction of quantities and locations of leakage 

from the impoundment. If rock zones where excessive leakage could occur are observed, grouting 

of the rock formations may be required. Houlsby (1990) and Weaver and Bruce (2007) discuss procedures and materials for grouting rock formations to reduce water flow.

6.4.3.11 Groundwater-Level Measurements

For new embankments, determination of the groundwater level in the planned construction area is 

important to construction requirements, particularly where excavations are planned. An understanding of the groundwater regime is essential in determining the direction and rate of possible seepage 

from an impoundment and may aid in estimating the overall hydraulic conductivity of the foundation materials.

Using piezometers to measure the phreatic surface level within an existing embankment is often 

the most important of the field tests used for evaluation of an existing coal refuse disposal facility, 

whether there is an impoundment or not. ASTM D 4750, “Standard Test Method for Determining 

Subsurface Liquid Levels in a Borehole or Monitoring Well (Observation Well),” describes procedures that should be followed in measuring groundwater levels. Piezometers or standpipes should 

generally be installed in exploration boreholes. Common techniques for installing piezometers and 

standpipes are presented in Chapter 13 along with monitoring procedures. The accuracy of data 

obtained from piezometers is directly related to the care taken in their installation. Therefore piezometer installation should always be under the supervision of a qualified engineer or geologist.
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6.4.3.12 Water Flow and Quality Tests

In addition to phreatic surface level measurement with piezometers, valuable information for an 

existing impounding refuse disposal facility can be gained from: (1) monitoring flows from nearby 

springs or seep areas with weirs and (2) evaluating water quality aspects of these flows. As an example, measuring the volume of flow from a spring below an impoundment during both wet and dry 

periods, and as the impoundment level changes, will provide an indication of the rate of seepage 

from the impoundment and the effect on the overall groundwater system. Likewise, simple field 

measurements of temperature and acidity of seepage will allow comparison with similar measurements for the water in the impoundment and/or the inflow groundwater. Flow measurements and 

related water quality data are generally less important in the design of a new disposal facility, but 

the data are useful for future evaluation of the effect of the impoundment on the local and regional 

groundwater and surface water quality.

The type of weir to be used for flow monitoring and related construction requirements are determined by the magnitude of flow to be measured and the type of material in which the weir will be 

placed. In the case of very small seeps, the flow volume can be estimated simply by observation. If the 

flow initially passes over a “natural weir,” such as a rock outcrop or through an existing pipe, estimates can be made without installing special instrumentation. Construction of weirs and methods for 

accurately measuring flows from weirs and pipes are discussed in detail in Chapter 13.

Field testing of water can be performed using portable equipment to obtain indicator parameters 

such as pH, specific conductance and temperature, as well as some other mining-related parameters. 

Where measurements of additional constituents and characteristics of seepage water (including sulfate, chloride, iron, manganese, acidity, alkalinity, and dissolved and suspended solids) are desirable, 

additional water sampling and laboratory testing can be performed.

6.4.3.13 Backfilling of Boreholes

Boreholes at coal refuse disposal facility sites should not be left open, particularly if they are located 

beneath embankments or impoundments, or if they can potentially provide a pathway for fluid flow 

that is detrimental to site safety. Open boreholes can be backfilled with drill cuttings, cement grout, 

bentonite, and other materials depending on the objectives. Where there is no concern related to 

fluid migration or the impact of seepage on ground conditions, backfilling with cuttings or other 

materials may be acceptable. As a practical matter, boreholes in cohesionless soils may collapse 

when not supported, and it may not be possible (or necessary) to backfill such boreholes. Grouting of an open borehole is generally performed for the purpose of constructing a barrier that will 

prevent the vertical migration of fluids between geologic units. At active mine sites, the purpose 

may be to maintain the barrier between the mine workings and other strata. Materials employed 

for backfilling boreholes include cement, bentonite slurries, dry bentonite, and fast-setting cement 

grouts. Placement can be accomplished by tremie, pumping, and surface pouring. Site-specific 

considerations for a grouting program include: (1) whether to grout, (2) where to grout, and (3) the 

method of deployment.

ASTM D 5299, “Decommissioning of Ground Water Wells, Vadose Zone Monitoring Devices, Boreholes, and Other Devices for Environmental Activities,” presents guidance on methods and materials for closing of boreholes. While this standard is primarily oriented to environmental activities, it 

can be used to decommission boreholes where no contamination is observed. Attributes of common 

borehole plugging materials are discussed in the standard.

Grouting of boreholes that penetrate mines requires special provisions for supporting the borehole 

plug above the mine void (and potentially in the mine floor, if the boring is advanced through the 

mine). Frequently, sacrificial casing is left in the mine void to support the plug, although a grout 

basket has been used to allow sealing the borehole and to permit grouting.
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6.4.4 Geophysical Methods

Applied geophysics is a rapidly evolving field, and the applicability of geophysical techniques to 

coal refuse disposal facilities will continue to advance with respect to the aspects of data gathering, processing, interpretation and presentation of the geophysical data. Two basic deployments of 

geophysics are available: (1) surface surveys and (2) measurements from boreholes. Airborne geophysical techniques are not discussed herein, as their application in terms of identifying features of 

interest with respect to coal refuse disposal facilities is still in the experimental stage. Nevertheless 

it is worth noting that in some cases airborne electromagnetic (EM) surveys have been used to map 

flooded, abandoned coal workings (Love et al., 2005), and aeromagnetic surveys have been used 

for many years to map abandoned well casings, which can be a significant hazard to coal mining 

(Frischknecht et al., 1985).

Numerous sources of information related to geophysics are available in the general literature. A good 

source of information is the Environmental and Engineering Geophysical Society (EEGS) in Denver, 

Colorado, which annually holds the Symposium on the Application of Geophysics to Engineering 

and Environmental Problems (SAGEEP). The SAGEEP proceedings are an excellent source of up-todate information on the application of engineering geophysics to the types of problems that could be 

encountered at a coal refuse disposal facility. Other comprehensive compilations of geophysical techniques for subsurface exploration for engineering applications include Ward (1990), USACE (1995a), 

Sabatini et al. (2002), Wightman et al. (2003), and Sirles (2006). The Federal Highway Administration 

(FHWA) presents summaries of geophysical techniques at their web site. This material is substantially based on the USACE (1995a) work.

MSHA (2008) is a summary report of mine void detection demonstration projects that were performed to evaluate the use of geophysical techniques for detection of underground mine workings. 

These projects include actual field demonstrations of void detection at mine sites using seismic methods, electrical resistivity, electromagnetics, and radar.

The following ASTM standards provide guidance for conducting geophysical exploration:

• D 6429, “Standard Guide for Selecting Surface Geophysical Methods”

• D 6430, “Standard Guide for Using the Gravity Method for Subsurface Investigation”

• D 6431, “Standard Guide for Using the Direct Current Resistivity Method for Subsurface Investigation”

• D 6432, “Standard Guide for Using the Surface Ground Penetrating Radar Method 

for Subsurface Investigation”

• D 5753, “Standard Guide for Planning and Conducting Borehole Geophysical Logging”

• D 6639, “Standard Guide for Using the Frequency Domain Electromagnetic Method 

for Subsurface Investigations”

• D 7128, “Standard Guide for Using the Seismic-Reflection Method for Shallow Subsurface Investigation”

• D 6820, “Standard Guide for Use of the Time Domain Electromagnetic Method for 

Subsurface Investigation”

While ASTM guides provide useful background information on geophysical techniques, they may be 

dated in terms of defining procedures for data acquisition, processing, interpretation and presentation. For example, ASTM D 6431, “Standard Guide for Using the Direct Current Resistivity Method 

for Subsurface Investigation,” discusses the technique in terms of acquisition with a four-electrode 

system and processing of one-dimensional data sets with computer programs developed in the 1970s. 

Modern resistivity surveys are commonly conducted with multi-electrode arrays, and the data are 

routinely processed and interpreted in terms of 2D profiles or 3D blocks.
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6.4.4.1 Surficial Geophysical Techniques

In general terms, surface geophysical testing can be used to create a general image of subsurface 

conditions that can be checked by intrusive means such as borings or test pits. Data from geophysical testing should always be correlated with information from direct methods of exploration. Often, 

a combination of geophysical and direct exploration methods provides the best approach for interpreting subsurface conditions. When conducted at the outset of a subsurface investigation program, 

geophysical exploration can prove to be cost-effective through reduction of the number of borings 

needed for characterization of a site.

Conventional applications of surface geophysics include: (1) establishing the stratification of subsurface materials, (2) mapping the top of bedrock, depth to groundwater, and extent and quantity of soil 

deposits, and (3) determining the rippability of hard soil and rock. Over the past several years some 

improvements in traditional geophysical techniques have enhanced capabilities for the detection of 

abandoned mine workings and the presence of karst-related voids.

As summarized in Table 6.23, surface geophysical testing offers some advantages and limitations that 

should be understood before a technique is selected for a specific application.

Table 6.24 presents an overview of surficial geophysical methods and techniques in relation to the 

physical parameters measured and exploration objectives for coal refuse disposal facilities. The following text describes the most commonly used geophysical techniques listed in the table.

6.4.4.1.1 Seismic Refraction

The seismic refraction technique consists of measuring the first arrival of P- and/or S-waves at varying distances from a seismic source. The most common application of this technique is the determination of the depth to bedrock, which requires that the upper layer velocity (soil/weathered or soft 

rock) is less than that of the lower layer (competent rock). If a high-velocity surface layer is present, 

the technique is not effective. For this reason, the technique is generally not applicable for detection 

of mine voids.

TABLE 6.23 ADVANTAGES AND LIMITATIONS OF SURFACE GEOPHYSICAL TESTING

Advantages Limitations

1. Many geophysical tests are non-invasive and thus offer 

significant benefits in cases where conventional drilling, testing, and sampling are difficult (e.g., deposits of 

gravel, talus deposits or access constraints).

2. Geophysical testing generally covers a relatively large 

area, thus providing the opportunity to characterize large 

areas with relatively limited testing. It is particularly well 

suited to projects having large areal extent (e.g., new 

refuse disposal facility).

3. Some types of geophysical measurement can assess 

the characteristics of soil and rock at very small strains 

(0.001%), thus providing information on truly elastic 

properties.

4. Most geophysical methods are relatively inexpensive 

when considering cost relative to the relatively large 

areas over which data can be obtained.

5. A properly performed geophysical survey can reduce the 

number of borings required for site characterization.

1. Geophysical testing, when applied to locating 

changes in soil and/or rock properties, will be 

effective only if a target of interest has a physical contrast with the surrounding ground.

2. Results are generally interpreted qualitatively. 

Useful results can only be obtained by an experienced engineer or geologist who is familiar 

with the particular testing method.

3. Specialized equipment is required, as compared to more conventional subsurface exploration methods.

4. Results from surface geophysical testing 

should be validated using direct methods of 

exploration such as borings.

(ADAPTED FROM SABATINI ET AL., 2002)
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Seismic waves are usually created using a sledge hammer for depths up to about 50 feet and with 

explosives for depths up to about 100 feet. Other sources such as vibrators are sometimes used. Initially, the seismic waves travel solely through the soil to arrive at geophones (vibration transducers) 

located away from the source. The seismic waves also propagate through the overburden and refract 

along the bedrock surface. While the waves are traveling along this surface, they continually refract 

seismic waves back to the ground surface that are also detected by the geophones and recorded 

with a seismograph. The result is the generation of travel-time curves, as shown in Figure 6.15. The 

method can often be a low-cost (compared to boreholes) method of bedrock mapping and overburden estimation. When measurements are obtained with a high degree of redundancy, the result is a 

reliable acoustic image of the subsurface in terms of layers and variations of seismic velocity within 

the individual layers.

Seismic refraction data can also be useful for determining the rippability of rock materials using heavy 

construction equipment. Companies such as Caterpillar have prepared graphs comparing rippability 

versus P-wave velocity for various equipment types, an example of which is shown in Figure 6.16.

The design of a seismic refraction survey involves locating the profiles where data are desired and 

determining the length of the array of geophones (the geophone spread) and the geophone spacing. 

TABLE 6.24 SURFACE GEOPHYSICAL METHODS IN RELATION TO 

TYPICAL INVESTIGATION OBJECTIVES

Geophysical Method Dependent Physical

 Property

Applications (see key below)

1 2 3 4 5 6 7 8 9 10

Seismic refraction Elastic moduli; density P P P S X X X M M S

Seismic reflection Elastic moduli; density S M S S X P X X P P

Resistivity Resistivity P X X P S P X P M S

Spontaneous potential (SP) Potential differences X X X X P X X X X X

Electromagnetics (EM) Conductivity; inductance M X X P S S P S P S

Ground penetrating radar Permittivity; conductivity M X X X M S M X S S

Gravity Density X X X X X P X X X X

Magnetics Magnetic susceptibility X X X X X M P X X X

Key to Techniques and 

Applications

Technique Applicability Applications

P – primary technique 1 – depth to bedrock 6 – location of mine workings or

 other subsurface voids

S – secondary technique 2 – rippability of rock and hard soil 7 – abandoned well detection

M – may be used but probably not

 the best approach

 3 – elastic properties of coal refuse,

 soil and rock

 8 – variations of coal refuse

 composition

X – not applicable 4 – hydrogeological investigations 9 – location of faults/fractures/

 geologic structures

 5 – location of seepage pathways in

 a dam

10 – determination of soil/bedrock

 stratigraphy

(ADAPTED FROM SIRLES, 2006)
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The length of the spread depends on the required depth of penetration. As a rule of thumb, the length 

of the spread needs to be at least four times the depth of interest. Seismic “noise” originates from 

ambient vibrations that can be caused by sources such as traffic or wind. Obtaining multiple recordings from the same location and summing (stacking) the results or covering the geophones with sand 

bags can reduce the effect of this type of interference. Improved results with the seismic refraction 

technique can also be obtained when multiple refractions from the same refractive interface generated by multiple shots are received at a given geophone. This multiplicity of data is obtained by 

using a single geophone spread with multiple shot points. Commercial engineering seismographs 

designed for the seismic refraction technique usually allow for simultaneous recording from spreads 

with either 12 or 24 geophones, although some commercial equipment will allow for recording with 

96 or more geophones.

There are several steps in the processing and interpretation of seismic refraction data. Firstly, it is 

necessary to pick the first arrival time for P-wave analysis and to tabulate this information. If the 

purpose is to obtain the S-wave velocity, then it is necessary to pick the onset of the S-wave arrival, 

which is more difficult than picking the P-wave arrival, because the S-waves lie within the wave train 

of the P-wave. Analytical procedures for processing the data commonly use either the generalized 

reciprocal method (GRM) or the delay-time method for the inversion and interpretation of refraction 

data. Both methods are suitable for resolving multilayer profiles with structural complexities (dips 

up to about 20 degrees). The acquisition of redundant data optimizes the accuracy of both interpretive techniques. The end result is a cross section of the ground with velocities assigned to each layer, 

as depicted in Figure 6.15.

FIGURE 6.15 TYPICAL SEISMIC REFRACTION DATA WITH INTERPRETATION

0 200 400 600 800 1000 1200 1400

200

150

DISTANCE (FT)

100

50

0

TIME (MSEC)

TIME-DISTANCE PLOT

UNSATURATED SEDIMENTS

BEDROCK

DISTANCE (FT)

0 200 400 600 800 1000 1200 1400

DEPTH (FT)

0

50

100

150

200

250

INTERPRETED CROSS SECTION

BORING BORING

SATURATED SEDIMENTS

(HAENI, 1988)

FIGURE 6.15 TYPICAL SEISMIC REFRACTION DATA WITH INTERPRETATION
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When S-wave arrivals are picked from a seismic refraction record, it is possible to use the S-wave 

velocity to calculate the elastic properties of the identified layers. In practice, it can be difficult to 

identify S-waves, even when a horizontally-polarized source and horizontal geophones are used. 

Although there are numerous published examples (Johnson and Clark, 1992; Ellefsen et al., 2005) of 

the successful calculation of S-wave velocity from refraction data, in practice the most reliable methods for measuring elastic properties of the subsurface are from crosshole or downhole surveys, as 

discussed in Section 6.4.4.2.

6.4.4.1.2 Seismic Reflection

Application of the seismic reflection technique involves measuring the travel time required for a 

seismic wave generated at or near the surface (P-wave or S-wave, depending on the survey setup) 

to return to surface or near-surface geophones after reflection from acoustic interfaces between subsurface layers (Figure 6.17). Seismic reflection is the most powerful of all geophysical techniques 

for mapping subsurface layering and is by far the most commonly applied method for oil and gas 

exploration. It is also the most sophisticated of all geophysical methods and requires highly specialized equipment and processing software for its successful application. For this reason, the techTOPSOIL

CLAY

IGNEOUS

GRANITE

BASALT

SEDIMENTARY

SHALE

SANDSTONE

SILTSTONE

CONGLOMERATE

BRECCIA

LIMESTONE

METAMORPHICS

SCHIST

SLATE

OTHER

COAL

IRON ORE

0 5 10 15

RIPPABLE SEISMIC VELOCITY (FT/SEC x 1000)

UNRIPPABLE

FIGURE 6.16 CORRELATION OF RIPPABILITY WITH P-WAVE

 VELOCITY FOR CATERPILLAR D9

(CATERPILLAR, 2008)

FIGURE 6.16 CORRELATION OF RIPPABILITY WITH P-WAVE VELOCITY 

FOR CATERPILLAR D9
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nique requires highly experienced practitioners, and no attempt has been made herein to describe the 

details of the data acquisition, processing and interpretation. Seismic reflection is not commonly used 

in environmental and engineering projects because of its relatively high cost. Contrary to most other 

geophysical methods, shallow seismic reflection studies are more expensive than the deep surveys 

conducted for oil and gas exploration because of the need for closely spaced measurements. Nevertheless, the method offers the potential for defining subsurface structure better than other methods. 

In cases where it is important to know the location of faults or other lithologic breaks or when the 

target is a deep abandoned mine working, the high-resolution seismic reflection technique may be 

the only practical means to obtain the desired data.

Seismic reflection has been applied to mapping the continuity of coal seams in advance of longwall 

mining, particularly in Europe where mines are commonly at depths greater than 1000 feet and it is 

difficult and expensive to characterize a coal seam with borings (D’Appolonia, 1982). The method has 

also been successfully applied to the mapping of mine voids (Clark et al., 1994; Johnson et al., 2002), 

but the experience base is limited and few practitioners are equipped to properly conduct this type 

of survey. An example of a high-resolution seismic reflection survey performed over shallow mine 

workings is presented in Figure 6.18.

FIGURE 6.17 SEISMIC REFLECTION PRINCIPLE AND SCHEMATIC

 OF REFLECTION DATA RECORD

 

GROUND SURFACE GEOPHONES

LAYER 1

LAYER 2

ENERGY SOURCE

CL

CL

X X

t = 0 t = 0

t

t

FIRST ARRIVALS

FIRST ARRIVALS

SE

SECOND ARRIVALS

COND ARRIVALS

(JOHNSON AND CLARK, 1992)

FIGURE 6.17 SEISMIC REFLECTION PRINCIPLE AND SCHEMATIC 

OF REFLECTION DATA RECORD
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FIGURE 6.18 SEISMIC REFLECTION SURVEY PROFILE OVER

 ABANDONED COAL MINE WORKINGS

10 50 100 150 200 240

0

50

100

150

200

BORING PENETRATING

MINE WORKINGS

POSSIBLE MINE

WORKING

CHANNEL FILL

CUTTING COAL

SEAM

INTACT COAL SEAM

DISTANCE (FT)

DEPTH (FT)

NOTE: DATA WERE OBTAINED USING

 VIBRATORY S-WAVE SOURCE. (JOHNSON ET AL., 2002)

 

6.4.4.1.3 Electrical Resistivity

The purpose of electrical resistivity surveying is to determine the subsurface resistivity distribution by making measurements at the ground surface. From these surface measurements, the true 

resistivity of the subsurface can be estimated, and variations or anomalies in the observed resistivity may indicate limits of surface deposits such as coal refuse, the bedrock surface, or flooded mine 

workings. Resistivity is typically described in units of ohm-meters or ohm-feet. Ground resistivity 

is affected by various physical parameters such as the mineral and fluid content, porosity, and the 

degree of saturation.

The measurement of electrical resistivity is normally performed using four electrodes, two that 

induce current into the ground and two that measure potential difference (voltage). Figure 6.19 

provides some examples of electrode configurations commonly used for electrical resistivity measurements. Electrical resistivity surveys have been performed for many decades as part of hydrogeological, mining and geotechnical investigations, but the use of this technique has recently increased 

due to improvements in both data acquisition and data processing technologies. Multi-electrode 

systems have greatly improved the efficiency of data acquisition, as measurements can now be made 

automatically without moving the current insertion and voltage measurement points. Also, the DC 

resistivity method had been limited by the need to perform complex calculations to model subsurface electrical properties. With the availability of high-speed personal computers and improved 2D 

and 3D processing software (Gan; 2004, 2005), the technique has seen increased interest from the 

mining industry, including as a means for detection of subsurface openings.

FIGURE 6.18 SEISMIC REFLECTION SURVEY PROFILE OVER ABANDONED 

COAL MINE WORKINGS
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FIGURE 6.19 COMMONLY USED ELECTRODE CONFIGURATIONS

 FOR GROUND ELECTRICAL MEASUREMENTS

V I

a na a

DIPOLE-DIPOLE CONFIGURATION

V

WENNER CONFIGURATION

a a a

M N A B A M N B

V

SCHLUMBERGER CONFIGURATON

na a na

V

A M N B

POLE-DIPOLE CONFIGURATION

I

M N

a

A B

na

I

I

NOTE: A AND B ARE CURRENT (I) INSERTION POINTS;

 M AND N ARE VOLTAGE (V) MEASUREMENT POINTS. (ADAPTED FROM GAN, 2004)

At coal refuse disposal sites, electrical resistivity surveys have proven useful for estimating the 

amount of coal refuse disposed at existing facilities by enabling location of the base of the refuse. 

Furthermore, the method also has the potential to differentiate zones with varying physical characteristics related to coal content within the coal refuse. An example of the application of electrical resistivity to generate profiles across a fine coal refuse deposit is presented in Figure 6.20. In the figure, the 

depth to the base of the coal refuse, which is in contact with bedrock, is clearly visible (yellow line), 

and variations in resistivity within the fine coal refuse can be attributed to physical differences such 

as the level of coal content. DEPTH (FT)

DISTANCE (FT)

0 100 200 300 400 500 600

10

20

30

40

50

60

70

80

RESISTIVITY (OHM-M)

5 25 150 1200 4500

FIGURE 6.19 COMMONLY USED ELECTRODE CONFIGURATIONS 

FOR GROUND ELECTRICAL MEASUREMENTS

FIGURE 6.20 RESISTIVITY PROFILE ACROSS FINE COAL REFUSE IMPOUNDMENT
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Another application of the electrical resistivity method at coal refuse sites is the detection of abandoned mine workings, as shown in Figure 6.21. The ability to detect voids is enhanced when the void 

has a physical contrast with the surrounding rock. If the void is empty (no water), it will be difficult to detect with electrical measurements. Air does not transmit an electrical current, and, unless the 

coal has an unusually low resistivity, it may be difficult to distinguish a void. The resistivity contrast 

between flooded mine voids and typical coal will approach two orders of magnitude (Johnson, 2003), 

thus allowing for detection of mine workings as resistivity lows. Project experience with electrical resistivity demonstrates that commercially available technology can be effective, especially for the detection 

of flooded mine workings at depths up to about 100 feet (Figure 6.21). D’Appolonia (2006) conducted 

a demonstration project for MSHA to illustrate the application of the method at the perimeter of an 

impoundment where abandoned workings in the 40- to 60-foot-depth range contained limited water. 

For deeper workings, the method has the potential to be effective, but theoretical models and practical 

experience indicate that the target size/depth ratio needs to be favorable and that the length of the resistivity profile required for acquiring deep images is often limited by surface interference. Therefore, the 

method is usually most effective for mine subsidence applications.

6.4.4.1.4 Spontaneous Potential (SP)

The spontaneous or self-potential (SP) method consists of measuring naturally occurring electrical 

potentials (voltage differences) in the subsurface. One of the sources of these electrical potentials is 

the movement of water through a porous medium, which produces electro-filtration or streaming 

potentials. These potentials can be used for the evaluation of seepage (Figure 6.22). As water flows 

through a capillary system, it collects and transports positive ions from surrounding materials. The 

positive ions accumulate at the exit point of the capillary, leaving a net positive charge. The untransported negative ions accumulate at the entry point of the capillary, leaving a net negative charge. If 

the streaming potentials developed by this process are of sufficient magnitude to be measured, the 

entry point and the exit point of zones of concentrated seepage can be determined due to the negative 

and positive (respectively) SP anomalies.

SP is measured with a pair of non-polarizing electrodes and a high-impedance voltmeter. One of 

the electrodes is placed in the ground at a convenient location and remains in place throughout the 

survey. This is referred to as the “base” electrode. It is connected to a multi-meter via an insulated, 

single-conductor wire mounted on a reel. This wire may be hundreds of meters long. The second 

DISTANCE (FT)

ELEVATION (FT)

ZONE WHERE PROFILE

CROSSES EDGE OF WORKINGS

ZONE WHERE PROFILE

CROSSES WORKINGS

RESISTIVITY (OHM-M) (JOHNSON, 2003)

0 500 1000 1500

0 100 200 300 400 500 600

1360

1400

1440

1480

BASE OF LOWER

KITTANNING COAL

FIGURE 6.21 ELECTRICAL SURVEY OF FLOODED MINE WORKINGS
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electrode, or “measuring electrode,” the reel, and the multi-meter are then moved from point to point 

in a planned grid. At each point of the grid, the electrical potential between the base and the measuring electrode is recorded.

If the potential differences are plotted in profile or contoured to identify zones with negative potential, areas of seepage can be identified. Additional information and examples of SP surveys used to 

locate seeps in embankment dams and tailings impoundments are provided by Butler et al. (1989), 

Butler et al. (1990), Bérubé (2004), Song et al. (2005), and Mainali (2006). Recent advances in the SP 

technique allow for predictive modeling of the SP anomalies associated with seepage, enhancing the 

interpretability of SP results (UBC-GIF, 2005; Bérubé, 2004).

6.4.4.1.5 Electromagnetics (EM)

Similar to electrical resistivity measurements, electromagnetic methods (EM) allow mapping of 

the distribution of subsurface electrical properties, except that the EM methods are designed for 

measurement of variations in conductivity, not resistivity. Resistivity and conductivity are different parameters related to the same physical property and are simply the inverse of one another. 

As noted above, the unit most commonly used to measure ground resistivity is the ohm-meter 

(ohm-m). The term “mho” reflects its inverse relationship to the ohm, but was discontinued in 

favor of the term “siemen” (symbol S) in the late 1970s. The corresponding unit of conductivity is 

the inverse of an ohm-meter, referred to as a mho (or siemen) per meter, or S/m. The most common 

unit of conductivity is the μS/cm, which is 0.0001 S/m. With this conversion, 10,000 μS/cm = 1 S/m 

= 1 ohm-m.

FIGURE 6.22 SCHEMATIC ILLUSTRATION OF USE OF THE SP METHOD

 TO IDENTIFY AN AREA OF EMBANKMENT SEEPAGE

MULTIMETER

REFERENCE

ELECTRODE

+

_

0

POTENTIAL

DISTANCE

IDENTIFIED

SEEPAGE AREA

NON-POLARIZABLE

ELECTRODE LOCATIONS

SEGMENT OF WATERIMPOUNDING EMBANKMENT

(ADAPTED FROM BUTLER ET AL., 1989)

FIGURE 6.22 ILLUSTRATION OF USE OF SP METHOD TO IDENTIFY AN AREA OF SEEPAGE
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An advantage of all EM systems as compared to the resistivity method is that it is not necessary to 

insert electrodes in the ground and thus the surveying is more rapid. Disadvantages of EM methods 

are: (1) they are generally not as good as the DC resistivity method in resolving variations of electrical 

properties with depth and (2) they are more subject to cultural interference from electrical lines and 

metallic objects. For these reasons, EM methods are most commonly used to rapidly measure lateral 

variations of soil electrical properties, as well as to delineate the distribution of metal objects.

Electromagnetic (EM) techniques can be grouped into active methods, where an active EM signal is 

induced in the ground by human activity, and passive systems, where measurements are made of 

natural variations of the earth’s EM field. Active systems are further grouped into frequency domain 

and time domain. Passive systems include very low frequency (VLF), and magnetotelluric methods. 

McNeill (1990) provides a discussion of various EM techniques.

EM methods have potential application for characterization of coal refuse sites (e.g., mapping 

abandoned workings) as long as the workings are flooded. For example, time-domain EM (TDEM) 

measurements have been used to map flooded workings, as shown in Figure 6.23. Where this 

technique has been attempted over workings that are not flooded, the method was less successful (MSHA, 2008). EM techniques that measure bulk ground conductivity are commonly used 

at operating or abandoned refuse disposal facilities to determine the migration of contaminated 

groundwater or to delineate the extent of waste deposition. EM techniques can also be used to 

characterize variations in the physical properties of existing coal refuse deposits related to coal 

content. Results of this type of survey with a commonly-used conductivity meter (Geonics EM-31) 

over an existing coal refuse deposit are shown in Figure 6.24. The plan location of the resistivity 

profile in Figure 6.20 is shown in Figure 6.24. In this example, the EM survey provides mapping 

of the near-surface horizontal variations of the coal refuse, while the resistivity profile in Figure 

6.20 shows vertical variations.

6.4.1.1.6 Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) has evolved over the past two decades into one of the most commonly applied techniques for imaging the shallow subsurface. The method offers the highest resolution of geophysical techniques commercially available today. In many cases, the time required for 

the acquisition of GPR profiles is minimal, and subsurface profiles can normally be generated in real 

time, making this tool very cost-effective. GPR works best in non-conductive soils, such as dry sand 

or sand saturated with fresh water.

The typical result of a GPR survey is a profile that presents radar wave amplitude as a function of 

distance along the line and two-way travel time. To determine the depth to a reflector, it is necessary 

to know the average propagation velocity from the ground surface. The velocity of a radar pulse in 

an earth material is dependent on the relative dielectric constant (εr) of the material according to the 

following relationship:

V = c / (εr) 0.5 (6-5)

where:

V = velocity in propagating material (m/sec)

c = speed of light (m/sec)

 εr = relative dielectric constant (dimensionless)

This velocity can sometimes be estimated from the characteristics of the subsurface lithology. Table 

6.25 presents typical velocities in terms of two-way travel time (nanoseconds/meter) for various earth 

materials along with their approximate relative dielectric constants.
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Until the advent of commercial systems with separate transmitting and receiving antennas, depth 

estimation based on subsurface material properties or from observations from reflectors of a known 

depth was the only means to interpret a GPR profile. Modern systems with the ability to record reflections at varying distances from the transmitting antenna allow for the calculation of the subsurface 

velocity profile by means of a normal moveout (NMO) correction based on hyperbolic reflections 

from subsurface features.

Depth of penetration depends on the selection of an appropriate antenna frequency. An antenna 

frequency of one gigahertz would be suitable for mapping rebar in concrete, but would only have at 

most a few feet of penetration in typical soil. Most GPR surveys in soil use antennas with frequencies 

FIGURE 6.23 DELINEATION OF FLOODED MINE WORKINGS WITH TDEM METHOD

0 100 200

1300

1350

1500

1400

1450

300 400 500 600

ELEVATION (FT)

DISTANCE (FT)

BASE OF LOWER

KITTANNING COAL

ZONE WHERE PROFILE CROSSES

EDGE OF WORKINGS

ZONE WHERE PROFILE

CROSSES WORKINGS

PIT

POND

ROAD

RESISTIVITY PROFILE

PILE

-700 -600 -500 -400 -300 -200 -100 0 +100

EASTING (FT)

NORTHING (FT)

0

-100

-200

-300

-400

-500

GROUND

RESISTIVITY

(OHM-M)

0

10

20

30

100

1000

2500

FIGURE 6.23 DELINEATION OF FLOODED MINE WORKINGS WITH TDEM METHOD

FIGURE 6.24 RESISTIVITY OF COAL REFUSE FROM EM-31 MEASUREMENTS
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between about 100 and 400 MHz, with the greatest penetration achieved with the 100-MHz antenna, 

but with a substantial loss of resolution as compared to the 400-MHz antenna.

Another factor affecting the depth of penetration of the GPR signal is attenuation. Attenuation is 

caused by spreading and scattering losses, as well as electrical losses. Scattering and electrical losses 

are due primarily to the conductivity of the subsurface materials, which in soils relates mainly to clay 

and moisture content. In dry sand, penetration can reach as much as 50 to 70 feet. In wet, saturated 

clay penetration may be as little as 3 to 7 feet.

The main limitation of the GPR technique is depth of penetration under conditions commonly 

encountered in areas with coal workings. The soils commonly encountered in coal mining areas are 

clays weathered from the claystones associated with the sedimentary sequences that include the coal, 

and these soils can severely restrict the effective penetration of the radar waves. Thus, use of GPR is 

generally limited to the identification of near-surface features such as buried waste, pipes, etc. Nevertheless, where abandoned mine workings are shallow, GPR can sometimes be used to detect these 

workings. An example of a GPR record with identified mine workings is shown in Figure 6.25.

6.4.4.1.7 Gravity

At mine sites the gravity method can detect shallow abandoned mine workings by measurement 

of minute changes in the earth’s gravity field resulting from the lack of near-surface mass associated with mine openings. The measurement of the gravity field for this application is referred to as 

microgravimetry and requires the use of specialized gravimeters with a sensitivity of one microgal 

(approximately one billionth of the earth’s gravity field). An air-filled mine void would in theory be 

detectable with commercial equipment at a depth of about 30 feet. In practice, it is time-consuming 

to acquire the data, and accurate elevation control is needed, and it is desirable to have a topographic 

survey crew accompany the geophysicist to measure the precise elevation of the instrument at each 

reading point. For a target as shallow as 30 feet, the surface width of the gravity anomaly is about 

100 feet. Thus, a survey requires a significant amount of accessible space, which is often not availTABLE 6.25 PHYSICAL PROPERTIES AND TYPICAL GROUND PENETRATING RADAR 

VELOCITIES FOR COMMON EARTH MATERIALS

 Material

Approximate

Conductivity

(mS/m)

Approximate Relative

Dielectric Constant

εr

(dimensionless)

Two-Way

Travel Time

(sec × 10-9/m)

Air 0 1 6.6

Fresh Water 10-1 – 30 81 59

Fresh-Water Ice 10-1 – 10 4 13

Permafrost 10-2 – 10 4 – 11 13 – 15

Limestone 10-6 – 1 6 – 8 22

Granite 10-6 – 1 5.6 – 8 18.7

Dry Sand 10-4 – 1 4 – 6 13 – 16

Saturated Sand (fresh water) 10-1 – 102 30 32 – 36

Saturated Silt (fresh water) 10 – l02 10 21

Saturated Clay (fresh water) 102 – 104 8 - 25 18.6 – 23

Average “Dirt” 10-1 – 102 16 20 – 30

(BENSON ET AL., 1984)
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able. Furthermore, it is often difficult to correct the gravity data for variations caused by surrounding 

topography, instrumental drift, and elevation. In particular, micro-topographic changes can significantly affect gravity readings. Unless the target is in a flat, open area and the depth does not exceed 

about 40 to 50 feet, the gravity method will probably not be practical. Nevertheless, if the mine workings are expected to be very shallow and air-filled, the gravity method is one of the few geophysical 

methods that can provide conclusive evidence of the presence of a mine void. A theoretical gravity 

response over an air-filled mine void is presented in Figure 6.26.

6.4.4.1.8 Magnetics

The primary application of the magnetic method in a coal refuse environment is the detection of metal, 

including abandoned metal well casings. Measurements are made by an instrument called a magnetometer, and the unit of magnetic intensity is the nanotesla (nT), sometimes referred to as a gamma. Differences 

in the normal value of the earth’s magnetic field correspond to magnetic anomalies that can be measured 

with a magnetometer. Surveys for well casings can be conducted from the ground or from the air, as previously noted. As shown in Figure 6.27, well casings produce very strong anomalies, detectable even when 

the magnetometer is located at an elevation of 250 feet above the well casing. The potential for detecting 

abandoned mines is minimal, unless mine openings are associated with metal, as might be the case if old 

mine rails are present. Coal has a low magnetic susceptibility when compared to most other rocks. A void 

in a coal seam, therefore, will not produce a significant disturbance to the earth’s natural magnetic field, 

but a sensitive magnetometer can detect old mine rails at a depth of several tens of feet.

6.4.4.2 Borehole Geophysical Techniques

Borehole logging includes numerous geophysical techniques involving the lowering of sensing 

devices into a borehole and continuously recording physical parameters associated with the sur112

126

TIME (NS)

DEPTH (FT)

0

14

28

42

56

70

84

98

4

8

12

16

20

0 50 100 150 200

DISTANCE (FT)

FIGURE 6.25 GPR RECORD OF SHALLOW COAL MINE WORKINGS

KNOWN MINE TUNNEL

SUSPECTED MINE TUNNELS

AIR WAVES REFLECTED

FROM SURFACE OBJECTS

LEGEND

(JOHNSON ET AL., 2002)

FIGURE 6.25 GPR RECORD OF SHALLOW COAL MINE WORKINGS
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FIGURE 6.26 THEORETICAL RESPONSE OF GRAVITY GEOPHYSICAL METHOD

 OVER 20-FOOT-DIAMETER, AIR-FILLED TUNNEL

LINE DISTANCE (FT)

GRAVITY (MICROGALS)

0

-10

-20

-30

-40

-50

-60

-100 -80 -60 -40 -20 0 20 40 60 80 100

30-FT DEPTH

90-FT DEPTH

60-FT DEPTH

APPROXIMATE LIMIT OF DETECTION WITH GRAVITY METHOD

NOTE: RESPONSE WOULD BE APPROXIMATELY

 ONE-THIRD OF THE ABOVE FOR A WATER-

 FILLED TUNNEL. (JOHNSON ET AL., 2002)

rounding rock, soil, pore fluids, or other physical parameters. The FHWA lists 23 borehole logging 

techniques on their web site.

A general grouping of the most commonly applied borehole techniques is provided in ASTM D 5753, 

“Standard Guide for Planning and Conducting Borehole Geophysical Logging.” This general grouping of borehole geophysical techniques is shown in Table 6.26, where a division is made in terms of: 

(1) acoustic logs intended to determine ground variations related to seismic velocity, (2) electrical and 

induction logs that identify lithologic and groundwater variations on the basis of resistivity/conductivity, (3) nuclear logs that relate to variations in natural or induced radioactivity, and (4) miscellaneous 

techniques that define the physical characteristics of boreholes and/or voids penetrated by boreholes.

For coal refuse facilities, most of the commonly used borehole geophysical methods are suitable for general site characterization in combination with conventional drilling and sampling. 

Techniques with particular relevance to coal refuse disposal facilities are those that provide the 

S-wave velocity as a function of depth (crosshole and downhole seismic methods), because this 

information is useful for the evaluation of the potential for seismically-induced liquefaction. 

Borehole logging and techniques that have potential for characterization of abandoned mine 

workings (video, laser imaging, sonic imaging) are also directly applicable to coal refuse disposal 

facility evaluations.

FIGURE 6.26 THEORETICAL RESPONSE OF GRAVITY GEOPHYSICAL METHOD 

OVER 20-FOOT-DIAMETER, AIR-FILLED TUNNEL
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A borehole technique with the potential for imaging abandoned mine workings is borehole GPR. 

Mine voids within approximately 10 ft can be detected from borehole GPR. Research has shown that 

crosshole GPR tomography can identify tunnels, but this is not a commonly applied technique and 

the difficulties of this technique in mapping coal mine voids is well described by the Colorado School 

of Mines in an experimental mine detection study for MSHA (CSM, 2007).

Information on commonly applied general borehole geophysical techniques can be found in USEPA 

(1993), USACE (1995a), Keys (1997), Sabatini et al. (2002), Wightman et al. (2003), and Sirles (2006). 

The following discussion focuses on techniques for measuring S-wave velocity that are primarily 

used for determining the seismic properties of coal refuse and soils for liquefaction analyses, but 

these techniques may also be useful for characterizing mine voids.

6.4.4.2.1 Crosshole and Downhole/Uphole Seismic Surveys

Crosshole and/or downhole/uphole seismic testing in boreholes is conducted for determining soil 

and rock properties (P- and S-wave velocities). The information obtained from these tests can be used 

to compute shear modulus, Young’s modulus, and Poisson’s ratio for use in static/dynamic analyses. 

The basic relationships are as follows:

E = r Vp

2 [(1 + u) (1 - 2 u) / (1 - u)] (6-6)

G = Vs

2 r (6-7)

where:

Vp = compressional (P-wave) velocity (length/time)

Vs = shear (S-wave) velocity (length/time)

E = Young’s modulus (force/length2)

G = shear modulus (force/length2)

r = mass density of soil (mass/length3

)

u = Poisson’s ratio of soil (dimensionless)

If both Vp and Vs are known, the Poisson’s ratio of the soil can be determined from the following relationship between E and G:

G = E / 2 (1 + u) (6-8)

With borehole seismic surveys, one or more boreholes are drilled into the soil to the desired depth 

of exploration. Wave sources and/or receivers (borehole geophones normally oriented to record both 

horizontal and vertical components of wave motion) are then lowered into the boreholes. There are 

three basic approaches to borehole seismic surveys:

• Crosshole Survey – In a crosshole survey, the energy source is located in one borehole and detectors are placed in another borehole at the same depth as the energy 

source. The energy source is usually a mechanical pulse instrument composed of a 

stationary part and a hammer. The pulse instrument is held against the side of the 

borehole by a pneumatic or hydraulic bladder. Travel times between the source and 

receivers are measured, allowing determination of wave velocities.

• Uphole Survey – Geophones are laid out on the ground surface in an array around 

the borehole. The energy source is set off within the borehole at successively decreas-
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FIGURE 6.27 MAGNETIC INTENSITY RECORDED AT VARIOUS

 HEIGHTS ABOVE WELL CASING

TOTAL FIELD (NANOTESLAS)

250 FT

200 FT

150 FT

100 FT

HEIGHT OF AIRPLANE

ABOVE GROUND

S N

5,000 0

DISTANCE (FT)

(FRISCHNECHT AND RAAB, 1984)

5,000

100 NT 100 NT

ing depths starting at the bottom of the hole. The travel times from the source to the 

surface are analyzed to determine the wave velocity versus depth.

• Downhole Survey – In a downhole survey, the energy source is located on the surface and a detector (geophone) is placed in a borehole. The travel time is measured 

with the geophone placed at progressively increasing depth, and a wave-velocity 

profile is generated.

Crosshole seismic surveys involve measurement of the travel time of seismic energy transmitted 

between two or preferably three boreholes to derive information relative to the elastic properties 

of the intervening materials. The travel times of the seismic waves are derived from the identified 

first-arrivals of the P- and S-waves on the seismic trace for each shot-receiver position and are used 

with the known distance (s) between the shot/receiver boreholes to calculate the apparent velocities (P- and S-wave) for each depth interval. The borings are usually cased and grouted to the surrounding soil/rock. PVC casing is normally used for the tests, so that the casing is not a seismic 

pathway. A typical field setup for a crosshole seismic survey is shown in Figure 6.28.

Crosshole geophysical testing is described in ASTM D 4428, “Standard Test Methods for Crosshole 

Seismic Testing.” Crosshole measurements are generally preferred to downhole measurements 

because they provide higher resolution and greater accuracy. However, the distances between the 

energy source and the detector must be measured precisely. An inclinometer survey is generally performed in crosshole test boreholes to correct the data for deviation of the boreholes from vertical. To 

calculate P- and S-wave velocity, the wave arrivals must be processed with a computer program that 

accounts for situations where the waves may be refracted between the boreholes according to Snell’s

Law. The data are then used to develop vertical profiles of the various elastic moduli.

FIGURE 6.27 MAGNETIC INTENSITY RECORDED AT VARIOUS HEIGHTS ABOVE WELL CASING
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TABLE 6.26 APPLICABILITY OF COMMON BOREHOLE GEOPHYSICAL METHODS

Borehole Geophysical

Method

Dependent Physical

Property

Application (see key below)

1 2 3 4 5 6 7 8 9 10

Acoustic Logs

In-hole acoustic velocity Elastic moduli, density A-2 A-2 X X A-2 X A-2 X X X

Crosshole S-wave velocity Elastic moduli, density A-4 X X X M X X X X M

Downhole S-wave velocity Elastic moduli, density A-4 X X X M X X X X M

Electric and Induction

Spontaneous potential Potential difference A-2 X X X A-2 X X X X X

Single-point resistance Resistance A-2 X X X A-2 X A-2 X X X

Multi-electrode resistivity Resistivity A-2 X A-2 X A-2 X A-2 X X X

Induction Conductivity A-4 X A-4 X M X X X A-4 X

Nuclear

Gamma Gamma radiation A-6 A-6 X X X X X X X X

Gamma-gamma Density A-6 X A-6 X A-6 A-6 X X X X

Neutron Hydrogen content A-6 X A-6 X A-6 X X X X X

Fluid logs

Borehole fluid 

characteristics

Resistivity/

conductivity X A-5 X A-5 A-5 X X X X X

Fluid flow Velocity X A-5 X X X X X X A-1 X

Temperature Temperature X A-5 X A-5 A-5 X X X A-1 X

Miscellaneous

Borehole deviation Inclination X X X X X X X X A-6 X

Video Visual characteristics M X X M A-6 X X A-6 4 A-6

Laser imaging Physical dimensions X X X X X X X X X A-3

Sonic imaging Physical dimensions X X X X X X X X X A-2

Caliper Physical dimensions X X X X X X X A-3 A-3 M

Applications Key

1. Lithology and correlation

2. Hydraulic Conductivity

3. Porosity

4. Fluid properties

5. Depth to groundwater

 6. Bulk density

 7. Rock structure

 8. Borehole parameters

 9. Elastic properties of coal refuse, soil and rock

10. Characterization of mine workings or other subsurface voids

Technique Applicability and Required Hole Conditions

A-1 Applicable (cased, fluid-filled hole)

A-2 Applicable (uncased, fluid-filled hole)

A-3 Applicable (uncased, dry hole)

A-4 Applicable (open or fluid-filled hole,

 non-conductive casing)

A-5 Applicable (screened or uncased, fluid-filled hole)

A-6 Applicable (any hole condition, but fluid must be clear for video)

M May be applicable, but probably not the best approach

X Not applicable

(ADAPTED FROM USACE, 1995)
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The uphole and downhole techniques are more economical alternatives to the crosshole technique 

because only one borehole is required. Downhole measurements are not as accurate as crosshole 

measurements, especially if the layers of interest are thin. However, if critical thin layers are not present, downhole and uphole measurements may be the preferred means for determining the variation 

of the P- and S-wave velocities with depth. Downhole surveys are generally preferred to uphole surveys, because it is usually more practical to induce a strong seismic signal at the surface than it is in 

a borehole. Figure 6.29 depicts the deployment for a downhole seismic survey. In terms of analysis, 

the downhole or uphole methods differ from the crosshole method in that it is necessary to calculate 

incremental velocities on the basis of differences in travel time between geophones at varying depths 

rather than from direct pathways.

Seismic tomography employing surveys from boreholes can be used as a tool for detecting abandoned mines. A vertical seismic profile (VSP) can be developed by deploying geophone sensors in a 

borehole and a seismic source at multiple surface locations. An emerging technology whereby a drill 

bit is used as a downhole source and seismic waves are recorded at the surface is referred to as reverse 

vertical seismic profile (RVSP). Although these techniques are generally available and are used in the 

oil and gas industry, they are rarely applied to investigations at coal refuse facilities because of their 

relatively high cost. Nevertheless, there is some experience in the application of these techniques as 

documented by the Colorado School of Mines (2007) and Gritto (2003).

FIGURE 6.28 FIELD SETUP FOR CROSSHOLE SEISMIC SURVEY

SOURCE

HOLE

RECEIVER

NO. 1

RECEIVER

NO. 2

LAYER 1

LAYER 2

LAYER 3

PVC CASING

GROUTED INTO

BOREHOLE

DOWNHOLE

SEISMIC

SOURCE BODY WAVES

GEOPHONE OR

HYDROPHONE

TRIGGER SWITCH

FOR IMPACT TIME

DIGITAL RECORDING

SYSTEM

CLAMPING DEVICE

NOTE: THE SEISMIC WAVES GENERATED MAY BE

 P-, SV-, OR SH BODY WAVES DEPENDING

 UPON THE TYPE OF SOURCE EMPLOYED. (SIRLES, 2006)

FIGURE 6.28 FIELD SETUP FOR CROSSHOLE SEISMIC SURVEY
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A specialized application of a cross borehole technique to identify mine voids is seam wave seismic. Because coal typically has a relatively low seismic velocity compared to the rock formations 

that confine coal seams, seismic waves can become “trapped” in a coal seam and propagate over 

long distances with relatively little attenuation if the coal is continuous. Conversely, obstructions 

to a coal seam such as abandoned mine workings will prevent the propagation of a seam wave. 

D’Appolonia (1982) describes the use of this technique, but since the publication of this report, 

seam wave technology has only rarely been used due to the expense and difficulty in interpreting 

the results. Additional discussion of this technology is provided by Marshall Miller & Associates 

(MM&A, 2006) and Pennsylvania State University (2006).

6.4.4.2.2 Video and Laser/Sonar Imaging

Imaging of portions of abandoned mine workings can be critical to an understanding of the orientation and condition of these mine workings with respect to an existing or planned coal refuse facility. 

Video imaging with a borehole camera is a mature technology that also allows for the identification 

of fractures and collapse zones from a borehole. A disadvantage of using a borehole video camera 

in an abandoned mine is that it is difficult to image very far into the mine because of lack of light. 

Another disadvantage is that only visual data are obtained, and it is often difficult to determine distances because of a lack of scale.

If a mine is not flooded, an alternative means to image abandoned workings is a laser, range-based 

geometric scanner inserted through a dry borehole. Once deployed into a mine void space, a pan and 

SEISMIC RECORDER

TRIGGERING

GEOPHONE

PNEUMATIC

PACKER

TRIAXIAL

GEOPHONE

PNEUMATIC PUMP

DYNAMIC

LOAD

STATIC LOAD

FIGURE 6.29 FIELD SETUP FOR DOWNHOLE SEISMIC SURVEY

FIGURE 6.29 FIELD SETUP FOR DOWNHOLE SEISMIC SURVEY
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tilt sequence is initialized, producing a scan of the void. The collected data set is then converted in the 

field into a 3D point cloud model of the void. The point cloud model is then converted into a 3D mesh 

model of the underground space. These data can subsequently be processed to produce plan views, 

sectional views, and volume estimations. Figure 6.30 is a 2D image of a mine entry obtained from a 

borehole laser scanner, and Figure 6.31 is a 3D laser image of mine workings.

If a mine is submerged, it is still possible to image mine openings with a submersible, sonar, rangebased scanner inserted into a borehole. Data collected can be oriented using an on-board compass. 

Through correlation of several scans at varying elevations, it has proven practical to prepare 3D 

models of the flooded space.

6.5 MATERIAL PROPERTY DETERMINATION THROUGH TESTING

Accurate and reliable laboratory soil, rock and materials testing requires selection of the appropriate 

tests and care in sample preparation and performing the tests. Laboratory test results must be carefully interpreted, based upon the: (1) sampling and testing procedures, (2) types of soil and rock at 

the site, (3) geologic history of the site, and (4) types of coal refuse present and their possible use in 

refuse disposal facility construction. Suggested references for soil, rock and materials testing procedures include the most current ASTM standards, Mayne et al. (2002), Bardet (1997), and Head (1980, 

1982, 1986)

Careful work and attention to detail in laboratory testing are important if accurate and representative 

results are to be obtained. This is true for all soils, but it is especially important for soils whose structure or fabric, and consequently their tested engineering characteristics, can be affected by disturbance. When undisturbed sample tests are to be conducted on fine-grained soils, sample disturbance 

must be minimized during sampling, transport, storage and testing. Similarly, some rock types (e.g., 

mudstones and claystones) can degrade following stress relief and exposure to the air following drilling. Such materials should be carefully stored and transported so that their in-situ moisture condition 

is preserved to the extent possible.

LENGTH (FT)

WIDTH (FT)

-120 -100 -80 -60 -40 -20 0 20 40

40

20

0

-20

-40

(WORKHORSE TECHNOLOGIES, 2006)

FIGURE 6.30 IMAGE OF MINE ENTRY FROM BOREHOLE LASER SCANNER
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Laboratory tests that provide essential data for the analysis and design of an earthen dam or coal 

refuse embankment are summarized in Tables 6.27 and 6.28. Table 6.27 lists typical soil laboratory 

tests for site characterization, and Table 6.28 summarizes typical laboratory soil tests applicable to 

the types of soil, rock and refuse materials used in the construction of embankments and other earth/

refuse structures. In most investigations, all of the classification or index property tests listed in the 

tables should be performed on representative samples. The need for other tests depends upon the 

purpose and subject of the investigation. The use of test data in the analysis and design of earthen 

dams and coal refuse impoundments is discussed in Section 6.6.

Standard testing procedures used in soil and rock mechanics are generally applicable to coal refuse, 

although modifications may be appropriate because of the characteristics of coal refuse. Tests for 

ash content, pyrite content and leachate water quality, as indicated in Table 6.28, are parameters not 

included in a typical embankment testing program. However, these parameters may be important in 

any portion of a disposal facility to be constructed from coal refuse. The ash content is an indication of 

the amount of coal remaining in the refuse and can be directly correlated with measurements of specific gravity and density. In cases where significant amounts of coal may remain in the refuse, there is 

a possibility of spontaneous combustion. Knowledge of pyrite content and leachate water quality can 

facilitate placement procedures that will minimize the potential for environmental impacts.

6.5.1 Selection of Samples for Testing

Samples used for laboratory testing include: (1) bulk and disturbed samples, (2) undisturbed samples, and (3) reconstituted samples. Reconstituted samples are samples created to have characteristics 

similar to in-situ properties and to meet specified test criteria (e.g., maximum particle dimension 

cannot be greater than some proportion of the minimum dimension of the test apparatus). The following text describes the three basic types of samples and their possible use for laboratory testing.

6.5.1.1 Bulk and Disturbed Samples

Representative bulk and disturbed samples of soils and refuse materials (for existing refuse disposal 

facilities) are collected from refuse delivered from the preparation plant, test pit excavations and 

disturbed sampling (e.g., split-barrel samples) for use in conducting laboratory index (e.g., classification, moisture content, Atterberg limits) and property characterization (e.g., compaction tests and 

FIGURE 6.31 LASER IMAGE OF MINE WORKINGS

(WORKHORSE TECHNOLOGIES, 2006)

FIGURE 6.31 LASER IMAGE OF MINE WORKINGS
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TABLE 6.27 TYPICAL SOIL AND ROCK LABORATORY TESTS FOR COAL 

REFUSE DISPOSAL SITE CHARACTERIZATION

Test Category Test Description ASTM 

Designation

Visual

Identification

Standard Practice for Description and Identification of Soils (Visual-Manual 

Procedure)

D 2488

Index

Properties

Standard Test Method for Laboratory Determination of Water (Moisture) 

Content of Soil and Rock by Mass

D 2216

Standard Test Methods for Specific Gravity of Soil Solids by Water 

Pycnometer

D 854

Standard Test Method for Particle-Size Analysis of Soils D 422

Standard Practice for Classification of Soils for Engineering Purposes 

(Unified Soil Classification System)

D 2487

Standard Test Methods for Amount of Material in Soils Finer than the No. 

200 (75-μm) Sieve

D 1140

Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index 

of Soils

D 4318

Corrosivity

Standard Test Method for pH of Soils D 4972

Standard Test Method for Measuring pH of Soil for Use in Corrosion 

Testing

G 51

Standard Test Method for Sulfate Ion in Water D 516

Standard Test Method for Field Measurement of Soil Resistivity Using the 

Wenner Four-Electrode Method

G 57

Standard Test Methods for Chloride Ion in Water D 512

Organic

Content

Standard Test Methods for Moisture, Ash, and Organic Matter of Peat and 

Other Organic Soils

D 2974

Compaction

Test

Standard Test Methods for Laboratory Compaction Characteristics of Soil 

Using Standard Effort (12,400 ft-lbf/ft3) (600 kN-m/m3)

D 698

Standard Test Methods for Laboratory Compaction Characteristics of Soil 

Using Modified Effort (56,000 ft-lbf/ft3) (2,700 kN-m/m3)

D 1557

Standard Test Methods for Maximum Index Density and Unit Weight of 

Soils Using a Vibratory Table

D 4253

Standard Test Methods for Minimum Index Density and Unit Weight of 

Soils and Calculation of Relative Density

D 4254

Hydraulic

Conductivity

Standard Test Method for Permeability of Granular Soils (Constant Head) D 2434

Standard Test Methods for Measurement of Hydraulic Conductivity of 

Saturated Porous Materials Using a Flexible Wall Permeameter

D 5084

Consolidation

Properties

Standard Test Methods for One-Dimensional Consolidation Properties of 

Soils Using Incremental Loading

D 2435

Standard Test Method for One-Dimensional Consolidation Properties of 

Saturated Cohesive Soils Using Controlled-Strain Loading

D 4186
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TABLE 6.27 TYPICAL SOIL AND ROCK LABORATORY TESTS FOR COAL 

REFUSE DISPOSAL SITE CHARACTERIZATION 

(Continued)

Test Category Test Description ASTM 

Designation

Static Strength 

Properties

Standard Test Method for Unconfined Compressive Strength of Cohesive 

Soil

D 2166

Standard Test Method for Unconsolidated-Undrained Triaxial Compression 

Test on Cohesive Soils 

D 2850

Standard Test Method for Consolidated-Undrained Triaxial Compression 

Test for Cohesive Soils

D 4767

Standard Test Method for Direct Shear Test of Soils under ConsolidatedDrained ConditionsD 3080

Standard Test Method for Laboratory Miniature Vane Shear Test for 

Saturated Fine-Grained Clayey Soil

D 4648

Cyclic/Dynamic 

Strength Properties

Standard Test Method for the Determination of the Modulus and Damping 

Properties of Soils Using the Cyclic Triaxial Apparatus

D 3999

Standard Test Methods for Modulus and Damping of Soils by the 

Resonant-Column Method

D 4015

Standard Test Method for Load-Controlled Cyclic Triaxial Strength

of Soil

D 5311

Rock Properties

Standard Test Method for Determination of the Point Load Strength Index 

of Rock

D 5731

Standard Test Method for Compressive Strength and Elastic Moduli 

of Intact Rock Core Specimens under Varying States of Stress and 

Temperatures

D 7012

Standard Test Method for Splitting Tensile Strength of Intact Rock Core 

Specimens

D 3967

Standard Test Method for the Slake Durability of Shales and Similar Weak 

Rocks

D 4644

(ADAPTED FROM SABATINI ET AL., 2002)

strength and compressibility testing of reconstituted samples) testing. Table 6.18 provides a summary of common sampling methods for obtaining bulk and disturbed soil samples.

Field personnel directing field sampling activities need to be aware that the quantity of material 

needed depends on the laboratory tests to be performed, the relative amount of coarse (> 3 inches) 

particles present, and the size limitations of the test equipment. ASTM D 420, “Standard Guide to Site 

Characterization for Engineering Design and Construction Purposes,” provides general guidelines 

for minimum sample weights. These guidelines are presented in Table 6.29. More specific guidance 

on minimum sample weight is provided in the instructions for individual test procedures.

For moisture-sensitive, fine-grained soils, samples should be retained in sealed containers, and bulk 

samples should be labeled, indicating information such as test pit number, depth below the ground 

surface, and date sampled.
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TABLE 6.28 TYPICAL LABORATORY SOIL TESTS FOR VARIOUS MATERIALS(1)

 Test

ASTM 

Test 

Method

Type of Material(2)

 Use in Design

Fine-Grained 

Soil

CoarseGrained Soil

Rock

Coarse

Refuse

Fine

Refuse

Combined 

Refuse

Classification or Index 

Property Tests Evaluation of feasible 

configuration

Correlation of materials

Selection of samples for 

other tests

Selection of borrow 

areas

Specification of 

construction procedures

Determination of 

filter and drainage 

requirements

 Moisture Content D 2216 a a – a a a

 Unit Weight – c – – – c c

 Specific Gravity D 854 b, c b, c b b b b

 Atterberg Limits D 4318 b, c b, c – – b, c –

 Particle-Size

 Analysis

D 422,

D 2217 b, c b, c b b b, c b

 Soil Classification D 2487 a a – – – –

Compaction Tests

Evaluation of sample 

preparation for other 

tests

Specification of 

placement requirements 

 Standard Proctor D 698 b – – b b b

 Modified Proctor D 1557 b – – b b b

 Relative Density D 4253, 

D 4254 – b b b – –

Hydraulic Conductivity D 2434, 

D 5084 c, d – – d c, d c, d

Seepage analyses

Determination of pore 

pressure for stability

Consolidation D 2435,

D 4186 c, d – – – c, d – Settlement analyses

Shear Strength

Stability analyses

Structure foundation 

design

 Direct Shear D 3080 c, d d d d c, d c, d

 Triaxial compression D 2850, 

D 4767 c, d d – d c, d c, d

 Unconfined

 compression D 2166 c, d – – – – –

 Vane Shear D 4648 c – – – c –

 Direct Simple Shear D 6528 – – – – – –
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TABLE 6.28 TYPICAL LABORATORY SOIL TESTS FOR VARIOUS MATERIALS(1)

(Continued)

 Test

ASTM 

Test 

Method

Type of Material(2)

 Use in Design

Fine-Grained 

Soil

CoarseGrained Soil

Rock

Coarse

Refuse

Fine

Refuse

Combined 

Refuse

Rock Property and 

Behavior Tests

Design of rock slopes

Stability of 

underground mine 

roofs, pillars and 

barriers

Evaluation of rock 

degradation

 Point Load D 5731 – – c – – –

 Unconfined

 Compression D 7012 – – c – – –

 Slake Durability D 4644 – – c – – –

 Indirect Tensile

 Strength D 3967 – – c – – –

Miscellaneous Tests

Evaluation of burning 

potential

Corrosion analyses

 Ash Content D 2415 – – – b b b

 Pyrite Content D 4239,

D 2492 – – b b b b

 Leachate Water

 Quality

D 1068,

D 858,

D 516

– – – b(3) b(3) b(3)

Note: 1. The testing program for all significant coal refuse embankments should be established by a qualified, 

experienced geotechnical engineer. The types and numbers of tests needed will vary depending of the 

purpose of the testing program and the condition being evaluated. Use of these guidelines should not be 

substituted for evaluation of specific site conditions by a qualified engineer. Additional discussion is pro-

 vided in Section 6.5.

2. a – tests normally conducted on all samples

 b – tests normally conducted on representative disturbed samples

 c – tests normally conducted on representative undisturbed samples

 d – tests should be conducted on specially-prepared samples to simulate as-constructed behavior

3. Discussion related to conducting water quality tests as part of the geotechnical investigation is provided

 in Section 6.4.4.

6.5.1.2 Undisturbed Samples

Undisturbed samples are obtained from cohesive soil strata for laboratory testing to determine 

properties such as strength, stratification, hydraulic conductivity, density, consolidation, dynamic 

behavior, and other engineering characteristics. Specialized procedures are required for obtaining 

undisturbed samples of granular soils, thus their application at coal refuse disposal sites is limited 

to locations where void ratio, density and strength tests are needed for seismic design. Undisturbed 

samples are obtained with specialized equipment designed to minimize the disturbance to the insitu structure and moisture content of the soils. Table 6.18 provides a summary of common methods 

for obtaining undisturbed soil samples. The importance of sample preservation during undisturbed 

sample recovery and transport is described in Section 6.4.3.5.
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6.5.1.3 Reconstituted Samples

Occasionally due to lack of adequate sample volume, difficulties encountered in the field in retrieving 

undisturbed samples, or dimensional requirements imposed by specific test methods, samples must 

be created or reconstituted in the laboratory for testing to establish engineering properties needed for 

design. The need to use reconstituted samples is more common for granular soils because undisturbed 

sampling of sands and gravels is difficult and costly and because the particle sizes in the coarser fraction of a sample may exceed particle-size limits in some tests. For example, the relative density of a 

saturated sand or of fine coal refuse can be estimated by in-situ testing, but an acceptably undisturbed 

sample for cyclic triaxial testing in the laboratory is difficult and costly to obtain. As a result, samples 

may need to be prepared in the laboratory to reasonably recreate the in-situ relative density or void 

ratio of the soil or sand-like refuse material. For predominantly coarse-grained soils, samples can be 

reconstituted by compaction in a mold. For sand-like refuse material, samples can be reconstituted 

by molding moist material or settling from a slurry. For clay-like fine refuse, the depositional history 

cannot be readily recreated in the laboratory, so reconstituted samples should not be used.

Reconstituted samples may also be necessary when the coarse particle-size fraction of a sample (usually a bulk sample) exceeds a dimensional limitation associated with the desired test. For example, 

ASTM D 3080, “Standard Test Method for Direct Shear Test of Soils under Consolidated Drained 

Conditions,” prescribes that the maximum particle size not exceed 0.1 times the tested sample diameter (for circular samples) or sample width (for square samples). With this criterion, the maximum 

sample particle size cannot exceed 0.2 inches (corresponding approximately to a No. 4 sieve) for a 

2-inch-diameter sample and 0.4 inches (corresponding approximately to a 3⁄8-inch sieve) for a 4-inchsquare sample. For this example, if the maximum particle size exceeded 0.4 inches, the particle-size 

distribution for the sample used for direct shear testing would need to be adjusted to accommodate 

the maximum-particle-size criterion. ASTM test methods identify such gradational limitations and 

describe sample preparation techniques and test result evaluation methods to account for the removal 

of over-size particle fractions.

6.5.2 Classification and Index Property Tests

To catalog soils and coal refuse materials that will form the foundation, embankment cross section, and impoundment of a coal refuse disposal facility, samples from the field testing program 

should be examined and accurately classified. The system of classification used by most geotechnical engineers and government agencies is the Unified Soil Classification System (USCS) as described 

in ASTM D 2487, “Standard Practice for Classification of Soils for Engineering Purposes (Unified 

Classification System).” This classification system for engineering purposes is based on laboratory 

determination of particle-size distribution and Atterberg limits. ASTM D 2488, “Standard Practice 

for Description and Identification of Soils (Visual-Manual Procedure),” provides a companion procedure for preliminary classification of soils based on visual and manual techniques available to 

field and laboratory personnel.

TABLE 6.29 GUIDELINES FOR MINIMUM SAMPLE WEIGHTS

 Test and Soil Characteristics Minimum Sample Weight

Visual classification 2 ounces to 1 pound

Soil constants and particle-size analysis of non-gravelly soil 1 to 5 pounds

Soil compaction tests and sieve analysis of gravelly soils 40 to 80 pounds

Aggregate properties 100 to 400 pounds

(ADAPTED FROM ASTM, 2008b,c)
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As shown in Table 6.30, the USCS divides soils into two main classes: coarse-grained and fine-grained 

soils. Highly organic soils form an additional division. Coarse-grained soils are soils composed of 

predominantly gravel- and/or sand-sized particles (greater than 50 percent retained on a No. 200 

sieve) that can be separated into eight groups based primarily on the coarseness, gradation, and percentage of fines and secondly on the plasticity of fines. Fine-grained soils are soils composed of predominantly silt- and/or clay-sized particles (greater than 50 percent passing a No. 200 sieve) that can 

be separated into six groups, based primarily on plasticity and secondly on coarseness, gradation, 

and percentage of coarse fractions, if present. Generally, the system is arranged so that any sample 

can be classified by visual observation and simple tests that often can be conducted in the field by the 

engineer or geologist supervising an exploration program. However, laboratory tests on representative samples are needed to confirm the visual classification of soil properties or to classify borderline 

cases. Table 6.30 provides numerical criteria used for classification based upon laboratory test results. 

Figure 6.32 presents the plasticity chart used to classify fine-grained soils based on the liquid limit 

(LL) and plasticity index (PI) of tested samples.

Table 6.8, as adapted from Sherard et al. (1963), presents an approximate correlation between the 

USCS classification and the engineering and design properties of soils. Although the table is not a 

substitute for detailed laboratory tests, it can be used to help determine which tests should be conducted and to preliminarily evaluate available embankment materials.

There are no categories in the USCS for coal refuse materials, and current practice is to classify and 

test them in the same manner as other soil materials. Each of the following discussions of classification tests concludes with information on coal refuse properties as compared to properties of other 

soils. The basis for the discussion includes published data, as cited, and project experience, although 

it should be recognized that substantial variation can occur due to site-specific conditions and mining 

and coal preparation practices.

6.5.2.1 Moisture Content

Moisture content tests are typically conducted on disturbed and undisturbed samples obtained 

at a site to: (1) better characterize in-situ conditions and evaluate other tests, (2) evaluate borrow 

material suitability through comparison of natural moisture content and the moisture content 

required for proper compaction, and (3) provide information for calculating the void ratio of 

saturated samples.

Void ratio is defined as the ratio of void space to the volume of the solid particles:

e = Vv /Vs (6-9)

where:

Vv = volume of voids (length3)

Vs = volume of solids (length3)

Properly obtained samples of fine-grained soils sealed in plastic, wax or airtight jars at the time of 

sampling can be accurately tested for moisture content at a later time in the laboratory. Testing of 

coarse-grained soils may not be accurate if water is lost by drainage during sampling. The field engineer should note whether moisture content measurements for coarse-grained soil samples may have 

been affected by the sampling.

As described in ASTM D 2216, “Standard Test Method for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass,” the procedure for measuring moisture content is to weigh 
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TABLE 6.30 SOIL CLASSIFICATION CHART (LABORATORY METHOD)

Criteria for Assigning Group Symbols and Group Names

Using Laboratory Tests(1)

Soil Classification

Group 

Symbol Group Name(2)

GRAVELS

≥ 50% of coarse 

fraction retained 

on No. 4 Sieve

CLEAN GRAVELS Cu ≥ 4 and 1 ≤ Cc ≤ 3(5) GW Well-graded Gravel

< 5% fines Cu < 4 and/or 1 > Cc > 3(5) GP Poorly-graded Gravel(6)

GRAVELS WITH 

FINES Fines classify as ML or MH GM Silty Gravel(6,7,8)

> 12% fines(3) Fines classify as CL or CH GC Clayey Gravel(6,7,8)

SANDS

≥ 50% of coarse 

fraction retained 

on No. 4 Sieve

CLEAN SANDS Cu ≥ 6 and 1 ≤ Cc ≤ 3(5) SW Well-graded Sand(9)

< 5% fines(4) Cu < 6 and 1 > Cc > 3(5) SP Poorly-graded Sandi

SANDS WITH 

FINES Fines classify as ML or MH SM Silty Sand(7,8,9)

> 12% fines(4) Fines classify as CL or CH SC Clayey Sand(7,8,9)

SILTS AND 

CLAYS

LL < 50

Inorganic

PI > 7 and plots on or above “A” 

line(10) CL Lean Clay(11,12, 13)

PI < 4 or plots below “A” line(10) ML Silt(11,12,13)

Organic LL after oven drying < 0.75 LL 

before oven drying

OL Organic Clay(11,12,13,14)

OL Organic Silt(11,12,13,15)

SILTS AND 

CLAYS

LL ≥ 50

Inorganic

PI plots on or above “A” line CH Fat Clay(11,12,13)

PI plots below “A” line MH Elastic Silt(11,12,13)

Organic LL after oven drying < 0.75 LL 

before oven drying

OH Organic Clay(11,12,13,16)

OH Organic Silt(11,12,13,17)

Highly fibrous 

organic soils

Primarily organic matter, dark in color, with

organic odor PT Peat andMuskeg

Note: 1. Based on the material passing the 3-in (75-mm) sieve.

2. If field sample contained cobbles or boulders, or both, add “with cobbles” or “with boulders” to group name.

3. Cu = D60/D10 = uniformity coefficient (UC);

 Cc = (D30)

2

 / (D60 x D10) = coefficient of curvature

4. If soil contains ≥ 15% sand, add “with sand” to group name.

5. Gravels with 5 to 12% fines require dual symbols:

GW-GM – well-graded gravel with silt

GW-GC – well-graded gravel with clay

GP-GM – poorly-graded gravel with silt

GP-GC – poorly-graded gravel with clay
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a wet sample, dry it in a constant-temperature oven at 105° C until the weight is constant (approximately 24 hours for small samples of fine-grained soils), and then weigh the dry sample. In soil 

mechanics practice, the moisture content is defined as the ratio of the weight of water (wet weight 

minus dry weight) to the dry weight. This is sometimes referred to as the dry-weight-basis moisture 

content. In other disciplines, moisture content may be defined on a wet-weight basis, i.e., moisture 

content is defined as the ratio of the weight of water to the wet weight of soil.

LIQUID LIMIT, LL

PLASTICITY INDEX, PI

(ASTM, 2008)

10

20

0

4

7

MH or OH

"A" LINE

"U" LINE

CL-ML ML or OL

0 10 16 20 30 40 50 60 70 80 90 100 110

30

40

50

60

FOR CLASSIFICATION OF FINE-GRAINED SOILS AND THE

FINE-GRAINED FRACTION OF COARSE-GRAINED SOILS,

THE FOLLOWING RELATIONSHIPS APPLY:

"A" LINE - HORIZONTAL AT PI=4 TO LL-25.5,

 THEN PI=0.73 (LL-20)

"U" LINE - VERTICAL AT LL = 16 to PI=7,

 THEN PI=0.9 (LL-8) CH or MH

CL or OL

TABLE 6.30 SOIL CLASSIFICATION CHART (LABORATORY METHOD)

(Continued)

Note 6. If fines classify as CL-ML, use dual symbol GC-GM or SC-SM.

7. If fines are organic, add “with organic fines” to group name.

8. If soil contains ≥ 15% gravel, add “with gravel” to group name.

9. Sands with 5 to 12% fines require dual symbols:

SW-SM – well-graded sand with silt

SW-SC – well-graded sand with clay

SP-SM – poorly-graded sand with silt

SP-SC – poorly-graded sand with clay

10. If Atterberg limits plot in the orange area in Figure 6.32, soil is a CL-ML (silty clay).

11. If soil contains 15 to 29% plus No. 200 sieve, add “with sand” or “with gravel,” whichever is predominant.

12. If soil contains ≥ 30% plus No. 200 sieve, predominantly sand, add “sand” to group name.

13. If soil contains ≥ 30% plus No. 200 sieve, predominantly gravel, add “gravelly” to group name.

14. PI ≥ 4 and plots on or above “A” line.

15. PI < 4 or plots below “A” line.

16. PI plots on or above “A” line.

17. PI plots below “A” line.

(ADAPTED FROM ASTM, 2008b,c)

FIGURE 6.32 PLASTICITY CHART
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If a soil sample contains a significant amount of organic material, this method of measuring moisture 

content is not always satisfactory, because heating the sample to 105° C may drive off some of the 

organic material in addition to the water. Alternatively, ASTM D 2216 permits oven drying at 60° C 

for the moisture content of organic soils and organic materials. Most coal refuse is not significantly 

affected by oven drying at 105° C, although this may need to be considered when working with lower 

grade coals such as lignite.

6.5.2.2 Specific Gravity

The specific gravity of a soil is the ratio of the weight of a given volume of soil solid particles to the 

weight of an equal volume of distilled water at 4° C. As used in geotechnical engineering, the term 

specific gravity refers to the average specific gravity of the individual soil particles in a sample rather 

than bulk specific gravity. Specific gravity is determined in the laboratory in accordance with ASTM 

D 854, “Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer.” The test is 

performed by weighing a calibrated bottle containing soil particles suspended in distilled water and 

comparing this to the weight of the same bottle containing an equal volume of distilled water only. 

Specific gravity is used to determine relationships between soil weight and soil volume. Specific 

gravity is used for: (1) computing the void ratio of a soil, (2) hydrometer analyses, and (3) predicting 

the unit weight of a soil. Occasionally, the specific gravity may be useful in soil mineral classifications 

(e.g., iron minerals have a higher specific gravity than silica).

Soils typically have a specific gravity ranging from 2.4 to 2.8. For many design purposes specific 

gravity can be estimated without testing. For coal refuse facilities, specific gravity is an important 

design parameter because coal refuse often contains a significant amount of materials with specific 

gravity in the range of 1.3 to 1.6. As a result, coarse coal refuse can have a specific gravity ranging 

from as low as 1.5 to as high as 2.8. The most common range is between 1.9 and 2.4. Similarly, specific gravity measured for fine coal refuse typically ranges from 1.4 to 2.3. Published data on specific gravity and unit weight of coarse and fine coal refuse from sites in the northern Appalachian 

region illustrating some of the variability in these parameters is presented in Tables 6.31 and 6.32, 

respectively, as compiled by Hegazy et al. (2004). The database for these summaries from Hegazy 

et al. (2004) was developed from geotechnical investigations of existing coal refuse disposal sites in 

western Pennsylvania and England. In-situ samples were collected from the sites using both disturbed methods (bucket samples from test pits and fine coal refuse deltas and split-barrel samples 

from boreholes) and undisturbed sampling methods (Shelby- and Dennison-tube samples).

The coefficient of variability (COV) is the ratio of the standard deviation of a set of data divided by 

the mean. Typically, values of COV below 10 percent are thought to be low, between 10 and 30 percent moderate, and above 30 percent high. Values of total unit weight γT, dry unit weight γD, and 

specific gravity Gs for coarse and fine coal refuse are provided in Tables 6.31 and 6.32. For coarse coal 

refuse, Table 6.31 indicates low variability for γT and γD and moderate variability for Gs. For fine coal 

TABLE 6.31 IN-PLACE UNIT WEIGHT AND SPECIFIC GRAVITY OF COARSE COAL REFUSE

Property Dimension Average Standard

Deviation

Coefficient of

Variation

γT lb/ft3 124 5.8 0.048

γD lb/ft3 115 5.5 0.047

Gs Dimensionless 2.02 0.31 0.154

(ADAPTED FROM HEGAZY ET AL., 2004)
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refuse, Table 6.32 indicates low variability for γT and moderate variability for γD and Gs. Figure 6.33

shows the effect of carbon content on the specific gravity of coal refuse materials.

Designers should recognize that values of specific gravity and unit weight for coal refuse are lower 

than for commonly encountered soils. The lower specific gravity of coal refuse results in lower densities, higher moisture contents at a given void ratio, and the potential for reduced stability with 

respect to seepage forces. These characteristics are discussed further in Section 6.6.4.

6.5.2.3 Atterberg Limits

The Atterberg Limits define the boundaries between four states of consistency (hardness or softness) 

of fine-grained soils. In order of decreasing moisture content, these states are: liquid, plastic, semisolid and solid. The boundaries or limits between these states are:

• Liquid limit (LL) – boundary between the liquid and plastic states

• Plastic limit (PL) – boundary between the plastic and semi-solid states

• Shrinkage limit (SL) – boundary between the semi-solid and solid states

The plasticity index (PI) is defined as LL minus PL. The liquid and plastic limits and plasticity index 

are determined in accordance with ASTM D 4318, “Standard Test Methods for Liquid Limit, Plastic 

Limit, and Plasticity Index of Soils.” The shrinkage limit is determined in accordance with D 4943, 

“Standard Test Method for Shrinkage Factors of Soils by the Wax Method.”

TABLE 6.32 UNIT WEIGHT AND SPECIFIC GRAVITY FOR FINE COAL REFUSE

Property Dimension Average Standard

Deviation Coefficient of Variation

γT lb/ft3 86 7.7 0.096

γD lb/ft3 62 9.1 0.162

Gs Dimensionless 1.52 0.25 0.165

(ADAPTED FROM HEGAZY ET AL., 2004)

The liquid limit is the moisture content at which the soil sample flows and closes a standard width 

groove when the sample is jarred in a specified way. For practical purposes, it is the moisture content at which the soil has essentially no shear strength (the soil becomes liquid). The test for the LL is 

conducted using a standard liquid-limit device and grooving tool. The plastic limit is the moisture 

content at which the soil begins to crumble when rolled by hand into 1⁄8-inch-diameter threads. The 

shrinkage limit (SL) is the moisture content sufficient to fill the pores of the soil at the minimum 

volume it will attain by drying. Other useful parameters from these tests are the plasticity index (PI) 

and the liquidity index (LI). The plasticity index is an indicator of soil plasticity, and the LI, which 

equals (w – PL)/PI, is an indicator of stress history and soil sensitivity. The liquidity index is approximately 1 for normally-consolidated soils and zero for over-consolidated soils. An LI greater than 1 

indicates high sensitivity.

Many properties of fine-grained clays and silts can be correlated to the Atterberg limits, and the plasticity chart shown in Figure 6.32 can be useful in interpreting the correlation. Procedures for using the 

plasticity chart for various soil types are discussed by Terzaghi et al. (1996).

Caution and considerable judgment should be used when applying the plasticity chart to coal refuse, 

because the chart is based on the behavior of natural, fine-grained soils. Tests conducted on only the 
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fines portion of coarse refuse have obtained LL’s in the range of 25 to 35 percent and PI’s typically 

below 12 percent. Table 6.4 cites published data characterizing the properties of fine coal refuse. Tests 

conducted on fine refuse samples from impoundments show LL’s in the range of 20 to 40 percent and 

PI’s generally below 15 percent, although higher PI’s have been reported. Factors affecting plasticity are discussed in Section 6.2.3.2. A summary of statistical properties related to Atterberg limits for 

fine coal refuse from northern Appalachian sites is provided in Table 6.33. Other data suggest that 

the plasticity of fine refuse from an impoundment increases with distance from the slurry discharge 

point, as the content of clay-size refuse particles increases.

6.5.2.4 Particle-size Distribution

The particle-size distribution of a coarse-grained soil is an important factor in its engineering behavior. The particle-size distribution of a coarse-grained soil can be used to classify the soil, to estimate 

hydraulic conductivity and to provide a qualitative indication of soil strength and deformation characteristics. For fine-grained soils, plasticity and moisture content are better indices of soil behavior than particle-size distribution. For both fine and coarse material, the particle-size distribution is 

needed for checking filter criteria requirements as part of prevention of piping of fines into coarser 

filter and drainage zones. Figure 6.34 shows typical gradation curves for several types of materials 

including coarse and fine coal refuse. Of particular note is the correlation between types of soil (clay, 

silt, sand and gravel) and sieve sizes and particle diameters.

For comparative purposes between soil types and for certain design applications, the uniformity of 

a soil can be expressed by the uniformity coefficient, which is the ratio of D60 to D10, where D60 is the 

particle diameter at which 60 percent of the soil weight is finer and D10 is the particle diameter at 

which 10 percent of the soil weight is finer. Sand and gravel soils having uniformity coefficients less 

than 6 and 4, respectively, are considered to be “uniform.” For example, the uniformity coefficients of

1.0 1.4 1.8 2.2 2.6 3.0

0

20

40

60

80

100

SPECIFIC GRAVITY, GS

FIXED CARBON PERCENT (DRY BASIS)

FIGURE 6.33 EFFECT OF CARBON CONTENT ON SPECIFIC

 GRAVITY OF COAL REFUSE MATERIAL

(HEGAZY ET AL., 2004)

FIGURE 6.33 EFFECT OF CARBON CONTENT ON SPECIFIC GRAVITY 

OF COAL REFUSE MATERIAL
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TABLE 6.33 STATISTICAL PROPERTIES FOR ATTERBERG LIMITS AND MOISTURE 

CONTENTS OF FINE COAL REFUSE AT NORTHERN APPALACHIAN SITES

Property Average

(%)

Standard

Deviation

Coefficient of

Variation

LL 31.2 5.2 0.17

PL 20.1 3.4 0.17

PI 11.2 3.1 0.28

w 33.0 11.5 0.35

(HEGAZY ET AL., 2004)

the two sandy soils shown in Figure 6.34 are about 6.1 and 1.4. These soils are termed “well-graded 

sand” and “uniform sand,” respectively.

As described in ASTM D 422, “Standard Test Method for Particle-Size Analysis of Soils,” the particlesize distribution of the portion of a soil sample coarser than a No. 200 sieve (0.074-mm-square openings) is generally determined by sieve analysis. This procedure consists of shaking a dry soil sample 

or washing a wet soil sample through a stack of wire screens of decreasing opening size. The diameter of an individual soil particle is defined as the minimum side dimension of a square hole through 

which the soil particle will pass.

The particle-size distribution of the portion of a sample finer than a No. 200 sieve can be measured 

with a hydrometer test as described in ASTM D 422. In this test, a sample of soil is vigorously mixed 

with water and a deflocculating agent to form a suspension. The suspension is then allowed to sit 

100 8 6 4 3 FIGURE 6.36 P 2 10 8 6ARTICLE-SIZE DISTRIBUTION 4 3 2 1

10

20

0

30

40

50

60

70

80

90

100

8 6 4 3 2 0.1 8 6 4 3 2 0.01 8 6 4 3 2 0.001

PERCENT FINER BY WEIGHT

4"

3"

2 1/2"

2"

1 1/2"

1"

3/4"

1/2"

3/8"

4

8

10

16

30

40

50

70

100

200

CLEAR SQUARE OPENINGS U.S. STANDARD SIEVE NUMBERS HYDROMETER ANALYSIS

PARTICLE DIAMETER (mm)

COBBLES GRAVEL SAND SILT OR CLAY (PLASTIC OR NON-PLASTIC) COARSE FINE COARSE MEDIUM FINE

FINE COAL REFUSE

COARSE COAL REFUSE WELL-GRADED

GRAVEL

POORLY-GRADED

GRAVEL

WELL-GRADED

SAND

UNIFORM SAND

FIGURE 6.34 PARTICLE SIZE DISTRIBUTION
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undisturbed in a 1000-ml glass cylinder to permit settlement of the suspended soil particles. Periodically, the change in the specific gravity of the suspension is measured with a calibrated hydrometer, 

allowing the approximate distribution of particle sizes to be calculated. The specific gravity of the 

suspension changes with time because the larger particles settle faster than smaller particles of the 

same specific gravity. The test result is only approximate because the calculation is based upon the 

assumption that the particles are spherical, of equal specific gravity, and do not interfere with each 

other during settlement. In actual tests, the particles are not spherical, there are variations in specific 

gravity, and considerable interference between particles likely occurs during settlement. Because the 

distributions determined by the hydrometer test are primarily used for comparative purposes, the 

accuracy of the test is generally not of major concern.

The effect of differences in specific gravity of various particles in the hydrometer test is greater for 

coal refuse than for other soils, because coal refuse consists of coal with specific gravity as low as 1.3 

and rock fragments with specific gravity as high as 2.8. For most analyses of coal refuse, it is appropriate to consider the average specific gravity of the entire sample. However, it should be understood 

that the variation in specific gravity of coal refuse particles adds greater than normal inaccuracy to 

the portion of the gradation curve determined by the hydrometer method.

Coarse coal refuse delivered to disposal facilities typically has the grain-size distribution of a wellgraded silty sand and gravel. Coal preparation usually limits the upper size to about 5 inches, 

although this size has more characteristically been less than 3 inches. Uniformity coefficients for 

coarse refuse range from about 20 to several hundred. Because many coal refuse particles are 

extremely friable and highly susceptible to both chemical and mechanical deterioration, the particle-size distribution changes during preparation, transportation and placement at the disposal 

facility. As delivered to the point of disposal, coarse refuse typically contains between 5 and 30 

percent of particles finer than the No. 200 sieve (0.075 mm). The clay-size fraction (finer than 0.002 

mm) is usually very small and often less than 2 percent. Sampling programs should be designed 

to evaluate the potential for particle degradation through collection of both as-delivered samples 

and after-placement samples.

Particle-size degradation occurs at the surface of a coarse refuse embankment or disposal facility, due 

to both chemical and mechanical deterioration. This behavior is described in Andrews et al. (1980) 

where the environmental effects of slaking of surface mine spoils in the eastern and central U.S. were 

evaluated. The study showed that degradation of surface mine spoil occurred over periods of years 

depending on the rock type(s) found in the spoil and their depth of burial. Field examination showed 

that degradation was more predominant with finer-grained, rock-type spoils (e.g., shales, mudstones 

and claystones) and was most prevalent in the upper 5 to 10 feet depending on the amount of compaction (if any) during spoil placement. For embankments constructed using coarse refuse, particlesize degradation is also affected by hauling equipment and mechanical compaction that occurs as the 

material is placed.

Fine coal refuse delivered to disposal facilities from preparation plants is typically a slightly clayey, 

sandy silt. Generally, 50 to 80 percent of the material will pass the No. 200 sieve, most of which is 

silt-size. Fine refuse segregates after being deposited, with the larger and heavier particles settling 

out of suspension near the discharge point. Farther from the discharge point, the settled refuse is 

predominantly finer-grained, and samples containing nearly 50 percent clay-size particles have been 

reported. A summary of statistical properties related to particle-size distribution for fine coal refuse 

samples from sites in northern Appalachia is provided in Table 6.34.

6.5.2.5 Chemical Characterization

Some soil and rock materials encountered at refuse disposal sites and the refuse being placed can have 

adverse chemical characteristics that may lead to undesirable environmental conditions or deteriora-
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tion of construction materials. Therefore, the chemical characteristics of these materials (e.g., corrosivity, organic content, ash content, pyrite content, and leachate water quality) should be determined. 

When deleterious conditions are encountered, appropriate amendment or containment/protection 

requirements should be implemented. Table 6.35 lists laboratory test methods that can be used for 

chemical characterization of soil, rock and refuse materials.

6.5.3 Compaction and Density Tests

Any soil placed as part of a structural fill, including coal refuse in embankments, is normally compacted to increase density and shear strength and to decrease compressibility and hydraulic conductivity. This makes relatively steep slopes possible, reduces seepage from impoundments, and reduces 

the potential for spontaneous combustion by reducing the flow of air and water into the embankment. In the field, compaction is accomplished with various types of rollers, including sheepsfoot, 

static and vibrating steel drum, and rubber-tired. The type of roller that is most appropriate depends 

upon the material being compacted, as discussed in Section 11.4.3.

6.5.3.1 Fine-grained Soils and Coal Refuse

Standard laboratory test procedures to control and evaluate the degree of compaction that can be 

achieved in the field during placement have been established. The test most commonly used for 

fine-grained soils and coal refuse utilizes impact compaction. The test procedure entails placing soil 

or refuse in several layers in a standard mold and compacting each layer by dropping a hammer of 

specified weight a specified distance for a specified number of times per layer.

The two standardized impact compaction tests are the standard Proctor and the modified Proctor tests. The standard Proctor compaction test was developed in the 1930s and was designed to 

approximate the compactive energy applied by field compaction equipment then available. As field 

equipment became larger and more efficient, the modified proctor compaction test was developed 

to approximate the greater compactive energy of the newer equipment. The test procedures for the 

standard and modified Proctor compaction tests are described in ASTM D 698, “Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft3

 (600 

kN-mm3))” and ASTM D 1557, “Standard Test Methods for Laboratory Compaction Characteristics 

of Soil Using Modified Effort (56,000 ft-lbf/ft3 (2,700 kN-mm3)),” respectively.

These test methods are suitable for soils and coal refuse that have 30 percent or less retained on the 

3⁄4-inch sieve and that have more than 15 percent by dry weight passing a No. 200 sieve. If more than 30 

percent is retained on a 3⁄4-inch sieve for either test, the unit weight and moisture content should be corrected in accordance with ASTM D 4718, “Standard Practice for Correction of Unit Weight and Water 

TABLE 6.34 PARTICLE-SIZE DISTRIBUTION FOR FINE COAL REFUSE

Particle Size or

Percent Passing Dimension Average StandardDeviation

Coefficient of

Variation

D10 mm 0.010 0.015 1.50

D30 mm 0.037 0.055 1.49

D50 mm 0.127 0.128 1.01

D60 mm 0.196 0.209 1.07

Passing No. 200

(0.075-mm) sieve % 57.7 25.0 0.43

(HEGAZY ET AL., 2004)
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Content for Soils Containing Oversize Particles.” Alternatively, a 12-inch-diameter compaction mold 

using standard Proctor compactive effort can be employed using the U.S. Army Corps of Engineers 

procedure, “Compaction Test for Earth-Rock Mixtures,” described in Section 11.5.1 (USACE, 1986). If 

less than 15 percent by dry weight passes the No. 200 sieve, the density of the soil may not be affected 

by changes in moisture, and the guidelines presented in Section 6.5.3.2 for coarse-grained soils may be 

applicable. Coarse coal refuse may contain less than 15 percent by dry weight passing a No. 200 sieve, 

but it generally does respond to changes in moisture content when compacted. Accordingly, in practice, 

the standard Proctor test is typically used to evaluate the compaction and density of coarse refuse.

Normally, a series of compaction tests is performed on several soil samples prepared at varying moisture contents. From the test results plotted as shown in Figure 6.35, the maximum density attainable 

and the moisture content at which the maximum density is attained (the optimum moisture content), 

can be determined. The greater compactive energy of the modified Proctor compaction test produces 

higher maximum densities at lower optimum moisture contents than the standard Proctor compaction test. The 100-percent-saturation (or zero-air-voids) curve to the right of the compaction curves in 

Figure 6.35 can be determined using the relationship:

γd = γw Gs / (1 + w Gs) (6-10)

where:

γd = dry density of soil (force/length3

)

γw = unit weight of water (force/length3

)

Gs = specific gravity of solids (dimensionless)

 w = moisture content expressed as a decimal value (dimensionless)

TABLE 6.35 TEST METHODS FOR CHEMICAL CHARACTERIZATION

OF SOIL, ROCK AND REFUSE MATERIALS

 Item Description ASTM Test Method

Corrosivity

Standard Test Method for pH of Soils

Standard Test Method for pH of Soil for Use in Corrosion Testing

Standard Test Method for Sulfate Ion in Brackish Water

Standard Test Method for Field Measurement of Soil Resistivity Using 

the Wenner Four-Electrode Method

Standard Test Methods for Chloride Ion in Water

D 4972

G 51

D 4130

G 57

D 512

Organic Content Standard Test Methods for Moisture, Ash, and Organic Matter of Peat 

and Other Organic Soils D 2974

Ash Content Standard Test Method for Ash in Coal Tar and Pitch D 2415

Pyrite Content Standard Test Method for Forms of Sulfur in Coal D 2492

Leachate Water

Quality(1)

Standard Test Method for Leaching Solid Material in a Column 

Apparatus

Standard Test Method for Shake Extraction of Mining Waste by the 

Synthetic Precipitation Leaching Procedure

D 4874

D 6234

 Note: 1. State regulatory agencies may require specific test procedures. Other standard test method references

 include EPA Method 1312, “Synthetic Precipitation Leaching Procedure (SPLP)” and EPA Method 1320,

 “Multiple Extraction Procedure (MEP).”
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 Equation 6-10 provides a check on the compaction test results to confirm that no compaction test result 

plots to the right of the 100-percent-saturation curve and that the shape of the dry density-moisture 

relationship wet of optimum moisture content generally parallels the 100-percent-saturation curve. 

Additionally, Equation 6-10 demonstrates the need to know the specific gravity of a material when 

determining compaction and density.

Specifications for construction of compacted fills usually require that the density attained in the field 

be equal or greater than a certain percentage of the maximum density attained in the laboratory 

compaction tests (for structural embankment zones, normally 95 percent of the maximum density at 

the optimum moisture content from the standard Proctor test). To help achieve good density control, 

specifications also usually require the fill to be placed at a moisture content near the optimum moisture content (normally in the range from 2 percent below optimum to 3 percent above optimum).

D’Appolonia (1973) reported a measured in-place dry unit weight for uncompacted coarse refuse of 

80 to 110 pounds per cubic foot (pcf), with a median value of 94 pcf based upon data from 200 tests. 

For compaction testing performed at mine sites in northern Appalachia, Hegazy et al. (2004) reported 

a median dry density for compacted coarse refuse of 115 pcf. While these results demonstrate dry 

densities encountered in some specific situations, there can be considerable variation depending 

upon geographic location and mining and coal processing activities.

Because of the degradation of coarse coal refuse due to equipment traffic during placement and weathering after placement, the percentage of fines in “aged” coarse coal refuse will likely be greater than in 

fresh or recently placed coarse coal refuse. Saxena et al. (1984) report that particle breakdown due to a 

combination of weathering and compaction produces better graded materials with higher density and 

strength and lower compressibility and hydraulic conductivity. Hegazy et al. (2004) presented the results 

of sieve analyses performed for fresh coarse coal refuse samples and repeated after compaction to determine the effect of compaction on the fines content (i.e., the percent passing the No. 200 sieve). For the 

tests plotted in Figure 6.36, the average increase of fines due to compaction was approximately 4 percent. 

WATER CONTENT (%)

DRY DENSITY (PCF)

FIGURE 6.35 TYPICAL MOISTURE-DENSITY RELATIONSHIPS

 FOR DYNAMIC COMPACTION

 100 PERCENT SATURATION

MAXIMUM DRY DENSITY

(MODIFIED PROCTOR)

MAXIMUM DRY DENSITY

(STANDARD PROCTOR)

OPTIMUM WATER CONTENT

(MODIFIED PROCTOR)

OPTIMUM WATER CONTENT

(STANDARD PROCTOR)

FIGURE 6.35 TYPICAL MOISTURE-DENSITY RELATIONSHIPS FOR DYNAMIC COMPACTION
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In applications sensitive to fines content, breakdown due to weathering and compaction can be evaluated through slake durability testing of fresh and weathered compacted samples (Section 6.5.9.4).

Specifications for clay core materials designed to restrict seepage through an embankment are 

normally based upon Proctor test results but these specifications often require that the material to 

be placed slightly wet of optimum. This results in lower strengths, but eliminates the potential for 

brittle soil behavior. The resulting core should be sufficiently flexible to allow for small amounts 

of differential movement without the development of cracks that would allow the passage of large 

volumes of water and cause piping. Sherard et al. (1963), Sherard (1973), and Lo (1990) report a 

number of dam failures caused by cracking, and they discuss the related importance of compaction 

specifications and control.

Slurry-placed fine coal refuse typically has a very low in-situ density because of low specific gravity 

and moisture contents near the liquid limit. Dry densities near 50 pcf have been reported. Coarser or 

dryer portions of the fine refuse may have dry densities of 70 pcf or higher.

6.5.3.2 Coarse-Grained Soils and Coal Refuse

Relative density is the dry density of a soil in relation to the minimum and maximum dry densities 

that can be achieved by specific laboratory procedures. The relative density test is applicable to freedraining, cohesionless soils with low fines content (i.e., less than 15 percent non-plastic fines passing 

the No. 200 sieve) that do not have a well-defined moisture-density relationship. The maximum dry 

density is determined in accordance with ASTM 4253, “Standard Test Methods for Maximum Index 

Density and Unit Weight of Soils Using a Vibratory Table,” and the minimum dry density and rela

0 20 30 40 50

0

10

20

30

40

50

PERCENT PASSING NO. 200 SIEVE BEFORE COMPACTION

PERCENT 

PASSING NO. 200 SIEVE

AFTER COMPACTION

(HEGAZY ET AL., 2004)

10

AVERAGE 4 PERCENT

INCREASE IN SAMPLE

PASSING NO. 200 SIEVE 

AFTER COMPACTION

FIGURE 6.36 PARTICLE-SIZE DISTRIBUTION OF COARSE COAL REFUSE AFTER COMPACTION
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tive density are determined in accordance with ASTM 4254, “Standard Test Methods for Minimum 

Index Density and Unit Weight of Soils and Calculation of Relative Density.”

For coal refuse embankments, the relative density test is normally applicable to materials used for 

granular drainage and filter zones that require compaction. Relative density tests are also conducted 

for evaluating the capability of a saturated, coarse-grained soil to resist seismic loadings. Seismic 

issues are discussed in Chapter 7.

The minimum density (zero percent relative density) in accordance with ASTM 4254 is obtained 

by placing dried soil as loosely as possible in a mold of known volume. The preferred method for 

placing soil in the mold requires using a pouring device that limits the height of free fall to no more 

than 1⁄2 inch. The weight of the known volume of soil is then measured and used in the determination of the minimum test dry density. The minimum density is the weight of soil divided by the 

volume of the mold.

The maximum dry density (100 percent relative density) in accordance with ASTM 4253 can be 

obtained using either dry soil (method A) or wet soil (method B). The soil is densified using either an 

electromagnetic, vertically vibrating table or an eccentric or cam-driven, vertically vibrating table. If 

method A is used, dry soil is placed in a mold using a scoop or funnel, a surcharge base plate is placed 

atop the level soil surface, the filled mold with surcharge weight is attached to the vibrating table, and 

the table is vibrated for 8 to 12 minutes depending on the frequency of vibration.

If method B is used, the mold is attached to the vibrating table, the table is turned on, and wet soil 

is placed in the mold over a 5- to 6-minute period during which care is taken to avoid excessive 

vibration that causes the soil to boil excessively. The table is then turned off, a surcharge is placed 

atop the soil, and the table is vibrated for 8 to 12 minutes as for Method A. After the table is turned 

off (both methods), the surcharge is removed and the height of the sample in the mold is measured. 

The material in the mold is then weighed. If the sample is wet, it is oven dried and weighed again 

after drying. The weight of the known volume of dried soil is used to determine the maximum dry 

density. The difference between the wet and dry weights can be used to determine the moisture 

content of the material tested.

The relative density is calculated by the relationship:

Dr = 

γdmax (γd – γdmin) (6-11)

 γd (γdmax – γdmin) x 100

where:

Dr = relative density (dimensionless)

γdmax = maximum dry density (force/length3

)

γdmin = minimum dry density (force/length3

)

γd = measured dry density of the sample (laboratory or in situ) (force/length3

)

The relative density Dr is used as a basis to confirm whether placement of coarse-grained soils in the 

field meets the minimum specified compaction criteria. The minimum acceptable relative density for 

coarse-grained soils typically ranges between about 70 to 85 percent depending upon performance 

requirements.

6.5.4 Hydraulic Conductivity Tests

The hydraulic conductivity (or permeability) of a soil is a measure of the rate at which water will flow 

through a soil under a particular pressure (or head). It is represented by the coefficient of hydraulic 
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conductivity k, which is normally expressed in units of distance per time. Hydraulic conductivity is 

essentially the volume of flow per unit time per unit of cross-sectional area for a unit pressure gradient. While laboratory measurement of hydraulic conductivity can be valuable in developing design 

criteria, field tests are generally more representative of in-situ materials if site conditions and access 

allow performance of the tests.

Hydraulic conductivity is measured in the laboratory by percolating water through a soil sample of 

known cross-sectional area and length. The constant head hydraulic conductivity test is conducted 

for coarse-grained soils in accordance with ASTM D 2434, “Standard Test Method for Permeability of Granular Soils (Constant Head).” This test method is most suitable for granular soils with a 

hydraulic conductivity greater than 10-4 cm/sec that might be used for drains or filter media and 

that do not have more than 10 percent passing a No. 200 sieve. Soils tested using this procedure 

are typically compacted in a permeameter to a density comparable to the relative density used for 

field placement.

For soils that have a hydraulic conductivity less than about 10-4 cm/sec, laboratory hydraulic conductivity tests should be conducted in accordance with ASTM D 5084, “Standard Test Methods for 

Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter.” This standard permits hydraulic conductivity testing by either the constant- or falling-head 

methods and is suitable for soils with hydraulic conductivities not less than about 10-9 cm/sec. Thus, 

the method and equipment are suitable for testing a wide range of soils. The test specimen is sealed 

within a flexible membrane and enclosed within a pressure cell similar to that used for triaxial testing (Section 6.5.7.4). This setup permits back pressuring to saturate the test sample and application 

of high hydraulic pressures (gradients) needed for testing low hydraulic conductivity soils and fine 

coal refuse within a reasonable time frame of several days to a few weeks.

Table 6.8 shows the probable range of hydraulic conductivity for various USCS soil classification 

groups. For coal refuse, hydraulic conductivity data are less well documented. Based upon a very 

limited number of field tests and observations, hydraulic conductivity values for coarse coal refuse 

range from 10-6 to 10-2 cm/sec (about 1.0 to 10,000 ft/year). Hegazy et al. (2004) report the results of 

falling head and rising head slug tests that were performed to estimate the hydraulic conductivity 

of the coarse refuse at coal refuse disposal facilities in western Pennsylvania. The average horizontal hydraulic conductivity kh was 3 x 10-5 cm/sec (about 30 ft/year), and the standard deviation and 

coefficient of variation were 2.7 x 10-5 and 0.9, respectively. Density is inversely related to hydraulic 

conductivity; thus equipment trafficking across the refuse surface and weathering due to exposure 

following placement tend to increase the density and lower the hydraulic conductivity. These environmental and construction processes can result in a vertical hydraulic conductivity on the order of 

10 times less than the horizontal hydraulic conductivity, as discussed in Section 6.6.2.1. For impounding coal refuse disposal facilities, it is important to be aware of the effects of density and grain-size 

distribution on the hydraulic conductivity of the refuse materials.

As with coarse refuse, the hydraulic conductivity of fine coal refuse varies greatly and is difficult 

to predict. Typical values based on the results of piezocone dissipation tests performed at northern 

Appalachian sites are presented in Table 6.36. The range of estimated kh indicates that fine coal 

refuse behaves similarly to very fine sands, silts and mixtures of sand, silt and clay. Some designers 

have reported the presence of moderate plasticity clay with very low vertical hydraulic conductivity at refuse disposal sites. In general, conservative assumptions should be made relative to the 

hydraulic conductivity of coal refuse, with consideration of field test data when available. Generally, conservative values are assumed based upon judgment and hydraulic conductivity values 

from the high end of the range determined from available test data, material classification and 

representative anisotropy values, resulting in either higher (more conservative) phreatic levels or 

seepage rates.
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6.5.5 Geosynthetic Materials Tests

Geosynthetic materials are polymeric sheet materials used with soil, rock, or other geotechnicallyrelated material as an integral part of a civil engineering project, structure, or system. Geotextiles, 

geomembranes and geosynthetic clay liners (GCLs) are types of geosynthetic materials that may be 

used in the design and construction of a coal refuse disposal facility to convey or limit seepage or to 

act as a filter medium.

A geotextile is a permeable geosynthetic made of textile materials. At refuse disposal facilities, geotextiles are used as filters in drainage applications, as well as for material separation applications 

such as beneath spillway linings and haul roads.

Geomembranes are continuous polymeric sheets with very low hydraulic conductivity (typically less 

than 10-12 cm/sec) in contrast to GCLs, which are sheets of very low hydraulic conductivity, composite 

barrier material. The geomembrane polymeric types used for refuse disposal applications include: (1) 

chlorinated polyethylene (CPE), (2) chlorosulfonated polyethylene (CSPE), also called “Hypalon,” (3) 

high-density polyethylene (HDPE) and (4) polyvinyl chloride (PVC). Of these types, PVC and HDPE 

are the most commonly used because they are lowest in cost and widely available.

GCLs consist of dry bentonite clay soil between two geotextiles or on a geomembrane carrier. Geomembranes and GCLs are manufactured in sheets and delivered to the site in rolls. The geotextiles 

used above and below the dry clay may or may not be connected with threads or fibers to increase 

the in-plane strength. Geomembrane rolls are seamed in the field using thermal methods or solvents, 

and GCLs are overlapped in the field to create a continuous barrier. At refuse disposal sites, geomembranes and GCLs are used as hydraulic barriers to limit seepage from impoundments into the 

groundwater and underground mines.

Although not required by MSHA, some state agencies that regulate coal refuse disposal facilities may 

require linings to control seepage. While placement of a low-hydraulic conductivity, compacted soil 

liner is permitted, some sites have insufficient material for constructing such a liner. Therefore, geomembrane and geosynthetic clay liners are more commonly used at these sites.

Leakage, rather than hydraulic conductivity, is the primary design concern for geomembrane-lined 

containment structures. Leakage can occur through poor field seams, poor factory seams, pinholes 

from manufacture, and puncture holes from handling, placement, or in-service activity. Leakage of 

geomembrane liner systems can be minimized by specification of an appropriate liner material and 

implementation of QA/QC procedures during installation.

TABLE 6.36 ESTIMATED HORIZONTAL HYDRAULIC CONDUCTIVITY OF FINE

COAL REFUSE BASED ON PIEZOCONE DISSIPATION TESTS

Test Depth

(m)

t50

(seconds)

ch

(cm2/s)

kh

(cm/s)

PCPT1 7.9 800 15×10-3 1.21×10-5

PCPT1 19.2 30,000 0.4×10-3 0.03×10-5

PCPT1 22.9 40 301×10-3 24.3×10-5

PCPT3 13.3 128 94×10-3 7.59×10-5

PCPT4 21.9 300 40×10-3 3.24×10-5

Note: Pool level was approximately 3 m below the ground surface.

(HEGAZY ET AL., 2004)
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The quality of geosynthetic material installation can be controlled by testing. Tables 6.37, 6.38 and 6.39

identify applicable quality control test methods published by ASTM or the Geosynthetic Research 

Institute (GRI) for geotextiles, geomembranes and GCLs, respectively. Laboratory testing such as the 

gradient ratio test described in ASTM D 5101, “Standard Test Method for Measuring the Soil-Geotextile System Clogging Potential by the Gradient Ratio,” can be employed to check for clogging of 

geotextiles used for filtration. “Geotextile Filter Performance via Long Term Flow (LTF) Tests” (GRI 

Method GT1) should also be considered. In the design of a slurry impoundment, if a geotextile is to 

be used instead of a granular filter in a location where clogging of the geotextile would adversely 

affect the safety of the embankment, testing should be performed with site-specific materials to demonstrate that significant clogging of the geotextile will not occur during the design life of the filter. 

Additionally, as discussed in Section 6.6.2.3.2, sufficient field instrumentation to monitor the phreatic 

level near critical drain and filter installations is recommended.

6.5.6 Consolidation Tests

Applying loads to coal refuse and underlying foundation materials causes compressive strains that 

are either immediate or time-dependent. Immediate strains are usually the result of elastic deformation of solids and compression of voids that are not held open temporarily by trapped pore water. 

Time-dependent strain is referred to as consolidation and is a function of the movement of pore water.

Immediate strain is common to soils with a low degree of saturation and/or a high hydraulic conductivity. This type of deformation is usually not an important part of the design of an embankment 

because the resulting movements are generally complete by the end of embankment construction. 

Situations where immediate strain should be considered include:

• Horizontal and vertical movement of a pipe within the embankment that could cause 

opening of joints, cracking of the pipe material or changes in the slope of drainage 

pipes. Special pipes are available that are flexible or have joints that allow longitudinal movement and slight bending.

• Movement beneath rigid structures, such as concrete spillways, that can be cracked 

or hydraulically affected by differential movement.

The behavior of saturated, fine-grained soils under stress can be very important in disposal facility 

design. Compressive strain in these soils can occur only through drainage of water from the pores 

because, at the stresses experienced during construction, both the soil particles and the pore water are 

essentially incompressible. Therefore, most of the strain occurs slowly, possibly over several months 

or years. This slow compressive strain is termed consolidation. With saturated fine-grained soils, the 

major portion of the total strain is due to consolidation. Until the drainage occurs, the excess porewater pressures can reduce the effective strength of the material and cause instability.

Consolidation can be an important design consideration because, in addition to causing damage to 

pipes and structures, it can affect the gradient of surface drainage structures and the integrity of a cap 

following abandonment. The problems it creates may not become apparent until after the disposal 

facility begins operation, when corrections are most expensive. If an embankment is constructed over 

soft clay deposits or previously settled fine refuse, the movements resulting from consolidation can 

be especially large and the excess pore-water pressures can cause instability. Additional problems 

that can result from consolidation include:

• Settlement of the embankment crest below the design elevation

• Differential settlement that disrupts internal drains

• Cracking of the embankment, particularly in areas where large differential settlements occur over small distances, such as where soft foundation materials abut 

harder soil or rock at the base of a valley wall
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TABLE 6.37 METHODS FOR QUALITY CONTROL TESTING OF GEOTEXTILES

Description Test Method

Standard Test Method for Biological Clogging of Geotextiles or Soil/Geotextile Filters ASTM D 1987

Standard Practice for Sampling of Geosynthetics for Testing ASTM D 4354

Standard Test Methods for Water Permeability of Geotextiles by Permittivity ASTM D 4491

Standard Test Method for Trapezoid Tearing Strength of Geotextiles ASTM D 4533

Standard Test Method for Tensile Properties of Geotextiles by the Wide-Width Strip Method ASTM D 4595

Standard Test Method for Grab Breaking Load and Elongation of Geotextiles ASTM D 4632

Test Method for Determining the (In-plane) Flow Rate per Unit Width and Hydraulic 

Transmissivity of a Geosynthetic Using a Constant Head ASTM D 4716

Standard Test Method for Determining Apparent Opening Size of a Geotextile ASTM D 4751

Standard Test Method for Index Puncture Resistance of Geotextiles, Geomembranes and 

Related Products ASTM D 4833

Standard Test Method for Measuring the Soil-Geotextile System Clogging Potential by the 

Gradient Ratio ASTM D 5101

Standard Test Method for Permittivity of Geotextiles Under Load ASTM D 5493

Standard Test Method for Hydraulic Conductivity Ratio (HCR) Testing of Soil/Geotextile 

Systems ASTM D 5567

Standard Test Method for Biological Clogging of Geotextile of Soil/Geotextile Filters ASTM D 1987

Geotextile Filter Performance via Long Term Flow (LTF) Tests GRI Test Method GT1

Fine Fraction Filtration Using Geotextile Filters GRI Test Method GT8

• Restrictions on the rate of fill placement over previously deposited fines

• Settlement of abandoned facilities that disrupts cap integrity and positive surface 

drainage control

The conventional laboratory test for measuring the consolidation characteristics of a fine-grained 

soil consists of trimming an approximately 1-inch-thick undisturbed soil sample in a metal ring that 

prevents lateral expansion. This test is described in ASTM D 2435, “Standard Test Methods for OneDimensional Consolidation Properties of Soils Using Incremental Loading.” Porous stones above and 

below the sample allow excess pore water to drain from the soil as it is subjected to a series of load 

and unload cycles. Application of a constant vertical load to the sample permits measurement of compression with time. When compression stops, the sample is said to be 100 percent consolidated under 

the applied load. The load is subsequently removed and the sample undergoes a small rebound. The 

load is then increased and held constant until the compression stops again. This procedure is continued for several cycles of loading and unloading, and a relationship is developed between the applied 

load and the compression produced. It is convenient to express the relationship between sample 

compression and load, as shown in Figure 6.37, such that the logarithm of the applied load is plotted 

on the horizontal axis and the compression is plotted on the vertical axis in terms of the void ratio of 

the sample (as the sample is compressed, the void ratio decreases).
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TABLE 6.38 METHODS FOR QUALITY CONTROL TESTING OF GEOMEMBRANES

Description ASTM Test 

Method

Standard Practice for Sampling of Geosynthetics for Testing D 4354

Standard Practice for Determining the Integrity of Field Seams Used in Joining Flexible Polymeric 

Sheet Geomembranes D 4437

Standard Practice for Determining the Integrity of Factory Seams Used in Joining Manufactured 

Flexible Sheet Geomembranes D 4545

Standard Test Method for Index Puncture Resistance of Geotextiles, Geomembranes and Related 

Products D 4833

Standard Test Method for Determining Performance Strength of Geomembranes by the Wide Strip 

Tensile Method D 4885

Standard Test Method for Determining the Coefficient of Soil and Geosynthetic or Geosynthetic and 

Geosynthetic Friction by the Direct Shear Method D 5321

Standard Test Method for the Determination of Pyramid Puncture Resistance of Unprotected and 

Protected Geomembranes D 5494

Standard Test Method for Large Scale Hydrostatic Puncture Testing of Geosynthetics D 5514

Standard Test Method for Multi-Axial Tension Test for Geosynthetics D 5617

Standard Practice for Geomembrane Seam Evaluation by Vacuum Chamber D 5641

Standard Practice for Pressurized Air Channel Evaluation of Dual Seamed Geomembranes D 5820

Standard Test Method for Determining Tearing Strength of Internally Reinforced Geomembranes D 5884

Standard Guide for Selection of Test Methods to Determine Rate of Fluid Permeation Through 

Geomembranes for Specific Applications D 5886

Standard Test Method for Measuring Core Thickness of Textured Geomembrane D 5994

Standard Test Method for Determining the Integrity of Field Seams Used in Joining Geomembranes 

by Chemical Fusion Methods D 6214

Standard Practice for the Nondestructive Testing of Geomembrane Seams using the Spark Test D 6365

Standard Test Method for Determining the Integrity of Non-reinforced Geomembrane Seams 

Produced Using Thermo-Fusion Methods

D 6392

Standard Guide for the Selection of Test Methods for Flexible Polypropylene (FPP) Geomembranes D 6434

Standard Guide for Selection of Techniques for Electrical Detection of Potential Leak Paths in 

Geomembrane D 6747

Standard Practice for Leak Location on Exposed Geomembranes Using the Water Puddle System D 7002

Standard Test Method for Strip Tensile Properties of Reinforced Geomembranes D 7003

Standard Test Method for Grab Tensile Properties of Reinforced Geomembranes D 7004

Standard Practice for Ultrasonic Testing of Geomembranes D 7006
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As shown in Figure 6.37, the steeper slope of the void ratio versus log effective stress plot is defined as 

the compression index Cc. The flatter slope of the void ratio versus log effective stress plot is defined 

as the recompression index Ccr. Typically, Ccr is in the range of 0.1 to 0.2 Cc. The compression index 

is typically used for estimating consolidation settlement of normally consolidated soils and fines. 

Procedures for determining Cc and Ccr are presented in ASTM D 2435. Presenting the test results in 

the form shown in Figure 6.37 simplifies computation of the estimated compression for a given increment of applied load. Most standard texts on soil mechanics include comprehensive discussions of 

this test and procedures for using the test data to predict settlement.

Consolidation testing also provides information regarding the time rate of settlement as a function of 

consolidation stress. The time rate of consolidation settlement is defined by the coefficient of vertical 

consolidation cv and the coefficient of horizontal consolidation ch. The parameter cv can be used for 

estimating vertical pore pressure dissipation with time, which is useful for evaluating the rate of consolidation settlement of fills that are large in areal extent. The parameter ch can be used for estimating horizontal pore pressure dissipation with time, which is important in the design of wick drains. 

Procedures for determining cv and ch are presented in ASTM D 2435. Values for cv and ch determined 

from laboratory consolidation results tend to be conservative (i.e., underpredict the rate of porepressure dissipation) and can vary significantly from in-situ values. More reliable values of cv and ch

can usually be obtained by conducting a dissipation test during piezocone testing (Section 6.4.3.7) or 

by monitoring piezometers.

Description ASTM Test 

Method

Standard Practices for Electrical Methods for Locating Leaks in Geomembranes Covered with 

Water or Earth Materials D 7007

Standard Test Method for Determining the Tensile Shear Strength of Pre-Fabricated Bituminous 

Geomembrane Seams D 7056

Standard Specification for Non-Reinforced Polyvinyl Chloride (PVC) Geomembranes Used in Buried 

Applications D 7176

Standard Specification for Air Channel Evaluation of Polyvinyl Chloride (PVC) Dual Track Seamed 

Geomembranes D 7177

Standard Practice for Leak Location using Geomembranes with an Insulating Layer in Intimate 

Contact with a Conductive Layer via Electrical Capacitance Technique (Conductive Geomembrane 

Spark Test)

D 7240

TABLE 6.38 METHODS FOR QUALITY CONTROL TESTING OF GEOMEMBRANES

(CONTINUED)

TABLE 6.39 METHODS FOR QUALITY CONTROL TESTING OF GEOSYNTHETIC CLAY LINERS

Description ASTM Test 

Method

Standard Guide for Storage and Handling of Geosynthetic Clay Liners D 5888

Standard Test Method for Determining the Internal and Interface Shear Resistance of 

Geosynthetic Clay Liner by the Direct Shear Method D 6243

Standard Guide for Acceptance Testing Requirements for Geosynthetic Clay Liners D 6495

Standard Test Method for Tensile Strength of Geosynthetic Clay Liners D 6768
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Coarse coal refuse is normally placed at a moisture content below saturation and is usually sufficiently 

coarse-grained that consolidation is not a consideration. On the other hand, consolidation of fine coal 

refuse is important, especially in the design of disposal facilities developed using the upstream method 

of construction. Consolidation effects should be considered if the incremental increase in height of an 

embankment constructed on settled fine coal refuse is greater than several feet and the embankment 

supports drainage structures or seepage barriers that could be impacted by differential settlement. Consolidation parameters may also be estimated from the results of triaxial shear strength tests.

Published data for typical ranges of consolidation parameters for coal refuse are limited. Almes and 

Butail (1976) report that Cc for saturated fine coal refuse varies between 0.2 and 0.3 at moisture contents 

ranging between 30 and 45 percent. Hegazy et al. (2004) present values for the horizontal coefficient 

of consolidation ch determined from piezocone testing in fine coal refuse deltas at disposal sites in 

western Pennsylvania. The values of ch range from 15 x 10-3 cm2/sec to 300 x 10-3 cm2/sec. These results 

are typical of coefficient of consolidation values reported for sandy silt to silty clay soils (Bardet, 1997).

6.5.7 Shear Strength and Related Tests

The shear strengths of soil and coal refuse materials used to construct an embankment, or used as the 

embankment foundation, are needed for stability analysis of the embankment. Stability analyses are 

discussed more extensively in Section 6.6.4. Shear strengths are also needed for determination of the 

allowable bearing pressure for structures founded on or within the embankment and for the stability 

of slopes cut during embankment construction.

Embankment and foundation stability may be evaluated using either “total stress” or “effective 

stress” methods. The method selected depends on the:

• Embankment material or materials

• Foundation conditions

FIGURE 6.37 RESULTS OF CONSOLIDATION TEST ON FINE COAL REFUSE

0.1 1.0 10 100

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

VOID RATIO, e

EFFECTIVE CONSOLIDATION STRESS, v (TSF)

ei

eo=em

Cc

1-cycle

Ccr

BORING B-1 DEPTH 7.8'

 Ccr

1+eo

= 0.003

Cc = 0.23

Ccr = 0.015

OCR = 1

INITIAL STATE w = 49.3%

 d = 79.73 PCF

CONSOLIDATION PARAMETERS

 eo = 0.81 vo = 0.38 TSF

 ei = 0.905 vm = 0.38 TSF

 em = 0.81 Cc

= 0.127 1+em

vo = vm = 0.38 TSF

1-cycle

FIGURE 6.37 RESULTS OF CONSOLIDATION TEST ON FINE COAL REFUSE
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• Magnitude of pore-water pressures within the embankment

• State of construction or use for which embankment stability is to be evaluated

Total stress is a combination of the stress between the individual soil grains, termed “effective stress,” 

and the pressure of the pore water, termed “pore pressure.” Because pore water has no shear resistance, all shear resistance is represented by the effective stress. The shear stress at failure (ultimate 

shear strength) on any surface within an embankment is directly related to the stress normal to the 

failure surface, because the failure mechanism involves friction of one body moving on another and 

apparent bonding. This relationship can be expressed as:

τmax = c′ + (σ - u) tan φ′ = c′ + σ′ tan φ′ (6-12)

where:

 τmax = shear stress on surface at failure (force/length2)

c′ = effective cohesion (force/length2)

 φ′ = angle of effective internal friction (degrees)

σ = total stress acting normal to the failure surface (force/length2)

u = pore-water pressure acting on the failure surface (force/length2)

 σ′ = effective stress acting normal to the failure surface (force/length2)

The following paragraphs describe the above two approaches and their application to refuse embankment design.

For shear strength tests conducted for a total stress analysis, water is not allowed to drain from the 

sample during shearing. This method of stability analysis and related types of analyses (Section 6.6.4) 

are generally considered most appropriate for evaluating relatively short-term conditions that would 

occur: (1) during and immediately after construction, (2) immediately following rapid changes in 

the impoundment level, (3) during pushouts, and (4) during seismic loadings. Tests typically used 

to develop strength parameters for a total-stress analysis include the vane shear test, the unconfined 

compression test and the unconsolidated-undrained (UU) triaxial test. For these tests, the strength 

does not increase with increasing normal stress if the soil is saturated. Total stress analysis parameters can also be calculated from the consolidated-undrained (CU) triaxial shear test.

Shear strength tests for an effective-stress analysis either allow water to drain from the samples 

during testing or provide for measurement of the pore pressures under loading and confining 

conditions that are intended to simulate actual field conditions. Effective-strength parameters 

apply to all soil types, including gravels, sands, silts, and clays. This method of stability analysis 

and related types of analyses (Section 6.6.4) are generally considered most appropriate for evaluating long-term conditions after the temporary effects of construction on pore pressures have 

dissipated and seepage rates become steady. The tests typically used to develop effective-stress 

strength parameters include: (1) consolidated-undrained triaxial shear tests with pore-water pressure measurements (CU), (2) consolidated-drained triaxial tests at slow strain rates (CD), or (3) 

drained direct shear tests. For these tests, the strength increases with increasing normal stress. For 

long-term analyses, the drained test strength parameters are the effective cohesion intercept c’ and 

effective friction angle φ’ from the effective stress Mohr-Coulomb envelope. The shear strength 

τmax is given by: 

τmax = c’ + σ’ tan φ’ (6-13)
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For analysis purposes, c’ is often assumed to be zero because laboratory tests are affected by loading 

rate and duration effects. In this situation, the cohesion component of strength can be likened to a 

bond that weathers with time (Mesri and Abdel-Ghaffar, 1993).

Sample preparation for laboratory shear strength tests is an essential aspect of a testing program, 

if the tests are to accurately reflect field conditions. Undisturbed, disturbed and remolded or compacted samples may be tested depending on the soil and material conditions to be modeled in the 

total- or effective-stress analyses. For shear strength tests on proposed embankment construction 

materials, the materials are compacted to the densities and moisture contents that are anticipated to 

occur within the embankment. Tests for the shear strength of coarse-grained foundation soils are performed on samples that are reconstituted in the laboratory to simulate the in-situ conditions. Tests for 

the shear strength of fine-grained foundation soils are performed on undisturbed samples obtained 

from borings or test pits.

Triaxial and direct shear test results are presented either as a series of Mohr’s circles or stress paths 

that reflect sample failure conditions. A Mohr’s circle presentation is typically used for UU triaxial 

and direct shear test results, while a stress path presentation is used for CU and CD triaxial test 

results. An example Mohr’s circle presentation is shown in Figure 6.38, and a stress path presentation 

is provided in Figure 6.39. As shown in Figure 6.38, a Mohr’s circle for each test is drawn on a plot of 

shear stress τ versus normal stress σ by drawing a circle that connects the maximum and minimum 

principal stresses on the sample at failure (i.e., σ1f and σ3f , respectively). Failure is then defined by a 

straight or curved line drawn tangent (or nearly tangent) to the series of circles on the plot. The inter

0 100 200 300 400 500 600 700 800

0

100

200

300

400

500

600

EFFECTIVE NORMAL STRESS, ' (KPA)

SHEAR STRESS, 

 (KPA)

'3f '1f

MOHR-COULOMB PARAMETERS

c' = 0 ' = 34

' tan ' = 0.675

FIGURE 6.38 EFFECTIVE STRESS MOHR’S CIRCLE FOR CU TRIAXIAL TEST 
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cept of the line with σ = 0 is referred to as the total stress cohesion intercept c and the angle of the line 

to the horizontal is the total stress angle of friction φ.

As shown in Figure 6.39, the stress path for each test is constructed on a plot of (σ1’+ σ3’)/2, or p, versus 

(σ1’- σ3’)/2, or q, where values of p and q are plotted for each load increment in each test. Failure is 

defined by a straight or curved line connecting values of (q/p)max for each stress path. The intercept 

of this line with the p axis (a) can be represented as a = c’ cos φ’, where the angle to the horizontal of 

the line connecting values of (q/p)max for each stress path is α and tan α = sin φ’. Then the effective 

angle of friction φ’ = arcsin (tan α) and c’ = a/cos φ’.

Many soils exhibit stress-strain behavior that varies with confinement. This behavior is referred to as 

stress dependency and can be characterized by the stress path method. A stress path is a numerical 

and graphical representation of the past, present and future state of stress on a representative soil 

element because it captures the geologic stress history of the element, the current stresses acting on 

the element, and the anticipated future changes in stress on the element. The stress path of a material 

is determined by plotting the effective strength from CU and CD triaxial tests for each load increment of the tests. Using the stress path method, the test results are then analyzed with respect to the 

approximate field stress and strain conditions before, during, and after construction (Lambe, 1967; 

Lambe and Marr, 1979).

Determining the appropriate strength parameters for evaluating the stability of any embankment, 

regardless of size or location, should be performed by a person experienced in the engineering behavior of soil, rock and refuse materials. The complexity of laboratory shear strength test procedures for 

modeling expected field conditions requires that laboratory shear strength testing be conducted with 

p (PSI)

q (PSI)

0 5 10 15 20 25 30 35 40 45 50 55 60

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

sin ' = tan 

c' = a/cos '

a



NOTE: PLOT IS NOT TO SCALE

q = 2

1 - 3

p =

1 + 3

2

FIGURE 6.39 EFFECTIVE STRESS PATHS AND FAILURE ENVELOPE FOR CU TRIAXIAL TEST
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greater care and more professional scrutiny than routine tests. Shear strength testing must be tailored 

to site conditions by a qualified geotechnical engineer familiar with the type of embankment to be 

constructed and foundation conditions. The tests should be conducted by laboratories with appropriate equipment and skilled technicians.

Standard shear strength test methods are: (1) vane-shear, (2) direct-shear, (3) unconfined-compression, and (4) triaxial-compression. The applicability of these tests to various soil types is presented in 

Table 6.40. Descriptions of these tests are provided in the following sections. Head (1982, 1986) and 

Bardet (1997) discuss other methods for determining shear strength.

6.5.7.1 Vane-Shear Test

The laboratory vane-shear test is used to determine the undrained shear strength su using the test 

method described in ASTM D 4648, “Standard Test Method for Laboratory Miniature Vane Shear 

Test for Saturated Fine-Grained Clayey Soil.” Similar to the field vane-shear test (Section 6.4.3.8), the 

laboratory vane-shear test is conducted on very soft to stiff, fine-grained, undisturbed, remolded or 

reconstituted, cohesive soil by inserting a four-bladed vane into a soil sample and rotating it such 

that shearing occurs along a cylindrical surface. The undrained shear strength is determined from the 

resistance to rotation. The miniature vane is similar to the field vane-shear device, except that it has a 

smaller blade diameter (0.5 inch) and blade height (1 inch). After su is determined, the residual (minimum) shear strength sur is determined by quickly rotating the vane 10 full rotations (to fully remold 

the soil) and then conducting a second shear test. The ratio of peak to remolded undrained shear 

strength is the sensitivity St. The laboratory vane test is typically conducted on a vertically oriented 

sample because that is the direction in which the soil sample is taken in the field. If the sample is 

rotated 90 degrees from the vertical, the laboratory vane test can be used to measure soil anisotropy. 

Laboratory vane shear testing of fine coal refuse is not recommended. Instead, the strength of fine 

coal refuse should be determined in situ using the CPT, PCPT methods or the field vane-shear test, 

as described in Section 6.4.3.8, or by laboratory testing using the direct-shear or triaxial-compression 

test methods described in Sections 6.5.7.2 and 6.5.7.4, respectively.

6.5.7.2 Direct-Shear Test

The direct-shear test is the oldest and simplest form of shear test. Direct-shear tests are used for testing 

reconstituted cohesionless soils and undisturbed cohesive soils. The test method is particularly useful 

if the residual strength at large strain is desired. The test is conducted in accordance with ASTM D 

3080, “Standard Test Method for Direct Shear Test of Soils under Consolidated Drained Conditions.”

As shown schematically in Figure 6.40, the direct-shear test is performed by placing a 1⁄2-inch-minimum-thickness specimen into a cylindrical (2-inch-minimum-diameter) or square-shaped (typically 

3 or 4 inches) shear box that is split along a horizontal plane. The test specimen is confined top and 

bottom by porous stones, and the shear box is placed in a container to permit submergence and 

saturation of the specimen during testing. A vertical (normal) load is applied over the specimen and 

allowed to consolidate. The test is conducted by holding the upper or lower part of the box stationary 

and applying a horizontal load on the other part of the box to shear the specimen along a predefined 

horizontal plane. The shearing load applied at failure divided by the cross-sectional area of the soil 

sample is considered to be the shear stress at failure. The normal load divided by the cross-sectional 

area of the sample is considered to be the normal stress at failure. Direct shear tests of cohesionless 

soils are considered drained tests because of the high sample hydraulic conductivity. Depending on 

the rate of shearing, direct-shear tests of cohesive soils can be either undrained or drained.

After the maximum shear strength has been determined, the residual shear strength (cr’ and σr’) can 

be determined by performing repeated and rapid cycles of shearing (usually a minimum of five full 

forward and reverse cycles) along the plane of failure mobilized during the initial portion of the test. 
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The repeated cycles of loading are intended to simulate large straining in the field that would be typical 

of a slope failure. Once the cycles of repeated shearing are complete, excess pore pressures in the test 

specimen are allowed to dissipate under constant normal load. When the excess pore-water pressures 

equilibrate (i.e., consolidation is complete), the test specimen is sheared as previously described.

As shown in Figure 6.41a, a series of direct-shear tests (typically three minimum) is conducted 

using varying normal stress σ’. Test results are plotted in the form of shear stress τ versus horizontal 

displacement δ. A plot of the peak or failure shear stress τ versus σ’ is used to determine the angle 

of internal friction and cohesion intercept, as shown in Figure 6.41b. For most soils, a line drawn 

through the points for each test is approximately straight. This line is termed the failure envelope. 

The angle that the line makes with the horizontal axis is a measure of the component of strength due 

TABLE 6.40 LABORATORY TESTS FOR DETERMINING SOIL SHEAR STRENGTH

Type of Test

Relative Frequency of Use(1) Preparation 

Prior to 

Applying

Load

Drainage 

Conditions 

During 

Test

Parameters 

Determined Remarks Coarsegrained 

Soils

Finegrained 

Soils

Coarse 

Refuse

Fine 

Refuse

Direct Shear

•	Drained 1 3 1 1

Consolidated 

Under

Normal

Load

Drained Effective 

Stress

Difficult 

to control 

rate of test 

to assure 

drained 

condition

•	Undrained NA 3 NA NA

Consolidated 

Under

Normal

Load

Undrained Approximate

Total Stress

Difficult to 

conduct 

quickly 

enough to 

assure no 

drainage

Triaxial

•	UnconsolidatedUndrained (UU) NA 2 NA 3 Unconsolidated Undrained Approximate 

Total Stress

Also called 

Quick (Q) 

test

•	ConsolidatedUndrained (CU) 1 1 1 1

Consolidated 

Under

Isotropic 

Pressure

Undrained

Total 

Stress and 

Effective 

Stress

Pore 

pressures 

are 

measured 

to give 

effective 

stress 

condition

•	ConsolidatedDrained (CD) 2 1 2 1

Consolidated 

Under

Isotropic 

Pressure

Drained Effective 

Stress

Also called 

Slow (S) 

test

Unconfined NA 2 2 NA Unconsolidated Undrained Approximate 

Total Stress

Sample 

must have 

sufficient 

cohesion 

to maintain 

shape 

without 

support

 Note: 1. 1 = frequently used 3 = applicable, but seldom used

 2 = occasionally used NA = not applicable
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FIGURE 6.40 DIRECT SHEAR TESTING ARRANGEMENT

T



P

H

(MAYNE ET AL., 2002)

PISTON

LOWER FRAME

POROUS STONE

POROUS STONE

UPPER FRAME

to friction between the soil particles and is termed the angle of effective internal friction φ’. When 

the line intercepts the vertical axis at a value greater than zero, the intercept is referred to as the 

effective cohesion c’.

Direct-shear tests are simple and can be performed relatively quickly. However, the test has several 

inherent shortcomings due to the forced plane of shearing:

• The failure plane is predefined and horizontal and may not be the weakest plane in 

the sample.

• There is little control over the drainage of the soil.

• The height cannot be defined for calculating shear strains, so a stress-strain modulus 

cannot be determined from the test.

• Stress conditions on the failure surface are non-uniform and failure develops progressively (the entire strength of the specimen is not mobilized simultaneously at all 

points on the failure surface), so measured strength values are lower than would be 

obtained under uniform stress conditions.

• Stress conditions are known only at failure.

6.5.7.3 Unconfined-Compression Test

The unconfined-compression test is conducted on cohesive soils in accordance with ASTM D 2166, 

“Standard Test Method for Unconfined Compressive Strength of Cohesive Soil.” Cohesive soil specimens are tested to failure by rapidly applying an axial load. Measurements of axial force and axial 

deformation are made during the test, and the test results are presented as a plot of axial stress versus 

axial strain as shown in Figure 6.42. The maximum measured force over the sample cross section qu

is the axial stress, and the peak value of axial stress divided by 2 is the undrained shear strength su. 

For a total stress analysis, the unconfined-compression test provides an approximate measure of the 

short-term, undrained strength of the soil at the density and moisture content of the sample. It provides no information about long-term strength properties appropriate for an effective stress analysis.

FIGURE 6.40 DIRECT SHEAR TESTING ARRANGEMENT
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NOTE: DATA BASED UPON DIRECT SHEAR TEST

 OF TRIASSIC CLAY IN RALEIGH, NC

EFFECTIVE NORMAL STRESS ' (KPA)

SHEAR STRESS, 

 (KPA)

6.41b SHEAR STRESS VS. EFFECTIVE NORMAL STRESS

FIGURE 6.41 RESULTS OF DRAINED DIRECT SHEAR TEST ON CLAY

(MAYNE ET AL., 2002)

DISPLACEMENT,  (MM)

SHEAR STRESS, 

 (KPA)

0 1 2 3 4 5 6 7 8 9 10

0

20

40

60

80

100

120

140

' = 214.5 KPA

' = 135.0 KPA

' = 39.3 KPA

6.41a SHEAR STRESS VS. DISPLACEMENT

0 50 100 150 200 250

0

20

40

60

80

100

120

140

STRENGTH PARAMETERS: c' = 0, ' = 26.0 o

' tan ' = 0.488

FIGURE 6.41 RESULTS OF DRAINED DIRECT SHEAR TEST ON CLAY
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The unconfined-compression test can be performed using undisturbed, remolded or compacted soil 

samples. The stress-strain curves and failure modes observed during testing provide an index value 

of the soil properties in addition to strength. For example, bulging or yielding of the sample signifies 

a relatively soft clay, while a sudden brittle failure indicates a desiccated clay or cemented material. 

The stress-strain curves developed from these tests should be used with caution when determining 

the soil modulus for input to numerical analyses (e.g., finite element analysis) because they are very 

sensitive to minor variations of the modulus.

Test specimens with inclined fissures, sand and silt lenses or slickensides have a tendency to fail prematurely along these weaker planes in unconfined compression tests. If these failure modes occur, 

more sophisticated testing, such as triaxial tests, may be needed to obtain a more realistic determination of the in-situ strength.

6.5.7.4 Triaxial-Compression Test

The triaxial test is used to determine strength and stress-strain behavior of undisturbed, remolded 

or reconstituted soil samples. To conduct a triaxial test, cylindrical samples are consolidated, usually 

isotropically, and then sheared in axial compression. Undrained and drained testing can be conducted, and pore-water pressures can be measured during undrained shear tests. Triaxial tests proFIGURE 6.42 UNCONFINED COMPRESSION TEST RESULTS

AXIAL STRAIN, a (PERCENT)

AXIAL STRESS, 

a (KPA)

0 1 2 3 4 5 6

0

1

2

3

4

5

6

(MAYNE ET AL., 2002)

UNDRAINED SHEAR STRENGTH

cu = su = 51 KPA

qu = 102 KPA = 2cu

FIGURE 6.42 UNCONFINED COMPRESSION TEST RESULTS
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vide a reliable means for determining: (1) the undrained strength of cohesive soils, (2) the angle of 

friction and cohesion intercept of undisturbed, reconstituted and compacted soils, and (3) the soil 

modulus at intermediate to large strains.

Test specimens are typically 1.4 to 2.8 inches in diameter with a height to width ratio between 2 and 

2.5. Selection of the sample diameter is governed by limitation of the maximum particle size in the 

test specimen to not more than one-sixth of the sample diameter. Thus, the maximum particle size for 

a 1.4-inch-diameter sample is about 1⁄4 inch and for a 2.8-inch-diameter specimen about 1⁄2 inch. If the 

soil to be tested has large-size particles, then sufficiently large-diameter specimens should be used 

so that the sample diameter is more than six times the maximum particle size. Otherwise, the tested 

sample should be modeled or scalped using the procedures recommended by Becker et al. (1972) and 

summarized in Duncan and Wright (2005)

Modeling entails creating a modeled particle-size distribution that parallels the original gradation, 

where the maximum particle size does not exceed one-sixth of the diameter of the tested sample. 

Using this approach, Becker et al. (1972) determined that strength test results using the modeled gradation were essentially the same as the strength of the original gradation that had been tested using 

sufficiently large diameter samples to meet the one-sixth criterion provided the test specimens were 

prepared to the same relative density (Section 6.5.3.2).

Scalping entails using that portion of the sample that remains after sieving to remove particle sizes 

that exceed the one-sixth criterion. As with modeling, Becker et al. (1972) determined that strength 

test results using a scalped gradation were essentially the same as the strength of the original gradation provided that the test specimens were prepared to the same relative density. However, if either 

modeling or scalping is used to achieve an acceptable particle-size distribution for testing, additional 

testing to determine the minimum and maximum densities of both the original and modeled or 

scalped materials must be conducted to verify that the relative density of the modeled or scalped 

material is approximately equal to the relative density of the original material.

Of the options for achieving an acceptable particle-size distribution for testing, scalping is probably 

the simpler and less costly approach. Scalping also does not result in an appreciable shifting of the 

fines content, which could affect the strength test results if a substantial portion of over-size material must be removed for testing. While these procedures can be used for coal refuse, they should be 

applied with caution, particularly in cases where the characteristics (e.g., material type, angularity, 

surface roughness) of the smaller particles differ materially from those of the larger particles that 

were removed from the sample.

To conduct the triaxial test, a sample is enclosed by a thin rubber membrane and placed inside a 

cylindrical pressure chamber that is usually filled with water. The sample is subjected to a uniform 

confining pressure σ3 by compression of the fluid in the chamber acting on the membrane. The range 

of σ3 for a triaxial test series is generally selected so that the confining pressures are higher, lower and 

about equal to anticipated in-situ value of σ3 at the end of construction. Using a range of σ3 where all 

confining pressures are less than or equal to the anticipated in-situ value of σ3 can result in overestimation of the strength and compressibility of the tested material, as compared to the in-situ material. 

Depending on the type of triaxial test conducted, a backpressure may be applied to the specimen 

through the end platens to saturate the specimen. The test sample is sheared to failure by applying an 

axial stress, typically referred to as the deviator stress (σ1 - σ3), through a vertical loading ram. Axial 

stress can be applied at a constant deformation rate (strain controlled) or by means of dead weight 

increments or hydraulic pressure (stress controlled) until the sample fails.

Triaxial tests can be used to simulate various in-situ loading conditions. The types of triaxial tests 

typically employed for this purpose include:
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• Unconsolidated-undrained (UU or Q) test

• Consolidated-undrained (CU test)

• Consolidated undrained (CU or R) test with pore-pressure measurement

• Consolidated-drained (CD or S) test

UU (also referred to as quick or Q) triaxial tests are conducted on cohesive soils in accordance with 

ASTM D 2850, “Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on 

Cohesive Soils.” The UU test provides only an approximation of the short-term or undrained strength 

for a total stress analysis and typically is conducted in order to provide data for preliminary analyses and for designing the final test program. The UU test does not provide data about the effective 

stress or long-term properties of the material. In a UU test, the specimen is not allowed to consolidate 

during application of σ3 or to drain during the testing, so the strength measured is the undrained 

shear strength su. The rate of axial deformation during shear is comparable to the rate used for unconfined compression tests. The results of undrained tests depend on the degree of saturation Sr of the 

specimens. If Sr ≈ 100 percent, testing of similar samples will provide similar values of su, because the 

shear strength of the test sample will not increase with increasing confining pressure. However, if Sr

≤ 95 percent, increasing σ3 may result in increasing values of su until the air voids compress and the 

sample becomes completely saturated.

If water is allowed to completely drain from a test sample when σ3 is applied, the sample becomes 

uniformly consolidated. Two types of tests can be performed on consolidated samples. In the consolidated-undrained (CU) test, the consolidated sample is sheared by application of an axial test load 

without allowing any additional water to drain during the loading. Because the sample does not 

drain during loading, the results are suitable only for total stress analyses. The rate of strain used for 

the CU triaxial test is similar to that for the UU triaxial test.

If the pore pressure is measured while the sample is loaded, effective stress parameters can be calculated. This test is called a consolidated undrained with pore-pressure measurement (CU or R) triaxial 

test. The CU triaxial test permits determination of both total stress and effective stress (c’ and φ’) parameters. The rate of strain used for the CU triaxial test is much slower than for the UU or CU triaxial tests 

so that excess pore pressures equilibrate throughout the test specimen and pore-water pressures can be 

reliably measured at the ends of the test specimen. The CU test is performed in accordance with ASTM 

D 4767, “Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive 

Soils.” The rate of strain used for the CU triaxial test is prescribed in ASTM D 4767 and is much slower 

than the rate of strain for UU triaxial tests. This permits equalization of excess pore pressures during 

undrained shearing so that the excess pore-water pressures measured at the end of the test specimen 

are not less than 95 percent of the excess pore-water pressure along the sample shear plane.

Consolidated-drained (CD) triaxial (also referred to as slow or S) tests also yield the effective stress 

parameters c’ and φ’. The primary difference between the CU and CD triaxial tests is that the sample 

is allowed to drain during the CD test. The rate of strain used for CD tests is usually much slower 

than the rate used for CU tests, so that the development of excess pore pressures in the test sample is 

less than a few percent of the initial effective confining pressure σ3’. The CD test measures only the 

effective stress parameters c’ and φ’.

Triaxial test results are typically presented in Mohr’s Circle or p-q plots, as described previously.

Typical values of the shear strength from triaxial testing of fine coal refuse, as reported by Hegazy 

et al. (2004) for sites in northern Appalachia, are summarized in Table 6.41. Drained-shear-strength 

parameters were determined using consolidated isotropic undrained compression (CIUC) triaxial 

tests with pore-pressure measurements and consolidated isotropic drained compression (CIDC) tri-
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axial tests. In this study, fine coal refuse samples from Shelby tubes were collected from beneath the 

upstream stages of a refuse embankment or from working platforms built over the fine coal refuse 

in the impoundment. Table 6.41 indicates that the variability of φ′ is low to moderate, while the variability of c′ is relatively high. The shear strength parameters presented in the table are peak values 

and were found to decrease with increasing fines contents, as shown in Figure 6.43.

Residual shear strength values at large strains determined in accordance with the cited ASTM standards are sometimes considered for the design of impoundments with upstream construction.

The test results in Table 6.41 and Figure 6.43 are presented for illustration purposes only, and strength 

characteristics will vary depending on the geology and coal extraction, processing, and disposal 

practices. 

TABLE 6.41 SUMMARY OF FINE COAL REFUSE SHEAR STRENGTH 

PARAMETERS BASED ON TRIAXIAL TEST RESULTS

Parameter Average Standard Deviation Coefficient of Variation

φ′ 33 degrees 4 0.12

c′ 11 kPa 14 1.30

φ′ (c′ = 0) 35 degrees 4 0.11

(HEGAZY ET AL., 2004)

0 20 40 60 80 100

25

30

35

40

45

' = -0.09(%-#200) + 40.5

r2

 = 0.25

PERCENT FINES PASSING NO. 200 SIEVE

EFFECTIVE ANGLE OF INTERNAL

FRICTION (DEGREES)

 

(HEGAZY ET AL., 2004)

FIGURE 6.43 VARIATION OF STRENGTH OF FINE COAL REFUSE WITH FINES CONTENT
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6.5.8 Seismic Property Characterization

Laboratory testing for determination of the seismic properties of soil or refuse materials generally 

involves the evaluation of potential strength loss associated with earthquake loading. This section 

presents testing methods for: (1) cyclic-triaxial testing, (2) cyclic loading followed by monotonic loading, and (3) resonant-column testing. Chapter 7 presents details of seismic stability and deformation analyses, including the application of the laboratory strain-based approach developed by Castro 

(1994), sometimes referred to as the residual- or steady-state-strength approach. Section 7.4.3.2 presents guidance for laboratory testing and application of the laboratory tests below.

6.5.8.1 Cyclic-Triaxial Test

The cyclic-triaxial test can be used to evaluate the cyclic strength (or liquefaction potential) of primarily cohesionless, free-draining soils in undrained shear. The samples tested are either undisturbed, or 

they are reconstituted to simulate the relative density of the in-situ soil. The test apparatus consists of 

a regular triaxial cell and a cyclic (often sinusoidal) loading machine attached to the loading piston. 

The sample is isotropically consolidated in the triaxial cell and then subjected to a cyclic axial load in 

extension and compression. The cyclic loading generally causes an increase in pore-water pressure 

and a decrease in the effective confining pressure with increasing cyclic deformation of the sample. 

Failure occurs when the excess pore-water pressure equals the initial effective confining pressure 

(sometimes called initial liquefaction) or when some limiting cyclic or permanent strain is mobilized. 

Details regarding the test method are described in ASTM D 5311, “Standard Test Method for Load 

Controlled Cyclic Triaxial Strength of Soil.”

There are limitations to use of cyclic triaxial tests for representing field conditions during earthquake 

loading, including:

• Non-uniform stress conditions imposed on the test sample by the end platens can 

cause a redistribution of the void ratio.

• There would be a continuous reorientation of the principal stresses in the field 

whereas the reorientation angle is either 0 or 90 degrees in the laboratory test.

• The laboratory test sample is isotropically consolidated, whereas the material 

sampled would be in an at-rest lateral earth pressure (Ko) condition in the field (i.e., 

lateral stress = Ko times vertical stress).

• Cyclic shear stress is applied on a horizontal plane in the field but on a 45-degree 

plane in the triaxial test.

• The mean normal stress in the field is constant while the mean normal stress in the 

laboratory varies cyclically.

Despite these limitations, the cyclic triaxial test has been used with reasonable success since the early 

1960s.

6.5.8.2 Cyclic Loading Followed by Monotonic Loading

The purpose of this type of testing is to evaluate the post-earthquake, residual, steady-state, undrained strength of clay-like materials for post-earthquake stability analyses. A limited number of 

loading cycles is applied to a test sample to model the straining induced by earthquake loading. 

Monotonic loading is then applied in order to determine the post-earthquake strength. The initial 

portion of the test consists of a cyclic triaxial test (Section 6.5.8.1) followed monotonic loading using 

the CU triaxial test (Section 6.5.7.4). Selection of the cyclic stress ratio and number of cycles to be 

applied during cyclic loading depends on requirements discussed in Section 7.4.3.2. Typically, there 

is a holding period between the end of cyclic loading and the beginning of monotonic loading to 

permit equilibration of excess pore-water pressures so that measurement of pore-water pressures at 
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the ends of the sample during CU testing are representative of pore-water pressures along the sample 

shear plane. In reality, the time needed to transition between cyclic and CU testing is sufficient to 

permit pore-water pressure equilibration in the sample. Application of this type of testing program 

for seismic design is discussed in Section 7.4.3.2.

6.5.8.3 Resonant-Column Test

Evaluation of the response of foundation and embankment soils to seismic ground amplifications 

requires information regarding shear modulus (Gmax or Go) and damping D. While field geophysical 

methods such as the crosshole, downhole, and surface wave techniques can provide direct in-situ 

measurements of shear wave velocity (Section 6.4.4), the resonant-column test permits an evaluation 

of the variation (decrease) of shear modulus with increasing shear strain γs and the increase of D with 

γs under controlled effective stress states. The test may yield lower values than those obtained from 

field testing due to the effect of soil aging.

The resonant-column test is conducted in accordance with ASTM D 4015, “Standard Test Methods for 

Modulus and Damping of Soils by the Resonant-Column Method.” The undisturbed or reconstituted 

test specimen is sealed in a flexible membrane and enclosed in a pressure cell similar to that used for 

triaxial testing (Section 6.5.7.4). This setup permits the use of back pressure to saturate the test specimen. The resonant-column device excites one end of the test sample in a fundamental mode of vibration by means of torsional or longitudinal motion. Both solid and hollow specimens can be used in 

the apparatus. Either a sinusoidal torque or a vertical compressional load is applied to the top of the 

sample through the top cap. The deformation of the top of the sample is measured, and the excitation 

frequency is adjusted until the sample resonates. The wave velocity or modulus is computed from the 

resonant frequency and the geometric properties of the sample and driving apparatus. Damping is 

determined by switching off the current to the driving coil at resonance and recording the amplitude 

of decay of the vibrations. The decay of the amplitude with time is used to determine the logarithmic 

decrement (the percentage decay over one log cycle of time), which is directly related to the viscous 

damping ratio. Typical test results are presented in Figure 6.44. Figure 6.44a shows the decrease in 

shear modulus with increasing strain amplitude, and Figure 6.44b shows the increase in damping 

with increasing strain amplitude for a clay soil.

The resonant-column test is generally limited to small to intermediate shear strains by the applied 

force requirements and resonant frequencies. At larger strains, hollow samples must be used to maintain a relatively constant shear strain across the sample. For these reasons, resonant column testing is 

primarily used to estimate shear modulus associated with small strains.

6.5.9 Rock Property Tests

Common laboratory tests for engineering properties of intact rocks and index testing of rock fragments include measurements of strength, stiffness, and durability. Table 6.42 presents a summary list 

of ASTM standards and procedures for laboratory rock testing. Additional discussion of the testing 

of coal is presented in Section 8.4.2.2.

6.5.9.1 Point-Load Index Test

Determination of rock strength is typically determined in the laboratory using specially prepared 

rock core and specialized test equipment. Because of the extensive sample preparation and equipment requirements, the point-load test was developed so that rock specimens from drilled core, 

cut blocks or irregular lumps could be tested using portable equipment suitable for the field or 

laboratory. The point-load test is conducted using an apparatus (Figure 6.45) that applies a concentrated load through a pair of spherically truncated, conical platens. The distance between the 

opposing specimen-platen contact points is recorded, and the load is steadily increased until the 

specimen fractures and the failure load is recorded. The point-load test is conducted in accordance with ASTM D 5731, “Standard Test Method for Determination of the Point Load Strength 
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FIGURE 6.44 TYPICAL RESULTS FOR RESONANT COLUMN TEST

ON FINE COAL REFUSE

10-5 10 10 -4 10-3 10-2 10-1

0

0.2

0.4

0.6

0.8

1.2

1.0

NORMALIZED SHEAR MODULUS G/GMAX

SHEAR STRAIN,  (%)

e = 0.80

c = 1 KG/CM2, GMAX = 280 KG/CM2

c = 2 KG/CM2, GMAX = 450 KG/CM2

c = 4 KG/CM2, GMAX = 714 KG/CM2

c = 5.5 KG/CM2, GMAX = 842 KG/CM2

RANGE OF

RESULTS

(ELLISON AND CHO, 1976)

6.44a DECREASE IN SHEAR MODULUS WITH STRAIN

0

2

4

6

8

10

12

e = 0.80

c = 1 KG/CM2

c = 2 KG/CM2

c = 4 KG/CM2

c = 5.5 KG/CM2

SHEAR STRAIN,  (%)

DAMPING, D (%)

10-5 10 10 -4 10-3 10-2 10-1

6.44b INCREASE IN DAMPING WITH STRAIN

FIGURE 6.44 TYPICAL RESULTS FOR RESONANT COLUMN TEST ON FINE COAL REFUSE
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Index of Rock.” The test is used to classify and characterize rock that has a compressive strength 

greater than 2,200 psi.

ASTM recommends that test samples conform to size and shape requirements. In general, for diametral tests, core specimens with a length-to-diameter ratio of 1.0 are adequate, while for axial tests, 

core specimens with length-to-diameter ratio of 0.3 to 1.0 are suitable. Specimens for the block and 

the irregular lump test should have a length of 50 ± 35 mm and a length-to-width ratio between 0.3 

and 1.0 (preferably close to 1.0). Samples are typically tested at their natural moisture content.

Size corrections are applied to obtain the point-load strength index Is(50) of a rock specimen. A strength 

anisotropy index Ia(50) is determined when Is(50) values are measured perpendicular and parallel to 

TABLE 6.42 STANDARDS AND PROCEDURES FOR LABORATORY TESTING OF INTACT ROCK

 Test Category Name of Test ASTM Test 

Method

Point Load 

Strength

Standard Test Method for Determination of the Point Load Strength Index of 

Rock and Application to Rock Strength Classification D 5731(1)

Compressive 

Strength

Standard Test Method for Compressive Strength and Elastic Moduli of Intact 

Rock Core Specimens under Varying States of Stress and Temperatures D 7012(1)

Creep Tests Standard Test Method for Creep of Rock Core Under Constant Stress and 

Temperature D 7070

Tensile Strength

Standard Test Method for Direct Tensile Strength of Intact Rock Core 

Specimens D 2936

Standard Test Method for Splitting Tensile Strength of Intact Rock Core 

Specimens D 3967(1)

Direct Shear Standard Test Method for Performing Laboratory Direct Shear Strength Tests of 

Rock Specimens under Constant Normal Force D 5607(1)

Hydraulic Cond. Standard Test Method for Permeability of Rocks by Flowing Air D 4525

Durability

Standard Test Method for Slake Durability of Shales and Similar Weak Rocks D 4644(1)

Standard Test Method for Testing Rock Slabs to Evaluate Soundness of Riprap 

by Use of Sodium Sulfate or Magnesium Sulfate D 5240

Standard Test Method for Evaluation of Durability of Rock for Erosion Control 

under Freezing and Thawing Conditions D 5312

Standard Test Method for Evaluation of Durability of Rock for Erosion Control 

under Wetting and Drying Conditions D 5313

Deformation

and Stiffness

Standard Test Method for Compressive Strength and Elastic Moduli of Intact 

Rock Core Specimens under Varying States of Stress and Temperatures D 7012

Standard Test Method for Laboratory Determination of Pulse Velocities and 

Ultrasonic Elastic Constants of Rock D 2845

Specimen

Preparation

Standard Practices for Preparing Rock Core as Cylindrical Test Specimens and 

Verifying Conformance to Dimensional and Shape Tolerances D 4543

Standard Practice for Preparation of Rock Slabs for Durability Testing D 5121

Note: 1. The rock test procedure associated with this ASTM standard is described in the Chapter 6 text. Additional

 discussion of the testing of coal is presented in Section 8.4.2.2.

 (ADAPTED FROM MAYNE ET AL., 2002)
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planes of weakness. The test can be performed in the field or in the laboratory. The point-load index 

is used to estimate the unconfined compressive strength qu using a relationship of the form:

qu = K Is(50) (6-14)

The value of the constant K has been reported to vary from 15 to 50 (especially for anisotropic rocks) 

depending upon the specific rock formation. Rusnak and Mark (2000) determined that K ≈ 21 for 

rocks associated with coal seams in the eastern, mid-western and western U.S. Additional discussion 

of the limitations of testing of coal is presented in Section 8.4.2.2.

6.5.9.2 Unconfined Compressive Strength Test

The unconfined compressive strength serves as an initial index of the competency of intact rock and 

represents the most direct method for determining rock strength. The test method is described in 

ASTM D 7012, “Standard Test Method for Compressive Strength and Elastic Moduli of Intact Rock 

Core Specimens under Varying States of Stress and Temperatures.” In this test, cylindrical rock specimens are tested in compression without lateral confinement. The test procedure is similar to the 

unconfined compression test for soils and concrete. The test specimen should be a rock cylinder of 

length-to-width ratio in the range of 2 to 2.5 and should have flat, smooth, and parallel ends cut perpendicular to the cylinder axis in accordance with ASTM D 4543, “Standard Practices for Preparing 

Rock Core Specimens and Determining Dimensional and Shape Tolerances.” The peak stress during 

unconfined loading is the uniaxial compressive strength qu. The results may be affected by: (1) moisture content, (2) rate of loading, (3) condition of both ends of the rock sample, and (4) the presence of 

FLEXIBLE

HYDRAULIC

HOSE

PRESSURE

GAGE

ROCK CORE

BEING TESTED

LOADING

FRAME

CONICAL

ENDS

HYDRAULIC

JACK

FIGURE 6.45 POINT-LOAD TEST APPARATUS

GRADUATED

SCALE

(ADAPTED FROM MAYNE ET AL., 2002)

FIGURE 6.45 POINT-LOAD TEST APPARATUS
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inclined fissures, intrusions, and other anomalies that may cause premature failures on the associated 

planes. These conditions should be noted so that other tests such as triaxial or direct shear tests can be 

performed, as appropriate. Because the tests are of necessity conducted using intact rock samples, the 

test results may not be representative of rock mass behavior due to the effects of in-situ discontinuities 

(e.g., bedding planes and joints), weathering and moisture. For these reasons, unconfined compressive 

strength testing should be limited to rock types and strata where tests on intact rock are reasonably 

representative of the rock mass. This is especially a problem in determining the compressive strength 

of coal through laboratory testing of small-diameter core (e.g., 2 to 3 inches) because the presence of 

defects in the coal results in unconservative strengths as compared to the strengths determined from 

tests on larger-diameter core (e.g., 6 to 12 inches) and in-situ tests on field-scale test volumes (Pariseau, 

2006). Additional discussion of the limitations of testing of coal is presented in Section 8.4.2.2.

The stress-strain behavior of intact rock samples can be measured during an unconfined-compression test in accordance with ASTM D 7012, “Standard Test Method for Compressive Strength and 

Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures.” For 

this test, specimen deformations are measured using strain gauges applied to the test specimen or 

linear voltage displacement transducers attached to the top and bottom load platens.

6.5.9.3 Indirect Tensile Strength Test

Rock is relatively weak in tension; thus, the tensile strength To of intact rock is approximately 5 percent of its compressive value (Mayne et al., 2002). Because of the difficulties involved in proper end 

preparation (Jaeger et al., 2007), the direct tensile strength testing of rock specimens is not a common 

laboratory procedure. Therefore, the tensile strength of rock is usually determined by indirect methods such as the indirect tensile (Brazilian) test. The indirect tensile strength of intact rock core σT is 

determined in accordance with ASTM D 3967, “Standard Test Method for Splitting Tensile Strength 

of Intact Rock Core Specimens.” Core specimens with length-to-diameter ratios between 2 and 2.5 

are placed in a compression loading machine with the load platens placed diametrically across the 

specimen. The maximum load to fracture the specimen is recorded and used to calculate the indirect 

tensile strength.

Alternatives to the indirect tensile strength test are the direct tensile strength test and the bending test. 

The direct tensile strength of intact rock core is determined in accordance with ASTM D 2936, “Standard Test Method for Direct Tensile Strength of Intact Rock Core Specimens.” The core specimens for 

direct tensile testing are prepared as described in ASTM D 4543 for compressive strength testing, but 

the test is conducted by cementing the specimen ends to the test load apparatus and applying tensile 

loads on the sample until it fails in tension. While there is no ASTM standard for bending tests, the 

test specimens need to be long relative to their thickness which makes their preparation difficult and 

expensive, especially when samples are taken from very jointed rock strata (Pariseau, 2006).

Thus, the indirect tensile strength test is significantly more convenient and practical for routine 

measurements than the direct tensile strength test and the bending test, and it provides very similar 

results to those obtained from direct tension tests (Jaeger et al., 2007). In many situations, the indirect tensile test provides a more fundamental measurement of rock material strength than compression tests because the failure mode is more representative of failures that occur (e.g., overstressing 

of roof strata) in underground mines. It should be noted that the point-load index test discussed in 

Section 6.5.9.1 is actually a variation of the indirect tensile strength test with results correlated to 

compressive strength.

6.5.9.4 Rock Durability

Rock used for slope protection and aggregate used for drainage purposes must have high durability 

to attain suitable long-term performance when exposed to construction and in-service environments. 
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Material deterioration mechanisms include cracking, spalling, delaminating or splitting, disaggregating, dissolving and disintegration (USACE, 1990a). The following ASTM test methods can be used to 

evaluate the durability of these types of materials:

• D 5240, “Rock Slab Testing For Riprap Soundness, Using Sodium/Magnesium Sulfate”

• D 5312, “Rock-Durability For Erosion Control Under Freezing/Thawing”

• D 5313, “Rock-Durability For Erosion Control Under Wetting/Drying”

Additional rock durability test methods that have been used for evaluation of the slake durability of 

surface mine spoils and earthen embankments constructed using shale and other slake prone sedimentary rocks are described in Andrews et al. (1980) and Michael and Superfesky (2007). Some of 

these test methods involve use of acid solutions.

Rock materials used for constructing earthen dam and coal refuse embankments are generally 

obtained from borrow sources near the construction area. Because coal is found in geologic settings where shales and other weak rocks are encountered as part of mining operations, the durability of these rock materials as compacted fill needs to be evaluated. The most problematic rock 

types are certain shales, mudstones, claystones and other weak rocks that degrade rapidly soon 

after they are exposed to atmospheric conditions. These materials can degrade rapidly, affecting 

the stability of an embankment fill, a rock cut, or foundation on which a refuse embankment is 

constructed.

The slake durability test was devised in the early 1970s (Franklin and Chandra, 1972) to provide 

a qualitative measure of these materials in service. As described in ASTM D 4644, “Standard Test 

Method for Slake Durability of Shales and Similar Weak Rocks,” representative rock fragments are 

subjected to cycles of wetting and drying, and the weight loss is measured after two cycles. Figure 

6.46 provides an illustration of the slake durability test apparatus.

140 MM 40 MM

20 MM

100 MM

WATER-FILLED TROUGH

DRUM CONTAINING

SAMPLE

(MAYNE ET AL., 2002)

FIGURE 6.46 ROTATING DRUM ASSEMBLY AND SETUP OF SLAKE DURABILITY EQUIPMENT
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Ten dried fragments of rock of known weight are placed in a drum fabricated with 2.0-mm-square 

mesh wire cloth. The drum is rotated in a horizontal position along its longitudinal axis while partially submerged in distilled water to promote wetting of the sample. The specimens and the drum 

are dried at the end of the rotation cycle (10 minutes at 20 rpm) and weighed. After two cycles of 

rotating and drying, the weight loss and the shape and size of the remaining rock fragments are 

recorded, and the slake-durability index (SDI) is calculated. As a qualitative measure of durability, 

Gamble (1971) proposed the classification system presented in Table 6.43 using the SDI determined 

after two cycles. Both the SDI and the description of the shape and size of the remaining particles are 

used to determine the durability of soft rocks. The application of the test for design and construction 

with mine spoil is presented in Section 8.8.2.

Limestone and calcite cemented sandstone can degrade relatively quickly where acidic leachates are 

present and should not be used in the construction of filters, underdrains and internal drains. A fizz 

test of aggregate and rock can be conducted by placing a dilute solution of hydrochloric acid on the 

material. If an effervescent or fizzing reaction occurs, the material is generally considered to be unacceptable. Laboratory testing may also be performed on aggregate materials to evaluate the calciumcarbonate portion in terms of percent of total content. The presence of sulfide minerals in aggregates 

can lead to the formation of sulfuric acid and sulfate minerals and should also be avoided. Generally, 

aggregate materials should contain less than 1 to 5 percent calcium carbonate and much less than 1 

percent sulfide materials in order to be acceptable for drain applications.

6.5.10 Structural Material Tests

This section identifies the quality control tests that should be considered in developing plans and 

specifications for disposal facilities when using portland cement concrete and controlled low-strength 

material (CLSM).

6.5.10.1 Concrete

Portland-cement concrete is used at coal refuse disposal facilities for construction of drainage control 

structures and structure foundations. Structural design of reinforced concrete is beyond the scope of 

this Manual. For additional guidance, the most current version of ACI 318, “Building Code Requirements for Structural Concrete and Commentary,” should be used. Table 6.44 presents ASTM test 

methods for quality control of fresh portland-cement concrete.

6.5.10.2 Controlled Low-Strength Materials (CLSM)

CLSM is used as a replacement for compacted earth fill and consists of various mixtures of pozzolan 

(e.g., fly ash), portland cement, aggregates (typically fine aggregate such as sand and/or bottom ash), 

select cohesionless soil, water, and occasionally chemical admixtures (Howard and Hitch, 1998). A 

low percentage of bentonite or attapulgite can also be included in CLSM for reduced hydraulic conductivity and enhanced plasticity, where such characteristics are important. CLSM has been used for 

general backfills, shallow cutoff trenches, structural fills, insulating and isolating fills, pavement bases, 

erosion control, pipe bedding, cradles and backfill, and void filling. It is the latter two applications 

that have found the greatest application in mining and mine refuse disposal. CLSM is processed and 

mixed much like fresh concrete and delivered as a fluid in ready-mix trucks or as a dry material in a 

dump truck. The quality control test methods used for CLSM are adapted from those used for fresh 

concrete and grout, accounting for the various constituents in the CLSM mix and its lower compressive strength (i.e., 1,200 psi or less). Table 6.45 presents test methods used for quality control of CLSM.

CLSM must be designed in accordance with performance criteria appropriate for the specific application. At coal refuse disposal facilities, CLSM is used primarily for bedding and backfilling of pipe. 

When pipe is used as part of a decant or other structure that extends through the embankment cross 

section, the bedding and backfill should have adequate strength to provide support for the pipe and 
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low hydraulic conductivity and suitable stiffness and shrinkage characteristics to limit the potential 

for seepage along the pipe. The bedding should also perform as intended relative to the desired pipe 

behavior (i.e., flexible versus rigid system). Typical CLSM mix designs have a target maximum compressive strength in the range of 50 to 200 psi for flexible pipe installations and up to 1,200 psi for structural applications. When CLSM is used in flexible pipe installations, a lower strength CLSM is specified 

in order to produce high quality, soil-like bedding, cradle, and backfill zones to preserve the flexible 

behavior of the pipe-backfill system while capitalizing on the benefits of CLSM (i.e., ease of placement, 

better protection of the pipe under construction traffic loadings, improved haunch support and seepage 

control due to more intimate contact with the pipe, resistance to piping and erosion, and better quality control). Testing of the uniaxial strength of CLSM samples for design of flexible pipe installations 

should follow ASTM C109 with the addition of strain measurements for determination of the modulus 

of elasticity in accordance with ASTM D7012. Table 6.46 provides a mix design guide for CLSM. It 

should be noted that the CLSM strength is also dependent on the cement, ash and mixing water characteristics. Application-specific design and testing is recommended for use of CLSM as pipe backfill, with 

strengths evaluated at 7, 14 and 28 days (similar to concrete) and also at 56 and 90 days.

TABLE 6.45 QUALITY CONTROL STANDARDS FOR CLSM

 Description ASTM Test

Method

Standard Test Method for Preparation and Testing of Controlled Low Strength Material (CLSM) 

Test Cylinders D 4832

Standard Practice for Sampling Freshly Mixed Controlled Low-Strength Material D 5971

Standard Test Method for Unit Weight, Yield, Cement Content, and Air Content (Gravimetric) of 

Controlled Low Strength Material (CLSM) D 6023

Standard Test Method for Flow Consistency of Controlled Low Strength Material (CLSM) D 6103

TABLE 6.43 SLAKE DURABILITY CLASSIFICATION

Durability Classification Percent Retained after Two 10-Minute Cycles

Very High > 98

High 95 to 98

Medium High 85 to 95

Medium 60 to 85

Low 30 to 60

Very Low < 30

(GAMBLE, 1971)

TABLE 6.44 QUALITY CONTROL STANDARDS FOR PORTLAND-CEMENT CONCRETE

 Description ASTM Test

Method

Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens C 39

Standard Test Method for Slump of Hydraulic Cement Concrete C 143

Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method C 173

Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method C 231
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6.5.11 Verification of Test Results

Unverified or unvalidated test data or engineering properties (e.g., parameters that are assumed 

from published or other available sources or adopted from other sites without evaluation of their 

appropriateness) should not be used in the design of refuse disposal facilities. The only exceptions 

are analyses for facilities with very simple embankments with low hazard potential and for embankments where very conservative slopes are acceptable. Therefore, it is important to verify test data, 

particularly those for shear strength.

If site access, time and budget were not an issue, the design engineer could obtain as many samples 

as deemed necessary and conduct as many laboratory or in-situ tests as desired to obtain a complete 

assessment of subsurface soil and rock conditions. Engineering properties would be quantified, and 

any inconsistent data would be set aside and additional testing would be initiated, as needed. Unfortunately, site access, schedule and budgets are major factors that designers must consider in making critical decisions throughout the design process to obtain the most reliable and realistic soil, rock and coal 

refuse property data. As described previously, a critical step in determining these properties lies in the 

selection of specific tests and proper interpretation of the test results. For a number of reasons (e.g., cost, 

sampling difficulties or lack or representative values), it may be difficult to determine values for specific 

parameters of interest. Fortunately, designers can sometimes use established and/or site-specific correlations to obtain values for the desired parameters. Correlations are useful for evaluating test results 

that do not appear to be representative, and they can also be very useful in preparing test programs, 

performing preliminary calculations, and serving as a quality assurance check on the test results.

Engineering property correlations come in many forms, but all have a common theme. Specifically, a 

useful correlation is developed from a large database of results based on past experience. In the best 

case, the correlation and experience have been developed or “calibrated” using specific site construction materials, or the correlation may be based on reportedly similar construction materials. The 

reliance upon or use of correlations to obtain soil, rock and coal refuse properties may be justified 

TABLE 6.46 CLSM MIX DESIGN GUIDE

Properties and Criteria Type A Type B Type C Type D

Mix Design(1) (per yd3)

Cement (lb)

Pozzolans (lb)

Bottom ash or coarse aggregate

or fine aggregate (lb)

Air entrainment

100

2000

0

50

300

2600

150 to 200

300

2600

300 to 700

100 to 400

(2)

Slump (in) – ASTM C 143 7 (min.) 7 (min.) 7 (min.) 7 (min.)

Density (lb/ft3) – ASTM D 6023 NA(3) NA NA 30 to 70 or as 

specified(4)

Water absorption of aggregate – 

AASHTO T 85 – – – 20% max.

Compressive strength (psi) 125 (max.) 125 (max.) 800 (max.) 90 to 400

Note: 1. Quantities may vary in order to adapt mix to density and strength requirements or to adapt to site

 conditions and material characteristics.

2. Requires use of suitable light-weight aggregate or air-entraining admixture.

3. Not applicable.

4. Approximate value; use of air-entraining agent may reduce this value.

(ADAPTED FROM PennDOT, 2007)
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in the following cases: (1) specific data are simply not available and the only possibility is indirect 

comparison to other properties, (2) a limited amount of data for the specific property of interest are 

available and the correlation will complement these limited data, or (3) the validity of certain data is 

in question and a comparison to previous test results allows the accuracy of the data to be evaluated. 

It is strongly emphasized that correlations should never be used as a substitute for an adequate site 

exploration program, but rather should complement and verify available test data. Examples of each 

of the three cases listed above are provided in the following:

• Specific data are unavailable – Several examples of this type exist. Most notable is the 

strength of uncemented clean sands. Undisturbed sampling may be problematic and 

prohibitively expensive, and correlations to SPT, CPT, and other in-situ tests results 

have been shown to be quite reliable.

• Limited data are available – If few high-quality consolidation tests are performed 

and compression properties are found to correlate well with the results of Atterberg 

limits tests, it may be concluded that Atterberg limits tests can be used for assessment of compression properties.

• Data validation – If results from tests on two similar materials are inconsistent, comparing the results to previous data for similar soils may allow determination as to 

whether the data are simply inconsistent or if some of the data are inaccurate.

There are several sources of correlation data for geotechnical materials and properties. Many geotechnical textbooks and references provide correlations, including Holtz and Kovacs (1981), DOD 

(2005) and Carter and Bentley (1991). The Electric Power Research Institute (EPRI) commissioned 

the preparation of useful documents that include several correlations for laboratory and in-situ tests 

(Kulhawy and Mayne, 1970). For information regarding the engineering properties of coal refuse 

materials, designers should refer to Hegazy et al. (2004), Chen (1979), DiGioia and Gray (1979), and 

Coates and Yu (1977).

The following comments on applicability and use of correlations are noted:

• A correlation is only as good as the data upon which it is based.

• There may be some scatter in the correlation data. The effects of data scatter should 

be accounted for by using upper and lower bound (i.e., best case/worst case) scenarios and factors of safety in the design calculations.

• A correlation will be most accurate if it is calibrated to site soil/material conditions.

• If calibration to site conditions and design-phase laboratory test data cannot be conducted in sufficient detail, consideration should be given to an expanded program of 

field inspection, performance monitoring and maintenance (Chapter 12) and instrumentation (Chapter 13).

6.6 GEOTECHNICAL ANALYSES

Sections 6.1 through 6.5 have addressed planning and design considerations, site exploration, and 

material property characterization required for development of a functional, safe and environmentally acceptable coal refuse disposal facility. This section discusses geotechnical analyses associated 

with refuse disposal facility design. Analytical procedures for seepage, settlement, slope stability, 

rock excavation, and conduit design are presented. These discussions are necessarily limited in both 

scope and depth. More extensive treatments of these subjects are available in references cited herein.

6.6.1 Analytical and Numerical Methods in Geotechnical Engineering

Many of the important areas of geotechnical engineering analysis are governed by differential 

equations for solving problems of elastic equilibrium, consolidation and steady-state seepage. 
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Limit analysis using classical techniques such as Rankine’s earth pressure theory, Terzaghi’s bearing capacity equation, or various methods of slices is another important approach for the solution 

of geotechnical problems involving the failure of soil masses for slope stability analysis. These 

“traditional” methods of analysis, as applied to the design of coal refuse disposal facilities, are 

discussed in the following text. Seismic analysis and design issues for refuse disposal facilities are 

addressed in Chapter 7.

The availability of powerful personal computers, augmented by improved tools for problem setup, 

constitutive modeling of material properties, and post-processing of computational results, has facilitated the use of numerical methods to solve complex geotechnical problems. Sophisticated methods, 

such as the finite element (FE) method, are available to perform complex analyses related to seepage, 

deformation, and slope stability. The FE method is a numerical method for obtaining solutions to differential equations, given appropriate boundary condition data. Some of the most useful features of 

the FE method are that it can:

• Simulate one-, two- and three-dimensional problems

• Accommodate complex geometries and construction sequencing

• Model soil property variability (e.g., nonlinear stress-strain behavior and anisotropic 

hydraulic conductivity)

Nearly all areas of geotechnical analysis can be solved using the FE method. However, the engineer 

must recognize the inherent uncertainty and variability in material properties. The possible effects of 

simplifying assumptions for material behavior in FE models must be carefully evaluated. An understanding of empirical relationships and the ability to apply the lessons of experience are key factors 

in successfully using FE models and interpreting the results obtained.

6.6.2 Seepage Analysis

6.6.2.1 Basic Assumptions, Conditions Requiring Seepage Analysis

All earth and coal refuse structures are to some degree pervious to water and therefore susceptible to seepage. Seepage is a concern in earthen dams and coal refuse embankments for three 

reasons: (1) the pore-water pressures in an embankment and its foundation affect the stability of 

the embankment, (2) an excessive hydraulic gradient on an embankment slope, at drain or fine/

coarse material interfaces, or at the toe can lead to piping, internal erosion, and destructive uplift 

pressures, and (3) water lost through or under a structure may require treatment or may be a valuable water source worth recirculating through the preparation plant. Seepage control systems are 

incorporated into embankments to minimize the potential for excessive or uncontrolled seepage. 

Practical concepts for seepage control in coal refuse embankments and their foundations are discussed in Section 6.3. This section discusses analytical methods for assessing the need for seepage 

control and for selecting materials and design dimensions to achieve that control. Additionally, 

operational controls to mitigate seepage, such as the deposition of slurry within impoundments, 

are also discussed.

Under steady-state conditions, seepage through an earth or coal refuse embankment and its foundation can be estimated from the relationship for flow through porous media, which is based on the 

following assumptions:

• The flow occurs through incompressible media.

• The flow is caused by gravity forces only.

• The media through which flow occurs is always saturated.

• The boundary conditions of the flow are known.
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For unsaturated flow and transient seepage analyses, additional assumptions may be required.

Whether or not the boundary conditions are actually known depends on the specific flow condition 

being analyzed. For flow through a pervious foundation under a relatively impervious embankment, 

the boundary conditions are readily apparent. However, not all boundary conditions are easily determined, and many boundary conditions must be approximated or determined by iterative analysis 

procedures (e.g., the seepage and phreatic condition in an embankment with internal drains).

Steady-state flow through porous media such as soils or coal refuse is normally laminar and therefore 

follows the Darcy equation:

Q = k i a (6-15)

where:

Q = flow rate (volume/time)

k = coefficient of hydraulic conductivity (length/time)

i = hydraulic gradient (dimensionless)

a = cross-sectional area through which flow occurs (area)

Selection of the coefficient of hydraulic conductivity should be based on field and laboratory testing, 

although evaluation of other factors may also be required, including:

• Published information (e.g., soil surveys) along with the results of site-specific index 

and classification tests for natural soils and coal refuse.

• Potential variability of existing fill or mine spoil, especially when there are no available data related to in-situ material or placement.

• Potential for anisotropic conditions (if not documented based upon site-specific 

data), particularly for coal refuse embankments where the horizontal hydraulic 

conductivity may be an order of magnitude greater than the vertical hydraulic 

conductivity.

• Sensitivity of flow rate and hydraulic gradient (and phreatic surface) to variation of 

assumed parameters.

• Observed site groundwater levels in natural soils and existing embankments (to 

assist in validating assumed values, particularly if multiple zones or anisotropic 

conditions are present).

Seepage analysis for coal refuse embankments should be based on conservative estimates of hydraulic conductivity, with values selected based on available test data (preferably field tests), material 

classifications and anisotropy ratios resulting in conservative (higher) phreatic levels and seepage 

rates. The ratio of horizontal to vertical hydraulic conductivity (anisotropy ratio) can vary due to 

the material type, placement and compaction in lifts, and for embankment dams has been reported 

to range from 1 to 100 (Fell et al., 2005). The U.S. Army Corps of Engineers (USACE, 1993) suggests 

an anisotropy ratio between 2 and 10 or greater for embankment dams constructed in compacted 

lifts, and MSHA (2007) recommends a ratio of at least 9 for coarse coal refuse or as based upon sitespecific conditions and materials. For an existing embankment, back calculation of monitored pool 

and piezometer levels to estimate anisotropy is recommended.

If fine coal refuse deposits are accounted for in the seepage analysis, an anisotropy ratio of between 

1 and 10 is usually applied. In typical situations where coarse coal refuse is more permeable than 
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the fine refuse, the anisotropy ratio of the fine refuse deposit may have little effect on the computed 

phreatic levels and seepage rates in downstream coarse refuse embankments. It is likely that the 

anisotropy ratio of hydraulically-placed fine refuse is quite variable (beyond the range cited above) 

such that design of seepage control and collection measures critical for stability of the facility should 

be effective over the likely range of anisotropy.

For impounding coal refuse embankments, seepage analyses should be performed for critical intermediate stages along with the final stage of development. The pool level used in the seepage analysis 

should be the maximum normal pool level for the stage (usually based on the decant invert level) or, 

if there is potential for saturation of the embankment due to retention of the design storm, the maximum design storm pool level.

Unsteady-state seepage analysis can be employed for evaluation of the extent of saturation of an 

embankment or to assess the impact of rapid drawdown on the upstream embankment slope. Also, 

unsaturated flow analysis is sometimes coupled with saturated flow analysis (generally using numerical models) to evaluate internal drainage systems, particularly where there is significant anisotropy.

Turbulent flow regimes may occur in rockfill and rock drains used in embankments. Based on earlier work by Cedergren (1989), the U.S. Army Corps of Engineers (1993) provides guidance for estimating the reduction in hydraulic conductivity of narrow size range aggregate caused by turbulent 

flow at large hydraulic gradients in underdrains (Figure 6.47). As indicated in Cedergren (1989), 

the reduction in hydraulic conductivity is of relatively little importance for flat-lying underdrains 

if hydraulic gradients are less than 0.02. Furthermore, the effect of increasing the hydraulic gradient 100 times (from 0.01 to 1.0) reduces the hydraulic conductivity to one-tenth of the laminar 

value. Alternatively, Leps (1973) developed the following empirical relationship for turbulent flow 

through rock which is sometimes applied:

Q = a W m0.5 i 0.54 [e / (1 + e)] (6-16)

where:

Q = flow rate (length3/time)

a = flow cross section area (length2)

 W = empirical constant for rockfill material, dependent on the shape and roughness 

of rock particles and the viscosity of water (length-time units) (Wilkins, 1956)

m = hydraulic mean radius (length)

i = hydraulic gradient (dimensionless)

e = void ratio (dimensionless)

Leps (1973) provides the following guidance on the determination of Wm0.5 based on rock size:

 Rock size (in) 3/4 2 6 8 24 48

 W m0.5 (in/sec) 10 16 28 32 58 84

While Equation 6-16 applies to uniformly-sized rock, Leps suggests that it can be adapted for graded 

materials by using the 50-percent rock size to compute Wm0.5, provided that the minus 1-inch material 

is less than 30 percent, and preferably less than 10 percent by weight, of the rockfill. If the percentage 

of minus 1-inch material is greater than 30, Equation 6-16 may not be applicable.
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6.6.2.2 Seepage Analysis Methods

There are several approaches for conducting seepage analyses. Prior to the widespread availability of computers, graphical hand-solutions (i.e., hand-drawn flow nets) and hand calculations were 

commonly used for modeling embankment and foundation seepage. However, the use of numerical methods (primarily finite element modeling) has become the most common method for seepage 

analyses. These approaches are discussed in the following sections.

Seepage in homogeneous coal refuse embankments can be readily analyzed using graphical and 

analytical methods, provided that the hydraulic conductivity and any anisotropy are known. Zoned 

embankments, variable foundation conditions, and the presence of the settled fine coal refuse introduce additional complexity, and numerical methods are better suited for detailed analysis of these 

situations. Coal refuse embankments are developed in stages, each with a specific configuration and 

internal drainage system designed on the basis of the projected fine coal refuse level and pool level. 

Thus, in addition to the final impounding stage, intermediate stages will likely need to be analyzed as 

part of the design of internal drainage structures and to establish the stage configuration in conjunction with the stability analysis.

With either the graphical, analytical or numerical modeling approach, it is essential that the 

boundary conditions for the analysis be accurately defined. These boundary conditions typically 

include:

• Appropriate reservoir/impoundment pool level

• Foundation conditions, including possible artesian conditions

• Location(s) and capacity(s) of internal drains

• Geometry of intermediate and post-construction cross sections

• Maximum and minimum potential inflow rate(s) to the reservoir/impoundment

0.025 EFFECTIVE SIZE (IN)

0.3

0.5

1.0

0.10

LIMITING 5.0

ENVELOPE

COMPLETE

TURBULENCE

FIGURE 6.47 APPROXIMATION FOR ESTIMATED REDUCTION IN HYDRAULIC 

 CONDUCTIVITY UNDER TURBULENT FLOW

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

HYDRAULIC GRADIENT, i

RELATIVE HYDRAULIC CONDUCTIVITY, k

(CEDERGREN, 1989)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

FIGURE 6.47 APPROXIMATION FOR ESTIMATED REDUCTION IN HYDRAULIC 

CONDUCTIVITY UNDER TRUBULENT FLOW
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Selection of the appropriate elevations for water and fine coal refuse in an impoundment are important if meaningful results are to be realized. Typically, normal pool or the maximum decant inlet 

level for a specific stage is selected. If the reservoir pool is above the settled fine coal refuse level and 

against the upstream slope of the impoundment, the effect on the phreatic surface elevation of the 

fine coal refuse will be limited, even if it has low hydraulic conductivity relative to the coarse refuse. 

If the impoundment is designed to retain a design storm that would cause saturation of the embankment due to an elevated reservoir level, then seepage associated with that reservoir level should be 

analyzed. Cedergren (1989) provides a method for estimating the approximate time for embankment 

saturation to occur for simple embankment geometry and homogeneous hydraulic conductivity. 

Unsteady seepage analyses can be performed for complicated geometries and zoned embankments, 

although it is recommended that the designer exercise caution and consider redundant seepage control measures in critical situations.

Site conditions may necessitate the evaluation of other features and boundary conditions. As one 

of the primary purposes of seepage analyses is to determine seepage conditions for use in stability 

analyses, the locations of the seepage cross sections should be consistent with the locations of stability sections (for 2D analyses). Also, the size, lateral extent and continuity of drainage features should 

be evaluated when deciding whether and how to incorporate them into the analysis. For example, a 

blanket drain that is limited in lateral extent to the center of a valley may not significantly affect the 

phreatic level at the abutments of an embankment. In such a case, it may be necessary to also analyze 

the influence of a higher saturation level in cross sections close to the abutments.

6.6.2.2.1 Graphical (Flow Net) Approach

A flow net is a set of orthogonal lines graphically representing the seepage conditions in an embankment. The seepage at any cross section of an earth or coal refuse embankment can be determined by 

constructing a flow net using free-hand trial-and-error sketching. One group of flow net lines represents paths of flowing water, and the other group represents lines of equal head (equipotential lines). 

The flow net must satisfy the flow boundary conditions. The flow lines and equipotential lines must 

intersect at right angles, and for each element the mean dimension parallel to the flow lines must 

equal the mean dimension parallel to the equipotential lines (or remain in the same proportion over 

the extent of the flow net). These dimensional requirements can be checked by drawing a circle in 

elements and determining if it is tangent to the mid-sides of the adjacent flow and equipotential lines. 

Figure 6.48 shows a typical flow net analysis.

To estimate the rate of seepage flow from a two-dimensional flow net of unit thickness, the Darcy 

equation can be written in the following form:

q = k h (Nf /Nd) (6-17)

where:

q = flow rate per unit width (length2/time)

k = coefficient of hydraulic conductivity (length/time)

h = total head loss across the system (length)

Nf = number of flowpaths through the system (dimensionless)

Nd = number of potential drop (divisions of head loss) across the system (dimensionless)

Examples of the application of this equation are provided in numerous references, including Cedegren (1989), Harr (1962) and USACE (1993).
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An example of the flow net graphical approach is illustrated in Figure 6.49a for a homogeneous dam 

section. Figure 6.49b illustrates the graphical transformations necessary to develop flow nets for a 

homogeneous dam embankment with anisotropic hydraulic conductivity. When a dam is composed 

of a number of different soils with varying anisotropic hydraulic conductivity, the graphical approach 

becomes considerably more complicated. In practice, the use of graphical methods may require that a 

number of simplifying assumptions be made to render the problem manageable. The construction of 

flow nets and their application are described in USDA (1973a) and Reddi (2003).

6.6.2.2.2 Analytical Solutions

The seepage and associated phreatic surface for an embankment dam with an underdrain resting on 

an impervious base, generally referred to as Kozeny’s basic parabola, can be determined analytically 

(Harr, 1962). The phreatic surface and minimum width of drain a0 (Figure 6.48) can be estimated 

from the following equations:

x = (– k y2/2q) + (q/2k) y0 = (d2 – h2) 0.5 – d (6-18)

q = k [(d2 + h2) 0.5 – d] (6-19)

where:

k = coefficient of hydraulic conductivity (length/time)

q = seepage rate per unit length of drain (length2/time)

Also, the d, h, x, and y dimensions (length) are as shown on Figure 6.48 and a0 = y0/2.

Harr (1962) provides analytical solutions for embankments with foundations at great depth (as well 

as finite depth) and with variable upstream slopes. These procedures allow for treatment of anisotropy using transformed sections (simple expansion or contraction of spatial coordinates to convert an 

anisotropic zone to an isotropic zone).

FIGURE 648 FLOW NET ANALYSIS

F

Y

C

X D

Z

y0

E B A

 

a0

IMPERMEABLE FOUNDATION = 0 = 0

= q

d

 = -kh

h

(HARR, 1962)

FIGURE 6.48 FLOW NET ANALYSIS
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Kashef (1986) and Mishra and Singh (2005) point out simplifying assumptions for Kozeny’s basic 

parabola and their impact on seepage rate and drain length. The results are compared with numerical 

methods for zoned embankments.

Coal refuse embankments frequently employ limited width and height internal drains positioned 

above the base of the dam to intercept seepage and control the phreatic surface. Van Zyl and Robertson (1980) have analyzed a drain of limited width on an impervious base. They estimate the 

effective width of the drain as approximately 0.1h for the condition where d/h > 2, provided the 

drain filter has sufficient capacity. They also recommend that the drain width be sized 50 percent 

larger than the width determined based on the capacity of the filter, with a minimum dimension 

of 2 meters.

Analytical solutions provide a means to estimate seepage and phreatic levels for some specific boundary conditions. They can be useful for initial development of flow nets and for interpreting results 

obtained with numerical modeling, as subsequently discussed.

6.6.2.2.3 Numerical Modeling Approach

As noted previously, the advent of powerful computers has facilitated the use of numerical modeling for evaluating seepage. There are a number of finite element programs that can be used to model 

seepage for two- and three-dimensional, anisotropic, unconfined and confined flow. The use of such 

computer programs has several advantages over the use of graphical methods to analyze seepage. 

Frequently, computer programs support the use of hydraulic conductivity functions (relationships 

between hydraulic conductivity and pore-water pressure) and thus allow the inclusion of unsaturated flow into the seepage model. Computer programs can more easily accommodate anisotropic 

FIGURE 6.49 FLOW NET ANALYSIS FOR ANISOTROPIC CONDITIONS 

PHREATIC SURFACE FOR kh = 16kv

(TRANSPOSED BACK FROM BELOW)

PHREATIC SURFACE FOR kh = kv

NOTE: FLOW NET IS FOR ISOTROPIC

 CASE (kh = kv)

k ~ 0 DRAIN

h

6.49a ACTUAL CROSS SECTION

(USDA, 1973a)

6.49b TRANSFORMED CROSS SECTION

DRAIN

h

k ~ 0

NOTE: HORIZONTAL TRANSFORMATION FACTOR kv

kn

= 1 = 16 = 0.25

WHERE: kv = VERTICAL HYDRAULIC CONDUCTIVITY

kh = HORIZONTAL HYDRAULIC CONDUCTIVITY

FIGURE 6.49 FLOW NET ANALYSIS FOR ANISOTROPIC CONDITIONS
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hydraulic conductivities and complicated soil/rock geometries, including varying anisotropy ratios. 

Transient analyses are also possible using numerical methods if water content data for each material 

in the model are available.

Computer programs utilizing numerical methods frequently provide several alternatives for modeling boundary conditions, including the ability to model internal drainage systems of limited width 

and height located above the base of the embankment. However, not all of these alternatives will 

result in a realistic model. Therefore considerable judgment must be used if realistic results are 

to be achieved. As noted previously, accurate geometry and realistic representation of boundary 

conditions are vitally important when setting up a seepage model. The resulting model should be 

checked using a more simple approach (e.g., flow nets) to determine whether the resulting flow 

paths and related directional orientation of velocity vectors, predicted total head, and predicted 

pressure heads are realistic.

The application of numerical methods for seepage analyses at fine coal refuse impoundments has 

been presented in publications by Snow et al. (2000) and Thacker (2000). These studies present 

models for steady-state flow conditions with anisotropic hydraulic conductivity for various material property zones, and they use specified heads in the embankment or as boundary conditions 

to simulate impoundment levels or drains. For numerical models of this type, flow vectors and 

hydraulic gradients should be checked to verify that values are reasonable and that mass balance 

is maintained.

6.6.2.3 Seepage Control Measures

6.6.2.3.1 Granular Drain and Filter Requirements

Drains and filters made of aggregate materials are used within and under coal refuse embankments 

to collect seepage and to lower pore-water pressures in parts of embankment where internal erosion or unstable conditions might otherwise develop. The use of drains and filters in coal refuse 

embankments is also discussed in Section 6.3. Technical requirements for drains are discussed in the 

following pages.

For a drainage system to be effective, the following criteria must be satisfied:

• The hydraulic conductivity must increase in the direction of flow.

• The particles in the system must be stable against the seepage forces.

• Material segregation during construction must be prevented so that 

grain-size characteristics are uniform throughout the drain.

• The materials must not be susceptible to clogging over time.

• The materials must be durable and not decompose over time.

Filter criteria must be satisfied for transitions between a drain and adjacent portions of an embankment and between zones of finer and coarser materials within the embankment. As water percolates through soil or refuse materials, seepage forces in the direction of flow are exerted on the 

soil/material particles. Where water flows from a fine soil into a coarse soil, it may transport fine 

particles into the voids in the coarser material. This erosion of the fine material can lead to piping, 

a phenomenon where a path of much higher hydraulic conductivity that could ultimately cause 

a failure is developed within an embankment. When the difference in grain-size distribution for 

adjacent embankment zones is too great to prevent piping, a filter or layer of material of intermediate particle size needs to be placed between the two zones. When adjacent embankment and drainage system zones differ greatly in particle size, two or more filter zones with graduated particle 

sizes may be needed.
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The U.S. Army Corps of Engineers (USACE, 1993) guidance for selecting materials for successive 

layers of a filter and drainage system, stated as rules, is provided in the following:

To prevent particles of a fine soil from washing into an adjacent coarser soil:

Rule 1: The criteria presented in Table 6.47 should be followed.

Rule 2: Ideally, grain-size curves for the base and filter soils should be approximately 

parallel. For gap-graded base materials (sometimes the case with coarse coal 

refuse), filter criteria that exclude the coarser portion of the base soil should be 

applied. Also adjustments can be made to exclude particles larger than the No. 

4 (4.75-mm) sieve. The filter should be designed to protect the fine matrix. For 

gap-graded and broadly-graded materials, considerable care and judgment 

must be employed in identifying the grain-size portion of the base soil to be 

filter-protected so that sufficient capacity to transmit seepage flow from the 

base soil is provided (USBR, 2007a). Alternatively, tests can be conducted in 

the laboratory to select the appropriate filter (Sherard and Dunnigan, 1985).

For hydraulic conductivity consistent with adequate discharge capacity:

Rule 3:

D15 filter soil ≥ 3 to 5 (6-20)

 D15 base soil

Rule 4: The portion passing the No. 200 sieve should be less than or equal to 5 percent,

and the fines should be cohesionless.

To prevent segregation during placement:

Rule 5: The restrictions on filter grain size presented in Table 6.48 should be followed.

Rule 6: Dmax ≤ 3 inches.

The USACE (1993) provides guidance for filters within gap-graded and broadly-graded soils. The 

latter condition may be particularly relevant because the grain-size distribution associated with 

coarse coal refuse embankments is frequently either gap-graded or broadly-graded. Additionally, 

the above guidelines (Rule 6) may require adjustment in some project-specific situations where large 

mine rock overburden is used in downstream zones.

Where a perforated drainpipe is the final element of a drainage system, a well-graded gravel that 

will not wash through the perforations or slots should be placed around the pipe. The D85 size of 

the gravel should be greater than the width of the perforations or slots. Experience has shown that 

drainpipes placed with perforations directed downward are less likely to clog. Geotextiles should 

not be placed directly against a perforated or slotted pipe due to the potential for clogging. Instead, a 

zone of sand or aggregate should be placed around the pipe and then the filter (geotextile or another 

granular layer) should be placed around the aggregate (France, 2004)

The discharge capacity of all portions of a drainage system must be checked for adequacy, particularly for thin horizontal drains. This can be accomplished by very simple approximate calculations 

using the Darcy equation presented earlier in this section. Cedergren (1989) contains several useful 

examples of this calculation.

The NRCS (1994) and the USBR (2007a) also provide guidance on filter criteria, including the use of 

perforated pipes in drains. These criteria may be applied to coal refuse disposal facilities. McCook 
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(2006) presents an overview of design and construction issues for pipes in drain systems, comparing 

criteria published by the USACE, NRCS and USBR and methods for estimating inflow capacity. Slotted pipe generally is less prone to clogging and has a much larger inflow capacity than circular perforated pipe. However, perforated pipe probably has higher strength than slotted or screened pipe, 

especially when the perforations are relatively small in diameter and widely spaced. Care should be 

exercised in selecting pipe materials, recognizing that: (1) slotted single-wall corrugated tubing has 

been vulnerable to crushing in some drain installations and (2) single-wall corrugated pipe is seldom 

used in the design of significant- and high-hazard-potential dams. Pipes in drains should generally 

be designed to withstand the maximum height of backfill over the pipe, and they should be protected 

against damage during construction. Care should be exercised in evaluating the load-carrying capacity of perforated pipe, as the perforations can affect the strength of the pipe. Since the outer rings of 

TABLE 6.48 RESTRICTIONS ON FILTER PARTICLE SIZE TO LIMIT SEGREGATION

Minimum D10 filter

(mm)

Maximum D90 filter

(mm)

< 0.5 20

0.5 to 1.0 25

1.0 to 2.0 30

2.0 to 5.0 40

5.0 to 10 50

10 to 50 60

 (USACE, 1993)

TABLE 6.47 FILTER CRITERIA AS A FUNCTION OF BASE SOIL AND PERCENT 

PASSING NO. 200 SIEVE

Base Soil 

Category

Base Soil 

Description

Percent Finer Than 

No. 200 (0.075-mm) 

Sieve(1)

Filter Criteria(2)

1 Fine silts and clays > 85 D15 filter soil ≤ 9 D85 base soil

D15 filter soil ≥ 0.2 mm

2

Sands, silts, clays, 

and silty to clayey 

sands

40 to 85 D15 filter soil ≤ 0.7 mm

3 Silty and clayey 

sands and gravels 15 to 39 D15 filter soil ≤ 0.7 mm + [4(D85 base soil) - 0.7mm]

x [(40 - A)/(40 - 15)]

(3,4)

4 Sands and Gravels < 15 D15 filter soil ≤ 4 to 5 D85 base soil(5)

Note: 1. Category designation for soil containing particles larger than 4.75 mm is determined from a gradation

 curve of the base soil that has been adjusted to 100 percent passing the No. 4 (4.75-mm) sieve.

2. Filters should have a maximum particle size of 3 inches (75 mm) and a maximum of 5 percent passing

 the No. 200 (0.075-mm) sieve with a PI = 0. To provide sufficient hydraulic conductivity, filters must have

 D15 filter soil ≥ 4 D15 base soil, but not less than 0.1 mm.

3. A is the percent passing the No. 200 sieve using the adjusted grain-size curve.

4. When 4 D85 base soil < 0.7 mm, use D15 filter soil = 0.7 mm.

5. In Category 4, the D15 filter soil ≤ 4 D85 base soil criterion should be used for filters beneath riprap

 subject to wave action and for drains that may be subjected to violent surging and vibration.

(USACE, 1993)
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corrugated pipe carry the majority of the load, the effect of perforations on the inner rings is thought 

to be negligible (< 1 percent). Double-wall, corrugated and slotted HDPE and PVC pipe is available 

in a variety of diameters.

It should also be noted that all materials used in drainage systems for a coal refuse embankment 

should be resistant to corrosion from exposure to seepage leachates. Guidelines for selecting such 

materials are presented in Chapter 11.

Drain Capacity and Grade

Drains should be sized with sufficient dimensions and grade (typical minimum slope of 1 percent) 

to convey the estimated drain demands based on the seepage analyses multiplied by a safety factor. 

It is recommended that drains have the capacity to convey 10 times the estimated drain demand 

(Cedergren, 1989). It is also recommended that drains installed in trenches should include a minimum width determined on the basis of capacity requirements and constructability considerations 

(typically 3 feet) of the drain core material surrounded by specified minimum thicknesses for the 

surrounding filter aggregate(s). For drains at the ground surface or the coal refuse working surface, 

the minimum width of the core material should be determined on the basis of constructability. Additional discussion of drain and filter dimensioning is available from the USBR (2007a).

Detailed review of the hydraulic gradients in the vicinity of a drain may lead to adjustments in drain 

dimensions. McCook (2002) provides a discussion of critical hydraulic gradients and notes that cohesive soils such as clays will sustain significant gradients when confined. Indraratna and Radampola 

(2002) provide further guidance relative to critical gradients at drain filters.

Drain Configurations

The above criteria for drain capacity and grade apply to aggregate drains of various configurations, 

including trench drains, finger drains, blanket drains, chimney drains and discrete longitudinal 

drains. In determining the thickness of drainage layers, the layer inclination and the method of placement in addition to hydraulic conductivity requirements must be considered. Drainage layers placed 

by machine should generally have a thickness of at least 18 inches. Layers placed very carefully by 

hand in confined spaces or machine-placed layers for which there is construction oversight and QC 

compliance monitoring should have a minimum thickness of 12 inches. If adjacent materials could 

migrate into drainage layers during placement, the thicknesses should be increased. Where machine 

placement is used for constructing steeply inclined drainage layers within an embankment, each layer 

should be sufficiently thick to permit efficient operation of the construction equipment. Filters and 

drainage structures that are so thin that a small amount of contamination during construction would 

reduce the effective size to below design requirements are generally considered to be inadequate.

Another factor that may impact the thickness of drainage layers is the source and grain size of the granular material. Granular drains should be compatible with the selected filter material. The minimum 

thickness of a well-graded rock drain must be more than the maximum size of the rock, and gradation 

requirements should be strictly monitored so that there is no concentration of large rock. If uniformlygraded rock is used for a drain, then an increase in the minimum thickness should be considered. A 

minimum thickness equal to twice the maximum rock size should provide predictable flow capacity.

6.6.2.3.2 Geotextile-Wrapped Drains

At some coal refuse embankments, geosynthetic materials have been used to separate granular drain 

material from soils or coal refuse as a replacement for or supplement to granular filters. In addition 

to some state dam safety agencies, the Nation Dam Safety Review Board currently recommends 

that geotextiles not be used as filters in locations where they would be critical to the safety of an 

embankment dam, citing concerns about the long-term capability of the geotextile to function with-
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out deterioration or clogging. Critical drain applications and areas of concern include internal drains 

for controlling the phreatic surface for embankment stability and drains and filters that are designed 

to minimize the potential for piping of susceptible soils. FEMA, in a publication planned for 2009, is 

addressing the use of geotextiles in embankment dams.

MSHA has generally permitted the use of geotextiles as filters in slurry impoundments provided that 

testing using site-specific materials demonstrates acceptable behavior with respect to clogging and there 

is sufficient instrumentation to monitor the phreatic level. In addition to monitoring of the phreatic level, 

the seepage quantity from geotextile-wrapped drains should be monitored as an additional indicator 

of how the geotextile is performing. In recognition of the potentially more critical seepage conditions 

that can exist in other dams with significant hydraulic head and narrow cross section (e.g., fresh water 

dams), and until the use of geotextiles is accepted in this application, MSHA advocates that granular 

filters be used in such significant- and high-hazard-potential dams where filters are critical to safety.

As discussed in Section 6.5.5, a geosynthetic is a planar polymeric material used with soil or rock 

as an integral part of a civil engineering project, structure, or system. Geotextiles are a subcategory 

of geosynthetics and are made from woven or nonwoven fabric that allows the passage of water. At 

refuse disposal facilities, geotextiles have often been used as filters in internal drains. The geotextiles 

restrict movement of soil particles as water flows into the drain. Typically, non-woven geotextiles are 

used for filtration purposes. However, woven monofilament geotextiles have performed well in filter 

applications, and knit geotextiles have been used around perforated pipes as part of a two-stage filter 

in combination with a primary sand filter layer.

AASHTO M288, “Standard Specification for Geotextile Specification for Highway Applications,” 

(AASHTO, 2008) provides reference information concerning material properties, applications related 

to highway use (including subsurface drainage) and construction guidance for the use of geotextiles 

in drainage applications. FHWA Publication No. HI-95-038, “Geosynthetic Design and Construction 

Guidelines,” (Holtz et al., 1998) is a comprehensive reference providing information on the retention 

criterion, hydraulic conductivity criterion, clogging resistance criterion and survivability criterion, as 

discussed in subsequent paragraphs.

Geotextiles, like graded granular filters, require engineering design if they are to perform as desired. 

Unless flow requirements, piping and clogging resistance, and constructability requirements are 

accurately specified, the geotextile/soil filtration system may not perform properly. Also, as with 

graded granular filters, construction using geotextiles must be monitored to verify that installation is 

performed correctly and that the geotextiles are not damaged during installation.

Design of geotextile filters is comparable to design of graded granular filters. A geotextile is similar 

to a soil in that it has openings (voids) and filaments and fibers (particles). However, the geometric 

relationship between filaments and openings is more complex than the relationship between particles and voids. In geotextiles, opening size can be measured directly, whereas for soils pore size is a 

function of particle size. Because pore size can be directly measured, relatively simple relationships 

have been developed between the pore sizes and particle sizes of the soil to be retained. Three filtration concepts are used in the design process:

1. If the size of the largest opening in the geotextile filter is smaller than the larger particles of embankment material, the filter will retain the embankment material.

2. If the smaller openings in the geotextile are sufficiently large enough to allow smaller 

particles of embankment material to pass through, then the geotextile will not clog.

3. The number of openings in the geotextile should be such that adequate flow can be 

maintained even if some of the openings become clogged.
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The above simple concepts and analogies to soil filter design criteria have been used to establish 

design criteria for geotextiles (Holtz et al., 1998). Specifically, these criteria require that geotextiles must:

• Be capable of retaining soil or other embankment material (retention criterion)

• Allow water to pass (hydraulic conductivity criterion)

• Be functional throughout the life of the structure (clogging resistance criterion)

• Survive the installation process (survivability criterion)

Retention Criterion

For steady-state flow conditions:

AOS ≤ B D85 soil (6-21)

AOS ≈ O95 (6-22)

where:

AOS = apparent opening size (length)

O95 = opening size in the geotextile for which 95 percent are smaller (length)

B = coefficient (dimensionless)

D85 = soil particle size for which 85 percent are smaller (length)

B ranges from 0.5 to 2 and is a function of the type of soil to be filtered, its density, the uniformity coefficient Cu (for granular soils), the type of geotextile (woven or nonwoven), and the flow conditions. 

For sands, gravelly sands, silty sands, and clayey sands (with < 50 percent passing the No. 200 sieve), 

B is a function of Cu as defined below:

 Cu ≤ 2 or ≥ 8: B = 1

 2 ≤ Cu ≤ 4: B = 0.5 Cu

 4 < Cu < 8: B = 8 Cu

where:

Cu = D60 /D10 (6-23)

For silts and clays, B is a function of the type of geotextile:

 Woven B = 1 O95 ≤ D85

 Nonwoven B = 1.8 O95 ≤ D85

 For both AOS or O95 ≤ 0.3 mm

Due to their random pore characteristics and, for some types, their “felt-like” nature, nonwoven 

geotextiles will generally retain finer particles than a woven geotextile of the same apparent opening size. Therefore, a value of B = 1 is more conservative for nonwoven geotextiles than it is for 

woven geotextiles.
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Hydraulic Conductivity Criterion

For non-critical applications (less severe conditions):

kgeotextile ≥ kretained material (6-24)

For critical applications (severe conditions):

kgeotextile ≥ 10 kretained material (6-25)

Guidelines for determining critical nature or severity for drainage applications are provided in 

Table 6.49. Geotextile permittivity ψ can be defined as:

ψ = k/t (6-26)

where:

k = Darcy coefficient of hydraulic conductivity (length/time)

t = geotextile fabric thickness (length)

Geotextile permittivity should meet the following requirements:

 ψ ≥ 0.5/sec for < 15 percent passing No. 200 sieve

 ψ ≥ 0.2/sec for 15 to 50 percent passing No. 200 sieve

 ψ ≥ 0.1/sec for > 50 percent passing No. 200 sieve

Clogging Resistance Criterion

For actual flow capacity, the hydraulic conductivity criterion for non-critical applications is conservative because an equal quantity of flow through a relatively thin geotextile takes significantly less time 

than flow through a thick granular filter. Where flow reduction is judged not to be a problem, such as 

in clean, medium to coarse sands and gravels, Equation 6-24 may be used. Even so, some pores in the 

geotextile may become blocked or plugged with time. Therefore, for critical or severe applications, 

Equation 6-25 should be used to provide an additional level of conservatism. FEMA, in a document 

to be published in 2009, suggests even greater conservatism with kgeotextile = 10 to 100 kretained material and 

indicates that the French (Degoutte and Fry, 2002) use 100 kretained material for dams.

The required flow rate through the system q should also be determined, and the geotextile and drainage aggregate should be selected to provide adequate capacity. As indicated previously, flow capacities should not be a problem for most applications. In some situations, however, such as where 

geotextiles are used in multiple stage filters around a perforated or slotted pipe (pipe wraps), portions 

of the geotextile may not function effectively. For these applications, the following criteria should be 

used together with the hydraulic conductivity criteria:

qrequired = qgeotextile (Ag /At) (6-27)

where:

Ag = geotextile area available for flow (length2)

At = total geotextile area (length2)
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TABLE 6.49 GUIDELINES FOR EVALUATING CRITICAL NATURE OR SEVERITY 

FOR DRAINAGE APPLICATIONS

 Category Critical Less Critical

A. Critical Nature of the Project

1. Risk of loss of life and/or structural damage due to 

drain failure High None

2. Repair costs versus installation costs of drain Much Higher Equal or Less

3. Evidence of drain clogging before potential catastrophic 

failure None Yes

B. Severity of the Conditions

1. Soil to be drained Gap-graded, pipable or 

dispersible Well-graded or uniform

2. Hydraulic gradient High Low

3. Flow conditions Dynamic, cyclic or 

pulsating Steady-state

(ADAPTED FROM CARROLL, 1983)

Clogging resistance for less critical/less severe conditions should be designed to meet:

O95 geotextile ≥ 3 D15 retained material (6-28)

This relationship applies to soils with Cu > 3. For Cu < 3, a geotextile with the maximum AOS value 

should be selected. In situations where clogging is possible (e.g., gap-graded or silty soils), the following optional qualifiers may be applied:

• For nonwovens – porosity of the geotextile n ≥ 50 percent

• For woven monofilament and slit-film wovens – percent open area (POA) ≥ 4 percent

Most common non-woven geotextiles have porosities much greater than 70 percent. While most 

woven monofilaments easily meet the criterion of POA ≥ 4 percent, tightly woven slit films do not 

and are therefore not recommended for subsurface drainage applications.

For critical/severe conditions, geotextiles should be selected in accordance with the retention and 

hydraulic conductivity criteria described previously. A filtration test should be conducted to check 

for clogging using samples of on-site materials and hydraulic conditions such as long-term flow tests 

or the gradient ratio test, which is described in ASTM D 5101, “Standard Test Method for Measuring the Soil-Geotextile System Clogging Potential by the Gradient Ratio.” Dam safety agencies have 

expressed concern about the use of geotextiles in critical filters and drains (e.g., structures critical for 

the control of internal erosion and piping failures) due to susceptibility to:

• Excessive clogging from buildup of fines at the face of the geotextile 

or from biological, fungal or mineral matter buildup

• Separation at interfaces, junctions, connections and boundaries

• Undetected damage during construction.
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These performance problems are related to the following mechanisms: (1) inability to support the 

seepage discharge face, (2) excessive clogging or piping, (3) stress-induced distortion, (4) environmental degradation, (5) slope instability, and (6) rupture. Designers should consider these concerns 

and performance issues when using geotextiles and, as part of the planning process, should discuss 

the acceptability of any proposed application with MSHA and state agencies.

Talbot et al. (2000) describe concerns about the use of geotextiles in dams, including the propensity 

for excessive clogging because seepage forces move the geotextile away from the base soil and into 

the voids of the adjacent drain. Thus, separation can occur between the base soil and geotextile allowing fine particles from the base soil to accumulate at the interface, thus “blinding” the filter. Blinding 

can also occur if there are open voids in the base soil or if the base soil surface is irregular and has 

poor contact with the geotextile. These concerns can be addressed through: (1) use of fine gravel 

(about 3⁄4-inch to 11⁄2-inch maximum size) for the drainage layer to improve contact with the base soil 

(Giroud, 1997; van Zyl and Robertson, 1980), (2) grading the base soil surface smooth and placing the 

geotextile in contact with the base soil with a minimum of wrinkles, and (3) minimizing vertical or 

steeply inclined slopes (FEMA, to be published in 2009).

The potential for particulate clogging can be addressed through application of filter criteria, and 

chemical and biological clogging can be addressed based on evaluation of the drain environment 

and/or testing, as discussed subsequently. Stress-induced distortion, environmental degradation, 

and stability can be addressed with design and laboratory testing procedures.

Chemical clogging of geotextiles at coal refuse disposal sites will most likely be associated with iron 

and manganese oxidation and deposition, although it is possible that calcium carbonate precipitation may also be encountered. These processes are affected by chemical reactions and biological 

activity that varies depending on whether the environment is anaerobic and aerobic. Factors that 

may lead to problems with clogging of drains are discussed by Smith and Hosler (2006). Research 

into predictive methods has generally concentrated on biological clogging and the role of microbial activity, and more work is necessary for prediction of potential chemical clogging. Long-term 

hydraulic conductivity testing procedures can be used to evaluate the potential effect of chemical 

clogging.

Koerner and Koerner (2005) provide drainage reduction factors for determination of the design flow 

rate or transmissivity of geotextiles. These reduction factors address soil clogging and blinding, 

reduction in void space, and chemical and biological clogging. Koerner and Koerner recommend 

reduction factors to the design flow capacity of a geotextile of between 1.2 and 1.5 for landfill filters 

to account for chemical clogging, and they suggest adoption of a high reduction factor where total 

suspended solids in the permeating liquid is greater than 5,000 mg/l. Similarly, they recommend 

reduction factors of between 2 and 5 to the design flow capacity of a geotextile to account for biological clogging and suggest adoption of a high reduction factor where biochemical oxygen demand 

(BOD) is greater than 5,000 mg/l.

The potential for biological clogging can be examined in accordance with ASTM D1987, “Standard 

Test Method for Biological Clogging of Geotextile or Soil/Geotextile Filters.” However, before using 

this test, Mackey and Koerner (1999) recommend that to facilitate interpretation of the results of the 

test the physical, chemical and biological processes at the site be evaluated and understood. If clogging is a concern, a higher-porosity geotextile can be used, and/or the drain design and operation can 

include an inspection and maintenance program for flushing the drainage system. For nonwoven 

geotextiles, Luettich et al. (1992) recommend using the largest porosity available, but not less than 

30 percent; for woven geotextiles they recommend using the largest POA, but not less than 4 percent. Because of concerns for clogging, geotextiles with the largest opening sizes that satisfy piping 

requirement should generally be used.
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Survivability Criterion

For filtration and drainage applications, if a geotextile is to survive the construction process, certain minimum strength and endurance properties are required. Table 6.50 provides these minimum 

requirements for less critical/less severe applications. It is important to understand that these minimum survivability parameters are based on empirical data from geotextiles that have performed satisfactorily in drainage applications. These parameters serve as guidelines for selecting geotextiles for 

most projects. The guidelines are not intended to replace site-specific evaluation, testing, and design.

Geotextile material should be covered subsequent to installation as soon as possible to prevent degradation from sunlight or in accordance with the manufacturer’s recommendations. Geotextile endurance 

TABLE 6.50 GEOTEXTILE STRENGTH PROPERTY REQUIREMENTS 

FOR DRAINAGE GEOTEXTILES(1,2,3,4)

Property ASTM Test 

Method Units

Geotextile Class 2(5)

Elongation

< 50%(6) ≥ 50%(6)

Grab Strength D 4632 N (lb) 1100 (247) 700 (157)

Sewn Seam Strength(7) D 4632 N (lb) 990 (223) 630 (142)

Tear Strength D 4533 N (lb) 400(8) (90) 250 (56)

Puncture Strength D 4833 N (lb) 400 (90) 250 (56)

Burst Strength D 3786 kPa (lb/in2) 2700 (392) 1300 (189)

Note: 1. Acceptance of geotextile material shall be based on ASTM D 4759, “Standard Practice for 

Determining the Specification Conformance of Geosynthetics.”

2. Acceptance shall be based upon testing of either conformance sampler obtained using 

Procedure A of ASTM D 4354, “Standard Practice for Sampling of Geosynthetics for Testing,” 

or on manufacturer’s certifications and testing of quality assurance samples obtained using 

Procedure B of ASTM D 4354.

3. Values apply to minimum strength; use value in weaker principal direction. All numerical values 

represent minimum average roll value (i.e., test results from any sampled roll in a lot shall meet 

or exceed the minimum values in the table). Lot samples according to ASTM D 4354.

4. Woven slit film geotextiles will not be allowed.

5. AASHTO Geotextile Class 2 is the default geotextile selection. The engineer may specify 

AASHTO Class 3 geotextiles for trench drain applications based on one or more of the following:

a) The engineer has found Class 3 geotextiles to have sufficient survivability based on field 

experience.

b) The engineer has found Class 3 geotextiles to have sufficient survivability based on 

laboratory testing and visual inspection of a geotextile sample removed from a field test 

section constructed under anticipated field conditions.

c) Subsurface drain depth is less than 2 m, drain aggregate diameter is less than 30 mm, 

and the compaction requirement is ≤ 95 percent of ASTM D 698, “Standard Test Methods for 

Laboratory Compaction Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft3 (600 

kN-mm3)).”

6. As measured in accordance with ASTM D 4632, “Standard Test Method for Grab Breaking Load 

and Elongation of Geotextiles.”

7. When seams are required. Values apply to both field and manufactured seams.

8. The required MARV tear strength for woven monofilament geotextiles is 250 N (56 lb).

(ADAPTED FROM AASHTO STANDARD SPECIFICATIONS FOR TRANSPORTATION MATERIALS AND METHODS OF TESTING, PART I SPECIFICATIONS, 2007, BY PERMISSION OF THE 

AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS, WASHINGTON, D.C. USED BY PERMISSION. DOCUMENT MAY BE PURCHASED FROM THE AASHTO 

BOOKSTORE AT 1-800-231-3475 OR ONLINE AT HTTP://BOOKSTORE.TRANSPORTATION.ORG)
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is related to longevity. Geotextiles have been shown to be basically inert materials in most environments and applications. However, some applications may expose the geotextile to chemical or biological activity that could dramatically affect filtration properties or durability. For example, in drains, 

granular filters and geotextiles can become chemically clogged by iron or carbonate precipitates and 

biologically clogged by algae and mosses. Biological clogging is a potential problem when filters and 

drains are periodically inundated then exposed to air. Excessive chemical and biological clogging can 

significantly affect filter and drain performance, and monitoring with piezometers is recommended.

6.6.2.3.3 Seepage Control along Conduits

Seepage along conduits that pass through coal refuse embankments should be controlled. Two methods for seepage control are filter diaphragms and anti-seep collars (also known as cutoff collars). 

Since the mid-1980s, the use of anti-seep collars has become less common and filter diaphragms have 

become more widely used (Van Aller, 1998). Many dam safety agencies require the use of filter diaphragms because of the many instances where seepage problems have occurred along conduits even 

when anti-seep collars were used. The primary advantage of filter diaphragms is the relative ease of 

construction as compared to anti-seep collars, particularly with respect to compaction around conduits. The presence of anti-seep collars complicates compaction around conduits, and accordingly 

they are more likely to function poorly due to construction flaws. Also, filter diaphragms are considered to be a better measure for mitigating the consequences of embankment cracking associated 

with the presence of a conduit. Filter diaphragms should be constructed with suitable filter materials, 

and careful placement is required during construction. The following sections present basic design 

considerations for both filter diaphragms and anti-seep collars.

Filter Diaphragms

A filter diaphragm is used for intercepting seepage through backfill pores or cracks and to prevent internal erosion of the backfill materials along buried conduit installations. To meet filtration 

and drainage requirements, filter diaphragms may consist of a single zone or multiple zones of 

granular material. The guidance for dimensioning filter diaphragms provided in the following text 

is taken from the USDA (1985) and NRCS (2007b). Van Aller (1998) discusses various aspects of 

filter diaphragm design, and McCook (2002) discusses site-specific conditions that should also be 

considered.

Filter diaphragms should be located approximately parallel to the centerline of the embankment and 

approximately perpendicular to the direction of seepage flow, and should extend horizontally and 

vertically into adjoining portions of the embankment and foundation. In homogeneous dams, filter 

diaphragms should be located using the following criteria:

• Downstream of the cutoff trench

• Downstream of the centerline of the dam if there is no cutoff trench

• Upstream of a point where the embankment cover (from the upstream face of the 

diaphragm to the downstream face of the embankment) is at least one-half of the 

difference between the elevation of the top of the filter diaphragm and the maximum 

potential impoundment water level

In zoned embankments, the filter diaphragm should be located downstream of the core zone and/or 

cutoff trench, in accordance with the minimum cover guidance for homogeneous dams. In instances 

where the downstream shell of a zoned embankment is more pervious than the diaphragm material, 

the diaphragm should be located at the downstream face of the core zone.

Provisions should be made for discharging seepage and groundwater collected by the filter diaphragm. 

These provisions could include tying the diaphragm into other drainage systems in the foundation, 
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tying into internal embankment drains or designing/constructing a separate outlet for the filter diaphragm. Such an outlet should be designed with the assumption that the hydraulic conductivity in 

the zone upstream of the filter diaphragm is 100 times the hydraulic conductivity in the compacted 

embankment material. This zone should have a cross-sectional area equal to the area of the filter diaphragm, and the length of the seepage path should equal the distance from the embankment upstream 

toe to the filter diaphragm. This higher hydraulic conductivity is intended to account for partiallyfilled cracks and openings in the upstream zone. An advantage of having a separate outlet for the filter 

diaphragm is that the seepage outflow can be monitored separately from other seepage flows and will 

provide feedback as to the performance of the diaphragm system over time.

For rigid conduits, the NRCS (2007b) recommends that filter diaphragms should extend the following minimum distances from the conduit surface (Figure 6.50):

1. Horizontally and vertically upward 3 times the outside diameter of circular conduits 

or the vertical dimension of rectangular box conduits except that:

a. The vertical extension need be no higher than the maximum potential 

impoundment level.

b. The horizontal extension need be no further than 5 feet from the sides 

and slopes of any excavation for installation of the conduit.

2. Vertically downward from the conduit:

a. Filter diaphragms should extend from the pipe support 1.5 times the outside diameter of circular conduits or outside vertical dimension of box 

conduits or to the top of rock, whichever is shallower.

b. Alternatively, for conduit settlement ratios δ of 0.7 and greater, filter 

diaphragms should extend the greater of 1 foot beyond the bottom of 

the trench excavation for the conduit or 2 feet. The diaphragm should 

be terminated at the surface of bedrock when it occurs within this distance. Additional control of general seepage through an upper zone of 

weathered rock may be needed. The conduit settlement ratio δ is defined 

in NRCS (2007a) and Technical Release-5 (TR-5) by the USDA (1958) and 

requires a complex computation. On firm foundations with δ = 0.7 or 

greater (the conduit settlement ratio for pipe on rock is 1.0), the filter diaphragm should extend below the pipe to rock, or at least 2 feet.

For flexible conduits, NRCS recommends that filter diaphragms be designed to extend in all directions a minimum of 2 times the outside diameter from the surface of the conduit, except that the diaphragm need not extend beyond the limits in 1a and 1b described above or beyond a bedrock surface 

beneath the conduit.

Filter diaphragms should have a minimum thickness of 3 feet, but the thickness specified should 

be appropriate for the level of quality control and supervision during construction. If a multi-zone 

system is employed to satisfy filter criteria, a minimum thickness of 1 foot should be used for any 

single zone. Greater thickness may be required as dictated by: (1) flow capacity requirements, (2) 

the need to tie the filter diaphragm into embankment internal or foundation drains, or (3) the need 

to accommodate construction methods. An example design for a filter diaphragm associated with a 

decant pipe is shown in Figure 6.51.

Some state regulatory agencies have developed specific guidance on filter diaphragms. Also, McCook 

(2002) discusses site-specific conditions that may warrant enlarging the filter diaphragm relative to 
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the minimum guidance cited above. These conditions include foundations with varying rock surfaces, soft soils, and situations where there is potential for differential settlement and related strain.

Anti-seep Collars

Cutoff or anti-seep collars are intended to minimize seepage along the contact between the outside 

surface of a conduit and an embankment. Van Aller (2004) and FEMA (2005a) provide compelling 

reasons to use filter diaphragms rather than anti-seep collars. For low-hazard-potential structures 

with dam heights of 35 feet or less and storage volume less than 3,000 acre-feet, the NRCS (2002) 

allows consideration of anti-seep collars. In coal refuse embankment dams, there may be site-specific 

reasons to use anti-seep collars. For example, during early stages of construction, installation of antiseep collars may be preferable to an internal drainage structure that, in the case of downstream construction, could ultimately be located relatively far upstream and perhaps beneath the impoundment 

during later stages of construction. In such situations, the pipe and collar backfill should be designed 

so as to minimize the potential for concentrated seepage zones and internal erosion.

Cutoff collars have been fabricated using concrete, steel, and plastic for consistency with conduit 

materials. The intent of their use is to increase the length of percolation along the conduit contact 

FIGURE 6.50 FILTER DIAPHRAGM DESIGN FOR CONDUIT

PROFILE THROUGH COAL REFUSE EMBANKMENT

COMPACTED COARSE

 COAL REFUSE

DROP INLET

PRINCIPAL SPILLWAY

(PIPE OUTLET) A

A

FILTER DIAPHRAGM

FILTER DIAPHRAGM

SECTION A - A

FLEXIBLE CONDUIT

2D

 2D 2D

STOP AT ROCK SURFACE IF LESS THAN

2D BENEATH STRUCTURE

2D

3D

D

1.5D

3D

STOP AT ROCK SURFACE IF LESS THAN

1.5D BENEATH STRUCTURE

SECTION A - A

RIGID CONDUIT

FILTER DIAPHRAGM

(USDA, 1985)

FIGURE 6.50 FILTER DIAPHRAGM DESIGN FOR CONDUIT
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FIGURE 6.51 EXAMPLE DESIGN CONFIGURATION FOR

FILTER DIAPHRAGM AND DECANT PIPE

SECTION B - B

FLOWABLE

BACKFILL

DECANT PIPE

PIPE SUPPORT

ON PERIODIC

SPACING

2.5 D (MAX)

4" SCHEDULE 80

DIAPHRAGM OUTLET PIPE

3"

C DECANT PIPE

D

L

PROTECTIVE COVER

AASHTO NO. 8

COARSE AGGREGATE

C L DECANT PIPE

PROJECTED LIMITS OF TRENCH

EXCAVATION FOR PIPE UPSTREAM

AND DOWNSTREAM OF FILTER

DIAPHRAGM

GEOTEXTILE

ANGLE OF

REPOSE

SECTION A - A

2D OR 5' (MIN)

AASHTO NO. 57

COARSE AGGREGATE 2D (MIN)

SAND

2D

(OR TO ROCK)

PROTECTIVE COVER

FLOWABLE BACKFILL

A

A

 SAND

4" O PERFORATED SCHEDULE 80 PVC PIPE

GEOTEXTILE

AASHTO NO. 8

COARSE AGGREGATE

ANGLE OF

REPOSE

COMPACTED

COAL

REFUSE

PROTECTIVE COVER AASHTO NO. 57

COARSE AGGREGATE

........................................................................................................................

........................................................................................................................

CAP

B

B

FIGURE 6.51 EXAMPLE DESIGN CONFIGURATION FOR FILTER DIAPHRAGM AND DECANT PIPE
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surface by 20 to 30 percent for significant- and high-hazard-potential dams (USBR, 1987a). For a 

conduit on an earth foundation, the cutoff collar should completely encircle the conduit. Where 

the foundation is sound rock and good contact along the base is expected, cutoff collars will need 

to extend only to a depth sufficient for keying into the rock foundation. Cutoff collars should be 

separated from rigid conduits using watertight fillers (gasket or seal) to avoid introducing concentrated stresses into the walls of the conduit. Non-rigid collars can be attached or clamped to flexible 

conduits using gaskets that accommodate conduit deformation such that a watertight connection 

is achieved.

For small, low-hazard-potential dams, the NRCS (2002) recommends the use of filter diaphragms 

unless it is determined that anti-seep collars will adequately serve the purpose. If anti-seep collars are 

used, the NRCS recommends increasing the flow path along the conduit by at least 15 percent, with 

10- to 25-foot spacing between collars.

6.6.2.3.4 Relief Wells

If a pervious layer underlies a relatively impervious layer beneath the toe of an embankment slope, 

pore-water pressures can build up in the pervious layer to produce an artesian effect if drainage 

downstream is impeded and the layer is recharged upstream by groundwater, seepage from the 

impoundment, or rainfall. If artesian water pressure must be reduced for stability, relief wells can be 

used, as shown in Figure 6.52.

Water collected by relief wells is usually conducted through a horizontal overflow pipe at the ground 

surface and discharged into a lined drainage ditch near the toe of the embankment. With this collection method, the phreatic surface can be lowered to the ground surface. If a greater lowering of head 

is required, the outfalls from the wells and the collection drain can be lowered below the ground surface provided that they can be connected to the collection drain through a discharge pipe. To allow 

for inspection and maintenance, relief well casings should extend to the ground surface.

The spacing required for relief wells depends on the geology and groundwater hydrology at the site. As 

with embankment seepage modeling, an analysis should be performed to evaluate the impact of relief 

wells on foundation pressures. Numerical modeling is well suited to such an analysis, but requires 

an understanding of groundwater conditions, including geometry, pre-construction piezometric levels 

and the connection of the pervious strata with recharge features such as the impoundment or local 

aquifers. In instances where the model can be sufficiently simplified, flow nets can be used successfully. 

As a check, an estimate of the required spacing of relief wells can be made using methods presented by 

Leonards (1962). Given the limitations inherent in this type of modeling, the adequate function of the 

relief wells should be monitored with piezometers. A relief well system can be readily expanded if the 

initial configuration fails to produce the desired reduction in piezometric head.

Attention should be given to relief well design details so that the wells meet performance requirements and can be properly inspected and maintained. The chemical content of the water to be recovered should be evaluated to determine if any special precautions are needed for preventing corrosion 

of any part of the relief well system. The diameter of the internal perforated pipe depends on the 

anticipated flow volume, but should be no less than 6 inches. To facilitate inflow and to prevent clogging, relief wells should be surrounded by a filter designed according to the requirements discussed 

earlier in this section.

The annulus around the portion of the relief well above the pervious layer should be sealed with an 

impervious material (such as hydrated bentonite or a cement-bentonite mixture) or concrete to prevent upward flow of water around the pipe. It may be necessary to temporarily lower the water level 

in the relief well during the construction of this seal.
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GROUND

SURFACE

CL

RISER

3' - 0"

FLANGED RISER COVER

OUTFALL

FLAPGATE OUTFALL

GUTTER

1' - 0"

CONCRETE BACKFILL

CONCRETE OR

BENTONITE BACKFILL

SAND BACKFILL

TOP OF FILTER PACK

4" (MIN.) FILTER PACK

SURROUNDING PIPE

6" (MIN.) I.D. SLOTTED SCREEN

MAY CONTAIN SECTIONS OF

RISER THROUGH VERY FINE

SAND OR SILT STRATA

BOTTOM PLUG

FILTER PACK

LENGTH

VARIES

FIGURE 6.52 TYPICAL RELIEF WELL INSTALLATION

6.6.2.3.5 Horizontal Drains

Horizontal drains perform a similar function to relief wells, but provide more effective drainage 

either in the foundation under the main body of an embankment or within the embankment. Most 

often, horizontal drains are used to reduce excessive pore-water pressure within or beneath an existing embankment. A typical horizontal drain installation and details are shown in Figure 6.53. Horizontal drains are normally sloped toward the discharge end.

Although horizontal drains 600 feet long and longer have been installed, lengths of 400 feet or less are 

more common. The drains, which are usually slotted plastic pipe, are normally installed inside steel 

drill rods, which are subsequently retracted. Because there is no soil filter around the slotted pipe, the 

slots must be sized to prevent infiltration of fines based upon the rules discussed in Section 6.6.2.3.1. 

In some cases, porous plastic pipe has been placed around the slotted pipe to limit the infiltration of 

fines. The spacing required between horizontal drains is difficult to determine accurately. Pore-water 

pressure should be monitored with piezometers to check the performance of an installation. A system 

FIGURE 6.52 TYPICAL RELIEF WELL INSTALLATION
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FIGURE 653 EXAMPLE HORIZONTAL DRAIN INSTALLATION

DETAIL 2 - DRAIN COLLAR AND DRAINAGE COLLECTION

DETAIL 1 - 1 1/2" I.D. PVC PIPE WITH 0.02"

 MACHINE-CUT SLOTS

MACHINE CUT SLOTS

EXTERNAL OR

INTERNAL COUPLING

NOMINAL 2" O.D.

SECTION A - A 

(SEE DETAIL 2)

GROUTED ANNULUS

3/4" I.D.

PVC PIPE

6" CASING

GROUT BACKFILL

1 1/2" I.D.

PVC PIPE

DAM

2'

3/4" PVC PIPE

WITH 0.01" SLOTS

BENTONITE AND BURLAP

ANNULAR PACKER

GROUTED ANNULUS

SURROUNDING 6" CASING

BACKFILL GROUT

3

1

SURFACE CASING FOR

DRILLING COLLAR

1 1/2" I.D. SOLID

 PVC PIPE DRAIN

3/4" I.D. PVC BLEEDER PIPE

2'

4'

COLLECTION

TANK

RUBBER

GASKETS

FLANGE

PLATE

FLANGED

OUTLET PIPE

STEEL FENCE

POSTS

A

A

NATIVE SOIL

DAM

SAND DRAINS

(MAY BE CONSTRUCTED

LATER, AS NEEDED)

140' LENGTH OF SLOTTED 1 1/2" I.D.

PVC PIPE (SEE DETAIL 1)

APPROXIMATE PHREATIC SURFACE

COLLECTOR PIPE AND DRAIN

 (SEE DETAIL 2)

SOLID 1 1/2" I.D.PVC PIPE

BEDROCK

~ 3 SLOPE

EMBANKMENT CROSS SECTION

NATIVE SOIL

FIGURE 6.53 EXAMPLE HORIZONTAL DRAIN INSTALLATION
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of horizontal drains can be readily expanded if the capacity of the initial installation fails to produce 

the required reduction in piezometric head.

The installation of horizontal drains can be dangerous in situations where large volumes of seepage 

could lead to a piping failure at the drain borehole during installation. Horizontal drain systems 

should be installed by an experienced contractor under expert supervision.

6.6.2.3.6 Impoundment Liners

To protect the groundwater some state agencies may require that seepage from fine coal refuse be 

limited. This can be accomplished by using a layer of low-hydraulic-conductivity soil or geomembrane liners. Geomembranes are manufactured, low-hydraulic-conductivity synthetic materials that 

function as barriers to liquids and vapors. When used as a liner on the bottom and sides of a refuse 

disposal impoundment, a geomembrane can impede leachate migration from overlying refuse into 

the underlying soil and groundwater and can be used to collect the leachate for treatment. When a 

geomembrane is used as a cap in the final cover over the impoundment, it prevents precipitation 

from infiltrating the coal refuse, thus minimizing or eliminating leachate generation.

Minimum requirements for geomembrane liners are often specified in state regulations. Minimum 

strength properties are provided in Table 6.51. However, if the application is on a slope or there is a 

possibility that differential settlement could occur, increasing stress and strain on the geomembrane, a 

more conservative choice of membrane thickness may be appropriate. The strength of the liner is usually 

reduced at seams. Standard tests for shear and peel strength should be performed on both factory and 

field seams. A determination must be made as to the minimum percentage of geomembrane material 

strength that the seam itself must possess. This minimum seam strength criterion is typically incorporated into the quality assurance/quality control (QA/QC) program for liner installation. For evaluation 

of survivability, the minimum seam strength should be determined early in the design stage if it is likely 

that the geomembrane will be subjected to ultraviolet radiation for a significant period of time. If that is 

the case, a material with a high resistance to ultraviolet light deterioration should be used.

TABLE 6.51 RECOMMENDED MINIMUM PROPERTIES FOR GENERAL 

GEOMEMBRANE INSTALLATION SURVIVABILITY

Property and Test Method

Required Degree of Survivability

Low(1) Medium(2) High(3) Very High(4)

Thickness – ASTM D 1593 (mils) 20 25 30 40

Tensile – ASTM D 882 (1-in strip) (lb/in) 30 40 50 60

Tear Resistance – D 1004 (Die C) (lb) 5 7.5 10 15

Bursting Strength – D 3787 (lb) 20 25 30 35

Impact Resistance – D 3998 (ft-lb) 10 12 15 20

Note: 1. Low refers to careful hand-placement on very uniform, well-graded subgrade with light loads of a static

 nature – typical of vapor barriers beneath building floor slabs.

2. Medium refers to hand- or machine-placement on machine-graded subgrade with medium loads –

 typical of canal liners.

3. High refers to hand- or machine-placement on machine-graded subgrade of poor texture with high loads

 – typical of landfill liners and covers.

4. Very high refers to hand- or machine-placement on machine-graded subgrade of very poor texture with

 high loads - typical of reservoir covers and liners for heap leach pads.

(ADAPTED FROM KOERNER, 2006)
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6.6.2.3.7 Foundation Seepage Cutoffs

In addition to measures to control seepage in embankments, foundation treatments such as cutoff 

trenches backfilled with compacted low-hydraulic-conductivity materials are also routinely incorporated into refuse facility designs. Foundation cutoff trenches are discussed in Section 6.3.

6.6.2.3.8 Impoundment Water and Slurry Deposition Management

Management of impoundment water and slurry deposition at a coal refuse disposal facility are operational measures for controlling and mitigating the effects of seepage. Maintenance of a low level 

of clarified water in an impoundment reduces the hydraulic head and the source volume for seepage through the embankment. Additionally, deposition of coal refuse slurry at or near the upstream 

embankment face and the resulting build up of a delta of fine refuse against the embankment will 

reduce seepage into the embankment if the fine refuse has a lower hydraulic conductivity than the 

embankment material. To develop and maintain an effective fine refuse delta across the upstream 

face of the embankment, periodic relocation of the slurry discharge point or use of multiple discharge 

points may be required. At some facilities, small cells have been constructed near the upstream end 

of a refuse impoundment for storage of water for recirculation to the preparation plant. Such provisions reduce the likelihood of water being impounded directly against a coarse refuse embankment 

and associated seepage concerns.

6.6.2.4 Seepage Measurements

When seepage rates or piezometric levels are an important factor in the performance of a disposal 

facility, estimates of the phreatic surface, magnitude and rate of dissipation of pore pressures, 

and quantity of seepage collected in internal drains should be made. These predictions should 

be checked during and after construction by instrumenting the refuse embankment (and foundation if necessary) and measuring changes in the phreatic surface and pore pressures. Procedures 

for monitoring groundwater levels and pore-water pressures are discussed in Chapter 13. Field 

piezometer data provide a basis for updating performance predictions and for possible modification of design or construction procedures. Weirs or parshall flumes at the outlets of internal drains 

can be used to measure the seepage collected from an embankment for comparison to results of 

seepage analyses associated with the internal drain design. In addition, instrumentation can provide a check on the in-situ embankment hydraulic conductivity, as compared to that assumed in 

the analyses and/or determined by laboratory or field hydraulic conductivity tests. Monitoring 

of seepage conditions is also important for detection of unanticipated changes in the saturation 

level or seepage quantity and can provide an early indication of a problem such as clogging of an 

underdrain.

6.6.3 Settlement Analysis

6.6.3.1 Conditions Requiring Deformation Analysis

Settlement of a coal refuse embankment occurs as a result of embankment compression, foundation 

compression, plastic deformation, differential settlements, mine subsidence or a combination of these 

effects. Settlement is usually important if an embankment will impound water or if it will serve as 

the foundation for future construction. The design of any impounding embankment must limit settlement of the crest so that the freeboard is not reduced below the allowable limit. Embankments should 

be cambered so that the crest is elevated relative to the abutments to compensate for the increased 

settlement that typically occurs near the center of the embankment where the foundation overburden 

and embankment height are the greatest. It is also important that the embankment does not settle so 

much that the hydraulic conductivity characteristics of the embankment are significantly changed or 

the potential for piping or internal erosion due to cracking is created. Embankment and foundation 

settlement can also affect the performance and structural integrity of conduits and other structures. 

The settlement that can be tolerated by a structure depends upon its function.
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The magnitude of settlement under self-weight experienced by a coal refuse embankment cannot 

be accurately estimated from fundamental stress-strain properties. The most useful information for 

computing embankment settlement is performance data from instrumented earth and rockfill dams. 

These data indicate that settlement of well-compacted earth dams due to embankment compression 

ranges from less than 1 percent for dams constructed of non-plastic soils to more than 4 percent for 

dams constructed of highly-plastic, fine-grained soils. Measured settlements of rockfill dams have 

ranged from essentially no settlement for well-compacted and sluiced rockfill placed in thin lifts to 10 

percent or more for unsluiced rock placed in thick lifts. Therefore, the amount a coal refuse embankment will settle due to compression can range from 1 percent or less for well-compacted materials to 

8 to 10 percent for uncompacted materials. However, a coal refuse embankment is often constructed 

over 20 to 30 or more years, as compared to one to three years for an earth or rockfill dam. Thus, 

nearly all coal refuse embankment settlement will likely occur during construction, and additional 

settlement after abandonment will be minimal.

If an embankment foundation consists of dense glacial till, dense sand and gravel, or rock, it will 

deform only slightly under the weight of the embankment because, at the stress levels experienced, 

both the foundation materials and any existing pore-water are essentially incompressible. Thus, settlement of such embankments due to foundation settlement will be minimal. If the foundation consists of saturated fine-grained soils, there may be significant settlement from consolidation due to 

the time-dependent expulsion of pore water from the soil. In such cases, the development of excess 

pore-water pressures can also affect foundation stability and these excess pressures need to be taken 

into account in embankment stability analyses.

Upstream construction of coal refuse disposal embankments over fine coal refuse deposits typically 

results in consolidation settlements that can give rise to elevated pore pressures if construction occurs 

rapidly. To prevent localized instability, construction procedures should be carefully planned. Monitoring of pore pressure may be needed as part of monitoring of embankment stability and controlling 

the rate of construction. These issues are discussed in Chapter 11.

Consolidation occurs slowly, sometimes over several months or years. Consolidation settlement can 

be a very important design factor because, in addition to causing damage to drainage pipes and 

structures, it can affect aspects of the entire disposal facility. Problems created by consolidation may 

not become apparent until after the disposal facility begins operation, at a time when the settlement 

could create a safety hazard and when corrections are most expensive. If consolidation is associated with embankment construction over soft clay deposits, the total settlement could be substantial. 

When a slurry impoundment is to be eliminated by backfilling the remaining reservoir, the settlement 

of the cap material must be considered in the design. Sufficient cap material must be placed such that 

following long-term settlement positive surface drainage will be maintained and a depression that 

will collect water is not created.

The magnitude of the foundation consolidation settlement depends on the weight of the embankment, the depth and thickness of the compressible strata in the foundation, and the compression 

indices of these compressible strata. Compression indices can be obtained from laboratory consolidation tests on undisturbed samples taken from the compressible strata. Compression indices from 

field settlement records for other disposal facilities underlain by similar compressible strata with 

similar moisture contents and index properties can also be used to predict the potential range of 

settlement.

The rate at which settlements occur is a function of the rate of change in vertical stress (directly proportional to the rate of construction), the hydraulic conductivity of the compressible material, and the 

drainage characteristics of the foundation. The rate of settlement is much more difficult to estimate 

than the magnitude of settlement. Computations based on laboratory consolidation test data may be 
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inaccurate because the rate at which foundation settlements occur is often controlled by minute geological characteristics that may not be detected even by carefully conducted foundation studies.

Consolidation and conventional laboratory tests used for measuring the consolidation characteristics 

of a soil are discussed in Section 6.5.6.

Plastic deformation of an embankment and its foundation represents only a portion of the observed 

settlement at a coal refuse embankment. Plastic deformation is the lateral spreading of an embankment at the base coincident with settlement. Although difficult to determine, it is important to allow for 

the amount of plastic deformation that will occur because it can cause extension of pipes constructed 

through the embankment. Although the magnitude of such an extension may be as much as several 

percent of the total length of the pipe, damage can usually be avoided if the extension occurs uniformly 

over the length of the pipe. However, if the extension is concentrated or if the pipe cannot tolerate significant extension, it may separate and leak or fracture and collapse. Such an outcome could seriously 

affect embankment stability. To minimize the risk of this type of failure, pipes should either be placed in 

the foundation below the plastic zone or should be designed to tolerate the anticipated extension.

Differential vertical settlements can also cause damage to a coal refuse embankment and to interior 

structures. Differential foundation settlements, particularly if they occur over small distances, can 

result in embankment cracks and lead to subsurface erosion. Differential settlements can also cause 

damage to pipe drains and decant lines installed within or beneath an embankment. When considering the potentially damaging effects of foundation settlements beneath a planned coal refuse embankment, it should be assumed that some areas of the foundation may have localized settlements at least 

twice as great as the predicted overall settlement. Therefore, if substantial foundation settlements are 

expected, placing pipelines within or beneath an embankment should be avoided. If a decant pipe is 

placed in an embankment, it should be located as far as possible from the areas where the settlement 

is anticipated to be greatest.

Embankments constructed above or adjacent to underground mine voids can experience settlement 

and lateral distortion caused by subsidence. Depending on the characteristics of the overburden rock, 

the depth and type of mining, and whether the mining is pre-existing or is occurring during disposal operations, the subsidence can have a significant effect on the stability of the embankment and 

the retained coal refuse. Additional discussion of subsidence issues can be found in the following 

Manual sections:

• Section 5.4 for mine subsidence considerations during facility design

• Section 8.4 for analysis of mine subsidence

• Section 8.5 for mine breakthrough potential evaluations

Sherard (1973) reports that earth dam cores of almost all soil types have experienced cracking. 

Therefore, designers should carefully consider the surface topography in which, and the foundation condition and materials on which, the embankment will be constructed. For dams in narrow 

valleys, cracking patterns such as those shown in Figure 6.54 have been observed. Transverse vertical cracks often develop within about 15 to 50 feet from the end of the crest, and in the central 

portion where the embankment is the highest, vertical cracks often develop on or near the crest, 

as shown in Figure 6.54.

Based on the results of laboratory testing reported by Leonards and Narain (1963) and statistical evaluation of earth dam performance by Biarez et al. (1970) and Londe (1970), as reported by 

Sherard (1973), the tensile strain at which first cracking occurs is in the range of 0.1 to 0.3 percent. 

In most of these case histories, the embankments were placed with a moisture content from 1 to 
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4 percent dry of optimum (Wilson and Squier, 1969). Tensile strains in earth dams should be considered, and precautionary measures to limit the magnitude of tensile strains should be incorporated, as necessary. Precautionary measures could include: (1) incorporating more crack resistant 

soils in the dam cross section where cracking is most likely, (2) taking added care in foundation 

preparation to avoid/minimize large changes in foundation grades, or (3) increasing the width of 

the embankment cross section. Regardless, designers should assess the potential effects of factors 

that can cause differential settlement and incorporate appropriate measures to limit the development of cracks during construction and long-term operations.

Broadly-graded coarse coal refuse, placed at or above optimum moisture content and at relatively 

low construction rates that allow settlement to occur as fill is gradually added, is generally considered to be less susceptible to cracking than earthen materials, which are frequently subject to longerterm consolidation, after completion of construction.

6.6.3.2 Settlement and Deformation Analysis

Embankment and foundation settlement and deformation are typically evaluated using principles of 

elastic behavior and/or Terzaghi consolidation theory. For most settlement and deformation analyses 

associated with coal refuse disposal facilities, stresses in foundations and embankments and changes in 

stress due to load application (e.g., embankment loading on the foundation) can be estimated using elastic theory. Closed-form equations and graphical methods for estimating stresses for a variety of geometric loading cases, homogeneous and layered subsurface profiles, and isotropic and anisotropic subgrade 

conditions are available in Poulos and Davis (1974). Methods for estimating settlement due to elastic 

compression and consolidation are described in numerous foundation engineering textbooks (Terzaghi 

et al., 1996; Hunt, 1986; Holtz and Kovacs, 1981) and design manuals (CGS, 2007; DOD, 2005).

Since the early 1970s, finite element (FE) methods that permit realistic deformation analysis of earthen 

embankments and foundations have been developed. As summarized by Duncan (1996), some of the 

special features of FE methods with application to embankments and foundations are:

OPEN CRACKS

PLAN VIEW A

OPEN CRACKS

PLAN VIEW B

FIGURE 6.54 CRACKING PATTERNS OBSERVED FOR DAMS IN NARROW VALLEYS

(FELL ET AL., 2005)

LONGITUDINAL SECTION A

EXAGGERATED CREST

SETTLEMENT

ROCK

PROJECTION

LONGITUDINAL SECTION B

OPEN OPEN CRACK

CRACKS

FIGURE 6.54 CRACKING PATTERNS OBSERVED FOR DAMS IN NARROW VALLEYS
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• Versatile tool for analysis of stresses and movements in earth masses, including:

‒ Stresses, deformations and pore pressures in embankments and foundations

‒ Conditions during construction such as consolidation and embankment 

compression due to self-weight

‒ Potential for cracking and hydraulic fracturing

• Can model nonlinear stress-strain behavior, non-homogeneous conditions, and 

changes in geometry such as embankment construction

• Software are available with graphical pre- and post-processors to facilitate data input 

and evaluate analysis results.

FE analyses require input data such as: (1) definition of the in-situ stress conditions of the foundation 

materials, (2) the stress-strain properties of the foundation and embankment materials, and (3) the 

sequence of construction.

For problems that involve a natural soil or rock deposit or an existing fill, the state of stress in the soil 

mass prior to the beginning of construction or loading must be specified because:

• For incremental analyses, the changes in stress calculated during each increment 

are added to the stresses at the beginning of the increment in order to evaluate the 

stresses at the end. To begin this process, it is necessary to know the initial in-situ 

stresses.

• The stiffness of the soil is a function of the stresses in the soil.

The in-situ stresses can be measured, but are usually estimated. For level ground where at-rest 

pressure conditions would be expected, the vertical stresses are usually assumed to be equal to the 

overburden pressure, and the horizontal stresses are assumed to be the at-rest lateral earth pressure 

coefficient Ko times the overburden pressure. The value of Ko is usually estimated based upon empirical relationships (Jaky, 1944; Mayne and Kulhawy, 1982). For initially sloping ground, one procedure 

that has been used is performing a gravity turn-on analysis (i.e., applying vertical forces representing 

the weight of the material to an initially unstressed mesh) and then changing the horizontal stresses 

to Ko times the calculated vertical stresses.

The stress-strain properties of modeled materials play a critical role in finite element analyses. 

For most deformation analyses of embankments and foundations where constituent materials 

are not stressed close to failure and strains are small, stress-strain behavior can be represented 

by a linear elastic model. However, for rock foundations, the modulus should reflect the deformation characteristics of the rock mass through modification of the deformation of intact (i.e., 

unfractured) rock using a rock mass classification system such as that proposed by Bieniawski 

(1989). The Bieniawski geomechanics classification system involves modification of the modulus 

of intact rock as determined from unconfined compression tests (Section 6.5.9.2) using the following relationship:

Em = 2 RMR – 100 (6-29)

where:

 Em = modulus of the rock mass (force/length2)

RMR = rock mass rating; this parameter accounts for the effects of intact rock 

strength, rock quality designation (RQD), joint spacing, joint condition, 

joint orientation and groundwater (dimensionless)
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RMR increases with rock quality with a range from 0 to 100. Equation 6-29 is valid for RMR ≥ 55. For softer 

rocks, the following relationship, which was proposed by Serafim and Pereira (1983), can be used:

Em = 10(RMR-10)/40 (6-30)

For soils it is often necessary to use stress-strain relationships that account for nonlinear behavior 

and the variation of soil modulus with confining pressure. Table 6.52 summarizes the types of stressstrain models typically used and their respective advantages and limitations.

TABLE 6.52 STRESS-STRAIN RELATIONSHIPS USED FOR FINITE ELEMENT 

DEFORMATION ANALYSES OF EMBANKMENTS AND FOUNDATIONS

Stress-Strain Relationship Advantages Limitations

Linear Elastic Simplicity Can only model real soil behavior at low 

stress levels and small strains

Multi-linear Elastic Can model any shape stress-strain 

curve for ductile materials

Must be developed on a case-by-case 

basis to approximate stress-strain 

behavior

Hyperbolic

Can model nonlinear behavior; parameters have physical significance and 

can be evaluated by triaxial testing

Inherently elastic; does not model plastic 

deformations in a fully logical manner

(ADAPTED FROM DUNCAN, 1996)

Analyses should simulate as closely as possible the actual construction or loading sequence associated with the structure being analyzed. This can be accomplished by adding elements to simulate fill placement, removing elements to simulate excavation, and applying loads in increments. 

Other processes that can be modeled in FE analyses include raising or lowering phreatic levels and 

consolidation.

Comparison of the results of FE analyses with field measurements shows there is a tendency for calculated deformations to be larger than measured deformations. Duncan (1996) offered the following 

reasons for this difference:

• Soils/materials in the field tend to be stiffer than laboratory test samples at the same 

density and moisture content due to aging effects.

• Average field densities are higher than the specified minimum dry density, which is 

often used as the target for preparing samples for laboratory testing.

• Soils/materials sampled in the field for laboratory testing are disturbed by the sampling process.

• Most field conditions approximate plane strain whereas triaxial tests are routinely 

used for laboratory characterization.

• 2D finite element analyses overestimate deformations of embankments constructed 

in V-shaped valleys with steep walls.

Analysis of embankments using the FE method has demonstrated the considerable potential of the 

approach and has identified the sources of uncertainty that engineers should be aware of. These 

uncertainties are primarily associated with difficulties in accurately predicting the in-place density 

and moisture content of soils/materials in the field and difficulties anticipating the sequence of operations that will be followed during construction.
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6.6.3.3 Deformation Control Measures

In a slurry impoundment, fine coal refuse typically has a low unit weight (as compared to typical soils), 

low hydraulic conductivity and low coefficient of consolidation. It is placed by peripheral discharge of lowsolids-content slurry, either by single-point discharge or using multiple discharge locations. Because of this 

method of placement, fine coal refuse tailings can be very soft and susceptible to long-term settlements.

The rate of consolidation at existing refuse disposal sites can be expedited by the installation of prefabricated vertical (PV) drains into the tailings at close vertical spacing to drain excess pore-water 

pressures. PV drains consist of a high-flow polymeric core wrapped in a non-woven geotextile. The 

drains are installed using static, vibratory or jetting methods. Guidelines for the engineering design 

of PV drains are presented in Rixner et al. (1986). Brown and Greenaway (1999) describe instances 

where PV drains were used to expedite consolidation of uranium mill tailings before construction 

of a clay cap required for abandonment. Prefabricated drains have also been placed horizontally on 

the surface of a coal refuse impoundment prior to upstream construction to speed consolidation and 

dissipation of pore pressures (Thacker et al., 1988). The effectiveness of the drain installations was 

demonstrated by the control of pore pressures and consolidation. Drilled horizontal drains have also 

been used to lower pore-water pressures in tailings impoundments.

Drainage measures to accommodate dissipation of pore pressures and to consolidate fine materials 

such as fine coal refuse can be incorporated into facility design, as discussed in Section 6.3.

Adaptations of shallow and deep soil mixing technologies developed for improving loose and soft 

soil deposits have been used for the in-place solidification of fine coal refuse. Bazán-Arias et al. (2002) 

describe the use of these methods to stabilize fine coal refuse using custom-designed equipment to 

blend a cement/fly ash slurry with coal refuse for supporting a highway embankment over a slurry 

pond. QC testing of cured samples resulted in a 28-day, unconfined compressive strength greater 

than 100 psi and peak shear strength parameters of φ’ > 45° and c’ > 13 psi.

Lime has also been used to stabilize fine coal refuse. Lime stabilization causes the refuse to behave 

as an overconsolidated material. However, when load is applied that exceeds the apparent maximum past consolidation pressure, the “stabilized” refuse tends to collapse and return to its previous 

“unstabilized” behavior. Therefore, it is recommended that potential use of lime stabilization of fine 

coal refuse be carefully evaluated through laboratory and field performance testing before implementation in the field.

6.6.3.4 Deformation Measurements

When predicted settlements are an important factor in the design of a coal refuse disposal facility, 

estimates of the magnitude and the rate of settlement should be made using sophisticated analyses 

with the best available data. The performance of a coal refuse disposal facility should be monitored 

through installation of an instrumentation system and comparison of observed data to predicted 

deformations. Depending on site conditions and project requirements, deformations and deformation rates can be monitored by:

• Surface monuments (vertical and horizontal displacements)

• Settlement gages and extensometers (vertical displacements)

• Inclinometers (lateral displacements in slopes)

• Piezometers (piezometric heads in the embankment and foundation)

As described in Chapter 13, instrumentation should be installed to verify that acceptable levels of 

performance are being achieved and to provide a check on design assumptions.
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6.6.4 Slope Stability Analysis

6.6.4.1 Conditions Requiring Stability Analysis

The design of a new coal refuse disposal facility or the expansion or modification of an existing 

facility requires that the stability of compacted embankments and natural soil and rock slopes and 

foundations be assessed. The most critical cross sections and cases must be analyzed. Factors considered in selecting the most critical cross sections include: (1) slope of the embankment, (2) height of 

the embankment, (3) foundation conditions, (4) pore-pressure conditions, and (5) presence of lower 

strength material zones within the embankment/impoundment (e.g., upstream construction cross 

sections). Critical cross sections can include upstream or downstream embankment slopes. Important cases include the following:

• Long-term or final embankment configuration

• Intermediate stages of development that may include critical cross sections 

related to slope, height, foundation, embankment material properties, or 

phreatic levels or pore-pressure conditions

• Short-term, end-of-construction conditions for stages on soft, compressible 

materials

• Rapid loading of fine coal refuse deposits during initiation of upstream 

construction that possibly leads to an unacceptably low factor of safety 

against bearing failure of the fine refuse underlying the area of embankment raising

• Rapid drawdown of the impoundment

• Seismic loading and strength loss or increases in pore pressure

Phreatic levels and pore-pressure conditions should be determined on the basis of seepage analyses 

and, for existing facilities, by correlations with piezometric measurements.

In selecting an upstream construction cross section for analysis, the interface between the coarse 

refuse embankment and fine refuse deposits should be determined based on subsurface exploration 

(borings and cone penetration tests are recommended) if an existing facility is being analyzed. For 

new facilities, analysis cross sections should be determined based on staging calculations considering 

upstream construction procedures and fine coal refuse behavior. Staging calculations will identify 

the approximate level of the settled fine refuse at the initiation of upstream construction. The following facility-specific issues must be considered:

• Type of settled fine coal refuse that will support the upstream stage (e.g., sandy or 

clayey fine refuse)

• Presence and location of impoundment pool relative to the extent of the upstream 

push out

• Staging area for coarse coal refuse to be used for the push out

• Presence of excess pore-water pressures in areas where the fine refuse is not fully 

consolidated

• Equipment and lift thickness that can be used for the push out

• Monitoring program that can be implemented and used in controlling the push out

The behavior of fine coal refuse in response to upstream construction may include consolidation, 

mixing with the coarse refuse during the initial push out, and development of a zone of assimilation. Appropriate material strengths and levels of excess pore-water pressure need to be used 
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in analyses. Experience at similar facilities can provide valuable insight into material behavior, 

construction procedures to be employed, and the resulting zone of mixing. The strength in this 

zone of mixing should be determined based on the relative properties of the coarse and fine refuse 

materials at the site. In some cases, an exploration program at an existing similar facility might be 

undertaken for determining the desired properties. Typically, this zone of mixing does not play 

a significant role in the static analysis of downstream embankment slopes, but can be critical for 

seismic analysis and upstream slopes. The following guidance has been developed for deciding 

whether potential upstream failure surfaces are critical to the seismic stability and deformation of 

the embankment (MSHA, 2007):

• Potential upstream slope stability failure surfaces that terminate on the crest of the 

embankment stage should provide an acceptable factor of safety such that the integrity of the embankment and impounding capacity of the facility are maintained (i.e., 

if a portion of the embankment becomes unstable, a sufficient section of the crest will 

remain intact to prevent release from the impoundment).

• Potential deformation of the crest of the embankment should not result in the threat 

of a release from the impoundment (i.e., sufficient freeboard must be available to 

compensate for the maximum amount of crest settlement).

Another critical case where fine coal refuse characteristics are important is related to recovery or 

re-mining of fine coal refuse within an impoundment. This situation typically involves the development of an excavation plan with interim and final coal refuse slopes that must have acceptable 

factors of safety.

Additional conditions requiring slope stability analyses may arise due to situations in which the 

shear strength decreases or stress level in an embankment increases. Duncan and Wright (2005) cite 

the following causes for a decrease in shear strength:

• Increased pore pressure (reduced effective stress) due principally to a rise in groundwater level or increased seepage during periods of heavy rainfall

• Cracking near the crest of a slope due to tension and factors such as soil desiccation

• Swelling of highly plastic and heavily overconsolidated clays

• Development of slickensides due to shear in highly plastic clays resulting from shear 

on distinct slip planes

• Decomposition of rock in fills due to inadequate breakdown during compaction and 

weathering as a result of wetting and drying

• Creep of highly plastic clays under sustained load

• Leaching of chemical constituents in the soil matrix

• Strain softening of brittle soils leading to progressive failure

• Weathering of rocks and indurated soils due to physical, chemical and biological 

processes

• Cyclic loading and loss of strength due to liquefaction (Chapter 7)

Possible causes for an increase in shear stress include:

• Increased loads at the top of a slope

• Water pressure in tension cracks at the crest of a slope

• Increase in soil weight due to increase in moisture content
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• Excavation at the bottom of a slope

• Rapid drawdown of an impoundment (upstream slope)

• Earthquake loading

Traditionally, slope stability has been evaluated using limit equilibrium analyses whereby the forces 

tending to decrease stability are compared to the forces tending to increase stability. These types of 

analyses are generally conducted using limit equilibrium slope stability computer programs. Since 

the early 1970s, finite element methods of analysis have improved to the point where realistic stability/deformation analysis of soil slopes is possible. The following section provides an overview of 

stability analyses for coal refuse disposal facility embankments.

6.6.4.2 Methods of Stability Analysis

6.6.4.2.1 Limit Equilibrium Stability Analysis

The stability of refuse embankments is usually solved by limit equilibrium methods of analysis. These 

analyses are conducted by calculating the minimum factor of safety (FS) for a slide surface through 

the slope as follows:

FS = Available shear strength = 

s (6-31)

 Equilibrium shear stress τ

If a large number of potential slide plane surfaces are assumed, the surface with the minimum factor 

of safety is a numerical representation of the relative safety of the slope. If FS = 1, a slope is in a state of 

“just-stable” limit equilibrium. Because of the uncertainty related to the geometry of the actual slide 

plane surface, the controlling soil properties, the pore-pressure distribution in the slope, and other 

factors that may affect the stability of a slope, slopes for water impounding embankments should 

be designed with FS equal to at least 1.5. A higher factor of safety should be used where factors that 

affect slope stability (e.g., limited testing has been performed) are less certain. The available shear 

strength (s) is defined in terms of the angle of friction (φ or φ’) and cohesion (c or c’) of the soil along 

the slide plane surface using soil properties determined from in-situ or laboratory tests.

Two approaches can be used to satisfy static equilibrium of a slope. The first and much less commonly used approach is to assume a single free-body bounded by the face of the slope and slide 

plane surface. Examples of this approach are the infinite slope, log spiral and Swedish slip circle 

methods. The second approach involves dividing the slope into a number of vertical slices that 

extend between the face of the slope and slide plane surface. Examples of this approach are the 

ordinary method of slices, simplified Bishop method and Spencer’s method. Regardless of the 

approach used there are more unknowns (e.g., forces, location of forces, FS) than equilibrium equations, so the problem is statically indeterminate. Therefore, assumptions must be made to render 

the problem determinate. Examples of such assumptions include inclination of interslice forces, the 

location of the normal force at the base of a slice and the relationship of interslice shear force to the 

interslice normal force.

Some slope stability analysis methods are based solely upon force-equilibrium principles, while other 

methods involve satisfaction of all conditions of equilibrium. The characteristics of various equilibrium methods for slope stability analysis are summarized in Table 6.53. If force-equilibrium methods 

are used (Lowe and Karafiath, 1960; USACE, 2003), the factor of safety is affected significantly by the 

assumed inclinations of the side forces between slices. Thus, force-equilibrium procedures are not 

as accurate as methods that satisfy all conditions of equilibrium (Janbu, 1973; Spencer, 1967; Morgenstern and Price, 1965). The maximum range of results for methods that satisfy all conditions of 

equilibrium is generally less than 12 percent. Thus, with an average accuracy of about plus or minus 

6 percent, a factor of safety calculated using procedures that satisfy all conditions of equilibrium can 
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be considered to be acceptably accurate, because for practical purposes key parameters such as slope 

geometry, unit weight, shear strength, and pore-water pressure cannot be defined with an accuracy of 

plus or minus 6 percent. Therefore, any method that satisfies all conditions of equilibrium should be 

sufficiently accurate for impoundment design and analysis. Additional information relative to selection of a slope stability analysis method can be found in Duncan and Wright (1980).

For slopes composed of nearly homogeneous materials, both analysis and observation of actual 

failures indicate that the failure surface can be approximated with sufficient accuracy by a circular 

arc. For such cases, procedures that do not satisfy all conditions of equilibrium may be acceptably 

TABLE 6.53 CHARACTERISTICS OF EQUILIBRIUM PROCEDURES 

FOR SLOPE STABILITY ANALYSIS

Procedure Application

Infinite Slope Homogeneous cohesionless slopes where stratigraphy restricts slip surface to 

shallow depths and parallel to slope face. Very accurate where applicable.

Logarithmic Spiral Applicable to homogenous slopes; accurate. Potentially useful for developing 

slope stability charts and used in some software for design of reinforced slopes.

Swedish Circle Applicable to slopes where φ = 0 and for relatively thick zones of weaker 

materials where slip surface can be approximated by a circle.

Ordinary Methods of Slices 

(Fellenius, 1922)

Applicable to non-homogeneous slopes and c - φ soils where slip surface 

can be approximated by a circle. Very convenient for hand calculations, but 

inaccurate for effective stress analyses with high pore pressures.

Simplified Bishop

(Bishop, 1955)

Applicable to non-homogeneous slopes and c - φ soils where slip surface can 

be approximated by a circle. More accurate than OMS, especially for effecttive 

stress analyses with high pore pressures. Calculations can be performed by 

hand or spreadsheet.

Force Equilibrium Methods

(Lowe and Karafiath, 1960; USACE, 

2003)

Applicable to virtually all slope geometries and soil profiles. The only procedure 

suitable for hand calculations with non-circular slip surfaces. Less accurate 

than complete equilibrium procedures, and results are sensitive to assumed 

inclination for interslice forces.

Janbu Generalized

Procedure of Slices

(Janbu, 1973)

Satisfies all conditions of equilibrium. Applicable for any shape of slip surface. 

Numerical problems are encountered more frequently than with some other 

methods.

Spencer

(Spencer, 1967)

Satisfies all conditions of equilibrium. Accurate procedure applicable to virtually 

all slope geometries and soil profiles. Simplest complete equilibrium procedure 

for computing factor of safety.

Morgenstern and Price (1965)

Satisfies all conditions of equilibrium. Accurate procedure applicable to virtually 

all slope geometries and soil profiles. Rigorous, well-established complete 

equilibrium procedure.

Chen and Morgenstern (1983)

Satisfies all conditions of equilibrium. An updated Morgenstern and Price 

procedure. Rigorous and accurate procedure applicable to any slip surface 

shape and slope geometry, loads and soil profiles.

Sarma (1973)

Satisfies all conditions of equilibrium. Accurate procedure applicable to 

virtually all slope geometries and soil profiles. Convenient complete equilibrium 

procedure for computing seismic coefficient required to produce a given factor 

of safety. Side force assumptions are difficult to implement for all but simple 

slopes.

(DUNCAN AND WRIGHT, 2005)
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accurate. For non-homogeneous slopes or embankments, embankments supported on a foundation 

with a weak zone and impoundments lined with geomembranes or geosynthetic clay liners, limitequilibrium procedures that are suitable for any shape slip surface and that satisfy all conditions of 

equilibrium must be used. Designers should be especially alert to the presence of a weak layer or 

layers upon which sliding may occur. In such cases, the factor of safety for wedge-type surfaces coincident with the weak layer must be evaluated in addition to the usual failure surfaces. If movement 

has already occurred in a zone of material that is included in a stability analysis, then residual shear 

strength may be applicable.

An implicit assumption in equilibrium analyses of slope stability is that the stress-strain behavior of the constituent material is ductile (i.e., it does not have a brittle stress-strain curve where 

the shearing resistance drops off after reaching a peak). This limitation results from the fact that 

limit-equilibrium methods provide no information regarding the magnitudes of the strains within 

a slope, nor any indication about how they may vary along the slip surface. Therefore, unless the 

strengths used in the analysis can be mobilized over a wide range of strains (i.e., the soil exhibits 

ductile stress-strain behavior) there is no guarantee that the peak strength can be mobilized simultaneously along the full length of the slip surface. Where multiple embankment or impoundment 

zones are traversed by the slip surface, strain compatibility for each material should be evaluated. 

For instance, coarse coal refuse typically mobilizes peak strength at a lower strain than fine coal 

refuse and cohesive foundation soils. Thus, the stability analysis should be based on strength at 

compatible strains, particularly if there is a drop-off in strength with large strains. If the shearing 

resistance of one material drops off after the peak is reached, progressive failure can occur, and the 

shearing resistance that can be mobilized at some parts of the failure surface may be smaller than 

the peak strength. For this situation, a reliable approach is to use the residual strength rather than 

the peak strength in the analysis.

For coal refuse, earth and rockfill embankments, the following critical embankment conditions should 

be evaluated:

1. High pore-water pressures are present in both the embankment and foundation. 

This condition occurs most often during or at the end of construction, particularly if 

construction is rapid, the slope materials have low hydraulic conductivity, and construction conditions are wet. For a coal refuse embankment, the rate of construction 

usually is not fast enough to cause high pore pressures in the foundation materials. 

An exception is when a thick layer of saturated clay underlies the embankment. For 

this case pore pressures during construction should be estimated, and piezometers 

should be installed to facilitate maintaining pore pressures within acceptable limits 

during construction. The rate of construction can also be an issue if an upstream 

construction pushout is constructed rapidly. Stability checks may be required both 

during construction and at the end of construction when an embankment is constructed over settled fine refuse using the upstream method.

2. Steady seepage has developed within the embankment and may have saturated a 

large part of the downstream slope. This condition occurs most often after long-term 

operation of an impounding embankment at full storage level, particularly if the 

slope materials had a high hydraulic conductivity. For compacted embankments, 

placement of refuse is usually slow enough that excess pore pressures will adequately dissipate. For situations where wet materials are placed in thick lifts, (e.g., 

filter cake or combined refuse in upstream embankment zones) excess pore pressures can develop, although generally the rate of construction is slow enough that 

the excess pore pressures will dissipate. Some slurry impoundments are designed to 
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store the runoff from the design storm and release it relatively slowly. In such cases, 

the water level may rise above the level of the slurry delta and be in direct contact 

with relatively permeable upstream slope material. If storm water is impounded 

against the upstream slope long enough for steady-state pore-water pressures to 

develop, this condition could represent a critical stability scenario.

3. The impoundment water level drops very quickly after steady seepage has developed within the embankment. This condition is generally referred to as rapid drawdown and can be critical to the stability of the upstream slope of the embankment. 

USBR (1987a) provides general guidance for water-detention and storage dams 

considering susceptibility of earth fill materials (based on USCS classification) to 

rapid drawdown loading (drawdown rate of 6 inches or more per day following 

prolonged storage at high reservoir levels). For most coal refuse embankments that 

impound slurry and runoff water, rapid drawdown is either not possible or is not 

an issue because the embankment material is generally free draining. For embankments constructed of low-hydraulic-conductivity material (less than 10-4 cm/sec) 

and designed to store storm runoff for subsequent release through a decant pipe, the 

potential effects of rapid drawdown should be considered. Another situation where 

rapid drawdown may need to be considered is during remining of an impoundment 

for recovery of additional coal from the fine coal refuse.

4. The embankment is subjected to earthquake loading during embankment construction, operation, or following abandonment. Issues related to the analysis and 

design of embankments that are subjected to earthquake loading are discussed in 

Chapter 7.

Analyses for the first three critical stability conditions listed above must reflect the rate of construction and pore-pressure conditions. The following analyses are typically employed:

• Total-Stress Analysis is used in situations where the pore-water pressure (u) that 

would act on the potential failure surface at failure is unknown and cannot be reliably estimated. The embankment stability is analyzed in terms of total stress (i.e., 

the stress between the individual soil grains plus the pore pressure). This method of 

stability analysis is generally considered most appropriate for evaluating relatively 

short-term loading conditions such as end-of-construction and rapid drawdown of 

the impoundment.

• Effective-Stress Analysis is used in cases where the pore-water pressure (u) that 

would act on the potential failure surface at failure is known or can be reliably 

estimated. The embankment stability is analyzed in terms of effective stress (i.e., the 

total stress minus the pore pressure). This method of stability analysis is generally 

considered most appropriate for evaluating long-term conditions after the transient 

effects related to construction and seepage have ended.

Selection of the appropriate conditions for analysis requires knowledge of soil behavior under 

drained and undrained conditions and evaluation of the conditions that will control drainage in the 

field. Shear strengths, water and pore pressures, and unit weights for slope-stability analyses are 

summarized in Table 6.54.

A useful guide for determining whether total- or effective-stress methods of analysis are applicable 

relates to whether the soils comprising the foundations and refuse embankment are free draining 

or impermeable. Free-draining soils are those able to drain completely during the construction or 

loading period. Impermeable soils are those that cannot drain completely during the construction or 
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loading period. Duncan (1996) recommends using the dimensionless time factor T from consolidation theory to estimate the degree of drainage that can occur during construction or loading using 

the relationship:

T = cv t/D2 (6-32)

where:

cv = coefficient of consolidation (length2/time)

 t = time

D = drainage path (length)

If T > 3, the material can be treated as drained. If T < 0.01, the material can be treated as undrained. 

If 0.01 < T < 3, both drained and undrained conditions should be evaluated. Duncan (1996) suggests 

that, if the data needed to calculate T are not available, soils with hydraulic conductivity k > 100 feet/

year can be considered drained and soils with k < 0.1 foot/year can be considered undrained.

Undrained conditions should be analyzed in terms of total stress in order to avoid having to rely on 

estimated, and sometimes unreliable, pore pressures for undrained loading conditions. Undrained 

strength can be determined using in-situ tests (e.g., vane shear), UU triaxial tests, or CU tests in conjunction with a strength normalizing procedure such as the SHANSEP (stress history and normalized 

soil engineering parameters) procedure (Ladd and Foott, 1994). For multi-stage construction (e.g., 

upstream pushouts), the undrained strength can be estimated using CU triaxial test results together 

with values of consolidation pressure determined from consolidation analyses (Ladd, 1991).

Drained conditions can be analyzed in terms of effective stresses using c’ and φ’ from drained triaxial 

or direct shear tests or from CU tests. Direct-shear or CU tests are more often used when testing clays, 

TABLE 6.54 SHEAR STRENGTHS, WATER PRESSURES, AND UNIT WEIGHTS 

FOR SLOPE-STABILITY ANALYSES

Soil Type Parameter

Condition

End-of-Construction Multi-stage Loading(1) Long-Term

All soils External Water 

Pressures Include Include Include

All soils Unit weights Total Total Total

Free-draining Shear Strength Effective stress c′

and φ′

Effective stress c′

and φ′

Effective stress c′

and φ′

Free-draining Internal Pore 

Pressures

u from steady-state 

seepage analyses

u from steady state 

seepage analyses

u from steady state 

seepage analyses

“Impermeable” Shear Strength

Total stress c and φ

from in-situ or UU or 

CU lab tests

Total stress c and φ

from in-situ or UU or

CU lab tests

Effective stress c′

and φ′

“Impermeable” Internal Pore 

Pressures

No internal pore 

pressures, set u = 0

in computer input

No internal pore 

pressures, set u = 0

in computer input

u from steady state 

seepage analyses

Note: 1. Multi-stage loading includes rapid drawdown, staged construction, and any other condition where a

 period of consolidation under one set of loads is followed by a change in load under undrained conditions.

(DUNCAN, 1996)
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because the time required for testing is shorter than for conducting CD tests. Values of c’ and φ’ from 

CU tests have been found to be nearly identical to values obtained from drained tests. Values of φ’ for 

natural deposits of cohesionless soils are usually estimated using correlations with SPT or CPT data.

In performing embankment stability analyses, the strength behavior of coal refuse must be carefully 

evaluated. There is evidence that the failure strength envelope of coarse refuse has considerable curvature in the stress range associated with high embankments due to crushing of the refuse particles. 

Thus, the cohesion and friction angle of the coal refuse may vary depending upon location in the 

embankment. Application of a bi-linear failure model may be appropriate for such cases.

TABLE 6.55 GUIDELINE FRICTION VALUES AND EFFICIENCIES FOR VARIOUS 

GEOSYNTHETIC AND SOIL COMBINATIONS

Soil-to-Geomembrane Friction Angle

Geomembrane

Soil Types

Concrete Sand

(φ = 30°)

Ottawa Sand

(φ = 28°)

Mica Schist Sand

(φ = 26°)

PVC

 Rough 27° (0.88)(1) 25° (0.96)

 Smooth 25° (0.81) 21° (0.79)

CSPE 25° (0.81) 21° (0.72) 23° (0.87)

HDPE 18° (0.56) 18° (0.61) 17° (0.63)

Geomembrane-to-Geotextile Friction Angle

Geotextile

Geomembrane

PVC

CSPE HDPE Rough Smooth

Nonwoven, Needle-Punched 23° 21° 15° 8°

Nonwoven, Melt-Bonded 20° 18° 21° 11°

Woven, Monofilament 11° 10° 9° 6°

Woven, Slit Film 28° 24° 13° 10°

Soil-to-Geotextile Friction Angle

Geotextile

Soil Types

Concrete Sand

(φ = 30°)

Ottawa Sand

(φ = 28°)

Mica Schist Sand

(φ = 26°)

Nonwoven, Needle-Punched 30° (1.00) 26° (0.92) 25° (0.96)

Nonwoven, Melt-Bonded 26° (0.84) – –

Woven, Monofilament 26° (0.84) – –

Woven, Slit Film 24° (0.77) 24° (0.84) 23° (0.87)

 Note: 1. Efficiency values in parentheses are based on the relationship E = tan δ / tan φ

(ADAPTED FROM KOERNER, 2006)
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For sites where impoundments are lined with geomembranes or geosynthetic clay liners (GCLs) to 

control seepage, the potential for slope failure between the liner and subgrade and between the liner 

and soil cover placed over the liner may need to be evaluated. Table 6.55 provides guideline friction 

angles that may be appropriate for preliminary design for various interfaces. However, final design 

should be based on more refined published data and manufacturers’ information for the specific geosynthetic materials under consideration, as well as laboratory interface testing between on-site soils 

and selected geosynthetic materials to simulate site-specific conditions.

When GCLs are used on slopes, their friction properties are important. Sodium bentonite, which is 

often used in GCLs, is a clay with a saturated, drained residual internal angle of friction of approximately 6° to 9°. However, significantly greater friction angles may be appropriate in GCLs that are 

needle-punched or stitched. Manufacturers should be consulted for design data, as these data may 

be product specific.

An important part of slope stability analysis is determining the slip surface with the lowest FS. Most 

computer programs that use an assumed circular failure surface systematically change the position 

of the center of the circle and the length of the radius to find the most critical (lowest FS) circle. For 

more complex geometries typical of most real-world situations, local minima may exist. Therefore, 

multiple searches should be conducted using multiple starting points and search strategies so that 

the overall minimum value of FS is determined. The results of slope stability analyses should be carefully examined to verify that the upstream and downstream limits of the search are not so restrictive as to exclude potentially critical failure surfaces. Locating a critical noncircular surface is more 

complex, and a variety of approaches have been developed. Methods such as random generation 

of kinematically admissible slip surfaces, coupled dynamic programming minimization techniques, 

and optimization have been used successfully to model slopes that do not have extremely complicated geometries. Regardless of the computational procedure used, tests of reasonableness should be 

applied to the results, and multiple searches should be performed to be certain that the critical slip 

surface has been located. Failure surfaces through weaker embankment or foundation layers should 

always be considered.

Most slope stability problems can be modeled in two dimensions because the geometry of a slope is 

typically relatively constant along its length. However, some slopes are: (1) curved in plan or contain corners (e.g., some diked embankments), (2) subjected to loads of limited extent at the top, or 

(3) constrained by physical boundaries such as a dam in a narrow-walled valley. For these situations, consideration may be given to conducting a three-dimensional (3D) limit-equilibrium analysis. 

Duncan (1996) reports that the factor of safety for 3D analysis is greater than the factor of safety for 

2D analysis (i.e., FS3D > FS2D) provided that: (1) FS2D is calculated for the most critical two-dimensional cross section of the slope and (2) the procedure used for 2D limit-equilibrium analysis satisfies 

all conditions of force and moment equilibrium. If 2D analyses of refuse embankment stability meet 

these criteria, a 3D analysis is not generally warranted.

Some limit-equilibrium computer programs include features that permit a probabilistic analysis of 

slope stability. Some common characteristics of these programs include:

• Simulation techniques (e.g., Monte Carlo) that allow the program to repeatedly 

sample values from probability distributions of the uncertain variables

• Modeling of input parameters as random variables (e.g., material properties, 

phreatic surface location, seismic load coefficient)

• Defining the probability density function of the random variables in terms of 

statistical distributions commonly used in geotechnics (e.g., normal, exponential, 

lognormal)
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• Using truncated distributions to define maximum and/or minimum values

• Defining correlation coefficients between correlated values (e.g., c and φ)

• Presentation of results in a variety of forms (e.g., histograms, cumulative plots, 

scatter plots)

Thus, probabilistic slope stability analysis accounts for variability and uncertainty associated with 

traditional limit-equilibrium methods. Figure 6.55 illustrates the results of a stability analysis of a 

cohesive soil slope using Spencer’s method. The figure shows a histogram of frequency distribution 

of the FS computed for 5,000 random analyses. The red bars at the left of the distribution represent 

analyses where FS is less than 1. From the histogram, the mean FS = 1.072, and the maximum and 

minimum FS are 1.298 and 0.860, respectively. However, Figure 6.55 illustrates another advantage of 

probabilistic analysis. The ratio of the 658 analyses where FS is less than 1 to the 5,000 total analyses 

is the probability of failure pf. For this analysis, pf = 13.2 percent, which provides a measure of the 

potential for failure separate from the factor of safety. For this case, the analysis shows that a low 

average FS results in a high probability of failure.

Note that no value of pf is recommended for design of embankment dams at the time of publication of this Manual, although proposed risk evaluation criteria and guidelines for significant- and 

high-hazard-potential dams are available (Von Thun, 1996). El-Ramly et al. (2003) present a probabilistic stability analysis of a tailings dike along with a general spread sheet model, and they note 

that computed values of pf for existing dams demonstrating satisfactory performance may not meet 

recommended values. However, a probabilistic analysis is useful in understanding the contribution 

of parameters affecting stability and in comparing conditions and configurations for establishing reliable design. Probabilistic acceptance criteria will become established as more analyses are conducted

and results published for both failed and satisfactorily performing slopes.

0.9 1.0 1.1 1.2

0

1

2

3

4

5

6

7

SPENCER FACTOR OF SAFETY

RELATIVE FREQUENCY

FIGURE 6.55 HISTOGRAM OF FACTOR OF SAFETY DISTRIBUTION
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Additional information relative to probabilistic methods for the stability analysis of slopes is provided in Hoek (2000) and Baecher and Christian (2003).

6.6.4.2.2 Stability/Deformation Analysis Using Finite Element Methods

Slope stability analyses are traditionally performed using classical techniques such as the method of 

slices. These approaches are based on the assumption of rigid-plastic stress-strain behavior with no 

deformation occurring prior to failure. However, if elastic deformations occur prior to failure, the 

finite element method can be used to solve the elastic differential equations, with failure defined by 

placing a limit on stresses using the Mohr-Coulomb (typical) or other failure criterion. The stressstrain behavior of the embankment material should be modeled using the simplest representation 

possible that is appropriate for the problem analyzed (Duncan, 1996). However, while simple linear 

elastic, multi-linear elastic or hyperbolic models may be appropriate for analyzing stress states well 

prior to failure, more complex stress-strain models (e.g., elasto-plastic and elasto-viscoplastic) are 

required to analyze slope behavior near failure.

The elasto-plastic and elasto-viscoplastic FE approach to slope stability analysis offers the following 

advantages over traditional methods (Griffiths and Lane, 1999):

• No a priori assumptions are needed relative to the shape or location of the failure 

surface. Failure occurs “naturally” through zones in the soil mass where the shear 

strength is unable to support the gravity-induced shear stresses.

• Because there is no concept of slices in the finite element approach, there is no need 

for assumptions about slice side forces, and the finite element method preserves 

global and local equilibrium until failure is reached.

• The finite element method can indicate progressive failure up to and including overall 

shear failure.

Slope stability analysis using the finite element method requires the following steps:

• Gravity loads are applied to the slope.

• An elastic analysis is performed to compute stresses.

• Stresses in each element are compared with the Mohr-Coulomb failure criterion.

• If stresses exceed the Mohr-Coulomb criterion, they are redistributed to neighboring 

elements that still have reserve strength.

• Slope failure occurs if stress redistribution cannot be accomplished to satisfy the 

Mohr-Coulomb criterion and global equilibrium. Failure is indicated by significantly 

increased nodal displacements.

Finite element analyses are not commonly used for the design of refuse embankments, but 

have application for the analysis of embankments where unexpectedly large deformations are 

observed or where unusually soft foundations may lead to unacceptable plastic deformations or 

differential settlements. Additionally, finite element analyses may prove to be a useful tool for 

evaluation of the effects of deformation on embankment stress and for the evaluation of mine 

subsidence effects.

6.6.4.3 Acceptable Factors of Safety

Selection of an acceptable factor of safety for the analysis or design of an embankment slope depends 

on the degree of uncertainty in calculating FS and the hazards or consequences should a slope fail. 

The calculation of the factor of safety for a coal refuse disposal facility embankment involves evaluation of many factors, including:
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• Uncertainties about the type and extent of sampling and testing to determine 

material strengths

• The variation of materials in the embankment and impoundment

• Uncertainties of embankment geometry

• The type of embankment (e.g., new or existing, impounding or non-impounding, 

constructed by upstream method or downstream method)

• The level of uncertainty concerning the location of the phreatic level and related 

excess pore-water pressures

• Embankment location (e.g., in a V-shaped valley, high seismic region)

• The accuracy of the stability analysis method(s) used

• Potential for future loads

• Potential for future changes in disposal operations and practices

The hazards or consequences associated with refuse embankment failure include:

• Potential for loss of life and/or property damage on site

• Potential for loss of life and/or property damage off site

• Economic cost to mining operations if disposal operations are lost

• Economic cost associated with restoring or safely abandoning disposal operations

• Economic cost of restoring environmental damage

The magnitude of the factor of safety to be used for coal refuse disposal facility design should be 

determined by the designer considering state and federal regulatory requirements and the completeness and accuracy of designer’s knowledge of the uncertainties and conditions identified previously. 

If these uncertainties and conditions are well defined, or if conservative assumptions have been made, 

a lower FS may be acceptable. Conversely, when many assumptions are made relative to forces and 

material strengths, a higher FS is appropriate.

In selecting an acceptable FS for the stability of embankment slopes for coal refuse disposal facility 

design, the practices of major engineering organizations, both governmental and private, should 

be considered. Most earth dams in the U.S. are designed based upon extensive laboratory test 

information and clearly identified loads and geometry. For these structures, a FS ≥ 1.5 is generally 

considered to be acceptable for permanent or sustained loading conditions. For special circumstances, such as soft embankment foundations, a higher FS is sometimes adopted to limit foundation or embankment deformations. For temporary loading conditions during construction, a lower 

FS is often acceptable. For transient loads, such as earthquakes, a FS of 1.2 is generally acceptable depending on the design earthquake magnitude, as discussed in Chapter 7. Minimizing the 

volume of material used for construction is not usually a design goal for refuse embankments as it 

is for earthen dams; thus, it may be possible to achieve somewhat higher factors of safety for refuse 

embankments without introducing special materials or operations, and the designer may be able 

to accommodate concerns about uncertainty of engineering properties or loads.

Table 6.56 presents recommended minimum factors of safety for the design of coal refuse embankments based on values adopted by the cited federal agencies (USSD, 2007). These values are 

generally consistent with factors of safety cited in the MSHA Impoundment Inspection and Plan 

Review Handbook (MSHA, 2007) and have been adopted by many state regulatory agencies.

The values of FS provided in Table 6.56 apply if the following conditions are met:
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TABLE 6.56 RECOMMENDED MINIMUM FACTORS OF SAFETY FOR DESIGN 

OF COAL REFUSE EMBANKMENTS

 Condition Design Basis Factor of 

Safety Source(1)

Long-term stability 

analysis with maximum 

storage pool

Design based on shear strength measured in laboratory and/

or field testing program reflecting long-term site development 

with steady-state seepage and maximum storage (operational) 

pool.

1.5

USACE, 

USBR, 

NRCS, 

FERC

Long-term stability 

analysis with maximum 

surcharge pool

Design based on shear strength measured in laboratory and/

or field testing program reflecting long-term site development 

with steady-state seepage and maximum surcharge (design 

storm) pool.

1.4 USACE, 

FERC

Intermediate-stage 

static analysis with 

maximum storage pool

Design based on shear strength measured in laboratory and/

or field testing program reflecting intermediate-stage critical 

configurations using long-term analysis with steady-state 

seepage and maximum storage (operational) pool.

1.5

Intermediate-stage 

static analysis with 

maximum surcharge 

pool

Design based on shear strength measured in laboratory and/

or field testing program reflecting intermediate-stage critical 

configurations using long-term analysis with steady-state 

seepage and maximum surcharge (design storm) pool.

1.4

Short-term, end-ofconstruction, undrained 

static analysis 

Design based on intermediate- or long-term configurations 

and supported by shear strength measured in laboratory and/

or field testing program using undrained strength parameters, 

as appropriate.

1.3

USACE, 

USBR, 

FERC, TVA

Rapid-drawdown 

analysis with maximum 

storage pool

Design based on intermediate or long-term configurations and 

supported by shear strength measured in laboratory and/or 

field testing program using drained or undrained analysis.

1.3 USACE, 

USBR

Seismic analysis Discussed in Chapter 7. – 

 Note: 1. From USSD (2007).

• The critical failure surface has been determined from stability analyses 

based on systematic searches and evaluation of defined planes of weakness.

• Stability analysis parameters are, with reasonable certainty, known to be 

representative of the actual conditions that will exist in the embankment.

• Sufficient control will be provided during construction to verify that materials placed within the embankment conform to the standards assumed or 

required by the disposal facility design.

• When pore-water pressure in an embankment and its foundation is a 

significant factor in the stability of the embankment, piezometers are 

installed and monitored, and the observed data are compared to design 

assumptions.

If an existing coal refuse disposal embankment is found to have a low FS, it may not be possible to 

modify the site sufficiently within a short time period to satisfy minimum FS criteria. In such cases, a 

slight, temporary reduction in the FS may be acceptable provided that:

• Monitoring of pore pressures in the embankment and movements of the embankment surface is performed on a scheduled basis.
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• A plan to improve the FS is developed and implemented (e.g., the impoundment 

level is lowered, the embankment upstream face is sealed to reduce seepage, pore 

pressures are measured, or other steps to reduce the impounding capacity or to 

implement abandonment are initiated).

When test data for embankment and foundation materials are limited or available test data are inconsistent, either conservative values of shear strength and pore-water pressure should be used in the 

stability analysis or an increased FS should be used in the embankment design.

6.6.4.4 Stability Control Measures

The most effective measures for stabilizing coal refuse embankments and other slopes are: (1) drainage control and (2) buttress fills. Drainage control is probably the most frequently used and often the 

most effective measure because slope failures are very often the result of increases in groundwater 

level, phreatic surface level or pore pressures. Also, drainage control is often the least expensive and 

most easily implemented of the options that are typically available. The types of drainage control 

measures (Section 6.6.2.3) that can be employed include:

• Surface control using ditches designed with a gradient and lining that limits infiltration into the slope and conveys surface flows to a drainage outlet.

• Lowering of the impoundment pool level, if feasible, and consideration of partial 

liner systems placed on the upstream slope to limit seepage.

• Occasional movement of the slurry discharge point so that fines are distributed along 

the upstream embankment slope, limiting seepage and pushing the free water further away from the embankment.

• Horizontal drains drilled at an upslope gradient into the downstream face of the 

embankment to intercept water up to 400 feet from the face. As discussed in Section 

6.6.3.3, prefabricated drains have been used to control pore pressures and improve 

consolidation of impoundment fines.

• Relief wells to intercept artesian water pressure at the downstream toe of the 

embankment. The wells may be furnished with pumps to convey flow to a drainage 

outlet.

• Trench drains extending below the toe of the downstream embankment slope or into 

a bench on the slope. Flow is conveyed by gravity and typically in pipes to a drainage outlet.

• Finger drains excavated perpendicular to and typically at a shallow depth into the 

downstream face of the embankment to intercept water flow at a shallow depth.

• Blanket drains placed at the surface of downstream embankment slopes where seepage or piping is occurring. Blanket drains, especially when incorporated into a buttress, also add weight to help increase the effective stress.

Structural buttress and gravity berm fills constructed at the toe of an embankment also serve to 

increase stability. A structural buttress fill constructed with well-compacted, high-strength material 

improves stability by providing both strength and weight. A gravity berm fill using uncompacted 

material improves stability by providing weight to reduce shear stresses at the toe of the embankment slope. The effectiveness of either type fill can be improved by placing a layer of free-draining 

material between the embankment face and the fill to convey water draining from the face of the 

embankment slope to a drainage outlet.

Other types of stabilization measures such as structural and ground improvement options may be 

considered, but these choices typically require greater time to implement and are more expensive 

and less effective than drainage or toe buttress stabilization measures.
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6.6.4.5 Stability Measurements

Parameters that influence the slope stability factors of safety should be monitored during and after 

each stage of construction by instrumentation to provide a basis for checking the design factor of 

safety so that modifications to the design or to construction procedures can be made, if needed. 

Depending on site conditions and project requirements, embankment performance monitoring 

instrumentation may include:

• Surface monuments for monitoring vertical and horizontal displacements

• Inclinometers for monitoring lateral displacements at depth

• Piezometers for monitoring piezometric head

In addition, monitoring of density test results is important to stability because it provides an indirect 

method to monitor the strength of the compacted fill. Thus, the minimum compaction requirement 

serves to ensure that the shear strength of the compacted fill is consistent with the strengths obtained 

from laboratory testing and used in the stability analysis.

An extensive discussion of instrumentation for coal refuse disposal facilities is provided in Chapter 13.

6.6.5 Rock Excavations

Excavations into rock at coal refuse disposal facilities are usually associated with: (1) spillway channels, (2) diversion ditches, and (3) haul roads. Less frequently, rock excavations are associated with 

decant structure installation and obtaining borrow material.

The degree of assurance that an excavated rock slope will remain stable and has a sufficient factor of 

safety for each of these purposes will depend on facility design requirements. This section provides a 

discussion of the stability of rock excavations, methods for minimizing the potential for a failure, and 

procedures for improving stability.

6.6.5.1 Conditions Requiring Stability Analysis

Major factors that must be considered when evaluating the stability of rock excavations include:

• Existing ground surface slope

• Overburden soil thickness, type and quality

• Bedrock surface conditions

• Rock type, quality and configuration (particularly the orientation of discontinuities)

• Groundwater conditions

6.6.5.1.1 Existing Ground Surface Slope

The steepness of the existing ground surface is an important factor in both the cost of excavation and 

the resulting stability. Figure 6.56 provides a guide for estimating both the vertical and horizontal 

extent of rock excavations as a function of existing slope, cut width, and cut slope. In steep areas 

where existing slopes are only marginally stable, deep excavations are very expensive because of the 

volume of material to be excavated and protection requirements.

6.6.5.1.2 Overburden Soil Thickness, Type and Quality

Usually the top portion of an excavation into rock extends through overlying soils. The need for a 

shallower slope and erosion protection in the overburden soil portion of the excavation must be considered in evaluating the limits of excavation and related costs.
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W L

H 1

SE

1

SC

40

30

20

10

0

10 0 10 20 30 40

HEIGHT, H (FEET)

HORIZONTAL EXTENT, L (FEET)

0.5:1

0.75:1

1:1

1.25:1

1.5:1

2:1

0.5:1

0.75:1

1:1

1.25:1

1.5:1

2:1

BASE WIDTH OF CUT

W

FIGURE 6.56 HORIZONTAL AND VERTICAL EXTENT OF

 CUTS FOR BASE WIDTH OF 10 FEET

LEGEND

EXISTING SLOPE (SE)

SLOPE OF CUT (SC)

(1) SE > SC

(3) H = 

L

SC

(2) L = W 

SC

SE - SC

FOR W = 10 FEET, MULTIPLY

THE VALUES OF L AND H

OBTAINED FROM THE GRAPH

BY COEFFICIENT C = W/10.

6.6.5.1.3 Bedrock Surface Conditions

As shown in Figure 6.57, the bedrock surface can be difficult to define because of weathering, downhill 

slope creep and loosening from stress relief. Except where rock is very massive, conditions normally 

change with depth from: (1) overburden soil consisting of colluvial and residual material to (2) soft, 

heavily weathered rock and then to (3) sound and competent rock. Heavily weathered, decomposed 

rock should be treated as soil to a depth determined by an experienced geologist or engineer. Particular caution is required for soft rocks (e.g., shale, siltstone, claystone and mudstone) that weather very 

rapidly when exposed to air, rainfall and freezing conditions.

FIGURE 6.56 HORIZONTAL AND VERTICAL EXTENT OF CUTS FOR BASE WIDTH OF 10 FEET
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SANDSTONE

SANDSTONE

SHALE

WEATHERED ROCK

RESIDUAL SOIL

COLLUVIUM

GROUND SURFACE

NOTE: INTERFACES CAN BE

 DIFFICULT TO DETERMINE.

FIGURE 6.57 SCHEMATIC GEOLOGICAL PROFILE OF A BEDROCK SLOPE

6.6.5.1.4 Rock Type, Quality and Configuration

Generally the design of a cut into competent rock is technically simple and requires only application of the designer’s judgment or general experience. However, when rock conditions are such 

that analyses are required for demonstrating adequate stability, detailed evaluation is frequently 

more complex than the evaluation of the stability of a soil slope. A rock mass may consist of: (1) a 

layered system of individual beds of different type rocks with variable bed thickness and strength 

and (2) discontinuities such as bedding planes, joints, fractures and faults that have much lower 

strengths than the intact rock. These discontinuities control rock slope stability, but can be difficult 

to define for purposes of analysis. The types and characteristics of rock most often found in coal 

mining areas are:

• Soft Rocks – Shale, Siltstone, Claystone and Mudstone – Rapid weathering of these 

types of rocks affects their strength, which eventually decreases to that of soil. Where 

these rocks are exposed, it must be expected that weathering will occur during the 

useful life of the facility. When exposed soft rock underlies massive, more competent 

rock, particular care must be taken to avoid instability as the soft rock weathers and 

provides less support. Also, erosion of soft rocks can be a problem, particularly when 

they are thinly-bedded and exposed to surface water flows for long periods.

Excavation of soft rocks can usually be accomplished with dozers, shovels, scrapers 

or backhoes. Even when competent, these rocks can be ripped without blasting using 

a bulldozer.

• Harder Rocks – Limestone and Sandstone – These types of rocks are more resistant 

to weathering. Strength is usually high if defects such as fractures or interbedded 

units of softer rocks do not occur at critical locations. Defects and weak cementation can also adversely affect the erosion resistance of these rocks, which is otherwise good to excellent.

The exploration and evaluation of rock defects is very important. The characteristics of 

rock defects reported as part of an exploration program generally include type, quality, thickness, strike and dip, continuity, extent, frequency, and relative strength. The 

orientation of rock defects relative to the excavation is critical. For example, Figure 6.58

illustrates two conditions where orientation of bedding planes dipping into a cut and 

“daylighted” by the excavation could cause bedrock sliding along these planes. The 

extent of a potential slide could be small (Figure 6.58a) or large (Figure 6.58b) dependFIGURE 6.57 SCHEMATIC GEOLOGICAL PROFILE OF A BEDROCK SLOPE
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    

  

    

  

POTENTIAL SLIP PLANES

POTENTIAL SLIP PLANES

POTENTIAL SLIDE

EXTENT

POTENTIAL SLIDE

EXTENT

6.58a EXTENT OF POTENTIAL SLIDE (SMALL)

6.58b EXTENT OF POTENTIAL SLIDE (SEVERE)

FIGURE 6.58 UNFAVORABLE ORIENTATION OF BEDDING PLANES
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ing on the interrelationship of the dip angles of the existing slope, cut and bedding 

planes. Figure 6.59 shows three cases of favorable bedding plane orientation. Even 

with a favorable bedding plane orientation there may be potential for failure if the rock 

is highly fractured (Figure 6.59b) or if a capable fault is present (Figure 6.59c).

Thinly layered strata of limestone and sandstone are usually rippable and are 

not extremely difficult to excavate. However, thick layers of competent rock will 

require blasting.

Because sedimentary strata are layered, harder more competent rock may overlie softer, less competent rock. Weathering of the softer rock will occur faster than weathering of the harder rock 

and can lead to overhangs of the more competent and more massive harder strata. For permanent 

excavations, slope conditions should be observed over time, and large and potentially dangerous overhangs should be eliminated by occasionally scaling the slope to make it more uniform in 

geometry. If loose rock should develop, it should be removed or the potential rockfall zone should 

be barricaded.

6.6.5.1.5 Groundwater Conditions

It is important to recognize that the flow of groundwater through a rock formation can cause 

distress. Excessive pressure along bedding planes or in fractures, can contribute to rock movements. Also, flowing water can cause erosion of defects either chemically from solutioning or 

mechanically from fines washing from joints and fractures. Both types of erosion can weaken the 

strength of a rock mass. Finally, freezing water in fissures and cracks can widen existing fractures 

and accelerate the weathering process or can act as a dam to create higher water pressures behind 

the face of the slope.

6.6.5.1.6 Other Factors

Other factors that can affect the stability of rock cuts include:

• Erosion from precipitation associated with large storms or long wet periods

• Infiltration associated with precipitation

• Vegetation growth

• Shocks and vibrations from blasting or earthquakes

• Changes in loading conditions

6.6.5.2 Slope Stability Analysis

The stability of cuts in rock can be analyzed using two basic approaches. Analyses similar to those 

used for soil slopes (Section 6.6.4) can be used if the rock mass can be assumed to act as a homogenous medium. This condition could apply when the rock strength is not governed by particular 

planes of weakness. In most cases, however, discontinuities govern rock behavior, and principles 

of rock mechanics can be used for stability analyses, as discussed by Wyllie and Mah (2004), Hoek 

(2000), and Coates and Yu (1977).

Most small rock excavations, where failure would not represent a severe hazard, can be designed 

based on experience with similar geological conditions. The slopes for these conditions usually 

range from 2 horizontal to 1 vertical (2H:1V) to 0.5H:1V for soft rocks and from 1H:1V to nearly vertical for hard rocks unless the orientation of weakness planes require flatter slopes. The uniformity 

of a rock slope is often broken by benches or berms for controlling runoff and catching loose rock 

fragments and debris.
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POTENTIAL SLIP PLANE

FAULT ZONE

(POTENTIAL SLIP PLANE)

6.59a ROCK IS COMPETENT

6.59b ROCK IS HIGHLY FRACTURED

6.59c FAULT ZONE IS GOVERNING FEATURE

FIGURE 6.59 FAVORABLE ORIENTATION OF BEDDING PLANES
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Guidance for rock cut slopes above roadways is presented by ODOT (2006), based on the slake durability index (SDI), rock quality designation (RQD) values, and unconfined compressive strength. 

Construction bench and debris width guidance is also provided.

6.6.5.3 Stability Control Measures

An approach to stability control developed by ODOT (2006) is illustrated in Figure 6.60. The advantage of this technique is that ample horizontal area is provided for debris collection and vegetation 

growth after construction. The edges of individual steps gradually break down and debris is collected at the benches so that vegetation growth helps to protect against sloughing and erosion. The 

individual benches and steps do not have to be spaced equally or be level. To minimize the cost, the 

steps should be cut during initial excavation.

Several remedial alternatives are available when difficulties with the stability of rock excavations are 

anticipated or encountered. Benching of the rock slope can be a simple means of improving stability. 

Other methods, as shown in Figure 6.61, include:

• Construction of a bench with optional retaining berm or wall at the toe of slope to 

catch falling rock debris.

• Construction of a retaining wall against all or a portion of the slope.

• Prevention of rock weathering and sloughing by anchoring a wire mesh (with 

optional shotcrete overlay to protect against weathering) to the rock slope.

• Containing loose rock on the cut face with rock bolts, possibly combined with mesh.

• Supporting the rock face with tiebacks anchored below potential slide planes.

• Placing concrete or masonry support to replace weathered soft rock supporting overlying layers of harder rock.

(ADAPTED FROM ODOT, 2006)

SANDSTONE

RQD = 91 PERCENT

qu = 4,200 PSI CUT SLOPE

ANGLE = 0.5:1

CUT SLOPE

ANGLE = 1.5:1

CUT SLOPE

ANGLE = 0.5:1

ORIGINAL GROUND

SURFACE SANDSTONE

RQD = 91 PERCENT

qu = 4,200 PSI

OVERBURDEN

ZONE

CL

SHALE

RQD = 52 PERCENT

SDI = 34 PERCENT

qu = 2,000 PSI CHANNEL

SLOPE HEIGHT = H

STEPPED

BENCH

FIGURE 6.60 STEPPED SLOPE EXAMPLE FOR CUTS IN WEATHERED ROCK
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DEBRIS RETAINING

STRUCTURE

VARIES

ROCK-FALL CATCHMENT RETAINING WALL

GRANULAR

DRAINAGE

MATERIAL

WEEP HOLES

TIE-BACK ANCHORS CONCRETE OR MASONRY

 SUPPORT

FIGURE 6.61 EXAMPLES OF ROCK CUT CONTROL

WEEP HOLES

HORIZONTAL

DRAINS

(OPTIONAL)

WIRE MESH WITH OR

 WITHOUT GUNITE

ROCK BOLTS

FIGURE 6.61 EXAMPLES OF ROCK-CUT CONTROL
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All of these support techniques are costly and require contractor expertise and monitoring by experienced engineering personnel for successful implementation.

Where stabilization of highwalls is not practical, safety zones where access is restricted should be 

considered. The extent of safety zones can be established using computer programs such as the Colorado Rockfall Simulation Model (Jones et al., 2000) to delineate the area that could be affected by 

rockfalls and to estimate the size of the berm that should be constructed to retain materials that fall 

from the rock face.

6.6.5.4 Stability Measurements

When the stability of a rock slope is important to the design of a coal refuse disposal facility, estimates 

of deformations should be made for comparison to field observations. Depending on site conditions 

and project requirements, rock slope performance can be monitored by:

• Surface monuments for monitoring vertical and horizontal displacements

• Inclinometers for monitoring subsurface lateral displacements within slopes

Instrumentation for mine refuse disposal sites is presented in Chapter 13.

6.6.6 Conduit Structural Design for Earthen Fill Loads

At coal refuse disposal facilities, conduits are used to convey water from an impoundment through, 

under, or around a refuse embankment dam in a controlled manner. They are also used to convey 

surface water under haul roads and to control surface flows at the periphery of impoundments and 

refuse embankments. Conduits through refuse embankments serve the following purposes:

• Convey stored waters to the coal preparation plant

• Provide emergency reservoir evacuation capability

• Provide a primary or secondary outlet for passing storm water flow

A conduit through a refuse- or water-impounding embankment creates a discontinuity in the embankment cross section. Therefore, the structural design and construction of conduits for these situations 

must address potential problem conditions that have led to the distress and/or failure of impounding 

embankments. Some of these potential problem conditions include:

• Differential settlement between the conduit and the surrounding embankment due 

to difference in stiffness between the conduit and surrounding material.

• Differences in compaction between the backfill around the conduit and the remainder of the embankment.

• Structural defects in the conduit resulting from deterioration, overstressing due to 

lack of proper backfilling under conduit haunches or excessive fill heights, cracking 

due to foundation compression or lateral extension, or joint separation due to poor 

design and construction.

• Water leaking from the conduit that can lead to increased seepage, especially under 

pressurized flow conditions, in the embankment.

• Seepage flowing into an open conduit joint or crack in a conduit leading to internal 

erosion in the embankment.

There are special design considerations for conduits through impounding embankments that 

are unlike those for conduit applications that do not involve dams or impoundments. These

include:
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• Limiting the number of joints to maintain water tightness and to minimize the potential for internal erosion, backward erosion piping, and structural deterioration.

• Limiting flow velocity to minimize the potential for cavitation and erosion.

• The need for seepage control structures along the outside of the conduit.

• Accommodating fluctuating flows due to seasonal conditions and operating 

requirements.

• Accommodating flow interruptions to meet inspection and maintenance 

requirements.

• Maintaining operating tolerances for gate and valve functionality.

The following sections describe the types of conduits typically used at coal refuse embankments, 

including their structural characteristics and methods used for structural design and conduit installation (FEMA; 2005a, 2007). Hydraulic design issues for coal refuse embankment conduits are presented in Chapter 9.

6.6.6.1 Conduit Types

The materials used for conduits at coal refuse disposal facilities include reinforced precast concrete, 

thermoplastics, and welded steel pipe. High-density polyethylene (HDPE) is currently popular due 

to its light weight, durability, corrosion resistance, lower cost and the use of fuse-weld pipe joints to 

make the constructed pipe virtually jointless except at upstream extensions of decant pipes. Precast, 

prestressed concrete cylinder pipe also has a long history of satisfactory performance and resistance 

to structural deterioration. However, product cost, heavy weight and the frequency of joints are limitations that must be considered. Coated steel pipe is also used at refuse disposal facilities. A general 

discussion of the characteristics of concrete, thermoplastic and metal conduits including advantages 

and disadvantages is presented in the following sections.

6.6.6.1.1 Concrete

Types of precast concrete pipe typically used for conveying water through dams and impounding refuse embankments include reinforced concrete pipe (RCP), reinforced concrete cylinder pipe 

(RCCP), and prestressed concrete cylinder pipe (PCCP). RCP is used to convey flows under gravity 

head. RCCP and PCCP have seals at the pipe joints and thus are able to convey flow under pressure head. Precast concrete pipes are typically circular in cross section. Rectangular (or box) precast 

conduits are seldom used in dams and impounding refuse embankments because watertight joints 

cannot be reliably constructed.

The advantages of using precast concrete pipe for conduits include:

• The pipe is manufactured in a controlled environment to tight tolerances.

• Installation is relatively quick.

• Varying settlement along the pipe length can be accommodated by articulated joints 

between sections.

The disadvantages of using precast concrete for conduits include:

• There is a potential for opening of joints due to embankment settlement or elongation because longitudinal reinforcement does not extend across joints.

• Shipping and handling limitations result in short pipe section lengths and numerous 

joints over the length of the pipe, leading to an increase in the number of potential 

leakage locations.
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• Gasketed joints are the only defense against leakage.

• Compaction of backfill is difficult under the pipe haunches.

While reinforced cast-in-place concrete pipe has a long history of successful performance for waterimpounding dams, it is seldom used at coal refuse disposal facilities due to the extended timeframes over 

which the lengthy decant conduits are constructed, the difficulty of delivering adequate volumes of fresh 

concrete (to what are often remote locations) during pipe construction, the complexity of formwork and 

related construction, and the susceptibility to cracking and other distress associated with settlement.

6.6.6.1.2 Thermoplastic

Thermoplastics are solid materials that change shape when heated, and they commonly include polyethylene (PE) and polyvinyl chloride (PVC). Thermoplastic pipe is produced by an extrusion process, 

in which molten polymer material is continuously forced through an angular die by a turning screw. 

The die shapes the molten material into a cylindrical shape. After a number of additional processes, 

the final product is cut into pipe lengths that are suitable for delivery and handling.

The thermoplastic material most commonly used in refuse facilities is solid-wall HDPE. HDPE pipe is 

relatively inert chemically and thus is not particularly prone to corrosion or deterioration, has a long 

service life, and requires little maintenance. This is especially important for small-diameter pipes that 

are not easily renovated and cannot be easily inspected. HDPE is typically available in sizes up to 63 

inches in diameter. Manufacturers can fabricate HDPE custom pipe fittings in addition to common 

fittings such as bends, flanges, reducers, and transitions.

The advantages of using thermoplastic pipe include:

• Its light weight facilitates relatively quick and simple installations.

• It resists corrosion and is not affected by most naturally occurring soil and water conditions.

• Its smooth interior surface limits friction loss and is resistant to the adherence/

buildup of minerals such as calcium carbonate.

• The pipe is relatively watertight pipe if pipe joints are properly heat fused.

• It is resistant to biological attack.

The disadvantages of using thermoplastic pipe include:

• Its susceptibility to damage or displacement by construction and compaction 

equipment.

• Proper compaction of backfill at pipe haunches is difficult.

• Heat fusion of pipe joints requires special equipment and an experienced 

operator.

6.6.6.1.3 Metal

Metal conduits used at coal refuse disposal facilities are typically limited to welded, coated steel pipe. 

Steel pipe with diameters of 24 inches and smaller (36 inches and smaller at some shops) is manufactured in standard wall thicknesses and diameters. Pipe that is greater than 24 inches in diameter can 

be custom manufactured to any desired diameter. Standard diameters for steel pipe with diameters 

greater than 24 inches are listed in Manual M11 published by the American Water Works Association 

(AWWA, 2004) . Minimum plate (wall) thickness for larger-diameter pipe is one-quarter inch. Available plate thicknesses increase by multiples of one-sixteenth inch.

Steel pipe can be protected with a variety of linings and coatings. Frequently, the interior lining is not 

the same as the exterior coating because of the difference in exposure conditions between the interior 
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and exterior surfaces. Typically, the interior surface is lined with the same coating regardless of location. The exterior surface coating will depend on location, encasement, and whether or not the pipe is 

submerged. The exterior surface is usually uncoated if it will be encased in concrete. Interior coatings 

and linings for mitigation of corrosion should be selected consistent with the anticipated fluid velocities within the pipe. Cement mortar should only be used on the interior surfaces of steel pipe where 

flow velocities will be low.

The advantages of using steel for conduits include:

• It is manufactured to tight tolerances in a controlled environment.

• It has a long service life, if proper linings and coatings are used.

• Welded joints provide watertightness.

• It can be constructed on compressible foundations.

• It has high compressive and tensile strength.

• It is flexible and deformable under stress.

• Its high modulus of elasticity provides resistance to buckling loads caused by 

external water pressures.

• A wide variety of special sections can be fabricated.

• It is easy to connect additional steel pipe in the future by tapping and welding.

• Flanges provide a rigid connection to gates and valves.

The disadvantages of using steel for conduits include:

• The material cost is high.

• Proper selection of linings and coatings and associated protection and maintenance measure is necessary in order to prevent corrosion.

• Proper compaction of backfill at pipe haunches is difficult.

• Special linings are required at impoundments where aggressive/corrosive water 

may be present (e.g., acidic mine drainage).

Corrugated Metal Pipe (CMP) applications are generally limited to surface drainage culverts and other near-surface installations where maintenance or replacement can readily be 

accomplished.

6.6.6.2 Soil-Structure Design

Conduits should be designed to withstand a variety of loads and pressures including:

• Internal fluid and vacuum pressures

• External hydrostatic loadings and buckling pressures

• Embankment loads

• Surface surcharge loads

• Construction loads from handling or equipment trafficking

• Operational and maintenance loadings

• Load combinations

Designers should also consider the effects of vertical and horizontal displacements that might occur 

due to settlement and spreading of the embankment and foundation during construction. These dis-
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placements could result in loads on the conduit that exceed the design loads listed previously. Excessive displacements, both vertical and lateral, can occur when conduits have foundations that are 

either weak, compressible, or both. Poorly compacted embankments or embankments supported on 

compressible foundations can deform due to shear forces, and this deformation can lead to lateral 

spreading of conduits (Rutledge and Gould, 1973).

FEMA (2005a) recommends that conduits be analyzed for the following loading conditions:

• Usual – Includes: (1) normal operating conditions with the reservoir at or near 

normal pool, involving combinations of vertical soil load (due to the weight of the fill 

above the pipe), (2) horizontal soil load, (3) external and internal hydrostatic pressure loads, and (4) the vertical foundation reaction (typically assumed to be equal to 

the vertical soil load plus the weight of the conduit).

• Unusual – Includes loads resulting from high reservoir levels and elevated discharges associated with flood conditions. Because floods typically have a short duration, conduits may not be significantly affected by increased external hydrostatic 

pressure. The difference between usual and unusual loading conditions could be 

limited to increased internal hydrostatic pressure.

• Extreme – Usual loading conditions plus earthquake loading. Depending on the 

consequences of failure of (or severe damage to) a conduit and vertical concrete riser 

intake (particularly, if of appreciable height) under seismic loading, a range of earthquake loadings may need to be considered, including the seismic loading associated 

with the maximum credible earthquake, if conduit operation is required for preventing overtopping during the design storm. Conduits are “low-profile” structures and 

tend to have a relatively high natural frequency. Unless a conduit is founded on deep 

layers of soil where peak ground accelerations could be magnified, unamplified 

peak ground accelerations are typically assumed to act on the conduit. If the natural frequency of the conduit is greater than 33 Hz, a pseudostatic analysis generally 

provides acceptable results. Other factors that may affect loading conditions are the 

type of foundation, method of bedding, flexibility of the pipe, and soil properties. 

Generally, conduits designed with an adequate static factor of safety are unlikely to 

buckle without a substantial stiffness reduction of the embedding soil under earthquake loading. Davis and Bardet (1998) describe a pseudostatic analysis and estimate 

backfill stiffness reduction, demonstrating that critical conditions would be associated with high pore-pressure buildup under low vertical effective stress (e.g., low 

depth of cover).

• Construction – Pertains to loads resulting from construction activities such as construction vehicles or equipment moving or working near or over the conduit.

The Marston theory (Moser, 2001) is typically used to calculate loads on a conduit that is partially or fully projecting above the original ground surface. The vertical load on the conduit is 

considered to be a combination of the weight of the fill directly above the conduit (i.e., prism 

load) and the frictional forces from adjacent fill acting upward or downward at the boundaries 

of the prism of earth above the conduit. This combined loading is also known as the “projection” 

condition. As illustrated in Figure 6.62, there are two projection loading conditions. The embankment condition shown in Figure 6.62a occurs when fill adjacent to a conduit settles more than the 

fill directly above the conduit. As a result, downward frictional forces act on the prism of earth 

above the conduit and can increase the resultant load on the conduit by as much as 50 percent of 

the weight of the fill above the conduit (i.e., 1.5 times the prism load, which has historically been 

adopted by the NRCS (2005a)). The trench condition shown in Figure 6.62b occurs when the fill 
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 NOTE: FRICTION FACTORS INCREASE EMBANKMENT

 LOAD ON THE CONDUIT AS ADJACENT FILL SETTLES

 MORE THAN EARTHFILL OVERLYING THE CONDUIT.

6.62a CONDUIT CONSTRUCTED PRIOR TO FILL PLACEMENT

TOP OF EMBANKMENT

FIGURE 6.62 DEFORMATIONS ASSOCIATED WITH

 CONDUITS IN EMBANKMENTS

TOP OF EMBANKMENT

TRENCH

EXCAVATION

LIMITS

6.62b CONDUIT CONSTRUCTED IN TRENCH EXCAVATED INTO EMBANKMENT

NOTE: FRICTION FACTORS DECREASE EMBANKMENT

 LOAD ON THE CONDUIT AS FILL OVER CONDUIT

 SETTLES RELATIVE TO ADJACENT EMBANKMENT.

(ADAPTED FROM FEMA, 2005a)

FIGURE 6.62 DEFORMATIONS ASSOCIATED WITH CONDUITS IN EMBANKMENTS
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adjacent to the conduit settles less than the overlying fill. For this case, the differential settlement 

results in an arching condition that can reduce the load on the conduit by as much as 50 percent 

of the weight of the fill above the conduit (i.e., 0.5 times the prism load).

FEMA (2005a) recommends that conduits in embankment dams not be installed in trenches with 

vertical side walls or steep side slopes because a reduction of contact between the fill and the 

conduit is possible and planes of reduced in-situ stress can develop due to the effects of arching. 

These phenomena can lead to concentrated seepage paths and conditions more prone to internal 

erosion of susceptible backfill along and around the conduit. With respect to plastic pipe, FEMA 

(2007) presents guidance for determining pipe loading (i.e., application of the Marston theory or 

reliance on the prism load immediately above the pipe) for applications with depths up to about 

50 feet and recommends other guidance from pipe manufacturers for situations of deeply buried 

pipes. FEMA’s recommendations are written in the context of earth embankment dams designed 

for water supply or flood protection. Trench installation of conduits has been successfully 

employed in the mining industry. However, unnecessarily deep trenches, trenches in potentially 

unstable ground, and installation details that are difficult to construct within a trench should be 

avoided. If trench installation of conduits is considered, the backfill should be designed to provide consistent lateral support for the pipe (e.g., through reliable compaction effort or the use 

of CLSM to enable positive contact of the backfill with the conduit and trench sides). Also, the 

design of trench installations for flexible conduits should take into account imposed impoundment pressures resulting in external hydrostatic loads and concentrated seepage. The need for a 

seepage barrier to limit seepage and pressures should be evaluated in conjunction with downstream filter and drainage provisions (e.g., filter diaphragm) along the conduit.

Caution should be exercised when the height of the overburden on a conduit is greater than about 

100 feet. Under such fill heights some of the previously mentioned guidance for estimating loads on 

conduits can result in very high stresses that require inordinately thick conduit walls and/or special 

reinforcement. More rigorous earth-structure interaction analyses (e.g., finite element methods) may 

be warranted and/or desirable in such cases and where relatively uniform firm lateral or subgrade 

support is not present.

The Marston theory is considered to be a very conservative approach for estimating earth loads associated with the conduit fully-projecting condition. However, more sophisticated soil-structure-interaction analytical methods that allow for 2D, 3D, and time-dependent analysis of conduits are available. 

These methods also allow modeling of the effects of nonlinear, stress-dependent soil (or coal refuse) 

stress-strain behavior. Computer programs such as CANDE (Culvert ANalysis and DEsign), FLAC 

(Fast Lagrangian Analysis of Continua) and Plaxis can be used to model excavation, conduit construction, conduit backfilling, and embankment construction over conduits. These programs permit 

calculation of backfill and conduit stresses and displacements for each stage of construction. They 

can also be used to evaluate the effects of conduit stiffness, backfill characteristics, foundation movements and other factors that can affect conduit performance and are not considered by Marston 

theory. Parameters that are typically required for conducting such an analysis include:

• Embankment/fill geometry and unit weight

• Trench geometry

• Undeflected pipe geometry

• Ground/embankment water levels

• Conduit internal water pressure

• Short- and long-term elastic modulus of conduit material

• Compressive strength of conduit material
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• Tensile strength of conduit material (metal conduits)

• Geometry, spacing and material properties of reinforcing steel (concrete conduits)

• Bedding factor (concrete conduits)

• Stress-strain properties of compacted backfill envelope (e.g., soil or CLSM) and 

embankment fill

Concrete conduits are typically designed as rigid structures, whereas plastic and metal conduits are 

designed as flexible structures. Concrete conduits deflect minimally under load and derive much 

of their external load capacity from the high strength of the conduit, although bedding and backfill 

support also contribute to this capacity. Flexible conduits deflect under load and derive their external 

load capacity from the ability of the conduit to deflect and develop soil support at the sides of the 

conduit. These very different behaviors must be accounted for during design.

Precast concrete cylinder pipe (RCCP, PCCP) is customarily structurally designed as a rigid conduit 

by the manufacturer. Internal, external and combinations of loads are applied to a unit length of 

pipe, and thrusts and moments at various points around the perimeter of the pipe are calculated. 

The required reinforced concrete design parameters, including concrete thickness, reinforcing steel 

amount, steel cylinder thickness, and prestress tension, are then determined. Detailed reinforced concrete design procedures with examples for RCCP are provided in Manual M9 (AWWA, 1995). Design 

procedures, with examples for PCCP, are presented in Standard C304-07, “Design of Prestressed 

Concrete Cylinder Pipe” (AWWA, 2007).

Thermoplastic and welded steel pipe are designed as flexible structural elements. Internal, external 

and combinations of loads are applied to a unit length of pipe, and thrusts and moments at various points around the perimeter of the pipe are determined using design guidelines such as National 

Engineering Handbook Chapter 52: Structural Design of Flexible Conduits (NRCS, 2005a). FEMA (2007) 

has also published a technical manual for plastic pipe used in embankment dams. The NRCS document includes detailed procedures and guidelines for design, inspection, maintenance, and repair of 

thermoplastic pipe. Detailed design procedures, including the application of methods for evaluating 

constrained pipe wall buckling, ring deflection, soil reaction modulus, and long-term deflection considerations and limits are provided in engineering manuals prepared by flexible pipe manufacturers 

(e.g., Performance Pipe, 2003). Additional references include Watkins and Anderson (1999).

Both thermoplastic and welded steel pipe, when backfilled with CLSM or concrete, provide resistance to seepage along the external perimeter of the pipe (FEMA, 2005a). Use of CLSM or concrete 

encasement, for example, can eliminate the problem of poor backfill compaction in the haunch area 

of circular pipes. The design of CLSM for pipe support is discussed in Section 6.6.6.3.3. The design of 

concrete encasement will need to address the structural interaction between the rigid concrete and 

flexible conduit, and it should demonstrate that the resulting stresses are acceptable.

Because of the critical nature of conduits extending through embankments, careful attention must 

be paid to construction quality control. Major conduit installation work should be monitored on a 

full-time basis by a person familiar with the installation requirements. This should include careful 

monitoring of conduit joint construction and sampling and testing of construction materials. In the 

post-construction period, major conduits should be periodically inspected (typically through internal camera surveys and, particularly for flexible conduits, inside diameter/deflected shape measurements) to verify acceptable performance of the conduit and to allow early detection of defects and 

signs of overstressing. Early detection is critical to timely planning and implementation of remedial 

measures for poorly functioning or distressed conduits. In multi-stage coal refuse disposal facilities, 

data from monitoring of early stages provides valuable information for projecting or verifying conduit performance under increased fill depths.
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The structural adequacy of riser inlets attached to the main conduit barrel should be evaluated. Typically, risers extend upward to relatively shallow depths below the impoundment surface during facility operation and then are decommissioned when no longer needed. Careful consideration should 

be given to the method of supporting and subsequently capping and decommissioning riser pipes, 

particularly when the capped riser will eventually be deeply buried. Decommissioned riser pipes 

represent a potentially weak point in the conduit system. Loads imposed on the riser pipes can be 

transferred to the main conduit (“barrel pipe”)and cause unacceptable deflection or stress at the 

location of the riser pipe connection. At present, there are few analytical tools available for analyzing 

the interaction of deeply buried risers (particularly in cases where the inlet sections are neither vertically nor horizontally oriented) with the surrounding fine coal refuse. Until better analytical tools 

are available, the support (or encasement) of inlet risers should be carefully evaluated. It may be 

beneficial to monitor the performance of the riser pipe where it joins the main conduit using internal 

camera surveys and by making inside diameter/deflected shape measurements at key stages during 

the life of the facility.

6.6.6.3 Conduit Installation and Design Details

6.6.6.3.1 Installation

Conduits are constructed or installed by placement at or near a level ground surface (embankment 

condition) or within a trench (trench condition). If the fill adjacent to the conduit settles more than 

the fill overlying the conduit, which is typical for an embankment installation, downward frictional 

forces are induced that can increase the load on the conduit by up to 50 percent of the weight of the 

fill above the conduit. However, if the fill placed directly above the conduit settles more than the adjacent fill, which can happen for conduits constructed in a trench, arching will occur and can reduce 

the load on the conduit by up to 50 percent of the weight of the fill above the conduit. FEMA (2005a, 

2007) presents guidance and recommended limitations for pipe loading and installation conditions 

for conduits and plastic pipe, respectively, in dams, although other references are applicable for plastic pipe at depths greater than about 50 feet.

Construction of conduits in trenches with vertical side walls or steep side slopes is not recommended 

(FEMA, 2005a) because a reduction of contact between the fill and the conduit is possible and planes 

of reduced in-situ stress can develop due to the effects of arching. As noted previously, FEMA’s recommendations are written in the context of earth embankment dams designed for water supply and 

flood protection. If trench installation of conduits is considered, the backfill should be designed to 

provide consistent lateral support for the pipe. Also, the design of trench installations for flexible 

conduits should account for imposed impoundment pressures resulting in external hydrostatic loads 

and concentrated seepage. Unnecessarily deep trenches, trenches in potentially unstable ground, and 

installation details that are difficult to construct within a trench should be avoided.

6.6.6.3.2 Rigid Conduit Support

Precast concrete circular pipe should be constructed on concrete or CLSM cradles to eliminate difficulties related to compaction beneath the haunches of the pipe. The cradle should be concrete or 

higher strength CLSM and should provide vertical, longitudinal, and lateral structural support to 

the pipe. The cradle should extend for the full length of the pipe and should also extend above the 

bottom of the pipe at least 25 percent of the pipe outside diameter (i.e., Class A bedding per the 

American Concrete Pipe Association (1987). A more substantial cradle may be desirable or may be 

required for enhanced support, especially on soil subgrade. For applications that allow for conduit 

support on soil, direct or indirect design methods are available (American Concrete Pipe Association, 2007; USACE, 1998a).

The design of the conduit bedding and backfill is dependent upon the compressibility of the foundation and the purpose for which the conduit will be used. Precast concrete pipe should not be used for 
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pressurized applications in significant- and high-hazard-potential embankments unless measures are 

taken to address the potential impact of failure of a pipe joint or joint gasket and resulting leakage of 

pressurized water into the embankment. Conduits should be constructed on rock or firm foundations 

whenever possible. If a pipe is founded on a compressible foundation, the design should accommodate 

the anticipated magnitude and distribution of settlement, because settlements can lead to joint failure.

Many approaches have been used to design rigid conduits on compressible foundations. A preferred 

approach utilizes a joint design for the cradle that allows articulation and spreading of both the pipe 

and the cradle. Cradle joints are placed at the locations of the pipe joints, and cradle reinforcement is 

not allowed to pass through the joints. Spaces between joints are filled with a compressible material, 

such as high-density sponge rubber or bituminous fiberboard, to keep extraneous material out of the 

joints and allow articulation. USDA (1958) provides design guidelines for pipe cradles.

A concrete cradle should bond to the conduit. In embankment installation configurations, the cradle 

should extend at least 4 inches beyond each side of the pipe, and the sides of the concrete cradle 

should always be sloped at 1H:10V or flatter through low-hydraulic-conductivity zones to allow 

construction equipment to compact earthfill directly against the cradle. There should be no sharp or 

protruding corners associated with the cradle that could cause undesirable stress concentrations in 

the fill. Measures should be taken to support the conduit on grade until the concrete cradle has been 

placed and cured.

Concrete bedding and cradles beneath circular conduits are typically designed to reach about 25 percent of 

the conduit outside height in order to provide support to the conduit and to facilitate compaction under the 

haunches. The bedding is often constructed with joints opposite the circular conduit joints so as to not interfere with potential conduit movement. Guidance for the use of bedding in conjunction with fully circular 

pipes is provided in USDA (1958) and USACE (1998a). The choice of whether to use cradles or bedding is 

typically a function of the height of the embankment, quality of the subgrade and the benefits gained from 

using a cradle versus bedding in reducing the structural requirements for the pipe. Cradles are often used 

in higher embankments where more lateral support is required. Regardless of whether a cradle or bedding 

is used, seepage control measures such as filter diaphragms should be employed, even for low-hazardpotential embankments and favorable conditions. A discussion of seepage control measures along conduits 

is provided in Section 6.6.2.3.3.

6.6.6.3.3 Flexible Conduit Support

As discussed in Section 6.6.6.1, the use of HDPE pipe at coal refuse disposal facilities has become prevalent. Concrete cradles and bedding are not generally used for flexible pipe installations because flexible 

pipes require some deflection to develop resistance from the surrounding backfill, and cradles and 

bedding limit this deflection. Thus, if flexible pipe is constrained by a rigid cradle, it may become overstressed. Therefore, HDPE pipe should not be supported or constrained by a rigid cradle or encased 

in normal-strength concrete that will limit the flexible pipe from deforming in its intended manner. 

However, HDPE pipe may be used as a liner within a structural encasement if the system is properly designed as a rigid conduit, with appropriate stiffness capable of withstanding potential external 

hydrostatic loads, and the heat of hydration of the concrete and wall thickness of the HDPE liner are 

balanced to limit the potential for separation of the liner from the encasement. Simply encasing a plastic 

liner pipe in concrete will not necessarily preclude development of significant hydrostatic pressure at 

the interface between the liner pipe and encasement. The potential for external hydrostatic loading is 

further addressed in FEMA (2007).

As an alternate to compacted soil backfill, flowable or controlled low-strength material (CLSM) backfill has been successfully used to provide support and encasement of flexible conduits. For plastic 

conduits, the flowable backfill/CLSM should be designed to have a relatively low cement content 
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(low unconfined compressive strength), but have substantial fractions of fine aggregate and pozzolan 

so that a mix design with a soil-like matrix, low heat of hydration, limited shrinkage potential, and 

relatively low hydraulic conductivity is achieved. As described in Section 6.5.10.2, flowable backfill 

consists of cement and other pozzolans (e.g., fly ash), sand and water. The target strength should be 

that of a very stiff to hard soil (in the range of 50 psi to 200 psi maximum) so that the conduit-backfill 

system behaves as intended. Mixes designed to produce a compressive strength much lower than 50 

psi may not have sufficient cement content to produce a conduit backfill with uniform characteristics 

(e.g., strength and compressibility) and erosion and weathering resistance. Pre-testing of prospective 

flowable backfill mixes is recommended so that a flowable material with relatively uniform strength 

and stiffness properties is achieved.

The hydraulic conductivity of the flowable fill material should be less than or equal to the hydraulic conductivity of the adjoining portions of the embankment. The addition of a small percentage 

of bentonite or attapulgite to the CLSM mix is an option in cases where a hydraulic conductivity 

much lower than 10-5 cm/sec is desired or required. For conduit installations in embankment dams, 

measures should be in place to intercept and drain potential seepage along the conduit. Typical 

measures include filter diaphragms, as described in Section 6.6.2.3.3. FEMA (2007) has expressed 

caution about the use of CLSM in significant- and high-hazard-potential embankment dams, pending research to evaluate its performance. As noted previously, FEMA’s concerns are presented in the 

context of earth embankment dams designed for water supply and flood protection. CLSM backfill 

installations of conduits are becoming more common for embankments used in transportation infrastructure and in the mining industry, and they are attractive because they provide effective support 

while addressing seepage issues.

The reaction modulus for CLSM backfill should be selected based on the mix design, as discussed 

above, and available test data (e.g., unconfined and uniaxial compressive strength tests), conservatively considering the CLSM to be equivalent to a very dense sand or gravel material. For deep conduit 

installations, the influence of earthen materials adjacent to the CLSM may also need to be incorporated into the evaluation of deflection and buckling. When used in the modified Iowa Formula for 

buried pipe deflection calculations, the reaction modulus is analogous to the soil reaction modulus E’

(Howard, 1977). Flowable fill employing a mix design to achieve a minimum 50 psi at 28 days, and 

incorporating the recommendations presented in Section 6.5.10.2, would presumably have a reaction 

modulus equal to or greater than highly compacted granular backfill given its compressive strength 

and low compressibility. Modeling based on testing of such mixes by McGrath and Hoopes (1998) 

determined an E’ of 3,000 psi after 28 days for CLSM with high air content. Reaction modulus values 

as high as 3,800 psi have been applied in the past for embankment dams in the mining industry and 

have yielded reasonably good predictions of actual deflections for deeply-buried, thick-walled HDPE 

pipe. However, the actual width of flowable fill on each side of the pipe installation relative to the 

pipe diameter and the consistency of the native sidefill soil or trench wall will influence the extent to 

which the flowable fill support/reaction predominates over the support/reaction of the sidefill soil or 

trench wall (as applicable).

Note that the modified Iowa Formula and the recommended modulus of soil reaction E’ values for 

application in the formula were developed from data on pipe installations up to about 50 feet deep, 

presuming that the prism load acts on the buried conduit. Therefore, for burial depths up to and 

somewhat greater than 50 feet, the prism load and recommended limiting soil reaction values should 

be adopted when applying the modified Iowa Formula. However, for burial depths much greater 

than 50 feet, if the vertical load on the conduit is truly increasing in proportion to the fill height, it 

follows that there would be some attendant increase in the soil reaction modulus. Currently, guidance for quantifying a soil reaction modulus or reaction modulus of non-soil backfill for simplified 

analysis of deeply buried conduits is limited.
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6.6.6.3.4 Watertightness

If the joints between conduit sections separate or deteriorate, the conduit may develop leaks that can 

lead to internal/backward erosion (piping) failure mechanisms. Accordingly, government agencies 

such as the USACE and the Bureau of Reclamation require that joints in conduits in embankment 

dams be watertight (FEMA, 2005a). The degree to which joints must be watertight depends on the 

anticipated hydrostatic head both inside and outside of the conduit. As shown in Figures 6.63 and 

6.64, RCCP and PCCP employ a rubber gasket confined between steel spigot rings and mastic in 

the joints to provide a waterstop. The watertightness of HDPE conduit is achieved by fuse welding 

the conduit joints. Similarly, the watertightness of steel conduit is achieved by welding the conduit 

joints. Details related to conduit joint construction and measures for achieving watertightness are 

provided in FEMA (2005a).

Conduits constructed on compressible foundations are vulnerable to joint spreading. Special attention should be given to evaluating the compressibility and shear strength of materials in compressible foundations. Section 6.6.3 provides a discussion of methods for predicting settlement. Technical 

Release 18 (TR-18) published by the USDA (1969) uses the predicted vertical strain beneath a conduit, the shear strength of foundation soils, and the geometry of the embankment and foundation to 

predict horizontal strain in the conduit. If the joints between the ends of conduit sections separate 

or develop other defects, a conduit may develop leaks. This leakage can lead to the development of 

internal erosion or backward erosion piping failure mechanisms.

Excessive lateral movement of an embankment/foundation system can occur when thin weak layers 

in the foundation are loaded beyond their shear strength. These movements may result in slope instability, and they can also cause damage to a conduit if it is located over the weak layer. Slope flattening 

and berms can be used to prevent such movements, but these remedies will result in a longer conduit. 

To minimize differential settlement and movement of conduit joints, foundations under conduits 

should have relatively uniform compressibility characteristics.

(FEMA, 2005a)

ROD REINFORCEMENT OR

WELDED WIRE FABRIC

STEEL BELL

MASTIC IN RING

JOINT

STEEL SPIGOT

RING

RUBBER

GASKET

MASTIC

IN JOINT

STEEL CYLINDER AND

SUPPORT ROD OR WIRE

FABRIC REINFORCEMENT

AS NEEDED

CONCRETE

CORE

FIGURE 6.63 REINFORCED CONCRETE CYLINDER PIPE (RCCP) DETAILS
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Variable foundation conditions can result in abrupt changes in conduit settlement that can lead to 

large relative movements and failure. A properly designed joint will limit vertical and transverse 

displacement of conduit sections relative to each other as an embankment settles and will also accommodate rotation and longitudinal movement while maintaining watertightness. For conduits on a 

compressible foundation, the maximum potential joint elongation that may occur as a result of the 

compressibility of the foundation should be determined as accurately as possible. The potential joint 

elongation is a function of:

• Shear strength of the foundation

• Estimated settlement of the foundation

• Configuration of the embankment dam

• Lengths of conduit sections

If the predicted elongation exceeds the magnitude that the joints can accommodate, design changes 

such as using shorter lengths of conduit, replacing compressible foundation soils with compacted 

backfill, and flattening the slopes of the embankment should be considered.

Embankment settlement may not always be as predicted by analyses and can sometimes vary over 

relatively short distances, resulting in abrupt joint displacements. Figure 6.65 illustrates that measured settlement can be significantly different from the calculated settlement. These variations in 

actual settlement can lead to joint displacements. Abrupt joint displacements are generally more 

likely for conduits that are constructed using precast concrete pipe than for conduits constructed 

with RCCP or PCCP. The reason for the difference is that RCCP and PCCP conduits are constructed 

with longitudinal reinforcement extending through the joints, allowing the conduits to bridge over 

weak foundation areas and to spread the effects related to variation in settlement. Welding of the conduit joints for HDPE and steel pipe results in higher strength and strain characteristics and increased 

resistance to distress caused by abrupt displacement. However, conduit foundations should be 

designed to provide uniform support.

Special precautions should be taken at joints where conduits connect to structures. The joint between 

an intake structure and a conduit is susceptible to differential settlement because of the potential 

FIGURE 6.64 PRESTRESSED CONCRETE CYLINDER PIPE

 (PCCP) DETAILS (LINED CYLINDER)

CONCRETE

CORE

MASTIC IN JOINT

PRESTRESS WIRE

CEMENT-MORTAR COATING

STEEL

CYLINDER

STEEL BELL

RING JOINT

MASTIC

IN JOINT

STEEL SPIGOT

 RING

RUBBER

GASKET

(FEMA, 2005a)

FIGURE 6.64 PRESTRESSED CONCRETE CYLINDER PIPE (PCCP) DETAILS (LINED CYLINDER)
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disparity in the weights and foundation conditions between the two structures. If both structures 

are constructed over relatively soft materials, the intake structure will tend to settle much more 

than the conduit. If the intake structure is constructed over engineered fill of low compressibility 

or on deep foundation elements, the conduit may tend to settle more than the intake structure. 

Either situation can lead to excessive deformation of the conduit joints in the vicinity of the intake 

structure. Under such circumstances, provisions to allow for relative movement may need to be 

incorporated into the design.

6.6.7 Blasting Impacts

Blasting must be conducted in such a manner as to prevent injury to site personnel and unacceptable impacts to structures associated with a coal refuse disposal facility, including refuse or earthen 

embankment dams and their impoundments. Impacts to off-site structures and properties must also 

be limited in accordance with applicable guidelines. Typically, embankment dams have very low natural frequencies (on the order of 1 hz) and thus are not particularly susceptible to damage due to blast 

vibrations, which have a much higher frequency range. If it is believed that blast effects could have a 

deleterious effect on site structures (such as for embankments developed by the upstream construction method), the impact of ground motion for the anticipated magnitude and frequency range of blast 

vibrations can be considered using the procedures for seismic stability described in Chapter 7. Some 

structures associated with fresh water dams, such as large concrete spillway channels, tall riser intake 

structures or pipelines under low confinement could possess natural frequencies similar to blast frequencies and thus be impacted by blasting. However, typical concrete structures and pipelines used at 

coal refuse facilities are normally not very susceptible to damage from blasting vibrations.

Structures are affected by blasting in relation to the peak particle velocity and frequency content of 

the ground motion induced by a blast. Simplified relationships are commonly used to determine 

FIGURE 6.65 COMPARISON OF PREDICTED AND

 OBSERVED CONDUIT SETTLEMENTS

INTAKE STRUCTURE

MAXIMUM WATER

SURFACE ELEV.

NORMAL WATER

SURFACE ELEV.

NON-PRESSURIZED

(FREE FLOW) CONDUIT

TERMINAL

STRUCTURE

ESTIMATED SETTLEMENT PROFILE

ACTUAL SETTLEMENT PROFILE BASED

ON MEASURED DISPLACEMENT OF

CONDUIT JOINTS)

DISTANCE (FEET)

SETTLEMENT (INCHES)

(FEMA, 2005a)

FIGURE 6.65 COMPARISON OF PREDICTED AND OBSERVED CONDUIT SETTLEMENTS
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the charge size relative to the horizontal distance to the monitoring point in order to meet acceptable motion (velocity) criteria. One such relationship for the peak particle velocity V resulting from 

a blast is (ISEE, 1998):

V = K(D/W 0.5 )-1.6 (6-33)

where:

K = site-specific constant determined from calibration test

D = distance to blast (length)

W = weight of charge (force)

Topographic and geologic variations between the blast location and observation point and the position of the blast horizon relative to the foundation of the structure can significantly affect the attenuation or amplification of blast-induced ground motions.

Acceptable vibration criteria are published in a number of sources: state regulatory programs typically provide guidance for peak particle velocity for common structure types, and general guidance 

can be found in Nichols et al. (1971), Siskind et al. (1980), ISEE (1998), and Hartman (1992). Blasting 

for excavation of rock materials is generally controlled to a peak particle velocity (PPV) of 4 inches per 

second for mass concrete structures (Hartman, 1992) and to 2 inches per second for typical steel and 

concrete superstructures. Notably, these PPV thresholds are generally very conservative and correlate 

to the possible onset of visible cosmetic damage. The noted structure types can typically tolerate much 

higher PPVs before structural damage occurs. Table 6.57 provides additional information on levels of 

damage to houses for specific particle velocities, and Figure 6.66 shows acceptable limits of vibration 

for houses, as recommended in Siskind (1980).

When blasting is planned within 500 feet of an active underground mine, the Surface Mining Control 

and Reclamation Act of 1977 requires approval of an operator’s blasting plan by OSM, or the appropriate state agency, and MSHA. Blasting regulations are provided in 30 CFR § 780.13 and 30 CFR § 

TABLE 6.57 COMMON RESIDENTIAL VELOCITY CRITERIA AND EFFECTS

Velocity Damage Level

0.5 in/sec Recommended limit to prevent threshold damage in plaster-on-lath construction near surface 

mines due to long-term, large-scale blasting operations (USBM, 1980).

0.75 in/sec Recommended limit to prevent threshold damage in sheetrock construction near surface 

mines (USBM, 1980).

1.0 in/sec Office of Surface Mining (OSM) regulatory limit for residences near surface mine operations at 

distances of 300 to 5,000 ft (long-term, large-scale blasting).

2.0 in/sec Widely accepted limit for residences near construction blasting and quarry blasting. Also 

allowed by OSM for frequencies above 30 hz (USBM; 1971, 1980).

5.4 in/sec Minor damage to the average house subjected to quarry blasting vibrations (USBM, 1971).

9.0 in/sec About 90 percent probability of minor damage from quarry blasting. Structural damage to some 

houses, depending on vibration source, characteristics and house construction.

20.0 in/sec For close-in construction blasting, minor damage to nearly all houses, structural damage to 

some. For low-frequency vibrations, structural damage to most houses.

(ADAPTED FROM ISEE, 1998)
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FREQUENCY (HZ)

PARTICLE VELOCITY (IN/SEC)

(ADAPTED FROM SISKIND ET AL., 1980)

FIGURE 6.66 ACCEPTABLE LIMITS OF VIBRA TION FOR HOUSES

1 10 100

0.1

1.0

10.0

4 15 40

DRYWALL

2.0 IN/SEC

PLASTER

0.50 IN/SEC

0.75 IN/SEC

816.61 through 816.68 and 816.79. State criteria may also be applicable. The blasting plan should be 

prepared by a professional licensed in the state where the blasting is to be performed. Potential concerns when an impoundment is present include fracturing of abutments, impacts to pipes and other 

rigid structures, and possibly impacts to upstream construction. The potential impacts should be 

evaluated, and monitoring of particle velocity with a seismograph may be appropriate. Monitoring 

of specific structures and features may be warranted and could include inspection of impounding 

embankment crests and slopes for evidence of cracks or displacements, review of piezometer data for 

evidence of water level fluctuations, and observation of concrete joints or crack apertures to verify the 

general integrity and to note any movements.

FIGURE 6.66 ACCEPTABLE LIMITS OF VIBRATION FOR HOUSES
Chapter 7

SEISMIC DESIGN: STABILITY

AND DEFORMATION ANALYSES

MAY 2009 7-1

7.1 GENERAL

7.1.1 Design Approach

Under certain conditions, seismic loadings from an earthquake or other source can cause embankment or foundation materials to lose strength, potentially causing a structure to become unstable. 

Coal refuse embankments constructed using the upstream construction method may be particularly 

susceptible to instability from an earthquake because a portion of the dam is constructed on soft or 

loose saturated, hydraulically-placed material. Designers should perform an evaluation (commensurate with the hazard potential of the structure) to confirm that dam and embankment designs provide 

an adequate margin of safety against seismically-induced instability.

The methods of exploration, testing, and analysis presented in this chapter are based on research 

and practice, publications, and experience on a variety of projects. They provide a variety of options 

for design. These methods, ranging from basic to sophisticated, have generally been applied on coal 

refuse disposal sites. Commentary is provided in the text to help explain the basis for the methods 

and the applicability of the methods to specific situations. It is recognized that refinements in the 

methods, as well as new methods, may be developed in the future. Designers are encouraged to evaluate such refinements, new methods, and other approaches that are technically sound, particularly if 

they better address site-specific materials or conditions. Designers are also cautioned that this subject 

is complex and that refinements of existing methods and development of new methods can require 

substantial research and investigation, as well as input from geologists, seismologists, geotechnical 

engineers, and other professionals.

This chapter refers to dams and embankments interchangeably. Also, references to “soil” or “material” encompass soils and coal refuse materials (e.g., fine coal refuse or tailings, filter cake, combined 

coal refuse, mixed refuse, coarse coal refuse, and amended refuse). As there have been no reported 

failures of coal refuse embankments due to seismic loading within the U.S. coalfields, the various 

studies of strength loss due to seismic loading are predominantly from sites that contain natural soils 

or other mine tailings. Thus, the physical behavior of coal refuse materials must be inferred from correlations, supported by laboratory testing.

The following published papers, studies and ongoing research on coal refuse materials and seismic 

design of disposal sites provide an overview of information specific to slurry impoundments and 

potential information on testing and parameters that may be available:
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• Gardner and Wu (2002) present an overview of challenges in evaluating strength loss 

at coal refuse disposal facilities from MSHA’s perspective. They summarize the available pore-pressure-based empirical methods (field standard penetration testing and 

cone penetration testing) and strain-based laboratory methods (undrained steadystate shear strength approach based on triaxial compression tests) for evaluation of 

potential strength loss at coal refuse disposal impoundments.

• Castro (2003) presents the undrained peak strengths and undrained steady-state 

strengths derived from cone penetration testing, field vane-shear tests, laboratory 

tests on undisturbed samples and laboratory-consolidated slurry samples. These 

strength data show that strength loss of fine tailings is noticeable and the undrained 

steady-state strength values are typically between one-half and one-fourth of the 

peak undrained strength. The paper also provides cyclic-triaxial test data for undisturbed samples of natural clayey silt of low plasticity, similar to fine tailings, to show 

the degradation of peak undrained strength with strain during cyclic loading.

• Genes et al. (2000) present the undrained steady-state shear strength approach for 

evaluation of strength loss at five coal refuse disposal facilities in West Virginia. 

Isotropically-consolidated, undrained triaxial compression tests of undisturbed and 

remolded samples of fine coal refuse from five different disposal sites are presented 

to show undrained steady-state shear strength variation with void ratio and effective 

vertical stress.

• Ulrich et al. (1991) present a pore-pressure-based evaluation using cyclic-triaxial tests 

on samples of fine coal refuse from sites in Kentucky, Ohio, and Tennessee. 

• Cowherd and Corda (1998) discuss pore-pressure-based empirical methods for triggering of strength loss at coal refuse dams and provide standard penetration tests 

data along with the measured seismic shear wave velocities for fine coal refuse, with 

a summary of cyclic-triaxial test results from four disposal sites.

• Hegazy et al. (2004) presents engineering properties for northern Appalachian coal 

refuse, including a summary of results of seismic piezocone testing and field vaneshear testing used for determining undrained shear strength.

• Kalinski and Phillips (2008) present a progress report on research being conducted 

at the University of Kentucky concerning development of dynamic properties of coal 

refuse. When completed, it will include field and laboratory testing on the dynamic 

behavior of coal refuse materials. Field standard penetration testing, cone-penetration testing, field vane-shear testing, seismic surface-wave testing, and downholeseismic testing are to be performed. Complementary laboratory cyclic-triaxial testing 

and resonant-column testing are also proposed for determining dynamic properties 

of coal refuse materials.

• Zeng and Goble (2008) present the results of laboratory testing (resonant-column and 

cyclic-triaxial tests) for determining dynamic properties (damping ratio and shear 

modulus) performed on Appalachian coal refuse at Case Western Reserve University.

Seismic design of dams and embankments involves two separate requirements:

1. Prevention of seismic instability (slides)

2. Prevention of excessive deformations (translation, settlement, and cracking)

7.1.1.1 Seismic Instability

The ground motion from an earthquake can result in a reduction in the shear strength of loose, saturated materials. Seismic instability may occur when post-earthquake shear strength is less than the 
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Seismic Design: Stability and Deformation Analyses

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pre-earthquake shear strength in one or more significant zones of an embankment or foundation. 

The driving force of the seismic instability is the static (gravity) weight of the embankment. Seismic 

instability is a particular concern for dams with substantial upstream construction because a portion 

of the dam is constructed on hydraulically-placed fine material. For seismic instability to occur, three 

conditions must develop:

1. The earthquake shaking must be strong enough to trigger undrained strength loss in 

one or more zones of material.

2. The strength loss must be significant enough that the post-earthquake shear 

strengths are less than the static driving shear stresses.

3. The location and amount of the material that experiences strength loss must be sufficient to generate instability.

Seismic stability is generally analyzed as a static (i.e., no seismic coefficient) limit-equilibrium, slopestability problem, using post-earthquake shear strengths for the materials in the embankment and 

foundation. The earthquake shaking causes the material in the embankment or foundation to lose 

strength, but the static gravity shear stresses drive the failure. Some instability failures have been 

observed to occur after the earthquake shaking has stopped (Seed et al., 2003; Seed and Harder, 1990; 

Marcuson, Hynes and Franklin, 1992).

Experience has shown that when significant strength loss occurs in critical sections of a structure: 

(1) failures are often rapid, (2) they occur with little warning, and (3) the resulting deformations are 

often very large. Experience has also shown that the trigger events can be quite small. Hence, seismic 

design for significant- to high-hazard-potential dams and embankments should be carried out with 

caution and care.

7.1.1.2 Excessive Deformations

If seismic stability analyses indicate that an embankment is unstable, then deformations should be 

considered to be unacceptably large. However, if seismic stability analyses indicate that an embankment is stable, then potential seismic deformations should be assessed. Seismic deformations occur 

primarily during earthquake shaking. The cyclic-shear stresses induced by the earthquake contribute directly to the deformations. This contrasts with the primary mechanisms of instability. In seismic instability, the earthquake shaking causes undrained strength loss, but the static gravity stresses 

drive the instability failure.

The material making up the dam or embankment, the fine coal refuse or tailings retained behind 

and sometimes underlying the embankment, and the natural soil below the embankment must all be 

evaluated as part of stability and deformation analyses.

The basic elements for seismic design and analysis require evaluation of:

• Susceptibility of materials to strength loss and post-earthquake strengths

• Seismic stability using post-earthquake strengths

• Whether the design earthquake will trigger strength loss

• Deformations

7.1.2 Seismic Design Considerations and Flow Chart

The following points were considered in developing the guidance and recommendations presented 

in this chapter:
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MAY 2009

• The levels of analysis that should be performed vary depending on the type of facility and the consequences of failure. So, for example, no seismic analysis is required 

for low-hazard-potential dams (provided static stability is satisfied), while seismic 

stability and deformation analyses are required for high-hazard-potential dams.

• Methods for evaluating the susceptibility of a material to strength loss during an 

earthquake and for evaluating the degree of strength loss depend partly on whether 

the material is sand-like or clay-like. Fine coal refuse within a structure, and natural 

soil deposits in the foundation, might include zones of both sand-like and clay-like 

material. Therefore, methods for evaluating both sand-like and clay-like material are 

provided. These methods apply to both coal refuse materials and soil.

• Straightforward screening methods should be available for differentiating zones that 

are potentially susceptible to seismically-induced strength loss from zones that are 

not susceptible. Further detailed investigation and evaluation can then be focused on 

the potentially-susceptible zones. This chapter presents screening methods for both 

clay-like and sand-like material that require only the basic information provided by 

Standard Penetration Test (SPT) or Cone Penetration Test (CPT) data, grain-size test 

results, and Atterberg-limit data.

• Relatively straightforward methods of analysis should be available to designers, as 

well as methods that are more sophisticated. The more sophisticated methods may 

allow for less conservatism in the design and might be worthwhile for achieving a 

more economical design. However, the more sophisticated methods are optional, not 

required. For example, relatively straightforward field testing methods can be used 

to estimate post-earthquake strength, as well as more sophisticated, optional, laboratory methods. Another example is that seismic-stability analyses can be performed by 

simply assuming that the design earthquake triggers strength loss in materials that are 

potentially susceptible to strength loss, or an optional triggering analysis can be performed to evaluate whether the design earthquake is in fact strong enough to trigger 

strength loss. The authors of this chapter note that triggering analyses are not considered to be appropriate for sand-like materials for design earthquakes that exceed 

certain criteria and therefore impose significant seismic stresses on the materials. In 

designing new structures, it is often prudent to design based on the relatively straightforward methods rather than using more sophisticated methods to justify a design.

• The level of detail required for evaluation of the seismicity of a site should depend 

on the level of seismic hazard at the site. Many coal mining regions in the U.S. are in 

areas of low seismic hazard. Minimum parameters for the design earthquake in these 

areas are provided, and a site-specific evaluation is not recommended. For sites in 

areas of higher seismic hazard, a site-specific seismicity evaluation is recommended.

• The various credible methods employed by geotechnical engineers experienced 

in the seismic design of dams should be available for use. Therefore, this chapter 

presents three methods for analyzing the triggering of strength loss in loose, sandlike material: (1) the pore-pressure-based approach developed by Seed and updated 

by Youd et al. (2001), (2) the strain-based approach developed by Castro (1994), and 

(3) the stress-based approach developed by Olson and Stark (2003). Several field 

and laboratory methods and correlations for estimating post-earthquake strength of 

materials that are susceptible to strength loss and several methods for performing 

deformation analyses are also presented.

• Structures should generally have a safety factor of at least 1.2 for seismic stability 

based on a static stability analysis using post-earthquake material strengths. This 

safety factor is intended to account for uncertainties in the geometry of the structure, 
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Seismic Design: Stability and Deformation Analyses

MAY 2009

in the shear strength, and in the delineation of zones that are potentially susceptible 

to strength loss, and it also helps achieve designs for which predicted seismic deformations are within acceptable limits.

• There may be special cases involving existing facilities for which the recommended 

design criteria can be relaxed. Examples of these special cases include minor modifications made as part of closure activities in which the progress toward closure 

will eventually improve seismic stability, and interim improvements for addressing 

a specific existing deficiency (e.g., adding an interim stage to provide needed freeboard).

The recommended steps for a seismic evaluation or design are illustrated in the flow chart presented 

in Figures 7.1a, 7.1b and 7.1c. These steps are described in detail in Section 7.4.4. A relatively straightforward path through the seismic stability portion of the flow chart (in which triggering analyses, 

sophisticated laboratory testing, and seismicity evaluations are avoided) is described in Section 7.1.5.

The steps in the flow chart in Figure 7.1a can be summarized as follows:

1. Classify the structure and foundation based on type, size, downstream hazard potential, and anticipated performance under seismic loading, per the criteria indicated 

in Boxes 1, 2, and 3 of the flow chart.

2. Considering the classification in Step 1, and a conservative evaluation of postearthquake stability (optional, Boxes 5 and 6), categorize the structure and foundation as either (1) further seismic evaluation is not needed (go to Box 4) or (2) potentially susceptible to seismic instability such that additional analysis is required (go 

to Box 7).

3. For those structures that are potentially susceptible to seismic instability, thoroughly 

characterize the soils and refuse in the structure and foundation (Box 7 of flow chart 

and Section 7.3 of text). This step generally requires a significant effort because the 

spatial distribution of the refuse materials can be variable. Identify zones in the 

structure and foundation that may be susceptible to strength loss due to earthquake 

shaking (Section 7.4.4.2).

4. Analyze the stability of the embankment using post-earthquake strengths (Section 7.4.3). Post-earthquake strengths will be lower than pre-earthquake (static) 

strengths for zones that are susceptible to strength loss. This analysis may be relatively straightforward based on field testing data and laboratory index testing (Boxes 

7, 7A, and 8). At coal refuse disposal sites where the ratio of coarse to fine refuse is 

large enough to allow design of massive (wide) embankments with broad crests, 

employing the basic and more straightforward methods are recommended because 

of their relative simplicity in design and regulatory review and their conservatism. 

Alternatively, sophisticated laboratory testing can be used to provide better estimates of post-earthquake strength, and relatively complex triggering analyses can be 

performed to evaluate whether the earthquake shaking is actually strong enough to 

trigger strength loss in the materials that are potentially susceptible to strength loss 

(Boxes 9 through 16 and Sections 7.4.2 and 7.4.3.2). A seismic hazard evaluation (Section 7.7 and Figure 7.23) may be needed as part of these more sophisticated testing 

and analysis methods to define the magnitude and peak ground acceleration of the 

design earthquake and to obtain representative time histories of acceleration. These 

more sophisticated testing and analysis methods are often less conservative than the 

basic and more straightforward methods. The added costs of the more sophisticated 

testing and analysis may be justified by designs that are more economical or more 

efficient from an operational standpoint.
7-6

Chapter 7

MAY 2009

Classify structure

and foundation

Initial Exploration and Site Characterization

For existing structures this will include review of construction records, some subsurface exploration and some laboratory testing. Generally more exploration and testing will be required for structures in Box 3 than in Boxes 1 and 2.

For new structures, properties of proposed materials must be estimated. See 30 CFR 77.216 for structures requiring a

design plan approved by MSHA.

Non-Impounding Embankments That Clearly Have No

Loose Zones of Significance

with Respect to Seismic

Stability.

 Coarse Coal Refuse

Embankments

 Combined Refuse

Embankments

 Isolated Slurry Cells/

Filter Cake Cells

Low-Hazard-Potential Dams

Non-Impounding Embankments That May Have or Clearly Have Significant

Loose Zones

 Segregated Refuse Embankments

 Stacked, Overlapping or High Slurry Cell/Filter Cake Cell Embankments

 Embankments with potentially Sensitive Foundations

 Upstream-Constructed Embankments or Embankment Stages

Small, Significant-Hazard-Potential Dams

Dams less than 40 feet high or impoundments with less than 1,000 acre-feet

capacity that are “well constructed” and are unlikely to have issues with respect to

seismic stability.

Evaluate Post-Earthquake Stability

For sand-like material, use conservative estimates of Sus for any suspect zones. For

sand-like material with N(1,60) > 15, use the drained strength. For soft clay-like

material (N < 6) use Sus. For stiff clay-like material use Sup. Differentiate clay-like

material from sand-like material based on gradation, Atterberg limits, and (if

available) CPT.

For existing embankments and dams, it may be necessary to obtain field data

related to the level of the phreatic surface, piezometric levels within the dam or

embankment, the geometry of fine and coarse refuse within the dam or embankment, and material properties, particularly the degree to which fine refuse may have

consolidated due to the weight of overlying material. Only limited data are required

at this point, as the evaluation is intended as a conservative screening analysis.

The purpose of this step is to decide whether analysis of the structure should start at

Box 1 or Box 3. This step may be skipped, in which case the analysis of the

structure starts at Box 3.

High-Hazard-Potential

Dams

Large, Significant-HazardPotential Dams

Definition of Problem Geometry and Material Characterization

If necessary, perform additional subsurface investigations to define the geometry of

embankment zones, obtain SPT and/or CPT data, identify the phreatic surface and

piezometric levels within the refuse to estimate the degree of fine refuse consolidation, and obtain samples for index testing. Classify materials as clay-like or sandlike. For zones of clay-like refuse, it may be desirable to obtain undisturbed tube

samples for laboratory tests to measure Sup and to perform field vane shear testing

for estimating Sup and Sus.

Initial Seismic Stability Analysis

Evaluate static stability using post earthquake strengths. For loose sand-like

material, use Sus values based on SPT and/or CPT data. For sand-like material with

N(1,60) > 15, use the drained strength. For soft clay-like material (N < 6) use Sus. For

stiff clay-like material use Su equal to 80 to 100 percent of Sup (Section 7.4.4.3).

Reconfigure cross section to

obtain FS > 1.2, or perform

triggering and seismic stability

analysis (Figure 7.1b)

Stability is acceptable. Perform

Deformation Screening and

Analysis (Figure 7.1c).

Confirm that Static

Stability is Adequate

Triggering and Seismic

Stability Analysis and

Deformation Screening

steps and Deformation

Analysis are not required.

MSHA (2007) cites that

an impounding structure

should normally have a

minimum seismic stability

factor of safety of 1.2 and

allows for pseudo-static

analysis where embankment or foundation

materials are not subject

to significant strength loss

as a result of seismic

loading.

Is FS > 1.2?

Is FS > 1.5?

YES

YES NO

NO

1 2 3

4

5

6

7

7A

8

FIGURE 7.1a SEISMIC STABILITY SCREENING
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Seismic Design: Stability and Deformation Analyses

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Steady-State Laboratory Testing Program

For loose, sand-like materials and N(1,60) < 15 or qt1 < 75, consider performing a steady-state laboratory testing program

on undisturbed and reconstituted samples to refine values of Sus.

This step and Box 10 may be deferred or skipped entirely, especially if loose, sand-like zones are not significant with

respect to stability. OK to skip to Box 11.

Perform cyclic tests followed by static

tests to obtain better estimates of

strength reduction due to earthquake.

Requires Design Earthquake number

of cycles and seismic shear stresses.

(Correlations of earthquake-induced

strain to post-EQ strength may be

developed in the future.)

Alternatively, use Sus.

Perform Triggering Analyses

Perform analyses to evaluate if earthquake will trigger strength loss. A seismic hazard assessment will be required for

evaluating the Magnitude and PGA for the Design Earthquake and possibly for obtaining ground motion records.

Alternatively, assume that the post-earthquake strength for loose sand-like zones and soft clay-like zones is Sus and skip

to Box 16.

If PGA < 0.2g or CSR in loose zone

< 0.15

Use Youd et al. (2001) to determine if

earthquake will trigger strength loss.

 FS > 1.4 - No strength loss

 FS < 1.0 - Strength loss to Sus

1.0 < FS <1.4 - Assume strength loss

to Sus unless strain-based or stressbased method indicates no strength

loss.

Perform strain-based triggering analysis

(possibly including laboratory testing) or

stress-based triggering analysis to

determine whether the earthquake is

large enough to trigger strength loss.

Alternatively, assume that triggering

occurs and skip to Box 16.

Structures in Area of Low Seismic Hazard (Section 7.7.3.7)

Modify design to provide stability and confidence that deformations or the consequences thereof would be appropriately

limited and therefore deformation screening is adequate as per Figure 7.1c. Alternatively, perform additional field and

laboratory testing and/or use alternatives presented in Boxes 9 through 15 to obtain less conservative estimates of

material properties.

Structures in Area of Moderate to High Seismic Hazard (Section 7.7.3.7)

Same as structures in area of low seismic hazard, except that a deformation analysis will be required.

In Certain Cases, Consider Relaxing Design Criteria

1. If making interim improvements to address a specific deficiency (e.g., adding an interim stage to improve freeboard).

2. For modifications as part of closure activities in cases where the closure activities will improve the seismic stability.

3. For relatively minor modifications to existing structures in areas of low seismic hazard.

9

11

Re-analyze post-EQ

stability considering both

SPT and CPT data and

results of Box 9.

Is FS > 1.2?

Re-analyze post-EQ

stability with new

information from Boxes 11

through 15.

Is FS > 1.2?

Evaluate PGA and

EQ magnitude and

compute CSR

If PGA > 0.2g and CSR in loose zone

> than 0.15

Assume that earthquake will trigger

strength loss.

Stability is Acceptable

Perform Deformation

Screening and

Analysis (Figure 7.1c)

For Loose Sand-Like Zones

For Soft Clay-Like Zones

NO

NO

YES

YES

10

17

16

18

12 14 15

13

FIGURE 7.1b TRIGGERING AND SEISMIC-STABILITY ANALYSIS
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Chapter 7

MAY 2009

Requires seismic hazard assessment (SHA) for evaluation of

Magnitude and PGA for Design

Earthquake and possibly obtaining

ground motion records.

Is the site in an area of low

seismic hazard

(Section 7.7.3.7)?

Is the predicted porepressure buildup in sandlike materials small (CRR/

CSR > 1.0)?

Is the safety factor for postearthquake stability > 1.2

for failure surface through

clay-like material?

SIMPLIFIED

DEFORMATION

ANALYSIS

Are criteria for use of the

pseudo-static procedure for

preliminary screening met?

Does the pseudo-static FS

meet required criteria?

Deformations are considered

acceptably small. No further

analysis is required.

DETAILED

DEFORMATION

ANALYSIS

Are the computed

deformations acceptable?

New and Existing Structures

Modify design to reduce deformations and/or to increase available freeboard. Re-evaluate modified design as

necessary.

In Certain Cases, Consider Relaxing Design Criteria

1. If making interim improvements to address a specific deficiency (e.g., adding an interim stage to improve freeboard).

2. For modifications as part of closure activities in cases where the closure activities will improve the seismic stability.

3. For relatively minor modifications to existing structures in areas of low seismic hazard,

DEFORMATION ANALYSIS

DEFORMATION SCREENING

FIGURE 7.1c DEFORMATION SCREENING AND ANALYSIS

Perform either:

1. Newmark Analysis if

 CRR/CSR > 1.0, or

2. Numerical Analysis without

 Cyclic Mobility if

 CRR/CSR > 1.0, or

3. Numerical Analysis with

 Cyclic Mobility if

 CRR/CSR < 1.0.

NO

YES

YES

YES

NO

NO

NO

NO

YES

YES

YES

NO

D1

D2

D3

D4

D5

D6

D7

D8

D9

5. If stability is acceptable (safety factor of 1.2 or higher), evaluate potential deformations (Step 7).

6. If stability is not acceptable, redesign or modify the embankment until stability is acceptable (Box 17).

7. Evaluate potential deformations of the embankment caused by the earthquake shaking (Boxes D1 through D8 and Section 7.5). The deformation analysis may involve a 

FIGURE 7.1c DEFORMATION SCREENING AND ANALYSIS
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Seismic Design: Stability and Deformation Analyses

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relatively simple screening analysis or may require sophisticated computer modeling. If not performed as part of Step 4, a seismic-hazard evaluation (Section 7.7 and 

Figure 7.23) will be needed as part of the deformation analysis.

8. If the estimated deformations are within an acceptable range, accept the design. Otherwise, redesign or modify the embankment (Box D9).

7.1.3 Sand-Like Versus Clay-Like Material

For many of the analyses described in this chapter, fine coal refuse and natural soils are referred to 

as sand-like or clay-like depending on whether they exhibit monotonic and cyclic undrained shear 

loading behavior that is fundamentally more similar to that of either sand or clay. The methods for 

evaluating susceptibility to strength loss, triggering, and post-earthquake strength are different for 

sand-like and clay-like materials. This differentiation is significant primarily if the material is loose 

enough (sands) or soft or sensitive enough (clays) that it is potentially susceptible to strength loss.

The key factors in differentiating loose sand-like material from soft or medium clay-like material, for 

the purposes of seismic stability and deformation analyses, are the strain at peak undrained strength 

and the abruptness of the drop-off in shearing resistance as strains increase beyond the strain at 

peak. Loose sands and highly sensitive clays can reach peak undrained strength Sup at small strains, 

and experience abrupt drop-off in resistance at higher strains. Most clays tend to reach Sup at higher 

strains, and tend to experience more gradual and limited drop-off in shearing resistance at higher 

strains. Fine coal refuse deposits often include materials falling within both classifications, and near 

the boundary of these two types of behavior.

Loose material with shear strain at peak strength of less than 2 percent in an undrained monotonic 

(non-cyclic) test, and a rapid drop-off in resistance after reaching peak strength, is generally considered sand-like (although highly sensitive clays may exhibit similar behavior). Loose or soft material 

with shear strain at peak strength of more than about 5 percent, and a gradual drop-off in resistance 

after reaching peak strength is considered clay-like. Figure 7.2 illustrates the associated stress-strain 

curves for these materials. Material with strain behavior between these descriptions is considered 

borderline. It should be noted that shear strain in an undrained triaxial test is 1.5 times axial strain. 

Peak strength refers to peak principal stress difference (σ1 ‒ σ3).

For the analyses in this chapter, it is generally more conservative to assume that a borderline material 

is sand-like than to assume it is clay-like. It is very difficult to obtain or prepare samples of in-situ 

low plasticity material for strength testing at its in-situ void ratio. Therefore, Atterberg-limits tests, 

gradation tests, and, preferably, CPT data should be used first as an index of stress-strain behavior to 

categorize materials as sand-like or clay-like. Laboratory stress-strain testing should be used to help 

categorize borderline materials.

The following criteria are recommended:

• Atterberg limits and gradation – Material should be treated as sand-like if the plasticity index of the material (measured from the portion passing the No. 40 sieve) is 

7 or less.

Material should be considered clay-like if all of the following criteria are met:

− The material has 35 percent or more by dry weight passing the No. 40 sieve.

− The material has 20 percent or more by dry weight passing the No. 200 sieve.

− The plasticity index of the material (as measured by the portion passing the 

No. 40 sieve) is 10 or higher.
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Chapter 7

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Material that does not fit either set of criteria should be treated as sand-like unless 

optional laboratory stress-strain testing demonstrates otherwise.

Commentary: In general, it is more conservative to treat a material as sand-like rather than as 

clay-like. The gradation and Atterberg-limit criteria provided above were selected as reasonably 

conservative recommended guidance by the authors of this chapter. These Atterberg-limit and 

gradation criteria may include as sand-like some materials that are considered clay-like by some 

investigators (e.g., Boulanger and Idriss (2004) who suggested a break between sand-like and 

clay-like behavior at a PI of 7). Seed et al. (2003) point out that there is no general agreement 

on Atterberg-limit criteria, and suggest a PI of 12 as one guideline for evaluating soil behavior, 

but they do not specifically refer to clay-like versus sand-like behavior.

• Cone penetration test data – CPT data can be used in conjunction with Atterberglimit and gradation data to differentiate sand-like from clay-like material. CPT Soil 

Behavior Type Index Ic values below 2.6 should be considered sand-like (Robertson 

and Wride, 1998; Youd et al., 2001). Ic values above 2.6 may be considered clay-like. 

Material with Ic values above 2.6, but with very low values of friction ratio, should 

be considered clay-like but potentially highly sensitive (Section 7.4.4.2). If CPT data 

conflict with Atterberg limits and gradation data, the Atterberg limits and gradation 

data should generally govern. CPT data may also be used as an index to compare 

zones where Atterberg limits and gradation were measured with zones where Atterberg limits and gradation were not measured.

Commentary: Soil Behavior Type is based on where CPT data fall on a log-log plot of normalized CPT tip resistance versus friction ratio. Olsen and Mitchell (1995), reproduced in Seed et 

al. (2003), suggested a similar soil characterization framework based on the same type of plot.

• Laboratory test data – As mentioned previously, borderline materials (PI between 7 

and 10 or otherwise not clearly sand-like or clay-like) should be classified as sandlike unless laboratory testing demonstrates otherwise. Laboratory testing for this 

purpose is optional. If performed, the testing should consist of isotropically-consolidated, strain-controlled, undrained triaxial compression tests with pore-pressure 

measurement performed on undisturbed samples. For the purpose of this testing, 

undisturbed sampling as described in Section 6.4.3.5 is recommended, considering also the discussion in Section 7.3 regarding sample quality. For this testing, the 

material has a PI of 7 or higher and is therefore less likely to densify during sampling 

than a less plastic sand-like material would be. Therefore, the detailed measurements 

of densification discussed in Appendices 7B and 7C for sand-like materials are not 

necessary for this sampling and testing. A consolidation stress of two-thirds of the 

in-situ vertical effective stress should be used to model in-situ conditions. If the axial 

strain at peak strength (peak principal stress difference, σ1 ‒ σ3) is less than or equal 

to 3.33 percent (5 percent shear strain), then the material should be considered sandlike. If the axial strain at peak strength is more than 3.33 percent (5 percent shear 

strain) then the material should be considered clay-like.

• Highly sensitive clay-like and borderline materials – In very rare occurrences, natural soils and (non-coal) mine tailings have been identified that have a Liquidity Index 

greater than 1 (i.e., a natural water content greater than their Liquid Limit), a very 

high sensitivity (i.e., significant and abrupt strength loss), and low shear strain at 

peak strength. If materials such as this are encountered, special evaluations outside 

the scope of this manual may be appropriate. As a conservative approach, these materials may be assigned a post-earthquake strength of 20 psf (Table 7.1).
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Seismic Design: Stability and Deformation Analyses

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Loose zones of coarse refuse (e.g., zones of uncontrolled placement and compaction) should be 

treated as sand-like for the analyses discussed in this chapter. Combined refuse and intermixed zones 

of coarse and fine refuse should first be differentiated as predominantly coarse or fine refuse based on 

grain size and whether the coarse or fine refuse will control the response to loading. If the combined 

or mixed refuse is considered to be predominantly fine material, it should then be classified as sandlike or clay-like for the analyses in this chapter.

Soils, rock fill, and mixtures of soil and rock fill that are not coal refuse, whether part of a natural 

deposit, mine spoil, or other fill, will be referred to as natural soils. Natural soils can be described as 

coarse, sand-like, or clay-like, using the same general criteria as for refuse.

As discussed in Section 6.2.3.2, fine coal refuse is often classified as a slightly plastic sandy silt or clay, 

or a slightly plastic to non-plastic silty sand. The Plasticity Index varies from non-plastic (PI = 0) to PI

> 10. In other words, fine coal refuse may be sand-like, clay-like, or borderline. Fine coal refuse (and 

some natural soils) may be highly stratified in very thin layers. CPT data are often useful in delineating this stratification. The designer must make a judgment as to whether clay-like or sand-like layers 

will control the behavior of individual zones. In general, zones consisting of stratified layers of sandlike and clay-like material should be considered sand-like, unless detailed field or laboratory data 

indicate otherwise.

7.1.4 Susceptibility to Strength Loss

As discussed in Section 7.4.4.2, saturated or nearly-saturated, loose, sand-like material with N1,60 

values less than 15 or qt1 values less than 75 tsf (as defined in Section 7.2) is considered potentially 

contractive (potentially susceptible to strength loss). As also discussed in Section 7.4.4.2, clay-like 

material with CPT values of tip resistance and side friction falling in certain ranges or with SPT 

N-values less than 6 (corrected for hammer efficiency per Youd et al. (2001), but not corrected for 

overburden pressure) is considered potentially susceptible to strength loss. These criteria are used to 

determine whether various zones within the embankment require further evaluation.

FIGURE 7.2 TYPICAL UNDRAINED STRESS-STRAIN CURVES

SHEAR STRAIN (%)

SOFT CLAY

LOOSE SAND

STRESS STRESS

0 5 10 15 20 25

FIGURE 7.2 TYPICAL UNDRAINED STRESS-STRAIN CURVE
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Chapter 7

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7.1.5 Simplified Steps for Seismic Stability

While a variety of methods for exploration, testing, and analysis are described in this chapter, simplified steps that provide a direct path for evaluating seismic stability are available. As introduced in 

Section 7.1.2, the recommended sequence of analysis is described in the flow charts in Figures 7.1a, 

7.1b and 7.1c, which allow the option of either skipping or incorporating sophisticated methods.

The intent of this section is to describe simplified steps for estimating soil and refuse properties for a 

relatively straightforward analysis without the need for extensive testing and analysis that will lead 

to safe designs based on supportable conservative assumptions and correlations. However, for existing structures there may be no simplified methodology for characterizing the existing conditions (i.e., 

finding out what types of soils are present and where). The simplified analyses and estimates of engineering soil properties are necessarily conservative, and in some cases a more detailed analysis may 

show them to be very conservative. Optional steps, which require additional testing and analysis to 

more accurately measure or predict material behavior and thereby reduce the conservatism in these 

simplified steps, can be considered and are discussed in other sections of this chapter.

The following basic steps describe a relatively straightforward sequence that can be followed for 

design or evaluation of a new or existing high-hazard-potential slurry impoundment dam (or other 

dam). (Box numbers refer to the flow charts in Figures 7.1a, 7.1b and 7.1c.) The steps described in the 

following text do not require sophisticated laboratory testing or relatively complex triggering analyses. A site-specific seismicity evaluation is only required for deformation analyses (not for seismic 

stability analyses) and may not be needed if the site is in a low-seismic-hazard area (Section 7.7.3.7).

7.1.5.1 Step 1

For a high-hazard-potential dam, proceed from Box 3 to Boxes 7 and 7A. Define the geometry of the 

dam and identify zones of clay-like and sand-like material (per Section 7.1.3). For an existing dam, 

the geometry will be based on construction records, a site survey, the results of borings and in-situ 

testing (preferably CPTs), and laboratory index testing (grain size and Atterberg limits). For design 

of a new dam, the foundation materials and properties will be based on exploration and testing programs, and embankment geometry will be based on the development plan and anticipated construction procedures and refuse properties.

7.1.5.2 Step 2

Characterize the fine refuse to address stratification, layering, distinction of sand-like and clay-like 

materials, and estimation of strength and associated properties. As discussed in Section 7.3, Cone 

Penetration Testing (CPT) in combination with recovery of samples (using SPTs, for example) is recommended for addressing these issues. 

7.1.5.3 Step 3

Characterize the fine refuse deposits (and other materials) as either clay-like or sand-like, per the 

criteria given in Section 7.1.3, based on laboratory grain-size and Atterberg-limits tests.

7.1.5.4 Step 4

Screen the various zones in the dam for susceptibility to strength loss based on the SPT and (preferably) CPT data. Basically (as detailed in Section 7.4.4.2.1), sand-like material with N1,60 values less 

than 15 or qt1 values less than 75 is considered potentially susceptible to strength loss. Clay-like material with N values less than 6 or CPT data in certain ranges is also considered potentially susceptible 

to strength loss. Screening to determine the susceptibility to strength loss should consider both CPT 

and SPT data when available. If such testing produces inconsistent results, it should be resolved in 

favor of the more reliable and robust data set considering the amount, consistency and quality of the 
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Seismic Design: Stability and Deformation Analyses

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data. For design of a proposed dam, zones potentially susceptible to strength loss should be defined 

based on experience, available data from nearby sites, and published data for similar facilities. For 

slurry impoundments, consider similarities in coal seams, mining and processing methods, and fine 

refuse deposition.

7.1.5.5 Step 5

Perform a limit-equilibrium stability analysis using conservative post-earthquake strengths (as 

detailed in Section 7.4.4.3). For zones that are screened as not being potentially susceptible to strength 

loss, the post-earthquake strengths are the same as those used in the static stability analysis.

For sand-like material considered susceptible to strength loss, the appropriate strength is the undrained shear strength at very high strain (undrained steady-state strength Sus), which can be obtained 

from correlations with SPT and/or CPT data (Section 7.4.3.1). Even more simply, as a lower bound, Sus

for sand-like material can be taken as 0.04 σ’v (if the material is non-plastic or has a liquidity index 

of less than 1), as discussed in Section 7.4.3.1 and as shown in Table 7.1. For design of a new dam, Sus

can be taken as 0.04 σ’v, or higher values may be used based on supporting data for similar materials 

for existing dams.

For clay-like material considered susceptible to strength loss (e.g., highly sensitive clay-like material), the appropriate post-earthquake strength is also Sus, which can be obtained from field vaneshear tests or from CPT data (as described in Section 7.4.3.3). A lower-bound, post-earthquake 

strength for clay-like material can be taken as 0.04 σ’v (if the material has a liquidity index less 

than 1), as discussed in Section 7.4.3.3 and as shown in Table 7.1. For design of a new dam, Sus can 

be taken as 0.04 σ’v, or higher values may be used based on supporting data for similar materials 

at existing dams. 

Commentary: The lower-bound, post-earthquake strength of 0.04 σ’v is considered conservative for either 

sand-like or clay-like fine coal refuse. As more testing and analysis of fine refuse becomes available, refinements 

to the 0.04 relationship can be expected (initial published work is discussed in Section 7.4.3.2.3). For proposed 

dams, strengths estimated during design must be validated during construction by performing field investigations and obtaining samples of the actual materials.

For clay-like material that is not considered susceptible to strength loss, the appropriate post-earthquake strength is typically much higher than Sus. Guidance for characterizing the post-earthquake 

undrained shear strength of such clay-like material, based primarily on consistency and strain and/or 

cyclic stress level considerations, is provided in later sections.

If the safety factor for the stability analysis using post-earthquake strengths is greater than or equal 

to 1.2, continue by performing a deformation analysis (from Box 8 go to Box D1). If the safety factor is 

less than 1.2, redesign the dam (skip from Box 8 to Box 17) to reduce the influence of material zones 

susceptible to strength loss. Alternatively, if the safety factor is less than 1.2, consider performing 

more detailed evaluations of triggering and post earthquake strength, as described in Sections 7.4.3

and 7.4.4.

7.1.5.6 Step 6

There are several methods of deformation analysis that can be used, depending on the site seismicity 

and the presence of material susceptible to strength loss. If the dam is in an area of low seismic hazard 

and is not influenced by materials that are potentially susceptible to strength loss, then a relatively 

simple pseudo-static procedure can be used (as described in Section 7.5.3). Otherwise, more complex 

analyses are required (Section 7.5 and portion of flow chart in Figure 7.1c), and the computed deformations must be compared to recommended tolerable values.
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Chapter 7

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 TABLE 7.1 COMPARISON OF BASIC CRITERIA FOR SAND-LIKE, CLAY-LIKE AND 

BORDERLINE MATERIALS

Sand-Like Clay-Like Borderline (Treat as 

Sand-Like)

Borderline (Treat as 

Clay-Like)

Atterberg limits

% passing No. 40 sieve

% passing No. 200 sieve

PI ≤ 7 PI ≥ 10

≥ 35

≥ 20

7 < PI < 10 7 < PI < 10

≥ 35

≥ 20

Triaxial tests on undisturbed samples to 

obtain stress-strain 

curve

Not Required Not Required Not Required

Shear strain at peak 

strength must exceed 

5%; otherwise treat as 

sand-like

Lower-bound postearthquake strength0.04 σ’v if non-plastic, 

or if LI is < 1.0

20 psf if LI ≥ 1.0,

but no higher than

0.04 σ’v

0.04 σ’v if LI is < 1.0

20 psf if LI ≥ 1.0,

but no higher than

0.04 σ’v

0.04 σ’v if LI is < 1.0

20 psf if LI ≥ 1.0,

but no higher than

0.04 σ’v

0.04 σ’v if LI is < 1.0

20 psf if LI ≥ 1.0,

but no higher than

0.04 σ’v

Other methods to obtain 

post-earthquake strength

1. Correlations with

 SPT/CPT

2. Steady-state lab

 testing

1. Field vane shear

 or CPT

2. Cyclic followed by

 static lab testing

Correlations with

SPT/CPT

1. Field vane shear

 or CPT

2. Cyclic followed by

 static lab testing

Field vane-shear testing 

for peak-undrained 

strength and Sus

Not Applicable Applicable

Potentially Applicable if 

it can be demonstrated 

that the test is undrained 

(Section 6.4.3.8)

Potentially Applicable if 

it can be demonstrated 

that the test is undrained 

(Section 6.4.3.8)

CPT to help identify 

layering and to 

differentiate sand-like 

from clay-like

Recommended Recommended Recommended Recommended

CPT to measure peak 

undrained strength and 

Sus

Not Applicable Applicable

Potentially Applicable if 

it can be demonstrated 

that the test is undrained

(Section 6.4.3.7)

Potentially Applicable if 

it can be demonstrated 

that the test is undrained

(Section 6.4.3.7)

 Note: 1. PI is the Plasticity Index.

 2. LI is the Liquidity Index.

7.2 TERMINOLOGY

The terms defined in the following list are specifically related to seismic stability and deformation 

analyses. These definitions should be reviewed carefully because in the literature authors may assign 

various meanings to the same terms. Basic terminology related to subsurface investigations, material 

strength, and static stability analysis is discussed in Chapter 6.

Clay-like material – This term is defined in Section 7.1.3.

Contractive soil or refuse – Contractive materials compress when sheared under drained 

conditions. They generate positive excess pore pressures when sheared under undrained 

conditions. Thus the undrained strengths are lower than the drained strengths. Loose sandlike material tends to be contractive. When sheared undrained, the peak strength is typically 

reached at relatively low strain, and strength can decrease (i.e. strain-soften) significantly at 

higher strains. Soft clay-like material also tends to be contractive. However, when sheared 

undrained, the peak strength develops at relatively high strains and the strength decreases 

gradually at strains beyond the strain at peak strength.

Cyclic mobility – Cyclic mobility is the progressive softening and resulting large cyclic 

strains that occur in saturated soils during undrained cyclic loading in which shear stress 
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Seismic Design: Stability and Deformation Analyses

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reversal occurs (Casagrande, 1971; Castro, 1975). In an undrained cyclic-triaxial test, 

stress reversal occurs during cyclic loading if the sample is loaded in extension as well as 

compression. In the field, stress reversal occurs when the cyclic shear stresses induced by 

the earthquake exceed the static shear stresses. When this occurs, the stress strain curve after 

some number of cycles develops an S-shape, and significant cyclic strains are generated. 

The condition of significant cyclic straining is defined as “cyclic mobility.” Robertson (1994) 

and Robertson and Wride (1998) suggested the term “cyclic liquefaction” to describe this 

phenomenon and suggested that the term “cyclic mobility” be used to describe the cyclic 

strain behavior of anisotropically consolidated soil subject to undrained cyclic loading 

without stress reversal. For the latter case, cyclic strains are smaller. However, the original 

definition of cyclic mobility will be used in this Manual.

Cyclic resistance ratio (CRR) – Used in the pore-pressure-based method of triggering 

analysis for sand-like material (Section 7.4.2.2). CRR is a measure of the resistance of a 

material to pore-pressure increase caused by earthquake shaking.

Cyclic stress ratio (CSR) – Used in the pore-pressure-based method of triggering analysis 

for sand-like material (Section 7.4.2.2). CSR is a measure of the shear stress generated by the 

earthquake shaking.

Dilative soil or refuse – Dilative materials expand at large strains when sheared under 

drained conditions, and they generate negative excess pore pressures when sheared 

under undrained conditions. Thus the undrained strengths can be higher than the drained 

strengths. The amount by which the undrained strength exceeds the drained strength 

is limited by possible cavitation of the pore water. However, it is customary to neglect 

strengths higher than the drained strength. Medium-dense and dense sand-like materials 

tend to be dilative at large strains. When sheared undrained, the strength nearly reaches 

its peak at moderate strain and then levels off or increases (i.e. strain-hardens) gradually at 

higher strains.

Commentary: During undrained monotonic (non-cyclic) loading, dilative soils normally generate 

small positive excess pore pressures at small strains, and then generate negative excess pore pressures 

at higher strains. During undrained cyclic loading, dilative soils can experience pore-pressure buildup. 

Depending on the cyclic strain, both positive and negative excess pore pressures can develop at different portions of the load cycle. But after each full load cycle there will normally be an increase in positive 

excess pore pressure. However, if loaded statically after cyclic loading, the soil tends to dilate as strains 

increase, the positive excess pore pressures diminish, the excess pore pressure becomes negative, and the 

soil strain-hardens. In other words, even though dilative material can develop positive excess pore pressures during cyclic loading, dilative material will not lose strength as a result of the cyclic loading.

Flow slide – If saturated material in an embankment or its foundation experiences strength 

loss, and the static (gravity) shear stresses (often referred to as the driving stresses) exceed 

the available shear strength, then the embankment can experience instability and failure 

(depending on the amount of material that experiences strength loss and the geometry of 

the structure). The failure is often referred to as a flow slide in cases where the material 

controlling the instability experiences an abrupt loss in strength such that: (1) it approaches 

the steady-state (residual) strength, (2) the slide mass appears to deform almost like a liquid, 

and (3) deformations propagate over a substantial zone, rather than along a well-defined 

failure surface. In such cases, the failure mass can slide a long distance before coming to rest. 

Although movements might commence along a well-defined failure surface, a substantial 

portion of the slide mass may experience strength loss as slope movements develop, and a 

flow slide can evolve. With sand-like materials, instability can be rapid and can occur with 

little warning, and deformations can be very large.
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Liquefaction – The term “liquefaction” has been used in the literature to describe several 

related but distinctly different phenomena. These phenomena include: (1) flow slide failures 

of embankments and dams, (2) lateral spreading of gently sloping ground, (3) development 

of 100 percent excess pore pressure during undrained cyclic loading often accompanied by 

the appearance of sand boils at the ground surface, and (4) the development of high shear 

strains and/or high excess pore pressures in cyclic laboratory tests. In this Manual, the term 

“liquefaction” is not used. Instead, more descriptive and precise terminology is used.

Commentary: The following terms are sometimes used in the literature, but are not used in this 

Manual. They are presented here for completeness.

Initial liquefaction – During cyclic loading, initial liquefaction is the first occurrence of 

momentary zero effective stress (pore pressure = 100 percent).

Cyclic failure – Term sometimes used (Boulanger and Idriss, 2004) to describe the onset of 

high excess pore pressures and large shear strains during undrained cyclic loading of clay-like 

soils. In Boulanger and Idriss’s usage, “cyclic failure” of clay-like materials is comparable to 

“liquefaction” of sand-like materials.

Cyclic liquefaction – Term suggested by Robertson (Robertson, 1994; Robertson and Wride, 1998) 

to describe the progressive softening and resulting large cyclic strains that occur in saturated sands 

during undrained cyclic loading where shear stress reversal occurs. Synonymous with the definition 

of “cyclic mobility” used by Casagrande and others in the 1970s.

Flow liquefaction – Term sometimes used (Robertson and Wride, 1998) for “flow slide.”

Liquidity Index (LI) ‒ The Liquidity Index is derived from the Atterberg limits. LI = (water 

content - PL)/(LL – PL). If LI = 1, the natural water content equals the liquid limit.

N – The Standard Penetration Test (SPT) N-value, uncorrected.

N1,60 – The SPT N-value, normalized to an effective overburden stress of one tsf (approximately 

one atmosphere) and normalized to a hammer efficiency of 60 percent (Youd et al., 2001).

Peak Ground Acceleration (PGA) – The maximum horizontal ground surface acceleration 

at a site caused by the design earthquake. Values of PGA obtained from seismic hazard 

assessments normally refer to accelerations measured at bedrock outcrops or at the ground 

surface of stiff soil profiles. These values of PGA are usually not directly applicable to the 

ground surface of sites with other types of soil profiles. Also, the PGA (maximum horizontal 

ground surface acceleration) at the top of an embankment will generally be different from 

the PGA at the ground surface at the base of the embankment. PGAs obtained from seismic 

hazard assessments are usually not directly applicable to the top of an embankment. A site 

response analysis can be used to estimate the PGA at the top of an embankment based on 

the PGA identified for a bedrock outcrop or stiff soil profile.

Post-earthquake shear strength – The shear strength available at the end of an earthquake, 

used in a static, limit-equilibrium stability analysis to evaluate post-earthquake stability. For 

each zone of coal refuse or natural soil, the strength used in the analysis is estimated based 

on the expected response of the material to earthquake shaking. The strength used could be 

the peak drained strength, the drained steady-state (residual) strength, the peak undrained 

strength, the undrained steady-state (residual) strength, or other intermediate strength.

qc – Measured Cone Penetration Test (CPT) tip resistance.

qt – Measured CPT tip resistance corrected for unequal end-area pore-pressure effects 

(Lunne et al., 1997). For CPT in clean to silty sands, qc and qt are essentially the same and are 

often used interchangeably.
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qt1 – CPT tip resistance (qt) normalized to an effective overburden stress of one atmosphere 

(typically 1 tsf), in units of stress: qt1 = 1.8 qt /(0.8 + σ′v /Pa). (Olson and Stark, 2002)

qt1N – Dimensionless CPT tip resistance, i.e., qt1 normalized by a reference pressure of one 

atmosphere and expressed in dimensionless terms. Sometimes referred to as Q or Qt.

For clay-like material: qt1N = (qt – σvo)/ σ′vo

For sand-like material, refer to Youd et al. (2001)

(qt1N)CS – qt1N corrected to an equivalent value for clean sand (Youd et al., 2001).

Sand-like material – This term is defined in Section 7.1.3.

Sensitivity – Sensitivity in clays is normally defined as undisturbed strength divided by 

remolded strength (Sup /Sus). In this chapter, the term “highly sensitive” is used to refer to 

clays that have undrained stress-strain behavior similar to loose sands. Highly sensitive clays 

have a relatively small strain at peak undrained strength and a significant drop-off in shearing resistance (strain-softening) after the strain at peak. One example of highly sensitive clay 

is Norwegian quick clay. Clay-like coal refuse is generally not highly sensitive.

Sdp, Sds, Sup, and Sus – Drained peak strength, drained steady-state (residual) strength, undrained peak strength, and undrained steady-state (residual) strength. In this Manual, these 

strengths are defined as being measured under monotonic loading unless noted otherwise. 

See also the definition of “undrained steady-state strength.”

Strength loss – The tendency for contractive soils to have lower undrained strength at high 

strains than at lower strains (Sus < Sup). The term strength loss is used to indicate a reduction in 

available strength from Sup to a lower value, not a complete loss to zero strength. If earthquake 

shaking causes shear strains and pore pressures to increase in a soil, the soil may experience 

strength loss (in the extreme, a reduction from Sup to Sus). Loose sand-like soil may experience 

a reduction in strength to Sus as the result of relatively small earthquake shaking because the 

strain at peak strength is relatively low. Clay-like materials (excluding highly sensitive clay 

soils) tend to experience less abrupt loss of strength because they reach both peak undrained 

strength and Sus at much higher strains than sand-like materials.

Commentary: In many cases the available strength before a failure has been the drained strength 

throughout the history of the facility, because all loads have been applied sufficiently slowly to allow 

drainage. During an earthquake (or other disturbance such as a relatively quick loading of additional 

fill or a relatively quick cut near the toe of an embankment), shear stresses are applied quickly enough 

that the soil behaves in an undrained manner. In contractive soils the peak undrained strength would 

be lower than the previously available drained strength, and thus shear deformations or failure might 

ensue. In these cases, strength loss has sometimes been defined as a reduction in available strength from 

Sdp to Sup (and then possibly to Sus). 

For the purposes of this chapter, references to strength loss do not include reductions from Sdp to Sup. 

In this chapter, strength loss only refers to a decrease in available strength from Sup to (or toward) Sus. 

As discussed in Section 6.6.4.2.1, this Manual includes analysis of the potential for strength reduction 

from Sdp to Sup as part of static stability analyses, with undrained peak strength (possibly reduced to 

account for potential progressive failure) used in zones of fine refuse. 

A rise in groundwater level (or rise in the phreatic surface) can reduce stability in several ways, including the following:

• A rise in groundwater level can result in a decrease in effective stress in what was previously an unsaturated zone. This decrease in effective stress may result in a decrease 
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in available drained strength within that zone. This by itself is not likely to cause a 

failure, because the saturated drained strength is normally adequate to maintain stability.

• In some cases of very loose materials in a previously unsaturated zone, a rise in groundwater level can trigger collapse of the material matrix. This results in a rapid rise in pore 

pressure and a decrease in available strength within that zone from drained to undrained. 

Since the material is very loose, the undrained strength can be substantially lower than the 

drained strength, and a failure could ensue if the zone of affected material is large enough. 

This is unlikely to be a concern for fine coal refuse that is deposited through water and 

consolidates while saturated.

• A rapid rise in reservoir level could cause a rapid increase in seepage forces and thus driving stresses such that the soil within the embankment behaves as undrained rather than 

drained. (In contractive materials, the available strength would decrease from Sdp to Sup, as 

discussed in the first paragraph of this commentary.)

The term strength loss has been used to describe these phenomena related to groundwater. However, in 

this chapter, the term strength loss only refers to a reduction from Sup to (or toward) Sus.

Triggering – Term used to indicate that earthquake shaking is strong enough to cause, 

or trigger, strength loss. Some engineers propose that triggering of strength loss occurs 

when excess pore pressures due to cyclic loading reach 100 percent (effective stress reaches 

zero). Some engineers propose that triggering of strength loss occurs when a certain level 

of shear strain is reached, whether or not pore pressures have reached 100 percent. And 

some engineers propose that triggering of strength loss occurs when shear stresses exceed a 

certain level. The various approaches are discussed in this Manual. Triggering might also be 

caused by loadings other than earthquake loadings, such as rapid increases in embankment 

height, excavation of toe materials due to re-mining of coal refuse in an impoundment, or 

rapid changes in groundwater level.

Undrained residual strength – Term often used in the literature to describe the undrained 

soil or coal refuse shear strength at very high strains. It is synonymous with “undrained 

steady-state strength.” The term “residual strength” is often used in the literature to refer to 

drained as well as undrained strength. In this chapter, residual strength normally refers to 

undrained steady state (residual) strength.

Undrained steady-state strength – The undrained steady-state shear strength is the 

undrained shear strength of soil or coal refuse at very high strains (Poulos, 1981). This is 

the minimum shear strength of the material at a given void ratio. It is synonymous with 

terms such as “undrained residual strength,” “critical state strength,” “ultimate undrained 

strength,” “minimum undrained strength” and “liquefied undrained strength.” This Manual 

will use the term “undrained steady-state (residual) strength” or Sus.

7.3 CHARACTERIZATION OF SUBSURFACE CONDITIONS AND MATERIAL PROPERTIES

Previous chapters in this Manual have discussed the field exploration programs and laboratory testing needed for characterizing subsurface conditions and material properties for analysis and design. 

This section discusses issues related specifically to seismic analysis. These issues include:

• Field testing and sampling quality – Special attention to the details of field sampling 

and testing is required for the procedures and analyses discussed in this chapter. 

Drillers and other field exploration contractors should be selected and engaged based 

on contract terms that encourage quality over speed. Proper use of casing, drilling 

mud, and upward deflecting drill bits and properly maintaining a fluid head on the 
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borehole are important in SPT sampling, vane-shear testing, and obtaining undisturbed tube samples.

• Field vane-shear testing – Field vane-shear tests can be used to measure the undrained peak strength and undrained steady-state (residual) strength of clay-like 

material (Sections 6.4.3.8 and 7.4.3.3). Field vane shear tests should not be used to 

measure the undrained strength of sand-like material because sand-like material will 

drain during the course of the test, and the measured strength may be very unconservative (Table 7.1). Details of testing procedures (which differ in some important 

respects from the ASTM procedures) and criteria for confirming that the material 

remains undrained during the test are discussed in Section 6.4.3.8.

• CPT and SPT – Both cone penetration tests and standard penetration tests are recommended for characterizing fine coal refuse. CPT data are generally more repeatable and less affected by equipment or operator variability. CPTs are generally faster 

and less expensive than SPTs for each exploration location, and CPTs provide continuous data. SPTs provide physical samples for visual classification and for index 

testing. CPTs are often used for initial site characterization. A more comprehensive 

site exploration program (including more CPTs, at least some SPTs, and other methods) can then be developed based on the results of the initial CPTs. When using the 

CPT, samples must also be obtained to verify soil type (by visual classification and 

index testing). Samples can be obtained using CPT push equipment, using SPTs, or 

by other methods. SPTs should be performed in general accordance with ASTM D 

6066, which includes special provisions for seismic analyses in addition to the basic 

SPT procedures contained in ASTM D 1586. For high-hazard-potential dams, the 

hammer energy should be verified, and a qualified engineer or geologist should be 

assigned on a full-time basis to each drill rig to observe the testing procedures and 

to log the borings.

Fine refuse should be sufficiently characterized in the field to determine stratification, layering, distinction of sand-like and clay-like materials, and estimation of 

strength and associated properties. CPTs, in combination with recovery of samples 

(e.g., SPT samples), are recommended for addressing these issues at impoundment 

sites. Using both the SPT and CPT provides an opportunity for collecting redundant 

and complementary data to define layers and associated strengths of soft or loose 

deposits with a degree of resolution generally not achievable by relying solely on 

SPTs. However, there may be sites that, because of the fine refuse characteristics (e.g., 

uniformity of deposit, penetration resistance, etc.) or available detailed records of 

dam design and construction, can be adequately characterized by the SPT alone.

• Index testing – Whenever engineering property testing is performed, index testing 

(grain-size, Atterberg-limits, and specific-gravity) should be performed to allow 

comparison and interpretation of test results on different samples and to improve the 

understanding of the relationship between index properties and engineering properties of coal refuse. Grain-size analyses, hydrometer analyses, Atterberg-limits tests, 

and other index tests should be performed on samples that have not been air-dried 

or oven-dried. Wet preparation methods should be used.

• Undisturbed sampling – Obtaining high quality, undisturbed samples of sand-like 

material requires specialized procedures. Thin-walled, fixed-piston tube samplers, 

both mechanically actuated and hydraulically actuated, have been used successfully. 

A detailed procedure for using fixed-piston samplers is provided in Appendix 7C. 

Freezing of sandy material for either sampling or transportation has also been used. 

Freezing must be performed in a manner that allows the liquid water to escape the 
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sample as the freezing front advances. If liquid water becomes trapped, it will expand as it freezes and cause loosening of the sample.

• Representative sampling – Samples for laboratory testing must be representative of 

the layer being investigated. Therefore sufficient CPT and/or SPT data must be obtained to delineate material zones. Unsuccessful attempts to obtain undisturbed tube 

samples should be documented in order to confirm that the loosest material is being 

taken into account in the evaluation.

• Variability and layering – Variability is best evaluated based on CPT data and depositional history. Estimation of the depositional history includes consideration of all 

available information, including observations of the surface and documented or 

anecdotal information on operation of the structure. Undisturbed sampling and testing is not the best way to study variability and is most effectively performed after the 

variability of the deposit has been characterized.

• Material zones – Many of the analyses discussed in this chapter refer to zones of 

material. Zones should be identified based on consideration of depositional environment, grain size, plasticity, and penetration resistance (density and strength). Loose 

material should not be combined with medium or dense material in a single zone. 

CPT is often the most efficient method for defining the extent of loose zones. Further 

guidelines for site characterization are provided in Chapter 6.

• Piezometric levels and degree of consolidation – Soil properties are often estimated 

as a function of effective consolidation stress. Materials within tailings impoundments are not always fully consolidated under the current conditions of fill height 

and groundwater level. Therefore, to confirm estimates of effective consolidation 

stresses, subsurface exploration programs should include installation of piezometers for measurement of piezometric levels within various material zones at various depths within the impoundment. The piezocone is also valuable for measuring 

piezometric levels.

• Laboratory testing on reconstituted samples – Obtaining high-quality, undisturbed 

samples is often difficult, especially for materials with low plasticity. It would be 

easier to measure the steady-state strength of sand-like materials by reconstituting 

samples in the laboratory, starting at a very loose condition and consolidating them 

to the in-situ confining pressures. Unfortunately, there is little or no literature to 

document that reconstituted samples of fine coal refuse have the same void ratios 

or strengths as undisturbed samples with the same confining pressures. Anecdotal 

evidence for some natural soils and other mine tailings indicate that in some cases 

the reconstituted samples have significantly lower void ratios (and therefore higher 

steady state strengths) than undisturbed specimens at the same confining pressures, 

apparently either because of time effects (the time over which confining pressures 

are applied) or fabric effects (the particle structure is different in the lab and field). 

As a result, testing of reconstituted samples is generally not recommended without 

some verification that the consolidated laboratory samples have void ratios similar to 

the in-situ material. This might be a fruitful area for future research.

Commentary: For sand-like materials, use of reconstituted samples is recommended for developing the slope of the steady-state line in Section 7.4.3.2.3 and for laboratory testing associated 

with the strain-based method of triggering analysis in Section 7.4.2.3.1. However, these are 

exceptions to the general rule for using undisturbed samples. In the first case, in-situ strength 

is not being measured directly with the remolded samples; the slope of the steady state line is 

being measured. To obtain the in-situ, steady-state strength, undisturbed samples are needed. 

In the second case, samples are remolded and consolidated to match the in-situ void ratio (not 
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the in-situ confining pressure), and undisturbed samples are also needed for measurement of 

the in-situ void ratio.

• Material Properties for Proposed Structures – For design of a new dam, material 

properties may be based on supporting data for similar materials for existing facilities. Zones potentially susceptible to strength loss should be defined based on experience, available data from nearby sites, and published data for similar facilities. For 

slurry impoundments, consideration should be given to similarities in coal seams, 

mining and processing methods, and fine refuse deposition. The lower bound postearthquake strength of 0.04 σ’v is considered conservative for either loose sand-like 

or soft clay-like fine coal refuse, assuming the liquidity index is less than one (Table 

7.1). As more testing and analysis of fine refuse becomes available, refinements to the 

0.04 σ’v relationship can be expected (initial published work is discussed in Section 

7.4.3.2.3). Strengths estimated during design must be validated during construction 

by performing field investigations and obtaining samples of the actual materials. 

These data may lead to the need to modify the design of the facility.

7.4 SEISMIC STABILITY ANALYSES

7.4.1 General Discussion

Seismic instability may occur when the overall average post-earthquake shear strength is less than 

pre-earthquake shear strength in one or more zones of an embankment. The driving force of the stability failure is the static (gravity) weight of the embankment. For seismic instability to occur, three 

conditions must be met:

1. The earthquake shaking must be strong enough to trigger undrained strength loss in 

one or more zones of material.

2. The post-earthquake strengths must be less than the static driving shear stresses.

3. The amount of material that experiences strength loss must be sufficient to cause 

instability.

There are three general components to a seismic stability analysis:

1. Evaluation of whether the earthquake is strong enough to cause strength loss in one 

or more zones of the embankment or foundation. This is referred to as a triggering 

analysis.

2. Evaluation of the post-earthquake strengths in the various soil zones that may be 

susceptible to strength loss.

3. Performance of static, limit-equilibrium, slope-stability analyses using post-earthquake strengths for the materials in the embankment and foundation.

7.4.1.1 Triggering Analyses

In saturated, sand-like material, a small shear strain (1 percent or less) can trigger sudden strength 

decrease from Sup to Sus. In clay-like material, high shear strain (5 percent or more) is generally needed 

to cause some strength loss, and very high shear strain is generally needed to cause strength loss all 

the way to Sus. Highly sensitive clays may behave similarly to sand-like material in that strength loss 

may occur at low shear strains. Three general approaches to triggering analysis are discussed in Section 7.4.2, and are summarized as follows:

• Pore-pressure-based approach (Sections 7.4.2.1 and 7.4.2.2) – Uses a method developed for evaluating whether earthquake shaking will generate high excess pore 
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pressures as an index of whether the earthquake shaking will trigger strength loss in 

sand-like materials. This approach is not applicable to clay-like materials.

• Strain-based approach – For clay-like material (Section 7.4.2.3.1), this approach 

considers post-earthquake strength as a function of shear strain that occurs during 

the earthquake. For sand-like material (Section 7.4.2.3.2), this approach assumes that 

strength loss is triggered when the cyclic shear strain induced by the earthquake 

exceeds a critical value (i.e., the triggering shear strain).

• Stress-based approach (Section 7.4.2.3.3) – This approach assumes that strength loss 

is triggered when shear stresses exceed the undrained yield strength (the peak undrained strength). This approach is not applicable to clay-like materials.

It is important to note that for loose, saturated, sand-like material, moderate to large earthquakes will 

almost always be large enough to trigger strength loss. Therefore, triggering analyses should not be 

used to confirm the adequacy of a dam or embankment if the PGA at the base of the embankment is 

higher than 0.2g and the CSR (Section 7.4.2.2.1) in the zone of loose sand-like material is greater than 

0.15. If the design earthquake causes a higher site PGA, then triggering of strength loss should simply 

be assumed for zones of loose, sand-like material with CSR > 0.15. The post-earthquake strength of 

the material in the loose zone should be assumed to be equal to Sus. A PGA of 0.2g and CSR of 0.15 

are recommended guidance. In evaluating potential measures to improve the stability of a dam or 

embankment where the CSR in a zone exceeds 0.15, it may be useful to perform a triggering analysis 

to help assess whether alternative stabilization schemes that would reduce the CSR might ultimately 

improve the dam or embankment stability.

7.4.1.2 Evaluation of Post-Earthquake Strength

If the earthquake shaking triggers strength loss, the post earthquake strength of sand-like material 

will be the undrained, steady-state strength Sus, which is primarily a function of void ratio and is very 

sensitive to small changes in void ratio. The void ratio of sand-like material is primarily a function of 

the method of deposition. Even within a seemingly uniform layer, sand-like material deposited by 

water (including both natural sands and fine coal refuse) has significant variability in void ratio. As a 

result, multiple samples of sand-like material from the same zone or layer are likely to have varying 

values of Sus. Therefore, methods of evaluating Sus of sand-like materials for stability analyses should 

account for the potential variability within zones or layers.

In contrast, the void ratio of clay-like material is primarily a function of consolidation pressure and 

stress history. Furthermore, the peak-undrained strength and the steady-state (residual) strength in 

clay-like material is less sensitive to void ratio than in sand-like material. As a result, post-earthquake 

strengths within a zone are likely to be more uniform for clay-like material than for sand-like material.

Three general approaches to evaluating the post-earthquake strengths of materials that may be susceptible to strength loss are discussed in Section 7.4.3. The three approaches are:

1. Empirical correlations of SPT and/or CPT values with back-figured, post-earthquake 

strengths from flow slides may be used to estimate Sus for sand-like material (Section 

7.4.3.1). These methods assume that the earthquake shaking triggered strength loss, 

so the post-earthquake strength is Sus.

2. Laboratory tests on high-quality samples from the field can be used to estimate postearthquake strengths for both clay-like and sand-like material (Section 7.4.3.2).

3. Field vane-shear tests and CPTs can be used to measure Sus for clay-like material 

(Section 7.4.3.3). As discussed in Section 7.4.2.3.1, Sus is often a conservative low estimate of the actual post-earthquake strength for clay-like material.
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7.4.1.3 Stability Analysis Using Post-Earthquake Strengths

A generalized approach presented as a series of steps for performing the triggering analyses, the 

post-earthquake strength evaluations, and the limit equilibrium stability computations is provided 

in Section 7.4.4.

7.4.2 Triggering Analyses

7.4.2.1 Pore-Pressure-Based Method for Triggering of Strength Loss in Clay-Like Material

There is general agreement that highly plastic clays (with the exception of highly sensitive clays) are 

not susceptible to either significant pore-pressure buildup or strength loss due to earthquake shaking 

(Seed et al., 2003).

For clays that are not highly plastic, several criteria for pore-pressure buildup and susceptibility to 

strength loss due to earthquake shaking, based on Atterberg limits and percent clay content, have 

been proposed (Seed et al., 2003). However, while there is general agreement that susceptibility to 

strength loss decreases with increasing plasticity and increasing SPT or CPT resistance, there is no 

general agreement on specific criteria at the time of publication of this Manual.

Screening-level evaluations of the susceptibility of clay-like materials to strength loss, based on CPT 

and SPT data, are discussed in Section 7.4.4.2. More detailed estimates of whether earthquake shaking is strong enough to trigger strength loss in clay-like material can be obtained by using the strainbased methods discussed in Sections 7.4.2.3.1 and 7.4.3.2.2.

7.4.2.2 Pore-Pressure-Based Method for Triggering of Strength Loss in Sand-Like Material

This method is used for evaluating whether the design earthquake is large enough to cause (trigger) 

strength loss. For loose/contractive sand-like material, the earthquake shaking required to trigger 

strength loss is normally small, and triggering causes strength loss all the way to Sus. Therefore, as 

discussed in Section 7.4.1, if the design peak ground acceleration (PGA) at the embankment site is 

larger than 0.2g and the CSR within the zone of loose sand-like material is greater than 0.15, then the 

triggering analysis described in the following paragraphs should not be performed, and triggering 

of strength loss should simply be assumed. The post-earthquake strength of the material in the loose 

zone should be assumed to be equal to Sus. In evaluating potential measures to improve the stability 

of a dam or embankment where the CSR in a zone exceeds 0.15, it may be useful to perform the triggering analysis to help determine whether alternative stabilization schemes that would reduce the 

CSR might ultimately improve the dam or embankment stability.

For sand-like materials, evaluating the potential for triggering of strength loss using pore-pressurebased methods is done using the “simplified method” first published by Seed and Idriss (1971). The 

method is referred to as “simplified” because it does not require cyclic laboratory testing or detailed 

analysis of site response to earthquake shaking. However, use of this method for coal refuse impoundments will often require a detailed analysis of site response. Additions and updates to the method 

have been published by numerous authors. A consensus on the state of practice for using the method 

was published by 21 technical practitioners (Youd et al., 2001).

Commentary: After Youd et al. (2001) was published, several investigators proposed refinements to various 

aspects of the analysis method. These include Seed et al. (2003) and others. These refinements can be considered 

for specific cases, or Youd et al. (2001) may be used directly.

The method was developed for evaluation of the potential for a zone of sand-like material to develop 

high (approaching 100 percent) excess pore pressures, but may also be used as an index of the susceptibility of the soil zone to lose strength.
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The method is based on observations of high pore-pressure generation during earthquakes, as evidenced primarily by the appearance of sand boils, ground fissures, or lateral spreads at level or 

gently sloping sites and involves computations of cyclic stress ratio (CSR) and cyclic resistance ratio 

(CRR). CSR is a measure of the seismic “demand” on a soil layer. CRR is a measure of the capacity of 

the soil to resist pore-pressure increases during earthquake shaking. 

The method was developed from case history data primarily from flat to gently-sloping sites underlain with Holocene (recent) alluvial or fluvial sediments at depths less than 50 ft. The method has been 

verified for, and is strictly applicable only to, these site conditions (Youd et al., 2001). However, the 

method is often extended to other site conditions with steeper slopes and deeper soils, such as coal 

refuse embankments, since no directly applicable performance history is available for such facilities.

The steps for evaluating the triggering of pore-pressure increase are:

1. For the location at which each value of either SPT N1,60 or CPT qt1N was obtained in 

zones of material that may be susceptible to strength loss, compute the cyclic stress 

ratio (CSR), which is the average cyclic shear stress caused by the design earthquake 

in the zone of material divided by the initial effective vertical stress in the zone of 

material (τav /σ’vo). Although the method was developed based on average SPT or 

CPT values, it should be applied to all measured values for added conservatism and 

improved delineation of loose zones.

2. Estimate the cyclic resistance ratio (CRR) from various published plots that give CRR

as a function of N1,60 or qt1N and fines content. Alternatively, CRR may be computed 

using spreadsheets or other computer programs based on equations that approximate the curves shown in the published plots.

Commentary: For some other analyses in this chapter, corrections for fines content are not recommended, but they should be considered for estimation of CRR based on N1,60 or qt1N values.

3. Compute the safety factor against triggering based on pore-pressure increase using 

CSR, CRR, and appropriate corrections for earthquake magnitude, overburden, and 

shear stress using the following relationship (Youd et al., 2001):

FS = (CRR7.5 /CSR) × MSF × Kσ × Kα (7-1)

Methods for evaluating CSR, CRR, MSF, Kσ and Kα are discussed below.

4. If the safety factor against pore-pressure increase in a zone is greater than 1.4, then 

it can be assumed that the earthquake shaking will not be strong enough to trigger 

strength loss in that zone. For seismic stability analyses, the peak undrained strength 

Sup (but not more than the peak drained strength) may be used in that zone. If the 

safety factor (CRR/CSR) in a zone is less than 1.0, then triggering of strength loss 

should be assumed, and for seismic stability analyses, the undrained, steady-state 

(residual) strength Sus should be used in that zone. For safety factors between 1.0 and 

1.4, triggering of strength loss is possible. Either assume that triggering of strength 

loss will occur, or perform more rigorous triggering analyses (strain-based or stressbased methods, as discussed in Section 7.4.2.3) to make a final evaluation of whether 

or not triggering occurs.

Commentary: The safety factor of 1.4 is recommended guidance and is intended to account for 

the strength loss that can be triggered even if the pore-pressure increase is substantially lower 

than 100 percent.
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The paragraphs that follow discuss evaluation of CSR, evaluation of CRR, and correction factors for 

earthquake magnitude, overburden stress, and sloping ground.

7.4.2.2.1 Evaluation of Cyclic Stress Ratio (CSR)

The cyclic stress ratio (CSR) at a particular depth is given by:

CSR = τav /σ’vo (7-2)

where:

τav = average cyclic shear stress induced by the earthquake = 0.65 τmax (force/length2)

σ’vo = effective vertical overburden stress (force/length2)

As discussed in the following text, computing τav in a zone of material requires a design earthquake 

PGA or a design-earthquake, acceleration time-history.

Three methods for computing CSR that are acceptable for various situations are:

1. CSR computation method 1 – The preferred method is to compute CSR by using 1D 

or 2D numerical site response software, such as SHAKE or QUAD4, that compute 

the variation in maximum shear stress τmax with depth. The value of τav at the depth 

of interest should be taken from the site response analyses as 0.65 τmax at that depth. 

Using a one-dimensional (1D) or two-dimensional (2D) numerical site response 

analysis requires, as input to the analysis, an acceleration time-history for the design 

earthquake as well as dynamic properties (shear modulus and damping) for each 

material type. This method is appropriate for all sites.

Commentary: Important material properties used in performing the analysis are the shear 

modulus and damping. They can be estimated using index and classification data, or sitespecific testing such as crosshole testing can be performed to measure shear wave velocity (to 

obtain the maximum shear modulus Gmax). Laboratory testing including resonant-column 

testing and cyclic-triaxial testing can also be utilized. The resonant-column test measures 

dynamic soil properties at a lower range of shear strain than the cyclic-triaxial test. Dynamic 

properties of coal refuse materials are the subject of research in Kalinski and Phillips (2008) 

and Zeng and Goble (2008). Whether 1D or 2D site response analyses should be used depends 

on the specific conditions for which the triggering analysis is being performed (locations of 

potential slope stability failure surfaces, variability of materials, geometric complexity, and 

distance from the suspect zone to the exposed slope). For analysis of upstream failures of coal 

refuse impoundments, the potential failure surfaces are often relatively far from the downstream face, such that the simplifications associated with 1D analyses are acceptable. For 

analysis of downstream failures of coal refuse impoundments with wide coarse refuse stages, 

the potential failure surfaces generally pass through the coarse refuse stages and possibly 

thin zones of fine refuse. So even though the ground surface is sloping, this geometry can be 

reasonably modeled with a series of 1D analyses. For analysis of downstream failures of coal 

refuse impoundments with narrow coarse refuse stages, the potential failure surfaces pass 

through highly variable materials, and the sloping ground effects (downstream face) are more 

pronounced. Therefore 2D analyses should be used to model these more complex conditions.

2. CSR computation method 2 (for shallow, level-ground sites) – For conditions of 

nearly level ground, uniform deposits, and depths up to 75 feet, CSR can be computed from the following equation (Youd et al., 2001):
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CSR = (τav /σ’vo ) = 0.65 (amax /g) (σvo /σ’vo) rd (7-3)

where:

amax = maximum horizontal acceleration at the ground surface 

(e.g., top of embankment) (mass × length/time2)

g = acceleration of gravity (mass × length/time2)

σvo = total overburden stress (force/length2)

σ’vo = effective vertical overburden stress (force/length2)

rd = stress reduction coefficient (1.0 at the ground surface, 

decreasing with depth) (dimensionless)

Commentary:

τav = 0.65 τmax

τmax = peak seismic shear stress at any depth = (σvo ) (amax /g) (rd)

τmax/σvo = (amax /g) (rd)

τmax is the product of the total overburden stress at the depth in question σvo times the peak 

acceleration of the overburden mass above the depth in question (amax /g)(rd). This is equivalent to saying that seismic shear force equals mass times seismic acceleration or seismic shear 

stress (force per square foot) equals total vertical stress (mass per square foot) times seismic 

acceleration.

The coefficient rd accounts for the deformability of the overburden mass, which results in the 

seismic shear stress at depth being lower than the product of the total overburden stress σvo

times the peak acceleration at the ground surface amax. In other words, rd represents the fact that 

the peak acceleration of the overburden mass (amax /g)(rd) at depth is lower than the maximum 

acceleration at the ground surface, and thus there is a reduction of the ratio of peak seismic 

shear stress to total overburden stress (τmax /σvo) with depth. Seed et al. (2003) present refinements for values of rd for specific cases, or Youd et al. (2001) may be used directly.

The computation of total overburden stress σvo should not include the weight of any free water 

that may exist above the ground surface. It is a relatively common error to include the weight 

of free water since it is part of the total overburden. However, it cannot be accelerated by the 

ground below, and thus it does not count as part of the mass being shaken.

3. CSR computation method 3 – For depths greater than about 75 feet or for irregular 

and sloping ground conditions (i.e., for most coal refuse embankments), the following simplified method can, in certain cases, be used instead of Method 1. Method 3 

can be used for cases where potential failure surfaces pass through relatively uniform materials, and the potential failure surfaces are relatively far from steep slopes. 

Examples are:

− Upstream failures of coal refuse impoundments where the potential failure 

surfaces pass through reasonably uniform materials (no abrupt changes from 

loose or soft material to dense or hard material), and the failure surfaces are 

relatively far from the steep downstream slope.

− Downstream failures of coal refuse impoundments with wide coarse refuse 

zones near the downstream slope, where the potential failure surfaces also 

pass through reasonably uniform materials.
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Method 3 uses the same equation for CSR as Method 2. However, the stress reduction coefficient rd

used in that equation is only appropriate for level ground. Therefore, above the depth of interest, 

replace the product (amax rd) with the maximum acceleration of the overburden mass kmax. The value 

of kmax can be estimated from published relationships of the depth-dependent variation in maximum 

acceleration ratio (kmax /amax versus y/h, where y is the depth below the top of the embankment and h

is the embankment height). The “average of all data” line from Figure 7.4 can be used.

The maximum acceleration at the ground surface (top of embankment) amax will normally be higher 

than the maximum acceleration at the ground surface at the base of the embankment (PGA). The 

value of amax can be estimated from the PGA of the design earthquake at the base of the embankment 

using Figure 7.3.

Figures 7.3 and 7.4 were developed for conventional dams with an approximately triangular shape. 

Therefore, the results for Method 3 should be compared with the results of Method 2. If the results 

are significantly different, the more conservative value should be used, or an analysis using Method 1 

should be performed.

PEAK TRANSVERSE BASE ACCELERATION, g

PEAK TRANSVERSE CREST ACCELERATION, g

(HARDER, 1991)

0 0.1 0.2 0.3 0.4 0.5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1989 LOMA PRIETA

EARTHQUAKE

PREVIOUS EARTHQUAKES

FIGURE 7.3 COMPARISON OF PEAK BASE AND CREST TRANSVERSE 

ACCELERATIONS MEASURED AT EARTH DAMS
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Chapter 7

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Commentary: It should be noted that the ratios of maximum acceleration at the base of the embankment to 

maximum acceleration at the top of the embankment are different in Figures 7.3 and 7.4. That is because Figure 

7.4 compares the maximum acceleration of the overburden mass above the base of the embankment to the maximum acceleration at the top of the embankment, while Figure 7.3 compares the maximum acceleration at the 

base of the embankment to the maximum acceleration at the top of the embankment. In other words, Figure 

7.4 considers that, because the overburden mass is not rigid, the accelerations at different depths within the 

overburden mass will be different. Figure 7.4 looks at the average acceleration of the overburden mass at each 

time increment and then picks the maximum value kmax. In Figure 7.4, kmax acts at the base of the overburden 

mass, but represents the average acceleration of the mass above the base. In Figure 7.3, the peak transverse base 

acceleration is the acceleration at the specific depth corresponding to the bottom of the embankment.

7.4.2.2.2 Evaluation of Cyclic Resistance Ratio (CRR)

The cyclic resistance ratio can be evaluated through the use of field test data including: (1) SPT data, 

(2) CPT data, and (3) shear-wave velocity (Vs) measurements. Procedures for computing CRR are 

described in Youd et al. (2001). It is good practice to obtain both SPT and CPT data. Shear-wave velocity data may also be helpful. A description of each of these approaches is provided in the following:

• CRR determined from SPT – Numerous plots have been published over the years 

showing CRR as a function of SPT blowcount N1,60. The plot from Youd et al. (2001) 

is shown in Figure 7.5. The plot shows zones where data from case histories suggest

0.0 0.2 0.4 0.6 0.8 1.0

1.0

0.8

0.6

0.4

0.2

0.0

MAXIMUM ACCELERATION RATIO, kmax/amax

"SHEAR SLICE"

(RANGE FOR ALL DATA)

FINITE ELEMENT

METHOD

AVERAGE OF

RELA

ALL DATA

TIVE DEPTH OF SLIDING MASS, y/h

(MAKDISI AND SEED, 1978)

FIGURE 7.4 VARIATION OF MAXIMUM ACCELERATION RATIO WITH DEPTH OF SLIDING MASS
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Seismic Design: Stability and Deformation Analyses

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that significant pore-pressure increase will occur and zones where significant porepressure increase is unlikely to occur. The plot includes curves developed for soils 

with various percentages of fines. The curve for less than 5 percent fines is considered the basic penetration criterion for the simplified method described in Youd et al. 

(2001) and is thus referred to as the “SPT clean sand base curve.” Youd et al. (2001) 

describes ways to adjust N1,60 to a clean sand value N1,60(CS). The curves are valid 

only for magnitude 7.5 earthquakes. Scaling factors to adjust for different magnitude 

earthquakes are discussed later.

Two corrections are normally made to the field blow count N. The first (CN) is made 

to normalize N to an overburden pressure of 1 atmosphere (about 1 tsf or 100 kPa). 

The second (CE) is made to normalize hammer energy to 60 percent of the energy 

generated by a 140-lb weight free-falling 30 inches. Other corrections less commonly 

made in practice include borehole diameter (CB), rod length (CR), and samplers with 

and without liners (CS). Thus:

N1,60 = N × CN × CE × CB × CR × CS (7-4)

The correction factors are discussed in Youd et al. (2001), which notes that the correction for overburden (CN) becomes highly uncertain for overburden pressures greater 

than 3 tsf.

The presence of gravel can interfere with the penetration of the SPT sampler and 

lead to higher penetration resistances. One way to correct for the presence of gravel 

in the SPT is to record the hammer blows for every inch of penetration (rather than 

the standard 6 inches) as proposed by Poulos in the 1970s and described in USBR 

(1989) and Seed et al. (2003).

• CRR determined from CPT – Robertson and Wride (1998) developed curves of CRR

as a function of dimensionless CPT tip resistance ratio qt1N, as shown in Figure 

7.6. The figure shows zones where data from case histories suggest that significant pore-pressure increase will occur and zones where significant pore-pressure 

increase is unlikely to occur. The field CPT tip resistance qt must be normalized to 

an overburden pressure of 1 atmosphere (about 1 tsf or 100 kPa) to obtain qt1N and 

must be corrected to an equivalent clean sand value qt1N(CS). Equations and charts 

for normalizing qt and correcting to an equivalent clean sand value are discussed in 

Youd et al. (2001) and updated by Robertson (2004), which also includes a discussion of using average CPT values in a layer versus using all measured values. An 

additional correction for thin soil layers can be made, if applicable (Youd et al., 

2001). The references by Robertson and Wride (1998) and Youd et al. (2001) use 

the uncorrected tip resistance qc whereas, more correctly, it should be qt. For CPT 

in clean to silty sands, qc and qt are essentially the same and are often used interchangeably.

CPTs have the advantage that they are faster to perform and typically less expensive 

per test than SPTs, especially in deep, soft or loose deposits, and CPTs provide a continuous, more reliable profile of penetration resistance. CPTs are particularly useful 

for fine coal refuse. Physical samples of the material where CPTs are conducted 

should be obtained for purposes of description and laboratory testing.

• CRR determined from shear-wave velocity – CRR criteria have been developed from 

field measurements of shear-wave velocity Vs. Shear-wave velocities can be measured 
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Chapter 7

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using the seismic CPT (SCPT), crosshole testing, or other downhole or surface wave 

techniques. Crosshole testing is generally more accurate, but more expensive than 

seismic CPT. Potential advantages of using the shear-wave velocity are: (1) it is possible 

to obtain data in soils that are difficult to penetrate with CPT or SPT and (2) the shear 

wave velocity is directly related to small-strain shear modulus, which is a parameter 

required for estimating dynamic soil response. As discussed by Youd et al. (2001), 

concerns with the use of Vs include: (1) shear wave velocity measurements are made 

at small strains, whereas pore-pressure buildup and strength loss are medium to high 

strain phenomena and (2) thin, low shear-wave velocity layers may not be detected. 

Vs data, if obtained, should generally be used in conjunction with data from other 

methods. The shear-wave velocity obtained from the field should be normalized to a 

reference overburden pressure of one atmosphere. A recommended chart of CRR as a 

function of overburden-corrected shear wave velocity is provided in Youd et al. (2001).

CORRECTED BLOW COUNT, N1,60

CYCLIC STRESS RATIO (CSR) OR CYCLIC RESISTANCE RATIO (CRR)

FIGURE 7.5 SPT CLEAN-SAND BASE CURVE FOR MAGNITUDE 7.5 EARTHQUAKES 

 WITH DATA FROM LIQUEFACTION CASE HISTORIES

(ADAPTED FROM YOUD ET AL., 2001)

MODIFIED CHINESE CODE PROPOSAL

(CLAY CONTENT = 5%)

FINES CONTENT > 5%

LIQUEFACTION

 MARGINAL

LIQUEFACTION

 NO

LIQUEFACTION

PAN-AMERICA DATA

JAPANESE DATA

CHINESE DATA

30

20 30

31

27 10 13

10

10

50+

60

75

27 75 67

13 22

30

80

10

20

20

25

50 92 10

10

12

12

26 20 48

20 40

10

20

10 10

80 50+ 60

27

50+

20

31

29

12

12

18

18

20

17

11 10

12

25

10

ADJUSTMENT

RECOMMENDED

BY WORKSHOP

SPT CLEAN SAND BASE CURVE

PERCENT FINES = 35 15 > 5

37

0 10 20 30 40 50

0

0.1

0.2

0.3

0.4

0.5

0.6

FIGURE 7.5 SPT CLEAN-SAND-BASED CURVE FOR MAGNITUDE 7.5 EARTHQUAKES 

WITH DATA FROM LIQUEFACTION CASE HISTORIES
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7.4.2.2.3 Correction Factors for Earthquake Magnitude, High Overburden, Sloping Ground 

and Age of Deposit

The curves used to obtain CRR from SPT, CPT and Vs only apply to magnitude 7.5 earthquakes. 

Magnitude scaling factors (MSF) have been developed to adjust the curves to smaller or larger magnitudes. The recommendations by Youd et al. (2001) represent a wide consensus among experts in 

the field and are preferred at this time unless project-specific factors indicate that other recommendations should be used.

The “simplified” method was developed based on case history data from gently sloping sites (low 

static shear stress) and depths less than about 50 feet. Correction factors (Kσ and Kα) have been developed for extrapolating the method to higher overburden pressures and steeper slopes (higher static 

shear stress) than those encountered in the case histories. Youd et al. (2001) caution that using these 

correction factors requires specialized expertise, because the current bases for selecting specific values 

of the correction factors are limited. If used, the cyclic resistance ratio becomes:

CRR = (CRR7.5 × MSF) Kσ Kα (7-5)

CORRECTED CPT TIP RESISTANCE, qc1N

CYCLIC STRESS RATIO (CSR) OR 

CYCLIC RESISTANCE RATIO (CRR)

FIELD PERFORMANCE

STARK & OLSON (1995)

SUZUKI ET AL.(1995b)

0 50 100 150 200 250 300

0

0.1

0.2

0.3

0.4

0.5

0.6

LIQUEFACTION

NO LIQUEFACTION

M = 7.5

CPT CLEAN SAND

 BASE CURVE

NCEER (1996)

 WORKSHOP

FIGURE 7.6 CALCULATION OF CRR FROM CPT DATA ALONG WITH EMPIRICAL

 LIQUEFACTION DATA FROM COMPILED CASE HISTORIES

(YOUD ET AL., 2001)

0.25 < D50 (mm) < 2.0

LIQ. NO LIQ.

FC (%) < 5

FIGURE 7.6 CALCULATION OF CRR FROM CPT DATA ALONG WITH EMPIRICAL

LIQUEFACTION DATA FROM COMPILED CASE HISTORIES
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Recommendations for Kσ are provided in Youd et al. (2001).

Both Youd et al. (2001) and Seed et al. (2003) discuss the wide range of values that have been proposed for Kα and explain that there are no generally agreed upon recommendations. Seed et al. (2003) 

recommend that the Kα values proposed by Harder and Boulanger (1997) be used where the initial 

vertical effective overburden pressure is less than 3 tsf. Until a consensus is reached, Kα should be 

taken as 1.0 unless site-specific information (laboratory test data) indicates otherwise.

It has been noted that older deposits (both older reconstituted laboratory samples and geologically 

older natural soil deposits) are more resistant to pore-pressure increase. However, verified correction 

factors have not been developed. Since it is conservative to ignore age corrections, age corrections are 

not recommended at this time.

7.4.2.3 Strain-Based and Stress-Based Methods to Evaluate Triggering of Strength Loss

7.4.2.3.1 Clay-Like Material: Strain-Based Method for Triggering

Soft, clay-like materials may experience strength loss during an earthquake, but the strength loss is 

generally not as severe as with sand-like materials (Thiers and Seed, 1968; Castro, 2003; Boulanger 

and Idriss, 2004). This is because clay-like materials generally reach both peak undrained strength 

and Sus at much higher strains than sand-like materials, except for highly sensitive clays where the 

strain to peak undrained strength can be small.

Therefore, the term “triggering” can be somewhat misleading for clay-like material. It is more correct to say that the post-earthquake strength for clays is a function of the shear strain that occurs 

during the earthquake. The post-earthquake strength may be the peak undrained strength or it may 

be a reduced strength. The post-earthquake strength could be as low as Sus in highly sensitive clays, 

but for most clay-like material the post-earthquake strength will be higher than Sus. The reduced 

strength, while higher than Sus, may still be low enough to result in a flow slide.

There are three approaches to estimating the post-earthquake shear strength of clay-like material:

1. Conservatively assume that the post-earthquake strength is equal to Sus, and estimate Sus from laboratory or field testing, as discussed in Sections 7.4.3.2.2 and 7.4.3.3.

2. Estimate the post-earthquake strength as a function of earthquake-induced shear 

strain, using the following procedure: 

• Estimate the accumulated shear strain at points along critical failure 

surfaces in the embankment during the earthquake. The accumulated 

shear strains can be estimated using Newmark-type analyses (Section 

7.5.4).

Commentary: This step of this method requires that a deformation analysis 

be performed as part of the evaluation of post-earthquake strength. A design 

earthquake motion (time history of acceleration) and dynamic soil properties 

(modulus and damping) are required for this step.

• Obtain a relationship of post-earthquake strength as a function of total 

strain based on laboratory testing or other correlations as discussed in 

Section 7.4.3.2.2.

• Use the computed total strains during the earthquake and the 

relationship of post-earthquake strength versus strain during the 

earthquake to estimate post-earthquake strength in each zone of 

clay-like material.
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3. Estimate the post-earthquake strength based on the earthquake-induced, cyclic shear 

stress. This approach involves performing cyclic-undrained, shear-strength testing 

(that models the earthquake loading) on undisturbed samples of the clay-like material and then loading the samples monotonically to evaluate how much the cyclic 

loading degraded the peak strength. This method is discussed in more detail in Section 7.4.3.2.2.

7.4.2.3.2 Sand-Like Material: Strain-Based Method for Triggering

The method described in the following text is used for evaluating whether the design earthquake 

will produce shear strains that are high enough to cause strength loss in a zone of sand-like material.

This method is only appropriate if the safety factor against triggering using the pore-pressure-based 

method (CRR/CSR) is between 1.0 and 1.4. If the safety factor is higher than 1.4, then triggering may 

be assumed to not occur. If the safety factor is less than 1.0, then triggering should be assumed. For 

safety factors between 1.0 and 1.4, the pore-pressure-based method indicates that triggering is likely, 

but this strain-based method may indicate that triggering will not occur.

As discussed in Section 7.4.1, if the design PGA is larger than 0.2g and the CSR in the loose zone is 

greater than 0.15, then the triggering analysis described in the following text should not be performed, 

and triggering of strength loss in the loose zone should simply be assumed. The post-earthquake 

strength of the material in the loose zone should be assumed to be equal to Sus. In evaluating potential 

measures to improve the stability of a dam or embankment where the CSR in a zone exceeds 0.15, it 

may be valuable to perform the triggering analysis to help assess whether alternative stabilization 

schemes that would reduce the CSR might ultimately improve the dam or embankment stability.

The discussion that follows is based primarily on Castro (1994), which should be consulted for further details. The steps in the method are:

1. Select one or more potential failure surfaces through the embankment. Estimate the 

total seismically-induced shear strain along the critical failure surfaces in the embankment during the earthquake. Failure surfaces that had the lowest safety factors 

in the post-earthquake stability analysis performed when triggering of loose material 

was assumed should be selected. The total shear strains include: (1) transient cyclic 

strains, which can be estimated by a 1D or 2D site-response analysis using SHAKE or 

QUAD4 and (2) accumulated strains, which can be estimated using Newmark-type 

analyses (Section 7.5.4) or numerical modeling (Section 7.5.5). To be conservative, the 

total seismically-induced shear strain should be computed by adding the maximum 

transient cyclic strain to the total accumulated strain.

If numerical modeling is used to estimate accumulated strain, the model output will 

include shear strains. If a Newmark-type analysis is used to estimate accumulated 

strains, the analysis will provide displacements. To convert displacements to shear 

strains, the thickness of the loose zone should be estimated, and the strains should be 

assumed to be uniform across the full thickness of the zone. In other words, the shear 

strain will be approximately equal to the displacement divided by the thickness of the 

zone. A Newmark-type analysis can be used instead of numerical modeling only for 

cases in which the loose zone is relatively thin compared to the overall failure mass.

Commentary: A design earthquake motion (time history of acceleration) is required for performing the deformation analysis required for this step.

2. The triggering shear strain of sand-like material is approximately equal to the shear 

strain at peak strength in undrained monotonic (non-cyclic) loading. However, for 
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the triggering analysis, conservatively assume that the triggering shear strain is 

equal to one-half the shear strain at peak strength in monotonic, strain-controlled 

undrained laboratory strength tests. (Shear strain is equal to 1.5 times the axial strain 

in triaxial tests.)

Perform undrained shear tests on samples consolidated anisotropically to stresses 

corresponding to the static anisotropic stresses along the potential failure plane 

through the embankment.

Undisturbed samples should normally not be used for this testing because of concern for densification during sampling, handling, and laboratory consolidation to the 

point that the samples are not as contractive in the laboratory as the in-situ material. Remolded samples should generally be used, attempting to bracket (after being 

anisotropically consolidated in the lab) void ratios (more correctly – relative densities) that reflect field conditions. Representative samples from the zone of interest 

should be mixed to obtain a batch of soil for remolded testing. The samples that are 

mixed should have similar grain size distribution. (As an example, clean sand material should not be mixed with material containing more than about 20 percent fines). 

Remolded samples can be prepared by moist tamping in multiple layers or by wet 

or dry pluviation. Pluviation may not be appropriate for silty samples because of the 

potential for material segregation by particle size.

Commentary: The strain to peak strength and the triggering strain vary with the degree 

of anisotropic consolidation. Therefore, to model field conditions, anisotropic consolidation is 

required for these tests. In contrast, for identical specimens consolidated to the same void ratio, 

the steady-state (residual) strength Sus measured in the laboratory will be the same for isotropically- and anisotropically-consolidated specimens. Therefore, for convenience, Sus testing is 

normally performed on isotropically-consolidated specimens, as discussed in Section 7.4.3.2.3.

The recommendation to estimate the triggering shear strain as being equal to onehalf the shear strain at peak undrained strength in a monotonic test is conservative. 

Being conservative at this step is appropriate because of: (1) the low shear strain 

required to reach peak undrained strength indicates a high potential for progressive 

failure and (2) the evaluation of seismically-induced shear strains is highly uncertain.

Instead of performing laboratory strength tests, the shear strain at peak undrained 

strength could be estimated as a function of the ratio of vertical to horizontal consolidation stress from published data for similar sand-like material (Castro 1994). The 

triggering shear strain would then be assumed to be equal to one-half the estimated 

shear strain at peak undrained strength. However, at this time, there is not adequate 

published data available to rely on, so either: (1) site-specific laboratory testing 

should be performed or (2) the triggering shear strain of the sand-like material can 

simply (conservatively) be assumed to be 0.25 percent. This recommended shear 

strain is a lower bound for the data presented in Castro (1994) for consolidation 

stress ratios typical of coal refuse impoundments.

3. If the total computed seismically-induced shear strain along the critical failure surface is less than the shear strain needed to trigger strength loss, as estimated from: 

(1) laboratory testing, (2) comparisons to published data on similar materials, or (3) 

simply taken as 0.25 percent, then strength loss will not be triggered. Use the peak 

undrained shear strength (or the drained strength, whichever is lower) in the stability analyses. If the total seismically-induced shear strain is greater than the triggering 

shear strain, use Sus in the stability analyses.
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7.4.2.3.3 Sand-Like Material: Stress-Based Method for Triggering

This method is used to evaluate whether the design earthquake will produce shear stresses in a zone 

of sand-like material that are high enough to cause strength loss in that zone. The basic concept of the 

method is that strength loss will be triggered by earthquake shaking if the sum of the static (gravity) 

shear stresses along a potential failure surface plus the seismic shear stresses exceed the yield (peak) 

undrained strength Su(yield).

This stress-based method for triggering is only appropriate if the safety factor (CRR/CSR) against 

triggering determined from the pore-pressure-based method is between 1.0 and 1.4. If the safety 

factor is greater than 1.4, then triggering can be assumed to not occur. If the safety factor is less than 

1.0, then it should be assumed that triggering will occur. For safety factors between 1.0 and 1.4, 

the pore-pressure-based method indicates that triggering is likely, but this stress-based method may 

indicate that triggering will not occur.

As discussed in Section 7.4.1, if the design PGA is greater than 0.2g and the CSR is greater than 0.15, 

then the triggering analysis described in the following text should not be performed, and strength loss 

for loose sand-like material should simply be assumed. The post-earthquake strength of the material 

in the loose zone should be assumed equal to Sus. If evaluating potential measures to improve the stability of a dam or embankment where the CSR in a zone exceeds 0.15, it may be valuable to perform 

the triggering analysis to help assess whether alternative stabilization schemes that would reduce the 

CSR might ultimately improve the dam or embankment stability.

The methodology described in the following steps is based primarily on Olson and Stark (2003), 

which should be consulted for further details. The steps are:

1. Perform a slope stability analysis to estimate the static shear stress τdriving in the loose 

zone of sand-like material. This is accomplished by varying the assumed undrained 

strength of the material in the zone until a safety factor of one is achieved. For denser 

soils, the peak drained or undrained strength should be used, as discussed in Section 

7.4.4.3.

2. Divide the critical failure surface into 10 to 15 segments.

3. Compute the weighted average σ’vo along the failure surface and calculate the average static shear stress ratio τdriving /σ’vo.

4. Estimate the average seismic shear stress τav, seismic applied to each segment of the 

failure surface using a 1D or 2D site response analysis as provided by SHAKE or 

QUAD4. The value of τav, seismic for each segment of the failure surface can be taken as 

0.65 times τmax obtained from the site response analysis.

5. Estimate the value of Su (yield)/σ’vo from CPT or SPT data using the proposed equations in Olson and Stark (2003). The equations give a range of ratios based on SPT or 

CPT data. The median value of SPT or CPT data should be used for each zone, and 

the lower bound of Olson and Stark’s equation should be used to compute the ratio 

of Su (yield)/σ’vo.

Commentary: The equations are based on back-calculations of yield (peak) undrained strength 

from failure case histories where the failures were triggered by static, not seismic, loading. 

Therefore, the back-calculated strengths represent yield (peak) undrained strengths.

6. Compute values of Su (yield) and τdriving for each segment along the failure surface, 

based on the values of Su (yield)/σ’vo and τdriving /σ’vo (average) and the value of σ’vo for 

each segment.
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7. Compute the factor of safety against triggering for each segment as:

FStriggering = Su(yield)/(τdriving + τav, seismic) (7-6)

8. Assume that triggering of strength loss occurs for segments where FStriggering is less 

than or equal to 1.0. Assume that triggering of strength loss does not occur for segments where FStriggering is greater than 1.0.

7.4.3 Evaluation of Post-Earthquake Strength

7.4.3.1 Correlations of SPT and CPT with Sus of Sand-Like Material

Loose, sand-like refuse and sand-like natural soil may experience significant strength loss due to 

earthquake shaking, if the shaking is strong enough. The reduced strength is the undrained steadystate (residual) strength Sus, which is often much lower than the peak undrained strength or the peak 

drained strength. Table 7.2 presents references for several correlations of Sus with SPT data (N1,60) and 

CPT data (qt1) for sand-like material.

The correlations are based on back-calculated values of Sus from actual flow slides and measured or 

estimated values of N1,60 or qt1. The correlations are generally limited to N1,60 values (uncorrected for 

fines) less than about 12 or qt1 less than about 75 tsf because no flow slides have been reported for soils 

with higher penetration resistances. For higher values of N1,60 and qt1, the drained strength should be 

used because, as discussed in Section 7.4.4.2, the material is dilative.

The Seed and Harder (1990) plot (Figure 7.7) is probably the most well known of the Sus versus SPT 

or CPT correlations. The plot is an update of an earlier plot presented in Seed (1987). The 1990 plot 

includes a correction to increase N1,60 values for silty materials to “equivalent clean sand” N1,60(CS)

values. The correction varies from 1 to 5 blows per foot depending on the percent fines. However, no 

basis for this correction is provided in either the 1990 or 1987 paper.

TABLE 7.2 REFERENCES FOR CORRELATIONS OF SUS WITH SPT AND CPT DATA

Reference Correlated Parameters

Seed (1987) Sus vs. N1,60

Davis, Castro and Poulos (1988) Sus vs. N1,60

Seed and Harder (1990) Sus vs. N1,60

Baziar and Dobry (1995) Sus and Sus /σ′v vs. N1,60

Castro (1995) Sus vs. N1,60

Wride, McRoberts and Robertson (1999) Sus and Sus /σ′v vs. N1,60

Yoshimine, Robertson and Wride (1999) Sus /σ′v vs. qc1N(CS)

Olson and Stark (2002) Sus /σ′v vs. N1,60 and qt1

Idriss and Boulanger (2007) Sus and Sus /σ′v vs. N1,60 and qc1N(CS)

 Note: Sus = Undrained steady state (residual) strength.

N1,60 = Standard Penetration Test (SPT) N-value, normalized to an effective overburden stress of one 

atmosphere (typically 1 tsf) and normalized to a hammer efficiency of 60 percent.

Sus /σ′v = Undrained steady state strength normalized to vertical effective stress.

qc1N(CS) = Cone Penetration Test (CPT) tip resistance normalized to a reference pressure of one atmosphere 

and corrected to an equivalent value for clean sand.

qt1 = CPT tip resistance normalized to an effective overburden stress of one atmosphere.
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Castro (1995) re-evaluated several of the case histories in Seed and Harder (1990) for which 

detailed data were available. He also collected re-evaluations of some of the earlier case histories 

performed by Poulos (1988) and Davis et al. (1988). He referred to the re-evaluations collectively 

as the GEI data. Castro then replotted the Seed and Harder data points (without a fines correction) 

and compared them to the GEI data points. The resulting plot of representative N1,60 values and 

back-figured Sus is shown in Figure 7.8. The original Seed and Harder (1990) lower-bound curve is 

somewhat lower than the Seed and Harder lower-bound curve shown in Castro (1995) because the 

original Seed and Harder curve included a fines correction while the Seed and Harder curve shown 

in Castro (1995) did not.

Correlations of Sus/σ’v versus SPT and CPT data, back-calculated from case histories, are listed in 

Table 7.3. Olson and Stark (2002) re-evaluated the Seed and Harder (1990) data and added new case 

histories for development of plots of Sus/σ’v versus both N1,60 and qt1. Olson and Stark did not include 

a fines correction to the N1,60 values as Seed and Harder did. Olson and Stark’s plot of Sus/σ’v versus 

qt1 is shown as Figure 7.9. A reasonable lower bound of the Olson and Stark data is a ratio of Sus/σ’v

equal to 0.04, which is independent of SPT or CPT value.

Idriss and Boulanger (2007) re-evaluated the Seed and Harder case histories and the Olson and Stark 

case histories, and included the fines correction to the N1,60 values as Seed and Harder did. Idriss and 

Boulanger recommended design curves for both Sus versus N1,60(CS) and Sus/σ’v versus N1,60(CS). Idriss 

and Boulanger’s design curve for Sus versus N1,60(CS) is in between the upper and lower bounds suggested by Seed and Harder in Figure 7.7.

EQUIVALENT CLEAN SAND SPT BLOWCOUNT, (N1,60(CS))

RESIDUAL UNDRAINED SHEAR STRENGTH, SUS (PSF)

FIGURE 7.7 Sus vs. N1,60(CS)

(ADAPTED FROM SEED AND HARDER, 1990)

0

400

800

1200

1600

2000

EARTHQUAKE-INDUCED LIQUEFACTION AND SLIDING

CASE HISTORIES WHERE SPT DATA AND RESIDUAL

STRENGTH PARAMETERS HAVE BEEN MEASURED.

EARTHQUAKE-INDUCED LIQUEFACTION AND SLIDING

CASE HISTORIES WHERE SPT DATA AND RESIDUAL

STRENGTH PARAMETERS HAVE BEEN ESTIMATED.

CONSTRUCTION-INDUCED LIQUEFACTION AND

SLIDING CASE HISTORIES.

LOWER SAN FERNANDO DAM

0 4 8 12 16 20 24

FIGURE 7.7 Sus versus N1,60(CS)
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N1, 60 (NO FINES CORRECTION)

FIGURE 7.8 Sus versus N1,60

SUS (PSF)

SEED AND HARDER (1990)

DAVIS et al. (1988) and

CASTRO (1995)

NO N DATA 1. JUVENILE HALL

2. LOWER SAN FERNANDO

3. MOCHIKOSHI SLIMES

4. LA MARQUESA, UPS

5. LA MARQUESA, DS

6. FORT PECK

7. CALAVERAS

8. HEBER ROAD

9. MOCHIKOSHI DIKE

10. LA PALMA

LOWER BOUND, GEI

LOWER BOUND,

SEED AND HARDER

0

500

1000

7

9

3

3

3

8

10 4

1

5

4

1

6

5

2

2

7

6

0 5 10 15

(CASTRO, 1995)

N DATA

GEI {

TABLE 7.3 CORRELATIONS OF SUS/Σ′V VERSUS SPT AND CPT DATA BACK-CALCULATED 

FROM CASE HISTORIES

Reference Reported 

Sus /σ′v

Types of Data and Materials

Baziar and Dobry 

(1995) 0.04 to 0.20

Back-calculated values from failure case histories for 9 sites with silty sand 

or sandy silt material (more than 10 percent fines). One of the 9 sites was 

identified as a tailings dam.

Yoshimine, 

Robertson and 

Wride (1999)

0.03 to 0.19

Back-calculated values from case histories involving multiple submarine 

slides at 3 sites. Materials were natural clean sand, silty sand, and sandy 

silt.

Olson and Stark 

(2002)

0.05 to 0.12 

(proposed 

limits of ratios)

Back-calculated values from 33 failure case histories, including re-evaluation 

of the Baziar and Dobry sites. Four of the 33 sites were identified as tailings 

dams as compared to dams or slopes consisting of natural soils. Four of the 

33 sites, including one of the tailings dam sites, had ratios of 0.02 to 0.04, 

outside their proposed boundary.

Idriss and 

Boulanger (2007) 0.05 to 0.22

Back-calculated values based on select case histories published by Seed 

(1987), Seed and Harder (1990), and Olson and Stark (2002) with adequate 

amount of in-situ measurements and reasonably complete geometric details 

(7 of the 35 case histories reviewed).

FIGURE 7.8 Sus versus N1,60
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Idriss and Boulanger’s design curve for Sus/σ’v versus N1,60(CS) is very close to the best fit line suggested by Olson and Stark in the range where N1,60(CS) is less than about 14. For N1,60(CS) values higher 

than about 12 to 14, which is beyond the range of the case history data, Idriss and Boulanger suggest 

two curves: one for the case where “void ratio redistribution effects” are negligible, and one for the 

case where void ratio redistribution effects may be significant. The difference between these two 

curves is discussed in the following commentary.

Commentary (Use of Sus/σ’v Ratios): Using the ratios of Sus/σ’v for confining pressures significantly higher 

than the confining pressures from the actual case histories may be unconservative. Twenty-eight of Olson and 

Stark’s 33 case histories had mean σ’v of 115 kPa or less. This corresponds to mean σ’v of up to 2,400 psf or, for an 

assumed effective unit weight of 55 pcf, a mean depth of up to 45 feet. Using Sus/σ’v ratios implies that increasing 

the confining pressure by a factor of 5, for example, would also increase Sus by a factor of 5. But laboratory testing 

has shown that the slope of the void ratio versus log Sus line is often steeper than the slope of the void ratio versus 

log σ’v line. In other words, increasing the confining pressure by a factor of 5 often results in increasing Sus by 

a factor less than 5. So using the Sus/σ’v ratios for high confining pressures may be unconservative. Therefore, 

recommended guidance is to consider only the lower-bound ratio of 0.04. The lower-bound ratio of 0.04 σ’v is considered conservative enough that it is acceptable even at high confining pressures.

The lower bound ratio of 0.04 is applicable to non-plastic materials and to materials with a liquidity index 

(LI) of less than one. Materials with LI > 1 are unusual, but may have even lower values of Sus. At this time, 

the recommended lower bound value of Sus for soils with LI > 1 is 20 psf, but no higher than obtained with a 

strength ratio of 0.04, based on the judgment of the authors of this chapter.

The correlations of Sus with N1,60 have uncertainty at high confining pressures because the correction factors 

used to correct N to N1 to account for increasing confining pressure are highly uncertain at confining stresses 

FIGURE 7.9 Sus/v' vs. qt1

NORMALIZED CPT TIP RESISTANCE, qt1 (MPa)

BACK-CALCULATED LIQUEFIED STRENGTH RATIO AND MEASURED CPT

BACK-CALCULATED LIQUEFIED STRENGTH RATIO AND CONVERTED CPT

FROM MEASURED SPT

BACK-CALCULATED LIQUEFIED STRENGTH RATIO AND ESTIMATED CPT

ESTIMATED LIQUEFIED STRENGTH RATIO AND MEASURED, CONVERTED

OR ESTIMATED CPT

NOTE: NUMBER BESIDE SYMBOL INDICATES AVERAGE FINES CONTENT

 IN PERCENT

74

7

86

50

13

1

0

100

30

40

15

16

7

20

3 3

20 32

20

55

7

7

7

6

50 36+

25

7

13

3

7

20 85

0 1 2 3 4 5 6 7 8 9 10

0

0.1

0.2

0.3

0.4

OLSON (1998)

PROPOSED

RELATIONSHIP

(ADAPTED FROM OLSON AND STARK, 2002)

Sus /v'

FIGURE 7.9 Sus /σ’v versus qt1
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higher than about 2 tsf (Youd et al., 2001). However, the uncertainty of applying the Sus versus N1,60 correlations at high confining stresses is less than the uncertainty of applying the correlations of SPT or CPT with 

Sus/σ’v ratios.

Commentary (Void Ratio Redistribution): The phenomenon of void ratio redistribution is discussed in Section 

7.4.5. It refers to the possibility that if a loose zone of sand-like material is overlain by an impervious zone, then 

earthquake shaking may cause pore water to migrate toward the interface of the two materials, which could 

result in the sand-like material becoming looser and therefore having a lower steady-state strength. There are 

no generally accepted methods for evaluating the potential for this to occur, and there is some controversy as to 

whether it actually occurs in the field at all.

The Idriss and Boulanger (2007) design curve for Sus/σ’v versus N1,60(CS) splits at values of N1,60(CS) higher 

than about 12 to 14. For the case where void ratio redistribution effects are negligible, Sus/σ’v increases rapidly as N1,60(CS) increases above 12 to 14. This is consistent with the fact that sand-like material with N1,60(CS)

values higher than 12 to 14 tend to be dilative and not susceptible to strength loss (as discussed in more detail 

in Section 7.4.4.2.1). For the case where void ratio redistribution effects may be significant, Sus/σ’v increases 

less quickly as N1,60(CS) increases. As Idriss and Boulanger explain, the curve for this case is largely conceptual, 

since there are no case history data at these N1,60 values.

For the purposes of this Manual, and as discussed further in Section 7.4.5, specific evaluations for potential 

void-ratio redistribution effects are not required. As discussed in Section 7.4.4.3, the post-earthquake strength 

of sand-like material with N1,60 greater than or equal to 15 may be based on the drained strength and not on 

correlations of Sus or Sus/σ’v to N1,60. However, if redistribution is a concern, the corresponding Idriss and 

Boulanger curve can be used.

For estimating Sus values of sand-like materials based on correlations to SPT and CPT data, the following three-step procedure is recommended:

1. For each zone of material, determine a representative value of N1,60. In general, the 

representative value should be the median value. For this step, CPT data can be converted to N1,60 using the relationships discussed in Section 6.4.3.7 (Lunne et al., 1997). 

2. Use either the “Lower-Bound, GEI” curve or the “Lower-Bound, Seed and Harder” 

curve from the Castro (1995) plot (Figure 7.8) to obtain Sus as a function of representative value of N1,60. Both curves can be extrapolated to values of N1,60 as high 

as 14 by extending the curves along approximately straight lines. As discussed in 

Section 7.4.4.3, drained strength, rather than Sus, can be used for N1,60 values of 15 

or higher.

3. For each zone, if the resulting value of Sus is less than 0.04 σ’v, use Sus = 0.04 σ’v instead.

Commentary: As discussed in Section 7.4.1, Sus of sand-like material is very sensitive to small 

changes in void ratio. CPTs and SPTs are not sensitive to small changes in void ratio, and 

therefore correlations of CPT and SPT to Sus are expected to have large scatter, as shown in Figures 7.7, 7.8, and 7.9. The Sus versus SPT or CPT correlations provide conservative estimates 

of Sus. Generally Sus is not directly related to either N1,60 or qt1. The case histories used in these 

correlations are cases where Sus was low enough that a flow slide or significant deformations 

occurred. However, there were almost certainly other sites where N1,60 or qt1 were similar, but 

flow slides or significant deformations did not occur because Sus was higher than obtained from 

the failure case histories. Cases where stability failure did not occur could provide lower-bound 

estimates of Sus (i.e., the minimum value of Sus needed to maintain stability), but these cases are 

not typically studied and would still provide only conservative estimates. Laboratory testing 

methods for estimating Sus may provide less conservative estimates of Sus.
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7.4.3.2 Laboratory Testing for Measuring Post-Earthquake Strength for Clay-Like Material 

and Sus for Sand-Like Material

7.4.3.2.1 Laboratory Testing Issues

Soil fabric has been shown in the literature to affect the peak undrained strength of sand-like material. This often results in different peak undrained strengths for undisturbed samples versus reconstituted samples. However, initial soil fabric should not affect Sus, because Sus is measured at high 

strains after the soil fabric has become remolded. An example of this is presented in Castro, Seed, 

Keller, and Seed (1992). Samples of hydraulic sand fill prepared from slurry and by moist tamping 

had the same steady-state line.

There is extensive published research that shows that, for peak-undrained strength of clayey soils:

Su (triaxial compression) > Su (direct simple shear) > Su (triaxial extension). However, there are no 

data to suggest whether or not steady-state (residual) strength Sus varies with test type. The difficulty is that test methods other than triaxial compression can not generally be run to high enough 

strain levels to reach Sus before non-uniformities in the specimen become so large that the test data 

lose meaning. Triaxial extension tests experience necking at relatively low strains. Direct simple 

shear tests have significant stress non-uniformities (at all strains) because there are no vertical shear 

stresses (and therefore no horizontal shear stresses) at the outside edges of the specimen. Investigators who have tried to evaluate the effect of test type on measured Sus of sand-like material have 

encountered these difficulties, which make it difficult to identify Sus on stress-strain curves from 

tests other than triaxial compression.

It is reasonable that the peak strength varies with test type, because the peak strength is very much 

dependent on soil structure and therefore on the method of loading. For Sus, however, the initial 

soil structure has been lost (remolded) due to the high strain, so the method of loading should not 

be as significant. While there might be a difference in Sus for sand-like material related to the fact 

that the intermediate principal stress varies depending on the test type, this difference will be small 

compared to the variation in steady-state strength that one should expect for different samples 

from the same layer or zone of material. Expected in-situ strength variation was previously discussed in Section 7.4.1.

For most clay-like material, Sus is such a conservative estimate of post-earthquake strength that minor 

differences that may or may not be a function of test type do not seem significant. In any event, the 

Sus value for clay-like soils cannot be measured using triaxial (compression or extension) or direct 

simple-shear tests, because the strain to Sus is so high. Testing of clay-like material to estimate postearthquake strengths that are closer to Sup involves using cyclic loading followed by static loading, 

as discussed in Section 7.4.3.2.2. For this testing procedure, differences in strength between triaxial 

compression and other test types should be small compared to the variation in strength one should 

expect for different samples from the same layer. However, one could, if desired, use direct simpleshear testing or correct the triaxial test strengths to equivalent direct simple-shear strengths using the 

information in Article 20 of Terzaghi, Peck, and Mesri (1996).

Based on the preceding discussion, the use of triaxial compression testing to estimate Sus for sandlike material, and to estimate post-earthquake strength for clay-like material, as discussed in Sections 

7.4.3.2.2 and 7.4.3.2.3, is reasonable.

7.4.3.2.2 Laboratory Testing of Soft Clay-like Material to Measure Post-Earthquake Strength

As discussed in Section 7.4.2.3.1, the post-earthquake strength for clay-like material is a function of 

the shear strains that occur during the earthquake. The post-earthquake strength may be the peak 

undrained strength Sup or it may be a reduced strength. The post-earthquake strength could possibly 

(but not likely) be as low as Sus.
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A conservative estimate of post-earthquake strength for soft clay-like materials is to use Sus, which 

can be obtained by performing field vane-shear tests and/or CPTs, as described in Section 7.4.3.3. A 

less conservative estimate can be obtained by performing laboratory testing to obtain an undrained 

strength that is appropriate based on considerations of accumulated strain and/or cyclic stress level. 

As discussed in Section 7.4.4.3, the post-earthquake strength of stiff clay-like material can be taken as 

the peak-undrained strength.

If laboratory testing is performed, strain-controlled, undrained-triaxial (or perhaps undrained direct 

simple-shear) tests should be performed to measure: (1) the peak undrained strength, (2) the shear 

strain to peak undrained strength, and (3) the drop-off in shearing resistance with continued strain 

after peak. It should be remembered that shear strain in the triaxial test is 1.5 times the axial strain 

and that peak strength refers to the peak principal stress difference. Test specimens should be consolidated anisotropically to stresses that model in-situ conditions.

If the shear strain at peak strength is less than about 5 percent, or if the shearing resistance drops 

off significantly after peak, then either: (1) measure Sus (in the laboratory or the field) and use it as a 

conservative estimate of post-earthquake strength, (2) perform a series of laboratory tests with cyclic 

loading followed by monotonic loading to obtain a less conservative estimate of post-earthquake 

strength based on the strain that occurs during cyclic loading, or (3) perform a series of laboratory 

tests with cyclic loading followed by monotonic loading to obtain a less conservative estimate of postearthquake strength based on the cyclic stress levels applied by the design earthquake. Details for 

these options are discussed in the following text. Note that the number of loading cycles in the cyclic 

loading sequence must be appropriate compared to the number of representative loading cycles associated with the design earthquake magnitude. Seed and Idriss (1982) correlated the number of representative loading cycles with earthquake magnitude (e.g., M = 6.5, N = 10; M = 7.5, N = 15; and M = 8.5, 

N = 26, where M is earthquake magnitude and N is the number of representative loading cycles).

If the shear strain at peak strength is more than 5 percent, and there is little drop-off in shearing 

resistance after peak, then options (1), (2), and (3) discussed in the preceding paragraph can be used. 

However, a simplified version of option (2) can also be used. If the seismically-induced shear strains, 

as described in Step 2a, are less than one-half the shear strain to peak strength in the monotonic 

(static) test, then the post-earthquake undrained strength can be assumed to be equal to the peak 

undrained strength Sup. This is based on previous testing of clay-like materials (Thiers and Seed, 

1968; Castro and Christian, 1976; Castro, 2003).

The three options for evaluating the post-earthquake strength of clay-like material are:

1. Conservatively assume that the post-earthquake strength is Sus and measure Sus. To 

measure Sus in the field, CPT or field vane tests can be used, as discussed in Section 

7.4.3.3. To measure Sus in the laboratory, vane-shear tests can be used. The strain rate 

should be high to ensure that the sample is being sheared undrained and that the 

undrained Sus is being measured (see discussion of field vane-shear rates in Section 

6.4.3.8). Sus cannot normally be reached for clay-like material (except highly sensitive 

clays) within the strain limitations of laboratory tests, other than possibly rotation 

shear. Since it is unlikely that the post-earthquake strength will be reduced to Sus

except in highly sensitive clays, laboratory testing to measure Sus of clay-like material 

may not be needed in practice.

2. Measure the relationship between cyclic strain and post-earthquake strength. Anisotropically-consolidated, undrained-triaxial tests (or perhaps undrained, direct 

simple-shear tests, also consolidated with an initial shear stress), with cyclic loading followed by monotonic loading, can be used to obtain the relationship between

accumulated shear strain during the earthquake and post-earthquake, undrained 
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Seismic Design: Stability and Deformation Analyses

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strength. Accumulated strain is a measure of the effects of cyclic loading on the postearthquake strength of clay-like material. After low accumulated strains, the postearthquake strength will be close to the peak-undrained strength. After very high 

accumulated strains, the post-earthquake strength may be close to Sus. Instead of accumulated strain, peak strain could also be used as a measure of the effects of cyclic 

loading. However, the use of accumulated strains is recommended. A procedure for 

performing this type of testing is described in Castro (2003). The basic steps are:

a. Select one or more potential failure surfaces through the embankment. 

Estimate the accumulated strains that develop due to earthquake shaking 

using either Newmark-type analyses (Section 7.5.4) or numerical modeling (Section 7.5.5). If numerical modeling is used to estimate accumulated 

strains, the model output will include shear strains. If a Newmark-type 

analysis is used to estimate accumulated strains, the analysis will compute displacements. To convert displacements to shear strains, the thickness of the loose zone must be estimated, and the strains should be assumed to be uniform across the full thickness of the zone. In other words, 

the accumulated shear strain will equal the displacement divided by the 

thickness of the zone. (A Newmark-type analysis can be used instead of 

numerical modeling only for cases in which the loose zone is relatively 

thin compared to the overall failure mass. CPT data are probably the best 

way of delineating the thickness of the loose zone. Another way to delineate the thickness of the loose zone, so that shear strain can be computed 

from displacement, is to identify the range of potential failure surfaces for 

which the safety factor is within perhaps 10 percent of the safety factor 

of the most critical failure surface. The thickness of the range of failure 

surfaces can be taken as the thickness of the loose zone.

Commentary: A design earthquake motion (time history of acceleration) and 

dynamic soil properties (modulus and damping) are required for performing the 

deformation analysis required for this step.

b. Perform a series of tests on undisturbed specimens consolidated anisotropically to in-situ stresses. First apply cyclic stresses to each specimen, 

and measure the accumulated strains. Then, without allowing for dissipation of the pore pressures generated by the cyclic loading, apply monotonic loading to each specimen to measure the peak strength. Perform 

tests at various cyclic stresses to bracket the accumulated strains indicated 

by the deformation analysis. The cyclic loading portion of the test should 

be load-controlled, and the monotonic loading portion of the test should 

be strain-controlled.

c. Plot the value of peak undrained strength under monotonic loading 

versus the accumulated strain during loading. There should be a trend of 

constant monotonic peak strength for small accumulated strains and then 

decreasing monotonic strength for higher accumulated strains. (In the 

example in Castro (2003), post-earthquake strength was plotted against 

maximum cyclic strain instead of accumulated strain, but the concept 

is the same. The monotonic strength began to decrease for cyclic shear 

strains exceeding about 7.5 percent.)

d. Select the post-earthquake strength as the value of monotonic strength 

that corresponds to the estimated accumulated shear strain from the de-
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formation analysis. For clay-like materials, 80 to 100 percent of the peakundrained strength should be used, as discussed in Section 7.4.4.3.

3. Measure the effect of cyclic loading on post-earthquake strength – This approach 

involves performing cyclic undrained shear strength testing on undisturbed samples 

of the clay-like material, which are consolidated anisotropically to stresses that model 

the more critical in-situ conditions (i.e., higher anisotropic stress ratio for materials 

along the critical potential failure zone). The samples should be cyclically loaded for a 

conservative number (not less than 20) of cycles, while bracketing a conservative range 

of cyclic stress or CSR based on the design earthquake. (Cyclic stress or CSR for the 

design earthquake can be obtained from 1D or 2D site response analyses, as described 

in Section 7.4.2.2.1, Method 1.) The samples should then be subject to undrained monotonic loading to measure the range in post-cycling Su for the range of CSR.

The intent is to test clay-like samples at the higher end of the in-situ anisotropic stress ratio, to a conservative number of cycles (not less than 20 cycles) of loading and a conservative range of CSR, not 

to obtain a unique relationship between cyclic loading and Su, but only to bracket a post-earthquake 

Su (often higher than Sus) that can be applied in post-earthquake limit equilibrium analyses. This 

approach might be helpful in instances where undrained triaxial or direct simple-shear monotonic 

tests indicate a shear strain at peak strength somewhat less than 5 percent, but the post-peak drop-off 

in shearing resistance is not dramatic or particularly significant. It may also be helpful when such 

testing indicates a shear strain at peak strength greater than 5 percent, but the post-peak behavior 

and/or the perceived sensitivity of the clay-like material warrants more direct evaluation of the undrained strength behavior following cyclic loading.

For material that is clay-like, or that borders between sand-like and clay-like behavior, laboratory 

testing of undisturbed samples of the site-specific materials, as described above, should be considered. The laboratory testing might be monotonic testing to obtain just Sup and strain to Sup, or (for 

critical structures) cyclic loading followed by monotonic testing to obtain the relationship between 

seismically-induced shear strain and post-earthquake strength. If quality undisturbed samples of 

site-specific materials can not practically be obtained, then testing of carefully reconstituted samples 

of the site-specific materials should be considered. However, data from reconstituted samples should 

only be used if they can be compared to data from at least some undisturbed samples or a thorough 

base of in-situ and laboratory test data, to confirm that the reconstituted samples are representative 

of and/or bracket in-situ conditions.

For proposed new facilities, where material for testing is not available, undisturbed or carefully 

reconstituted samples of similar materials from other facilities can be used. Once a significant base of 

testing of fine coal refuse is built up, then it may be possible to estimate strength reduction based on 

correlations with previous testing. Provisions should be made to confirm soil properties of proposed 

facilities by performing testing as the facility is constructed.

7.4.3.2.3 Laboratory Testing of Sand-like Material to Measure Sus

In loose sand-like material, if the earthquake shaking triggers strength loss, then the post-earthquake strength will be equal to the undrained steady state (residual) strengths Sus. Estimates of 

Sus for sand-like material should first be made based on correlations with SPT and/or CPT data, 

as discussed in Section 7.4.3.1. If desired, laboratory testing to measure Sus can also be performed. 

Laboratory testing, while complex, often results in less conservative estimates of Sus than the SPT 

and CPT correlations. If laboratory testing to measure Sus is planned, the designer should make the 

parties responsible for obtaining, transporting, and testing the samples fully aware of the methods 

to be followed. For example, since Sus is highly sensitive to void ratio, high quality undisturbed 

samples are required and changes in void ratio must be tracked during material sampling, trans-
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portation, and testing. Failure to follow the testing guidance may bring laboratory measured values 

of Sus into question. The designer should consider discussing the approach and methods with MSHA 

prior to initiating sampling and testing.

The discussion that follows is based primarily on Poulos, Castro, and France (1985), which should be 

consulted for further details. In addition, Castro, Seed, Keller, and Seed (1992) discuss application of 

this laboratory testing method to the 1971 slide at the Lower San Fernando Dam. Additional information is provided in Appendices 7A, 7B, and 7C of this Manual.

At this time, an upper bound is recommended for the values of undrained steady state (residual) 

strength Sus obtained from laboratory testing of sand-like material. That upper bound is an envelope 

based on the median N1,60 value of the loose layer being tested, as indicated in Table 7.4. The purpose 

of the upper bound is to make sure that unreasonably high values of Sus based on laboratory testing 

of sand-like material are not proposed.

TABLE 7.4 UPPER-BOUND VALUES OF SUS FOR CORRESPONDING VALUES OF N1,60

N1,60

Maximum Allowable Value of Sus

Based on Laboratory Testing

0 200 psf

5 500 psf

10 1100 psf

14 1700 psf

Some basic concepts associated with laboratory testing for measuring Sus of sand-like material are:

1. The undrained steady state (residual) strength Sus is equal to the undrained 

strength at high strains, after the initial structure or fabric of the material has been 

fully remolded.

2. Sus is very sensitive to void ratio and to minor changes in grain-size distribution. As 

discussed in Section 7.4.1, deposits of sand-like material have inherent variations in 

both. Therefore, multiple samples from the same layer or zone are expected to have 

varying values of Sus.

3. Strain-controlled, undrained triaxial compression tests with pore-pressure measurement are typically used. Experience has shown that isotropic and anisotropic 

consolidation in the triaxial cell yield the same values of Sus. Figure 9 in Castro et 

al., (1992) provides a good example. Isotropic consolidation is commonly used for 

simplicity.

4. At strains beyond about 20 percent in the triaxial test, non-uniformities in specimen 

stress and strain become significant.

5. Sus is obtained from the triaxial test, as shown in the following. Note that these equations are derived from the Mohr-Coulomb failure envelope in Appendix 7A.

qs = (σ1s ‒ σ3s)/2, where qs is one-half the principal stress difference at steady state

sin φ’s = qs/(σ’3s + qs), where φ’s is the effective stress friction angle at steady state

Sus = qs cos φ’s

6. For different samples of a given material (all with exactly the same grain-size distribution), Sus is a function only of void ratio. The relationship between void ratio and 
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Sus for a given soil, with Sus plotted on a log scale, is referred to as the steady-state 

line (Figure 7.10).

7. Since Sus and σ’3s are related by φ’s (Appendix 7A), the steady-state line can be plotted as either void ratio versus Sus or void ratio versus σ’3s. Plotting void ratio versus 

σ’3 makes it possible to plot the state of the sample before and after consolidation in 

the triaxial cell, as well as at steady state.

8. It has been shown experimentally that similar materials with the same grain shape 

and mineralogy, but slightly different grain-size distributions, will have steady-state 

lines that are parallel but not coincident. Examples are provided in Figures 3, 4, and 

5 of Poulos, Castro, and France (1985).

9. High quality undisturbed samples are used to measure Sus of in-situ material. Because of unavoidable densification that occurs during sampling, handling, and especially during consolidation in the triaxial cell, Sus measured in the laboratory will 

be higher than the in-situ Sus. Correcting the value of Sus measured in the laboratory 

back to the in-situ Sus for a given sample requires two things. First, careful measurements must be made during sampling, handling, and testing to measure the total 

change in void ratio from the in-situ condition to the laboratory-consolidated condition. Second, the slope of the steady-state line must be measured.

100 1,000 10,000

STEADY-STATE STRENGTH, SUS (PSF)

VOID RATIO, e

FIGURE 7.10 STEADY-STATE LINE

0.6

0.7

0.8

The basic procedural steps in a steady-state (residual) strength laboratory testing program for 

sand-like material are provided in the following list (for each zone of interest). Detailed procedures 

are presented in Appendix 7B.

1. Identify the zone of loose material based on SPT, CPT, and/or other desired field 

methods. Obtain enough disturbed or undisturbed samples from which a batch mix 

can be created that will contain enough soil for at least 5 tests to measure the slope of 

the steady-state line. Obtain enough high quality undisturbed samples to perform at 

least 8 tests on undisturbed samples from each zone of interest to measure the in-situ 

Sus. A detailed sampling procedure that has been used with fixed-piston sampling to 

record sample densification during sampling is provided in Appendix 7C.

FIGURE 7.10 STEADY-STATE LINE
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2. Perform grain-size, hydrometer, specific-gravity, and (if the material has any plasticity) Atterberg-limits tests on material from the batch mix. Also perform grain-size 

tests on the trimmings from all undisturbed samples used to measure the in-situ Sus.

3. To measure the slope of the steady-state line, prepare at least 5 very loose uniform 

specimens from the batch mix for triaxial testing. Moist tamping in 10 layers has 

worked well for 3-inch-diameter by 7-inch-tall specimens. Isotropically consolidate 

the specimens to varying consolidation pressures such that a range of final void ratios is obtained. Carefully measure the volume of the sample in the triaxial cell after 

saturation but before beginning consolidation. This is typically done by applying a 

small vacuum to the sample so that it will maintain its shape with no cell pressure 

applied. After measuring the sample volume, carefully keep track of all volume 

change during consolidation. Shear the samples in undrained strain-controlled compression, and measure Sus at high strains. (See testing Note 2 below for how to identify whether Sus has been reached.) After the test, oven dry the entire tested sample 

to measure its water content. Using the water content and the specific gravity of the 

material, compute the void ratio during shear. Then plot the void ratio during shear 

versus Sus for each test. All the tests should plot close to a straight-line fit of the data 

(Figure 7.10). This is the steady-state line for the batch mix.

4. Set up, saturate, and consolidate each undisturbed sample. Measure the sample 

volume before consolidation and volume changes during consolidation, as discussed 

in the previous step. Shear each undisturbed sample in undrained, strain-controlled 

compression and measure Sus at high strains. After the test, oven dry the entire tested 

sample to measure its water content and dry weight. Alternatively, measure the entire wet weight, then use half the sample for water content measurement and half the 

sample for grain-size and/or specific-gravity testing. For fine coal refuse, a specific 

gravity measurement must be performed for each undisturbed test sample. Using 

the water content, the dry weight, and the specific gravity of the material compute 

the void ratio of the sample during undrained shear (Appendix 7B). The measured 

value of Sus corresponds to the void ratio during undrained shear in the triaxial cell. 

To estimate the in-situ value of Sus for that sample, first plot the result of the test on 

a diagram of Sus versus void ratio. Then draw a line upward and to the left from that 

data point, parallel to the steady-state line measured for the batch mix. Where the 

line drawn reaches the in-situ void ratio, select the value of in-situ Sus (Figure 7.11). 

Refer to Appendix 7B for detailed procedures for measuring the as-tested and in-situ 

void ratios of each sample.

5. The eight or more tests on undisturbed samples will result in a range of estimated 

in-situ values of Sus. Select a design value of Sus that is lower than two-thirds of the 

estimated in-situ values and higher than one third of the values. The appropriate total number of tests that should be performed for each zone depends primarily on the 

variability of that zone. An example is shown in Figure 7.12. As shown in the figure, 

one should not expect the as-tested Sus values from the undisturbed samples to plot 

on a straight line of void ratio versus log Sus, because slight variability in grain-size 

distribution from sample to sample may result in significant variability in the vertical 

position, but not the slope of the correlation of Sus versus void ratio.

Some notes on testing include the following:

1. Accuracy in measuring void ratio is critical to proper measurement of Sus. Appendices 7B and 7C discuss the required precision of various measurements in the field 

and laboratory. A given error in void ratio will result in a higher error in Sus for mate-
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rials with relatively flat steady-state lines. In other words, materials with low slopes 

of the steady-state line (low values of Δe/ΔlogSus), are more sensitive to errors in the 

measurement of void ratio.

The range of slopes of steady-state lines (Δe/Δlog Sus) for coal refuse reported for 

seven sites in West Virginia and one site in Kentucky was 0.090 to 0.140 (personal 

communication, GEI, 2007). For the flattest slope of 0.090 (the most sensitive case), 

errors in void ratio measurement would lead to errors in Sus, as shown in Table 7.5

As discussed in Appendix 7B, void ratios at the various steps of sampling and testing should be computed to the nearest 0.001. The goal is that the computed in-situ 

and as-tested void ratios should be correct to within 0.010. The resulting values of Sus

should then be correct to within 30 percent, as indicated in Table 7.5.

2. When looking at a stress-strain curve to identify whether the steady state has been 

reached, curves for contractive samples are more definitive than curves for dilative 

samples. Contractive samples will typically exhibit peak strength at small strains 

followed by a drop-off in strength and a flattening out at the steady state within the 

strain limits of the triaxial test. Dilative samples, on the other hand, typically show a 

gradually increasing strength and may not reach steady state within the strain limits 

of the test. Also, dilative samples are more likely to develop failure planes during 

shear, meaning that steady state is reached only within the thin failure zone where 

the void ratio is unknown. Therefore, contractive samples are preferred.

For an undisturbed sample with a given void ratio, consolidating the sample to a 

higher effective confining stress before undrained shear is likely to make the sample 

more contractive during undrained shear, as explained further in the commentary 

that follows. Contractive samples tend to have stress-strain curves that are more 

definitive for interpretation of Sus, as explained in the previous paragraph. Therefore, 

it is often good practice to consolidate the sample to a high confining pressure before 

undrained shear. The disadvantage is that consolidating the sample to a high confining pressure also decreases the void ratio of the sample in the test and requires a 

bigger correction to obtain the in-situ value of Sus.

100 1,000 10,000 100,000

0.5

0.6

0.7

0.8

0.9

1.0

STEADY-STATE SHEAR STRENGTH, SUS (PSF)

VOID RATIO, 

e

FIGURE 7.11 METHOD FOR CORRECTING LABORATORY SUS TO IN-SITU SUS

IN-SITU VOID RATIO

OF "UNDISTURBED"

SPECIMEN

B

A

VOID RATIO OF

"UNDISTURBED"

SPECIMEN DURING

SHEAR

SUS OF "UNDISTURBED"

 SPECIMEN

ESTIMATED

IN-SITU SUS

STEADY-STATE

LINE FOR RECONSTITUTED

SPECIMENS FROM BATCH MIX

FIGURE 7.11 METHOD FOR CORRECTING LABORATORY Sus TO IN-SITU Sus
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It is not necessary to consolidate the sample to stresses that model in situ conditions. 

This is because the steady-state strength is the strength at high strains and is not 

affected by stress history. Therefore, isotropic consolidation is normally used.

Commentary: Consolidated samples that plot well to the right of the steady-state line will be 

contractive, as shown in Figure 6 of Poulos et al. (1985). Experience with laboratory testing 

tells us that the e – log σ’3 consolidation curve is usually slightly flatter than the steady-state 

line (plotted as e versus log σ’3 per item 7 on page 7-46). Therefore, increasing the consolidation 

stress tends to move the sample further to the right compared to the steady-state line, and the 

sample tends to become more contractive.

At the steady state, both the principal stress difference and σ’3 should be constant 

with strain.

100 1,000 10,000

STEADY-STATE STRENGTH, SUS

SUS MEASURED AFTER

LABORATORY CONSOLIDATION

SUS AFTER CORRECTION

TO IN-SITU VOID RATIO

STEADY STATE LINE

FROM BATCH MIX

VOID RATIO, 

e

FIGURE 7.12 INTERPRETATION OF LABORATORY SUS TESTING

0.5

0.6

0.7

0.8

0.9

TABLE 7.5 ERROR IN VOID RATIO AND CORRESPONDING PERCENT ERROR IN SUS

Error in void 

ratio (Δe) ΔlogSus

(1) Sus(measured)/

Sus(actual)(2) Error in Sus (%)

0.005 0.0555 1.14 14

0.010 0.1111 1.29 29

-0.005 -0.0555 0.88 12

-0.010 -0.1111 0.77 23

 Note: 1. ΔlogSus = Δe/0.090 = [log Sus(measured)] - [log Sus(actual)] = log [Sus(measured)/Sus(actual)]

 2. Sus(measured)/Sus(actual) = 10(ΔlogSus)

FIGURE 7.12 INTERPRETATION OF LABORATORY Sus TESTING
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3. It is helpful to plot the compression curves (e - log σ’3) along with the values at 

steady state. Especially for the samples tested from the batch mix to measure the 

steady-state line, these plots will help determine what consolidation stresses are 

needed to achieve contractive samples at desired void ratios. (Figure 7.13)

4. Published data for coal refuse, based on laboratory testing as described above, include: (1) a reported range of 0.06 to 0.27 for Sus/σ’v and (2) a reported slope of the 

steady-state line Δe/Δlog Sus in the range of 0.11 to 0.13 based on laboratory testing 

of 34 undisturbed samples (PI in the range of 0 to 12) of fine coal refuse from five 

West Virginia sites (Genes et al., 2000). Also, reported slopes of the steady-state line 

Δe/Δlog Sus for one site in West Virginia and one site in Kentucky were found to be 

0.09 and 0.14 (GEI, 2007).

Because the preceding parameters are based on limited data and a wide range of ratios, the data 

should be used carefully. Also, as discussed in Section 7.4.3.1, using the ratios of Sus/σ’v for confining 

pressures significantly higher than the confining pressures from the actual samples may be unconservative. However, if values of Sus are measured using laboratory testing of site-specific materials or 

comparable deposits at comparable confining pressures, then the site-specific ratios may be valuable 

for adjusting the measured Sus values for other portions of the embankment, or for future construction stages (Genes et al., 2000).

Over time, if a database of strength ratios for fine coal refuse is developed, it should become possible 

to use these ratios with more confidence.

Estimation of strengths using strength ratios requires an estimate of the consolidation pressure. One 

should not make the assumption a priori that the soils are normally consolidated without obtaining confirming data and monitoring. In areas where fine coal refuse will serve as the foundation 

for embankment construction, the design plan should include provisions for monitoring pore pressures and, if significant upstream construction is planned, controlling the rate of construction to 

mitigate excess pore pressure development and/or reducing the likelihood that significant zones of 

fine refuse will be under consolidated as construction proceeds. When evaluating existing embankments, piezometer data should be obtained to allow estimation of effective consolidation stresses. 

Also, pore-pressure-dissipation tests performed with the piezocone at various depths are an effective 

CONSOLIDATION IN

TRIAXIAL CELL

UNDRAINED SHEAR

STEADY-STATE LINE

VOID RATIO, 

e

0.9

0.8

0.7

0.6

0.5

100 1000 10,000 100,000

MINOR EFFECTIVE PRINCIPAL STRESS, 3

FIGURE 7.13 CONSOLIDATION CURVE AND STEADY-STATE LINE
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way of obtaining the pore-pressure profile at a given location. Consolidation tests on undisturbed 

samples of clayey layers can also be used to determine the current consolidation stresses on the 

clayey layer and adjacent more sandy layers.

7.4.3.3 Field testing to Measure Sus of Soft Clay-Like Material

Field vane-shear testing can be used to estimate Sus of clay-like material. For the field vane measurements to be valid, they must be performed to high enough strains to reach Sus and must be performed 

quickly to be confident that the material is being sheared undrained. Recommended vane shear testing procedures are provided in Chapter 6, Section 6.4.3.8. It is a good idea to pair field vane-shear 

tests with CPTs, to confirm that the field vane is being performed in a layer of clay-like material.

CPT sleeve friction measurements can also be used for estimating Sus of clay-like material (Lunne et 

al., 1997), although comparison field vane tests are recommended because the CPT sleeve friction 

is less reliable in sensitive clays. CPTs should be performed with pore-pressure measurements to 

confirm that the pore-pressure response indicates undrained shear. Lack of elevated pore-pressure 

response may indicate that the material is behaving as drained rather than undrained.

Commentary: For most clay-like materials the post-earthquake strength will be higher than Sus. Therefore, 

using Sus as an estimate of post-earthquake strength may be overly conservative. Laboratory methods of estimating post-earthquake strength of clays, as discussed in Section 7.4.3.2.2, may provide less conservative estimates of post-earthquake strength.

As a lower bound, clay-like material with a liquidity index of less than 1.0 can be considered to have 

a post-earthquake strength of 0.04 σ’v. If the liquidity index is greater than or equal to one, a lower 

bound post-earthquake strength of 20 psf, but no higher than 0.04 σ’v, may be used. These values 

are guidance based on the judgment of the authors. Clay-like materials with a liquidity index less 

than 1.0 are unlikely to lose strength all the way to Sus due to earthquake shaking. A reduction in 

undrained strength for these materials from a pre-earthquake peak strength of 0.2 σ’v (a value within 

the representative range for natural clays) to a post-earthquake strength of 0.04 σ’v is considered conservative. Clay-like material with a liquidity index of 1.0 or higher may act like a “quick” clay with 

a very low remolded or post-earthquake strength. Therefore, a lower bound of just 20 psf, but no 

higher than 0.04 σ’v, is considered appropriate. 

7.4.4 Analysis Steps

Engineers use various methods for evaluating triggering and post-earthquake strength. Engineers 

also perform these evaluations and the related limit-equilibrium, slope-stability analyses in varying 

sequences. The following steps represent a generalized approach that can be adjusted for individual 

projects. Reference should be made to the flow chart in Figures 7.1a, 7.1b and 7.1c.

7.4.4.1 Step 1 - Define Embankment Geometry

Define the geometry of the embankment and of the material zones within the embankment. Identify 

zones of clay-like versus sand-like materials.

7.4.4.2 Step 2 - Screen for Potential Strength Loss

Review the subsurface conditions at the embankment to evaluate whether any zones have the potential for strength loss due to earthquake shaking.

7.4.4.2.1 Sand-Like Material

For this initial screening step, saturated to nearly-saturated, sand-like materials with N1,60 values less 

than 15, or qt1 values less than 75 tsf, should be considered potentially susceptible to strength loss.
7-52

Chapter 7

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Commentary: The screening criteria that sand-like materials with N1,60 values greater than 15 or qt1 values 

greater than 75 tsf are not susceptible to strength loss are based on case-history data from large earthquakes 

indicating that flow slides have only occurred where N1,60 values were 13 or lower (Seed and Harder, 1990; 

Castro, 1995; Wride et al., 1999) and qc1 values were less than 65 tsf (Olson and Stark, 2002). Olson and Stark 

(2003) present curves showing dilative versus contractive behavior for sands. The boundary for dilative versus 

contractive in terms of N1,60 values varies from 10 to 15. The boundary in terms of qc1 values varies from 65 to 

85. Also, Seed (1979) presents data showing that sands with relative densities higher than about 50 percent or 

N1 values higher than about 15 have limited strain potential and therefore may experience cyclic mobility but 

not strength loss, even though pore pressures may have reached 100 percent.

Many experts agree that a fines correction might be reasonable in this screening analysis for sand-like material. 

That is, a silty sand may be less likely to be contractive than a clean sand with the same N-value. However, the 

data to support this are sparse and vague. Therefore, a fines correction should not be applied here unless additional data is published in the future that can justify it.

It is interesting to note that the curves presented in Youd et al. (2001) indicate that “liquefaction” may occur 

during large earthquakes in sand-like material having N1,60 values as high as 30, and qc1N values as high as 

150. This is because, in Youd et al. (2001), “liquefaction” refers to increases in pore pressure but not necessarily 

to strength loss. As stated in the introduction to Youd et al. (2001): “In moderately dense to dense materials, 

liquefaction leads to transient softening and increased cyclic shear strains, but a tendency to dilate during shear 

inhibits major strength loss and large ground deformations.” (The term “increased cyclic shear strains” is 

referred to herein as cyclic mobility.) In other words, moderately dense to dense sand-like materials (N1,60 values 

higher than 15 or qt1 values higher than 75 tsf) may experience high excess pore pressures and cyclic mobility, 

but they will not experience strength loss.

The criterion that sand-like materials with N1,60 values greater than 15, or qt1 values greater than 75 tsf, be considered not susceptible to strength loss is a conservative criterion. Many materials with N1,60 values less than 

15, or qt1 values less than 75 tsf, will be dilative and not susceptible to strength loss. Therefore, if almost all N1,60

values exceed 13 and the mean N1,60 value exceeds 15 within a zone, or almost all qt1 values exceed 65 tsf and the 

mean qt1 value exceeds 75 tsf within a zone, then the zone can be considered not susceptible to strength loss.

Conservative criteria and conservative engineering judgments are appropriate and intended at this step for 

three reasons: (1) little if any of the data supporting these criteria are based on coal refuse materials (although, 

considering that coal refuse particles are more compressible than natural soil particles, one might expect these 

criteria to be even more conservative, because the compressibility of the coal refuse may mean that SPT and 

CPT values are lower for coal refuse than for natural soils at the same void ratio), (2) there is some uncertainty 

in the overburden corrections used to convert measured field values to N1,60 and qt1 values, and (3) no further 

analysis is recommended for materials that satisfy these criteria.

Thorough site-specific field and laboratory testing should ultimately yield data other than N1,60 and qt1 to better 

evaluate whether sand-like materials at a site are or are not susceptible to strength loss. If in-situ and laboratory 

test data are available during the initial screening phase, it should also be used in classifying sand-like zones as 

susceptible or not susceptible to strength loss.

7.4.4.2.2 Clay-Like Material

For this initial screening step, clay-like material may be screened for zones with potential for significant strength loss based on CPT data, as shown in Figure 7.14 (Robertson, 2008). This figure is a 

slightly more conservative version of a similar figure presented in Robertson and Wride (1998). In this 

figure, Q is equivalent to qc1N and F is the friction ratio. Computation of both Q and F is discussed in 

Youd et al. (2001). CPT data in Zone B indicate clay-like material for which significant strength loss is 

unlikely. CPT data in Zone C indicate potentially highly sensitive clay-like material for which significant strength loss can occur (Zones A1 and A2 indicate sand-like material). If CPT data are not avail-
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Seismic Design: Stability and Deformation Analyses

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able, then SPT data can be used along with Atterberg limits to confirm that the material is clay-like. 

N > 6 corresponds to Zone B and N < 6 corresponds to zone C. For this screening step, the N-values 

of the clay-like material should be corrected for hammer efficiency per Youd et al. (2001), but should 

not be corrected for overburden pressure. Overburden pressure corrections are applicable only to 

sand-like material, not clay-like material. Undrained strength data, if available, can also be used. 

Peak undrained strength of 1500 psf or higher corresponds to Zone B, and peak undrained strength 

less than 1,500 psf corresponds to zone C.

Commentary: These screening criteria for clay-like material are recommended guidance. No generally accepted 

screening criteria for susceptibility to strength loss were found in the literature. Selection of these criteria is 

based on the authors’ judgment and experience, and considering published data from sites where liquefaction 

(strength loss or excess pore pressure) was observed in soils with significant fines content. NORMALIZED CPT TIP RESISTANCE, Q

0.1 1 10

1

10

100

1000

Ic = 2.6

A2

2 3

C

7

6

A1

5 B

4

NORMALLY

CONSOLIDATED

INCREASING

OCR, AGE,

CEMENTATION

INCREASING

OCR AND AGE

INCREASING

SENSITIVITY

NORMALIZED FRICTION RATIO, x 100%

NOTE: 1. VALUES OF Q AND F ARE COMPUTED FROM CPT DATA AT THE DEPTHS OF INTEREST,

 AS DESCRIBED IN YOUD ET AL. (2001).

 2. CPT DATA THAT PLOT IN ZONES A1 AND A2 INDICATE MATERIAL THAT IS NOT CONSIDERED

 CLAY-LIKE, SO THIS SCREENING METHOD IS NOT APPLICABLE.

 3. CPT DATA THAT PLOT IN ZONE B INDICATE CLAY-LIKE MATERIAL THAT IS NOT SUSCEPTIBLE

 TO STRENGTH LOSS.

 4. CPT DATA THAT PLOT IN ZONE C INDICATE CLAY-LIKE MATERIAL THAT MAY BE SUSCEPTIBLE

 TO STRENGTH LOSS.

 5. NUMBERED ZONES SEPARATED BY DASHED BLUE LINES ARE FOR REFERENCE ONLY AND

 ARE A GUIDE TO SOIL TYPES, AS DESCRIBED IN YOUD ET AL. (2001). FOR EXAMPLE, DATA IN

 ZONE 3 INDICATE SILTY CLAY TO CLAY WHILE DATA IN ZONE 6 INDICATE CLEAN SAND TO

 SILTY SAND. THE LINE SEPARATING SAND-LIKE FROM CLAY-LIKE MATERIAL CORRESPONDS

 TO A SOIL BEHAVIOR TYPE INDEX (IC) OF 2.6, AS ALSO DESCRIBED IN YOUD ET AL. (2001).

fs

qt - v0

(ADAPTED FROM ROBERTSON, 2008; © 2008 NRC CANADA OR ITS LICENSORS; REPRODUCED WITH PERMISSION.)

FIGURE 7.14 USE OF CPT DATA TO SCREEN CLAY-LIKE MATERIAL
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In applying these criteria, CPT data are considered the most reliable, laboratory strength data the next most 

reliable, and SPT data the least reliable.

7.4.4.2.3 Screening Results

If no zones within the embankment or foundation are susceptible to strength loss, then the seismic stability of the embankment is acceptable (assuming, of course, that the static stability has been 

analyzed and is acceptable, including cases where undrained strengths are considered). No further 

analyses of seismic stability are needed. The next step is to evaluate seismic deformations.

If one or more zones within the embankment or foundation are susceptible to strength loss, then further analyses should be performed as discussed in Step 3, which follows.

7.4.4.3 Step 3 - Define Post-Earthquake Strengths for Limit Equilibrium Stability Analyses

Stability analyses are typically performed using 2D limit-equilibrium, slope-stability software for 

one or more potentially critical cross sections of the embankment. The stability analyses should be 

static analyses using post-earthquake strengths. Analyses should be performed for both potential 

upstream and potential downstream failures.

Upstream failures may or may not pose a risk of uncontrolled release, depending on whether the 

location of the critical surface leaves adequate freeboard and crest width in place. Guidelines for 

deciding whether potential upstream failures pose a safety hazard are discussed in Sections 6.6.4.1

and 6.6.4.3.

Values of post-earthquake strength for various zones of the embankment should be selected as discussed in the following:

• Dense sand-like materials (N1,60 > 15 and qt1 > 75 tsf) such as compacted coarse refuse 

and dense sand-like natural soils tend to be dilative when they are sheared. That is, the 

undrained strength tends to be higher than the drained strength. Also, these materials do not experience strength loss due to earthquake shaking. For post-earthquake 

stability analysis, one cannot be certain whether the material will act as if it is drained 

or undrained, and the negative pore pressures required to mobilize a higher strength 

may not develop because they cause cavitation. Therefore, it is reasonable and conservative to use the drained strength for these materials, as discussed in Chapter 6.

• Stiff clay-like materials (SPT N > 6 and CPT data in Zone B) tend to have high 

shear strain up to the peak undrained strength and limited drop-off in shearing 

resistance after the peak, so they should not experience significant strength loss 

due to earthquake shaking. Unlike dense sand-like materials, stiff clay-like materials should act as undrained during the most critical earthquake and post-earthquake period. For these materials, 80 to 100 percent of the peak undrained strength 

should be used.

Commentary: Available data indicate that the cyclic stresses and strains caused by earthquakes are unlikely to cause significant strength reduction in stiff clays. However, some practitioners have commonly used 80 percent of the peak undrained strength for clays that are subject 

to significant seismic loading, presumably to account for possible strength degradation and 

other factors. Therefore, although it is perhaps overly conservative, the 80-percent value is recommended in areas of moderate to high seismic hazard potential. Use of 100 percent of the peak 

undrained strength is reasonable in areas of low seismic hazard potential and when applying 

a factor of safety of 1.5 in initial post-earthquake stability evaluations to classify the structure 

and foundation (Figure 7.1a, Boxes 5 and 6).
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• For loose saturated sand-like material, as in most fine coal refuse impoundments, 

strength should be selected based on the results of triggering analyses, and Sus estimates should be made in accordance with the previous sections of this chapter. If 

triggering of strength loss is assumed or computed to occur, use Sus. If triggering is 

assumed or computed to not occur, use the peak undrained strength, but no higher 

than the drained strength. The Sup values for sand-like materials can be obtained 

from published data such as Castro (2003) and Olson and Stark (2003). Alternatively, 

the Sup values can be obtained from the testing on remolded samples performed as 

part of the testing program described in Section 7.4.3.2.3. Tests on remolded samples 

with values of steady-state strength and confining pressure similar to in-situ samples 

can be considered representative of in-situ conditions.

Drained strengths for these materials can be used if they are clearly determined to be 

unsaturated (saturation ratio less than 80 percent), and it can be demonstrated that 

they will remain unsaturated. (When the degree of saturation is higher than 80 percent, strength loss is still possible for loose, sand-like material.) Materials above the 

phreatic surface should not necessarily be assumed to be unsaturated. Materials above 

the phreatic surface may be saturated or close to saturated, particularly if the water 

level was previously higher than the current phreatic surface.

• For soft or sensitive clay-like material (SPT N < 6 or CPT data in Zone C), strength 

should be selected based on the results of strength-loss estimates made in accordance 

with Sections 7.4.2.3.1, 7.4.3.2.2, and 7.4.3.3. If triggering (significant strain during the 

earthquake) is assumed or computed to occur, use a value lower than Sup, but probably not as low as Sus. (If the zones of this material are relatively small and probably 

not significant to the seismic stability, then Sus can be used as the post-earthquake 

strength even though it is probably overly conservative. For highly sensitive clays, 

Sus may be the appropriate post-earthquake strength.) If triggering (significant strain 

during the earthquake) is computed to not occur, use 80 percent of Sup. Sup for claylike material can be obtained from CPT data, field vane shear testing, or laboratory 

testing. Sup for borderline material can be obtained from CPT data (if the push is 

undrained based on pore-pressure response) or from laboratory testing or possibly 

from field vane-shear tests performed at high strain rates.

Coal refuse impoundments tend to be stratified. Variations in post-earthquake undrained strengths 

may be more significant than variations in drained strengths. Therefore, post-earthquake strengths 

should be selected conservatively. For zones of mixed coarse and fine refuse (such as the lower portion of an upstream stage), use strength properties weighted toward the properties of the overlying 

coarse refuse zone or the underlying fine refuse zone, based on the prevalence and extent of each 

zone, the index properties of each zone, and a realistic estimation of the upstream embankment 

cross section (based on subsurface conditions and past experience with displacement of fines during 

upstream construction pushouts).

Commentary: Strain compatibility is not an issue for these stability analyses, because the strengths used all 

represent reasonably conservative estimates of the undrained strength that the soil might have at high strains 

(5 percent to 10 percent). As discussed previously, the recommended strengths are:

• Loose sand-like material – Undrained steady-state (residual) strength. This is the lowest 

resistance that the material can have at any strain beyond a few percent.

• Dense sand-like material – Drained strength. The undrained stress strain curve of a dense, 

dilative, sand-like material increases steadily with strain with no drop-off at high strain. For 

these materials, the drained strength is a conservative estimate of the undrained resistance at 

any strain beyond a few percent.
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• Soft clay-like material (SPT N < 6 or CPT data in Zone C) – Either: (1) the undrained steady 

state (residual) strength as a very conservative estimate of the available strength, (2) the 

post-earthquake peak strength estimated from cyclic tests followed by monotonic (static) tests, 

or (3) 80 percent of the peak-undrained strength. But option 2 or 3 can only be used if the 

clay-like material has a broad peak to the stress-strain curve, with no rapid drop-off in resistance after peak, which is the case for most clay-like material. So, the strength selected for use 

should be appropriate for a wide range of strain.

• Stiff clay-like material (SPT N > 6 and CPT data in Zone B) – 80 to 100 percent of undrained 

peak strength. The undrained stress-strain curve of stiff clay-like material reaches nearly its 

peak strength at a few percent strain and then levels off or increases slowly with strain with 

no drop-off at high strain. So, again, the strength used is appropriate for a wide range of 

strain. As discussed above, the 80-percent value is recommended in areas of moderate to high 

seismic-hazard potential. Use of 100 percent of the peak-undrained strength is reasonable in 

areas of low seismic-hazard potential, and when applying a factor of safety of 1.5 in initial 

post-earthquake stability evaluations to classify the structure and foundation (Figure 7.1a, 

Boxes 5 and 6).

7.4.4.4 Step 4 - Perform Initial Stability Analysis

First perform an initial, conservative, post-earthquake stability analysis. This initial stability analysis 

is performed with the assumption that the earthquake shaking is strong enough that soils potentially 

susceptible to strength loss do in fact experience strength loss.

Use Sus for sand-like material in zones of the embankment that, based on the screening analysis in 

Section 7.4.4.2, have low N-values and CPT values and may therefore be susceptible to strength loss. 

For this analysis, Sus for sand-like materials can be conservatively estimated from correlations with 

SPT and/or CPT data (Section 7.4.3.1). Also, the post-earthquake strength of clay-like material considered not susceptible to significant strength loss (Zone B on the CPT chart or N > 6) can be estimated 

as 80 to 100 percent of Sup, as discussed in Section 7.4.4.3. The post-earthquake strength of clays 

considered potentially susceptible to significant strength loss should be taken as an estimated value 

of Sus. Sup and Sus for clays can be estimated from CPT data, field vane shear data, or laboratory data 

(Chapter 6 and Section 7.4.3.3).

The slope stability analysis is a total-stress analysis using undrained strength, which is a function 

of the pre-earthquake consolidation stress. The location of the phreatic surface affects the pre-earthquake consolidation stresses and thus the undrained strength. The estimated location of the phreatic 

surface at the time of the earthquake should be used in the stability analysis. Elevated pore pressures 

caused by earthquake shaking need not be considered.

If these initial stability analyses (for various potential upstream and downstream failure surfaces) 

indicate a minimum safety factor of at least 1.2, then seismic stability is considered acceptable, and 

the next step is to perform a deformation analysis.

If the minimum safety factor is below 1.2, then the embankment geometry should be modified to 

make the factor of safety acceptable, or else more sophisticated (and less conservative) evaluations 

of post-earthquake shear strength and triggering, as subsequently discussed, may be appropriate. 

If the safety factors are very low, then it may be clear at this initial stage that some modification to 

the embankment design is needed. It is often useful to vary the assumed values of Sus in the zones 

potentially susceptible to strength loss to find the values of Sus that would be needed to obtain a 

safety factor against instability of 1.2, and then judge if the materials in question could possess such 

strengths before undertaking more detailed evaluations of Sus.
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Commentary: A safety factor of 1.2 is recommended for this analysis because, even though the shear strengths 

selected for this analysis are fairly conservative, there may be significant uncertainties in the geometry of the 

embankment and the various zones within the embankment and foundation.

7.4.4.5 Optional Step 5 - Perform More Detailed Evaluation of Sus for Sand-Like Material

Obtain representative high quality undisturbed samples of sand-like material in zones that may 

experience strength loss (Section 7.4.4.2). Perform a laboratory steady-state shear strength testing 

program involving monotonic undrained tests on both undisturbed and reconstituted specimens 

(Section 7.4.3.2.3) to obtain better estimates of Sus.

Re-run the limit equilibrium stability analyses using the better estimates of Sus. If the minimum safety 

factor is at least 1.2, then the seismic stability of the embankment is acceptable, and seismic deformations should be evaluated next.

Commentary: The preceding discussion refers to laboratory testing to measure Sus for sand-like material. 

Performing this testing before undertaking triggering analyses should be considered, because the measured 

values of Sus might result in satisfactory seismic stability, and thus triggering would not be an issue. Also, the 

Sus testing will give information on stress-strain behavior, including strain at peak-undrained strength, which 

may be used in the triggering analysis. However, this step involves complex sampling and testing, and it can 

be deferred until after a triggering analysis is performed. This step can also be skipped entirely, especially if the 

post-earthquake strength of sand-like materials was not a significant factor in the initial stability analyses.

7.4.4.6 Optional Step 6 - Perform Triggering Analysis

If the minimum safety factor is still less than 1.2 after a more detailed evaluation of Sus, then for loose 

sand-like material, consider performing a triggering analysis, as discussed in Section 7.4.2, to evaluate whether the earthquake is large enough to trigger strength loss in the critical zones.

For sand-like materials, triggering analyses should only be performed if the design earthquake is 

relatively small. If the design earthquake produces a peak ground acceleration at the embankment 

site of more than 0.2g and causes a cyclic stress ratio (CSR) within the zone of interest of more than 

0.15, then triggering of strength loss in sand-like material potentially susceptible to strength loss 

should simply be assumed.

If triggering analyses are performed for sand-like materials, the pore-pressure-based method, as discussed in Section 7.4.2.1 (Youd et al., 2001) should be performed first. If the safety factor against triggering using the pore-pressure-based method (CRR/CSR) is higher than 1.4, then triggering can be 

assumed to not occur. If the safety factor is less than 1.0, then triggering of strength loss should be 

assumed. For safety factors of between 1.0 and 1.4 calculated using the pore-pressure-based method, 

triggering of strength loss is possible. Either assume that triggering of strength loss will occur, or perform more rigorous triggering analyses (strain-based or stress-based methods as discussed in Section 

7.4.2.3) to make a final evaluation of whether or not triggering occurs.

For clay-like material, a strain-based triggering analysis (possibly involving laboratory testing with 

cyclic loading followed by monotonic loading as discussed in Section 7.4.3.2.2) can be performed to 

evaluate whether the earthquake shaking is strong enough to trigger a decrease in Sup and to determine what the decrease will be.

If strength loss is not triggered in one or more of the zones that had previously been assumed to experience strength loss, then re-run the stability analysis using the appropriate, higher strength in those 

zones as discussed in Step 3, Section 7.4.4.3. If the safety factor is at least 1.2, then the seismic stability 

is acceptable, and seismic deformations should be evaluated next.
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If the safety factors are still less than 1.2, then the embankment is not seismically stable. Additional 

investigations should be performed to better characterize the geometry and materials in the embankment or the embankment must be redesigned or modified.

7.4.5 Comments on Methods for Evaluating Triggering and Strength

All of the methods described in this chapter require special expertise, and should only be applied by 

geotechnical engineers with experience in seismic stability and deformation analyses.

Laboratory steady-state (residual) strength testing programs provide site-specific values of Sus that 

are often less conservative than the values obtained from empirical correlations with SPT and CPT 

data, but these testing programs require sophisticated undisturbed sampling and laboratory testing.

Strain-based triggering analyses require sophisticated laboratory testing and sophisticated analyses 

of shear strains induced by the design earthquake for evaluation of whether the earthquake shaking is 

strong enough to cause strength loss down to Sus. Thus, steady-state laboratory programs and strainbased triggering analyses are normally performed only for large embankments where the additional 

testing and analysis costs are relatively small compared to the cost of redesigning or modifying the 

embankment.

If contractive zones of material are confined by overlying layers of material with low hydraulic conductivity, then drainage of excess pore water generated by earthquake shaking can be impeded. 

There is some indication from centrifuge model tests that this may result in migration of pore water 

toward the overlying low-hydraulic-conductivity layer, with consequent redistribution of void ratio 

and loosening of the material near the interface with the low-hydraulic-conductivity layer. This in 

turn reduces the post-earthquake strength of the material near the interface. This mechanism was 

suggested by Whitman in the 1980s. Recent work on the subject includes Malvick et al. (2006) and 

Naesgaard et al. (2005). There are currently (2008) no generally accepted methods for analyzing this 

potential mechanism, and therefore it need not be directly considered in stability analyses at this time. 

A method for including the potential effects of void ratio redistribution on post-earthquake strength 

has been proposed by Idriss and Boulanger (2007). As discussed in Section 7.4.3.1, this method can be 

used for sand-like material with N1,60 > 15 if one has a concern for the possibility of void-ratio redistribution (contractive zones overlain by lower-hydraulic-conductivity materials that would impede 

dissipation of excess pore-water pressure). In such a case drainage provisions (such as wick drains, 

as discussed in Section 6.6.3.3) could be incorporated. The need for such drainage provisions for dissipation of pore pressure in fine coal refuse at a slurry impoundment could be assessed based on 

pore-pressure monitoring during upstream construction.

Commentary: Pore-water migration and void-ratio redistribution (which may or may not actually occur 

outside laboratory centrifuge tests) are not considered in either laboratory testing or SPT/CPT methods for 

estimating post-earthquake strength. The laboratory testing relates to pre-earthquake void ratios. If there is 

pore-water migration, the relationship of void ratio to strength obtained from the laboratory testing would 

allow one to quantify the effects of a given amount of void ratio change (assuming that one day we will have 

methods to estimate to what extent, if any, pore-water migration takes place). SPT and CPT data also reflect 

pre-earthquake conditions. One cannot expect that SPT or CPT data will be able to predict whether or not, 

and to what extent, pore-water migration might take place, and what impact the pore-water migration might 

have on post-earthquake strength.

At the present time (2008), it is debatable whether pore-water migration did or did not occur for the 

case histories used to develop the relationships between SPT/CPT data and post-earthquake strength. For 

example, extensive investigations of the Lower San Fernando Dam slide indicate that the characteristics of 

the slide can be explained without assuming pore-water migration (Castro, Seed, Keller, and Seed, 1992).
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7.5 SEISMIC DEFORMATION ANALYSES

7.5.1 General Discussion

The procedures described in Sections 7.5.2 through 7.5.6 are appropriate for sites where seismic stability has been shown to be adequate based on analyses discussed in the previous sections of this chapter.

Even if the seismic stability is adequate, a dam or embankment will deform during earthquake shaking because of the development of accumulated strains along a potential sliding surface due to the 

superposition of seismic and static (driving) shear stresses. Two mechanisms for developing accumulated strains should be considered:

• Yielding mechanism – When seismic accelerations in an embankment are high 

enough, total shear stresses (static plus cyclic) along a potential failure surface 

may tend to exceed the available shear strength. The soil along the potential failure 

surface yields during short earthquake time increments when the total shear stress 

tends to exceed the available shear strength. Another way of saying this is that the 

soil along the potential failure surface yields during the short time increments of the 

earthquake when accelerations are higher than the acceleration that can be transmitted by the soil. This mechanism can occur in both sand-like and clay-like material. 

The resulting deformations can be evaluated using Newmark-type analyses.

• Ratcheting mechanism – As shown in Figure 7.15, cyclic stress-strain curves are 

softer in loading than in unloading. When there is a static shear stress, repeated 

cyclic loading will result in accumulated strains (deformations) in the direction of the 

static shear stress, even if the available shear strength is not exceeded. These ratcheting accumulated strains (deformations) are generally significant only if cyclic mobil

0.6

0.4

0.2

0.0

-0.2

SHEAR STRAIN (%)

SHEAR STRESS NORMALIZED 

T

O

 CONSOLIDATION PRESSURE

(SEED ET AL., 2003)

-2 0 2 4 6 8 10 12

FIGURE 7.15 CYCLIC STRESS-STRAIN CURVE DEMONSTRATING RACHETING MECHANISM
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ity occurs. If the cyclic loading involves stress reversal (i.e., the cyclic shear stress 

exceeds the static consolidation shear stress), and if enough cycles of loading occur 

that pore pressures approach 100 percent, then the cyclic stress-strain curve will 

develop an S-shape, and the cyclic strains will become significant. This type of cyclic 

stress-strain behavior is referred to as cyclic mobility. If there is no static shear stress, 

then cyclic mobility will not cause significant accumulated strain. Similarly, if there is 

a static shear stress, but cyclic mobility does not occur, then accumulated strains will 

generally be insignificant. To evaluate whether a material will develop cyclic mobility due to earthquake shaking, the Youd et al. (2001) procedure should be used to 

evaluate whether excess pore pressures will approach 100 percent. For the purpose 

of deformation analyses, a safety factor (CRR/CSR) of 1.0 or less should be considered to result in cyclic mobility.

For both the yielding and ratcheting mechanisms, the deformations are driven by the earthquake 

shaking. Therefore, the magnitude of the deformations is related to the intensity of the earthquake. This is in contrast to seismic instability, where the instability failure may be triggered by 

the earthquake shaking, but the instability failure is driven by the static (gravity) weight of the 

embankment.

Deformations may involve the embankment crest and both the upstream and downstream embankment slopes. Deformations along the upstream slope may or may not pose a risk of uncontrolled 

release, depending on whether the location and magnitude of the potential movement leaves adequate freeboard and crest width in place. Guidelines for deciding whether potential upstream slope 

movements pose a safety hazard are discussed in Sections 6.6.4.1 and 6.6.4.3. If these guidelines are 

met, detailed deformation analyses may not be required (i.e., if deformation screening or simplified analysis indicates acceptable performance for potential sliding masses that include the crest or 

provide overall dam freeboard and integrity, then detailed analyses may not be needed for other 

potential sliding masses that do not affect the integrity of the dam). While this point may also apply 

to potential movements on the downstream slope, the deformation, strain and stability of the remaining embankment should be evaluated considering both undrained and drained strength conditions 

and the potential for progressive failure.

Commentary: If seismic stability and deformation screening or simplified analysis demonstrate that a substantive portion of the embankment and crest would be preserved and would be sufficient to preclude a release 

from the impoundment (although other portions of the embankment might be compromised), then detailed 

deformation analysis to assess the significance of other zones that might be subject to cyclic mobility may not be 

warranted. This judgment should be based on the site-specific cross-section and material properties with careful consideration of: (1) the presence, extent and depth of zones of potential cyclic mobility and (2) mitigating 

factors that would limit large deformations or otherwise provide support for the adjoining embankment (e.g., 

the configuration of the upstream slope and impoundment might restrict the amount of displacement that could 

be expected, thus providing some remaining confinement and support to preserve a substantial portion of the 

foundation and embankment).

Limit-equilibrium slope stability analyses, employing post-earthquake strengths, might be useful in identifying the foundation-embankment zone most prone to significant deformation and in judging what portion of the 

foundation and embankment would remain to preclude a release from the impoundment. For broader dams and 

broader upstream embankment stages, limit-equilibrium analyses with post-earthquake strengths may be adequate to justify that sufficient embankment breadth, crest width and freeboard would remain such that detailed 

deformation analyses are not warranted.However, for narrower dams and narrower upstream embankment 

stages founded on significant zones subject to potential cyclic mobility, the initiation and progression of excessive deformation and breaching of the crest may be a concern, and detailed deformation analyses are likely to be 

unavoidable. The distinction between “broader” and “narrower” dams and embankments is not definitive, but 
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professional judgments can be supported by the margin of safety suggested by the static and seismic (post-earthquake) stability analyses, considering potential failure surfaces within portions of the embankment-foundation 

that are anticipated to be preserved (e.g., factors of safety much greater than the minimum recommended values 

of 1.5 and 1.2, respectively) and supported by a limited extent of potential failure reflected by surfaces with low 

factor of safety (e.g., near the recommended minimum). Generally, to justify such judgment, potential failure 

surfaces with a low factor of safety should be limited in extent relative to the entire embankment breadth.

7.5.2 Preliminary Screening

If the site is in a low seismic-hazard area, which is defined in Section 7.7.3.7 as an area where the 

National Seismic Hazard Map of the United States (Frankel et al., 2002) indicates that the PGA with 

a return period in 2,500 years is less than or equal to 0.10g, and if the seismically-induced, pore-pressure buildup does not approach 100 percent (which could lead to cyclic mobility), then deformations 

will be small enough that no further evaluation of deformations is required. As noted in Section 7.5.1, 

all deformation evaluations, including this preliminary screening, are applicable only if the embankment has first been determined to be seismically stable. Therefore, for preliminary screening, the following steps can be followed, as indicated in the flow chart in Figure 7.1c:

1. Is this a structure-foundation system for which no deformation analysis at all is 

required (e.g., structures of the type indicated in Figure 7.1a, Box 1)? If the answer is 

“Yes,” then no seismic deformation analysis is required. If the answer is “No,” then 

continue to Step 2.

2. If the site is in an area of low seismic hazard potential (as defined in Section 7.7.3.7), 

then continue to Step 3. If the site is in an area of higher seismic hazard area, then skip 

to Step 5.

3. In sand-like materials, evaluate the potential for pore-pressure increase using the 

methods discussed in Section 7.4.2.2 (Youd et al., 2001). If the safety factor against 

100 percent pore-pressure increase in any zone (CRR/CSR) is greater than 1.0, then 

cyclic mobility will not develop in that zone. For clay-like materials, confirm the 

safety factor from the static stability analysis using post-earthquake strengths. If the 

safety factor is greater than 1.2, then significant deformations in zones of clay-like 

material are unlikely.

4. If the analyses in Step 3 do not indicate cyclic mobility in sand-like material and the 

analyses in Step 3 indicate high safety factors for static stability in clay-like material, 

and if the site is in a low-seismic-hazard-potential area, then deformations will be 

small enough that no further evaluation of deformations is required.

Commentary: These criteria are intended to be conservative and represent recommended guidance based on experience with deformation analyses.

5. If the criteria in Step 3 are not met, or if the site is not in an area of low seismic hazard potential, then permanent deformations should be evaluated, as described in 

Sections 7.5.3 through 7.5.6, which follow.

7.5.3 Pseudo-Static Procedure for Screening

A relatively simple pseudo-static procedure (Hynes-Griffin and Franklin, 1984) can be used for 

screening of seismically-induced deformations. This procedure is applicable only if all of the following conditions are met:

• Design earthquake of magnitude less than 8 (M < 8).

• No significant zones susceptible to strength loss, as determined during the seismic 
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stability analyses (Sections 7.4.2 or 7.4.4.2).

• Small displacements (less than 3 feet) are not significant to the performance of the 

dam or embankment.

For this procedure, limit-equilibrium slope-stability analyses are performed with a pseudo-static 

seismic coefficient of one-half the PGA determined at the base of the embankment. For clay-like 

material, 80 percent of the peak undrained shear strength should be used. For sand-like material, 80 

percent of the peak undrained shear strength, but no higher than 80 percent of the drained strength 

should be used. If the factor of safety is greater than 1.0 (based upon a thorough scope of geotechnical 

investigation or a high confidence level in material characterizations) or greater than 1.2 (based upon 

a moderate geotechnical investigation or less certain material characterization), then the deformations can be assumed to be less than 3 feet.

Commentary: Hynes-Griffin and Franklin (1984) suggested that a safety factor of 1.0 is adequate for assuming deformations would be less than 3 feet. The 1.2 safety factor has been added for cases where there is limited 

confidence in the site characterization. The method was developed based solely on earthquake records from the 

1971 San Fernando earthquake. However, it is considered acceptable to apply the method to eastern as well as 

western sites. This screening analysis applies to any height of embankment. The height is taken into account 

indirectly by the pseudo-static limit-equilibrium analysis.

7.5.4 Newmark-Type Analysis (No Cyclic Mobility)

This method of analysis addresses the yielding mechanism of strain accumulation discussed in Section 7.5.1. In these Newmark-type analyses (Newmark, 1965), deformations are analyzed by considering movements of a sliding mass over a sliding surface caused by yielding of the soil at its available 

strength. The basic assumption is that movement (or yielding) occurs when the sum of the static plus 

the seismic shear stresses along the sliding surface reaches the value of the available shear strength. 

Thus, the acceleration that can be transmitted to the sliding mass is limited to a value referred to as 

the yield acceleration.

A Newmark-type analysis is not appropriate for very large deformations because it is based on the 

assumption that the embankment geometry is the same before and after the earthquake.

The basic steps are as follows:

1. For a given sliding mass, perform a pseudo-static stability analysis to determine 

the horizontal acceleration that produces a safety factor of one. This is the yield acceleration ky, and it has the units of gravity. Critical sliding masses may be selected 

by searching for the most critical mass (critical sliding surface) while varying the 

pseudo-static horizontal acceleration until a safety factor of one is achieved. The estimated location of the phreatic surface at the time of the earthquake should be used. 

Elevated pore pressures caused by earthquake shaking need not be considered. The 

material strengths for the pseudo-static stability analyses should be the available undrained strengths determined as indicated in Sections 7.4.1 through 7.4.3 and 7.4.4.3. 

In general, the following strengths should be used:

• Loose sand-like material – Undrained peak strength Sup or undrained steadystate (residual) strength Sus depending on results of triggering analysis

• Dense sand-like material – Peak undrained strength Sup but no higher 

than peak drained strength Sdp

• Soft to medium clay-like material – 80 percent of Sup

• Stiff clay-like material – 80 percent of Sup or 80 percent of the post-earthquake strength determined as per Section 7.4.3
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Commentary: The use of 80 percent of Sup instead of 100 percent of Sup for stiff 

clay-like materials in Newmark deformation analyses is based on a recommendation in Makdisi and Seed (1978). The recommendation accounts for the fact that 

some deformation accumulates when the peak stresses exceed about 80 percent of 

Sup. The Newmark analysis assumes that there is no deformation until the selected 

strength is reached. The 80-percent value results in some computed deformation 

when stresses exceed 80 percent of peak.

2. Define a time history of average acceleration of the sliding mass k(t) during the design earthquake. Values of k(t) are in units of gravity. The maximum value of k(t) is 

referred to as k(max). One- or two-dimensional computer programs such as SHAKE, 

FLUSH, or QUAD4 can be used to compute k(t) by modeling the propagation of the 

earthquake motion from bedrock to the zone of interest. The k(t) time history is not 

the same as the acceleration time history output by SHAKE (or other programs) at the 

elevation of the base of the sliding mass. Rather, to obtain k(t), one should take the 

time history of seismic shear stress output by the computer program at the base of the 

sliding mass and divide it by the total overburden pressure at that elevation (acceleration equals force/mass or stress/pressure). In this way, k(t) will represent the average 

acceleration of the sliding mass and not the acceleration of individual layers or elements at the base of the sliding mass. Note that the total overburden pressure must 

not include the overburden stress caused by any free water above the ground level.

Commentary: In SHAKE, or other programs, potential yielding of the soil is ignored, so 

the computed accelerations, k(t) can be higher than the yield acceleration. The analysis is 

said to be decoupled because it separates the computations of accelerations and the computations of deformations due to yielding, when in fact they occur simultaneously. However, 

comparisons with more rigorous coupled methods indicate that generally the uncoupled 

method is satisfactory. Wartman et al. (2004) conclude that uncoupled analyses are satisfactory for cases where the predominant frequency of the motion is at least 1.3 times the 

natural frequency of the soil mass, which is generally the case for earthquake shaking of

dams and embankments.

3. During time intervals that k(t) exceeds ky, displacements are initiated. Displacements 

continue even after k(t) becomes less than ky, until the velocity of the mass becomes 

zero. Compute the total displacement of the sliding mass by double-integrating 

the portion of the k(t) curve above and below ky for time intervals where velocity is 

greater than zero (Figure 7.16).

4. If performing one-dimensional analyses at different points along the potential failure 

surface to compute k(t), then the resulting displacements must be combined in order 

to estimate the displacement of the overall potential failure wedge. A method for 

combining the computed displacements is presented in Appendix 7D.

The deformations computed using a Newmark-type analyses are appropriate only if there is no significant softening (loss of stiffness) of the soil caused by cyclic mobility, as described in the second 

bullet point (ratcheting mechanism) in Section 7.5.1. This is because the Newmark-type analyses are 

based on the assumption that no displacements are initiated during time intervals when k(t) is less 

than ky, but significant displacements could occur during those time intervals if cyclic mobility and 

resulting accumulated strains are occurring.

Commentary: Newmark-type analyses should generally not be used if cyclic mobility is expected (i.e., if 

CRR/CSR ≤ 1.0). Newmark-type analyses are based upon the assumption that there is no deformation when 

the sum of static plus seismic shear stresses is less than the available shear strength. But if cyclic mobility 

occurs, then significant deformations may occur due to the ratcheting mechanism described in Section 7.5.1
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even when the sum of the shear stresses is less than the available strength. Therefore, if cyclic mobility occurs, 

the Newmark-type analyses will under-predict deformations. In areas of low seismic hazard potential, the 

earthquake shaking may not be high enough to cause cyclic mobility. For small earthquakes, CRR/CSR may 

be greater than 1, such that Newmark-type analyses may be valid even for cases of upstream construction 

with loose sand-like material.

A simplified procedure for evaluating embankment deformations using the Newmark method 

was developed by Makdisi and Seed (1978). The method provides estimates of embankment 

deformations if the following information is known or a representative range of values can be 

established:

• The natural period of the embankment

• The maximum crest acceleration due to earthquake shaking

• The earthquake magnitude

The method was developed for embankments in the range of 100 to 200 feet high with a natural 

period ranging between 0.26 seconds and 5.22 seconds, based on several western earthquake records 

with magnitudes of about 6.5 to 8.25. A simplified formula for estimating the natural period of the 

embankment (T = 2.6 H/Vs where H is the height of the embankment and Vs is the representative 

shear-wave velocity) is provided in Makdisi and Seed (1977). This simplified formula for the natural 

period is applicable to embankments with a cross section that is approximately triangular and therefore may not be applicable to non-trapezoidal coal refuse embankments. For embankment configurations that deviate from a triangular cross section, a range in natural period may be preferable to an 

approximate single value. Because of the limited range of conditions upon which it was based, the 

Makdisi and Seed method is not considered appropriate for evaluation of significant- or high-haz

FIGURE 7.16 DOUBLE INTEGRATION TO OBTAIN DISPLACEMENT DISPLACEMENT VELOCITY ACCELERATION

ta tb tc

A A A

B

B

B

t’a t’b t’c t”a t”b t”c

FIGURE 7.16 DOUBLE INTEGRATION TO OBTAIN DISPLACEMENT
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ard-potential coal refuse impoundments unless the embankment under consideration approximates 

a triangular configuration or more conservative configuration (with respect to seismically-induced 

deformation) and is founded on materials that are not susceptible to cyclic mobility.

7.5.5 Numerical Modeling with No Cyclic Mobility

If stress reversal and cyclic mobility are not likely (i.e., if CRR/CSR > 1.0, as discussed in Section 7.5.1), 

then Newmark analyses may be used. An alternative to Newmark analyses for this case is computer 

modeling based on finite-element or finite-difference analyses.

With finite-element modeling, the cross section of the structure is divided into a grid of discrete 

(finite) elements that connect to each other at node points. The elements are assigned material properties such as unit weight, Poisson’s ratio, and stress-strain properties. Loads (forces) are applied at 

the node points, and displacements at the node points are computed by solving the stiffness matrix 

for the assembly of finite elements. Strains and stresses within elements are computed based on the 

displacements at the node points and the material properties.

With finite-difference modeling, the cross section of the structure is divided into a grid of lumped 

masses that interact with each other in accordance with constitutive equations based on the material properties. The displacements resulting from forces applied to the lumped masses in the grid 

are computed by solving the equations of motion (force equals mass times acceleration) at sequential time steps.

One of the key issues is assigning appropriate material properties (especially stress-strain properties) to the elements. Various non-linear soil models have been developed that, when used properly, 

can reasonably model material behavior when cyclic mobility does not occur. The soil models can 

be complex, and using them without fully understanding them can lead to errors. The results of 

the analyses are often sensitive to relatively minor variations in the parameters used to define the 

stress-strain properties. Checking the sensitivity by varying the parameters is an important part of 

the analysis. It is also important to check the model output to be sure that the pattern of stresses and 

strains across the cross section of the structure appears reasonable.

7.5.6 Numerical Modeling Considering Cyclic Mobility

If stress reversal and cyclic mobility are likely, (i.e., if CRR/CSR < 1.0) then the evaluation of deformations becomes highly uncertain, and Newmark analyses (Section 7.5.4) are not appropriate. Finiteelement or finite-difference numerical models can be used, but the stress-strain models must have 

special provisions to account for cyclic mobility. These models are particularly complex because they 

model the S-shaped, stress-strain curve associated with cyclic mobility (very low stiffness at low 

to moderate strains followed by dilation and increasing stiffness at high strains). An example of 

such a stress-strain model is presented in Byrne et al. (2004). It is particularly important to check 

the output of these analyses for sensitivity to modeling parameters and for reasonableness. The U.S. 

Army Corps of Engineers demonstrated a procedure using commercially available finite-difference 

software (FLAC) and a modified version of the UBSAND model (Byrne et al., 2004) to conduct a 

deformation analysis for a dam in California, including comparison of the results with somewhat 

more simplified methods (Perlea et al., 2009).

Cyclic mobility is most likely to be a concern for cases of upstream construction in areas of medium 

to high seismic risk. Deformation analyses of complex embankment-foundation configurations and 

situations where significant zones of the structure are subject to cyclic mobility require experience 

with numerical analysis techniques (e.g., finite element and finite difference) and soil behavior constitutive models. An experienced user of the selected modeling software for deformation analyses 

should be involved in the modeling and/or validation of the results.
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7.5.7 Acceptable Deformations

Once estimates of deformation have been made, the question arises as to what values of deformation 

are considered acceptable and what values are considered excessive. For embankments that do not 

retain liquids, fine coal refuse, or similar materials, deformations are not normally a problem so long 

as the embankment is stable. For dams or embankments that do retain liquids, fine coal refuse, or 

similar materials, deformations should be small enough that:

• Freeboard is not significantly compromised.

• Cracking through the dam, which affects the integrity of the dam for retaining liquids or might allow a release of material, does not occur.

• Appurtenant structures that affect dam safety are not severely damaged.

• Affected structures can be repaired before they might deteriorate further and pose a 

greater hazard.

The following are generally considered acceptable:

• Downstream analyses – Computed displacements or deformations less than 25 percent of the available freeboard and less than 3 feet.

• Upstream analyses – Computed displacements or deformations less than 25 percent 

of the available freeboard and less than 6 feet.

7.6 EMBANKMENT MODIFICATIONS FOR IMPROVING STABILITY OR RESISTANCE TO 

DEFORMATIONS

Measures to improve the seismic stability and the resistance to deformations of new and existing 

facilities can be grouped into the following general categories:

• Changes to embankment geometry (e.g., flatter slopes, buttressing, wider coarse 

refuse zone)

• Drainage control measures (including both internal and surface drainage)

• Soil/refuse improvement or reinforcement

• Increased freeboard

These measures are discussed in Chapter 6.

7.7 SEISMIC HAZARD ASSESSMENT (SEISMICITY)

The purpose of a seismic hazard assessment is to estimate the appropriate design earthquake(s) for a 

site in terms of earthquake magnitude (M) and peak ground acceleration (PGA) and, when needed, 

to define representative time histories of ground motions.

This discussion of recommended procedures for deriving the seismic hazard is intended to be applicable in the U.S. where coal is mined. Figure 7.17 shows the various regions where anthracite, bituminous and lignite coals are mined. Although the greatest amount of coal production is from the 

western U.S., the type of coal processing in that region does not typically require the construction 

of tailings dams for coal refuse. Accordingly, the procedures defined to derive seismic hazard place 

emphasis on eastern U.S. conditions, which are different from the western U.S. in many respects, as 

discussed in Section 7.7.1.

Anthracite coal is mined in eastern Pennsylvania. Bituminous coal deposits in the eastern U.S. are 

found in the Appalachian, Illinois, Michigan, and the Western Interior Basins, with some bituminous 

coal also found within the Gulf Coast lignite deposits (Figure 7.17). These sources, primarily anthra-
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cite in eastern Pennsylvania and bituminous in the Appalachian and Illinois Basins, are where coal 

needs to be processed and where coal refuse impoundments are required. Within these large areas, 

the seismic hazard is not uniform and can range from being nearly insignificant to an important 

design consideration.

The U.S. Geological Survey (USGS) has prepared national seismic hazard maps based on probabilistic analyses that define the variation of seismic hazard throughout the conterminous U.S., Alaska, 

Hawaii, Puerto Rico and the U.S. Virgin Islands, among other regions. Figures 7.18 and 7.19 present 

USGS seismic hazard maps respectively for the PGA with a 10 percent and 2 percent probability of 

exceedance in 50 years (~ 500 year and ~ 2,500-year return periods, respectively) in the conterminous U.S. Surface or near-surface active faults are the main sources of seismic hazard in the western 

U.S., whereas the eastern U.S. coal fields are affected primarily by three less-well-defined seismic 

sources: the New Madrid Seismic Zone, the Eastern Tennessee Seismic Zone, and the Charleston, 

South Carolina Seismic Zone (Figure 7.20). The coal fields with the most significant seismic hazard 

LEGEND

ANTHRACITE/OTHER USES

ANTHRACITE/POTENTIALLY MINABLE

LIGNITE/OTHER USES

LIGNITE/POTENTIALLY MINABLE

LOW VOLATILITY BITUMINOUS/OTHER USES

LOW VOLATILITY BITUMINOUS/POTENTIALLY MINABLE

MEDIUM AND HIGH VOLATILITY BITUMINOUS/OTHER USES

MEDIUM AND HIGH VOLATILITY BITUMINOUS/POTENTIALLY MINABLE

SUB-BITUMINOUS/OTHER USES

SUB-BITUMINOUS/POTENTIALLY MINABLE

FIGURE 7.17 LOCATION OF U.S. COAL RESOURCES

NOTE: IMAGE WAS OBTAINED FROM THE

 USGS “NATIONAL ATLAS” WEB SITE

 AND WAS COMPILED BY THE USGS

 EASTERN ENERGY RESOURCES

 TEAM (EERT).

FIGURE 7.17 LOCATION OF U.S. COAL RESOURCES
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in the eastern U.S. are those in the southern half of the Illinois Basin. Portions of the coal fields in the 

southern Appalachians have moderate hazard. The Michigan and Western Interior Basins have very 

low hazard.

Seismic evaluation of coal refuse facilities and embankment dams requires the estimation of at least 

two ground motion parameters (earthquake magnitude and PGA). Another factor that needs to be 

considered in association with these two parameters is frequency content. Earthquake magnitude is 

critical because of the general relationship of magnitude and duration, as well as the strength of ground 

motion and related parameters such as frequency content, attenuation characteristics, etc. In essence, 

the larger the earthquake magnitude, the longer the earthquake, which implies that the soil will have 

to resist a greater number of ground-motion cycles of greater magnitude. Magnitude is not the only 

consideration in evaluating earthquake duration, as variations in fault mechanism, distance from the 

source, and local geologic conditions also contribute to duration, but acceptable practice is to consider 

LEGEND

20

15

10

9

8

7

6

5

4

3

2

1

0

FIGURE 7.18 SEISMIC HAZARD MAP OF THE U.S. WITH OVERLAY

 OF MAIN BITUMINOUS COAL FIELDS

NOTE: 1. IMAGE WAS OBTAINED FROM THE

 USGS “NATIONAL ATLAS” WEB SITE

 AND WAS COMPILED BY THE USGS

 GEOLOGIC HAZARDS TEAM.

2. ANTHRACITE AND BITUMINOUS

 COAL FIELDS ARE DENOTED BY

 RED BORDERS.

PEAK HORIZONTAL GROUND ACCELERATION

IN % g WITH A 10% PROBABILITY OF

EXCEEDANCE IN A 50-YEAR PERIOD

FIGURE 7.18 SEISMIC HAZARD MAP OF THE U.S. WITH OVERLAY OF MAIN 

BITUMINOUS COAL FIELDS
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duration as a function of magnitude (Youd et al., 2001). PGA is the most commonly used parameter to 

define the strength of ground motion, but its significance also depends on frequency content. The actual 

seismic hazard will depend on the combination of duration and ground motion strength. With respect 

to dams and embankments, a single cycle of high PGA at a high frequency (in the range of 0.25 to 0.45g 

at a predominant frequency of 25 Hz) will likely not be as significant as a larger number of cycles of 

lower acceleration (in the range of 0.10 to 0.15g at 1 Hz). The reason for this is that for a dam to be significantly excited by earthquake ground motion, some portion of the ground motion must be close to 

the predominant frequency of the dam. A simple method for estimating the predominant frequency of 

an embankment based on height and shear-wave velocity of the embankment materials is provided by 

Makdisi and Seed (1977). For a typical coal refuse dam ranging in height from 100 to 400 feet and with FIGURE 7.19 ILLUSTRATION OF MODERATE- TO HIGH-HAZARD REGION

 

LEGEND

300

200

160

120

80

60

50

40

30

20

18

18

16

14

12

10

8

6

4

2

0

PEAK HORIZONTAL GROUND ACCELERATION

IN % g WITH 2% PROBABILITY OF EXCEEDANCE

IN A 50-YEAR PERIOD (2475-YEAR RETURN PERIOD).

NOTE: 1. IMAGE REDRAWN FROM 2002 HAZARD

 MAP FOR A 2% PROBABILITY OF EXCEED-

 ANCE IN 50 YEARS FROM THE USGS

 "NATIONAL ATLAS" WEB SITE. 

 

ZONE WHERE COMPLETE DSHA OR

PSHA IS REQUIRED FOR EVALUATION

OF SEISMIC HAZARD. BY INFERENCE,

A SIMPLIFIED EVALUATION OF SEISMIC

HAZARD IS PERMISSIBLE IN ALL OTHER

AREAS.

2. ANTHRACITE AND BITUMINOUS

 COAL FIELDS ARE DENOTED BY

 RED BORDERS.

FIGURE 7.19 ILLUSTRATION OF MODERATE- TO HIGH-HAZARD REGION
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average shear wave velocities ranging from 500 to 1,600 ft/sec, the predominant frequency for the first 

mode of vibration could range from about 0.5 to 6 Hz. Accordingly, coal refuse dams are more likely 

to be affected by low-frequency ground motion, implying that more than one earthquake source may 

need to be considered in order to fully characterize the seismic hazard at a given site.

FEMA (2005b) defines the following design earthquakes:

• Maximum Credible Earthquake (MCE) – The MCE is the largest earthquake magnitude that could occur along a recognized fault or within a particular seismotectonic 

province or source area under the current tectonic framework. 

• Maximum Design Earthquake (MDE) or Safety Evaluation Earthquake (SEE) – This 

is the earthquake that produces the maximum level of ground motion for which 

a structure is to be designed or evaluated. The MDE or SEE can be set equal to 

the MCE or to a design earthquake less than the MCE, depending on the circumLEGEND

AGE OF QUATERNARY FAULTING

HISTORIC

HOLOCENE (< 15,000 YEARS AGO)

LATE QUATERNARY (< 130,000 YEARS AGO)

OBSERVED EVIDENCE OF PALEOLIQUEFACTION

MID-LATE QUATERNARY (< 750,000 YEARS AGO)

QUATERNARY (< 1.6 MILLION YEARS AGO)

POSSIBLY OLDER THAN QUATERNARY

NOTE: 1. IMAGE WAS OBTAINED FROM THE

 USGS “NATIONAL ATLAS” WEB SITE

 AND WAS COMPILED BY THE USGS

 GEOLOGIC HAZARDS TEAM.

2. BITUMINOUS COAL FIELDS ARE

 DENOTED BY RED BORDERS.

FIGURE 7.20 QUATERNARY FAULTS AND SIGNIFICANT SEISMIC SOURCES

 WITH OVERLAY OF MAIN BITUMINOUS COAL FIELDS

CHARLESTON SEISMIC ZONE -

TWO INTERPRETATIONS GIVEN

EQUAL PROBABILITY OF BEING

CORRECT IN THE 2002 UPDATE

FOR THE NATIONAL SEISMIC

HAZARD MAP.

PEMBROKE FAULTS/GILES

COUNTY SEISMIC ZONE -

THE LARGEST APPALACHIAN

EARTHQUAKE IS IN THIS

ZONE, BUT THE FAULTING

APPEARS TO BE PREHOLOCENE

WABASH VALLEY SEISMIC ZONE -

IDENTIFIED BASED ON PALEOLIQUEFACTION EVENTS INTERPRETED

TO NOT BE RELATED TO THE NEW 

MADRID SEISMIC ZONE THAT 

OCCURRED ~ 6,100 YEARS BP.

EASTERN TENNESSEE

SEISMIC ZONE - SOURCE

USED FOR THE 1996 NATIONAL

SEISMIC HAZARD MAP.

NEW MADRID SEISMIC ZONE -

THREE INTERPRETATIONS OF

A POSSIBLE FAULT LOCATION

TO ACCOUNT FOR HISTORICAL

SEISMICITY. THE MIDDLE FAULT

LOCATION IS CONSIDERED TO

BE TWICE AS LIKELY AS THE

OUTER TWO IN THE 2002 UPDATE

FOR THE NATIONAL SEISMIC

HAZARD MAP.

FIGURE 7.20 QUATERNARY FAULTS AND SIGNIFICANT SEISMIC SOURCES WITH 

OVERLAY OF MAIN BITUMINOUS COAL FIELDS
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stances. Factors to consider in establishing the size of MDE or SEE are the hazard 

potential classification of the dam (FEMA 2004a), criticality of the project function 

(water supply, recreation, flood control, protection of the environment, etc.), and 

the turnaround time to restore the facility to operability. In general, the associated 

performance requirement for the MDE or SEE is that the project performs without 

catastrophic failure (such as uncontrolled release of a reservoir) although significant 

damage or economic loss may be experienced. If the dam contains a critical water 

supply reservoir, the expected damage should be limited to an extent that allows the 

project to be restored to operation within an acceptable timeframe.

• Operating Basis Earthquake (OBE) – The OBE is an earthquake that produces ground 

motions at the site that can reasonably be expected to occur within the service life 

of the project. The associated performance requirement is that the project functions 

with little or no damage and without interruption of function. The purpose of the 

OBE is to protect against economic losses from damage or loss of service. Therefore, 

the return period for the OBE may be based on economic considerations.

Coal refuse impoundments and other mining dams often have significant or high hazard potential, 

and recommendations for selection of the MDE and seismic hazard assessment are presented in Table 

7.6. To protect site operational personnel, the public and the environment, it is necessary to design 

coal refuse impoundments having high hazard potential so that catastrophic failure does not occur. 

This is normally accomplished by considering the MCE when developing the design earthquake. 

The MCE might be represented by more than one earthquake event (i.e., from near-field and farfield sources) for which the structure should fail catastrophically. For coal refuse impoundments and 

dams having significant hazard potential, some federal dam safety agencies are giving consideration 

to events with return periods of the order of 2,500 years for design earthquakes. Further guidance 

on return-period criteria for significant- and high-hazard-potential dams is anticipated in the future 

from federal dam safety agencies.

Although the OBE does not typically govern the safe design of a coal refuse impoundment, it is still 

recommended that an OBE be defined on the basis of its probability of occurrence during the life 

of the dam. A 500-year (or more precisely 475-year) return period is consistent with the basis for 

defining an OBE in some conventional building codes and corresponds to one of the hazard maps 

published by the USGS (corresponding to a 10-percent probability of non-exceedance event for a 

50-year lifetime), as shown on Figure 7.18. The OBE earthquake parameters should be used to check 

the reasonableness of the design MCE. It is expected that the MCE will be a remote event with a PGA 

substantially higher than the OBE.

7.7.1 Analytical Procedures

Two approaches, probabilistic seismic hazard analysis (PSHA) and deterministic seismic hazard 

analysis (DSHA), are commonly applied to estimation of earthquake ground motions at a site. Fundamentally, only a DSHA can be used to estimate the Maximum Credible Earthquake (MCE). In 

PSHA, an MDE is associated with a return period far beyond the life span of the structure and is 

associated with a very low probability of occurrence.

7.7.1.1 Deterministic Seismic Hazard Analysis (DSHA)

The basic steps in a DSHA are as follows:

1. Develop a seismotectonic model. Review and summarize the basic geology and 

tectonics of a region surrounding the site (approximately 322-km radius) with particular attention to specific seismic sources, both area sources (provinces) and linear 
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sources (faults). Note that a 332-km (200-mile) potential radius of influence has been 

chosen in order to account for the susceptibility of fine tailings to liquefaction effects 

from large, distant earthquakes.

2. Conduct a review of the seismic history of the site’s region; locate the epicenters of all significant earthquakes and plot these earthquakes on a map that shows magnitude values.

3. Relate these epicenters to the mapped sources defined in Step 1.

4. Based on the results of Step 3, postulate a group of conceivable MCEs by selecting 

the most severe earthquake within or along each source that could be larger than the 

largest historical earthquake and “move” each earthquake to the point on the respective source nearest to the site.

5. Derive the ground motion at the site in terms of PGA on the basis of regionally applicable attenuation functions (i.e., relationships that allow the estimation of ground 

motion parameters at the site as a function of earthquake magnitude, source-to-site 

distance, and soil conditions at the site).

6. From the various PGAs obtained in Step 5 for the group of conceivable MCEs from 

Step 4 (applying the minimum distances from source-to-site), determine the most 

severe ground motion and classify this motion as the MCE. In the case of dams and 

embankments, the most severe earthquake event is not simply a function of the 

site-specific PGA, so it might not be apparent that a single event controls. A distant, 

TABLE 7.6 MDE AND SEISMIC HAZARD ASSESSMENT RECOMMENDATIONS

Dam Hazard 

Potential 

Classification

Maximum Design 

Earthquake

(MDE)

Seismic Hazard Assessment

Low-Seismic-Hazard-Potential 

Area(1)

Moderate- to High-SeismicHazard-Potential Area(1)

High Hazard 

Potential MCE(2)

Simplified Minimum EQ and 

Simplified Design Ground Motion(3)

or

Site-specific DSHA(2)

supported by PSHA(4)

Site-specific DSHA(2)

supported by PSHA(4)

Significant Hazard 

Potential

MDE ~ 2500 return 

period(5)

USGS EQ Hazard Maps

or

Site-specific PSHA

USGS EQ Hazard Maps

or

Site-specific PSHA

 Note: 1. Low-seismic-hazard-potential areas, as defined in this Manual, are distinguished using the USGS 

 Earthquake Hazard Maps for a return period of 2,500 years with a PGA ≤ 0.1g and M ≤ 5.5. All other

 areas are considered moderate- to high-seismic-hazard-potential areas.

2. The Maximum Credible Earthquake (MCE) derived on the basis of a DSHA, as described in Section 

7.7.1.1, is recommended as the MDE for high-hazard-potential dams based on FEMA (2005b). If a 

PSHA is used to establish the MDE for a high-hazard-potential facility, the designer should be able 

to demonstrate that the probability of occurrence is very remote (the actual return period may be a 

function of the seismic hazard area and tectonic conditions).

3. See Section 7.7.3.7 for Simplified Design Ground Motion and Simplified Minimum Earthquake (EQ) 

description.

4. The results of a Probabilistic Seismic Hazard Assessment (PSHA) can aid in demonstrating that 

the probability of catastrophic failure is very remote and in deciding whether mean, or mean-plusone standard deviation or greater estimates of ground motion, would be justified for the MDE from 

the deterministic ground motion analysis to achieve an acceptably low probability of exceedance 

(FEMA, 2005b).

5. For a significant-hazard-potential facility, the designer should be able to demonstrate that the probability of catastrophic failure is remote. The actual return period may be a function of the seismichazard area and tectonic conditions (e.g., consideration of a return period of the order of 2,500 

years may be reasonable for low-seismic-hazard areas).
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large-magnitude earthquake might not be associated with a PGA as high as for a 

smaller magnitude, nearer event, but such a distant earthquake could pose a greater 

threat because of its greater energy content and potential to induce more cycles of 

significant vibrations around the natural damped frequency of vibration of the structure-foundation system. Consequently, an examination of representative ground 

motions for more than one event might be necessary (Step 7).

7. Associate the MCE ground motion(s) to time history(ies) of ground motion consistent with the magnitude of the source and, with lesser emphasis, the calculated PGA

from that source.

The ground conditions for which the selected attenuation functions in Step 5 are applicable require 

consideration when determining where on or in the ground the derived ground motion should be 

applied in site-specific dynamic analyses of the structure-foundation system. For example, some 

attenuation functions are only applicable to site conditions characterized by firm rock and thin, 

very stiff soil. Hence, the derived ground motion at a site with less stiff or thicker soil overburden 

should be applied at the top of rock, rather than at the ground surface. Depending on the site conditions, this derivation of ground motion at the top of relatively soft ground could be accomplished 

using the SHAKE program (Schnabel et al., 1972) or other similar approach as further discussed in 

Section 7.4.2.2.1 and 7.5.

7.7.1.2 Probabilistic Seismic Hazard Analysis (PSHA)

A PSHA requires the same basic input as the DSHA in terms of the definition of seismotectonic models 

and attenuation functions to characterize site ground motion. However, the results from PSHA and 

DSHA are fundamentally different. PSHA addresses the chance of a given level of ground motion 

being exceeded from all possible earthquakes.

The methodology used for the PSHA is well established in the literature (Cornell, 1968; McGuire, 

1976; McGuire, 1978). PSHA is the methodology used by the USGS to derive the seismic hazard of the 

United States, and details of the procedures followed are described by Frankel et al. (1996). Calculation 

of the seismic hazard requires specification of the same inputs as required for the DSHA, except with 

the additional requirement that the uncertainties associated with each input need to be quantified:

• Source geometry – the geographic description of the seismic sources in the region of 

influence around the site. A seismic source is a portion of the earth associated with 

a tectonic fault or (if individual faults cannot be identified) with an area/province of 

homogeneous seismicity. Source geometry determines the probability distribution of 

distance R from the earthquake to the site fR(r).

• Seismicity – the rate of occurrence ν and the magnitude distribution fM(m) of earthquakes within each seismic source.

• Attenuation functions – the definition of attenuation functions is the same as with the 

DSHA, except that it is important to define the uncertainty of each possible function.

One consideration related to uncertainties with each input is how to weight the events within a 

seismic source and weight the influence of a source. The USGS report, “Documentation for the 2002 

Update of the National Seismic Hazard Maps,” prepared by Frankel et al. (2002), explains the data, 

evaluations, judgments and assumptions behind their PSHAs for different regions. 

Mathematically, the calculation of whether the annual probability λY > y the ground-motion characteristic parameter Y , expressed in terms of peak ground acceleration PGA or spectral acceleration 

Sa for various frequencies of vibration, exceeds level y involves the summation, over seismic sources 

that may pose a threat to the site, of the following relationship:
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 N rmax mu

λY > y = ∑ νi { ∫ ∫ P[Y>y|m,r] fM,R(m,r) dm dr }i (7-7)

 i=1 rmin ml

where:

N = number of the nearby seismic sources, including both line sources (i.e., active 

faults) and area sources

νi = annual mean rate of occurrence of earthquakes generated by source i with 

magnitudes between ml and mu

rmin = minimum source-to-site distance for source i

rmax = maximum source-to-site distance for source i

ml = lower-bound values of magnitude for source i

mu = upper-bound values of magnitude for source i

and:

• P[Y>y|m,r] is the conditional probability that the site may be struck by a ground 

motion with parameter Y > y generated by an earthquake of magnitude M = m with 

epicenter in source i at distance R = r from the site.

• fM,R(m,r) is the joint probability density function of magnitude M and distance R for 

source i

Hence the expression within brackets in Equation 7-7 is the probability that at the site an earthquake 

with parameter Y > y will occur because a seismic event of magnitude m (ml ≤ m ≤ mu) originated 

anywhere in source i.

Ground motion at the site is modelled by an attenuation function, which is a mathematical equation that defines the relationship between the ground-motion parameter Y, earthquake magnitude, 

source-to-site distance and local soil condition at the site. The attenuation function is used to compute 

the conditional probability in the integrand of Equation 7-7. Note that the USGS hazard maps are for 

a “firm-rock site condition, where the shear-wave velocity averaged over the top 30 meters (Vs30) is 

760 meters per second (boundary of NEHRP site classes B and C).” On a practical level, this means 

that the USGS has not tried to model site-specific conditions. If there are site-specific conditions that 

need to be accounted for, an extra step in the analysis is required.

If a scalar parameter such as peak ground acceleration (PGA) is chosen, the repetition of these calculations for different values of the threshold level y allows the construction of the seismic hazard curve 

for the site (a plot of annual probability of exceedance, or return period, versus the parameter level). 

If the parameter of the ground motion is expressed in terms of spectral acceleration, Sa(f,ξ), a family 

of seismic hazard curves for the site can be evaluated for various values of f and ξ (f is the frequency 

and ξ is the percentage of the critical damping). These results based on spectral ordinates can also be 

rearranged in order to obtain Uniform Hazard Spectra (UHS) (McGuire, 1974), which represent the 

response spectra whose ordinates are associated with the same probabilities of exceedance at the site. 

These methods that make use of spectral quantities include those that employ only a high-frequency 

parameter such as PGA.

7.7.1.3 DSHA versus PSHA

Regardless of whether seismic hazard is defined by a DSHA or PSHA methodology, the results still 

have some degree of uncertainty. The advantage of PSHA is that it can incorporate a range of uncer-
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tainties inherent in the seismotectonic model, occurrence frequency, and ground-motion attenuation 

relationships, that is not possible with a DSHA approach. However, PSHA has some significant limitations, especially in terms of selecting ground motion parameters that can be considered representative of a maximum credible event (MCE). A discussion of the limitations of the PSHA approach for 

defining the seismic input for critical facilities is presented by Krinitzsky (1995).

Selection of the design ground motion parameters from a PSHA is complex because there are infinite 

points (choices) on the hazard curve. It is not practical to pick a point from a hazard curve and be 

able to relate it to the ground motion representing a reasonable worst case. Furthermore, the ground 

motion derived from PSHA does not have a clear physical meaning (Wang et al., 2003), because 

the total hazard (total annual frequency of exceedance) at a site is the sum of the individual hazards (annual frequency of exceedance) and is not associated with any individual earthquake, but 

potentially many earthquakes. For example, on the 2002 USGS national seismic hazard maps, the 

total seismic hazard in Chicago, Illinois was derived from a series of earthquakes with magnitudes 

ranging from 5.0 to 8.0 at distances ranging from 0 to 500 km (Harmsen et al., 1999). Therefore, it can 

be difficult to identify from the results of a PSHA a specific earthquake that represents a worst case 

scenario.

Although the earthquake magnitude and PGA are not physically related, the USGS Earthquake Hazards Program now provides probability mapping for earthquake magnitude (Moment Magnitude M

within 50 km of a site as discussed in Section 7.7.2.5) as well as PGA, which allows for definition of a 

local earthquake event by M and PGA for a selected return period or probability. (The USGS provides 

maps of PGA for return periods up to a 2,475-year event, although the PGA for a 4,975-year return 

period can be computed on the basis of available site-specific deaggregations.) The use of the USGS 

hazard mapping to arrive at these parameters does not provide any specific insight to the controlling 

source and earthquake event, or a representative ground motion, but should provide a reasonable set 

of design parameters for lower seismic hazard sites for a given return period or probability, as each 

variable would be statistically represented by the maximum mean value. Then actual ground motion 

records for locations within 50 km of past earthquakes of magnitude “M” not less than the design M

can be adopted in design, in lieu of attempting to identify the controlling source earthquakes and to 

derive attenuated ground motions from each of those events.

The primary advantage of the DSHA approach in terms of defining the seismic hazard for a structure 

like a dam or embankment is that it deduces a particular seismic scenario, consisting of the postulated occurrence of an earthquake of a specified size at a specified location upon which a groundmotion-hazard evaluation is based. A DSHA approach provides ground-motion parameters from 

those earthquakes that have the most significant impacts. This is particularly advantageous for seismic design and analysis of dams and embankments.

As noted previously, the design basis for high-hazard-potential dams and impounding embankments 

is generally the MCE. Fundamentally, probability is not a consideration for the MCE, except as verification that the MCE is an extremely remote event. Previous designs for some coal refuse impoundments in some areas of the northern Appalachian and Illinois Basins have associated a 10,000-year 

return period with a MCE event, absent a detailed site-specific study or the availability of a detailed 

study from a similarly critical nearby structure/facility. This return period is at the upper end of typical recommendations (USCOLD, 1999). Table 7.6 presents recommendations for the seismic hazard 

assessment (and design earthquake), and the subsequent sections of this chapter describe development of the design ground motion based on a DSHA, which yields an earthquake magnitude, peak 

horizontal ground acceleration, and associated time histories/response spectra. The procedures presented herein include verification that the design earthquake is an extremely remote event, through 

comparison with published return periods or results of a PSHA. If a designer elects to base the design 

for a high-hazard-potential dam on a PSHA, the return period should be established such that the 
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MDE can be equated to the controlling MCE, as cited in FEMA (2005b). Further PSHA guidance on 

return-period criteria for significant- and high-hazard-potential dams is anticipated in the future 

from federal dam safety agencies.

7.7.2 Seismotectonic Modeling

Most seismotectonic studies and publications focus on the western U.S. where there is a relative 

abundance of data and observations. The seismotectonic modeling of the central and eastern U.S. 

requires that the differences between the eastern and western U.S. be appreciated. The following 

characteristics of eastern U.S. earthquakes represent significant differences when compared to the 

western U.S.

• More than an order of magnitude lower seismicity rates (longer recurrence periods 

for the same magnitudes).

• General lack of surface faulting, such that it is difficult to define source models

• Slower attenuation of ground motions with distance, which implies larger areas of 

damage for the same earthquake magnitude.

• Higher high-frequency content of seismic ground motions to larger distances.

• Relatively higher site amplification where soft soil is over rock (considered to be 

more significant in glaciated areas where the contact with highly competent rock is 

abrupt; Youd et al., 2001).

• Greater uncertainty in quantitative hazard assessments because the historic seismicity record (300 years) is too short compared to the recurrence periods of major damaging events.

• Few sets of eastern strong-motion data exist. They marginally constrain the attenuation of shaking with distance, as well as the dependence of local ground motion on 

magnitude, distance and depth of the earthquake.

• Higher frequency content of eastern earthquakes may lead to a greater number of 

cycles for the same magnitude.

In spite of the differences between eastern and western U.S. seismic events and the limitations of 

the eastern U.S. database, these factors can be accounted for in a seismic hazard analysis. The highfrequency motions associated with eastern earthquakes are generally limited to near-field rock sites. 

These motions tend to attenuate rapidly when they propagate through soil, which effectively reduces 

the amount of energy and the number of pertinent ground motion cycles that could contribute to the 

hazards of seismically-induced strength loss or ground deformation. Therefore, as noted by Youd et 

al. (2001), the duration differences between eastern and western soil sites are not likely to be significant when evaluating strength loss and ground deformations.

Compared with a plate boundary environment like California, an intra-plate environment like the 

central and eastern U.S. is difficult to characterize. Although there have been many recent advances 

in the understanding of earthquake occurrence based mainly on discoveries of paleoliquefaction phenomena, many gaps still exist in the knowledge of why and how often earthquakes occur in the central and eastern U.S., as summarized by the USGS Geologic Hazards Team (Crone and Wheeler, 2000) 

that supports the National Earthquake Hazard Reduction Program (NEHRP).

Defining a seismotectonic model for the central and eastern U.S. requires considerable judgment and 

is a subject of ongoing research. It is anticipated that it will be necessary to review the status of ongoing research whenever new seismic hazard assessments are required. This Manual includes some of 

the concepts and sources of information suitable for the derivation of seismic hazard at the time of 

publication, as summarized in the following subsections.
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7.7.2.1 Regional Tectonic Framework

One of the major accomplishments with respect to the definition of seismic hazard in the central and 

eastern U.S. over the past 20 years is the definition of potential seismic sources on the basis of tectonic 

evidence. The zones where there has been Quaternary fault movement are summarized in Figure 

7.20. Fault sources are the dominant means for deriving seismic hazard in the western U.S. Fault 

sources are poorly defined in the central and eastern U.S., but recent work has identified a tectonic 

framework for earthquake occurrence, as summarized in Crone and Wheeler (2000). Recent data for 

the New Madrid Seismic Zone (NMSZ) is presented in Tuttle et al. (2002).

In terms of conducting a deterministic analysis, there is no agreement regarding the actual definition 

of the main seismic sources affecting the central and eastern U.S. Nevertheless, it is practical to use 

the sources that are currently defined by the USGS Geologic Hazards Team. The zones where Quaternary tectonic movements have been documented are predominantly in the New Madrid Seismic 

Zone, and the modeling of this zone has been considered to be as fault zones of varying probability, 

as shown in Figure 7.20. This zone has significant influence to the southern Illinois Basin, as documented in the probabilistic hazard map shown in Figure 7.18 for a 500-year return period and in 

Figure 7.19 for a 2,475-year return period.

Another relatively recent discovery is the presence of paleoliquefaction in southern Illinois and Indiana (Figure 7.20), which was identified by Crone and Wheeler (2000) as the Wabash Seismic Zone. 

These widespread paleoliquefaction effects have been documented and interpreted to be associated 

with a large earthquake (M~ 7.5) possibly centered in the Wabash Valley area between southern Illinois and Indiana that occurred about 6,100 years before the present (BP). Specific faults that produced the paleoliquefaction features have not yet been identified.

Within the Appalachian Basin coal fields, the most significant source is the Eastern Tennessee Seismic Zone, a belt of seismicity in northeastern Alabama, northwestern Georgia and much of eastern 

Tennessee (Figure 7.20). The largest historical shock was magnitude 4.6, and occurred in 1973. No 

evidence for larger prehistoric shocks has been discovered, yet the microearthquake data suggest 

coherent stress accumulation within a large volume (Chapman et al., 2002). The largest earthquake 

within the Appalachian system had a Modified Mercali Intensity (MMI) equal to VIII (M = 5.9) and 

occurred on May 31, 1897 in Giles County, Virginia in the general area where evidence of Quaternary 

faulting has been uncovered. This faulting is referred to as the Pembroke Fault zone by Crone and 

Wheeler (2000). There is no surface expression of these faults and it is uncertain if they are due to 

tectonic movements or from solution collapse.

The third significant earthquake source in the eastern U.S. is the Charleston, South Carolina Seismic Zone. Paleoliquefaction data indicate that for characteristic large earthquakes in the Charleston, 

South Carolina region, the recurrence interval is approximately 550 years, as presented by Talwani 

and Schaeffer (2001). The USGS (Frankel et al., 2002) have assigned two source zones for this area 

with an equal probability of their validity. Although a significant seismic source in the eastern U.S., 

the effects of this zone do not significantly affect the Appalachian coal fields.

The definition of seismic source zones in the central and eastern U.S. based strictly on tectonics is still 

very much an evolving science. In conducting a deterministic analysis it will be necessary to carefully 

evaluate the most recent work of the USGS and other organizations, especially universities, involved 

with ongoing research into the identification of seismogenic tectonic structures. For example, the 

Mid-America Earthquake Center (MAE Center) is a good source of information for the central and 

eastern U.S. seismic hazard. The MAE Center, headquartered at the University of Illinois at UrbanaChampaign, is a consortium of nine core institutions funded by the NSF and each core university, as 

well as through joint collaborative projects with industry and other affiliations.
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7.7.2.2 Historical Seismicity

The seismic hazard derived for locations in the central and eastern U.S. needs to be at least as 

severe as has been historically recorded. The National Earthquake Information Center of the U.S. 

Geological Survey and the Engineering Research and Development Center of the U.S. Army Corps 

of Engineers compile source and magnitude information for earthquake events. These sources 

also provide access to isoseismal maps of historical events and available strong motion records, 

although few are available for the central and eastern U.S. The seismic hazard evaluation process 

should consider the historical maximum Modified Mercalli Intensity experienced at the site, and 

the design ground motion should not be less than that corresponding to the maximum MMI. The 

correlation of MMI to PGA is not an exact science, but correlations developed for California such 

as developed by Wald et al. (1999) and adapted for use in the central U.S. by Atkinson and Kaka 

(2006) are worth considering.

7.7.2.3 Selection of Seismic Sources

To define the source zones for deriving seismic hazard, the distribution of earthquakes needs to be 

evaluated in the context of available information regarding Quaternary tectonic activity. In the western U.S., careful evaluation of surface and near-surface faults should be conducted in association 

with a review of historical seismicity, which is often of limited quality. It may be necessary to conduct 

site-specific studies of faults whose rupture could control the seismic design in order to determine 

their degree of activity and their physical characteristics, as summarized in detail by Slemmons and 

DePolo (1986). Such studies are seldom practical in the central and eastern U.S.

In many cases in the central and eastern U.S., earthquake distribution by itself is the basis for defining a seismic source. For example, the Eastern Tennessee Seismic Zone is defined primarily based on 

the distribution of instrumentally determined epicenters. The overall active zone in the New Madrid 

area has been similarly defined.

The seismic sources most critical to the central and eastern U.S. coal fields are those identified in 

Figure 7.20. The fault sources that are significant to the definition of seismic hazard in the western 

U.S. coalfields are too numerous to describe in this Manual, and the reader is referred to lists and 

discussions provided by the USGS or state geological surveys. The interpretation presented in 

Figure 7.20, which represents the current best interpretation of the USGS, and forms the basis for 

the National Seismic Hazard Maps (Frankel et al., 2002), examples of which are shown in Figures 

7.18 and 7.19 , illustrates the current uncertainty with respect to the main sources affecting central 

and eastern U.S. coal fields. It should be emphasized that the presence of earthquakes is not an 

absolute criterion for delineating a seismic source zone. For example, the Wabash Valley Seismic 

Zone shown in Figure 7.20 was developed based on the identification of paleoliquefaction phenomena. Other than for special areas of relatively high seismic hazard, the approach of the USGS 

to defining seismic hazard has been to construct hazard maps directly from the historic seismicity 

data following the procedure in Frankel (1995). The number of events greater than the minimum 

magnitude are counted on a grid with spacing of 0.1° in latitude and longitude. Accordingly, to 

develop the seismic hazard following a DSHA, it is necessary to develop source models more fully 

than the main sources defined by the USGS.

In addition to the USGS, there has been considerable effort by other organizations to delineate seismic 

sources in the central and eastern U.S. In particular, from 1981 to 1989, Lawrence Livermore National 

Laboratory (LLNL) developed a PSHA methodology for the eastern United States (Bernreuter et al, 

1989), followed in 1993 by improvements in the handling of the uncertainties (USNRC, 1993). Differences between these results and those of a utilities-sponsored study (Electric Power Research Institute, 1989) led to the formation of the Senior Seismic Hazard Analysis Committee (SSHAC) to identify 

the sources of differences and give guidance on how to perform a state-of the-art PSHA (USNRC, 
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1997). This work continues to be reviewed and updated (Savy et al., 2002). These documents provide 

different expert opinions regarding appropriate source models for the central and eastern U.S., and 

they are important sources for detailed information regarding the seismotectonics of the central and 

eastern U.S. Nevertheless, it is cautioned that the USGS modeling is not one of the source models 

considered, and that some of the experts involved in development of the seismic source models for 

the LLNL studies hold widely divergent interpretations, as documented in the review of the seismic 

hazard at USDOE sites (USDOE, 1996).

Another source of seismotectonic modeling information is the U.S. Army Corps of Engineers (USACE). 

The USACE seismic design manual, Response Spectra and Seismic Analysis for Concrete Hydraulic Structures (USACE, 1999), provides a detailed seismotectonic interpretation of the central U.S. in terms of 

a source model based on a comprehensive review of seismotectonic data, and it is a notable reference 

for the definition of seismic source zones in this area.

7.7.2.4 Maximum Magnitude

In either a deterministic or probabilistic analysis, it is necessary to identify the strongest earthquake 

that has occurred within each source and then determine if there is a reason to consider that a larger 

earthquake could reasonably be expected to occur. Substantial judgment is required for this assessment. For example, it may be reasonable to assume that the 1811-1812 earthquakes in the New Madrid 

Seismic Zone are a maximum event. However, the known historical seismicity in southern Illinois 

does not encompass the largest earthquake that could be produced in that area, given the widespread 

paleoliquefaction phenomena that have been mapped.

In an intraplate environment, the evaluation of maximum magnitude is especially problematic 

because the source mechanisms are poorly defined. Where source characteristics are well defined, 

such as along a mapped fault, common practice is to use relationships developed between earthquake 

magnitude and total fault length, rupture area, maximum displacement per event, fault slip rate and 

seismic moment (Slemmons and Depolo, 1986; Wells and Coppersmith, 1994 or several others). With 

some exceptions such as the Meers Fault in Oklahoma, these relationships can be applied only with 

considerable judgment in the central and eastern U.S.

DSHA is intended to define the worst-case ground motion and should therefore be based on the least 

favorable combination of earthquake source characteristics and location, and the strongest ground 

motion that could be generated by this scenario. In practice, DSHA generally uses the logarithmic 

mean or mean-plus-one-standard-deviation level of ground motion from predictive equations (Krinitzsky, 2002). If a mean plus a standard deviation is used, the ground motion will be conservative 84 

percent of the time. A more detailed discussion of the difficulties in defining maximum magnitude is 

summarized by Bommer et al. (2004). Because of the uncertainties in the definition of maximum magnitude, the best approach is one of targeting a consensus opinion, such as is provided by the USGS 

Geologic Hazards Team (Frankel et al., 2002).

Another potential source of information regarding maximum earthquake magnitude is to consider 

probability of occurrence. In addition to the PGA hazard maps, the USGS (Frankel et al., 2002) also 

provides maps of the probability of occurrence of earthquakes of various magnitudes within specific 

distances of a given location as a function of return period. An example of a map depicting the probabilities (in terms of return periods) that an earthquake with M > 5.5 will occur within a distance of 

50 km of different locations within the central and eastern U.S. is presented in Figure 7.21. As this 

map was prepared by considering earthquake recurrence with a return period up to 10,000 years, the 

results show that large earthquakes are only reasonably conceivable in limited areas of the central 

and eastern U.S. and, conversely, that earthquakes of M > 5.5 are a remote possibility in most areas 

of the central and eastern U.S.
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FIGURE 7.21 RETURN PERIOD FOR M > 5.5 EVENT AT 

 A DISTANCE LESS THAN 50 KM

LEGEND

RETURN PERIOD IN YEARS FOR AN

EARTHQUAKE OF M > 5.5 AT A DISTANCE

OF LESS THAN 50 KM.

9999

7500

4000

3000

2000

1000

500

300

NOTE: 1. IMAGE REDRAWN FROM 2002 MAP

 BY THE USGS OF RETURN PERIOD

 FOR AN M > 5.5 EVENT AT A DISTANCE

 OF LESS THAN 50 KM.

2. ANTHRACITE AND BITUMINOUS

 COAL FIELDS ARE DENOTED BY

 RED BORDERS.

FIGURE 7.21 RETURN PERIOD FOR M > 5.5 EVENT AT A DISTANCE LESS THAN 50 KM
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7.7.2.5 Earthquake Recurrence

The evaluation of earthquake recurrence for each source considered is a requirement of the PSHA. 

Recurrence is of interest to a DSHA if the distribution of earthquakes within a source as determined 

from the historical seismicity and geologic data as appropriate is suggestive that a maximum magnitude other than the historic maximum should be considered.

The determination of earthquake recurrence requires several steps:

1. Identification of both historical and instrumentally-located earthquakes within each 

postulated source.

2. Conversion of the available data into a common magnitude base. There are several 

magnitude scales, but for about the past 25 years it has been common practice to 

define magnitude in terms of seismic moment, as defined by Kanamori (1977) and 

Hanks and Kanamori (1979). Moment magnitude is the scale most commonly used 

for engineering applications and is the scale preferred for calculation of liquefaction resistance. The relationship between moment magnitude M and other commonly used magnitude scales can be reviewed as a chart published by Youd et 

al. (2001). In the central and eastern U.S., the most commonly derived magnitude 

scale is mblg, which was developed by Nuttli (1973). Frankel et al. (1996) provide 

the following conversion to moment magnitude:

M = 3.45 – 0.473 mblg + 0.145 (mblg)

2 (7-8)

3. Evaluate the completeness of the earthquake catalog after the removal of statistically 

dependent events – aftershocks and foreshocks. Determination of catalog completeness is a function of location (e.g., earthquake history is better defined for a longer 

period in Charleston, South Carolina than in central Montana). Procedures for evaluating completeness are presented by Veneziano and Van Dyck (1985).

4. Formulate earthquake recurrence for each source. The most common means of defining recurrence is the Richter formula (Richter, 1958):

log N = a – b m (7-9)

where:

m = earthquake magnitude

N = number of earthquakes of magnitude m or greater per year per unit area

a, b = constant coefficients defining a linear relationship between log N and m

This simple exponential recurrence model has been found to have good applicability for area 

sources, but there are many other ways to define the probability of earthquake recurrence, particularly when specific faults are considered (Schwartz and Coppersmith, 1986). It should also be noted 

that the simple Richter formula can be non-linear for the highest magnitudes and that caution 

should be used when estimating the recurrence of large magnitude events on the basis of limited 

historical data.

7.7.3 Design Ground Motion

The basic seismic data needed for determination of an MDE, including the MCE, is a time history(ies) 

representative of the source magnitude of the earthquake and a duration (number of cycles of strong 

ground motion) consistent with the source magnitude. The derivation of an MCE for a high-hazard-
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potential dam involves a seismic-hazard assessment, and this Manual describes procedures for performing a seismic-hazard assessment using a DSHA with comparison to published probabilistic 

hazard assessments, such as those available from the USGS or the USNRC. The MCE should be shown 

to be an extremely remote event. FEMA (2005b) indicates that a PSHA can aid in deciding whether 

mean, or mean-plus-one-standard-deviation, or even greater estimates of ground motion are justified in a deterministic ground motion analysis for the MDE to achieve an acceptably low probability 

of exceedance. Until published guidance from federal dam safety agencies is available, the designer 

should consider that for high-hazard-potential facilities in areas of medium to high seismic hazard, 

the primary role of a PSHA is to serve as technical support in the selection of an MCE consistent with 

FEMA (2005b).

The OBE, as well as the MDE for significant-hazard-potential dams, can be defined on the basis of a 

probabilistic analysis, which could simply be obtained from published results of the USGS. As noted 

in the introduction to this chapter, a probabilistic analysis considering a 500-year return period can 

serve as the basis for defining an OBE, and a return period of the order of 2,500 years may be reasonable for establishing the MDE for a significant-hazard-potential dam.

Another means for evaluation of the results of a DSHA is to review the design basis for nuclear 

power plants, which have all been designed on the basis of a DSHA. The location of nuclear power 

plants with respect to U.S. coal fields is presented in Figure 7.22. Table 7.7 provides the Safe Shutdown Earthquake (SSE) peak horizontal ground accelerations (PGA) that formed the basis for seismic 

design for each nuclear plant. The SSE PGA values for these nuclear plants should be considered as 

minimum values for a MCE. The conservatism associated with the seismic input for a nuclear power 

plant has proven to be the overall spectral shape of their seismic input, rather than the PGA. Furthermore, earthquake magnitude values associated with the SSE are generally not defined, because the 

practice for defining seismic hazard in the 1960s and 1970s, when these nuclear power plants were 

licensed, was to base the design on Modified Mercalli intensity (MMI) relationships.

As noted in Section 7.7.2, much of the areas mined for coal in the central and eastern U.S. have 

a low seismic hazard. For these areas a simplified approach is recommended for defining a minimum standard for seismic design. Section 7.7.3.7 of this Manual proposes standard seismic inputs for 

design in areas of low seismic hazard, and these inputs are believed to be reasonably conservative for 

coal refuse impoundments. If the designer judges that less conservative inputs than recommended 

herein are appropriate, it is acceptable to perform a complete hazard analysis to justify less conservative project-specific and/or site-specific seismic design inputs. The following sections present more 

detailed information regarding the definition of design ground motion for an MCE for a high-hazardpotential dam.

7.7.3.1 Selection of Design Earthquakes

The MCE for each seismotectonic structure or source area within the region examined needs to be 

defined by a common parameter, preferably by moment magnitude M, but in a manner that is compatible with the attenuation function used to derive site ground motion. It is recommended that the 

maximum historical site MMI be evaluated as an additional estimate to site ground motion, but it is 

recommended that the historical MMI be converted to magnitude based on scientific evaluations of 

the ground effects. Such studies are usually available for the most significant historical events. For 

example, using a new method for evaluating magnitude by directly inverting observations of intensities, Bakun and Hopper (2004) determined a moment magnitude M of 7.4 (7.0 to 7.7 at a 95 percent confidence level) for the largest New Madrid earthquake in the 1811-12 sequence. Scientific publications for 

evaluation of the magnitude of historical earthquakes are generally available.

Where the earthquake history is incomplete or where there is no geologic evidence regarding past 

earthquake activity, judgment is required in assigning a maximum magnitude to each source. Where 
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fault sources are inferred to be present, fault movement within the range of 35,000 to 100,000 years 

BP is considered recent enough to warrant an “active” or “capable” classification, and they should be 

modeled as sources according to their fault dimensions and histories. The assignment of a maximum 

magnitude to each source should consider the concepts presented in Section 7.7.2.4.

The MCEs identified for each source potentially affecting the site are candidates for one or more controlling MCEs at the site. It is also important to look at a variety of earthquakes that have a long duration 

and are rich in lower frequency contents, but do not necessarily cause the highest peak acceleration at 

the coal refuse site. For coal refuse impoundments, this longer-duration earthquake may be the controlling event if it triggers strength loss of the embankment/foundation materials. Seismic input for a coal 

refuse impoundment facility may be defined by both near-field and far-field events. For purposes of 

clarification, a near-field event is an earthquake that is postulated to have an epicenter at or very near a 

site of interest and a far-field event is one where the earthquake is not expected to occur near the site.

7.7.3.2 Ground Motion Attenuation

The difference in ground-motion attenuation between the western and eastern U.S. is significant and 

needs to be accounted for in the hazard analysis. This is a topic that has been extensively researched 

and there are a number of attenuation functions that have been developed for eastern U.S. ground 

motion. Examples include Toro et al. (1997), Frankel et al. (1996), Atkinson and Boore (1995, 2006), 

Somerville et al. (2001) and Campbell (2003). The attenuation relationships were developed based on 

FIGURE 7.22 LOCATION OF NUCLEAR POWER PLANTS

 WITH RESPECT TO U.S. COAL FIELDS

NOTE: 1. THE SAFE SHUTDOWN EARTHQUAKE (SSE)

 DESIGN PEAK HORIZONTAL GROUND ACCELER-

 ATIONS FOR THESE NUCLEAR POWER PLANTS

 ARE PROVIDED IN TABLE 7.5.

TURKEY POINT 3,4

ST. LUCIE 1,2

CRYSTAL RIVER

SAN

ONOFRE 2,3

DIABLO

CANYON 1,2

PALO VERDE 1,2,3

COLUMBIA

CALVERT CLIFF 1,2

GINNA

FITZPATRICK

VERMONT

YANKEE

PILGRIM

SEABROOK

NINE MILE

POINT 1,2

OYSTER CREEK

MILLSTONE

 2,3

HARRIS

BRUNSWICK 1,2

VOGTLE

INDIAN

POINT 2,3

SOUTH

TEXAS 1,2

COMMANCHE

PEAK 1,2

FARLEY 1,2

McGUIRE 1,2

HOPE CREEK

SALEM 1,2

RIVER BEND

 ARKANSAS

NUCLEAR ONE

 1,2

GRAND

GULF 1

MONTICELLO

PRAIRIE

ISLAND 1,2

SLURRY 1,2

NORTH

ANNA 1,2

SUMMER 1

FORT CALHOUN 1

COOPER 1

WOLF

CREEK

DUANE

ARNOLD

KEWAUNEE

POINT

BEACH 1,2

LIMERICK 1,2

OCONEE

 1,2,3

ROBINSON 2

CATAWBA 1,2 BROWN’S

FERRY

SEQUOYAH 1,2

WATTS BAR 1

HATCH 1,2

CLINTON 1

CALLAWAY 1

PERRY 1

FERMI 2

DAVIS

DC COOK BESSE 1

 1,2

PALISADES

BYRON

 1,2

QUAD

CITIES 1,2

LASALLE 1,2

DRESDEN

 2,3

BRAIDWOOD 1,2

BEAVER

VALLEY 1,2

SUSQUEHANNA 1,2

PEACH BOTTOM 2,3

3 MILE ISLAND 1

2. ANTHRACITE AND BITUMINOUS COAL FIELDS

 ARE DENOTED BY RED BORDERS.

FIGURE 7.22 LOCATION OF NUCLEAR POWER PLANTS WITH RESPECT TO U.S. COAL FIELDS
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numerical modeling and sparse strong-motion records from small earthquakes, and while subject to 

uncertainty, they are considered to be appropriate. 

7.7.3.3 Selection of Peak Ground Motion Parameters

The MCE is defined as the most severe ground motion calculated from the selected ground motion 

attenuation relationship(s) from the various potential MCEs identified from the possible sources. 

The credible severe combination(s) of magnitude (preferably moment magnitude) and distance will 

define the basis for the MCE. The ground motion associated with the credible severe combinations of 

magnitude and distance is normally defined in terms of peak ground acceleration (PGA).

In many cases the most severe PGA is associated with a relatively small nearby earthquake, especially in 

the areas of relatively low seismic hazard. Where there is moderate to high risk, just deriving the credible severe PGA from a small local earthquake is not sufficient, and it is necessary to supplement this 

MCE with a credible severe earthquake from a relatively distant large magnitude source where the PGA 

might be lower than a small local event, but will be associated with more cycles of ground motion.

7.7.3.4 Selection of Acceleration Time Histories

A time-history representation of the seismic input is generally presented in terms of the ground 

accelerations (accelerograms) where the variation of ground motion is plotted as a function of time. 

Actual strong motion recordings of earthquakes are time histories of the earthquake ground motion. 

Ultimately, the goal of selecting a time history for use in design is to be able to match the MDE ground 

motion with a PGA and number of cycles of ground motion defined for design. There are two ways 

to obtain time histories for engineering design and analysis: (1) actual ground motion records or (2) 

synthetic ground motions.

Actual ground motion records can be obtained from the USGS or COSMOS (Consortium of Organizations for Strong-Motion Observation Systems) web sites. If actual strong motion recordings are 

used as seismic input, they need to be selected to match as closely as possible the source magnitude 

and expected faulting mechanism, recognizing that faulting mechanisms may not be known if the 

site is in the central or eastern U.S. Although distance is a consideration in the selection of the ground 

motion, it is more significant that the earthquake simulate the PGA derived for the design MDE(s) 

and that the duration/number of cycles of ground motion be consistent with the number of cycles 

derived with the simplified approach for liquefaction analysis. In addition, care should be taken to 

select records that are well represented in the frequency range of interest to an embankment dam 

(approximately 0.5 to 1.5 Hz). In order to effectively bound the range of possible ground motions, 

several hypothetical design ground motions, or a synthetic time history rich in all possible frequencies of interest, should be used.

As noted in Section 7.7.3.2, most of the strong motion records available as potential design time 

histories in the range of a central and eastern U.S. MCE will be from areas with different attenuation characteristics (e.g., the western U.S.). There are very few records from the central and eastern 

U.S., especially for moderate to strong earthquakes of magnitude M greater than 5.0. Nevertheless, 

as noted in Section 7.7.2, in spite of the differences associated with western U.S. records and limitations to the eastern U.S. database, these differences can be accounted for in a seismic hazard analysis because, within soils where strength loss occurs, it is expected that western and eastern ground 

motions will be similar (Youd et al., 2001). If actual strong ground motion records are used as the 

basis for defining the MCE, it is recommended that at least three records be selected for the analysis 

and that justification provided for their selection. If the site is in an area of medium to high seismic 

hazard, more than three time histories should be used.

Although real time histories are the preferred means for defining ground motion, an alternative 

approach for defining a design time history that is often used in the central and eastern U.S. is to derive 
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TABLE 7.7 SAFE SHUTDOWN EARTHQUAKE (SSE) PEAK HORIZONTAL GROUND 

ACCELERATIONS FOR U.S. NUCLEAR POWER PLANTS

Plant SSE PGA (g) Plant SSE PGA (g)

Arkansas Nuclear 1, 2 0.21 Millstone 2, 3 0.17

Beaver Valley 1, 2 0.13 Monticello 0.13

Braidwood 1, 2 0.19 Nine Mile Point, 1, 2 0.15

Browns Ferry 1, 2, 3 0.21 North Anna, 1, 2 0.13

Brunswick 1 0.17 Oconee 1, 2, 3 0.11

Byron 1, 2 0.19 Oyster Creek 0.27

Callaway 0.19 Palisades 0.19

Calvert Cliffs 1, 2 0.15 Palo Verde 1, 2, 3 0.19

Catawba 1, 2 0.17 Peach Bottom, 2, 3 0.15

Clinton 0.21 Perry 1 0.15

Columbia Generating Station 0.25 Pilgrim 1 0.15

Comanche Peak 1, 2 0.13 Point Beach 1, 2 0.13

Cooper 0.21 Prairie Island 1, 2 0.13

Crystal River 3 0.11 Quad Cities 1, 2 0.27

D.C. Cook 1, 2 0.21 River Bend 1 0.11

Davis-Besse 0.15 Robinson 2 0.19

Diablo Canyon 1, 2 0.80 Saint Lucie 1, 2 0.11

Dresden 2, 3 0.21 Salem 1, 2 0.21

Farley 1, 2 0.11 San Onofre 2, 3 0.63

Fermi 2 0.15 Seabrook 1 0.25

FitzPatrick 0.15 Sequoyah 1, 2 0.17

Fort Calhoun 0.19 South Texas 1, 2 0.11

Ginna 0.19 Summer 0.15

Grand Gulf 1 0.15 Surry 1, 2 0.15

Harris 1 0.15 Susquehanna 1, 2 0.11

Hatch 1, 2 0.15 Three Mile Island 1 0.15

Hope Creek 1 0.19 Turkey Point 3, 4 0.15

Indian Point 2, 3 0.15 Vermont Yankee 0.19

Kewaunee 0.13 Vogtle 1, 2 0.19

La Salle 1, 2 0.21 Waterford 3 0.11

Limerick 1, 2 0.15 Watts Bar 1 0.19

McGuire 1, 2 0.15 Wolf Creek 1 0.13

 (ADAPTED FROM SOBEL, 1994)

synthetic ground motions. The most widely used methods to generate synthetic ground motion are 

the stochastic point- and finite-source models (Hanks and McGuire, 1981; Toro and McGuire, 1987) 

and the stochastic finite-fault model, which can simulate some of the near-source effects (Atkinson 

and Silva, 1997; Beresnev and Atkinson, 1997). With the improvement of computers, it has become 

possible to use more sophisticated numerical methods for simulating strong ground motion based on 

empirical or theoretical source functions and two- and three-dimensional wave propagation theory, 

which has been successfully used in ground-motion simulations for many earthquakes (Somerville 

et al., 2001; Saikia and Somerville, 1997). Zeng et al. (1994) also developed a composite source model 

that has been recommended for use in generating synthetic ground motions for seismic design and 

analysis of highway bridges in Kentucky.
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7.7.3.5 Applicability of Design Ground Motion

The end result of a seismic hazard assessment is to derive the ground motion that will affect the base 

of the impounding structure. Most ground-motion-attenuation relationships derive motion for bedrock or stiff (firm) soils. Where the site foundation consists of less stiff/dense or deeper soil deposits, 

or otherwise deviate from the site conditions for which the attenuation relationships are applicable, 

the derived ground motion should be applied at the top of rock and the ground motion (at the ground 

surface) estimated from local site response analyses based on computer simulation with programs 

such as SHAKE, QUAD4M, or DESRA (Schnabel et al., 1972; Finn et al., 1977). For these analyses it is 

necessary to use a time history input as discussed in Section 7.5.

7.7.3.6 Application of Seismic Parameters in Design Process 

Figure 7.23 presents a flow diagram for seismic hazard assessment to determine the design earthquake inputs (M, PGA, ground motion). At a minimum, the design M and PGA are required, as 

these data are used in the Youd, et al. (2001) pore-pressure-based triggering analysis, to establish a 

minimum number of cycles of loading for cyclic shear strength testing, in very simplified dynamic 

response analyses, and for other general purposes. More rigorous dynamic response, seismic stability, and deformation analyses require design ground motions (acceleration versus time histories) to 

estimate the amplification or de-amplification of the ground motion from the appropriate input horizon (e.g., top of rock, top of stiff soil, or base of structure), up through the foundation and overlying 

structure; calculate peak accelerations at critical locations within the foundation and structure; conduct more refined and sophisticated triggering analyses; calculate seismically-induced stresses and 

strains; and identify the likely modes and estimate the range in magnitude of deformation.

7.7.3.7 Simplified Design Ground Motion for Areas of Low Seismic Hazard

As noted in the introduction to Section 7.7.3, a substantial portion of the areas mined for coal in the 

central and eastern U.S. have a low seismic hazard. The definition of what constitutes low seismic 

hazard is judgmental, but coal refuse disposal facilities in the area where the Documentation for the 

2002 Update of the National Seismic Hazard Map (Frankel et al., 2002) indicates that the PGA with a 

return period in 2,500 years is less than or equal to 0.10g can be considered to be representative of 

low seismic hazard (Figure 7.19). This zonation is effectively similar to the area where the return 

period for an earthquake of M > 5.5 within 50 km of a given location is greater than 10,000 years 

(Figure 7.21).

For structures in areas of low seismic hazard that warrant evaluation for a MCE, a simplified 

approach can be used. The simplified minimum design earthquake should have a moment magnitude M of at least 5.5 (M ≥ 5.5) at a distance no farther than 50 km from the source. For evaluations 

of triggering or deformations requiring both M and PGA, adopt 150 percent of the PGA associated 

with a 2,500-year return period based on the USGS PSHA hazard maps, but not less than PGA = 

0.10g. When needed, select earthquake ground motion records based on M and the other considerations outlined in Section 7.7.3.4. In areas of low seismic hazard, it is not necessary to consider 

more distant earthquakes with M > 5.5.

Commentary: A simplified approach is recommended by the authors to define a minimum standard for seismic 

design in low-seismic-hazard areas, with inputs believed to be reasonably conservative for coal refuse impoundments. It is based on published seismic hazard mapping, subject to periodic updating and widely available from 

the USGS. Based on the project site location, a designer can directly determine the M and PGA using the above 

procedures for the simplified minimum design earthquake. If a designer judges that other (or potentially less 

conservative) inputs are appropriate, it is acceptable to perform a seismic hazard assessment. As indicated in 

Table 7.6, the minimum standard is appropriate for projects only in low-seismic-hazard areas, and a seismic 

hazard assessment is necessary in moderate to high seismic hazard areas.
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Screening of site seismic hazard based on a return period of 2,500 years

1. Determine PGA at site from USGS seismic hazard maps.

2. Determine M (within 50 km) from USGS Earthquake Probability Maps.

YES NO

DETERMINATION OF EARTHQUAKE PARAMETERS

IS THE PGA < 0.10g

AND

M < 5.5?

Select 3 actual earthquake ground motion records for

locations within 50 km of an earthquake event with M no

less than 5.5 (or design M, if higher). Potential sources

include USGS and COSMOS. In selecting ground

motions, consider duration of motion, frequency content

and ground/foundation characteristics comparable to

those found in the area of interest.

LOW SEISMIC HAZARD

DETERMINE DESIGN EARTHQUAKE PARAMETERS

MODERATE TO HIGH SEISMIC HAZARD

DESIGN OF NEW HIGH-HAZARD DAMS, EXPANSION OF

EXISTING HIGH-HAZARD DAMS AND ASSESSMENT

OF DESIGN STANDARDS FOR EXISTING HIGH-HAZARD DAMS

Apply PSHA data from USGS or data from available study

for similarly critical nearby structure, previous site-specific

seismic hazard study (whether from PSHA or DSHA) if still

valid, or from a site-specific PSHA or DSHA study.

1. Perform a site-specific DSHA of PSHA.

2. Verify that the PGA from a site-specific DSHA is

 an extremely remote event and not less than a Safe

Shutdown Earthquake (SSE) for a nuclear power

plant in a similar seismic zone, if such data are

available. If not, reassess postulated earthquakes

and DSHA evaluations.

3. Verify that the PGA from a site-specific PSHA is an

extremely remote event and can be equated to the

controlling MCE.

NOTE:

1. As support to development of a site-specific DSHA,

alternative approaches for demonstrating the

conservativeness of the DSHA assumptions,

including PSHA are encouraged.

2. Several representative ground motion records

covering the potential range of worst-case ground

motions consistent with the results of the DSHA and

accounting for site-specific ground conditions should

be selected to define the seismic input for new high

hazard dams in an area of medium to high seismic

hazard.

OTHER CASES

Adopt the following simplified minimum design earthquake

for the site:

1. Earthquake Magnitude M > 5.5

2. PGA = 1.5 times PGA for 2500-year return period >

 0.10g

Use available earthquake record (see box below) or DSHA

or PSHA study for similar critical or high hazard structure.

SELECT REPRESENTATIVE DESIGN EARTHQUAKE

GROUND MOTION RECORDS

FIGURE 7.23 SEISMIC HAZARD ASSESSMENT FLOW DIAGRAM
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Selection of time histories of acceleration for structures in areas of low seismic hazard should consider earthquake records that are reflective of the site-specific design parameters (M and PGA) and 

foundation conditions. Attributes of the time histories of acceleration should include: (1) the M and 

PGA are at least equivalent to the site-specific design parameters, (2) near-field event with hypocentral distance of 50 kilometers or less, (3) foundation conditions similar to site-specific conditions 

(otherwise, some adjustment of the time history may be warranted), (4) several cycles with a representative band of acceleration in proportion to the design PGA, and (5) some cycles at or near the 

frequencies of interest for the structure (low frequencies for dams). Section 7.7.3.4 presents reference 

sources for ground motion histories. The following time histories illustrate the selection of candidate 

records for low-seismic-hazard areas:

• Example 1 (Figure 7.24) – Time history for low-seismic-hazard area, and the site is 

distant from widely recognized seismic source zones. This example illustrates a time 

history that can be considered for a site in the eastern or central U.S., with a design 

M and PGA of at least 5.5 and 0.1g, respectively. This example has a high PGA (much 

greater than the site specific design PGA) and lacks cycles of substantive acceleration 

at lower frequencies, which is not uncommon.

• Example 2 (Figure 7.25) – Time history for low-seismic-hazard area, but the site is 

close to a boundary of moderate seismic hazard (as reflected by a site-specific PGA

much greater than 0.1g). This example illustrates a time history that may be considered for a site in the eastern or central U.S. (close to a recognized seismic source 

zone) with a design M of at least 5.5 and a design PGA much greater than 0.1g (e.g., 

0.12 to 0.15g) with significant low frequency content. This example could also be 

conservatively applied to sites in low-seismic-hazard areas that are distant from 

widely recognized source zones.

• Example 3 (Figure 7.26) – Time history for low-seismic-hazard area, and the site is 

distant from widely recognized seismic source zones. This example illustrates a time 

history for an actual eastern North American earthquake that could be considered 

for a low-seismic-hazard site in the eastern US.

7.8 SEISMIC DESIGN OVERVIEW

Gardner and Wu (2002) presented an overview of the challenges in evaluating strength loss and seismic stability at coal refuse disposal facilities, including a summary of available pore-pressure-based 

empirical methods and strain-based laboratory methods that MSHA has used in their review and 

approval of impoundment plans. Wu et al. (2003) cite common MSHA review issues with seismic 

stability including: (1) defining materials subject to strength loss, (2) sampling and testing protocols, (3) defining ground motion parameters applicable to the site, (4) determining the appropriate 

margin of safety, (5) and dealing with questions concerning the applicability of available technical information for fine coal refuse. There are no documented case histories of a fine coal refuse 

impoundment being affected by significant earthquake loading, and as a result, there is little direct 

confirmation of methods that have been employed for estimating strength loss. Thus, MSHA has 

been faced with reviewing impoundment design plans with minimal factors of safety, often without mitigating design features that would enhance seismic stability such as provisions for internal 

drainage of fine refuse deposits.

This chapter has presented credible, documented methods and procedures for the evaluation of seismic design including stability and deformation analyses considered applicable for materials encountered at coal refuse disposal facilities. A flow chart illustrating the steps involved in evaluating seismic 

design, stability and deformation was presented in Figures 7.1a, 7.1b and 7.1c, and Section 7.1.2 provides a discussion of the basis for the design guidance, including:
7-89

Seismic Design: Stability and Deformation Analyses

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WHITTIER NARROWS - STATION: 24399 MT. WILSON - CIT SEISMIC STATION

OCTOBER 1, 1987 M = 5.99, DISTANCE FROM FAULT RUPTURE = 21.2 KM, V = 822 M/SEC (ROCK)

7.24a OVERALL RECORD

7.24b SAMPLE TIME SEGMENT

ATTRIBUTES: 1. M AND PGA > SITE-SPECIFIC DESIGN M AND PGA

 2. NEARER FIELD, FAULT-DRIVEN EVENT WITHIN 50 KM

 3. FIRM ROCK COMPARABLE TO PROJECT SITE CONDITIONS

 4. MANY CYCLES WITHIN REPRESENTATIVE BAND OF PGA.

 5. LIMITED CYCLES AT LOWER FREQUENCY.

 

FIGURE 7.24 EXAMPLE 1: TIME HISTORY FOR LOW SEISMIC HAZARD AREA

PGA (FRACTION OF g)

TIME (SEC)

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0 5 10 15 20 25 30 35 40

TIME (SEC)

PGA (FRACTION OF g)

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

1.5 2.5 3.5 4.5 5.5 6.5

NOTE: SITE IS DISTANT FROM WIDELY RECOGNIZED SEISMIC SOURCE ZONES

(UNIV. OF CAL., 2007a)

FIGURE 7.24 EXAMPLE 1: TIME HISTORY FOR LOW-SEISMIC-HAZARD AREA
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FIGURE 7.25 EXAMPLE 2: TIME HISTORY FOR LOW SEISMIC HAZARD AREA 

7.25a OVERALL RECORD

TIME (SEC)

PGA (FRACTION OF g)

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0 5 10 15 20 25

7.25b SAMPLE TIME SEGMENT

TIME (SEC)

PGA (FRACTION OF g)

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

2 3 4 5 6 7

COYOTE LAKE - GILROY ARROY #6, 230 (CDMG STATION 57383)

AUGUST 6, 1979 M = 5.7, HYPOCENTRAL DISTANCE = 9 km, V = 663 m/sec (SOFT ROCK)

ATTRIBUTES: 1. M AND PGA > SITE-SPECIFIC DESIGN M AND PGA

 2. NEAR FIELD EVENT MUCH CLOSER THAN 50 km

 3. SOFT ROCK COMPARABLE TO PROJECT SITE CONDITIONS

 4. INCLUDES PERTINENT LOW FREQUENCY CYCLES.

 5. SIGNIFICANT ACCELERATION SPIKES AT LOWER FREQUENCY.

 

NOTE: SITE IS CLOSE TO THE BOUNDARY OF MODERATE SEISMIC HAZARD

(UNIV. OF CAL., 2007b)

FIGURE 7.25 EXAMPLE 2: TIME HISTORY FOR LOW-SEISMIC-HAZARD AREA
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Seismic Design: Stability and Deformation Analyses

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7.26b SAMPLE TIME SEGMENT

FIGURE 7.26 EXAMPLE 3: TIME HISTORY FOR LOW SEISMIC HAZARD AREA

7.26a OVERALL RECORD

TIME (SEC)

PGA (FRACTION OF g)

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0 5 10 15 20 25 30 35

TIME (SEC)

PGA (FRACTION OF g)

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

10 11 12 13 14 15

SAGUENAY - W. CHICOUTIMI NORD (SITE 16 T)

NOVEMBER 25, 1988 M = 5.9, HYPOCENTRAL DISTANCE = 50 KM (HARD ROCK)

ATTRIBUTES: 1. M AND PGA > SITE-SPECIFIC DESIGN M AND PGA

 2. EASTERN NORTH AMERICA

 3. ROCK RECORD

 4. MANY CYCLES WITHIN REPRESENTATIVE BAND OF PGA

 5. RELATIVELY LONG DURATION OF STRONG SHAKING > 0.05g (~12 SEC)

NOTE: EASTERN U.S. EARTHQUAKE, DISTANT FROM WIDELY RECOGNIZED SEISMIC SOURCE ZONES

(UNIV. OF CAL., 2007c)

FIGURE 7.26 EXAMPLE 3: TIME HISTORY FOR LOW-SEISMIC-HAZARD AREA
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• Appropriate levels of analysis, depending on the type of facility.

• Methods for identifying and evaluating material susceptibility to strength loss, 

including available field and laboratory techniques.

• Simplified methods for estimating post-earthquake strength, as well as more sophisticated methods for evaluating if triggering of strength loss occurs.

• Alternatives for evaluating seismicity depending on the level of seismic hazard of 

the region.

• A recommended factor of safety for seismic stability of 1.2 based on a static stability analysis using post-earthquake strengths, which also helps achieve designs with 

predicted deformations within acceptable limits.

Section 7.1.5 provides recognized, simplified steps and procedures that utilize basic site data and 

component material properties for development of conservative embankment dam designs in regions 

of low seismic hazard that constitute most of the areas of coal mining, as discussed in Section 7.7. 

More sophisticated analyses and procedures may be needed for some facilities built by upstream 

construction, and these are also discussed in Section 7.1.5, with additional details presented in Section 7.4. These more sophisticated analyses and procedures can reduce conservatism and potentially 

provide the experienced designer with more expertise additional options for design. These types of 

analyses may be needed in those limited regions where seismic hazard is moderate to high. Basic 

screening methods for evaluating deformations are discussed in Section 7.1.5, and the applicability 

of both screening and analysis procedures is presented in Section 7.5.

Other credible methods and procedures may be acceptable or may become accepted in practice, provided that documentation and testing support their application, and these methods should not be 

precluded from use in seismic design.
Appendix 7A

DERIVATIONS OF BASIC EQUATIONS FOR

STEADY-STATE LABORATORY TESTING

MAY 2009 7A–1

7A.1 COMPARISON OF α AND φ ENVELOPES AT STEADY STATE

a





3 1

b





3 1

MOHR'S CIRCLE AT STEADY STATE

 ENVELOPE DRAWN THROUGH MAXIMUM SHEAR

 STRESS ON MOHR'S CIRCLE

 ENVELOPE DRAWN TANGENT TO MOHR'S CIRCLE

2 DEFINE p = MEASURED IN TRIAXIAL TEST AT STEADY STATE

1 + 3

c c

2 DEFINE q = MEASURED IN TRIAXIAL TEST AT STEADY STATE

1 3

a = b = q

tan  = 

a

c

sin  =

b

c

THEREFORE sin  = tan 

c = p = 3 + q

a = b

 = sin-1 q

p

AND
7A–2

Appendix 7A

MAY 2009

7A.2 RELATIONSHIP BETWEEN SUS AND σ3 AT STEADY STATE

b

a 





3 3+ q 1

MOHR'S CIRCLE AT STEADY STATE

b = q = 

1 - 3

2 = RADIUS OF MOHR'S CIRCLE

a = b cos  = SUS

THEREFORE: SUS = q cos 

FROM TRIANGLE GEOMETRY: sin  =

b

3 + q = q

3 + q

sin  =

3 + 

= cos 

cos 

3 cos  SUS +

3 cos  sin  + sin  =

3 cos  sin  = SUS (1 - sin 

THEREFORE: 

 SUS

SUS

 SUS

 SUS

 SUS

3 = SUS

1 - sin 

cos  sin 

 SUS

q
Appendix 7B

VOID-RATIO MEASUREMENTS DURING UNDISTURBED 

TUBE SAMPLING AND LABORATORY TESTING

MAY 2009 7B–1

7B.1 INTRODUCTION

The purpose of this appendix is to present methods for accurate measurement of the void ratio of 

triaxial specimens and the changes in void ratio of undisturbed tube samples from conditions in situ 

to conditions during shear in the triaxial test. Accurate void ratio measurements are needed in order 

to correct the undrained steady-state (residual) strengths measured in the triaxial test to obtain insitu strengths using the procedures described in Poulos et al. (1985). Specifically, the required voidratio values are:

• Undisturbed specimens – both the void ratio in situ and the void ratio during 

undrained shear in the triaxial test must be determined. To obtain these values one 

measures the void ratio changes that occur at various stages of the sampling and 

testing process, as subsequently discussed.

• Remolded specimens (tested to obtain the slope of the steady-state line) – the actual 

void ratio in the triaxial cell during shear is needed. For drained tests, the void ratio 

changes during shear, and the value when the specimen reaches steady state is 

needed.

In the discussion that follows, undisturbed specimens are addressed first, and then the differences 

in the procedures for remolded specimens are presented. The procedures discussed below include 

some measurements that are not directly needed in the Poulos et al. (1985) procedure. However, 

these extra measurements provide important checks of the measurements that are directly needed. 

These procedures relate to soils ranging from slightly silty sands to slightly clayey silts. Note that, 

in this appendix, the term “sample” is used to refer to an entire tube sample obtained from the 

field. The term “specimen” refers to a portion of the sample trimmed from a tube or prepared in 

the laboratory.

7B.2 USE OF AVERAGE VOID RATIO

In all cases it is assumed that the average void ratio of the entire specimen during shear is representative 

of the strengths and pore-pressure measurements being made in the triaxial test. If localized shear 

straining develops, as evidenced by shear planes or localized bulging, this assumption is no longer 

valid because the average void ratio of the specimen is no longer representative of the soil being 

sheared. Tests with shear planes or localized bulging cannot be used unless localized measurements 

of void ratio are carried out using esoteric techniques such as impregnation with resins followed by 

microscopic examination.

Similarly, tests on specimens with significant stratification within the specimen (e.g., layers of slightly 

silty sand alternating with layers of slightly clayey silt) cannot be used, because the average void 

ratio is not representative of either soil type. Past experience with coal refuse indicates that careful 
7B–2

Appendix 7B

MAY 2009

selection of sampling zones based on CPT data has resulted in stratification not being an issue and 

not causing concentration of deformations in any zone within the test specimen.

7B.3 UNDISTURBED TUBE SAMPLING

Sampling in borings can be accomplished using a thin-wall tube (3-inch O.D. with wall thickness 

of one-sixteenth inch) with either a fixed-piston sampler with piston rods extending to the ground 

surface (Hvorslev sampler) or a hydraulically-actuated piston sampler (Gus or Osterberg sampler). 

Hand-carved samples can be obtained in test pits using a tripod tube sampler. Detailed sampling 

procedures for the Hvorslev sampler are presented in Appendix 7C. Similar procedures should be 

used for other samplers.

High quality galvanized-steel, brass, or stainless-steel tubes should be used. The cutting edge should 

have a clearance ratio of between 0.5 and 1.0 percent (Appendix 7C) and should be filed sharp and 

smooth immediately prior to use.

Measurements required during sampling are:

• Sampler penetration into the material (to the nearest millimeter).

• Length of recovered sample (to the nearest millimeter).

• Inside diameter of the sample tube cutting edge (to nearest 0.1 millimeter, using calipers).

• Inside diameter of the sample tube (to nearest 0.1 millimeter, using calipers).

The volume change during sampling is computed from:

• In-situ sample volume = distance pushed times tube area at cutting edge

• In-tube sample volume = sample length recovered times inside area of tube

When measuring the length recovered, it is expected that the bottom of the sample is flush with the 

bottom of the tube. If a section of the bottom of the sample is missing, it can be assumed that it fell 

during withdrawal of the sampler from the borehole and thus should be considered to be part of the 

measured length recovered.

There are potential errors in this part of the procedure. In general, the errors overstate any decrease 

in volume. For example, the sample may be flush with the bottom, but may have slid downwards. 

The shorter measured recovery would be assumed to be a volume decrease with an associated 

decrease in void ratio, which is conservative in the overall estimation of in-situ strengths. However, 

very large apparent void ratio decreases should be considered unreasonable, and the sample should 

not be used for testing.

Experience indicates that it is possible to keep volume changes during sampling generally below 

one percent, with the change being usually in compression, but some slightly clayey silts may actually 

expand slightly during sampling.

After the length of recovered sample is measured, a section of the sample at the bottom should be 

removed to allow installation of the packer. The tube should be maintained in an upright position 

at all times. The packer may have drain holes to allow drainage in sands, but should be solid for 

soils exhibiting any plasticity. The top should be cleaned and a loose packer or other disk placed 

on top of the sample. The distance from the top of the tube to the disk should then be accurately 

measured. This measurement should later be rechecked to determine if the sample settled during 

transport.
7B–3

Void-Ratio Measurements During Undisturbed Tube Sampling and Laboratory Testing

MAY 2009

7B.4 HANDLING AND TRANSPORTATION

Sample tubes must kept upright at all stages of handling and transportation. The tubes should be 

transported in private vehicles and cushioning should be provided around each tube and between 

the tube rack and the vehicle to minimize the transmission of vibrations from the vehicle to the 

samples. Freezing of the samples must be avoided. Any volume change that occurs during handling 

and transportation can be computed as the change in vertical distance from the top of the tube to the 

top of the disk resting on the sample. Volume changes that occur during transportation and handling 

are normally negligible even for sand, if reasonable care is taken.

7B.5 TUBE CUTTING AND EXTRUSION

Tube cutting must be performed using a method that does not produce vibration or deformations 

of the sample tube. Typically a tube cutter is used. Low pressure is applied to the tube, and stiffener 

rings are placed above and below the cut to minimize tube deformation. The tube must be maintained 

in a vertical position. The distance to the top of the sample will indicate if volume changes have 

occurred. If the described procedure is carefully followed, the volume changes during tube cutting 

should be negligible.

After the tube section containing the triaxial test specimen is cut, the ends of the specimen are typically 

trimmed away from each end of the tube section by about one-half inch, so that: (1) the ends of the 

tube section can be deburred and (2) a packer can be installed in the tube to keep the sample in place. 

The following measurements should be made:

• Total weight of the sample and tube (this is a back-up measurement)

• Length of tube section – length of the tube minus the distance from each end of 

tube to the ends of specimen (all measurements to the nearest 0.5 millimeter; each 

measurement should be the average of three points taken around the circumference 

of the tube).

• Inside diameter of the tube section (to the nearest 0.1 millimeter, using calipers).

The volume of the specimen in the sample tube after cutting and trimming can be computed from the 

above measurements.

From this point on, all soil in the tube section must be recovered and the dry weight measured. 

This requires that extreme care be taken because: (1) some soil will stick to the sample tube 

during extrusion, (2) some will stick to the membrane after triaxial shear, and (3) some will be 

used for index testing after the triaxial shear testing is performed. The total dry weight of soil, 

encompassing all of the above, and the measured volume of the specimen should be used to 

compute the void ratio.

The specimen must be extruded upward from the tube, i.e., in the same direction that it entered the 

tube during sampling. During extrusion, any soil that stuck to the tube and any other soil that for 

whatever reason did not make it to the triaxial cell must be recovered.

Depending on the ability of the soil sample to stand vertically, it may need to be extruded directly into 

a close-tolerance membrane stretcher using a membrane that is then used to transfer the sample to the 

triaxial cell pedestal. A small vacuum may need to be applied to the sample to complete placement 

of a second membrane and attachment of the drainage lines to the top cap. Membrane thicknesses 

should be measured prior to use. After the sample is safely in the triaxial cell under a small vacuum, 

measurements of sample length and diameter should be made, with the latter requiring correction for 

membrane thickness. Note that these are backup measurements.
7B–4

Appendix 7B

MAY 2009

7B.6 SATURATION

Once the triaxial cell is assembled, a cell pressure must be applied at the same time that the vacuum 

is released so that the effective stresses in the specimen remain about constant. As the vacuum is 

released, some water may enter the specimen as air bubbles are reduced in size, but this does not 

mean that there was a volume change.

Backpressure should then be applied keeping the effective confining stress constant. Note that the 

process must be done very slowly so that: (1) the backpressure is equalized at all times throughout 

the sample and (2) the pressure regulators are keeping up with the required pressures. The water that 

enters the specimen in this process does not correspond to a volume change but to compression of 

air bubbles in the voids of the soil and eventually their solution into the pore water. For undisturbed 

samples obtained below the water table, the volume change due to saturation should be negligible. 

For remolded specimens prepared at low water contents, there may be a collapse due to saturation 

and a substantial volume change, particularly for silts or very silty sands.

7B.7 CONSOLIDATION

Volume change during consolidation of the saturated specimens can be accurately determined from 

the water expelled from the samples. If the loading piston is attached to the top cap, height change 

measurements can also be made accurately, provided that an axial load is applied to the piston to 

compensate for the upward force in the piston caused by the cell pressure (in order to maintain the 

desired consolidation stress ratio – typically one). It is advisable to consolidate the sample in stages 

so that a compression curve (e versus log p) can be obtained. Comparison of compression curves 

among the various tests can be used as a means of judging unusual results. The changes in void ratio 

during the consolidation phase are by far larger than for any of the other stages including sampling, 

transportation, and extrusion.

7B.8 SHEAR AND DISASSEMBLY OF TRIAXIAL CELL

Since undisturbed samples are usually sheared undrained, there is no volume change during shear 

of saturated samples.

The procedure for disassembling the triaxial cell and removing the sample at the end of a test 

should allow for two key measurements, namely the water content and the dry weight of the full 

specimen. The specimen should be sliced in half longitudinally, and any stratification should be 

noted. A vertical slice should be taken for specific gravity measurement. For coal tailings, a specific 

gravity measurement should be made for every test specimen. For natural soil, 2 or 3 measurements 

should be adequate for a given soil layer. Other vertical slices may be taken for Atterberg-limits, 

grain-size, or hydrometer tests, but at least half of the specimen should be used for the water 

content measurement. The wet weight of the water content specimen should be measured quickly, 

before it begins to dry.

As mentioned above, all soil from the triaxial specimen must be recovered so that the full dry weight 

can be determined. This includes the soil used for index testing, soil stuck to the membrane or end 

caps, and any other soil trimmings. The wet weight of the water content specimen should be measured 

to the nearest 0.01 gram. The dry weight of the water content specimen and of all other material 

from the triaxial specimen and from the specimen removed during tube trimming should also be 

measured to the nearest 0.01 gram.

To make the test specimen firmer during disassembly of the triaxial apparatus, it may be desirable 

to reconsolidate the specimen at the end of the test, but, if this is done, the amount of water expelled 

during reconsolidation must be recorded. The water expelled during reconsolidation must be included 

in the computation of the water content of the specimen during shear.
7B–5

Void-Ratio Measurements During Undisturbed Tube Sampling and Laboratory Testing

MAY 2009

7B.9 COMPUTATIONS OF VOID RATIO

Void ratio computations are discussed in the following text and are summarized in Table 7B.1. The 

following notation is used in the table:

Ws dry weight

D diameter

H height or length

V volume

w water content

e void ratio

γ unit weight

γw unit weight of water

Gs specific gravity of soil grains

Δ incremental change

Stages of sampling and testing are noted by the following:

is in situ

t in tube after sampling

tt in tube after transportation

ts in tube section after cutting the sampling tube and trimming the ends

of the test specimen

txi in triaxial cell – initial condition under small vacuum

txc in triaxial cell – after consolidation and during undrained shear

The required values of void ratio are:

• Void ratio during shear in the triaxial cell

• Void ratio in situ

The void ratio during triaxial shear is computed simply from the measured water content of the 

specimen at the end of the test, using the relationship Gs w = e (assuming 100 percent saturation). As 

mentioned previously, for coal tailings a specific gravity measurement should be made for every 

test specimen. For natural soil, 2 or 3 specific-gravity measurements should be adequate for a given 

soil layer.

Sampling, transportation and tube cutting can result in changes in void ratio from the in-situ condition 

that can be determined directly from the measured changes in volume, using the formula:

Δe/(1 + e) = ΔV/V

Changes in void ratio should be averaged for the full tube sample. Subsequent to tube cutting, volume 

changes and resulting void ratio changes are associated with the tube section from which the triaxial 

specimen is extruded.

The table at the end of this appendix lists the measured and computed quantities considered to be 

the primary values, the formulas used, and the secondary values that can be used to compare with 

the primary measured or computed values. The primary measured and computed quantities should 
7B–6

Appendix 7B

MAY 2009

TABLE 7B.1 VOID RATIO MEASUREMENT FOR UNDISTURBED TUBE SAMPLES(1)

 Stage Volume (V) Dry Weight 

(Ws)

Void 

Ratio 

(e)

Comments

1. In situ

Change ∆V (meas.)

e(is)

 ↑

∆e (2)

 ↑

e(is) = e(t) + ∆e (during sampling)

Measure ∆V from V (sampled) versus V (recovered).

Compute ∆e.

2. Full tube after

 sampling

Change ∆V (meas.)

 ↑

e(t)

 ↑

∆e (2)

 ↑

e(t) = e(tt) + ∆e (during transport)

Measure ∆V from the height change of the sample 

during transport between the field and the laboratory

3. Full tube after

 transport, but

 before cutting

Change ∆V

 ↑

e(tt)

 ↑

 ↑

∆e (2)

 ↑

e(tt) = e(ts) + ∆e (during cutting)

Measure ∆V from the height change of the sample 

measured during tube cutting. ∆V is normally zero.

Compute ∆e (normally zero).

4. Tube section

 after cutting

 and trimming

Change

V(ts) → → → Ws(ts) → →

 ↑

 ↑

 ↑

 ↑

 ↑

 ↑

∆Ws

 ↑

e(ts) (3)

Measure V(ts) from the I.D. of the tube and sample 

length. Compute e(ts) for V(ts) and Ws(ts). From this 

point on, all solids should be saved so that Ws(ts) can 

be measured. The sample in the tube section should 

also be weighed. This can later be used to estimate 

the water content and saturation of the sample at this 

stage.

∆Ws is the soil left in the tube during extrusion.

5. In triaxial cell,

 following ex-

 trusion from

 tube

Change

V(txi) (meas.)

V(txi) (comp.)

 ↑

 ↑

 ↑

∆V

 ↑

≈ Ws(txi)

= Ws(txi)

e(txi) (3)

V(txi) (meas.) is from measurements of triaxial 

sample when set up in the cell under small vacuum.

V(txi) (comp.) is from V(txc) and ∆V during 

consolidation. Large differences between the two 

indicate a potential problem.

∆V is measured during consolidation.

6. In triaxial cell

 after consolid

 ation and during

 undrained shear

 ↑

V(txc) (3,4) ←

(Not necessarily needed, 

but good 

practice.)

Ws(txc) → →

w(txc)

(Computation

starts after 

above are 

measured.)

e(txc) (4)

Use w and Gs after test (txc) to compute consolidated 

as-tested void ratio, considering that the sample is 

saturated at this point. This is the primary method 

for obtaining e(txc). Also measure Ws for the entire 

sample. Ws is needed for computation of e(ts) in 

Stage 4. Ws may also be used to obtain V(txc). V(txc) 

and ∆V measured during consolidation can then be 

used as a check on V(txi).

 Note: 1. Measurement of ∆V are made from Stage 1 onward, as sampling and testing proceed. The arrows indicate

 the order of void ratio computations. Computations begin following measurement of Ws(txc) and w(txc) in

 Stage 6. Computations proceed along the path of the arrows.

2. ∆e = (1 + e) ∆V/V

3. e = (GsγwV/Ws) – 1

 V = Ws (1 + e)/(Gsγw)

4. e = Gs w (w is in percent, assumed to be 100 corresponding to complete saturation.)

be used for estimating in-situ strengths. However, there may be cases where comparison of primary 

and secondary quantities may indicate that the secondary quantities are more believable and should 

be used. Some measurements mentioned in the text are not included in the table, but they can also be 

used to compare to the primary values.
7B–7

Void-Ratio Measurements During Undisturbed Tube Sampling and Laboratory Testing

MAY 2009

As previously discussed, consolidation should be performed in stages. However, for clarity, 

consolidation is treated as a one-stage process in the table.

7B.10 REMOLDED SAMPLES

The purpose of testing remolded samples prepared at various void ratios from a uniform batch of 

soil is to determine the steady-state line for the soil batch, the slope of which is needed for correcting 

the results from testing of the undisturbed samples. Thus, primarily, we are interested in the void 

ratio during shear, i.e., the last line in Table 7B.1. However, in planning the tests one targets specific 

after-consolidation void ratios and confining pressures at which to perform tests so as to achieve 

sufficiently wide coverage of void ratio and stresses in the steady-state plot. For this purpose, it is 

important to develop the compression curves for each test (by consolidating in stages) and also to 

estimate void ratio changes, if any, that would be expected upon saturation.

Assuming that the samples are prepared in a mold of known volume, the process of computing void 

ratios at various stages is similar to the one shown in the table for undisturbed samples, except that 

it starts at the “tube section after trimming” stage, which is equivalent to an “in the compaction mold 

stage” for the remolded samples.

Often when samples are compacted at relatively low water contents, there is substantial compression 

upon saturation. Thus the volume change from the txi to the txc stage includes not only the effect of 

consolidation, but also the effect of saturation. Since saturation would occur under backpressure, it 

would occur with the cell assembled, and thus there would not be access to the sample to measure its 

height and diameter after saturation but prior to consolidation. An alternative procedure would be to 

cause near-saturation by circulating de-aired water very slowly through the specimen with vacuum 

applied at the upper end while slightly opening the lower valve to allow the de-aired water to be 

sucked in, but not opening the valve so much that the vacuum is lost at the lower end of the sample. 

This process will not result in full saturation, but it will cause collapse and thus allow measurement 

of the sample prior to cell assembly and the application of cell and backpressures.
Appendix 7C

PROCEDURE FOR UNDISTURBED, FIXED-PISTON

SAMPLING OF COHESIONLESS SOIL

MAY 2009 7C–1

The procedures presented herein have been developed for a fixed-piston sampler with piston rods (actuating rods) that extend up to the ground surface (often referred to as a Hvorslev sampler). Similar procedures should be employed for sampling using hydraulically-actuated samplers or tripod-tube samplers.

7C.1 ADVANCING THE HOLE

7C.1.1 General

1. The hole should be started as close to vertical as possible. This is necessary because the 

actuating rods will be placed through a bracket 10 feet directly above the borehole. If the 

hole is not vertical, the actuating rods will bend. Also, when the drill rods are pushed, 

they may slip off the drill head if they are not vertical.

2. The drill head should be almost all the way forward when starting the hole. This allows for 

maximum horizontal travel of the head. Usually, drillers will do this as routine procedure.

7C.1.2 Casing

1. It is best to use 4-inch-I.D. casing. If larger-diameter casing is used, the velocity of the 

drill fluid return is reduced, which increases the amount of coarse wash in the borehole. 

If the casing is smaller, the sampler will be a tight fit. Either flush-joint casing or hollowstem augers can be used depending on soil conditions.

2. In general, the casing should be advanced to within about 1 foot of the proposed sampling depth. However, this may not be necessary if thick drilling fluid is used and the 

borehole remains open below the casing. This is referred to as “open-hole” drilling. If 

Revert drilling fluid is used in open-hole drilling, it is prudent to advance the casing to 

the bottom of the borehole at the end of the work shift.

7C.1.3 Drilling Fluid

1. It is generally best to use a drilling fluid that has a high unit weight, such as bentonite 

mixed with water, rather than plain water. This will help to carry cuttings to the surface 

and will increase the pressure on the bottom of the sample during withdrawal from the 

borehole. Usually, the governing factor in preparing the drill fluid is the ability of the 

pumping equipment to circulate the fluid through the drill rods. It is advantageous to 

use a drill fluid that has a slippery feel.

2. When drilling through a dam, the potential for hydraulic fracturing at the bottom of the 

borehole should be evaluated. If hydraulic fracturing is possible, special drilling procedures should be used. These procedures may involve maintaining the fluid level in the 

borehole well below the ground surface and using augers to advance the boring. Tube 

samples can be obtained through large hollow-stem augers, or small-diameter solid 

augers may be used to clean out the hole inside casing.
7C–2

Appendix 7C

MAY 2009

3. Several types of drilling fluids are available. Some of these include Revert, Quick Gel, 

and Kim-Mud. Revert is an organic material that decomposes a few days after mixing 

with water. It is commonly used if piezometers or a groundwater observation well are 

to be installed in the borehole. All three of these muds have worked well for removing 

sand-sized particles from boreholes.

7C.1.4 Mud Tub

It is important to have a mud tub with baffles that create settling basins for the cuttings. A mud tub 

with three or more settling basins is preferred. Cuttings should be shoveled out of the mud tub at 

frequent intervals to keep the drill fluid clean.

7C.1.5 Hole Advancement

1. Drill bits must have deflectors that prevent drill fluid from jetting downward and disturbing the material below the drill bit. Tricone roller bits are available with a deflector 

attached to one of the cones that prevents drill fluid from jetting directly downward. It 

may be necessary to weld a bead on fishtail or chopping bits to deflect the drill fluid. 

Some bits have side or upward discharge of the drill fluid. The bit should be checked by 

flushing water through it above the ground surface.

2. Drill bits should be slightly smaller than the I.D. of the casing. If 4-inch casing is used, 

the drill rods should be N or NW size to reduce the annular space between the casing 

and drill rods. This will increase the upward velocity of cuttings and will help to reduce 

wash at the bottom of the borehole.

3. Advancement of the bit should be done at a slow rate of about 1 foot per minute maximum. Drill fluid pressure during drilling should be kept low to minimize disturbance 

to the material. After the required depth is reached, the bit should slowly be lifted a distance of about 3 feet above the bottom of the borehole. Then the drill fluid pressure can 

be raised to wash cuttings out of the hole.

4. The drill bit and rods should never be allowed to rest on the bottom of the borehole. 

They should always be suspended from the drill rig. This is important so that potentially 

loose material is not densified or disturbed by the weight of the drill rods.

7C.2 PREPARATION OF SAMPLER

7C.2.1 Definition of Sampler Parts

 TABLE 7C.1 STATIONARY PISTON SAMPLER WITH STEEL TUBE

Size Sample Tube Length Rod

Conn. Part No. Weight

(lb) (kg)

2” O.D. x 17⁄8” I.D.(1)

(50.8 x 47.6 mm)

30”

762.0 mm AW 22056-16 25.0 11.3

21⁄2” O.D. x 23⁄8” I.D.

(63.5 x 60.3 mm)

30”

762.0 mm AW 22053-16 28.0 12.6

3” O.D. x 27⁄8” I.D.(1)

(76.2 x 73.0 mm)

30”

762.0 mm NW 22041-43 30.0 13.6

31⁄2” O.D. x 33⁄8” I.D.

(88.9 x 85.7 mm)

30”

762.0 mm NW 22057-34 32.0 14.5

41⁄2” O.D. x 403⁄8” I.D.

(113.7 x 110.5 mm)

30”

762.0 mm NW 22065-34 36.0 16.3

Note: 1. Meets ASTM, AASHTO, DCDMA Standards for tubes.
7C–3

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7C.2.2 General Operation of Sampler

With the sampler at the bottom of the borehole and the actuating rods fixed relative to the ground 

surface, the drill rods are pushed down. All of the parts, except the actuating rods and piston, move 

down. The locking cone allows one-way (upward) movement of the actuating rods and piston relative to the sampling tube. After the push, the piston cannot move down because the locking cone jams 

the actuating rods in place.

7C.2.3 Sampler Tube Preparation

7C.2.3.1 Cutting Edge

1. Before the drilling program begins, the cutting edge of each sampling tube should be 

machined to achieve the desired clearance ratio (CR) defined as:

CR = 

DIT - DCE

 DCE

where:

DIT = inside diameter of tube (should be 2 7⁄8 inch)

DCE = inside diameter of cutting edge

Usually, there is less chance of sand sliding out the bottom of the tube if the clearance 

ratio is relatively high. However, more disturbance of the sand occurs when the clearance 

ratio is relatively high because the sand expands outward to meet the sides of the tube.

Typically, sampling tubes obtained from Acker Drilling Company have clearance ratios 

of about 1.3 to 1.5 percent. For sand sampling, this clearance ratio is too high. Tubes 

should be machined to have clearance ratios between about 0.5 and 1.0 percent.
7C–4

Appendix 7C

MAY 2009

TABLE 7C.2 OPTIONS AND SPARE PARTS – STATIONARY PISTON SAMPLER(1)

No.

Diameter and 

Head Thread 

Connection

2” O.D. 21⁄2” O.D. 3” O.D. 31⁄2” O.D. 41⁄2” O.D.

AW Wt.

(lb)

(kg)

AW Wt.

(lb)

(kg)

NW Wt.

(lb)

(kg)

NW Wt.

(lb)

(kg)

NW Wt.

(lb)

Name Part No. Part No. Part No. Part No. Part No. (kg)

1 Head 120140-9 2.0

0.9 1201140-7 6.02.7 120140-10 6.02.7 120140-12 8.03.6 120140-16 11.04.9

2 Clamp Spring 120136 (1) 120136 (1) 120136 (1) 120136 (1) 120136 (1)

3 Cone Clamp 

Assembly 220042-1 (1) 22042-1 (1) 22042-1 (1) 22042-1 (1) 22042-1 (1)

4 Plug 120221 3.0

1.3 120189 5.02.2 120142 7.03.1 120226 10.04.5 120243 12.05.4

5

Socket Head 

Cap Screw

(4 required)

120660 (1) 120652 (1) 120652 (1) 120652 (1) 120652 (1)

6 Packing Cup

(2 required) 150045 (1) 150045-14 (1) 150045-11 (1) 150045-12 (1) 150045-18 (1)

7 Piston Spacer

(2 required) 120222 (1) 120190 (1) 120138 1.00.45 120145 1.00.45 120297 1.00.45

8 Piston 120223 (1) 120295 1.0

0.45 120143 1.00.45 120225 1.00.45 120242 2.00.90

9 Lock washer 90399-04 (1) 90399-065 (1) 90399-08 (1) 90399-10 (1) 90399-15 (1)

10 Locknut 90400-04 (1) 90400-064 (1) 90400-08 (1) 90400-10 (1) 90400-15 (1)

11 Steel Tube – 30” 120021-4 3.0

1.3 120086-4 4.01.8 120037-4 5.02.2 120093-11 7.03.1 120095-4 7.03.1

11 Brass Tube 

– 30” 120245-4 3.51.5 120246-4 4.001.8 120038-4 6.02.7 120092-10 6.02.7 120094-4 7.03.1

11 Stainless Steel 

Tube – 30” 120245-4 3.51.5 120246-4 4.01.8 120230-1 5.02.2 120027-7 6.52.9 120244-4 7.03.1

12 Piston RodMaster 120139-1 1.5

0.67 120139-1 1.50.67 120139-1 1.50.67 120139-1 1.50.67 120139-1 1.50.67

(3) Act. Rod – 2 ft 120219-2 1.5

0.67 120219-2 1.50.67 120219-2 1.50.67 120219-2 1.50.67 120219-2 1.50.67

(3) Act. Rod – 5 ft 120219-4 5.0

2.2 120219-4 5.02.2 120219-4 5.02.2 120219-4 5.02.2 120219-4 5.02.2

(3) Act. Rod –

10 ft. 120219-5 7.03.1 120219-5 7.03.1 210219-5 7.03.1 210219-5 7.03.1 210219-5 7.03.1

Note: 1. For optional tubes, see Item 11.

1. Less than one pound or 0.45 kilogram.

2. Not shown.

(REPRODUCED FROM ACKER DRILL COMPANY INC. SOIL SAMPLING TOOLS CATALOG, 1978)

2. Any nicks in the cutting edge should be repaired. A small machinist’s file is useful for 

this purpose.

3. The cutting edge I.D. should be measured at three to four locations so that an average 

cutting edge diameter can be determined. Also, the I.D. and O.D. of the tube should 

be checked. They are normally exactly 27⁄8 and 3 inches. Field calipers should be used. 

Make sure that the calipers are aligned properly.

7C.2.3.2 Deburring and Cleaning

1. The four holes at the top of the tube should be deburred using a file so that the plug 

can be inserted into the top of the tube.

2. Wash all steel filings from the inside of the tube.
7C–5

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7C.2.4 Sampler Assembly

7C.2.4.1 Initial Cleaning of Parts

1. Locking Cone – should be free of all sand. Ball bearings should rotate freely. Spray with 

WD-40 (or similar light lubricant) to lubricate bearings.

2. Spring – a new spring should have the ends turned inward so that the ends do not 

become jammed between the locking cone and the plug. Clean spring of all sand.

3. Piston Rod – make sure that the beveled edge occurs on the threads of the piston rod 

only. The bevel should not extend below the threads. The piston rod must be deburred 

(filed smooth) above the threads so that the locking cone slides freely along the shaft. 

This should be done between each sampling attempt. The female threaded end of the 

piston rod should be thoroughly cleaned and lubricated with grease.

4. Plug – squirt water through the ports to clean out the sand and mud. Clean the female 

threads in the four side holes. Make sure the beveled hole at the top is free of sand and 

that the male threads are free of sand.

5. Head – clean thoroughly. Make sure there is no sand in the female threads.

6. Piston – make sure that sand is removed from the area behind the leathers. Clean the 

female threads with water. Make sure that the piston rod screws into the piston very 

easily (do not grease these threads). Leathers, which are referred to as packing cups (Part 

6 in Table 7c.2), should be kept in water so that they remain pliable. Leathers should not 

have major cracks. The leathers should be coated with a thin film of high vacuum grease.

Initially, the piston and piston rod should be checked to make sure they are able to hold a vacuum 

beneath the piston. Put the piston near the bottom of a sampling tube with the piston rod screwed in 

position and place in a bucket of water. Pull up on the piston rod to check if water can be raised into 

the sampling tube.

7C.2.4.2 Assembly

1. Screw the piston rod tightly to the piston.

2. Place the tube horizontally on a table.

3. Push the piston from the top to the bottom of the tube. Make sure the piston bottom is 

flush with the cutting edge.

4. Install the plug and four screws. The screws should be finger tight.

5. Slide the cone into position. Use a screwdriver to push it in as far as possible.

6. Push hard on the piston rod to make sure the cone is seated. Check that piston has not 

moved out of the tube more than one-sixteenth inch.

7. Measure the stick-up of the piston rod above the plug top. This measurement will be 

used to determine the actual movement of the piston during sampling.

8. Install a rubber or plastic seal around the actuating rod and push it down over the locking cone. This seal can be made from a plastic end cap typically used to seal the ends 

of the tubes or from an old rubber glove. This seal minimizes the entry of sand into the 

cone. Failure of the locking cone has been attributed to sand getting below the cone preventing movement of the cone required for locking the piston rod.

9. Install the spring over the cone.

10. Install the head. Make sure that the spring seats properly into the recess in the head. Put 

thick strings between the head and plug and grease the threads. Tighten as tight as possible by hand. Later, this connection will be broken while soil is in tube. The string and 

grease make it easier to break the connection without shaking the tube.
7C–6

Appendix 7C

MAY 2009

11. If an NW-AW sub is used, record the stick-up of the actuating rod above the sub. This 

will be used to determine if the actuating rods and piston moved up in the tube while 

the tube was being lowered down the borehole.

12. Put the plastic cap over the cutting edge of the tube to protect it.

13. Do not rest the sampler on the piston because the piston can move up into tube.

7C.3 SAMPLING PROCEDURE

7C.3.1 Preparation of Actuating Rods inside Drill Rods

All actuating rod threads should be cleaned and greased.

7C.3.1.1 Separate Drill Stem and Sampling Stem

It is often more efficient to use two sets of drill rods (with only one set down the hole at any one 

time). One set would have actuating rods inside and be used for sampling only. Using two sets of 

drill rods is especially beneficial when the depth of the hole extends beyond 30 feet.

7C.3.1.2 One Drill Stem

1. After drilling is completed, actuating rods can be placed down the drill rods and rest on 

the roller bit so that the entire string can be lifted out as a unit.

2. Protection for the male actuating rod thread at the bottom should be provided.

3. It is important to have a bleeder pipe (2-foot length of rod with holes through the side) 

just above the drill bit to allow the escape of drilling fluid from the rods.

4. It is desirable to have a short length of actuating rod at the bottom of the actuating rod 

string so that the actuating rods stick up above each corresponding drill rod by about 2 

to 18 inches.

7C.3.2 Attaching Sampler to Drill Rods

1. Have drill rods ready with the actuating rods inside.

2. Make sure that the bleeder pipe is at the bottom of the drill stem.

3. Screw the sub part way onto the bottom of the drill rods.

4. Put a vise grip at the bottom of the actuating rods and a vise grip on the piston rod.

5. Raise the drill rods high enough to attach the sampler.

6. The driller should hold drill rods so they don’t move. They usually extend far above the 

drill rig and must be held securely.

7. The helper and inspector attach the tube – do not allow the bottom of tube to rest on any 

surface.

a. Connect the actuating rod to the piston rod with vise grips, making sure the 

piston rod is not turned.

b. Unscrew the sub from the drill rods and screw the sub onto the sampler 

head. Make sure a string is in the joint.

c. Lift the sampler and the actuating rods that have been attached to the piston 

rod, and screw the sampler to the drill rods (this is the hardest part).

8. Remove the plastic cap and check that the piston has not moved. Note how much the 

piston is protruding below the cutting edge. This procedure is easier if NW drill rods are 

used because then an NW-AW sub is not required. The procedure for attaching the sampler to the drill stem will vary from one project to another and with the available drilling 

equipment.
7C–7

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7C.3.3 Lowering Sampler down Hole

1. Carefully place the tube into the hole without nicking the cutting edge. If the cutting 

edge is nicked, start over.

2. Slowly lower the sampler down the hole. When connecting actuating rods, never 

turn the lower actuating rod as this will release the vacuum port in the piston. Measurements of the stick-up of the actuating rods above the drill rods should be made 

whenever drill rods are joined as a check on the location of the piston in the sampling tube.

3. Stop lowering sampler about 1 to 2 feet above the bottom of the hole.

4. Measure the stick-up on the actuating rods. Is it correct? If the sampler hit a lot of 

wash, the actuating rods will be higher. String between joints of drill rods may make 

the drill stem slightly longer. Measure the drill rods if a problem is suspected.

5. Fix the actuating rods to the drill rods with a special coupling with a set screw. See 

sketch below.

6. Put a keel line just above the coupling.

7. Very slowly, with the hydraulic cable (or cathead and rope) and pull ring, lower the 

sampler until it rests on the bottom. First put a keel mark on the drill rods so that the 

keel mark will line up with the top of the casing when sampler is supposed to reach 

bottom. This will warn when the sampler is getting close.

8. Check to make sure that the actuating rods did not slip in the coupling.

7C.3.4 Preparation for Push

Make sure that there is a stable surface for making measurements of the drill rod relative to the 

casing. Use the top of the casing itself, or the pull-plate resting on the casing.

1. As soon as the sampler is resting on bottom of borehole, measure the actual depth of 

the bottom of the piston sampler.

2. Put a mark on the drill rod 24 inches above reference. Do this immediately, in case 

the sampler creeps down before push.

3. Set up frame to fix actuating rods. The frame shown on the following page is 

suggested:
7C–8

Appendix 7C

MAY 2009

The 2-ft AW rods driven into the ground should be as close as possible to the mud tub to minimize 

the span length of the upper cross bracket.

Fixing the actuating rods to the rig has proven unreliable because the rig can move (lift up) during 

sampling. Using cables to hold the actuating rods is very bad because they allow movement due to 

slack in cables.

4. Tighten set screw in top cross-bracket (very tight). Place a vise grip very tightly on 

actuating rod just above top coupling as a safety measure to keep actuating rods from 

moving down.

5. Measure the distance from the bottom of the top coupling to the reference plate (a1) to 

the nearest one-sixteenth inch.

6. Measure distance from reference plate to an arbitrary point on the actuating rod just 

above drill rods (b1) to nearest one-sixteenth inch. This will give the actual movement of 

the actuating rod, including slippage if it occurs.
7C–9

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7. Check that mark on drill rods is still 24 inches (c1) above reference plate. This will be 

used to determine actual push length.

8. Measure distance from a reference plate on the ground to a mark on the AW rods (d1). 

This will be used to determine whether the support footings move during sampling.

9. Determine the required rate of push and have driller set controls to achieved desired 

rate. The driller can experiment with rig controls.

On many drilling rigs, the hydraulic pressure gauge indicating downward pressure on 

the drill head will be very useful. If gravel is present in sandy soils, the tube may crumple if the cutting edge hits a large piece of gravel. By experimentation, a limit hydraulic 

pressure can be set which, if exceeded, would mean that the tube was hitting gravel, 

and, if not exceeded, would mean that the tub penetrated relatively easily for the entire 

push length.

On some drilling rigs, the hydraulic pressure gauge will not be informative because it 

is not sensitive to resistance against the drill head. Before sampling, determine how the 

hydraulic pressure gauge works.
7C–10

Appendix 7C

MAY 2009

a. Determine hydraulic pressure for the drill head moving down with no 

resistance (reference pressure).

b. Determine hydraulic pressure change for the case of the rig weight 

acting on the drill stem.

c. Decide if monitoring the gauge during sampling will be useful

10. It is important to have a stable connection between the drill head and the drill rods 

during the push.

On most drilling rigs, the drill heads are rounded and do not provide a good pushing 

surface. A special bracket can be fabricated to allow the head to push down on the drill 

rods, as shown in the sketch below:

Alternatively, a right-angle block that allows direct pushing on the drill rods can be 

attached to the drill head.

7C.3.5 Advancing Tube

7C.3.5.1 From Weight of Rods

With the actuating rods fixed to top bracket, release the actuating rods from the drill rods. Then remeasure a, b, c, and d and record on data sheet. Compute the following:

1. Penetration due to weight of rods = c1 – c2

2. Movement of actuating rods = b1 – b2

3. Movement of cross bar = a1 – a2

4. Movement of support rods = d1 – d2
7C–11

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7C.3.5.2 From Drill Head Push

1. The tube should be pushed in a smooth continuous motion, if possible.

2. The driller should be given a mark on the drill stem to watch.

3. It is prudent to push only 22 to 23 inches because there is not much space (less than 1 

inch) between the top of the piston and the bottom of the plug when a 24-inch push is 

completed. If fines have settled out on top of the piston and the fines encounter the plug, 

the soil in the tube will be compressed and the actuating rods will probably slip in the 

cross frame.

4. The inspector or helper should time the push. If necessary, the hydraulic pressure gauge 

should be observed by the inspector. The driller should watch the mark on the rods.

5. After pushing the tube, back off the drill head of the rig.

7C.3.6 Post-Push Measurements

1. Total Penetration – The total penetration (c1 – c3) includes penetration from the 

weight of the drill rods.

2. Movement of Actuating Rods – Measure and record b3. Record the net movement of 

the actuating rods

3. Movement of Actuating Rod Support – The actuating rods are now fixed to the 

bracket and are in tension and may have pulled the support bracket down. Measure 

and record a3. Record the net movement of actuating rod supports.

4. Movement of Support Footings – Measure d3 and record. Also record the net movement of the support footings.

5. Release Actuating Rods from Bracket – Have the driller release the actuating rods 

from the top of the bracket. Measure and record a4 (top bracket may have sprung 

upward when it was released from the act. rods). Measure and record d4. Record net 

movements of a, b, and d.
7C–12

Appendix 7C

MAY 2009

7C.3.7 Lifting Sampler up Borehole

1. Disassemble actuating rod support frame.

2. Put a full head of drill fluid in the borehole.

3. Rotate tube about twice – initially, the drill rod joints will come together. The actuating 

rods should not be tightened to drill rods. As soon as the actuating rods begin turning 

along with the drill rods, stop rotation. Measure b5 and c5 (initial distances from a reference to points on the actuator rods and the drill rods). Rotate the rods two full revolutions (or less, if specified), then re-measure b6 and c6. Compute c5 - c6 (penetration 

during shear). Rotation should be performed more than 10 minutes after the push.

4. Lifting the tube the first 2 feet is one of the most critical parts of the operation. As the 

sampler is lifted, a vacuum will develop at the bottom of the tube tending to suck the soil 

out of the tube. The tube should be lifted as slowly as possible without jarring or vibrating the drill stem. At the same time that the tube is being lifted, the drill stem should be 

rotated carefully in a manner such that the rods do not whip and the wrench does not 

slip. Lifting the first 2 feet has been accomplished as follows:

a. A swivel can be attached to the drill stem and hydraulic cables used to lift the 

tube. The hydraulic cables should be operated slowly enough to lift the tube 

in 20 to 30 seconds.

b. On some drilling rigs, the cable hydraulics are not sensitive enough to lift the 

tube smoothly. In such cases, a rope can be wrapped around the drill head 

and tied around the drill rods. The drill head can then be raised as slowly 

as possible, using the rope to facilitate rotation of the drill stem. A full head 

of drilling fluid should be maintained in the borehole during this operation. Note if the drill fluid drops temporarily when the tube has been lifted 

approximately 24 inches. If the fluid drops, this may indicate that the soil is 

in the tube and the fluid has dropped into the hole formed.

5. If two drill rod stems are being used, slowly lift the drill rods and actuating rods out of 

hole at less than 1 foot per second. Maintain head. If the rod joints must be broken, make 

sure that rope has been placed in the joints. When breaking actuating rods, use vise grips 

to make sure that lowermost actuating rod does not move.

6. If one drill rod stem is being used, before lifting rods, remove as many actuating rods 

from hole as possible. If this procedure is used, the actuating rod joint at the top of the 
7C–13

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

piston rod should have been made the loosest and be well greased and not tightened 

all the way. Other actuating rod joints should have been tightened very tight with vise 

grips. Then, when unscrewing the actuating rods at the surface, all the actuating rods 

above the piston rod can be removed. This speeds up the operation considerably.

7C.4 SURFACE HANDLING OF SOIL-FILLED SAMPLER

7C.4.1 Removal of Sampler from Borehole

1. Maintain a head of drilling fluid at all times. When the sampler nears the surface, make 

sure that the water flowing into the casing is not too turbulent, which could cause erosion of the soil at the bottom of the tube. A stocking over the end of the hose can be used 

to reduce turbulence.

2. When the bleeder pipe is observed, reduce the rate of movement to as slow as possible, 

but do not decelerate too rapidly.

3. As soon as the bottom of the sampler surfaces, slide a plate under the sampler to prevent 

any soil from falling out. Do not put fingers under the cutting edge of the tube. Wait 

until the drill stem is stable.

4. Start sliding the plate away. Be very careful that fingers are not placed directly under the 

cutting edge.

a. If soil starts falling out, put the plate back. Have a piece of foam rubber 

approximately 3 inches in diameter and approximately 2 inches thick ready 

for placing at the bottom of the tube. Make a quick switch of plate and foam 

rubber and place a plastic cap at the bottom of the tube. Tape the plastic cap 

in place. The foam rubber will conform to the soil surface and provide resistance to slippage of the soil.

b. If soil stays in, which it probably will, make a quick observation of the shape 

of the bottom soil surface and record later. If soil is flush with the bottom, 

install plastic cap and tape. If soil is missing up to 2 inches from the cutting 

edge, put a cylindrical piece of foam in the bottom and cap. If soil is far up 

into tube, install a vented packer with an extension used for tightening. Filter 

paper should be placed on the packer.

5. Disconnecting the Sampler. Make sure that the sampler does not rest on a surface with the 

weight of the drill rods on it, because it could bend or kink.

a. If the actuating rods are left inside the drill rods during withdrawal of the 

sampler, the following procedure is suggested: (a) break the joint at the 

bottom of the 2-foot bleeder pipe, (b) lift the drill stem with rig hydraulics, and (c) unscrew the actuating rods.

Do not let the sampler drop. A piece of foam can be placed under the 

sampler so that when it is unscrewed it will drop approximately 1⁄4 to 1⁄2 

inch onto foam (there may be considerable weight above from the actuating rods and soil in the tube).

b. If the actuating rods are removed prior to bringing the sampler to ground 

surface, unscrew the sampler at a convenient joint.

7C.4.2 Installation of Bottom Packer

The bottom packer is installed while the piston is still in place so that a vacuum holds the soil in the 

tube. Do not turn the tube upside down.
7C–14

Appendix 7C

MAY 2009

The sampler must be suspended above ground so that the bottom can be worked on. This can be done 

by mounting a chain clamp to a stationary object free from vibrations (i.e., a work bench or tree, but 

not the drill rig or other equipment). The head of the sampler (not the tube should be strapped in, 

allowing the bottom to be worked on.

Alternatively, the sampler can be suspended by a rope from the derrick of the drill rig. The 2-foot 

bleeder pipe should be kept on the sampler so that a swivel can be attached. The rope should be tied 

off at the rig. This method is less efficient than the chain clamp method because the drillers cannot 

advance the hole while the sample is being worked on by the inspector.

1. Remove temporary bottom cap. Note condition of cutting edge and sketch if it is 

distorted.

2. Measure distance from cutting edge to soil surface. Sketch profile. Record.

3. Remove soil with putty knife. Save cuttings in baggie and jar.

4. Prepare final bottom surface with trimming tool. Make flat. Final surface should be 

13⁄8 inch or more from cutting edge for packer to fit.

5. Clean inside wall of tube so that packer will not slip.

6. Measure to soil at three locations.

7. Install packer, vented, with filter paper. Pull down hard on packer to confirm that it 

is tight.

8. Measure at three locations to bottom of packer.

9. Install bottom plastic cap with holes for drainage.

7C.4.3 Removal of Piston

Carefully place the sampler on foam and support it so it is stable and wrapped with foam or 

mounted in a chain clamp to minimize vibrations.

Carefully unscrew the joint between the head and the plug. Remove the head.

1. Remove spring.

2. Clean top of plug and around cone. A squeeze bottle may be used.

3. Measure from top of plug to top of actuating rod. Record distance from tube bottom 

to piston. Is this the same as the penetration distance?

4. Remove cone: using vise grips, rotate piston rod clockwise while putting sideways 

force on piston rod. The cone will climb the piston rod. After removing the cone, 

screw the piston rod back down to plug piston.

5. Remove plug.

6. Measure distance from top of tube to top of wash resting on piston. Did the wash 

push up against plug?
7C–15

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7. Remove fluid and wash resting on top of piston.

a. This can be accomplished by sucking into a long tube and/or using a 

squeeze bottle.

b. Be sure to clean the piston around the piston rod so that air (not fluid) can 

travel around piston rod to release the vacuum.

8. Unscrew piston rod to release vacuum. Screw piston rod back in part way to pull out 

piston.

9. Pull out piston slowly. Use piston rod to loosen leathers by swaying piston rod back 

and forth. Can use vise grips and wood block to lift if it cannot be done by hand. 

If vacuum is not releasing, soil may be stuck to piston. Stick a wire through port to 

pierce soil. It is important to prevent soil and fluid above piston from getting below 

piston.

10. When the piston is removed, note if soil is stuck to the piston and record thickness. 

This is part of gross recovery.

7C.4.4 Cleanout of Top

1. Measure distance from top of tube to fluid above soil and record.

2. Measure distance from top of tube to soil at three locations and record.

3. Record gross recovery and net recovery

4. Was the fluid level at the same locations as the bottom of the piston? Remove a 

sample of the drill fluid above the soil. Is it water or drilling fluid? Is it from the 

pores of the soil?

5. Remove all fluid resting on top of soil.

6. Use trimming tool to remove wash from top.

7. Note: If suction developed below the piston while the piston was being removed, 

some soil may have lifted. Slight pressure from a ruler will cause this to drop down.

8. Use foam rubber to remove slop from top of soil. Make sure top of soil is nearly 

level.

9. Measure to top of soil in three places. Record.

7C.4.5 Installation of Top Packer

1. Prepare the bottom half of a vented packer with a filter paper and a wire for future 

retrieval.

2. Place packer on top of soil. This provides a good level surface for measuring.
7C–16

Appendix 7C

MAY 2009

3. Measure accurately at three locations. These data will later be used for monitoring 

height changes during handling and shipment. Record.

4. Put a moistened paper towel in top.

5. Put a vented plastic cap on top and tape in place.
7C–17

Procedure for Undisturbed, Fixed-Piston Sampling of Cohesionless Soil

MAY 2009

7C.4.6 Preparation for Shipment

1. Label with the following:

• Top

• Project Number 

• Project Name

• Boring Number, Sample Number

• Depths

• Penetration, Gross Recovery, Net Recovery

2. If needed, apply vacuum below bottom packer to draw water out of sample. This 

will increase the stiffness of sand without significantly changing the void ratio.
Appendix 7D

COMBINING NEWMARK-TYPE DISPLACEMENTS

COMPUTED USING 1D SITE-RESPONSE ANALYSES

MAY 2009 7D–1

The computer program SHAKE calculates time histories of horizontal shear stress acting at the 

boundaries of soil layers. To calculate a time history of acceleration for a wedge, and then a displacement, the following procedure can be used (Figure 7D.1):

1. Perform SHAKE analyses for two representative profiles through the potential 

wedge to obtain horizontal shear stress time histories τh(t) at two points A and B on 

the sliding surface.

2. Calculate the time history of the average acceleration k(t) of the soil above the two 

points:

kA(t) = τhA(t)/σvA kB(t) = τhB(t)/σvB

where:

τhA(t) = earthquake shear stress at point A

τhB(t) = earthquake shear stress at point B

σvA = total vertical stress at point A

σvB = total vertical stress at point B

3. Integrate the kA(t) and kB(t) time histories for various values of ky to develop curves of 

ky /kmax versus displacement (Figure 7D.1).

4. Compute kOVERALLmax for the entire wedge:

kOVERALLmax = 

kAmax WA + kBmax WB

 WA + WB

where:

kAmax = maximum value of kA(t)

kBmax = maximum value of kB(t)

WA,WB = weights of portions A and B of failure wedge

5. Using the value of ky computed from the pseudostatic stability analyses, calculate 

ky /kmax and enter the curves developed in Step 3 to obtain a displacement.
7D–2

Appendix 7D

MAY 2009

DISPLACEMENT

A

B

LOG ky /kmax kOVERALLmax = 

kAmax WA + kBmax WB

WA + WB

FIGURE 7.D.1 DEFORMATION ANALYSIS USING SHAKE

SHAKE

PROFILE A

SHAKE

 V PROFILE B A

VB

WA

WB

kA (t) = kAmax = maximum of kA(t)  VA

h (t) A

kB(t) = kBmax = maximum of kB(t) VB

h (t) B

h (t) B

h (t) A

ky /kOVERALLmax

where: h (t) = time history of horizontal shear stress

 = total vertical stress

k(t) = time history of average acceleration of mass

 above point of interest

W = weight

V

Final Draft Chapter 7 - Appendix 7D July 31, 2007

Page 2 of 2

FIGURE 7D.1 DEFORMATION ANALYSIS USING SHAKE
Chapter 8

SITE MINING AND

FOUNDATION ISSUES

MAY 2009 8-1

The construction of embankments and impoundments for coal refuse disposal and process water 

supply is an integral part of most coal mining operations. However, in many coal mining areas, 

it is difficult to locate a site for construction that has not been or will not be undermined to some 

extent, especially near the coal preparation plants that the impoundments serve. When mining is 

planned or is occurring in the vicinity of an impoundment, measures must be taken to evaluate 

the potential impact of mining-induced movement on both the safety of the embankment and the 

integrity of the impoundment. The potential for water or slurry from the impoundment to affect 

the safety of underground mine workings must also be considered. Even for non-impounding 

embankments, there may be performance concerns related to drainage structures that could be 

affected by surface movement induced by underground mining. Additionally, environmental considerations with respect to the effects of mining on the hydrogeologic regime may be important.

It can be difficult to identify and evaluate the potential subsidence mechanisms that may induce 

ground movement and affect the integrity of a refuse embankment or impoundment area because 

of uncertainties or inaccuracies related to the location of mining and the characterization of overburden and protective features such as barriers and mine seals. Accordingly, designers need to 

take steps to accurately assess these features and to provide appropriate protective measures.

An assessment of the potential for mining to affect the safety of an impoundment should be 

addressed in plans submitted to MSHA for approval as per 30 CFR § 77.216-2. Applicable MSHA 

permit requirements related to the potential for an impoundment to affect the safety of underground mine workings can be found in 30 CFR § 75.1716, which identifies requirements for obtaining permits when mining is planned under a body of water of sufficient size to present a hazard to 

miners. Because mining in the vicinity of an impoundment can present a hazard due to fracturing 

of the overburden beyond the horizontal limits of mining, MSHA may require a permit even in 

situations where the mine workings do not extend directly under the impoundment.

This chapter focuses on coal refuse impoundment design issues caused by the presence of underground mine workings, mine spoil materials, and mine highwalls. Based on past experience, these 

design issues include:

• Mine subsidence where the load imposed by overburden and/or an impoundment 

causes ground movement from the caving of overburden strata and/or crushing or 
Chapter 8

8-2 MAY 2009

bearing capacity failure of pillars. This response can impact the hydrogeology (i.e., 

seepage rates and paths), cause differential movements in the dam or impoundment 

reservoir area, result in loss of impoundment freeboard, and disrupt internal drainage systems. Mine subsidence can also aggravate pre-existing conditions and create a 

weakened or worse state that increases susceptibility to other failure mechanisms.

• Mine breakthrough where the load imposed by the overburden and impoundment 

or deterioration due to weathering, degradation or seepage forces causes collapse of 

overburden strata and uncontrolled release of fluid fine coal refuse and water into 

mine workings and beyond. Breakthroughs have also occurred where an impoundment intersects a coal seam or overburden barrier left in place to protect active or 

abandoned mine workings and the hydraulic load imposed by the impoundment 

exceeds the resistance provided by the barrier.

• Mine blowouts where water impounded in an underground mine breaks through a 

sealed mine opening and/or a section of the coal outcrop barrier. A blowout can be a 

secondary consequence of impoundment breakthrough into an underground mine 

or a result of natural accumulation of water or of underground slurry disposal.

• Seepage, internal erosion and stability concerns when an embankment is to be 

constructed over an area where there may be mine openings related to highwall 

mining and auger holes.

• Seepage and internal erosion concerns where an embankment is to be constructed on 

top of spoil and the spoil was placed under uncontrolled conditions.

• Seepage and internal erosion concerns where an embankment abuts a steep rock 

slope or highwall.

For non-impounding embankments, mine subsidence can cause disturbance to internal drainage systems, surface water control structures, slopes and liner systems. Additionally, surface cracking can 

lead to increased infiltration of water. Thus, potential underground mining impacts can be important 

design considerations for non-impounding structures.

Descriptions related to the circumstances behind and lessons learned from 11 mine breakthrough and 

four mine blowout events are presented in Appendix D of an MSHA (2003) report to congress titled, 

Guidance for Evaluating the Potential for Breakthroughs from Impoundments into Underground 

Mine Workings and Breakthrough Prevention Measures. This chapter draws extensively from the 

2003 MSHA report for guidance related to underground mine issues.

8.1 GENERAL CONSIDERATIONS

In addition to other geotechnical considerations associated with coal refuse disposal facilities, there 

are siting factors and structural analysis and design issues that need to be considered when a facility 

is sited above or adjacent to abandoned, active or planned future underground mine workings. These 

factors and issues include:

• Availability of information resources including mine maps, coal contour and outcrop 

maps and the interpretation and accuracy of such maps (i.e., certified as accurate or 

uncertified, includes the latest mining activity, etc.).

• Availability of on-site reconnaissance and surface exploration data related to the 

identification and mapping of geologic features, soil and rock overburden and natural outcrop barriers, and underground mine voids. These data may originate from 

surficial mapping, geophysical methods, long-hole directional drilling, and conventional drilling and sampling.
8-3

Site Mining and Foundation Issues

MAY 2009

• Evaluation of the potential for mine subsidence and breakthrough and mine seal barrier failure considering possible failure mechanisms and methods of evaluation and 

analysis.

• Available methods and design measures for mitigating or controlling mine subsidence and breakthrough and mine seal barrier failure.

• Impoundment construction and operations monitoring guidelines.

• Surface mine spoil properties, if such material is present in foundation zones or used 

in constructing embankments.

• Surface mine bench/highwall issues.

8.2 AVAILABLE SOURCES OF SITE INFORMATION

MSHA (2003) developed guidelines for evaluation of sites with potential for breakthrough into underground mines, covering preliminary investigation, on-site reconnaissance, and direct/detailed investigation. Table 8.1 presents sources for information related to mine maps, conditions, and records. 

Further discussion is presented in the following paragraphs.

A general discussion regarding the location, content and accuracy of mine maps with respect to the 

planning of site reconnaissance and exploration programs and the geotechnical analysis and design 

of coal refuse disposal facilities is presented in Section 6.4.1.5 of this Manual. The discussion presented herein relates to the general usefulness of mine maps with respect to locating coal contours 

and outcrops and to mine map interpretation and accuracy based on information from MSHA (2003) 

and NRC (2002).

8.2.1 Mine Maps

Underground mine maps can be used to locate a mine with respect to the surface or other underground mines and to determine the dimensions of pillars and mine openings. Information provided 

on most underground mine maps includes:

• Pillared, worked out, and abandoned areas; pillar locations; extraction heights; 

sealed areas; future mining projections; adjacent mine workings within 1,000 feet; 

locations of surface or auger mining; mined areas of the coal bed; and the extent of 

pooled water at the time the map was prepared.

• Dates of mining, coal seam sections, coal seam elevations, and survey data and 

markers.

• Surface features, coal outcrop, and 100-foot overburden contour or other prescribed 

mining limit; mineral lease boundaries; surface property or mine boundary lines, 

and identification of coal ownership.

• Areas used for underground injection of coal refuse slurry, acid mine drainage 

sludge, or other sediments or wastewater.

Sources for mine maps include local mining and mineral rights holding companies, local engineering 

and surveying firms, as well as the OSM National Mine Map Repository and state regulatory authorities. Other sources could include museums, mining schools, and mineral resource organizations. 

Mine maps should be assessed to determine their reliability (e.g., determining if mine production 

continued after the date of the map). Table 8.1 cites potential information sources or methods for 

corroborating mine maps, and Table 8.2 lists federal and state agencies that may have data bases and 

can provide assistance in locating mine maps. Exploration and reconnaissance methods for evaluating underground workings when there is no mapping or when available mapping is suspect are 

presented in Section 8.3.
Chapter 8

8-4 MAY 2009

8.2.2 Coal Contour Maps and Outcrops

A coal outcrop is generally regarded as the location (horizontal or at a slope equivalent to the dip of 

the seam) where the bottom of the coal seam intersects the surface, and it is commonly represented 

as the coal outcrop line on contour maps. Because the actual coal seam is generally not visible at the 

surface, the outcrop line is usually shown as a contour corresponding to the bottom elevation of the 

coal seam in that area. Subcrop locations, which are interfaces with other geologic media, are sometimes shown as lines on mine maps where the coal crops out below glacial, alluvial or other sediment 

deposits or volcanic material.

At some sites, coal outcrops are visible because of surface mining activities, abandoned portals, outcrop sample areas, auger mining, highway cuts, house and building excavations, and house-coal 

openings. However, locating coal outcrops in steeply sloped areas may be impractical because of 

difficult access to the outcrop areas and the need for surface excavation. Field location of outcrops 

can also be complicated by the presence of unmined coal horizons; multiple coal seams in the same 

vicinity; and spoil, refuse, backfill, or other fill material placement.

The steepness of the ground surface, stress relief fracturing, weathering, hillside creep, landslides, 

and man-made disturbance can all affect the coal at a projected outcrop location. These effects, couTABLE 8.1 SOURCES OF INFORMATION FOR EVALUATION OF BREAKTHROUGH POTENTIAL

1. Search available mine maps for the area. The local mining companies should be contacted. Other potential 

sources of mine maps include OSM’s National Mine Map Repository, museums, mining schools, and state 

agencies that deal with mine safety and reclamation.

2. Where there are multiple coal seams, the mine maps for each seam should be obtained.

3. Available aerial photographs of the area should be checked for indications of mine openings, auger openings, 

other types of surface disturbance, or the presence of mining facilities.

4. Topographic maps should be checked for indications of mine openings or mine facilities.

5. Geologic maps and cross-sections should be checked for the presence of coal seams in the area.

6. Available information on the condition of, and surface disturbance to, the outcrop barrier and overburden should 

be collected and reviewed. This would include geologic maps, aerial photographs, publications available from 

government agencies, and previous engineering reports, plans, or permits pertaining to the area.

7. Review boring logs from drill holes in the area, potentially for coal or gas exploration as well as geotechnical 

purposes.

8. Collect and review available information on the mining conditions and practices for workings under or near the site. 

Include information on: roof falls; roof support measures, especially supplemental support used near the outcrop 

or under shallow cover; pillar stability, particularly information on second mining of pillars; and floor conditions, 

especially information on punching or heaving.

9. Interview miners, especially older or retired miners who worked in the area, concerning the mining conditions, 

including roof, pillar, floor, and water conditions; the practices used when mining near the outcrop; and whether the 

available mine maps are consistent with their knowledge of the mining that took place.

10. Interview mine surveyors who may have worked in the area about their knowledge of mining in the surrounding 

area.

11. Search available information on auger mining. Often the depth of auger holes are not shown on the mine map, or 

only approximate depths are indicated.

12. Compare production records with the data on mine maps to help validate that additional mining did not occur after 

the mine map was produced.

(MSHA, 2003)
8-5

Site Mining and Foundation Issues

MAY 2009

pled with the possibility that the projected coal outcrop location may be inaccurate because of local 

changes in coal structure, may complicate coal barrier analyses.

The surface topography maps submitted to MSHA, OSM, and state regulatory authorities delineating 

coal outcrop locations are usually scaled at 500 feet or less to the inch. The maps are typically based 

on aerial photography or USGS mapping. Coal outcrop locations are usually projected based on the 

structure of the coal and its inferred (and sometimes measured) intersection with the ground surface. 

It is recommended that the coal outcrop location be measured and the distance from the ground surface to intact coal determined at impoundment sites where breakthrough potential is a concern.

Most mid- and large-size coal companies currently employ precise surveying techniques and mine 

planning software to generate mine maps. This has been prompted by the need to accurately locate 

ventilation shafts, to prevent mine pool blowouts, and to have accurate locations should a mine rescue 

be necessary. These software codes utilize data from borehole geotechnical and geologic logs, horizontal drilling, surface geophysical exploration, outcrop exploration, land and in-mine surveys and 

topographic mapping to generate an outcrop line on maps. The results are generally more accurate 

than previous methods; some mining companies make these maps available.

One of the key steps in delineating coal outcrop locations on surface topographic maps is the coordination of survey control points. Common control points should be used for both the underground 

mine map and the surface topography map. The accuracy of each map generated from these common 

control points is then a function of the equipment or method used to create the elevations and contours. Where practical, and at critical locations, surface topographic surveys should be conducted 

along the coal outcrop as part of permit submittals. These surveys should fully document existing 

locations where the coal seam is already exposed due to natural or man-made activities. The surveys 

should be tied to either the U.S. Geological Survey or the U.S. Coast and Geodetic Survey benchmark 

system used for the underground mine surveys. By using common control points for the surface and 

underground surveys, accurate vertical and horizontal control can be achieved.

8.2.3 Interpretation and Accuracy of Mine Maps

Mine mapping is a key element in evaluating breakthrough potential. As described in Section 6.4.1.5, 

the accuracy and completeness of the information on mine maps can vary widely. In times past some 

mines were not mapped at all, and the accuracy of older mine maps was questionable. Often there are 

significant differences in the accuracy and completeness of underground mine maps (especially those 

prepared before 1969) because of the varying requirements associated with their development. Sometimes the horizontal and vertical (overburden) distances between mined barriers and an impoundment are not accurately shown.

TABLE 8.2 SOURCES FOR MINE MAPS

OSM National Mine Map Repository Indiana Geological Survey

Virginia Department of Mines, Minerals, and Energy Ohio Division of Geological Survey

West Virginia Geological Survey Ohio Division of Natural Resources

Kentucky Map Information Center Maryland Coal Mine Mapping Project

Pennsylvania Department of Environmental Protection Utah Geological Survey

Illinois Department of Natural Resources
Chapter 8

8-6 MAY 2009

Accidents have occurred where an active mine has broken into old/abandoned mine workings that 

either were not shown on a map or were shown as being hundreds of feet from their actual location. 

These incidents may have occurred because the complete workings were not surveyed, the area was 

inaccurately surveyed, the data were not properly recorded, or coal was “robbed” by others after 

the mine closed. It is important to determine if the mine map being relied upon is current and not 

an earlier (interim) map. This can be accomplished by cross-referencing dates on the mine map with 

available production records. Problems can also occur when maps for adjacent mines are not referenced to a common coordinate system or when data from a map are inaccurately transposed from 

one coordinate system to another. Many potential problems can be avoided by referencing all mine 

maps to the state plane coordinate system. Unfortunately, some maps and records for older mines 

have been lost or destroyed.

When mining occurs near an outcrop, subsurface conditions may exacerbate problems related to 

inaccurate mine maps. It is not uncommon for roof conditions to deteriorate as mining approaches 

an outcrop. This occurs because the mining encounters less cover, more weathered roof strata, more 

frequent jointing, and possibly highly weathered joints near an outcrop (sometimes referred to as 

hillseams, as discussed in Section 6.4.1.2). Furthermore, the last cut made toward an outcrop typically 

is not provided with roof support. Because surveyors will not have direct access to the unsupported 

section of the entry, distances may be estimated rather than directly surveyed. Also, if a roof fall 

occurs or an area is restricted from access after mining, it may not be accurately surveyed. Dashed 

lines on a mine map indicate that the area was not surveyed and that locations are determined based 

on the best available information, which may be estimates from the section foreman’s map.

Areas of secondary recovery mining or partial mining of pillars and barriers are typically not accurately located, as these areas cannot be surveyed. The extent of secondary mining indicated on mine 

maps is generally based on estimates from foreman reports. Reconnaissance and exploration methods for evaluation of such conditions are discussed in Section 8.3.

8.3 ON-SITE RECONNAISSANCE AND EXPLORATION

The accuracy of available mapping for coal contours and outcrops can vary depending on the age, location and source of the mapping. Therefore, when earthen dams and refuse impoundments are to be 

constructed over or in proximity to active and abandoned underground mine workings, the accuracy 

of this mapping information must be verified as part of site reconnaissance studies. The following sections provide a description of surficial reconnaissance and geophysical and geotechnical exploration 

methods that can be used to confirm the location and configuration of underground mine workings.

8.3.1 Surficial Reconnaissance

Surficial reconnaissance consists of walking the disposal facility site and vicinity (both dam and 

impoundment areas to be developed) and observing topography, rock and coal outcrops, surface 

cracks and subsidence features, soil types, vegetative cover, spring discharges, perennial and intermittent watercourse locations, and any other information that may contribute to more complete 

knowledge of the site. This type of surficial reconnaissance and geologic mapping generally requires 

an experienced geologist or engineer with knowledge of refuse disposal and embankment design. If 

possible, the reconnaissance should be conducted during times when vegetation is dormant so that 

site features are more visible. Additional details regarding the types of features that should be documented during a surficial reconnaissance are discussed in Section 6.4.2.

Test pits can greatly enhance surficial reconnaissance, as areas can be excavated to: (1) determine the 

relationship of seeps to the coal seam(s), (2) verify coal seam outcrop(s), and (3) establish the depth of 

weathering, presence of fill, etc. Test pits can be cost effective particularly when active mining operations are nearby and on-site equipment can be employed.
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Site Mining and Foundation Issues

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8.3.2 Geophysical Methods

Where there is uncertainty regarding the presence and extent of mining or the accuracy of available 

mine maps, some geophysical exploration methods can be employed as a logical first step for site 

evaluation and reconnaissance. Geophysical methods, through detection of anomalies, can indicate 

whether mine workings are present and if detected mine workings generally correspond to available 

mine maps. Geophysical methods that have been used for locating mine workings include seismic, 

electromagnetic and electrical resistivity techniques. A detailed description of these and other geophysical methods is provided in Section 6.4.4. The advantage of geophysical methods is that a large 

area can be quickly explored, as compared to drilling programs where information is obtained one 

borehole at a time. Geophysical methods entail the use of sophisticated equipment and data processing techniques. Therefore, these studies should be planned, implemented, and the data interpreted 

only by persons experienced in this specialty field. Results of geophysical site exploration should be 

confirmed by subsurface drilling or other independent technique.

Geophysical methods have limitations related to void and mine depths, overburden type and condition, and whether voids contain water. No single method has proven successful in a majority of cases, 

even where depth and mine void filling is ideal for the method being employed. Thus, geophysical 

methods should be viewed as a tool to identify potential anomalies that can be further defined by 

other methods such as drilling.

8.3.3 Long-Hole Directional Drilling

Directional or long-hole drilling generally refers to in-mine drilling operations used for identifying and understanding geological and mining conditions in advance of mining. Directional holes 

have been drilled for horizontal distances of more than 5000 feet (Kravits and Schwoebel, 1994). 

Long-hole drilling can also be performed from the surface, independent of mining operations, and 

thus can be employed near abandoned mines. Advances in directional, long-hole drilling include 

a technique that can be used for locating or verifying the absence of mine workings. The position 

of the end of the hole is determined using a surveying tool that is an integral part of the drilling 

system. The borehole can be guided or steered from underground mine workings or from the surface to determine whether there are mine workings in a particular area. This technique can also be 

used to advance a hole within the coal seam and roughly parallel to a coal-outcrop line to establish 

the thickness of solid outcrop barrier around an impoundment site. Reportedly, long-hole drilling 

can achieve accuracies of better than ± 1° of azimuth and ± 0.25° of inclination. Long-hole drilling is 

also discussed in Section 6.4.3.9. Boreholes associated with such drilling should be properly sealed, 

as they can become seepage paths or increase breakthrough potential.

8.3.4 Conventional Drilling and Sampling

Drilling boreholes is probably the most effective method for determining the location, extent and 

variability of subsurface conditions at specific points. The difficulty with drilling boreholes is determining how many are needed to adequately characterize a site and where the boreholes should be 

located. Section 6.4.3 describes subsurface exploration and in-situ test planning for earthen dams and 

refuse impoundments. Geotechnical exploration for an impoundment site is much more complicated 

when there are underground mine workings at or in the vicinity of the site. For this situation, a substantial number of borings is often needed for characterizing conditions near the embankment and 

impoundment, and sometimes angle or horizontal drilling may be useful. Not only does the accuracy 

of available mine maps need to be determined, but the nature and variability of the overburden also 

need to be defined. An advantage of using geophysical methods as a preliminary step in site investigation is that the results of the geophysical work may allow the designer to optimize the number 

and location of boreholes and minimize associated costs. In general, the scope of the drilling needs 

to be such that the accuracy of the mapping information can be verified and the geologic, soil, and 

water conditions can be determined sufficiently to enable evaluation of: (1) the adequacy of barriers 
Chapter 8

8-8 MAY 2009

TABLE 8.3 EXPLORATION CONSIDERATIONS FOR ASSESSING IMPOUNDMENT 

BREAKTHROUGH POTENTIAL TO UNDERGROUND MINES

1. Identify locations under the reservoir, and around the perimeter of the reservoir, where the conditions appear to be 

most critical. Verification should be based on the locations where mapping or geophysics indicates that the total 

cover or the outcrop barrier widths are minimums, and other locations where the competent rock portion of the 

overburden is reduced.

2. These critical locations should be explored by drilling or test pits. The number of locations explored will depend on: 

(a) how far the mining is or is expected to be from the impoundment, (b) the level of uncertainty, (c) the results that 

are found, and (d) the degree of conservatism associated with the design or remedial measures being considered.

•	 In an outcrop barrier, the main concerns are to: (1) verify how close to the surface mining has occurred, (2) 

determine whether the pillars have been first-mined only, and (3) establish whether auger or highwall mining has 

reduced the size of the barrier. Exploration should be conducted at several of the more critical areas to verify the 

extent of mining and determine the cover conditions, in particular, the thickness and condition of the rock strata. If 

the mapping is found to be inaccurate in any of those areas, then additional areas will need to be checked.

•	 Where mining has occurred below the level of the bottom of the reservoir (below drainage), the main concern 

will be to confirm the depth to mining, the mining method, the extent of mining, the characteristics of the 

overburden, and the size and condition of the pillars.

•	 During exploration, evidence should be collected which helps to corroborate available information on the type 

of mining and characteristics of gob materials, age of mining, presence of water and pools, and past injection of 

slurry or other materials in the mine workings.

3. Where practical, selected borings that are drilled for other purposes, such as foundation investigation, should be 

extended to the coal seam level(s) to provide additional points to confirm whether or not mining has occurred. 

Where longwall or retreat mining has occurred, borings can provide information on mine convergence and gob 

materials (backfill classification and penetration resistance.)

4. For each area investigated, sufficient information on the geologic conditions (e.g., rock quality, joints, weathering), 

and the engineering properties of the materials, should be obtained.

5. A sufficient number of vertical drill holes should be drilled at critical locations to bracket the farthest extent of the 

mining. Where practical, horizontal and angle holes may be helpful in locating and mapping conditions.

6. In determining where holes should be drilled, the lateral extent should consider possible zones of subsidence. The 

lateral extent of the area impacted by subsidence is larger than the mined area and is generally defined by the 

angle of draw from mine workings that are in proximity to the impoundment.

7. If the designer determines that multiple seam mining has occurred in the area, information should be collected on 

the mining in the other seams including mine map overlays and the nature of the interburden between the seams.

8. The extent of augering and highwall mining is usually not accurately mapped and should be evaluated to assess 

the extent of penetration of the coal barrier.

9. Borehole cameras and imaging systems such as borehole sonar (flooded mine) or laser scanners (dry mine) can 

be used to obtain additional information about the conditions at mine level, such as mine void dimensions and 

orientation and rib characteristics, as well as the amount of subsidence or collapse that has occurred, and the 

conditions at discontinuities intersected by the borehole.

(ADAPTED FROM MSHA, 2003)

and overburden with respect to breakthrough and (2) assessment of the potential for subsidence to 

affect the dam. Table 8.3 presents exploration considerations for assessing breakthrough potential at 

impoundment sites (MSHA, 2003) and also provides a discussion of the use of test pits to verify coal 

seam outcrops. Table 8.4 presents exploration considerations for outcrop barriers developed by OSM 

(Kohli and Block, 2007).

Exploration borings that are advanced from an impoundment site into underground mine workings 

could be a potential breakthrough path if not properly plugged. Guidance related to sealing of boreholes is presented in Section 6.4.3.13.
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Site Mining and Foundation Issues

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TABLE 8.4 OUTCROP BARRIER EXPLORATION CONSIDERATIONS

1. Determine the minimum overburden thickness above the mine workings closest to the mine side of the barrier. 

Both the rock thickness and the thickness of unconsolidated material should be considered.

2. Inspect the surface area close to the proposed outcrop barrier for natural benches and other surface features (e.g., 

road cuts) that could reduce the overburden thickness.

3. Verify the correct location of the outcrop on the mine maps using land or GPS surveys. The outcrop may have 

been plotted by the mine survey or from the topographic maps.

4. Look for evidence of any water seepage above the barrier and from adjacent mine workings. Determine the 

quantity of discharge and its locations.

5. Determine if there are adits or auger holes in the mine barrier that are not shown on the map.

6. Identify other features that may impact the integrity of the barrier.

7. Look for evidence of subsidence or sinkhole cracks or other zones of weakness in the overburden above the mine 

workings adjacent to the barrier.

(KOHLI AND BLOCK, 2007)

8.4 EVALUATION OF MINE SUBSIDENCE AND BREAKTHROUGH

Mine subsidence can impact coal refuse disposal facilities directly by decreasing the stability of an 

embankment or impoundment leading to movement or release of refuse materials and water, or 

indirectly by affecting the grade or structural integrity of a drainage component. Ways in which 

mine subsidence can have an adverse effect on a refuse embankment include causing cracking, 

providing paths for seepage, disrupting internal drains, damaging decant pipes, and reducing freeboard. Sinkholes beneath a dam can cause internal erosion that can lead to dam failure. When mine 

subsidence leads to fracturing or sinkhole development extending to a surface impoundment, the 

possibility of a breakthrough into underground mine workings is an important consideration. A 

breakthrough is a sudden, uncontrolled release of water and/or fine coal refuse from an impoundment into an underground coal mine. Even when a slurry impoundment is no longer in use, there 

may still be potential for a breakthrough if the buried fine refuse remains in a loose, saturated 

condition.

The ability of overburden strata, outcrop barriers, and mine bulkheads to prevent breakthrough 

depends on many factors. These factors include:

• Thickness of strata above the mine workings

• Engineering properties of strata above the mine workings

• Width and integrity of the outcrop barrier

• Thickness and integrity of the material above and below the coal barrier

• Hydraulic conductivity of the coal and surrounding strata

• Piping potential of any natural soil, mine spoil, or refuse surrounding the outcrop 

barrier

• Size, depth and location of mine voids

• Hydrostatic and earth pressures

• Water and fine coal refuse levels

• Flowability of the fine coal refuse

• Presence and orientation of stress relief fractures, rock discontinuities, and weathered joints
Chapter 8

8-10 MAY 2009

• Impacts of any physical disturbance (e.g., landslides, road cuts, auger or highwall 

mining) on the material in the outcrop area

• Presence of mine pools or slurry within the workings and the effect of slurry injection on the mine floor and barrier

Differential movements associated with mine subsidence, even if they do not represent an impoundment breakthrough concern, need to be evaluated to assess potential effects on surface and subsurface 

drainage structures. Subsidence and potential breakthrough failure mechanisms should be evaluated 

based on site-specific data. Collection and evaluation of information and field data on the mine, 

overburden and disposal embankment may be necessary for evaluating the potential for subsidence 

and breakthrough and for assessing possible mitigation measures. The need for collecting these data 

is dependent upon site conditions; however, the data review will minimize assumptions associated 

with the identification of failure mechanisms.

Table 8.5 presents a summary of guidelines and associated references for evaluating potential impacts 

from subsidence. Additionally, MSHA (2003) presents guidance for evaluating the potential of breakthrough from impoundments to mine workings and breakthrough prevention measures. The following sections present and discuss information from this guidance document.

8.4.1 Mine Subsidence Considerations

The possible impacts of mine subsidence on an impoundment and mine workings must be evaluated 

whenever a refuse disposal facility or other impoundment is to be located in the vicinity of existing 

underground workings or planned underground mining. Subsidence generally entails both vertical 

and horizontal movement, strain, tilt and curvature and may manifest on the surface as: (1) cracks, 

fissures or fractures; (2) pits or sinkholes; and (3) troughs or sags. Surface fractures may occur where 

there are areas of tension or shear stresses in the ground. When the area of surface subsidence is 

relatively small and the workings are close to the surface (normally 100 feet or less but as deep as 

150 feet), the subsidence feature may be of pit or sinkhole size. Larger areas of surface subsidence 

are referred to as troughs or sags and are typical of deeper and secondary mining operations. Figure 

8.1 illustrates the disturbance of geologic strata over mine workings, including some of the following 

effects (Kendorski; 1993, 2006):

• Floor heave – Upward thrust of the floor in the mine working area.

• Caved zone – Caving of the overburden directly over a mine void and bulking of the 

caved material leading to support of overlying strata generally extending to a height 

of 3 to 10 times the extraction thickness.

• Fractured zone – A zone of vertical fracturing and bed separations. Overburden 

in this zone moves vertically in large blocks along existing joints and new vertical 

fractures. Typically this zone extends no more than 24 times the extraction thickness 

above the mine, but can reach 30 times the extraction thickness.

• Main roof – This zone, which is sometimes subdivided into the Dilated Zone and the 

Constrained Zone, is an area of no significant increase in vertical hydraulic conductivity. This zone has been characterized as extending above the Fractured Zone up to 

60 times the extraction thickness.

• Surface zone – Surface cracks are typically present in this zone and are generally 

limited to areas placed in tension by subsidence. Cracks can be created in dry clayey 

soil and joints can open in massive sandstones. Such cracks can extend downward to 

a depth of 50 feet.

Figure 8.2 illustrates the influence of extraction width on subsidence at the surface. For the maximum 

subsidence to be observed at the surface, the coal extraction width must typically reach the critical 
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Site Mining and Foundation Issues

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TABLE 8.5 SUMMARY OF GUIDELINES FOR MINING UNDER OR NEAR BODIES OF WATER

Reference Description Criterion

Room and Pillar (first mining only)

Babcock and 

Hooker, 1977

Minimum Solid Overburden

Minimum Solid Overburden where competent bed of 

sandstone or other rock is present.

10t or 5s, whichever is greater

< 10t or 5s, provided competent

 bed is > 1.75s

Panel and Pillar

Babcock and 

Hooker, 1977

Minimum Solid Overburden

Maximum Width of Extraction Panel (p). For multiple 

seams, superimpose panels and pillars with panel 

widths being determined from the depth of the 

uppermost seam and pillar width being determined 

by reference to the thickest and/or deepest seam, 

whichever gives the greater dimension. Where panel 

and pillar system are employed in upper seam, apply 

above criterion considering upper seam or body of 

water.

270 feet or 3p, whichever is greater

p ≤ 1/3 H

Total Extraction

Skelly and Loy, 1977 Minimum Overburden (H)

Surface Tensile Strain (εt)

100t or ≥ 700 ft, whichever is greater

< 0.010

Babcock and 

Hooker, 1977 Minimum Solid Overburden 60t

Kendorski et al., 

1979

Minimum Overburden

• Catastrophic-Size Water Body

• Major-Size Water body with Limited Potential

Surface Tensile Strain

• Catastrophic-Size Water Body

• Major-Size Water Body with Limited Potential

varies from 60t to 117t

varies from 37t to 105t

≤ 0.010

≤ 0.015

Note: These guidelines generally apply to the prevention of significant impacts to overburden and inflow into mines.

 Lower strains than those indicated can cause cracking of embankment dams.

 H = thickness of rock overburden (ft)

t = average extraction thickness (ft)

 s = entry width (ft) εt = surface tensile strain (dim)

width shown. If the mined width is less than critical, it is termed subcritical, and the amount of subsidence is less than the maximum subsidence experienced with wider extraction areas. If the mined 

width is greater than the critical width, supercritical conditions prevail, and the subsidence trough 

will exhibit a flat bottom depression, as shown in Figure 8.2. The figure also shows the angle of inclination between the vertical at the edge of the workings and the point of zero vertical displacement at 

the edge of the trough, which is termed the limit angle or angle of draw. Depending on the prediction method used, survey standards employed, or criticality of a surface structure, determination 

of the angle of draw is typically based on a minimal vertical movement of between 0.01 and 0.1 feet 

(for predictive measures that assume an asymptotic approach to zero such as the tangent-hyperbolic 

function). The “coal extraction width” can be a result of longwall mining, second mining of pillars, or 

pillars crushing or punching into the floor.
Chapter 8

8-12 MAY 2009

 Subsidence lowers surface grades and elevations and can affect surface drainage and alter impoundment freeboard. Strains and horizontal displacements associated with subsidence can impact the 

structural integrity of surface structures such as dams (including internal drains and decants), 

embankments, and bridges and buried structures such as pipelines and large culverts. Zones of surface tension and compression develop during mining, resulting in horizontal movement profiles that 

are primarily a function of the extraction width, as depicted in Figure 8.3. The most severe subsidence 

impacts on many impoundment structures occur where the tensile stain is highest, while for other 

structures it is the area of greatest tilt or subsidence-induced slope. The existing condition of a structure and slope of the terrain on which the structure is situated also affect subsidence impacts.

SUPERCRITICAL WIDTH

CRITICAL WIDTH

SUBCRITICAL WIDTH

DEPTH OF

SEAM (D)

COAL SEAM

t

ANGLE OF

DRAW



SMAX

SUBSIDENCE FOR

SUBCRITICAL WIDTH

SUBSIDENCE FOR

CRITICAL WIDTH

SUBSIDENCE FOR

SUPERCRITICAL WIDTH

COAL SEAM

(SINGH, 1992)

FIGURE 8.1 STRATA DISTURBANCE AND SUBSIDENCE CAUSED BY MINING

 

(ADAPTED FROM SINGH AND KENDORSKI, 1981;

 PENG AND CHIANG, 1984)

ORIGINAL GROUND SURFACE

SURFACE

CRACKING

CAVING

t

3 to 10t

COAL

FLOOR

HEAVE

BED SEPARATION

SURFACE ZONE

MAIN ROOF

(CONTINUOUS

DEFORMATION

ZONE)

FRACTURED

ZONE

CAVED

ZONE

FLOOR

HEAVE

FIGURE 8.2 INFLUENCE OF EXTRACTION WIDTH ON SUBSIDENCE

FIGURE 8.1 STRATA DISTURBANCE AND SUBSIDENCE CAUSED BY MINING
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Site Mining and Foundation Issues

MAY 2009

FIGURE 8.3 DISPLACEMENT AND STRAIN FOR VARIOUS EXTRACTION WIDTHS

8.3a SUBCRITICAL WIDTH

S < SMAX

COAL SEAM

SUBCRITICAL WIDTH

EXTRACTION ZONE

HORIZONTAL

DISPLACEMENT

ANGLE OF

DRAW

HORIZONTAL

STRAIN

ANGLE OF

BREAK

SUBSIDENCE PROFILE

GROUND

SURFACE

 TENSION

COMPRESSION

- -

+ +

8.3b CRITICAL WIDTH

CRITICAL WIDTH

COAL SEAM

SUBSIDENCE PROFILE

SMAX

EXTRACTION ZONE

ANGLE OF

DRAW

ANGLE OF

BREAK

 TENSION

COMPRESSION

GROUND

SURFACE

HORIZONTAL

DISPLACEMENT

HORIZONTAL

STRAIN +

- -

+

8.3c SUPERCRITICAL WIDTH

HORIZONTAL

STRAIN

HORIZONTAL

DISPLACEMENT

SMAX SMAX

SUBSIDENCE PROFILE

EXTRACTION ZONE

COAL SEAM

SUPERCRITICAL WIDTH

ANGLE OF

DRAW

ANGLE OF

BREAK

GROUND

SURFACE

 TENSION

COMPRESSION

+

-

+

-

(SINGH, 1992)

FIGURE 8.3 DISPLACEMENT AND STRAIN FOR VARIOUS EXTRACTION WIDTHS
Chapter 8

8-14 MAY 2009

For total extraction mining, subsidence follows the advance of the mine workings, and horizontal tensile and compressive strain regions move laterally with the mining, as shown in Figure 8.4. Therefore, 

determination of the impacts of full extraction mining beneath embankments and impoundments 

must be based upon subsidence development (i.e., dynamic subsidence) and the associated traveling 

strain curve and not just the final subsidence profile. Areas of greatest impact generally occur near 

the boundaries of the extraction zone. Figure 8.5 depicts the ground movements caused by subsidence. Singh (1992) summarizes factors that affect mine subsidence, including: (1) seam thickness, 

depth, and dip; (2) mine floor, roof and overburden; (3) geologic discontinuities and in-situ stresses; 

(4) surface topography; (5) groundwater; and (6) percent of extraction, advance rate, backfilling, and 

elapsed time.

8.4.2 Potential Subsidence and Failure Mechanisms

There are several potential mechanisms that are associated with mine subsidence and mine breakthroughs. Subsidence can occur as a result of caving and fracturing of the mine overburden and 

migration of the disturbance to the surface (e.g., sinkhole development), pillar crushing, or pillar 

punching. Also, planned subsidence can occur as a result of: (1) total extraction mining using longwall methods or secondary mining of pillars or (2) room and pillar mining with partial extraction 

leaving pillar remnants designed to yield. The condition of the coal and overburden and the mine 

extraction method will determine the extent to which subsidence will propagate upward toward the 

surface and the degree of fracturing and deformation that can occur at the surface. Longwall subsidA

B C D E F G H I J K

TENSION

COMPRESSION

8.4a SUBSIDENCE DEVELOPMENT

8.4b TRAVELING STRAIN CURVE

(RELLENSMANN AND WAGNER, 1957)

FACE

ADVANCE A B C D E F G H I J K

FINAL PROFILE AFTER TIMEDEPENDENT SUBSIDENCE

A

C

E

G

I

K

FIGURE 8.4 DEVELOPMENT OF SUBSIDENCE TROUGH AND STRAINS WITH FACE ADVANCE
8-15

Site Mining and Foundation Issues

MAY 2009

ence occurs shortly after mining, but room and pillar subsidence may occur years or decades after 

mining, as conditions or in-mine support deteriorate.

Subsidence of underground mine workings can lead to breakthrough if an impoundment is located 

directly over or near the mine. If the caved or fracture zone intersects with the surface or surface soil 

layer, a breakthrough can occur. The driving force for a breakthrough is the pressure gradient (earth 

and water pressure) between the impoundment and the underground mine workings. The added 

weight from the embankment and reservoir, combined with increased seepage heads, increases the 

stress on the underlying strata. Mechanisms that can cause a breakthrough include internal erosion 

or piping, outcrop barrier failure, hillside movement and disturbance, mine seal failure, or barrier 

(coal, soil and/or rock) decomposition. These mechanisms are frequently interrelated and should be 

carefully evaluated as part of the analysis of the potential for breakthrough.

8.4.2.1 Sinkholes

A sinkhole is a depression or opening in the ground surface above an underground mine void where 

the mine roof has fractured and fallen/caved and the disturbance has intersected the surface or soil 

mantel. The fracturing of the mine roof material can eventually extend high enough that an opening, 

or at least a weakened area, is created at the ground surface, particularly where the overburden is 

thin. A sinkhole can serve as a direct conduit from an impoundment to underground mine workings. Factors contributing to sinkhole development include: (1) the presence of a mine void, (2) low 

overburden thickness, (3) mine roof material that is not sufficiently strong or durable to span the 

mine opening, (4) fractures in the mine roof, (5) unconsolidated soil and weathered rock above the 

mine roof, and (6) pressure exerted by refuse and/or water at the surface. The action of water flowing 

through fractured strata can cause deterioration and can enlarge a sinkhole beyond the depth and 

extent that would occur in the absence of water.

FIGURE 8.5 GROUND MOVEMENTS CAUSED BY SUBSIDENCE

 

CURVATURE (S")

INFLECTION POINTS

HORIZONTAL

STRAIN (V')

DEPTH OF

OVERBURDEN

COAL SEAM

ANGLE OF DIP

BREAK

ANGLE

DRAW

ANGLE

VERTICAL

DISPLACEMENT (S)

Smax

TENSION

COMPRESSION

INFLECTION

POINT

HORIZONTAL

DISPLACEMENT (V)

SLOPE (S')

(SINGH, 1978)

INFLECTION

POINT

EXTRACTION ZONE

FIGURE 8.5 GROUND MOVEMENTS CAUSED BY SUBSIDENCE
Chapter 8

8-16 MAY 2009

A sinkhole will not develop if the rock strata above the mine are either strong enough to span openings without collapsing, the strata above the mine are thick enough that an arch forms over openings and prevents the collapse feature from propagating to the surface, or the swell of falling rock 

checks the propagation of the collapse before it affects the mantle of unconsolidated material. One 

study in Pennsylvania (Gray et al., 1977) found that 81 percent of sinkholes occurred where there 

was less than 100 feet of cover, with most occurring at 50 to 60 feet of cover. However, there have 

been unusual cases, such as a sinkhole in Illinois (DuMontelle et al., 1981) where the cover was 

165 feet, although the overburden thickness in this case included a substantial amount of glacial 

till. The mine depth associated with sinkholes reported from mines located primarily in Colorado, 

Utah and Wyoming (Dunrud and Osterwald, 1980) was generally 10 to 15 times the seam thickness.

Analysis of roof strata to determine if they will indefinitely span mine entries is difficult because: 

(1) the strength of the rock is not easily defined, (2) the effect of joints on the integrity of the roof 

(especially at shallow depths) adds considerable uncertainty, and (3) long-term integrity of coal pillars and roof support system can be an issue. For these reasons, the adequacy of the mine overburden to prevent sinkhole development is commonly assessed by applying certain “rule-of-thumb” 

type guidelines that have been developed based on experience. Guidelines developed by Babcock 

and Hooker (1977) are illustrated in Figure 8.6. These guidelines are generally considered to be 

conservative, and they should be carefully reviewed before detailed analyses to assess the potential 

for subsidence and related differential movement and strain are performed. Mine development 

work has been performed and production areas have been successfully implemented following 

the guidelines shown in Figure 8.6. However, where subsidence has the potential to affect a highhazard-potential impounding embankment or to allow a breakthrough that could affect the safety 

of miners or the public, these guidelines should be used only in conjunction with more rigorous 

site-specific analysis.

With respect to sinkholes, the guidelines recommend for first mining only that the thickness of solid 

strata should be equal to at least 5 times the entry width or 10 times the extraction height, whichever is greater. These guidelines are consistent with findings that compression arches in overburden are normally stable if the mining width is limited to one-fourth to one-half of the overburden 

height. In other words, the compression arch is typically stable if the thickness of the rock strata 

above the mine is from 2 times (strong strata) to 4 times (weaker strata) the entry width. Adding a 

margin of safety, the “rule-of-thumb” is that the overburden thickness should be 5 times the entry 

width. Similarly, the criterion related to the extraction height is based on adding a margin of safety 

to the observation that the height of collapse above a mine entry generally does not exceed 3 to 5 

times the extraction height, likely because of the swell of the collapsed material.

A key point in the use of these guidelines is that the strata thickness refers to “solid overburden 

strata.” Soil, weathered rock, or weak rock should not be included in the solid strata thickness since 

they may not provide the strength needed to resist sinkhole development (MSHA, 2003).

The guidelines in Babcock and Hooker (1977) suggest that a lesser strata thickness may be acceptable if the overlying strata consist of competent rock (e.g., competent bed of sandstone or similar 

material) with a thickness of at least 1.75 times the entry width. Competent rock is normally taken 

to mean a homogeneous, massive layer of sandstone or limestone (MSHA, 2003). This guideline is 

based on analyses of the overlying strata as a beam with minimum tensile strength (20 psi). While 

any intact piece of rock will likely exceed this tensile strength, joints or discontinuities will reduce 

the effective strength of the beam. Because the potential impact of joints and weathering, especially 

near an outcrop, cannot be modeled with confidence, this approach should normally not be used for 

a critical subsidence or breakthrough situation. Furthermore, there is a possibility that the strength of 

the overlying strata will deteriorate over time due to seepage and weathering, especially as the mine 

pool level rises.
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Site Mining and Foundation Issues

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8.6a SAFETY ZONE BENEATH BODY OF SURFACE WATER

FIGURE 8.6 SAFETY ZONE GUIDELINES FOR MINING IN THE VICINTY

OF DAMS AND IMPOUNDMENTS

SURFACE WATER

(BABCOCK AND HOOKER, 1977)

ROOM AND PILLAR

PANEL

TOTAL EXTRACTION

D = 5s or 10t, whichever is larger

D = 3p or 270 feet, whichever is larger

D = 60t

s = maximum entry width (ft)

p = panel width (ft)

t = extraction height (ft)

8.6b SAFETY ZONE BENEATH DAM AND IMPOUNDED BODY OF SURFACE WATER

NOTE: THE ZONE OF NO EXTRACTION TO DEPTH "D" IS PRESUMED TO COMPRISE SOLID ROCK STRATA (IF MATERIAL

 OTHER THAN SOLID ROCK COVER IS INCLUDED, IT IS NECESSARY TO DEMONSTRATE THE NATURE AND

 PERMEABILITY OF SUCH MATERIAL). THE ZONE OF NO EXTRACTION MAY BE INCREASED OR DECREASED,

 IF JUSTIFIED BY LOCAL OBSERVATION AND/OR EXPERIENCE.

ROOM AND PILLAR

PANEL

TOTAL EXTRACTION

D = 5s or 10t, whichever is larger

D = 3p or 270 feet, whichever is larger

D = 60t

s = maximum entry width (ft)

p = panel width (ft)

t = extraction height (ft)

D

NORMAL

EXTRACTION

PERMITTED ZONE OF NO EXTRACTION 350'

350'

65

ZONE OF EXTRACTION

USING GUIDELINES

65

NORMAL

EXTRACTION

PERMITTED

ZONE OF NO EXTRACTION

D

200'

200'

200'

IMPOUNDMENT

200'

NORMAL

EXTRACTION

PERMITTED

NORMAL

EXTRACTION

PERMITTED

200'

350'

D

350'

ZONE OF NO

EXTRACTION

ZONE OF NO

EXTRACTION TO

PROTECT DAM

350'

ZONE OF EXTRACTION

USING GUIDELINES

65

DAM

65 65

FIGURE 8.6 SAFETY ZONE GUIDELINES FOR MINING IN THE VICINITY 

OF DAMS AND IMPOUNDMENTS
Chapter 8

8-18 MAY 2009

In guidance for evaluating breakthrough potential, MSHA (2003) indicates that any location where 

the cover over a mine entry is less than 100 feet of solid strata (especially at locations near the outcrop 

where additional weathering and stress relief has occurred) is a concern for sinkhole development. 

Accordingly, if sinkhole development cannot be reliably ruled out, preventive measures should be 

considered.

8.4.2.2 Pillar Failure

Loss of support due to coal pillar failure causes a mine roof to sag or collapse. This can create or open 

fractures in the overburden. These fractures may cause a roof fall and consequent sinkholes, or the 

fractures may create zones where internal erosion can occur. Furthermore, failure of one pillar transfers the load to surrounding pillars and may lead to progressive pillar failure (sudden or gradual) or 

excessive displacements over a relatively large area.

Pillar crushing occurs when the load on a pillar exceeds its strength. This can be caused by existing loads, additional loading from impounded slurry and/or water, loss of strength in the coal from 

chemical decomposition or from slow oxidation, mine fire, or loss of buoyant pressure resulting 

from a lowered mine pool. In addition to pillar strength, the pillar width to height ratio (w/h) is also 

important (Mark, 2006). For “slender” pillars (w/h < 4), failure often results in nearly complete loss 

of load-bearing capacity, sometimes with sudden and total collapse. Pillars with w/h between about 4 

and 10 are largely elastic with a possible plastic core, and failures tend to occur gradually with postfailure residual strength essentially constant. The pillars deform until they have shed enough load to 

stop the process. Pillars with w/h greater than 10 (referred to as “squat”) have a plastic core and may 

strain harden once the loss of initial strength due to crushing or yielding of the outer elastic portion of 

the pillar occurs. After this initial crushing, the pillars gain strength as they deform. The implications 

for surface structures of the failure of slender pillars with shallow cover are much more significant 

than those associated with yielding of squat pillars at great depth.

A number of formulas for analyzing the strength of a pillar have been developed, and computer programs for performing pillar analyses are available. One example is the program ARMPS (Analysis of 

Retreat Mining Pillar Stability) developed by National Institute for Occupational Safety and Health 

(NIOSH). This program uses the Mark-Bieniawski formula to determine pillar strength, and it has the 

capability to account for loadings on barriers and abutment pressures.

Pillar stability formulas can be divided into two categories – analytical and empirical. Analytical 

formulas require extensive material testing, an understanding of loading under varying conditions, 

and a safety factor (typically about 2) based on knowledge and understanding of all variables. These 

relationships are best applied by engineers who are experienced in mining rock mechanics. One such 

relationship, Wilson’s equation, which is one of the first analytical models developed for estimating 

pillar strength, is directly calculated, thus making it more flexible and adaptable to actual conditions 

than any empirical equation. It can be used to estimate the stress distribution from the edge of a pillar 

to the center based on the confined core theory (Wilson and Ashwin, 1972). Wilson’s equation uses 

the Mohr-Coulomb failure criterion for modeling the coal and surrounding rock; however, at high 

confinement (high w/h ratio) coal strength is not linear with the result that it overestimates pillar 

strengths. Scovazzo (1995) modified Wilson’s equation to incorporate more appropriate coal and rock 

failure criteria, specifically those presented by Kalamaras and Bieniawski (1993).

The most commonly used pillar stability formulas are empirical equations where few parameters 

need to be defined. Since empirical equations are based on statistical analysis of failures and successful designs, a stability factor (not to be confused with safety factor) is employed. One commonly used 

empirical equation is the Mark-Bieniawski formula. For the Mark-Bieniawski formula, the recommended stability factor is 1.5 for mines less than 750 feet deep (Mark, 2006). A smaller stability factor 
8-19

Site Mining and Foundation Issues

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is generally used for mines greater than 750 feet deep; for example, a stability factor of 0.9 is used for 

pillars with over 1,250 feet of cover. These recommended stability factors are based on the assumption of equal area loading. Empirical equations are applicable to specific mining regions and coal 

seams and should not be applied outside the region for which they were developed unless statistical 

analyses are performed for the region in which the equation is intended to be used. While some longterm pillar instability can be tolerated in certain mining situations, impoundment designers should 

consider a higher margin of safety for overburden support and control of detrimental differential 

movements within the dam and foundation.

Conservative coal strengths based on statistical analysis of failure should be used for pillar failure 

analysis. Laboratory tests for verifying the strength of the coal can be conducted, although this rarely 

occurs and is not encouraged. The limited use of testing is in part due to the scatter typically encountered in uniaxial rock tests, a problem that is exacerbated by coal cleating and softness, which make 

test samples difficult to prepare. A laboratory testing program should include sufficient samples and 

statistical analysis to verify that data are reliable. As a result, the mass uniaxial compressive strength 

of coal is often assumed to be 900 psi.

The uniaxial compressive strength of coal determined in the laboratory is many times the in-situ 

strength of coal in a pillar. This size effect is caused by flaws in the coal that are present in the larger 

mass of the pillar but not in sample sizes tested in the laboratory. It would take test samples with the 

impractical minimum dimension in the range of 3 feet (Pariseau, 1975) to 5 feet (Bieniawski, 1968) for 

accurate determination of the mass uniaxial compressive strength of coal. The most accepted method 

for reducing laboratory compressive strengths for coal to reflect in-situ strength of the coal was developed by Hustrulid (1976) and is recommended by Bieniawski (1992) for use with his pillar formula.

In any analysis of pillar stability, it is best if the method being used is calibrated to conditions at the 

subject mine. Thus, the model should be applied to a number of locations in the mine to determine 

how well it reflects actual conditions or the actual performance of mine pillars. Pillar analysis is more 

complicated when multiple seams are mined. In such cases, the loading conditions are much more 

complex due to load transfer and the potential for stress concentrations. Simplified software models, 

such as LAMODEL, are available for performing this type of analysis. Higher pillar stability factors 

should be used for multiple seam analyses to account for the additional uncertainty. Both LAMODEL 

and ARMPS are available from the NIOSH web site.

8.4.2.3 Pillar Punching (Floor Failure)

Pillar punching (pillar foundation failure) occurs when a pillar pushes into the mine floor allowing 

the roof to sag. This sagging can create fractures and/or open existing joints in the overburden. If the 

punching occurs over a large area, the surface will be affected in a similar manner (normally without 

bed separation) to the situation where total extraction of a thinner seam has occurred.

Pillar punching occurs when the load on a pillar exceeds the bearing capacity of the mine floor beneath 

it. Pillar punching can be caused by existing loads, saturation of the floor material causing softening 

and loss of strength, additional loading from impounded slurry and/or water, and loss of buoyant 

pressure from a lowered mine pool.

A key factor to consider in evaluating the potential for pillar punching is experience elsewhere in the 

mine. A review of floor and pillar performance data, particularly for wet conditions, should be made 

as part of foundation bearing capacity analyses for evaluation of pillar punching. It may be appropriate to reduce the floor strength when both slender pillars and wet conditions are present or are anticipated as the result of construction of an impoundment. Additional information on pillar punching 

and associated floor heave can be found in Ganow (1975) and in Adler and Sun (1968).
Chapter 8

8-20 MAY 2009

8.4.2.4 Outcrop Barrier Failure by Shear or Punching

In an outcrop barrier situation, a potential failure mode that must be considered is whether the pressure from water and/or slurry in the impoundment may become high enough to overcome the shear 

strength that is holding in place the plug of material separating the impoundment from the mine 

works. Failure can occur through the coal seam itself, through the strata above the coal seam, through 

a weakened floor layer, or along an interface between strata. Figure 8.7 illustrates a typical outcrop 

barrier cross section, including the coal barrier, jointing and stress relief fractures. Surface features 

such as slope instability and regrading for access roads are also shown on the figure.

Analysis of this failure mode for a postulated plug separating the impoundment from the mine 

involves comparison of the cumulative force tending to push the plug into the mine to the available resistance from shear forces around the perimeter of the plug. The pressure driving the plug is 

normally taken as the hydrostatic head from the impoundment plus applicable lateral earth pressure from settled fines. A major issue with this type of analysis is judging what to use for the shear 

strength along the top, bottom and sides of the plug. Factors such as the presence of weathered 

joints, the presence of cleats in the coal, the softening of the floor from saturation, and the difficulty 

of obtaining test data can result in uncertainties with respect to shear strength. Furthermore, as 

seepage into the mine occurs, the strength along potential shearing surfaces may degrade over 

time, and, if sloughing or a roof fall occurs, the thickness of the plug may be reduced. Additionally, 

the potential hydraulic gradient across the plug must be carefully considered, as water pressures 

may be elevated on planes of discontinuity within the plug and present a more severe loading condition than at the plug boundary. Given these uncertainties, any shear-type failure analysis should 

be based upon conservative values for shear strength and a conservative factor of safety of at least 

2.0 (MSHA, 2003).

8.4.2.5 Internal Erosion

Internal erosion, or piping, is the movement of material (typically soil particles) under seepage forces, 

and it can occur where the gradation is such that particles are dislodged and carried along by seepage and drag forces. When soil overlies rock, movement of material can occur along or into joints, 

stress relief fractures, or subsidence fractures. Figure 8.7 illustrates typical conditions encountered 

near a coal seam outcrop where weathered joints may represent seepage pathways from a future 

impoundment. The presence of stress relief fractures or subsidence fractures would exacerbate the 

concern for seepage and internal erosion. Seeping water creates a drag force on the material that it is 

seeping through or around. If this drag force is greater than the frictional or cohesive forces holding 

material particles in place, they will be transported by the seeping water. As smaller particles are carried away, additional flow occurs, increasing the drag force and dislodging even larger particles. The 

process can continue until a channel (also referred to as a “pipe”) is formed. Piping into foundation 

discontinuities can lead to failure of an embankment. In a breakthrough situation, where the source 

of the seepage is an impoundment and internal erosion occurs through the material between the 

pool and the mine workings, the pipe can gradually enlarge and lengthen to the point where there 

is a significant discharge into the mine. Piping is dependent on the type, gradation, consistency, and 

cementation of the intervening material; relative hydraulic conductivities of the materials along the 

critical seepage front; and the hydraulic gradient.

One technique for preventing internal erosion is to incorporate filter layers (material with grain-size 

distributions that prevent particles from moving along a seepage path) at critical locations. Granular filter material or various geosynthetic materials can be used for this purpose. Filter criteria are 

discussed in Section 6.6.2.3. Also, the gradient can be reduced by using lower-hydraulic-conductivity materials along the upstream portion of the seepage path to increase head loss before the water 

reaches a critical zone. Excess water pressure can also be controlled by providing drainage to reduce 

pressures upstream of more erodible zones, such as by installing an internal drain protected by filters.
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Site Mining and Foundation Issues

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In breakthrough situations, seepage into an underground mine can occur through the roof, floor, 

or coal seam. If the material at the location(s) where the seepage enters the mine is weathered, finegrained, or loosened as coal may be along a face or rib, then the force of the seeping water may carry 

particles away and/or cause the in-situ material to slake or unravel. Over time, this can cause the area 

to weaken, slough, or progressively deteriorate. This is especially a concern near an outcrop barrier, 

where the internal erosion could cause the ground to give way or a pipe to form through the affected 

zone. Either situation could lead to an uncontrolled hydraulic connection between an impoundment 

and the mine workings.

Whenever seepage is flowing outward and exiting at an unconfined surface, as may be the case 

near the toe of a dam or impounding embankment, the value of the critical gradient with respect to 

piping is approximately one. However, when seepage is generally horizontal, such as through a rib, 

or vertically downward, such as through the mine roof, internal erosion can develop under smaller 

gradients.

Determining the hydraulic gradient between an impoundment and a mine requires accurate characterization of the permeabilities of the intervening materials. Near an outcrop, the seepage may be 

governed by flow along weathered joints and thus may be difficult to characterize and model. Gradients can be estimated by drawing flow nets or by using a finite element seepage program. Once 

the gradient has been estimated, the effect of the seepage force on the seepage medium can be determined. The seepage force, per unit volume of seepage medium, is equal to the product of the gradient 

and the unit weight of water. Whether the medium material will be dislodged by a combination of 

seepage and gravity forces depends on the frictional and cohesive strength of the material.

Cohesionless soils, particularly silts and fine sands, are most susceptible to piping. Clays are more 

resistant to piping because the cohesive strength of these soils helps to prevent particles from being 

carried away. However, clayey soils are not immune to internal erosion, especially if they abut open

ACCESS

ROAD

STEEP SLOPE AT JOINT

IN RESISTANT ROCK

SHALE

SANDSTONE WITH

INTERBEDDED SHALE

COAL SEAM

SHALE

REMAINING COAL BARRIER MINE WORKINGS

SHALE

SANDSTONE

SANDSTONE

SANDSTONE WITH

INTERBEDDED SHALE

SANDSTONE

TALUS

UNDERCLAY

HIGHLY WEATHERED

MATERIAL

JOINTS

FIGURE 8.7 HILLSIDE DISTURBANCE AT COAL OUTCROP

(ADAPTED FROM SAMES AND MOEBS, 1992)

FIGURE 8.7 HILLSIDE DISTURBANCE AT COAL OUTCROP
Chapter 8

8-22 MAY 2009

graded materials, open joints or fractures. Also, soft rocks, such as weakly cemented sandstones, have 

been associated with piping failures. Even shales, which are usually considered resistant to piping, 

have developed piping voids under conditions of very high gradients (Sowers and Sowers, 1970). In 

these unusual cases involving rock strata, weaker characteristics of the rock mass presumably led to 

pathways of increased seepage that intersected and then led to piping of more erodible materials.

If the materials between the mine works and an impoundment are prone to internal erosion or are 

sensitive to progressive deterioration, then the impoundment should be designed: (1) with suitable 

filters and drains so that seepage can be collected and released in a controlled fashion, (2) with seepage 

barriers so that pressures are minimized, or (3) with some combination of these two approaches.

8.4.2.6 Bulkhead Failure

If there is a mine opening in a potential breakthrough area, then the potential for failure of the bulkhead used to block the mine opening must be evaluated. Depending on their thickness and shape, 

bulkheads can fail when the pressure acting on them causes the bending or shear strength of the 

bulkhead material to be exceeded. Bulkheads can also fail if the material that they are anchored or 

keyed into is not strong enough to resist the applied pressures and the pressures from water seeping 

around the bulkhead, or if the bulkhead is not adequately anchored to the surrounding rock strata. 

Hydraulic fracturing within floor strata can aggravate seepage and transmission of pressures resulting in the typical recommendation to remove underclay/claystone at bulkhead locations. Analysis of 

bulkheads is addressed in Section 8.5.2.

8.4.2.7 Trough Subsidence and Subsidence Cracks

If room and pillar workings are located near the footprint of an impoundment (with inadequate stability factors on the remaining pillars), or if either longwall mining or secondary mining of pillars has 

occurred, an analysis of the potential impact of trough subsidence should be performed. In this situation, zones of tension and compression stress or deformation extend from the mine workings to the 

ground surface. The area at the ground surface affected by total extraction mining is typically larger 

than the mined area and is related to the draw angle. The assumed draw angle should be consistent 

with local experience, with past subsidence associated with mining in the coal seam, site topography, 

and the method of mining.

The effect that subsidence has on steep natural slopes is a matter of debate (Luo et al., 1996). It 

appears that the effect is greater than the generally accepted regional angle of draw and is dependent upon local geologic conditions and the direction of mining. The approach of mining from the 

downslope and towards the upslope appears to result in the greatest horizontal displacement and 

accompanying stress.

If mining is approaching an impoundment, operations should be terminated at a point where ground 

deformations will not adversely affect the impoundment. It is rare for the angle of draw to exceed 

25° in the U.S., and typical values range from 10 to 25°. In the Northern Appalachian Coalfield, the 

maximum angle of draw is normally 25°, while in the Black Warrior Basin and Southern Appalachian 

Coalfield, it is typically around 12°. Work by Scovazzo (2008) at three mines in the Arkoma Basin 

places this value at 19°. For the Illinois Basin, the angle of draw typically is in the range of 20 to 25° in 

the southern portion, increasing to 30° in the northern portion as glacial till thickness increases. The 

angle of draw varies with the dip of the coal seam and the slope of the ground surface and decreases 

as the percentage of hard rock (sandstone and limestone) in the overburden increases. No pattern for 

the western U.S. has been identified.

For total extraction mining (longwall mining or secondary mining of pillars) directly below an 

impoundment, the U.S. Bureau of Mines (USBM) guidelines provided in Babcock and Hooker (1977) 
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Site Mining and Foundation Issues

MAY 2009

indicate that the amount of cover should be at least 60 times the mining height, as illustrated in Figure 

8.6. This guideline was based on studies that looked at disturbance of the rock strata, and the change in 

the hydraulic conductivity of these strata, above mined areas. Data indicate that it is unlikely that the 

rock strata above an area of total extraction mining will be disturbed for more than 25 to 35 times the 

mining height. This thickness guideline (Babcock and Hooker, 1977) is based on the concept of a constrained zone. When total extraction occurs, cracks and joints open at the surface and in the mine roof, 

but if the overburden is thick enough, the induced stress is absorbed or resisted without fracturing. It 

should be noted that the Babcock and Hooker (1977) strata-thickness criterion is for evaluation of the 

potential for developing a hydraulic connection between the surface and the mine; it does not address 

the potential for other adverse effects such as tensile strains at the surface and loss of freeboard.

Another USBM research report titled, Criteria for Determining When a Body of Surface Water Constitutes 

a Hazard to Mining (Kendorski et al., 1979), recommends cover thicknesses greater than 60 times the 

mining height when the mining height is less than 7.5 feet. For example, this report recommends that 

the overburden thickness should be 71, 80, 95, and 117 times the mining height for mining heights of 

6, 5, 4, and 3 feet, respectively. The report also indicates that, where inflow can be tolerated, the thickness of cover can be reduced if certain types of strata (e.g., claystone or shales that are less prone to 

cracking and have low hydraulic conductivity) are present.

The preceding guidelines refer to bodies of surface water and do not specifically consider mitigating factors or measures such as fine refuse deposits that may be associated with a slurry impoundment. These guidelines are important historical literature and should be reviewed as part of initial 

evaluations; however, it is recommended that subsidence analyses and prediction models be used 

for evaluating potential deformations and strains on embankments and impoundments. On the basis 

of these analyses, smaller overburden buffers than identified in the aforementioned references may 

be sufficient to prevent a breakthrough to a reservoir or slurry impoundment and may satisfactorily 

limit impoundment leakage/seepage.

A number of subsidence prediction models are available. Two commonly used models include SDPS 

(Surface Deformation Prediction System) developed at Virginia Polytechnic Institute and State University and CISPM (Comprehensive and Integrated Subsidence Prediction Model) developed at West 

Virginia University. These programs can be used to estimate surface subsidence and ground strain 

caused by mining. Two cautions are offered concerning the use of these types of programs: (1) they 

should only be used for the type of topography and mining conditions for which they were developed 

and (2) the strain computations by these programs typically do not account for strain concentration 

along existing discontinuities. Concerning the second point, studies have shown that site topography may have a substantial effect on the development and concentration of horizontal strain.

Furthermore, where the confinement and continuity in the overburden is diminished, such as on 

hillsides and in highly weathered and fractured material, the tensile strain that is induced by mining 

may accumulate along one or more of the joints rather than being more evenly distributed. This can 

be significant in subsidence and breakthrough potential evaluations where open joints can provide 

seepage pathways. For total extraction mining, the strata and ground surface above the mine are 

affected regardless of the mining depth, and the effect of tensile strains and strata disturbance on 

dam stability and breakthrough potential should be evaluated.

8.4.2.8 Failures Related to Auger and Highwall Mining

Auger or highwall mining openings in an abutment can have adverse effects on an impounding 

embankment. Deformation of the abutment and embankment can occur if the webs between the 

holes deteriorate or fail. The holes can also provide seepage paths. Breakthroughs can occur through 

auger or highwall mining holes, through the coal remaining at the ends of auger holes, or as a result 
Chapter 8

8-24 MAY 2009

of the collapse of these holes. The importance of identifying the locations of these holes during site 

exploration cannot be overemphasized. During highwall reclamation these holes are typically covered without backfilling. If they are not discovered and accounted for in the impoundment design, 

then seepage pressures under high impoundment head can cause movement of backfill and water 

into auger holes. If the auger holes are accidentally connected to the mine, then there is a direct path 

for a breakthrough or piping. If the auger holes were terminated close to the mine workings, the 

remaining coal barrier can potentially fail in a shear or punch-type mode. Also, the narrow pillars or 

webs of coal between these holes may be marginally stable and may subsequently deteriorate and 

collapse, resulting in highwall instability and ground deformations that could adversely affect the 

embankment or lead to a breakthrough. NIOSH has a computer program (ARMPS-HWM) that performs an empirical analysis of highwall mine pillars.

8.4.2.9 Hillside Movement or Disturbance

Hillside movement and disturbance (landslide, creep, or human impacts) can occur at and above a coal 

outcrop. Weathering tends to reduce the strength of surface soils and rocks while gravity provides a 

driving force to move soil and rock down the hillside. Surface mining (contour mining) and site development activities, such as road and channel construction, can accelerate the process through removal 

of soil and rock from the hillside or by aggravating or causing landslides. Any movement or removal of 

soil and rock from the hillside can reduce and/or disturb the outcrop and overburden barriers between 

the underground mine and an overlying impoundment, thus contributing to the occurrence of a sinkhole, internal erosion, or shear-type failure. Figure 8.7 illustrates the effect of hillside disturbance on the 

coal outcrop barrier.

8.4.2.10 Mine Blowout

A mine blowout occurs when water impounded in an underground mine breaks out through either 

a sealed mine opening or a thin section of the coal outcrop barrier. Such a blowout may be a secondary consequence of an impoundment breaking into an underground mine. Additionally, a mine 

blowout in an impoundment watershed may represent an inflow source that should be considered 

in impoundment design. Such an event could cause a significant amount of water to flow into the 

impoundment, damage impoundment structures, and potentially affect impoundment safety. A 

guidance manual for design published by the Office of Surface Mining (OSM) is, Outcrop Barrier 

Design for Above Drainage Coal Mines, by Kohli and Block (2007). This document presents compiled information on case histories as well as design studies by Pearson et al. (1981). If seepage from 

an underground mine is occurring at an embankment or impoundment site, the conditions and 

potential effects should be evaluated. As a minimum, the facility design should account for collecting seepage and discharging it in a controlled manner that will not adversely affect the embankment 

or other facility structures.

8.4.2.11 Seismic and Blasting Events

Typically, earthquakes and surface blasting do not affect underground mines. The exception is 

when a fault crosses through a mine or when surface waves can affect the stability of the portals at 

or near their contact with the surface. The presence of an active fault passing through a U.S. coal 

mine is not a situation that normally occurs. If such a fault is identified, it would be necessary to 

conduct comprehensive stability analyses not just for the mine but also for the associated surface 

facilities including the refuse embankment and impoundment. Bhabdaru and Arora (1987), Nicholls et al. (1971), Rupert and Clark (1977), Fourie and Green (1993), and Fernandez and Van Der 

Heever (1996) describe blasting and earthquake impacts on mines.

When blasting is planned within 500 feet of an active underground mine, the Surface Mining 

Control and Reclamation Act of 1977 requires that the Operator’s blasting plan be approved by 

OSM (or appropriate state agency) and MSHA. Blasting regulations are provided in 30 CFR § 
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Site Mining and Foundation Issues

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780.13 and 30 CFR § 816.61 through 816.68 and also 816.79. State criteria may also be applicable. 

The blasting plan should be prepared by a professional licensed in the state where the blasting is 

to be performed. Potential concerns when an impoundment is present include fracturing of abutments, impacts to pipes and other rigid structures, or possibly upstream construction. Where an 

impoundment is present, the potential impacts should be evaluated, and monitoring of particle 

velocity with a seismograph may be appropriate. Section 6.6.7 addresses potential blasting impacts 

on impoundment structures.

8.4.3 Mine Subsidence Potential and Analysis

8.4.3.1 Pillar Evaluation and Analysis

For room and pillar mining, subsidence potential can be evaluated through analysis of the overburden stress imposed on coal pillars and comparison of this stress to the pillar strength. To determine 

the average stress on a coal pillar, the tributary area approach may be used with the assumption that 

the overburden pressure increases at a rate of 1.1 psi per foot of depth:

Sp = 1.1 H [(w + B)/w][(L + B)/L] (8-1)

where:

Sp = average pillar stress (psi)

H = depth below ground surface (ft)

w = pillar width (ft)

L = pillar length (ft)

B = opening width (ft)

Other assumptions inherent in this formula include:

• The coal seam is subject only to vertical pressure.

• Each pillar supports the column of rock and other overburden overlying the pillar 

and a proportionate share of surrounding openings.

• The load is distributed uniformly over the pillar cross section.

Most empirical pillar equations have been developed and the results statistically analyzed based on 

these assumptions, particularly with respect to tributary area loading. Therefore, empirical equations should only be used for uniformly distributed tributary loading. Numerical models such as 

LAMODEL can be used to analyze non-uniform pillar loading (transfer of load to flanking barrier pillars). However, if pillars are to be analyzed without considering tributary area loading, then the pillar 

loads should be determined through a detailed rock mechanics analysis using numerical modeling 

software. Typically, a higher safety factor is used for such a pillar analysis because of uncertainty as 

to the loading and the variability in the numerical methods typically employed.

Pillar strength formulas used for design of ground control measures are discussed in Bieniawski 

(1992). For subsidence evaluations for coal refuse disposal facilities, pillar strength formulas consistent with conditions in the underlying mine should be adopted; otherwise formulas applicable to 

local mines should be employed. Several empirical formulas that have been used to determine pillar 

strength are presented in Figure 8.8. The Pittsburgh Coal seam with a pillar height of 10 feet is used 

as an example of the influence of the width to height ratio on the overall pillar strength. The potential 

for subsidence can be determined by dividing the pillar strength by the pillar load. An important 

consideration when comparing the pillar strength formulas is the value of the recommended stabil-
Chapter 8

8-26 MAY 2009

FIGURE 8.8 COMPARISON OF PILLAR STRENGTH FORMULAS

WITH RESPECT TO WIDTHTOHEIGHT RATIO

PILLAR WIDTH-TO-HEIGHT RATIO, w/h

= Experimental data from actual mine

1 2 3 4 5 6 7 8 9 10 11 12

2

3

4

5

6

BIENIAWSKI FORMULA p = 0.64 + 0.36 (w/h)

p = w/h

OBERT-DUVALL FORMULA p = 0.78 + 0.22 (w/h)

1

10 20 30 40 50 60 70 80 90

0

1000

2000

3000

PILLAR WIDTH, w (feet)

PILLAR STRENGTH, 

p (psi)

PITTSBURGH SEAM

p = k w

OBER

h

T-DUVALL FORMULA

w

h

p = 1 (0.778 + 0.222 )

BIENIAWSKI FORMULA

 p = 1 (0.64 + 0.36 w ) h

HOLLAND FORMULA p = 1 

w

h

SALAMON-MUNRO p = 

w0.46

h

 k 0.66

12

1 = SAMPLE STRENGTH = 930 psi

k = SCALING FACTOR = 5580

PILLAR HEIGHT = 10 FT.

(BIENAWSKI, 1987)

NORMALIZED PILLAR STRENGTH, 

p 

1 /

FIGURE 8.8 COMPARISON OF PILLAR STRENGTH FORMULAS WITH RESPECT 

TO WIDTH-TO-HEIGHT RATIO
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Site Mining and Foundation Issues

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ity factor, which varies depending on the empirical formula used (Bieniawski, 1992). Mark (2006), in 

addressing the stability factor for the Mark-Bieniawski Formula, recommends a stability factor of 1.5 

for pillars with under 750 feet of cover and 0.9 for pillars with over 1,250 feet of cover. As previously 

noted, the term “stability factor” should not be confused with “safety factor.” While some long-term 

pillar stability can be tolerated in certain mining situations, impoundment designers should consider 

a higher margin of safety for overburden support and control of detrimental differential movement 

within the dam and foundation.

While the occurrence of subsidence over mine workings in some areas is a function of time, the general assumption for pillar analyses is that coal does not deteriorate over time. The reason is that coal is 

highly jointed due to cleating, and additional cracking of the coal does not reduce the confinement of 

the roof and floor, nor does it markedly reduce the coal’s mass strength. Initial pillar designs typically 

take into account spalling for coal seams susceptible to spalling. This is done by reducing the effective 

dimensions of the pillars by the anticipated spalled width and widening the opening width by this 

spalled dimension. Oxidation of the coal can occur for seams above drainage, but this effect tends to 

be limited to the yielded outer portions of the pillar. Once pumping and ventilation is discontinued, 

mines below drainage will flood, thus mitigating further deterioration of the coal.

The weathering of soft (high-clay-content) partings (non-coal layers in the pillars) reduces pillar 

strength over the long term, although the impact appears minor and may not occur after the mine is 

flooded (Biswas et al., 1995). Biswas et al. (1999) presented an approach for estimating time-dependent deterioration of partings, as well as coal. The authors indicated that much work remains before 

time effects on coal pillar strength are fully understood.

Computer programs for pillar stability analysis include the ARMPS and LAMODEL models. These 

programs can be used to evaluate stress distribution and factor of safety. Analyses can be performed 

considering the pillar sizes and arrangement within the mine, recognizing that some smaller pillars 

may shed load to larger pillars.

In addition to pillar stability, floor stability is also related to subsidence potential. Bearing capacity 

analyses have been applied to evaluation of pillar punching into soft floor materials, using the following formulas (Bieniawski, 1992; Brady and Brown, 2004):

For long (L/B > 10) pillars:

Qu = 0.5 (γBN1) + (cNc ) (8-2)

For pillars with L/B < 10:

Qu = 0.5 (γBN1S1) + (c cot φ Nq Sq – c cot φ) (8-3)

where:

Nc = (Nq – 1) cot φ

N1 = 1.5 (Nq + 1) tan φ

Nq = exp [π tan φ] [tan2 (π/4 + φ/2]

S1 = 1.0 – 0.4 (B/L) (dimensionless shape factor)

Sq = 1.0 – sin φ (B/L) (dimensionless shape factor)

γ = density of floor strata (pcf)
Chapter 8

8-28 MAY 2009

B = width of pillar (ft)

L = length of pillar (ft)

c = cohesion of the floor strata (psf)

φ = friction angle of the floor strata (degrees)

Bowles (1996) presents several other bearing capacity equations that are applicable to conditions/

situations not reflected in the above equations, including: (1) evaluation of strength parameters for 

rock for use in bearing capacity analysis, (2) the presence of adjacent pillars, (3) the presence of layered foundations, and (4) the effects of horizontal movement of very soft materials below the pillars. Equations developed by Terzaghi, Vesic, and others include parameters to account for these 

situations. Bowles (1996) presents bearing capacity factors for rock that are recommended for use 

in the Terzaghi equation. Bowles also presents bearing capacity failure modes for evaluating heave 

considering the effect of adjacent pillars (using an excavation trench model) for layered strata and for 

the case when the opening widths are less than 0.7 times the pillar width. Bieniawski (1987) presents 

equations based on the work of Vesic for analyzing multilayered conditions. Vesic’s equation is commonly used for the Illinois Basin because of work by Y. P. Chugh from Southern Illinois University. 

Similar to pillar partings, floors of mines that are high in clay content tend to deteriorate over time, 

but this effect is limited to the unconfined floor in openings and near pillar edges (Chugh et al., 1987).

The strength parameters (c and φ) of the mine floor strata can be affected by moisture. Just as the 

evaluation of pillar strength should reflect pillar performance at local mines, available records of 

floor performance should be reviewed when selecting strength data for analysis. For evaluating pillar 

punching potential, Bieniawski (1992) recommends a factor of safety against bearing capacity failure (qu /Sp) of 2.0 based upon the assumptions associated with computing the average pillar stress, 

although there are situations where a lower factor of safety (generally at least 1.5) is acceptable to 

provide long-term stability.

It is also possible that very soft material can be squeezed from beneath a pillar. This type of foundation 

failure is appropriately called a squeeze failure and can be analyzed as described in Bowles (1996).

8.4.3.2 Subsidence Evaluation and Analysis

Empirical approaches for subsidence analysis include: (1) the graphical method, (2) profile functions 

and (3) influence functions. The graphical method involves use of charts or nomographs and is generally applied in only a few geologically-similar regions. The profile functions approach involves the 

derivation of mathematical functions that describe the profile of the subsidence trough at the surface 

based on observed data. This method can be applied to geologically dissimilar conditions by modifying the profile constants, but the method is limited to predicting subsidence at specific points. The 

use of influence functions involves application of the principal of superposition to evaluate surface 

subsidence at any point influenced by the underground mine, based upon measured profile data. 

This method is based upon the assumption of homogeneous, isotropic overburden material, but it 

has been found to be suitable for subsidence prediction over underground workings with irregular 

or complex geometries. Singh (1992) provides selected profile and influence functions and examples. 

Continuum mechanics utilizing various elastic-plastic models has also been employed to analyze 

subsidence, although the complexity involved in depicting overburden response to mining has limited its application.

Computer programs such as SDPS and CISPM can be used for evaluation of stress at mine areas with 

varying overburden conditions, pillar and barrier arrangements, and subsidence parameters (settlement, horizontal displacement, curvature and tilt). Based on empirical or site-specific regional parameters, SDPS calculates the ground deformation indices using both the profile function method and 

the influence function method. The profile function method requires the following minimum input: 
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Site Mining and Foundation Issues

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panel width, overburden depth, seam thickness, and percent of hard rock within the overburden. The 

influence function method requires that the mine plan and measured subsidence survey information 

applicable to the area be input, although average parameters applicable for eastern U.S. coal fields 

can be selected. The results can be plotted in relation to mine or surface structure geometry.

Procedures for the evaluation of subsidence at coal refuse impoundments overlying mine workings 

were discussed by Newman (2003) and Karmis and Agiotantis (2004) and include the following steps:

• Digitizing the mine layout plan and ground surface topography.

• Inputting the mine layout plan and topography into software such as SDPS.

• Evaluation of the geologic and overburden properties for classifying and establishing 

the dimensions of hard rock strata.

• Calibration of the model using regional parameters (e.g., influence angle, strain coefficient, edge effect).

• Establishment of extraction characteristics and the mining height of each panel.

• Adjustment of the extraction geometries based on mine characteristics.

• Calculation of the pertinent deformation indices.

• Contouring and superposition of the results on mine and topographic maps.

• Evaluation of the results.

8.4.3.3 Subsidence Damage Criteria

The strain criteria presented in Table 8.5 have been used in assessing the potential for subsidence 

impacts on earthen embankment dams and water bodies at the ground surface. Subsidence settlement can affect drainage features and freeboard, as well as impounding structure stability, thus 

site-specific subsidence parameters need to be established. Impacts on the internal integrity of 

structures are not clearly related to any specific magnitude of subsidence, but rather to differential 

movements. For undermined impoundments, the National Coal Board (1975), Babcock and Hooker 

(1977), and Whittaker and Reddish (1989) have published guidelines based on case studies for 

limiting surface tensile strain in the overburden to a range of 0.5 to 1.5 percent with the objective 

of minimizing the inundation hazard to mines. These guidelines, which are empirical, are based 

upon data from reportedly successful mine operations beneath bodies of water. When sufficient 

engineering data and mining experience are available, these conservative guidelines should be 

updated based upon the new data. While strain levels are a rational basis for assessing structural 

impacts, the difficulty in accurate prediction and measurement of strains suggests that prudence is 

warranted with respect to establishing tolerable strain and assessing safety zone offsets based on 

a strain criterion.

Additionally, a refuse embankment or impounding structure may also be subject to damage and failure due to surface tensile strain. Tensile strain has been reported as a gauge of impending cracking in 

earth embankments by Sherard (1973), who reported that initial embankment cracks generally occur 

in the range of 0.1 and 0.3 percent tensile strain, as discussed in Section 6.6.3.1. A study of the effects 

of subsidence on spoil heaps in England found that cracking occurred where observed tensile ground 

strains exceeded 0.3 percent (Forrester and Whitaker, 1976). Cracking can endanger a dam by reducing the strength of an embankment slope and by creating paths for seepage and material movement 

resulting in internal erosion. This criterion (as opposed to loss of strength) is particularly relevant to 

embankment features that control seepage such as liners and cohesive soil cores.

Other structures that may be part of refuse embankments include conduits and pipes, which may be 

sensitive to low levels of strain depending on their constituent materials and the direction of straining relative to their orientation. Peng and Luo (1993) report that the critical tensile strain for strain-
Chapter 8

8-30 MAY 2009

sensitive structures is approximately 0.2 percent, which is the same as the strain associated with the 

appearance of visible cracks in masonry walls.

Nieto (1979) published a case history for central Illinois illustrating an evaluation of underground 

mining and subsidence at a depth of 620 feet. The study involved assessment of the potential for 

damage to an earth dam associated with adoption of a limiting tension strain of 0.25 percent, consistent with the references cited above and in Section 6.6.3.1.

8.4.4 Mine Breakthrough Potential and Analysis

In this section, mine breakthrough potential and analysis are discussed for: (1) a coal seam with 

mine workings outcropping into the impoundment area (outcrop barriers) and (2) a coal seam with 

mine workings extending beneath all or a portion of the impoundment pool (overburden barriers).

8.4.4.1 Outcrop Barriers

Outcrop barriers between mine workings and an impoundment, where the coal seam outcrops into 

the impoundment, may include:

• Natural soil, coal, and rock overburden between the mine workings and the ground 

surface

• Coal barriers separating auger holes and house-coal adits from underground mine 

workings

• Bulkheads and seals consisting of shot rock, aggregate, coarse refuse, grout and/or 

concrete used to backfill “punch-outs,” auger holes, portals, ventilation boreholes, 

and rock dust or utility boreholes.

The primary modes of outcrop barrier failure include: (1) uplift failure, (2) shear plug failure, and 

(3) fractures initiated through discontinuities or subsidence that become enlarged as a result of seepage and internal erosion. Uplift failure occurs when buoyant forces reduce the overburden weight 

to the point where the differential pressure on the outcrop barrier causes failure. It can be analyzed 

separately or in conjunction with shear plug failure. Shear plug failure analysis is generally applied 

to outcrop barriers along coal-floor and coal-roof planes, assuming that the overburden is completely 

saturated. Because of uncertainties about the limits of mine workings, as well as overburden material 

properties, conservative assumptions are generally adopted. Figure 8.9 illustrates a sliding wedge 

analysis at an outcrop barrier and the associated free body diagram for computation of forces using 

the approach proposed by Newman (2003). Following this approach, which considers only the resistance along the coal-floor surface, calculation of the factor of safety FS for a unit width along the sliding wedge can be determined from:

FS = 

[Fslurry + Frock + Fcoal] tan φ (8-4)

 Fwater + FKo

where:

 Fslurry = [(w hslurry) – w2

] (γslurry – γw) 2S

 

 Frock = [ w2

] (γrock – γw) 

2S
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Site Mining and Foundation Issues

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 Fcoal = [w hseam ] (γcoal – γw)

 Fwater = hw hseam γw

and:

Fslurry = effective weight of slurrry (lb/ft)

Frock = effective weight of overburden rock (lb/ft)

Fcoal = effective weight of coal (lb/ft)

Fwater = effective force of water (lb/ft)

FKo = lateral force on barrier from overburden (lb/ft)

hw = net hydrostatic head on coal seam (ft)

hseam = coal seam height (ft)

γw = unit weight of water (pcf)

γslurry = unit weight of slurry (pcf)

γrock = unit weight of overburden rock (pcf)

γcoal = unit weight of coal (pcf)

φ = shear strength for barrier contact (degrees)

w = outcrop barrier width (ft)

S = ground slope at outcrop (dimensionless)

The effective lateral force on the barrier from overburden FKo is given by the following relationship, 

based on the properties of the slurry and conservatively treating the soil/weathered coal as slurry:

FKo = [(γslurry – γw) hslurry hseam ](1 – sin φslurry ) (8-5)

where:

hslurry = depth of slurry to barrier (ft)

hseam = coal seam height (ft)

φslurry = shear strength of slurry (degrees)

The preceding equations can be modified to include resistance along the roof-coal interface and to 

incorporate protective embankments at the outcrop. In applying the analysis, the weight of overburden rock and coal may be taken as the total weight only if the mine workings are not and will not be 

flooded. The effective (or buoyant) weight is applicable if the coal and overburden are affected by 

seepage and the mine workings are flooded. Conservative assumptions associated with this analysis 

include:

• Hydrostatic impoundment head is assumed to be acting at the barrier.

• There is no cohesion along the coal-floor interface, and the resistance along the roofcoal interface is ignored consistent with Figure 8.9.

• Three-dimensional effects are not considered (side resistance of the wedge of coal 

should generally be ignored because of the potential for discontinuities).
Chapter 8

8-32 MAY 2009

Selection of the barrier width w requires a thorough evaluation of mine maps and exploration information to identify the representative minimum dimension, based on the following:

• Extent of weathering of the outcrop, which can best be evaluated by exploration and 

sampling.

• Potential for spalling and deterioration of the barrier rib within the mine workings, which can best be judged based on pillar performance within the mine. 

Biswas et al. (1999) introduce a method for estimation of the decline of pillar 

strength with time.

FIGURE 8.9 OUTCROP BARRIER BREAKTHROUGH

 ANALYSIS FOR SHEAR FAILURE

8.9b SIMPLIFIED FREE-BODY DIAGRAM OF EFFECTIVE BARRIER SEGMENT

(ADAPTED FROM NEWMAN, 2003)

8.9a ILLUSTRATION OF OUTCROP BARRIER

MINE VOID MINE WORKINGS

SLURRY

SOIL OVERBURDEN

ROCK OVERBURDEN

UNWEATHERED COAL

POOL LEVEL

hSLURRY

hSEAM

BARRIER SEGMENT

w

WEATHERED

COAL

OUTCROP BARRIER

S

1

NOTE: SLURRY AND SOIL ARE

 TREATED TOGETHER

 AS SLURRY.

FSLURRY

FCOAL

FROCK

FWATER

SHEAR RESISTANCE =  (VERTICAL FORCES) TAN 

MINE VOID

FKO

= (whSLURRY) - (w2/2S) (SLURRY - W)

= w2/2S (ROCK - W)

wh (COAL - W) = SEAM

FIGURE 8.9 OUTCROP BARRIER BREAKTHROUGH ANALYSIS FOR SHEAR FAILURE
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Site Mining and Foundation Issues

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• Discontinuities in coal seam or overburden (e.g., stress relief fractures to surface) that 

may allow hydrostatic pressures to reach the interior of the barrier, the potential for 

which can be evaluated based on local geologic studies and surficial reconnaissance 

above the outcrop.

The preceding analysis is based on the assumption of a horizontal outcrop barrier. Adjustments to 

account for the dip of the coal seam and barrier may be warranted as well as evaluation of possible 

wedge failure across overburden bedding planes where fracture systems with such orientation are 

present.

If a man-made barrier instead of coal is being evaluated, the preceding analyses should be performed 

using the properties of the barrier. The following guidance is provided:

• Earthen materials and aggregate barriers – The strength properties of earthen materials or aggregate should be established based upon how the material was placed to 

form the barrier and accounting for resistance at the barrier-floor interface only (i.e., 

resistance between the barrier and the coal seam or roof is ignored).

• Concrete, concrete block, and grouted aggregate barriers – Construction details 

associated with the barrier keys should be included in the analysis of resistance to 

failure, along with the interface friction between the barrier and floor/roof, as subsequently discussed in Section 8.5.

Fractures in overburden above an outcrop may have the potential for seepage and internal erosion 

that could propagate to an impoundment and lead to breakthrough. Natural stress relief fractures, if 

close to mine workings or subsidence cracks can lead to the potential for breakthrough from internal 

erosion. Breakthrough potential for this mode of failure can be determined by evaluating: (1) the 

presence of or potential for fractures and their influence on mine stability, (2) the occurrence and 

magnitude of subsidence strains and deformations, and (3) the type and effectiveness of any mitigation measures employed. Mitigation measures may include features to control susceptibility to internal erosion such as self-healing granular soils within the overburden and design of fills and filters to 

prevent piping.

8.4.4.2 Overburden Barriers

When mine workings extend below drainage and beneath the bottom of an impoundment, attention 

should be focused on the overburden, roof, pillar, and floor stability, as illustrated in Figure 8.10. The 

potential for breakthrough could be associated with the following scenarios:

• Sinkhole subsidence under relatively thin overburden strata where a roof fall propagates into the overburden and eventually daylights into the impoundment.

• Fracturing of the overburden due to pillar collapse or floor failure, leading to internal erosion propagating to the impoundment.

Analysis of roof stability and sinkhole development is addressed in Section 8.4.2. The potential for 

these phenomena can be determined empirically using the Rock Mass Rating (RMR) (Bieniawski, 

1992) or the Coal Mine Roof Rating (CMRR) (Molinda et al., 2001) and analytically by calculating 

beam or arch safety factors for each stratum. The analysis of pillar and floor stability, which was 

addressed in Section 8.4.3, is based on the stability factors of pillars that may lie within a reasonable 

angle of draw around the periphery of the impoundment, considering room and pillar dimensions, 

mining height, overburden, and coal strength. Newman (2003) presents an analysis methodology 

and associated case history for evaluation of breakthrough potential for an overburden barrier.
Chapter 8

8-34 MAY 2009

8.5 SUBSIDENCE AND BREAKTHROUGH MITIGATION METHODS

Mitigation measures to address the potential impacts of subsidence or breakthrough may include 

some combination of:

• Providing a safety zone around the embankment and impoundment, limiting 

mining to only entry development beneath the dam.

• Providing support by backfilling portions of the mine.

8.10a ILLUSTRATION OF OVERBURDEN BARRIER

FIGURE 8.10 OVERBURDEN BARRIER ILLUSTRATION

ROCK OVERBURDEN

MINE WORKINGS IN COAL SEAM

SLURRY

SOIL

8.10b SCHEMATIC PLAN OF PILLARS

PILLAR TRIBUTARY

SUPPORT AREA

B

B

8.10c TYPICAL PILLAR DETAIL

W

L

0.5B

FIGURE 8.10 ILLUSTRATION OF OVERBURDEN BARRIER
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Site Mining and Foundation Issues

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• Improving the in-situ materials by grouting.

• Constructing an engineered barrier.

• Isolating the structure from the area of influence of the mining or altering the 

mining sequence or plan.

• Constructing secondary defense measures such as bulkheads to contain a 

breakthrough within the mine.

• Other engineered measures or impoundment operating procedures to control 

seepage and reduce pressures in the areas of potential breakthrough.

Subsidence mitigation measures are dependent on the structure that must be protected. Mine workings beneath a dam must be stabilized such that support for the impounding embankment and abutments is achieved. Overburden or outcrop barriers beneath an impoundment must be sufficient to 

prevent breakthrough. Mitigation measures for breakthroughs are discussed in MSHA (2003) and 

NRC (2002) and are summarized in the following sections, which describe potential mitigation alternatives and methods for the design of bulkheads to seal mine entries.

8.5.1 Mine Subsidence and Breakthrough Mitigation

8.5.1.1 Use of Safety Zones

For new impoundments, the most effective method for preventing damage to the dam or a breakthrough is to leave an unmined safety zone between the mine workings and the impoundment 

so that any mining-induced ground disturbance cannot cause a breakthrough or other significant 

adverse effects. At new facilities, siting of the impoundment at locations that are not or will not 

be undermined is preferred. If mine workings cannot be avoided, other mitigation measures may 

be feasible provided that support for the impounding embankment and appurtenant structures 

is achieved.

For existing impoundments where the mine workings are already close enough to potentially cause 

a problem, a safety zone can be created (if necessary) by backfilling the mine workings. Guidelines 

for sizing safety zones around impoundments are provided in Babcock and Hooker (1977). Figure 8.6

provides an illustration of these guidelines. Kendorski et al. (1979) provide criteria for determining 

when a surface water body represents a hazard to mining. Peng and Luo (1993) provide guidance for 

establishing safety zones for sensitive structures.

8.5.1.2 Mine Backfilling

If the thickness and natural characteristics of the overburden barrier cannot be relied on to prevent 

a sinkhole, subsidence cracks, or other subsidence-related failure mechanisms, filling previouslymined areas with grout or other material (commonly referred to as “stowing”) may be a necessary 

remedy. The lateral extent of backfilling must include critical areas within the angle of draw based on 

the results of subsidence analyses described in Section 8.4.2.

The mine backfill material can have minimal strength, and even a partial backfill will offer confinement to mine pillars, thereby reducing spalling and dramatically increasing pillar strength. However, 

it is good practice to backfill to the roof of the mine, using material with sufficient strength (typically 

above 100 psi) to reduce consolidation and prevent erosion. This can be difficult to accomplish, as full 

contact with the mine roof is often not possible because of roof irregularity and the rolling nature of 

coal seams. However, partial roof contact will reduce roof falls and will dramatically limit roof fall 

propagation into the overburden.

Pozzolan slurry makes a good backfill, because it readily flows into irregular spaces. Because of the 

high sulfate environment in a coal mine, any fill containing cement should employ sulfate-resistant 
Chapter 8

8-36 MAY 2009

cement, preferably Type V, but in some mine environments Type II cement will suffice. Depending 

on cement availability, Type II or Type I cement in combination with pozzolans (e.g., fly ash, slag 

cement) may be required in order to provide adequate sulfate resistance. The extent of the backfilled 

areas must be sufficient to support the overburden and/or hillside that could potentially be inundated by the impoundment and to protect existing or planned embankment dams and mine seepage 

barriers from adverse subsidence effects. Boreholes should be advanced to determine if the backfilling program has achieved performance criteria. These boreholes can also be used to locate remaining 

voids, to perform secondary grouting, and to obtain samples from within the backfilled mine workings for strength testing.

To verify that the backfill strength and coverage is adequate, subsidence and/or pillar analyses should 

be conducted as part of the mine backfilling design. As a minimum, the following should be specified 

as part of the backfill design:

• Strength of the backfill material

• Area to be backfilled

• Methods for verification that the design strength 

and area of backfilling are acceptable

When used with cement, most types of fly ash improve flowability, increase sulfate resistance, and 

sometimes increase the compressive strength of backfill grout. Fly ash improves the flowability of 

grout because the spherical particles act like ball bearings that allow the grout to move more freely 

and the small particle size promotes better filling of voids. In addition, the pozzolanic properties of 

most types of fly ash increase compressive strength. Use of fly ash also reduces shrinkage and slows 

setup time, which is important if grout pumping must be interrupted for a few hours. The properties of fly ash will vary dependent upon the coal source used at the originating power plant. Fly ash 

properties are generally determined by the power companies that generated it and these properties 

can often be obtained from them.

Fly ash can pose potential environmental and health risks because it contains trace amounts of toxic 

metals such as boron, molybdenum, selenium, and arsenic. Portland cement also contains these elements, and they can occur naturally in soil and water. (USEPA, 2000) discusses environmental impact 

considerations associated with the use of fly ash in mine environments.

8.5.1.3 Stowing of Mine Openings and Associated Barrier Construction

It has been common practice to seal mine entries by stowing them with competent rock or other fill 

in conjunction with constructing a compacted earth or coarse coal refuse embankment on the outside against the openings. This approach can also be used with auger holes. The openings should be 

filled far enough back into the mine to prevent adverse subsidence or sinkholes that could extend to 

the ground surface under the dam or impoundment. Typically, aggregate or other rock materials are 

pushed or rammed into openings to the extent practical with construction equipment. If possible, 

stowing should extend into the opening for a distance sufficient to mitigate subsidence. In some 

applications, including areas beneath a dam or abutment, grouting is performed in order to extend 

the backfilling of the opening, support the roof, and mitigate seepage. In an impoundment area, 

when it is impractical or infeasible to mitigate subsidence by stowing, other options can be considered such as: (1) earthen barriers designed to provide protection against potential sinkholes or cracks 

and (2) overexcavation to remove shallow overburden subject to sinkholes and establish a highwall 

cut. Monitoring systems for detecting movement and/or hydraulic pressure should be considered.

Earth or coarse coal refuse barriers should be protected by appropriate geotextile or graded filters 

to prevent piping into the mine, open joints, and stowing material. Drains should be installed as 
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MINE OPENING

STOWED WITH ROCK

GEOTEXTILE

COMPACTED

BACKFILL

8.11a BACKFILL BARRIER WITH GEOTEXTILE

OVERBURDEN

COAL SEAM

WITH WORKINGS

FINE AGGREGATE

(e.g., AASHTO NO. 57)

8.11b BACKFILL BARRIER WITH GEOMEMBRANE

OVERBURDEN

COAL SEAM

WITH WORKINGS

GEOMEMBRANE

COMPACTED

BACKFILL

FINE AGGREGATE

MINE OPENING (e.g., AASHTO NO. 57)

STOWED WITH ROCK

FINE AGGREGATE

CUSHION LAYER 

(e.g., ASTM C33) 

needed to control the hydrostatic pressure. The system of the stowed and external barrier materials must be designed to have sufficient strength and dimensions to resist shear or punching failure 

into the mine resulting from hydraulic and earth-load forces. The stowed material must also have 

sufficient bulk and material gradation to provide adequate seepage resistance and thus prevent 

failure of the embankment or adjoining strata due to piping or hydraulic fracturing into the mine. 

Figure 8.11a illustrates a stowed opening protected with geotextile and backfill. The fine-aggregate 

filter shown in the figure should cover the area where fractures or open joints are present or are 

likely to be a concern.

If a coarse coal refuse embankment is to be constructed over a coal seam outcrop with workings, the 

exposed coal seam must be covered and sealed with soil or other inert material to provide a fire barrier and to minimize the potential for spontaneous combustion. If water is draining out of the mine 

from an opening that is to be covered, a drain to prevent water from building up in the mine may be 

needed. For these cases, the sealing cover should include a drainage system that will release the mine 

water and prevent hydrostatic pressures from building up and causing problems with respect to 

saturation, piping, or structural instability. A discussion of internal drain and filter design is provided 

in Section 6.6.2. If the mine discharge is acidic, drain materials capable of resisting degradation will 

be required (Section 11.7), and downstream water collection and treatment could also be required 

(Section 10.3.3).

FIGURE 8.11 STOWED MINE OPENINGS AND BARRIER CONSTRUCTION
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Impervious membranes have been used in conjunction with mine opening seals. A membrane should 

extend a sufficient distance past the perimeter of an opening to provide an effective barrier encompassing mining-induced fractures and open joints. To prevent seepage from flowing around the 

membrane, the edges of the membrane should be anchored or embedded. The membrane should be 

surrounded by a layer of finer-grained, cushioning material to prevent puncture by sharp rocks that 

may be present in the embankment or highwall. Figure 8.11b illustrates the use of a geomembrane as 

part of a mine opening seal. Liner systems are discussed Section 10.4.

If mine openings are safely accessible to workers or can be rehabilitated so that they are safely accessible and plans for entry are submitted and approved by MSHA (30 CFR Subchapters G, H I and 

O), form-work or bulkheads can be constructed to restrict the extent of stowed material to a desired 

depth into the mine. In this situation, workers could enter the mine to install grout pipes or to do 

other needed work. This approach has been used successfully for both the pneumatic stowing and 

grouting of gravel and for installation of a grouted rock plug. Pneumatic stowing and grouting methods are described in the U.S. Bureau of Mines Information Circular 9359 (Walker, 1993). The following considerations apply: (1) some material types or uncontrolled (uncompacted) placement can be 

subject to erosion under water flow and (2) the operation exposes personnel to dust. These concerns 

can be overcome by grouting, although the use of cement adds cost, and environmental impacts 

can be an issue. If the mine opening can not be made safely accessible to workers, rams mounted on 

heavy construction equipment can be used to pack the stowing material into the opening, but the 

depth to which the opening can be backfilled is limited.

In addition to stowing the mine opening, measures to isolate the sealed opening from the impoundment and to control hydraulic pressure may also be needed. A compacted embankment fill with 

an internal drain protected by a filter discharging beyond the limits of the impoundment provides 

additional protection and redundancy for mitigating breakthrough. Figure 8.12 shows an example of 

a compacted fill (prior to refuse placement) placed against an outcrop and with a drain wrapped in 

geotextile and spoil. Drain and filter design requirements are discussed in Section 6.6.2.

8.5.1.4 Bulkhead Construction

Construction of bulkheads inside a mine or at mine portals can be a feature of a breakthrough prevention program. Bulkheads are designed to withstand fluid and/or earth pressures. They may be 

relatively thin reinforced-concrete structures or relatively thick structures consisting of concrete, 

grouted rock or polyurethane foam.

COAL SEAM WITH WORKINGS BENCH

REFUSE

SPOIL

LOW PERMEABILITY BARRIER DRAIN

GEOTEXTILE

(ADAPTED FROM COWHERD ET AL., 2002)

OVERBURDEN

FIGURE 8.12 BARRIER AGAINST OUTCROP WITH LOW-PERMEABILITY FILL AND DRAIN
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In situations where there is uncertainty about the level of breakthrough protection provided by other 

preventative measures, remote bulkheads can be used as a secondary defense against the discharge 

of water or slurry through and from a mine. These bulkheads are constructed in mine openings 

where the water/slurry would flow to (and flow out of) in the event of a breakthrough. For such an 

application, designers need to consider the rapid build up of air pressure and subsequent impact of 

water and debris against the secondary structure. Furthermore, the impounding water or slurry in a 

mine creates the potential for a blowout of an outcrop barrier or bulkhead that should be evaluated. 

The design of bulkhead seals is presented in Section 8.5.2.

8.5.1.5 Construction of Compacted Earthen Barriers on the Surface

Construction of a compacted earth-fill barrier around the bottom or perimeter of the impoundment 

area is a design measure that has been used to mitigate potential breakthrough conditions. A compacted earth-fill barrier provides additional bulk between the impoundment and the mine workings 

and, in combination with properly designed internal drainage, can lower the water pressure against 

an outcrop barrier. The water pressure can be reduced if internal drains are used to draw down the 

hydrostatic level in the fill and also to provide an outlet so that seepage discharges in a controlled 

manner. Compacted earth-fill barriers can be placed and raised as the impoundment level rises. The 

design of a compacted earth-fill barrier should take into account the potential effect of subsidence, 

including sinkholes resulting from underlying mine workings. Measures that have been incorporated into barriers to resist potential subsidence movement, cracking and internal erosion include 

geogrid reinforcement and graded filters.

For designing a compacted earth-fill barrier as a breakthrough prevention measure, one approach 

is to conduct reconnaissance and excavation of the coal seam in the vicinity of the reservoir that is 

known or suspected to be constructed over mine voids. The concept is to identify and intercept any 

mine voids, and thereby expose known or suspected workings near the outcrop. Weathered or fractured strata near the coal seam outcrop should be removed as part of the excavation, thus allowing 

the barrier to be founded on competent rock. Surface mining of suspect areas is a comprehensive 

approach for addressing large areas with unknown conditions. The stability of highwalls should also 

be evaluated, particularly when workings are encountered during construction.

Another approach is to construct the earth-fill barrier over the existing coal seam outcrop without 

excavation of the seam. The support and internal stability of the earth-fill barrier due to the presence 

of mine voids should be evaluated. For this approach, there may be exposed highwall if the coal seam 

has previously been surface-mined, augered or highwall-mined, or there may be an undisturbed outcrop that has not been affected by surface mining.

The construction of compacted earth-fill and coarse coal refuse barriers should be based upon the 

same concepts regarding material selection and engineering properties, foundation preparation, provisions for internal drainage, slope stability and development of construction specifications used for 

the design and construction of earth-fill dams and refuse embankments. Where feasible, fine coal 

refuse slurry should be deposited around these barriers, as the development of a delta of fine coal 

refuse above the normal pool level will provide additional resistance to breakthrough.

8.5.1.6 Conversion to Slurry Cells

As described in Section 3.4, slurry cells can be used to dispose of fine coal refuse. One approach that 

can be taken to reduce the breakthrough hazard is to convert from a full slurry impoundment into 

a slurry cell configuration using compacted coarse refuse to construct small, individual cells or a 

series of cells over previously placed fines. Some cell designs have reached depths of 12 feet (8 feet 

of fine refuse covered by 4 feet of compacted coarse refuse). The depth and number of active cells 

(i.e., cells that are not capped with backfill) at any given time is usually determined by the volume of 
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8-40 MAY 2009

storage and cell configuration that will result in a low-hazard potential classification for the facility, 

as described in Section 3.4. The benefit is that the coarse refuse dikes and covering layers combined 

with the thin layers of fines allow the fines to dewater and consolidate and make the total mass less 

flowable. Additionally, with the fines compartmentalized, a problem at one cell location is less likely 

to affect the entire facility or result in a catastrophic event.

The downstream containment structure (i.e., structural zone) for slurry cells is designed in the same 

manner as a dam embankment with appropriate width, slopes, benches, internal drainage system, 

and embankment-material strengths needed to achieve suitable safety factors for slope stability. 

Slurry cells are most efficient when the depth of fines in the cells is kept relatively shallow, thus promoting drainage from the fines before and during covering.

Some disadvantages of slurry cells include:

• Frequent construction of diversion ditches, new cells, and cell spillways is required 

as the site elevation increases.

• There is limited flexibility in that a relatively large ratio of coarse refuse to fine refuse 

is required to keep cell construction ahead of slurry placement, and the fine refuse 

must settle quickly with limited clarified water retention.

• Close planning and supervision of the site is required so that the construction, filling, 

and backfilling of cells is accomplished in the proper sequence to make the system 

function as intended.

• Slurry cell operations are not generally compatible with high production rates at 

some coal preparation plants.

• Slurry cell operations are not generally compatible with sites that employ conveyor 

belt/dozer push disposal techniques.

• If a slurry cell site is large enough to be classified as having high-hazard potential, 

the facility spillways must be designed to handle the runoff from the associated 

design storm (e.g., PMF).

8.5.1.7 Sealing Sources of Leakage

Discontinuities such as open joints or cracks in a refuse embankment foundation or impoundment 

area may be treated by grouting or other measures to prevent them from transmitting high hydrostatic 

pressures and to eliminate potential paths for internal erosion. As a secondary protection measure 

against leakage, fine coal waste (slurry) can be deposited to form a delta that provides an additional 

layer of material between the impoundment and potential seepage problem areas. This technique 

has been used successfully to reduce leakage through embankment and abutment areas and around 

the perimeter of the impoundment. The long-term benefit of this secondary measure is limited to 

situations where the bedding planes or joints are relatively small and the sand-sized portion of the 

slurry is sufficient to allow the gradual formation of a natural filter in joint openings and the eventual 

sealing of the openings with the clay- and silt-sized particles. If a seepage problem develops after 

an impoundment is constructed and the technique of distributing the slurry upstream of the seepage area does not correct the problem, then grouting, construction of an impervious barrier, or other 

measures may need to be taken.

8.5.1.8 Stabilization of Fines

The potential for breakthrough of the contents of a slurry impoundment into underground mine 

workings can be significantly lessened through stabilization of the fine refuse. Stabilization alternatives include providing drainage measures or treating the fines with additives to increase their 

strength and/or reduce their water content.
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Stabilization of coal-waste fines has sometimes been accomplished by the addition of portland cement 

or lime-based products. Since the late 1970s and continuing more recently (Fiscor, 2002), studies for 

development and evaluation of the performance of stabilizer additives have been undertaken. Laboratory studies performed in the late 1970s and early 1980s showed that mixing fine coal refuse with 

lime-based products may cause the material to appear to have increased strength and stiffness in 

comparison to untreated material. After samples were mixed and cured, the treated refuse exhibited a 

relatively stiff load-deformation behavior during initial loading, but then when loading exceeded the 

apparent maximum past consolidation pressure, the “stabilized” refuse collapsed and the load-deformation characteristics returned to the previous “unstabilized” behavior. In addition, these laboratory 

studies showed that 5 percent or more by weight of the stabilizers were needed, which made the additives prohibitively expensive except for small treatment volumes. While not all stabilizer additives 

result in similar behavior, they should always be carefully investigated and used with caution.

Shallow or deep soil mixing methodology using fly ash and cement grout are also methods for stabilizing fine deposits that would be effective for mitigating breakthrough potential. Although not 

for the purpose of breakthrough mitigation, a portion of a slurry impoundment in Pennsylvania was 

stabilized in place by shallow and deep mixing with fly ash and cement grout (Bazan-Arias et al., 

2002), as also discussed in Section 6.6.3.3. The stabilized fines were then used as the foundation for a 

highway embankment that crossed the upstream end of the impoundment. 

For an existing impoundment, an approach that might be taken is to show that the settled fines have 

consolidated and gained sufficient strength that they will not flow. Whether settled fines will flow 

depends on factors such as their degree of consolidation and cohesive strength, pore pressures, the 

potential for excess pore-water pressures to be induced, and the size of opening associated with a 

potential breakthrough feature. One qualitative measure of flowability is the moisture content of the 

fines as compared to their liquid limit (LL) and plastic limit (PL). If the moisture content of fines near 

the bottom of an impoundment is below the LL and close to the PL, they are less likely to flow and 

progress to a breakthrough unless the openings in the overburden above or adjacent to the mine are 

sufficiently large to affect more flowable fines at shallower depths in the impoundment.

A change in conditions in an impoundment, such as inflow of runoff from a large storm, could result 

in the liquefaction of some portions of the fines leading to an unplanned release of slurry and water 

through underlying, connected mine workings. Additionally, if subsidence occurs beneath saturated, 

hydraulically placed fines, the sudden increase in shear stress in the fines may increase pore-water 

pressure, triggering static liquefaction and causing the fines to flow (Davies et al., 2002). While there 

may be mitigating conditions in such a situation, measures to drain the fines can be helpful in reducing or eliminating the consequences of a breakthrough because partially saturated fine refuse is likely 

to densify under load and thus would be more resistant to liquefaction.

8.5.1.9 Overexcavation and Induced Subsidence

Overexcavation of the overburden and mine workings around impoundment perimeters and beneath 

embankment structural zones may be feasible, particularly in areas of thin overburden such as along 

outcrops. When applied, this approach includes removal of sufficient overburden to mitigate concern 

for sinkhole subsidence over the remaining workings by creating a highwall. Barriers must then be 

designed for any exposed entries found at the base of the highwall. Additionally, highwall stability 

will need to be addressed as part of the overexcavation plan.

Inducing subsidence through controlled blasting may also be a feasible approach. Guidelines for 

evaluating the technical and cost feasibility of using controlled blasting to induce subsidence are 

provided by Workman and Satchwell (1987). In areas where old, unmapped, or poorly mapped 

mine workings make evaluation of mine breakthrough potential difficult, collapsing the workings 

by controlled blasting may be an option. Controlled blasting can prevent future mine subsidence 
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8-42 MAY 2009

from inducing cracks or sinkholes or from compromising planned mine seepage barriers. However, 

not all rock overburden strata are suitable for this approach; implementation requires: (1) a geologic 

investigation for determining the character of the strata, (2) a test area for initial demonstration, and 

(3) monitoring programs for evaluating performance and addressing safety issues associated with 

blasting and impacts to adjacent areas. Obviously, this approach should be evaluated with considerable diligence and should only be implemented under the guidance of persons with expertise and 

experience in dam safety, explosives, and the blasting characteristics of local rock strata.

8.5.1.10 Monitoring Provisions

Whenever there is potential for subsidence to affect a dam or for a breakthrough, critical parameters 

should be identified and a monitoring program should be implemented to determine whether the dam 

and overburden is performing/behaving as anticipated. Typical monitoring might include instrumentation for measuring reservoir and piezometric levels, discharge rates from mine workings and drains, 

seepage quantity and quality, water levels in the mine, ground movement, and weather conditions. 

Use of such instrumentation is discussed in Sections 13.2.1 and 13.2.2. Acceptable ranges and warning 

or action levels should be established for all monitored values. Monitoring programs should include 

requirements for plotting and evaluating data in a timely manner and review by an engineer familiar 

with the design of the facility. Such practice permits detection of trends and correlations to other data.

8.5.2 Mine Entry Bulkhead Seal Design

Bulkheads, also known as hydraulic seals, may be installed across underground mine entries or in 

mine openings at coal seam outcrops. Normally, bulkheads are installed across underground mine 

entries to control groundwater in abandoned workings, to prevent rapid inundation of active mine 

areas in the event of a breakthrough, or to serve as a retention dam in an underground disposal system 

for fine coal waste. At coal seam outcrops, bulkheads have typically been used to prevent access and 

to control acid mine drainage. Bulkheads have also been installed at outcrop openings within the 

footprint of a refuse embankment or within the refuse impoundment to prevent stored water or 

slurry from flowing into active or inactive underground mine workings. Also, when coal barriers are 

found to be inadequate, bulkheads may be added on the mine side of the barrier to enhance safety. 

Design considerations include: site preparation, seal type selection, design load assessment, structural resistance of the bulkhead and surrounding strata, and safety monitoring. Additional guidance 

is presented in Guidelines for Permitting, Construction and Monitoring of Retention Bulkheads in Underground Mines by Harteis, et al. (2008)

8.5.2.1 Site Preparation

As problems with bulkheads are often associated with seepage along the bulkhead and rock interface 

or through the surrounding strata, the nature of the rock strata at a bulkhead location is important. To 

the extent practical, bulkheads should be located in the most competent and least fractured area available (normally away from pillar corners), so that problems are minimized. The information gathered 

as part of the evaluation of a potential bulkhead location should include the type and strength of the 

rock in the roof and floor and the strength of the coal at the location under consideration. Data related 

to roof falls, pillar punching, floor heave, or other unusual conditions should be reviewed and evaluated. Coal pillars adjacent to the bulkheads should have a high factor of safety against failure, taking 

into account all loading factors including transfer stress due to overlying or underlying mining and 

stresses imposed by the dam and impoundment. Another consideration is that once a bulkhead is 

constructed, active mining in the vicinity, whether in the same seam or in other seams, can affect the 

surrounding strata and the load on the bulkhead itself.

As part of the evaluation, fractures or joints in the surrounding rock should be located and characterized. Geophysical techniques can be employed to supplement visual observation for locating 

rock discontinuities. The potential for subsidence and sinkhole formation over entries should also be 
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assessed. If joints, fractures, or subsidence cracks are present, then chemical or cement-based grouting 

measures may be necessary in order to minimize seepage pathways that could lead to piping problems or deterioration of the rock strata. To maintain the integrity of a bulkhead, it may be necessary 

to construct a grout curtain around the perimeter of the planned location. In addition to grouting, 

loose, cracked, and weak floor, roof, and rib material should be removed. Regardless of the bulkhead 

type, if the surrounding material consists of weathered or soft rock, then piping (internal erosion) 

and hydro-fracturing of this material should be considered as potential failure mechanisms.

In-mine bulkheads have failed because of softening of the floor material. These failures occur with a 

“rooted” claystone referred to as fireclay. When subjected to water, this material breaks down from 

a rock-like material to a soft soil. The presence of water causes the electrostatic charge on the component clay particles to break down. This process results in dispersion of the clay particles, loss of 

cohesive strength, and potential erosion from seeping water. For this reason, claystone and fireclay 

floors should be removed from beneath the planned footprint of a bulkhead by trenching to a depth 

sufficient to expose hard, competent rock. Trenching should minimize the potential for a floor failure 

directly beneath a bulkhead. Floor trenches should be backfilled with a seepage-resistant material 

such as a cement-based or polyurethane-foam product. The surface of fireclay running under the 

pillars should be sealed and the bulkhead should be constructed as soon as possible after the floor is 

exposed in order to limit the tendency for the fireclay to swell or absorb moisture from the air.

Shales in mine roofs, floors, and partings tend to be pyritic and thus prone to swelling. Bulkheads in 

shale should be designed to accommodate additional load due to swelling and should also accommodate deterioration of the shale over time due to weathering and moisture effects. High-clay-content shales can break down in the same manner as fireclays, and similar measures to those discussed 

above can be used to mitigate potential problems.

The integrity of a bulkhead system can be affected by hydraulic fracturing. This type of fracturing can 

occur when pressure from seeping water is sufficiently great to cause cracks in rock strata to widen 

and grow. When coal is mined to create an entry, the associated stress relief can result in stress fractures or the opening of natural discontinuities in the mine roof, ribs, and floor. Specifically, the floor 

may heave upward in some mines. This opening of the rock strata can lead to additional hydraulic 

fracturing if seepage pressures elevate. Strata grouting and notching of bulkheads into competent 

material are methods for mitigating potential bulkhead damage due to hydraulic fracturing.

Prior to construction, supplemental roof support should be provided near the interior (inby) and exterior (outby) sides of the bulkhead location. Roof support alternatives include roof jacks and cribbing 

on both sides of the bulkhead. Unlike typical mine supports, these supplemental roof supports are 

considered permanent installations. Therefore, corrosion protection should be incorporated into all 

steel supports, and concrete or other durable cribbing material should be used. Soft floor materials 

should be removed before installing supplemental roof supports. Notching of the bulkhead into the 

surrounding rock strata is also recommended. A notch will create a longer flow path for seepage and 

will increase the resistance to a contact failure between the bulkhead and the rock strata. In most cases, 

notching will place the bulkhead in contact with more competent material as loose material is removed 

to construct the notch. When roof notching is not feasible, a steel structural-angle member (with corrosion protection) can be bolted to the roof on the outby side of the bulkhead to provide lateral support. 

Angles with dimensions of 6 inches by 6 inches by 1⁄2 inch have been used for this purpose, but the 

angle should be sized based on the design loads. Seepage resistance can also be increased by treating 

the surrounding rock strata with a low-hydraulic-conductivity coating such as shotcrete.

8.5.2.2 Bulkhead Types

Several types of materials and physical arrangements have been used for bulkhead construction. 

Figures 8.13 and 8.14 illustrate entry and drift opening bulkhead concepts, including bulkheads 
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8-44 MAY 2009

installed at mine entrances to prevent water from entering. Bulkhead arrangements typically are 

either thick plugs with a straight or tapered length or thin walls with a straight or arched shape. Each 

type of construction has advantages and disadvantages. In general, the thicker the bulkhead, the 

more resistance there will be to seepage and piping around the perimeter.

Kirkwood and Wu (1995) present technical considerations for mine seals to withstand hydraulic heads 

in underground mines. Solid concrete block walls can be designed to withstand these loads and have 

been tested under hydraulic pressure of 40 psi (92 feet of head), but it is recommended that the maximum hydraulic pressure be limited to 2.6 psi (6 feet of head) because of long-term effects of strata 

movement and deterioration surrounding the seal (Chekan, 1985). Reinforced-concrete walls can be 

designed for significant hydraulic loads, provided it is recognized that the maximum safe head is 

controlled by the capacity of the surrounding rock. While notching a seal will help increase the total 

shearing resistance provided by the surrounding rock and increase the seepage path, the bearing 

8.13a WALL KEYED INTO RIBS, ROOF AND FLOOR

FIGURE 8.13 BASIC BULKHEAD DESIGN CONCEPTS

8.13b TAPERED PLUG

8.13c PARALLEL PLUG

= DIRECTION OF HYDROSTATIC PRESSURE

(HARTEIS AND DOLINAR, 2006)

FIGURE 8.13 BASIC BULKHEAD DESIGN CONCEPTS
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capacity of the floor and adjacent ribs must be addressed. The difficulty of achieving adequate contact 

with the roof and dealing with the limited access for working the concrete around the reinforcing can 

best be handled by using experienced work crews. When constructed as a drift opening bulkhead, a 

reinforced-concrete wall (Figure 8.14a) can be used to cap off an entry, but contact with the mine roof 

may require notching, dowels, or other measures to improve the seal and structural integrity.

Thick concrete-plug seals overcome these limitations through development of thicker barriers with 

significantly more contact with the mine ribs, floor and roof. Typically concrete-block walls serve 

as forms at either ends of the seal, and concrete or cement-foam materials are used to construct the 

plug, as shown in Figure 8.15. Grouted-rock seals are similarly constructed, but include rock (gravel 

to large hand-placed rock) that is grouted in place, as shown in Figure 8.14b. Grout materials vary 

from cement to rigid polyurethane foam. Grouted-rock seals are generally as thick as or thicker than 

concrete-plug seals.

When cement-based products are used to construct thick plugs, consideration should be given to the 

possibility of heat buildup within the fresh concrete mass during curing (Figure 8.15). This buildup 

8.14a REINFORCED CONCRETE "CAP" BULKHEAD

8.14b GROUTED BULKHEAD

FIGURE 8.14 DRIFT OPENING BULKHEAD DESIGN CONCEPTS

(HARTEIS AND DOLINAR, 2006)

MINE OPENING

REINFORCEDCONCRETE

WALL

MINE FLOOR

MINE ROOF

MINE OPENING

BLOCK RETAINING

WALL (BOTH SIDES)

MINE ROOF

MINE FLOOR

GROUTED ROCK

(e.g., POLYURETHANE-

 LIMESTONE CORE)

FIGURE 8.14 DRIFT OPENING BULKHEAD DESIGN CONCEPTS
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8-46 MAY 2009

of temperature is termed the heat of hydration and, if not accounted for, can lead to internal cracking within the plug and to property changes in the surrounding rock strata during the curing period. 

Measures to minimize heat buildup include using low-heat cements, replacing a portion of the cement 

with certain pozzolans, using larger aggregate, and cooling the mix water. Guidance can be found in 

ACI 207.1, Guide to Mass Concrete (ACI, 2005a). Polyurethane foam will also generate heat as it cures. 

The potential for this heating to cause a fire due to contact with combustible material must be taken 

into account.

Also, polyurethane foam is highly sensitive to moisture that may be present at a bulkhead construction site. Moisture can cause the foam to expand more than intended, resulting in a lower density and 

lower strength material. The product manufacturer should be consulted regarding measures to prevent 

moisture from adversely affecting the foam. 

Regardless of type of material selected for bulkhead construction, resistance to deterioration from acid 

mine water should be evaluated. For example, sulfates in groundwater, coal and the surrounding rock 

strata can cause deterioration or spalling of certain types of cements that are not inherently resistant to 

sulfate attack. Because of the high-sulfate environment in a coal mine, any cement used in fill should 

be sulfate resistant. Type V cement is preferable, but in some mine environments, Type II cement will 

suffice. Depending on cement availability, Type II or Type I cement in combination with pozzolans (e.g., 

fly ash, slag cement) may be used in order to provide adequate sulfate resistance.

When formwork is used for bulkhead construction, it should be adequately braced for structural support, and vents should be installed at the top of the formwork to release entrapped air and to prevent 

the formation of voids. Because concrete can shrink during curing, remaining voids between the surrounding rock strata and the bulkhead should be filled by contact grouting.

In selecting the type of bulkhead, consideration should be given to the potential for roof convergence. 

Some types of materials (e.g., low-density cementitious foams, lightweight concretes, and polyurethane foams) can accommodate this compression by deforming without cracking. 

FIGURE 8.15 BULKHEAD CONFIGURATION SHOWING CONCRETE- 

 BLOCK RETAINING WALLS AND CONCRETE CENTER

(ADAPTED FROM CHEKAN, 1985)

ENTRY

CONCRETE BLOCK

RETAINING WALL

 

10 TO 35 FT

RIB

RIB

CONCRETE BLOCK

RETAINING WALL

 

CONCRETE

CONCRETE

FIGURE 8.15 BULKHEAD CONFIGURATION SHOWING CONCRETE-BLOCK 

RETAINING WALLS AND CONCRETE CENTER
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8.5.2.3 Design Loads

Bulkheads should be designed to resist the estimated maximum hydraulic and geologic loads. The 

hydraulic pressure is affected by the projected head of water or slurry behind the bulkhead. This level 

may be influenced by such factors as:

• Other mine drift openings

• Shaft or borehole openings

• Flooded overlying mines

• Flooded adjacent mines with inadequate barrier pillars

• Partial height seals located up-dip that act as a weir spillway and divert the 

water elsewhere in the mine

• Changes in the mine floor slope that divert water into other parts of the mine

• Maximum seasonal groundwater levels or effects of slurry injection

• Maximum design storm water level in an overlying impoundment

The potential water level in overlying flooded mines is a concern if cracks in the interburden provide a hydraulic pathway between mines. There have also been instances where flooded mines located 

beneath the subject seam dip such that the flood water heads in the seam below are greater than the 

elevation of the bulkhead in the overlying mine opening or entry. In this case, if cracks are present in the 

interburden strata, the water level in the underlying mine could control the bulkhead design head. The 

inlet elevation of drainage pipes extending through up-dip ventilation seals is normally not considered 

the limiting design water pool level because these pipes can clog. Because of these considerations, an 

accurate, up-to-date contoured mine map is essential for predicting the design pool level.

In addition to static hydraulic loads, seismic loads may need to be considered if a site is located 

in a seismically active area. In this event, bulkhead design should account for both the inertial 

and the hydrodynamic forces that could result from an earthquake. The hydrodynamic forces 

would result from an increase in static pressure caused by acceleration of the water mass behind 

the bulkhead.

The roof in a mine entry can exert a compressive load on the top of a bulkhead seal. This compressive 

force should be determined based on experience with convergence or by estimating the stress that 

could occur in the zone of material forming the pressure arch over the entry.

A bulkhead should be designed to have the structural capacity to resist the forces acting on it with 

a factor of safety consistent with the degree of uncertainty associated with the type and magnitude 

of load and the consequences of failure. A bulkhead must be able to resist the shear and bending 

stresses caused by the pressures acting on its face. The bending stresses between the roof and floor 

and between the adjacent ribs can be calculated based on the width and length dimensions and the 

type of edge restraint (Timoshenko and Woinowsky-Krieger, 1959; Young and Budynas, 2001). For 

bulkheads that are thick relative to their span, Young and Budynas (2001) provide guidance on stress 

multipliers. In addition to resisting lateral loads, the bulkhead should have the capacity to resist 

vertical bearing loads caused by the mine roof convergence and stress transfer. With relatively thin 

bulkheads, grouting may be necessary in order to prevent excessive seepage from adversely affecting 

the anchorage of the bulkhead or the surrounding strata.

8.5.2.4 Design of Solid Concrete Block Bulkheads

The analysis of a solid concrete block bulkhead requires estimation of the shear strength around the 

perimeter of the wall and evaluation of the structural capacity of the wall itself. The structural capac-
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ity of the wall is dependent on the bending stresses near the center of the entry. Because a concrete 

block wall has no steel reinforcement, it has relatively poor flexural strength, and, except in the case 

of very narrow entry widths, the lack of flexural strength limits the maximum hydraulic head more 

than the strength of the surrounding strata. Kendorski et al. (1979) provide guidance for block wall 

design and the estimation of flexural stress. The structural capacity of the concrete block wall will 

also depend on the strength of the block, the strength of the mortar, joint thickness, and the quality of 

construction. Masonry design and construction guidance such as ACI 530, Building Code Requirements 

for Masonry Structures (ACI, 2008), should be followed.

8.5.2.5 Design of Concrete Bulkheads

For thin bulkheads installed with an entry width-to-height ratio greater than 2, a reinforced-concrete 

bulkhead can be designed as a one-way slab spanning between the roof and the floor if adequate 

edge connections are provided at the mine roof and floor. To account for temperature and shrinkage 

stresses, reinforcement is required in the rib-to-rib direction if the bulkhead is designed for one-way 

behavior in the roof-to-floor direction. For an entry width-to-height ratio less than 2, the bulkhead can 

be designed for two-way behavior provided there is adequate anchorage on all four sides. However, 

reinforcement should be sized to separately carry the full bending moment if there is concern that 

either the roof or floor or ribs may not provide adequate resistance. Regardless of the width-to-height 

ratio, diagonal reinforcement steel should be placed in the bulkhead corners to control cracking from 

torsional moments. Regardless of the load path direction assumed, if the mine roof, floor, and ribs are 

notched into the surrounding rock strata or if the steel bar reinforcing mats near the inby and outby 

faces are doweled into the surrounding strata, then it is possible to develop negative moment bending stresses at the edges of the bulkhead slab. Negative steel (i.e., the reinforcement near the inby 

“wet side” face) should be sized to resist negative moment bending stresses.

Reinforced-concrete structures should be designed in accordance with the most recent version of 

ACI 318, Building Code Requirements for Structural Concrete (ACI, 2005b) and ACI 350 (where applicable). The codes are based on ultimate strength design in which factors are applied to the loads and 

strength to account for the uncertainty of these parameters in structural design. Using this approach, 

a safe design is achieved when the factored design strength of a structure component exceeds the factored loads. For fluid pressure loading, the load factor is 1.4 when the maximum height of the water 

or slurry is controllable or conservatively estimated. Thick concrete bulkheads with a thickness-toheight ratio greater than 1 should be designed in accordance with Section 8.5.2.5.2.

8.5.2.5.1 Design of Reinforced-Concrete Bulkheads for Flexure

Where the span-to-thickness ratio of the bulkhead is greater than 2, the design flexural strength of a 

reinforced-concrete member (ACI, 2005b) is determined using the relationship:

Md = ΦAs fy [d – a

] (8-6)

 2

where:

 a = 

As fy (8-7)

 0.85 f’c b

Md = flexural design strength (lb-in)

Φ = strength reduction factor = 0.90 (dimensionless)

As = area of tension reinforcement (in2/ft)

fy = yield strength of reinforcing steel (psi)

d = distance from extreme compression fiber to centroid of tension reinforcement (in)
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a = depth of rectangular stress block at failure (in)

f’

c = specified minimum compressive strength of concrete (psi)

b = width of compression face of member, normally taken as 12 in for slabs (in)

For thick bulkheads with a span-to-thickness ratio less than or equal to two, the flexural design 

strength from Equation 8-6 should be modified to reflect the behavior of a thick flexural member. 

Because thicker members have low span to thickness ratios, a linear stress distribution is no longer 

valid. According to Park and Paulay (1975), for simply supported members with span-to-depth (thickness) ratios less than or equal to two, the internal lever arm can be calculated as:

z = 0.2 (l + 2h) 1 ≤ l/h ≤ 2 (8-8)

z = 0.6 l l/h ≤ 1 (8-9)

where:

l = centerline-to-centerline span distance of two bearing points or 1.15 times 

the clear span, whichever is smaller (in)

h = thickness of bulkhead (in)

z = internal lever arm (in)

Applying the revised lever arm value to the standard flexural equation (Eq. 8-6), the capacity can be 

estimated using:

Md = ФAs fy z (8-10)

The value of z should not exceed d – a/2. If the end supports are assumed to be fixed, rather than 

simply supported, the value of z should be further adjusted (Park and Paulay, 1975). Nilson et al. 

(2002) recommend that tension steel in a deep flexural member be distributed over the bottom third 

of the member depth.

8.5.2.5.2 Design of Unreinforced-Concrete Plug Bulkheads for Shear

Unreinforced-concrete bulkheads typically have a thickness-to-height ratio greater than 1. The shear 

resistance of these bulkheads may be governed by the:

• Shear strength of the seal material

• Shear strength of the surrounding strata

• Contact interface shear strength between the seal and the strata

In cases where there is no notching of the bulkhead into the surrounding strata, the interface resistance will be governed by adhesion or friction. The South African plug formulas, which are based on 

the shear strength and bearing capacity of the bulkhead material and surrounding strata, are often 

used to evaluate the required length of thick bulkheads (Garrett and Campbell Pitt, 1961):

l = p a b (8-11)

 2(a + b) fs

l = p a b (8-12)

 (a + b) fc
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8-50 MAY 2009

where:

l = length of bulkhead (ft)

p = hydraulic pressure on bulkhead (psi)

a = width of entry (ft)

b = height of entry (ft)

fs = minimum allowable shear strength of strata or plug material, whichever is less (psi)

fc = minimum allowable compressive strength of strata rock or plug material, 

whichever is less (psi)

The values of fs and fc obtained from sampling may not conservatively reflect the strength of the destressed edges of the coal pillars. The designer should select the bulkhead length based on the larger 

of the values obtained from Equations 8-11 and 8-12. These equations are most applicable to highhead situations where the bulkhead acts as a massive plug. If the bulkhead span-to-thickness ratio 

is greater than one, then flexural reinforcement should be provided, as discussed in Section 8.5.2.5.1.

Selecting shear strengths for coal is problematic, as such values are typically not determined and 

research in this area is limited. Coal is generally crushed or yielded at ribs and extending some distance into pillars. Some designers use the cohesive strength of the coal if the ribs are scaled and the 

bulkhead is constructed before substantial additional yielding takes place. If this is not possible, the 

residual cohesive strength of coal is taken as its shear strength. A common estimate for the cohesive 

strength for coal is 145 psi, which, depending on the practitioner asked, may have resulted from 

the calculation of 16 percent of the typical compressive strength (0.16 x 900 psi = 144 psi) or from 

an assumed minimum value of 1 MPa (145 psi). Sixteen percent is a rule-of-thumb sometimes used 

to estimate the cohesive strength of rock from the compressive strength. It can be difficult to obtain 

representative samples of coal for testing, but a method of estimating the shear strength of coal is to 

conduct direct shear tests with coal samples oriented to model in-mine conditions.

Methods for increasing bulkhead resistance include notching the bulkheads into the surrounding rock strata, tapering the plug, and/or installing corrosion resistant, epoxy-coated dowel rods 

into the surrounding rock strata and allowing the rods to protrude into the bulkhead. The dowel 

rods should have an embedded length into the surrounding strata and the bulkhead sufficient to 

develop the strength of the dowel rod without a bond failure. The minimum required development 

length in concrete should be determined in accordance with the most recent version of ACI 318, 

Section 12.2.

Thick concrete bulkheads with a thickness-to-height ratio greater than 1 should be designed in accordance with the most recent version of ACI 207.1. Reinforcement should be provided to control temperature and shrinkage stresses at the surface and should be designed in accordance with the most 

recent version of ACI 318. Reinforcement is not necessary when the bulkhead thickness is greater 

than 6 feet, provided that steps are taken to control the effects of temperature and shrinkage. Heat of 

hydration can be controlled by using low-strength concrete, low-heat cement, or replacing a portion 

of the cement with pozzolans. It can also be controlled by reducing the placement temperature using 

cooled mix water or aggregates. Accelerating admixtures should not be used.

8.5.2.5.3 Design of Reinforced-Concrete Bulkheads for Shear

While Equations 8-11 and 8-12 can be used to determine the required length of thick bulkhead plugs 

in order to prevent failure in shear, Equation 8-13 can be used to calculate the design shear strength 

of concrete for thinner, reinforced-concrete bulkheads:
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Vc = 2 Ф √ f’

c bw d (8-13)

where:

Vc = shear strength of the concrete bulkhead per unit width (lbs)

Ф = shear strength reduction factor = 0.75 (dimensionless)

bw = unit width of bulkhead (12 in)

d = distance from inby side of bulkhead to centroid of tensile reinforcement (in)

If Equation 8-13 indicates inadequate concrete shear strength, a more rigorous formulation should be 

obtained from the most recent version of ACI 318, Section 11.3. In addition, the contribution of steel 

shear reinforcement, as discussed in ACI 318, Section 11.5, can be added to the value obtained for the 

concrete to obtain a combined shear strength for the bulkhead.

For thick, reinforced-concrete bulkheads where the ratio of the clear span distance ln to the depth d

from the inby side of the bulkhead to the centroid of the tensile steel reinforcement is less than four, 

the most recent version of ACI 318, Section 11.8 should be applied. It should be noted that the spanto-depth ratios are different for shear design than they are for flexural design.

If lightweight concrete with a density of 100 to 110 pcf is used, values of Vc obtained using √ f’

c in the 

Equation 8-13 should be multiplied by 0.75 for “all-lightweight” concrete and by 0.85 for “sand-lightweight” concrete, or should be in accordance with the most recent version of ACI 318, Section 11.2.

8.5.2.6 Monitoring of Bulkheads

The water pressure behind a bulkhead should be monitored so that it can be compared to the design 

pressure. Warning levels that would warrant drawing down the mine pool or initiating an emergency 

action plan should be established. To lower the water pressure, a corrosion-resistant pipe should be 

installed through the bulkhead with a “U-trap” and a pressure relief valve on the downstream end 

and with provisions to prevent clogging (such as a riser and trash rack) on the upstream end. Pipes 

extending through a bulkhead should be equipped with external collars to cut off seepage and minimize the potential for a pipe blowout.

A possible safety measure for regulating the maximum hydraulic pressure is to install an additional 

pipe through the bulkhead with a corrosion-resistant rupture disk attached to the downstream end. 

If the hydraulic pressure on the bulkhead reaches the rupture strength of the disk, it will fail and 

thus limit the load on the bulkhead. The outlet end of the pipe should project downward to prevent 

injuries to anyone near the pipe if the disk should suddenly rupture. If water could collect and flood 

workings, inhibiting escape for miners, an evaluation of the consequences of an unexpected water 

release should be conducted.

Seepage can occur through a bulkhead or the surrounding strata, but the presence of seepage is not 

necessarily a sign of distress. Since unexpected increases in seepage could indicate deterioration of 

the bulkhead or adjacent strata, seepage through the bulkhead and the surrounding strata along with 

the corresponding head behind the bulkhead should be monitored and evaluated. To this end, seepage on the active side of the bulkhead is often channelized and monitored using a weir to facilitate 

measurement of changes in quantity.

A final safety measure is to establish an inspection schedule for long-term monitoring of the bulkhead and to have a contingency evacuation plan in place for situations where problems are encoun-
Chapter 8

8-52 MAY 2009

tered with the integrity of the bulkhead or surrounding strata. To determine the impact of a bulkhead 

failure on active in-mine escapeways, the potential inundation area should be identified.

8.6 FOUNDATION-RELATED CONSTRUCTION AND OPERATIONS MONITORING

Observations of foundation conditions during construction and operation of impounding facilities 

are critical. During construction, foundation conditions including mine-related features are exposed, 

allowing the facility operator and engineer to assess conditions for consistency with expectations and 

to evaluate specific foundation preparation design measures. The operator and engineer responsible 

for construction certification should involve the designer during critical tasks such as construction/

implementation of bulkheads and seals, cutoff trenches, and grouting programs. During construction, survey control to confirm locations of key features and the dimensions of associated structures 

is essential. Documentation of conditions with photographs and as-built drawings and reports of 

construction activity is important for certification of the work and for subsequent evaluation, if necessary. Additional discussion of construction monitoring is presented in Chapter 12. A discussion of 

construction and operations procedures and monitoring for bulkheads is provided in MSHA (2003), 

and key guidance from the MSHA document is presented in this section.

Where there are mine workings near an impoundment, the manner in which an impoundment is 

operated can affect the breakthrough potential. For example, measures should be taken to design 

and operate the impoundment in a manner that minimizes the presence of free water. Thus, decant 

raising should be staged so that the water level rises incrementally. Pumping can also be employed 

to minimize the volume of water in the impoundment. Mine personnel who work on or around the 

impoundment should be cognizant of key components of the operation plan and particularly of any 

unusual operational requirements.

A site-specific monitoring plan oriented toward breakthrough prevention and assessment of the potential for subsidence to affect the dam should be developed. Monitoring involves collection of information 

from both visual inspection of the impoundment and from instrumentation. Coal company personnel 

who inspect the impoundment, or who routinely work on or around the impoundment, should be trained 

to observe potential signs of trouble that could be related to subsidence effects or a breakthrough. These 

personnel should be aware of where underground mining has occurred and where to look for cracks or 

other evidence of subsidence. Signs that they should look for include: (1) unusual sudden drops in the 

pool level, (2) the presence of a whirlpool or bubbles in the pool, (3) cracking or sudden displacement 

in embankment surfaces, (4) unusual readings in piezometers, (5) changes in seepage conditions, and 

(6) changes in the quantity of flow and the amount of sedimentation in discharges from mine openings, 

backfilled mine openings, or outcrop areas. Instruments such as staff gages, weirs, survey monuments 

and inclinometers are useful for monitoring these conditions and detecting changes.

In determining what instrumentation should be installed and monitored, the designer should identify the parameters that will indicate how the site is performing with respect to potential failure 

modes. This will facilitate taking action if the facility does not perform as expected. The following 

items should be considered in developing a monitoring plan:

• Seepage – Seepage from the impoundment should be monitored, including seepage through the embankment, through any internal drainage systems, and through 

underground mines that receive seepage from the impoundment. Weirs or other 

devices should be installed so that flow rates can be easily and consistently measured. Changes in water quantity and quality in seeps and discharges (including 

mine/pump discharges) that are hydraulically connected to the impoundment 

should be monitored. Changes in seepage quantity, particularly when not correlated 

to rainfall or pool water levels, may indicate deteriorating or adverse conditions warranting additional investigation.
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Site Mining and Foundation Issues

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• Water levels – The pool levels in the impoundment and in underground mines 

should be monitored, and unexplained changes should be investigated. If there are 

bulkheads in the mine, the water pressure against them should be monitored. Where 

conditions with respect to breakthrough potential are uncertain, instrumentation 

can be installed to provide an alarm in the event of a sudden drop in the impoundment water level. The alarm would alert mine personnel to check on the situation 

and would facilitate early warning and emergency response in the event of a breakthrough failure.

• Piezometric levels – Saturation levels and water pressures in the refuse embankment, 

as well as in any other earthen barriers, should be monitored to determine whether 

hydrostatic pressures are within design limits and whether any changes or trends 

are reasonable. In situations where it is critical to be able to measure rapid or sudden 

changes in pore water pressure, a closed system such as a vibrating-wire piezometer 

should be used.

• Rainfall data – It is good practice to install a rain gauge in the vicinity of an 

impoundment, but it is especially important in situations where there is breakthrough potential and where discharges from a mine can be traced to seepage from 

the impoundment. In such cases, rainfall data should be routinely collected so that it 

can be determined whether changes in the water flow or water level data correlate to 

rainfall or may be occurring for other reasons.

• Ground movement – When there is potential for subsidence in the vicinity of an 

impoundment, the ground surface should be monitored for movement. Both horizontal and vertical movements should be measured.

The types and suitability of instrumentation for accomplishing these monitoring objectives are discussed in Chapter 13. The timely review and interpretation of instrumentation data by someone 

knowledgeable in the design and performance of impoundments is critical to an effective monitoring 

program.

The type and frequency of monitoring required depend on impoundment conditions. Typically, monitoring is performed during regular weekly inspections. Where conditions warrant, more frequent or 

even continuous automated monitoring may be needed. Monitoring plans should include provisions 

for plotting and evaluating the observed data in a timely manner. When a potentially hazardous 

condition develops, more frequent monitoring is required. 30 CFR § 77.216-3(b)(4) requires, in part, 

that when a potentially hazardous condition develops, the mine operator shall immediately direct a 

qualified person to monitor all instruments and examine the structure at least once every eight hours 

or more often, if requested by an authorized representative of MSHA.

8.7 MINE BACKFILLING DESIGN

Backfilling of mine workings is performed in order to provide localized support for pillars and the 

mine roof and to reduce the volume of open space that could potentially be filled with collapsed 

material, thus tending to minimize the deformation of the surrounding rock mass. Support for pillars 

and volume reduction of open space can be achieved by a variety of backfill types. While deformation and bulking of the roof strata can provide support for mine overburden, backfilling with grout 

to improve contact with the roof may be a desirable option. The most common types of fill material 

are waste rock, mill tailings, quarried rock, sand and gravel, and fly and bottom ash. Additives such 

as portland cement, lime, fly ash, and pastes can also be mixed with the fill to alter the characteristics 

and to improve the effectiveness of the backfill. Placement alternatives include stowing by hand, 

gravity, mechanical, pneumatic and hydraulic methods. Hydraulic placement is generally effective 

for the varying conditions encountered in mine workings.
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Backfilling of mines has significantly reduced surface damage from subsidence by lending lateral 

support to pillars and by limiting the volume of voids (National Academy of Sciences, 1975). The 

most important consideration in mine backfill design is the planned backfill material. The mechanics 

of uncemented fill can be analyzed using soil mechanics principles (Coates, 1981). The strength of 

most uncemented hydraulic fills is related to frictional resistance to sliding between particles and can 

be affected by pore water pressure and erosion, as well as dynamic loads such as blasts or sudden 

fluctuations in the water table. Additionally, the compressibility of the backfill material is related to 

its ability to provide support for the pillars and roof.

Cemented backfill has cohesion resistance gained through the addition of cement or pozzolanic 

admixtures that render the backfill relatively incompressible. Backfill strength usually increases linearly with cement content, which can range up to 10 percent. The grain-size distribution of the fill 

may also be important. Well-graded material generally has a greater strength than uniformly graded 

material, although the fines content (i.e., minus 200 mesh portion) can adversely affect the strength. 

The inclusion of pozzolanic material additives such as fly ash can reduce the volume and associated 

cost of portland cement in a mine backfill while significantly increasing strength and providing other 

beneficial characteristics such as improved fluidity during placement. The strength and placeability 

of candidate cement mixes should be evaluated through laboratory testing.

Typically, mine backfill provides substantial filling of mine voids, such that relatively little bulking 

from roof materials is needed to mitigate fracturing of the overburden and the advance of subsidence 

in the overburden. In critical situations, the space between the backfill and the roof can be grouted. In 

instances where mine backfill provides support for the coal pillars but not direct support for the overburden, the mechanical behavior of the cemented fill can be modeled as providing lateral restraint 

according to the relationship (Cai, 1983):

σh = n

γ H a (8-14)

 Kp L

where:

σh = passive earth pressure (psf)

n = correction coefficient (dimensionless)

γ = unit weight of overburden (pcf)

H = depth of overburden (ft)

a = width of open space (ft)

Kpl = coefficient of passive earth pressure of pillar (dimensionless)

L = width of adjacent pillar (ft)

With lateral support from cemented backfill, the strength of a pillar increases according to the following formula (Guang-Xu and Mao-Yuan, 1983):

σ’1 = σ1 + σh Kpl (8-15)

where:

σ’1 = strength of pillar supported by fill (psf)

σ1 = strength of pillar (psf)

σh = passive earth pressure provided by fill (psf)
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Site Mining and Foundation Issues

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The supported pillars can then be analyzed for stability based on the procedures discussed in Section 8.4.3. For instances of thin overburden, Mitchell (1983) and Wizniak and Mitchell (1987) present 

analytical procedures for estimating surface subsidence deformation for backfill placed to the mine 

roof wherein the roof is modeled as a beam on an elastic foundation.

The design of mine backfill generally depends on the availability of fill materials and fly ash and their 

associated costs.

8.8 SURFACE MINE SPOIL ISSUES

At some refuse disposal sites, surface mine spoil may be available in the foundation area, and some 

of this spoil may be suitable for use in constructing structural portions of refuse embankments such 

as the starter dam. Mine spoil is typically quite variable in terms of its composition of soil and rock 

materials and also relative to maximum particle or fragment size and distribution, durability and 

moisture content. Typical methods of spoil placement can also result in considerable segregation. 

This variability makes it difficult to characterize the engineering behavior of mine spoil, and thus the 

field and laboratory procedures described in Sections 6.4 and 6.5 may need to be modified to accommodate its special characteristics. This variability also can lead to difficulties in estimating engineering properties such as compressibility, shear strength and hydraulic conductivity. Piping in mine 

spoil materials is a concern and has been identified as the cause of sinkholes and black-water releases 

at slurry impoundment sites founded on mine spoil. When a coal refuse facility is to be constructed 

over or using surface mine spoil, designers should recognize the variable character and composition 

of the material and understand the potential impacts that it can have on the long-term performance 

of a coal refuse embankment.

8.8.1 Surface Mine Spoil Characteristics

Surface mine spoils result from excavation and placement of overburden and interburden materials. 

These operations typically range in size from several hundred to several thousand acres, and mine 

life is typically five to 30 years or more. Overburden removal is generally accomplished by continuous bucket-wheel excavators, walking draglines, hydraulic excavators, stripping shovels, scrapers, 

dozers, or cast blasting. Where unconsolidated materials such as glacial till or loess represent a significant portion of the overburden, bucket-wheel excavators are generally used.

Spoil placement is typically accomplished by dropping the spoil materials at the angle of repose to form 

a ridge of piles parallel to the active pit or by placing the materials in lifts where the lift thickness and 

degree of compaction depend on the equipment type, weight and number of passes.

Uncontrolled spoil placement is typically associated with contour and area mining, while placement 

in lifts is generally associated with haul-back mining and head-of-hollow and valley-fill construction. 

Before the late 1970s to early 1980s, spoil ridgetops were left as deposited or were graded with a single 

pass of a dozer that resulted in a ridge and trough topography. With the advent of state and federal 

regulatory programs to return mining operations to near their original contours, spoil ridges have 

been extensively graded with dozers leading to increased spoil handling and machinery traffic, greater 

breakdown of the spoil, higher density and improved engineering behavior of the in-place spoil. A discussion of the influence of mining method on the bulk density of spoil based on testing in the Eastern 

Coal Province is presented in Phelps et al. (1981).

Because of the material handling that occurs during excavation, deposition and placement, mine spoil 

materials experience significant changes in physical integrity. These changes are related to variations in 

geologic characteristics, moisture, stress regimes, mining and reclamation methods, and other environmental aspects of the materials. The physical deterioration of geologic materials caused by changes in 

stress conditions or strength characteristics is referred to as slaking. The most distinctive aspect of the 

slaking process is a relatively rapid decrease in the particle or fragment size of the material.
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The decrease in particle or fragment size caused by slaking can have a wide range of effects on the 

behavior of the material in a spoil pile. These effects will depend on the gross characteristics of the 

spoil pile (e.g., physical dimensions and configuration of the pile and the degree of compaction), the 

durability of the pile materials, the proportion of slakable materials, and changes in the pile surficial 

or internal moisture regimes. Possible adverse effects of spoil slaking include: (1) decreases in material strength that can reduce the stability of slopes and (2) increases in moisture content and decreases 

in particle and fragment size that can increase settlement and surface erosion and affect hydrologic 

regimes and vegetation. In a study of the environmental effects of slaking of surface mine spoils in 

the eastern and central U.S., Andrews et al. (1980) observed that:

• The rate and degree of particle breakdown is directly related to the material characteristics (e.g., durability) and local environmental conditions (e.g., depth of burial).

• The most active zone of slaking occurs within about three feet of the exposed mine 

spoil surface.

• The major observed effect of mine spoil slaking is a decrease in particle or fragment 

size that results in changes to the hydrogeologic characteristics (e.g., rate of infiltration, hydraulic conductivity, rate of groundwater flow) of spoil piles

• The significance of slaking seems to be minimized by the mixing of slakable (e.g., 

mudstone and shale) and nonslakable (e.g., limestone) materials that usually occurs 

during typical spoiling operations.

• No gross environmental damages due to slaking were apparent at the sites visited.

Andrews et al. (1980) developed these observations through laboratory testing of bulk samples 

obtained from test pits excavated in 2-, 5-, and 10-year-old spoils from four mine sites located in the 

Appalachian Basin. Based on extensive qualitative and semi-quantitative data collected relative to the 

behavior, causes, and effect of slaking materials, the study identified three field slaking modes:

1. Slaking to a constituent grain size that typically occurs in mudstones and occasionally sandstones

2. Chip slaking to thin, platy fragments that generally occurs in shales, siltstones and 

occasionally thinly-bedded sandstones

3. Slab or block slaking to large, approximately equidimensional fragments that generally occurs in sandstones and limestones.

The lithology, bedding and mineralogical characteristics of the spoil materials were found to have 

a major effect on the mode, rate and degree of slaking. In general, mudstones, siltstones and shales 

were found to be the most slake-prone lithologies. Slaking of sandstone and limestone was variable, 

but generally minor. Bedding characteristics were the primary factor in the mode of slaking (i.e., 

rocks with thin bedding typically exhibited chip slaking whereas rock with a massive structure were 

prone to block or slab slaking, or to slake to their constituent particle size). Spoils that slake to their 

constituent particle size were found to be less stable, while spoils in which chip slaking or slab or 

block slaking dominates generally were found not to be associated with stability issues.

The engineering behavior of mine spoil may be related to mining techniques. In general, overburden removal by blasting, material transport and dumping by trucks, and placement and leveling by 

dozers results in mechanical breakdown that leads to reduced slaking once the material is placed.

8.8.2 Design and Construction Considerations

The compressibility, shear strength and hydraulic conductivity of mine spoil can vary considerably 

depending on the proportion of slakable and nonslakable materials and the methods used to place 

and reclaim the spoil. Test methods that can be used to assess the slaking potential of mine spoil are 
8-57

Site Mining and Foundation Issues

MAY 2009

presented in Section 6.5.9.4. The application of these test methods as part of an overall management 

control process are described in Andrews et al. (1980). Figure 8.16 presents a system of classification 

and interpretation of spoil durability based on index and slake durability testing using the degradation index (DI), where DI = 1 – Id and Id is the slake durability index. Based on conclusions and recommendations from the study, designers should consider the following measures in planning and 

conducting site reconnaissance, site exploration and laboratory testing programs and in developing 

designs and preparing construction documents:

• Review of available surface mining records in order to document the mining and reclamation practices used, the stratigraphy of overburden and interburden materials 

excavated, and the time frame for mining operations.

• Use of test pits to observe and document the general proportion of slakable and 

nonslakable materials, the type and amount of slakable material breakdown, the 

grain-size distribution and moisture content of bulk samples, and the existence of a 

permanent or perched water level in the spoil mass.

• Provisions to control seepage and prevent internal erosion through mine spoil zones 

under refuse facility structures, including cutoff trenches through spoil deposits or 

impervious blankets with filter layers.

• Measures to monitor the settlement and lateral displacement of foundation spoil 

materials as the refuse embankment is raised. A discussion of instrumentation is provided in Chapter 13.

If mine spoil is to be used for embankment fill, the variability of the spoil should be determined so 

that the embankment can be constructed in a manner that will allow the desired performance in 

terms of strength and hydraulic conductivity to be achieved. The strength of mine spoil is typically 

estimated by: (1) evaluating its angle of repose, (2) correlating with published values for rock fill if 

the material is durable, or (3) by testing the finer portion of the material, which typically results in a 

conservative estimation. Internal stability of non-uniform material can be a concern for mine spoil, 

particularly if it is gap-graded. The hydraulic conductivity of mine spoil is typically estimated based 

on grain size, and measures such as zoning may be required in order to address the variability of the 

material. Designers should consider the following measures, if mine spoil is to be used to construct 

some or all of a coal refuse embankment:

• The composition and variability of the mine spoil used for borrow should be determined using test pits to observe and document the general proportion of slakable 

and nonslakable materials, the type and amount of slakable material breakdown, 

the grain-size distribution and moisture content of bulk samples, and the chemical 

composition (i.e., pH and content of sulfide minerals in fine-grained constituents 

such as shale) of the spoil mass. Placement of slakable material in upstream zones 

(particularly when fine-grained) and nonslakable material in downstream zones in 

an embankment can provide a means of isolating the materials and taking advantage 

of their properties.

• If oversize material (e.g., greater than 12 inches in maximum dimension) is present, 

provisions to crush, mechanically degrade or isolate oversize materials should be 

incorporated into the construction specifications.

• If the chemical composition of any portion of the spoil mass is not acceptable, these 

materials should be isolated or excluded from the embankment.

The presence of mine spoil in the foundation zone of an impounding embankment may require measures to address settlement and the potential for internal erosion. The significance of such effects 

should be evaluated based on exploration, testing and analysis of the foundation and embankment 
Chapter 8

8-58 MAY 2009

PRELIMINARY FIELD RECONNAISSANCE

EXAMINE ROCK EXPOSURES, IDENTIFY MAJOR

LITHOLOGIC UNITS; CHECK EXTENT AND MODE

OF WEATHERING (E.G., BLOCK FALLS, CHIPPING,

MUD FLOWS); CHECK LOCAL MINE SPOILING

OPERATIONS FOR SLAKING EFFECTS

DETERMINE LITHOLOGIC TYPES AND CORRELATE

WITH RECONNAISSANCE PROGRAM. CHECK FOR

CARBONATES USING HYDROCHLORIC ACID.

LIMESTONES AND NON-CALCAREOUS CEMENTED

SANDSTONES

EXPLORATION PROGRAM

NO FURTHER TESTING OR SPECIAL DESIGN

MEASURES RECOMMENDED ALL FINE-GRAINED LITHOTYPES AND CALCAREOUS

CEMENTED SANDSTONES

PRELIMINARY LABORATORY PROGRAM

CONDUCT 1:1 pH AND CEC TESTS ON CRUSHED

POWDERED SAMPLES

1:1 pH > 7.8 or CEC > 15 MEQ /100g

DURABILITY TESTING PROGRAM

CONDUCT 5-CYCLE DURABILITY TEST (WET/DRY

OR RATE OF SLAKE), FRAGMENT SIZE > 1-1⁄2 INCH

(38 mm) AND SLAKE FLUID: WATER. OBSERVE

PATTERN AND RATE OF BREAKDOWN (NONE TO

SLIGHT, CHIP, PARTIAL TO COMPLETE DISAGGREGATION), COMPUTE 5-CYCLE DI

1:1 pH < 7.8 AND CEC < 15 MEQ /100g

NO FURTHER TESTING OR SPECIAL DESIGN

MEASURES RECOMMENDED

SETTLEMENT AND STABILITY PROBLEMS MAY DEVELOP; MECHANICAL

BREAKAGE DURING EXCAVATION,

HANDLING AND PLACEMENT MAY

BE SUFFICIENT TO MINIMIZE SLAKING AFTER BURIAL. NO SPECIAL

SEDIMENT CONTROL REQUIRED.

SETTLEMENT, STABILITY AND SEDIMENT PROBLEMS MAY DEVELOP

IF LITHOTYPE COMPRISES MORE

THAN 50% OF THE OVERALL SPOIL

COMPOSITION.(1) SPECIAL DESIGN

AND MANAGEMENT TECHNIQUES

MAY BE REQUIRED.

SETTLEMENT, STABILITY AND SEDIMENT PROBLEMS MAY DEVELOP

IF LITHOTYPE COMPRISES MORE

THAN 75% OF OVERALL SOIL COMPOSITION.(1) SPECIAL DESIGN AND

MANAGEMENT TECHNIQUES MAY

BE REQUIRED.

NO SPECIAL DESIGN MEASURES

RECOMMENDED

“CHIP SLAKE”

PARTIAL OR

COMPLETE

DISAGGREGATION

“CHIP SLAKE” PARTIAL

DISAGGREGATION

BREAKDOWN

SLIGHT TO NONE

FIGURE 8.16 CLASSIFICATION AND INTERPRETATION OF SPOIL DURABILITY

NOTE: 1. BASED ON HOMOGENEOUS MIXING

 OF LITHOTYPES DURING PLACEMENT

(ANDREWS ET AL., 1980)

SLAKING MODE

DI > 50 DI < 50

SLAKING MODE

TESTING RESULTS

CEC = CATION EXCHANGE CAPACITY

MEQ = MILLIEQUIVALENT

 DI = DEGRADATION INDEX = (1 - Id)

 Id = SLAKE DURABILITY INDEX

FIGURE 8.16 CLASSIFICATION AND INTERPRETATION OF SPOIL DURABILITY
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Site Mining and Foundation Issues

MAY 2009

materials. Typical measures that are employed include: (1) compensating for settlements through staged 

construction by loading foundation zones using broad embankment widths and (2) providing sufficient 

gradient on drainage structures. The potential for internal erosion can be addressed by a variety of site 

preparation measures including the use of cutoff trenches with seepage barriers and filters.

8.9 SURFACE MINE HIGHWALL ISSUES

Surface mine highwalls represent foundation concerns for coal refuse disposal facilities, in that the 

change in rock surface elevation may represent an abrupt transition in an embankment abutment 

with the potential for: (1) embankment cracking due to differential settlement or (2) a zone of concentrated seepage due to the difficulty of placement and compaction of fill materials close to a nearvertical highwall. Additionally, surface mine benches and highwalls may contain fractured materials, 

particularly if they have been subjected to auger or highwall mining activities. Subsidence associated 

with such mine openings and remedial measures are discussed in Sections 8.4.2.8 and 8.5.1.3. The 

discussion in this section focuses on abrupt rock transitions and associated seepage cutoffs.

Whenever a coal refuse embankment or earthen dam abuts a surface mine bench or highwall, there 

is a potential for performance problems due to the abrupt transition where the rock bench/highwall 

adjoins the earthen embankment. These potential performance problems include:

• Inadequate compaction of embankment materials against the rock surface sufficient to provide a low-hydraulic-conductivity contact between the earthfill and 

rock surfaces.

• Incomplete filling of surface cavities and depressions in rock surfaces with compacted 

embankment materials that can be sources of differential settlement and seepage.

• Overly steep rock surfaces and overhangs that limit the opportunity for maintaining positive compressive pressure over the full contact between earthfill and rock 

surfaces.

• Earthfill placed over rock surfaces with steep or abrupt slope transitions that can 

lead to differential settlement, cracking and elevated seepage in the earthfill.

• Highly fractured highwalls or benches resulting from mining or subsidence that represent potential seepage pathways that could impact downstream embankment and 

abutment areas.

To minimize these potential performance problems, rock benches and highwalls should be inspected 

during construction and should be treated or modified so that unacceptable conditions and performance problems do not occur. For earth embankment dams with an impervious core, these problems are of concern mostly where the impervious core abuts the rock surface. For coal refuse or 

other embankments that are constructed as homogeneous dams, the concern may be more significant 

because these structures do not have an impervious zone to serve as a barrier to internal seepage or 

seepage along abutment contacts.

If the control measures described in the following list cannot be fully implemented in a constructed 

embankment, special drainage features for intercepting and conveying seepage flows from the embankment should be considered. Fell et al. (2005), USBR (1984) and Sherard et al. (1963) describe a number 

of treatment and modification options for such performance problems, which may be encountered at 

water retention dams. Additionally, alternate measures are described that may be appropriate for coal 

refuse impoundments, but any such measure should be evaluated with respect to site-specific conditions. Guidance for addressing a number of conditions is provided in the following:

• Loose and weathered materials – The existing overburden and weathered rock 

should be excavated to competent rock, and the resulting rock slope should be 
Chapter 8

8-60 MAY 2009

trimmed to a regular surface to eliminate depressions, overhangs, pinnacles or sharp 

transitions. This treatment should focus on the abutment cutoff zone, as illustrated 

in Figure 8.17, and should extend as warranted by site conditions and the embankment cross section. Sherard et al. (1963) noted that the most effective way to obtain 

a tight bond between earthfill and a rock surface is to slope the rock surface sufficiently to permit each embankment layer to be compacted directly against the rock 

using heavy compactors. Equipment operation above or below a highwall should be 

preceded by an evaluation of the highwall stability and development of appropriate 

procedures, if necessary, to address the threat of rockfalls or instability during grading and excavation.

• Overly steep rock slope – If the slope of a rock surface that will abut an earthfill is 

steeper than 0.5H:1V, the rock surface should be flattened by excavation or backfilling with concrete, or other measures should be taken to effectively place and compact the embankment materials. Alternate measures could include incorporation of 

impervious zones, incorporation of internal drainage features, or broadening of the 

embankment. The same cautions discussed above relative to working near a highwall apply.

• Hydraulic cutoff – If flattening is not practical due to the height of the rock slope, a 

cutoff keyway can be excavated into rock, or other measures to control seepage along 

the rock interface may be implemented. The depth of a cutoff keyway should extend 

a sufficient depth (e.g., six feet as per Fell et al., 2005) into competent rock, and the 

width of the keyway should be sufficient to permit compaction of earthfill in the 

cutoff in accordance with embankment criteria. Alternate measures to mitigate seepage could include incorporation of internal drainage features or broadening of the 

embankment-abutment contact.

ORIGINAL OVERBURDEN

SURFACE

(PRATT ET AL., 1972)

WEATHERED

ROCK SURFACE

1

 1

 2

1

1

2

NOTE: SEEPAGE CUTOFF NOT SHOWN.

SOUND

ROCK

SURFACE

FIGURE 8.17 EXCAVATION SLOPES FOR PREPARATION OF DAM ABUTMENTS IN ROCK
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Site Mining and Foundation Issues

MAY 2009

• Steep, abrupt transitions in rock grade – Steep, abrupt transitions in rock grade 

should be trimmed (Figure 8.18) to minimize the potential for impacts of differential settlement in compacted earthfill susceptible to cracking (e.g., cohesive fill) 

above the transition. The same cautions discussed above relative to working near a 

highwall apply.

• Clay-filled seams or very weathered rock – Clay-filled seams or very weathered 

rock should be excavated and filled with concrete (or grouted) to prevent erosion of 

the seams. Treatment should focus on the abutment cutoff zone and should extend 

upstream and downstream to the extent warranted by the site conditions and the 

embankment cross section. The U.S. Bureau of Reclamation (USBR, 1984) recommends that seams narrower than 2 inches be cleaned to a depth of three times the 

seam width and that seams between 2 inches and 5 feet wide be cleaned to a depth 

of three times the seam width or to a depth where the seam is 1⁄2 inch wide or less. To 

avoid unnecessary excavation of stable seam material that would be held in place by 

concrete and subsequent earthfill, engineering judgment should be used to determine the reasonableness of these guidelines in light of site-specific conditions. Perin 

(2000) presents a case history that addresses treatment of stress relief fractures at a 

coal refuse disposal facility.

• Irregularities on slopes not steeper than 0.5H:1V – These irregularities may be 

treated using dental concrete, pneumatically-applied mortar or slush concrete grout, 

as illustrated in Figure 8.19. The extent of treatment can be limited to the abutment 

cutoff zone, as warranted, or may extend throughout the abutment area depending 

on site-specific conditions and the breadth of and material types in the embankment 

cross section. Generally the following treatments are considered for structures that 

(FELL ET AL., 2005)

EMBANKMENT CREST

CRACKING DUE

TO DIFFERENTIAL

SETTLEMENT

POSSIBLE

ADDITIONAL

EXCAVATION

EXCAVATE

TO ELIMINATE

CRACKING

FIGURE 8.18 SLOPE MODIFICATION TO REDUCE DIFFERENTIAL SETTLEMENT 

AND POTENTIAL FOR CRACKING
Chapter 8

8-62 MAY 2009

may impose significant hydraulic head in the vicinity of the irregularity (e.g., water 

retention dams):

Dental concrete should be used to fill joints, bedding, sheared zones, overhangs or excavated surfaces, particularly in the abutment cutoff zone. Fell 

et al. (2005) recommend that dental concrete slabs have a minimum thickness of 6 inches, a minimum 28-day compressive strength of 3,000 psi, and 

a maximum aggregate size not more than one-third the depth of the slab or 

one-fifth the narrowest dimension between the rock surface and the edge of 

the form. Feathering at the ends of slabs should not be permitted, and slab 

edges should be sloped no flatter than 45 degrees. To achieve good bond 

between the rock surface and the concrete, the rock surface should be thoroughly cleaned and moistened prior to concrete placement. The finished concrete surface should have a roughened, broomed surface to facilitate earthfill 

placement. Sulfate-resistant cement should be used in the concrete.

Pneumatically-applied mortar (shotcrete) can be used as an alternate to dental 

concrete provided that care is taken that it is applied in a manner consistent 

with the recommendations for dental concrete.

Slush concrete grout is a neat cement grout or sand-cement slurry used to fill 

narrow surface cracks or to serve as a temporary cover over slakable materials that degrade rapidly upon exposure to air and water. Slush grout may be 

applied by brooming, troweling, pouring, rodding, or funneling into individual 

cracks (Fell et al., 2005). To facilitate adequate bond, the rock surface and cracks 

should be cleaned and moistened before the slush concrete grout is applied.

(FELL ET AL., 2005)

SHALE

SANDSTONE

SLAKING SHALE

INTERBEDDED

SILTSTONE

AND SHALE

SANDSTONE

CLEAN OUT WEATHERED SHALE AND SHOTCRETE TO FILL UNDERCUTTING

TRIM AND COVER WITH SLUSH

CONCRETE OR SHOTCRETE

FILL WITH CONCRETE

CLEAN OUT WEATHERED SEAM

TO DEPTH = 3X WIDTH OF SEAM

AND BACKFILL WITH CONCRETE

EMBANKMENT CREST

FLATTEN TO 0.5H TO 1V (LINE)

DRILL AND BLAST LIGHTLY

REMOVE SOIL, SOFT ROCK

TO CUT-OFF FOUNDATION

FILL WITH

CONCRETE

FLATTEN TO 0.5H TO 1V (LINE)

DRILL AND BLAST LIGHTLY

In some situations, grouting of extensively fractured surface mine benches and highwalls may be 

more appropriate than other treatments discussed above because of the depth or extent of disturbance. Table 8.6 presents general guidance for cement grout programs (Fell et al., 2005) that may 

be useful for highly fractured rock foundations. USACE (1984a) and Fell et al. (2005) provide further guidance for evaluation, design and implementation of grouting programs, including the use of 

chemical grouts for limited applications.

FIGURE 8.19 SLOPE MODIFICATION AND SEAM TREATMENT FOR SEDIMENTARY ROCK STRATA
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Site Mining and Foundation Issues

MAY 2009

TABLE 8.6 GUIDELINES FOR CEMENT GROUT PROGRAMS

Staging of Grout 

Program

 Downstage

Top section of hole is drilled and washed, pressure tested, grouted and allowed 

to set for 24 hours. Top section of grout is then washed out then second stage 

is drilled and washed, pressure tested, grouted and allowed to set for 24 

hours. Upper sections of grout are then washed out then third stage is drilled 

and washed, pressure tested and grouted. Use of packer allows increased 

pressures.

 Upstage

Hole is drilled to full depth and washed, packer is seated at the top of the 

bottom stage, pressure tested, grouted, and allowed to set for 6 hours. Set 

packer at top of second bottom stage, pressure test, grout, and allow to set for 

6 hours. Continue remaining stages.

 Full Depth Hole is drilled to full depth and washed, pressure tested, grouted. Only 

recommended for consolidation grouting.

Erodibility of 

Foundation(1)

Pressure Test 

Value Before 

Grouting 

(Lugeon)(3)

Reduction in 

Lugeon Value or 

Grout Take from 

Previous Stage(2)

(Lugeon)(3)

All Grout 

Takes 

(kg cement/

m)

Grout Hole

Spacing (m)

Closure Criteria Low/Non < 10 < 20 < 25 < 1.5

High < 7 < 15 < 25 < 1.5

 Note: 1. Erodible foundations would include extremely or highly weathered rock and rock with

 clay-filled joints that might erode under seepage flows.

 2. For rock with joints closer than 0.5m.

 3. Tabulated values are for Type F portland cement; for Type A portland cement,

 adopt Lugeon values 20 percent greater. One Lugeon is a flow of 1 liter/minute/

 meter of borehole under a pressure of 1000 kPa. In a 75-mm borehole, one Lugeon

 equivalent to approximately 1.3x10-7 m/sec hydraulic conductivity.

Depth and Lateral 

Extent

So far as practical, grout holes should be taken to the depth and extent necessary to meet 

closure criteria. Rules of thumb are not recommended. In nearly horizontally layered rock, 

geologic interpretations may provide a guide where testing is unavailable in the valley floor, but 

may be at different depths and orientations around the abutments due to the influence of stress 

relief, weathering, and rock types.

Cement Grout 

Particle Size

Cement particles are mostly silt size, but include some fine sand particles 

in conventional cement. With plasticizers, Type A and C Portland cements 

have a maximum particle size of about 0.05 to 0.08 mm, while microfine 

cements may be about 0.02 mm.

Fracture Size Minimum Lugeon values are indicative of rock that will accept 

cement grout

Cement 1 Fracture/m 2 Fractures/m 4 Fractures/m

Type A 8 16 32

Type C 5 10 20

MC-500 (microfine) 3 5 10

Type A with dispersant 8 16 32

Type C with dispersant 5 10 20

MC-500 dispersant 1 2 4

Note: Fractures are assumed to be rough and uniform width and grout is assumed to have 

been treated with plasticizer.

Grouting

Effectiveness

No further grouting is needed when:
Chapter 8

8-64 MAY 2009

TABLE 8.6 GUIDELINES FOR CEMENT GROUT PROGRAMS

(Continued)

 Approximate Penetration from Borehole of Grout (m)

Fracture Spacing

Grout

Penetration

Lugeons 1m 0.5m 0.25m

100 20 12 4

50 12 3 2

20 3 1.5 1

10 2 1 NP

5 1 NP NP

1 NP NP NP

 Note: NP indicates that grout will not penetrate the fractures.

Practical Aspects

Grout Holes 30 to 60 mm percussion drilling and washing of borehole.

Standpipes Threaded galvanized pipe just larger than drill size, grouted into borehole to 

enable near surface grouting.

Grout caps Necessary when grouting closely fractured or low strength rock where 

standpipe cannot be sealed into rock.

Grout Mixers High speed, high shear, colloidal mixers.

Agitators Slow speed designed to prevent cement particle settling.

Grout Pumps Helical screw pumps or ram type pumps.

Packers Mechanical or inflatable.

Water Cement 

Ratio

Starting mix: most sites – 2:1, for rock < 5 Lugeons – 3:1, for rock > 30 

Lugeons – 1:1; for very high losses – 0.8:1; for heavily fractured, dry rock 

– 4:1, and for above water table where excess water is absorbed by dry 

rock – 5:1.

Thicken mix: (1) to deal with severe leaks, (2) after 11⁄2 hours with continued 

take, or (3) if hole is rapidly taking grout (e.g., > 500 liters in 15 minutes).

Grout Pressure Recommend avoiding rock fracturing. Start at 100 kPa or less for 5 minutes, 

then steadily increase over next 25 minutes. Occurrence of fracturing 

can be detected by sudden loss of grout pressure at top of hole due to 

increased take. Recommend grouting to refusal or minimum take.

Monitoring Parameters: (1) hole location, orientation, and depth, (2) stage depths, 

(3) water pressure test value for each stage, (4) grout mixes, (5) grout 

pressures (e.g., 15-minute intervals), (6) grouting times, (7) leaks, uplift,

(8) total grout take for each stage, (9) amount of cement in these takes,

and (10) cement takes/unit length of hole.

Water Pressure 

Test

Before grouting, apply water pressure and monitor for 15 minutes.

Stage Lengths Based on geologic conditions, minimum drill run, allowable pressures in 

upper part of hole, rock fracture conditions and hole stability, water flows 

into the hole, and large water pressure tests or grout takes.

(ADAPTED FROM FELL ET AL., 2005)
Chapter 9

HYDROLOGY AND

HYDRAULICS

MAY 2009 9-1

Coal refuse impoundments and embankments must handle the runoff from precipitation that occurs 

over the contributing watershed area. If not properly controlled, runoff can jeopardize the collection 

and conveyance system (channels and conduits). For impoundments, runoff can cause the embankment to be overtopped with the potential for failure. The principles of hydrology and hydraulics can 

be used to determine and design the required combination of flow capacity and freeboard and to 

select durable channel lining systems. The discussion of technical issues in this chapter is based on 

the assumption that the reader is experienced in the technical areas of hydrology and hydraulics and 

is familiar with the selection of hydrologic and hydraulic design parameters and the use of related 

computer software. A number of traditional design concepts are reviewed herein, and reference is 

made to additional resource materials.

The design of coal refuse disposal facilities requires a somewhat specialized approach. There are 

many possible combinations of disposal facility configuration, facility staging, environmental considerations and unique characteristics and properties associated with each site. Therefore, one of the 

major aims of this chapter is to relate fundamental engineering principles to the unique requirements 

of refuse disposal facility site design. While primarily focused on slurry impoundments, the contents 

of this chapter are also applicable to other mining dams and impoundments.

The hydrologic and hydraulic information and design procedures presented in this chapter fall into 

five interrelated categories, as follows:

• Basic definitions and principles – Sections 9.1 and 9.2 define basic terms and conditions applicable to coal refuse disposal facilities that relate to hydrologic and hydraulic features. Table 9.1 presents a complete summary of hydrologic and hydraulic 

planning and design procedures. The table also serves as an outline of this chapter 

and a summary of supplemental references. The fundamental interrelationships of 

runoff, reservoir storage, and outflow are established. The major elements that may 

affect these interrelationships at coal refuse disposal facilities are also discussed.

• General design considerations – Section 9.3 identifies regional and site conditions 

that affect the suitability of various hydraulic conveyance structures for coal refuse 

disposal facilities. In Section 9.4, these broad concepts are extended to consider the 

effect of disposal facility configuration upon selection of suitable hydraulic convey-
9-2

Chapter 9

MAY 2009

ance structures. Characteristics that distinguish coal refuse disposal facilities from 

conventional embankment dams are emphasized.

• Design-storm criteria – Section 9.5 presents design storm precipitation criteria for 

coal refuse disposal facilities. Factors such as location, facility size, and hazard 

potential are discussed. Design storm criteria for short-term conditions and for 

minor hydraulic structures are also addressed.

• Procedures for analysis – Sections 9.6, 9.7 and 9.8 discuss analytical procedures for 

evaluation and design of coal refuse disposal facility hydraulic structures. Methods 

for determining runoff based on predicted precipitation are first established, followed by reservoir storage and outflow capacity requirements. Various components 

of outflow structures are discussed in detail. Procedures for routing storm runoff 

through an impounding disposal facility and optimizing reservoir storage and outflow are presented.

• Dam-breach analysis – Section 9.9 discusses procedures for evaluation of dam breach 

and potential downstream inundation for the determination of hazard potential and 

for Emergency Action Plan (EAP) preparation.

9.1 GENERAL CONSIDERATIONS

The hydrologic and hydraulic design and analysis procedures discussed in this chapter apply to both 

existing and new coal refuse disposal facilities. The sequence presented in Table 9.1 is normally followed either for modifying an existing disposal facility or for constructing a completely new disposal 

facility. It should be recognized that sequencing of a modification to an existing coal refuse disposal 

facility should be continually coordinated with the ongoing mining and coal preparation operations.

The designer of a new coal refuse disposal facility normally has flexibility in site selection, staging of the embankment growth and long-term planning of related hydraulic structures. Given this 

flexibility, design flood requirements can typically be met throughout the entire life of the disposal 

facility. Often the designer is able to optimize the relationships between refuse disposal operations, 

embankment design, hydraulic structure construction, and the overall mining and coal preparation 

operations.

A designer modifying an existing disposal facility should first determine its conformance with current design storm criteria and should then assess options for any necessary upgrade of the runoff 

collection and control system. Sometimes a facility has limited storage or hydraulic conveyance capability, may not satisfy current design and regulatory requirements, and cannot be easily modified in a 

short period of time. An effective solution may be to perform a staged modification program, as part 

of continued refuse disposal operations, which may in fact provide materials necessary for increasing 

freeboard and constructing diversions, thus improving hydraulic capacity. Under such conditions, 

the modifications to the facility are usually required to meet or exceed MSHA’s short-term hydrologic 

design criteria, as subsequently described in Section 9.5.2.

9.2 HYDROLOGY AND HYDRAULICS PRINCIPLES

Hydrology is the study of climatic and physical conditions that govern natural flows in rivers, streams 

and channels. Hydrologic analyses are used to determine the probable and possible direct runoff to 

a particular site from natural causes such as precipitation or snow melt. Hydraulics is the study of 

water flows in channels and conduits. Hydraulic engineering is used in the design of decant systems, 

outlet works, spillways, ditches, channels, diversion structures, and other systems for controlling 

flowing waters. An integrated application of hydrology and hydraulics is necessary for the development of safe, economical and environmentally acceptable coal refuse disposal facilities.
9-3

Hydrology and Hydraulics

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TABLE 9.1 HYDROLOGIC AND HYDRAULIC DESIGN PROCEDURES FOR 

COAL REFUSE DISPOSAL FACILITIES

Design Considerations

Applicability Manual 

Sections for 

Reference

Supplemental 

All References

Facilities

Impounding 

Facilities

I. Determine Importance of Hydrologic and 

Hydraulic Considerations

Type of facility X Chapter 3, 9.4 USBR (1987a)

Impounding vs. non-impounding 9.4 USBR (1987a)

Site conditions X Chapter 5, 9.5 USBR (1987a)

Downstream conditions X (Of particular 

concern)

9.3, 9.5 USBR (1987a)

Startup, operation, abandonment 

requirements

X Chapters 4, 

6, 9

II. Establish Preliminary Facility 

Configuration and Hydraulic Systems

Select structure type X Chapter 3, 9.4

Balance availability of materials for 

embankment construction with facility staging

X Chapter 5,

9.3, 9.4

Determine size and potential hazard 

classification based on dam breach analysis 

and downstream inundation

X 9.5, 9.9 FEMA (2004a)

Determine appropriate design storm for longterm operationX 9.5 MSHA (2007)

Determine if separate design consideration 

should be given to short-term conditions with 

lesser design storm at any time during the 

operational period of the facility

X 9.4, 9.5 MSHA (2007)

Calculate watershed contributing to major 

hydraulic systems

X 9.3

Determine approximate inflow rates and 

volumes to be controlled by major hydraulic 

systems from design storm criteria

X 9.6 NWS (2006a,b)

NRCS (2004b)

Evaluate alternative combinations of spillway 

outflow and impoundment storage capacities

X 9.6 to 9.8 USBR (1987a)

NRCS (2004b)

Brater et al. (1996) 

Determine preliminary spillway type, location 

and approximate size (for all stages of 

operation)

X Chapter 5,

9.6 to 9.8

USBR (1987a)

Brater et al. (1996)

Determine preliminary decant type, location 

and approximate size (for all stages of 

operation)

X Chapter 5,

9.6 to 9.8

USBR (1987a)

Brater et al. (1996)
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Chapter 9

MAY 2009

Design Considerations

Applicability Manual 

Sections for 

Reference

Supplemental 

All References

Facilities

Impounding 

Facilities

Determine magnitude of storm that can be 

controlled and compare with appropriate 

design storm for facility size and potential 

hazard classification

X 9.5 to 9.9 USBR (1987a)

NRCS (2004b)

Evaluate modifications to be made to 

improve the facility’s hydraulic system

X Chapter 6,

9.5 to 9.8

USBR (1987a)

Evaluate advantages and disadvantages of 

modifying the facility for continued use or to 

a satisfactory configuration for abandonment

X Chapter 6,

9.4 to 9.8

Assign appropriate long-term design storm or 

abandonment criteria

X 9.5 FEMA (2004c)

III. Determine Design Inflow Rates and 

Volumes for Major Hydraulic Systems

Determine if key parameter curves are 

suitable for final design for any or all stages, 

including abandonment

X 9.4, 9.6

Determine inflow hydrograph parameters, if 

required, for any stage of development

X 9.6 NRCS (2004b)

USBR (1987a)

IV. Design Major Hydraulic Systems

Design major diversion system to insure 

against failure during appropriate design 

storm

•	 Collection of inlet area X 9.7 Chow (1959)

•	 Establish control section of flow (inlet, 

transport section or outlet)

X 9.7 USBR (1987a)

Henderson (1966)

Brater et al. (1996)

•	 Determine requirements to prevent 

failure by overtopping, erosion or 

clogging

X 9.6 to 9.8 USBR (1987a)

•	 Determine downstream outlet and/

or discharge requirements to avoid 

unacceptable damage at design flow

X 9.7, 9.8 Chow (1959)

USBR (1987a)

Brater et al. (1996)

Determine optimum combination of storage 

and outflow for each stage of development 

(for impoundments)

•	 Perform reservoir routing analysis of 

inflow hydrograph

X 9.6 to 9.8 USBR (1987a)

Design the spillway system for the 

appropriate design storm for each stage of 

development

TABLE 9.1 HYDROLOGIC AND HYDRAULIC DESIGN PROCEDURES FOR 

COAL REFUSE DISPOSAL FACILITIES 

(CONTINUED)
9-5

Hydrology and Hydraulics

MAY 2009

Design Considerations

Applicability Manual 

Sections for 

Reference

Supplemental 

All References

Facilities

Impounding 

Facilities

•	 Establish control section for all flow 

conditions to assure adequate capacity

X 9.6 to 9.8 Chow (1959)

USBR (1987a)

Henderson (1966)

Brater et al. (1996)

•	 Design the inlet including provisions to 

prevent clogging

X 9.8 USBR (1987a)

•	 Design the outlet to prevent 

unacceptable damage at magnitude of 

flow

X 9.8 FHWA (2006)

USBR (1987a)

Design the decant system for normal 

operating conditions and to evaluate 

impoundment storage of design storm

•	 Establish flow control for all storage 

levels to assure adequate capacity

X 9.6, 9.8 USBR (1987a)

Brater et al. (1996)

•	 Design the inlet, including provisions to 

avoid clogging

X 9.8 USBR (1987a)

Brater et al. (1996)

•	 Design the transport section, 

considering structural stability, 

corrosion resistance, and capacity

X 9.8 USBR (1987a)

Brater et al. (1996)

FHWA (2005b)

•	 Design the outlet to prevent 

unacceptable damage

X 9.8 USBR (1987a)

FHWA (2006)

Perform dam breach analysis and evaluate 

downstream inundation

X 9.9 FEMA (2004c)

V. Design Minor Hydraulic Systems

Surface drainage ditches that are not critical 

to safety during design storm

X 9.6, 9.8 USBR (1987a)

FHWA (2005a)

FHWA (2006)

Minor roadway culverts X 9.6, 9.8 FHWA (2005b)

FHWA (2006)

Weirs to separate seepage from large flows, 

if required, for environmental control

X 9.8 Henderson (1966)

TABLE 9.1 HYDROLOGIC AND HYDRAULIC DESIGN PROCEDURES FOR 

COAL REFUSE DISPOSAL FACILITIES 

(CONTINUED)

9.2.1 Basic Design Principles

The fundamental principle governing the hydrologic and hydraulic design of a coal refuse disposal 

facility is that runoff, natural drainage and process water must be conveyed past the embankment, 

stored within the facility impoundment(s), or handled by a combination of these two methods. The 

hydrologic characteristics of the applicable watershed (rainfall, tributary area, land use cover conditions, soil type, slope, etc.) determine the runoff hydrograph, while the physical dimensions and 

hydraulic characteristics of the facility and hydraulic structures determine the required conveyance 

and storage capacity. Table 9.2 presents a summary of the application of the basic design principles 

to coal refuse disposal facilities.
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TABLE 9.2 HYDROLOGIC AND HYDRAULIC DESIGN CONSIDERATIONS 

FOR COAL REFUSE DISPOSAL FACILITIES

Embankment Type(1) Runoff, Outflow and Storage Considerations

Valley-Fill and Side-Hill

Non-Impounding

Embankments

If placement of the embankment is started at the upper end of the valley, runoff 

from the natural watershed can be diverted around the embankment and no 

water has to be stored. Precipitation on the embankment can be directed 

downstream.

If placement is started by forming a downstream embankment, it will have a 

temporary character with interim diversion ditches sequentially replaced as the 

fill is raised (sometimes the final diversion ditches are installed initially). Precipitation and runoff on the embankment are directly discharged downstream 

with the intervening drainage between the final diversion ditch and the interim 

diversion ditches.

Ridge and Heaped

Non-Impounding Embankments

Ridge and heaped embankments that are constructed above the natural 

topography only have inflow associated with direct rainfall onto the disposal area. Precipitation and runoff on the embankment can be directed 

downstream.

Cross-Valley

Impounding

Embankment

The cross-valley impounding embankment presents a variety of alternatives for 

handling hydrologic events. Inflow may include precipitation from upstream of 

the embankment, including the drainage area above diversion ditches, unless 

the ditches are designed not to fail from the design storm.

For a cross-valley impoundment, the three possibilities for handling design 

storm inflow are:

1. If the embankment crest elevation is maintained sufficiently high above the 

pool level, all runoff from the design storm can be stored, such that outflow 

is not a requirement during the design storm. The impounded water can 

then be lowered gradually by flow through a decant system.

2. If a spillway of adequate size is constructed with its crest at the normal 

pool level of the impoundment, all of the storm runoff can be passed 

directly through the disposal area and the storage requirement will be 

minimal.

3. If the spillway crest is located above the normal pool level, but the storage 

volume between the pool and spillway elevations is less than the inflow 

volume, the spillway must be designed to conduct a volume equal to the 

difference between inflow volume and storage volume in an appropriate 

time interval.

Side-Hill Impounding Embankment

A side-hill impounding embankment can have all of the alternatives of a crossvalley impoundment except that the smaller watershed and the potential for 

diversion significantly reduce the storage and outflow requirements associated 

with the design storm.

Diked-Pond

Embankment

Normally, a diked-pond embankment will have inflow equal to the precipitation 

falling directly into the impoundment. Total storage with limited or no outflow 

during the design storm is normally the best solution, although the drawdown 

requirement must be met by either a spillway, decant pipe, or pumping.

Incised Pond An incised pond has a water surface below the normal ground surface, and 

inflow runoff, storage and outflow generally are not critical to safety.

 Note: 1. Embankment types are discussed in Chapter 3.
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Hydrology and Hydraulics

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In general, for non-impounding coal refuse disposal facilities or the downstream or perimeter portions 

of impounding facilities or slurry cells, runoff is conveyed around the facility without retention and 

storage. On the other hand, impounding embankments are designed to temporarily store runoff from 

upstream areas and to convey excess flows past the embankment with decant pipes and spillways.

Design criteria for impounding and non-impounding coal refuse disposal facilities include the total 

volume of runoff from the design storm, as discussed in Section 9.5. For a non-impounding coal 

refuse disposal facility, the peak runoff rate caused by a flood or the design storm is of prime concern. 

For an impounding facility, both the peak runoff rate and the total volume of runoff are of concern. 

In the first case, the hydraulic facilities must be sized to pass the peak runoff rate, while in the latter 

case, the impoundment and hydraulic structures must be designed to store and pass the total volume 

of runoff.

The runoff and outflow elements are influenced by a number of critical factors, as discussed in the 

following section.

9.2.2 Definition and Discussion of Key Runoff Elements

Sources of impoundment inflow are shown in Figure 9.1. These sources also include ancillary flow 

contributions such as process water (water or water-slurry mixture pumped from the mine or the 

coal processing plant), indirect runoff from adjacent watersheds, or other diverted flows such as from 

underground mines. The sources of impoundment inflow can be categorized as follows:

Major Sources

• Direct precipitation – rain or snow falling directly onto the disposal site

• Runoff – from precipitation falling on areas upstream or upgradient from the site 

and within the watershed associated with the facility

Minor Sources

• Springs from groundwater flow

• Base flow in a stream passing through or by the site that is relatively independent of 

the most recent rainfall events, but directly related to infiltration associated with earlier rainfall events

• Process water and other pumped flows

Minor sources of flow are typically much smaller than the major sources of runoff. The volumes 

associated with minor sources can be determined with relative accuracy. However, the amount of 

runoff resulting from a storm will vary depending upon site location. Geographic location, climatic 

conditions and watershed characteristics all contribute to storm runoff, as discussed in the following sections.

9.2.2.1 Watershed Boundary and Area

The watershed is all of the catchment area that drains toward a particular point of interest. Watershed 

boundaries are typically determined from site-specific topographic maps (Section 6.4.1.1) or USGS 

topographic quadrangle maps, as shown in Figure 9.2.

9.2.2.2 Precipitation

Runoff results from precipitation falling on the watershed, melting of snow already on the ground 

and outflow from upstream impoundments in the watershed. Snowmelt is usually a minor portion of 

runoff in small watersheds, such as those usually associated with a coal refuse disposal facility. The 

effects of upstream impoundments should be considered on an individual site basis.
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NET INFLOW TO

IMPOUNDMENT

FIGURE 9.1 RUNOFF AND IMPOUNDMENT INFLOW SOURCES

WATERSHED

RUNOFF

PRECIPITATION

RAINFALL

SNOWMELT

OUTFLOW

SPILLWAYS & DECANTS

STORAGE

RETENTION IN

IMPOUNDMENT

UPSTREAM

IMPOUNDMENT

RELEASE

SPRINGS, PROCESS

WATER, STREAM INFLOW

GAINS

INFILTRATION, SURFACE

RETENTION, UPSTREAM

IMPOUNDMENT

DIVERSION AROUND

IMPOUNDMENT

LOSSES

RELEASED AT

CONTROLLED RATE

FIGURE 9.1 RUNOFF AND IMPOUNDMENT INFLOW SOURCES
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Hydrology and Hydraulics

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9.2.2.2.1 Rainfall Curves

Calculation of the design storm rainfall (Section 9.5) involves determination of the total amount and 

distribution of rainfall for the entire storm duration. The relationships between total precipitation 

(cumulative rainfall depth), storm duration, and storm intensity (slope of the rainfall distribution 

curve) have a direct effect on the runoff rate and volume (Sections 9.6.1 and 9.6.2). For example, a 

sudden short rainfall can result in a high runoff rate and a small total volume of runoff, while a prolonged rainfall of low intensity can produce a large total volume of runoff with a relatively low runoff 

rate. Coal refuse disposal facilities should be designed to accommodate all possible precipitation/

runoff conditions associated with the design storm.

9.2.2.2.2 Rainfall Intensity

The relationships between rainfall intensity, duration of the rainfall event and frequency (i.e., intensity-duration-frequency or I-D-F) can be used to determine the peak runoff, and are useful in the FIGURE 9.2 WATERSHED BOUNDARY DELINEATED

ON USGS TOPOGRAPHIC MAP 

LIMITS OF

WATERSHED

LONGEST 

WATERCOURSE

POINT OF INTEREST

FIGURE 9.2 WATERSHED BOUNDARY DELINEATED ON USGS TOPOGRAPHIC MAP
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design of hydraulic structures such as culverts, channels and ditches (Sections 9.6.3 and 9.6.4). Only 

the most intense portion of the rainfall, not the entire storm history, governs the selection of culvert 

size, the most efficient ditch or channel configuration, and the required erosion protection associated 

with the runoff flow velocity.

9.2.2.3 Watershed Characteristics

A portion of the precipitation falling on a watershed is retained in the soil and by vegetation or may 

be retained in upstream impoundments. The portion of the precipitation that flows to the point of 

interest is termed the runoff. The watershed characteristics that determine the difference between the 

amount of precipitation falling on the watershed and the amount that becomes runoff include: (1) the 

types of surficial soils and their effect on infiltration; (2) the condition of the ground surface (e.g., wet, 

dry, snow-covered or frozen) prior to the precipitation (termed the antecedent moisture condition); 

(3) the type and density of vegetation; (4) development features such as paved surfaces, channeling, 

storm sewers, etc.; and (5) the presence of dams, lakes, ponds or swamps upstream from the disposal 

facility that can either store water and release it at a slow rate or fail and release large volumes of 

stored water at a high rate.

The runoff hydrograph at the point of interest will vary as a function of the intensity distribution of 

precipitation and the geometric shape and slope conditions of the watershed area. Inflow and outflow hydrographs for a typical impoundment are shown in Figure 9.3. The figure also shows the net 

inflow and volume of impoundment storage.

INFLOW

HYDROGRAPH

FLOW RATE

TIME

VOLUME OF INFLOW

VOLUME OF STORAGE

VOLUME OF OUTFLOW

PEAK INFLOW RATE

OUTFLOW

HYDROGRAPH

PEAK OUTFLOW RATE

FIGURE 9.3 TYPICAL IMPOUNDMENT INFLOW AND OUTFLOW HYDROGRAPHS
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Hydrology and Hydraulics

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9.2.3 Key Storage and Outflow Elements

The principal factors governing the storage capacity of a reservoir or impoundment are the physical 

dimensions of the embankment and ground surface and the current level of water/slurry. The outflow capacity is determined by the types and sizes of the hydraulic structures.

9.2.3.1 Impoundment Capacity

The storm storage capacity of an impoundment is the volume of runoff that can be temporarily 

retained during the applicable design storm. If a refuse disposal facility has minimal storage capacity, 

the outflow hydrograph will be about the same as the inflow hydrograph, and the hydraulic structures must be designed to transport the peak runoff rate. The primary benefit of impoundment storage is that the outflow rate can typically be reduced, permitting use of smaller hydraulic structures. 

An additional benefit is that the downstream flooding risk is not exacerbated by an increase in runoff 

from disturbed areas in the watershed. The potential difference in peak flow rates is illustrated by the 

inflow and outflow hydrographs shown in Figure 9.3.

Figure 9.4 shows a typical impoundment capacity curve relating storage volume to pool elevation. 

The figure also shows the relationship between reservoir surface area and pool elevation. Such curves 

are used to evaluate the storage conditions at any given pool elevation and are prepared as part of the 

design of an impounding structure (Section 9.7).

Two terms used to describe the limits of acceptable pool elevation are “surcharge” and “freeboard.” 

Surcharge is the vertical distance between the usual operations level of the impoundment and the 

maximum allowable water surface elevation. Normal freeboard is the vertical distance between the 

pool elevation and the top of the embankment at its lowest point (where the dam would begin to 

be overtopped). Design storm freeboard is the vertical distance between the maximum water surface elevation during the design storm and the top of the embankment. The minimum design storm 

freeboard is an impoundment design criterion and should be such that waves do not overtop the 

embankment crest during the design storm. Freeboard also serves to compensate for uncertainty in 

hydrologic parameters.

9.2.3.2 Decants, Principal Spillways and Auxiliary Spillways

Decants are conduits that extend through an embankment and discharge under controlled conditions 

at or beyond the embankment toe. As the term “decant” would imply, impoundment water typically 

enters the conduit by flowing over the top edge of the upstream end. At coal refuse disposal facilities, decants are generally not intended to discharge at high flow rates, but are designed to remove 

clarified process water, pass base stream flows or to drain the impoundment of stored water after a 

storm. However, a decant must be sufficiently large that stored water from the design storm can be 

drained within a reasonable period of time, so that the storage volume needed for a subsequent storm 

is available. Several types of decant systems are shown in Figure 9.5.

Principal spillways are generally designed to control the discharge associated with large design 

storms, to limit discharges and associated impacts downstream, and to limit the frequency and duration of flow through the emergency (auxiliary) spillway. Principal spillways are most often associated with fresh water impoundments and sedimentation and treatment ponds, and state regulatory 

agencies typically provide specific design storm criteria that govern the size and capacity of these 

structures. In some coal refuse facility designs, the decant may also function as a principal spillway. 

Principal spillways are designed to: (1) release runoff at a controlled rate, (2) provide settling time for 

the removal of sediment or process water solids prior to discharge, (3) provide runoff detention, and 

(4) function as decants to control the impoundment operational pool level. Decant systems are generally not considered in design storm flood routing analyses for determining maximum impoundment pool level, as they do not have significant discharge capacity. If considered in the flood routing 
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analysis, the decant pipe should be of sufficient size that clogging is unlikely (typically, greater than 

12 inches in diameter) and should be equipped with a properly designed trashrack.

Auxiliary (emergency) spillways are open channels generally used to discharge that portion of the 

runoff volume that cannot be stored in the impoundment or routed through the principal spillway. 

Auxiliary spillways typically are capable of discharging: (1) moderate flows from storms much 

smaller than the design storm (Section 9.5.1) with little or no damage or (2) large flows resulting from 

the design storm, where some localized damage may occur, but without the threat of failure of the 

entire impounding embankment. Typical auxiliary spillway systems are shown in Figure 9.6.

STORAGE VOLUME (ACRE-FT)

IMPOUNDMENT SURFACE AREA (ACRES)

WATER SURFACE ELEVATION (FEET, MSL)

0 100 200 300 400 500

V

50 40 30 20 10 0

675

670

665

660

655

650

645

STORAGE VOLUME IMPOUNDMENT

SURFACE AREA

STORAGE VOLUME (V) AT WATER

SURFACE ELEVATION (h) 

h

FIGURE 9.4 TYPICAL IMPOUNDMENT AREA AND STORAGE VOLUME CURVES
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Hydrology and Hydraulics

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DECANT PIPE

OUTLET

RIPRAP

9.5b DECANT THROUGH REFUSE EMBANKMENT

FIGURE 9.5 TYPICAL DECANT SYSTEMS

9.5c DECANT THROUGH SADDLE

RIPRAP

DECANT PIPE

OUTLET

DECANT PIPE

INLET

DECANT INLET ON

HILLSIDE SLOPE

9.5a DECANT FOR DIKED IMPOUNDMENT 

NOTE: SPILLWAY TYPICALLY

 NOT REQUIRED

RIPRAP

DECANT PIPE OUTLET

DECANT INLET ON

UPSTREAM SLOPE

FIGURE 9.5 TYPICAL DECANT SYSTEMS
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The design of auxiliary spillways, principal spillways and decants normally requires evaluation of 

three basic components: the inlet, the transport section and the outlet. Key processes in the design of 

these systems include:

• Determining which component controls the outflow rate for various flow conditions.

• Sizing each component to function properly for the anticipated range of flow conditions.

• Specifying materials for each component that will not erode excessively under the 

anticipated flow velocities.

SPILLWAY CUT

INTO ROCK

DECANT INLET

POSITIONED ON 

HILLSIDE SLOPE

DECANT PIPE OUTLET

FIGURE 9.6 TYPICAL DECANT AND SPILLWAY SYSTEMS

DECANT PIPE OUTLET EXCAVATED GRASS-LINED

SPILLWAY TO ADJACENT VALLEY

DECANT INLET

POSITIONED ON

HILLSIDE SLOPE

FIGURE 9.6 TYPICAL DECANT AND SPILLWAY SYSTEMS
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Hydrology and Hydraulics

MAY 2009

• Designing inlet or transport sections that will not become clogged or otherwise fail, 

causing major downstream damage or failure of the impounding embankment.

• Arranging the outlet location so that the release of water does not lead to failure of 

the impoundment embankment or major downstream damage.

The design of outflow systems is further discussed in Section 9.7.

9.3 GENERAL CONSIDERATIONS FOR COAL REFUSE DISPOSAL FACILITIES

9.3.1 Special Characteristics

Table 9.3 lists characteristics that distinguish the design of typical coal refuse disposal facilities from 

many other structures with appurtenant hydraulic structures. In addition to the special characteristics indicated in Table 9.3, the hydrologic and hydraulic design of coal refuse disposal facilities is also 

governed by the considerations discussed in the following sections.

9.3.2 Site Conditions

Site selection impacts the cost and difficulty in providing adequate hydraulic appurtenant structures 

for use during the disposal period and subsequent abandonment of a coal refuse disposal facility. 

Based upon hydrologic and hydraulic considerations, the best site will almost always have the smallest possible upstream watershed. In some cases, however, hydrologic/hydraulic considerations are 

secondary to preparation plant location and materials handling requirements. Even if this appears to 

be the case, the designer should evaluate the needed hydraulic structures considering downstream 

hazard potential, environmental control and construction costs prior to finalizing the location for a 

disposal facility. Large initial costs associated with the construction of hydraulic structures may be 

justified if this allows materials transportation costs to be lowered.

TABLE 9.3 FACILITY CHARACTERISTICS INFLUENCING HYDRAULIC SYSTEM DESIGN

 Characteristic Significance In Design

The facility is designed for disposing coal refuse, 

with active operations taking place for an associated 

period of time, and not to collect water for flood 

prevention, water supply, power, or recreation.

Greater flexibility in choosing location, configuration and 

construction sequence for appurtenant hydraulic structures.

The facility covers a large area, with the gradient or 

drainage slope primarily in one direction.

Providing diversion facilities not subject to localized failures or 

controlled overtopping during large storms is often not economically practical.

The placement of refuse occurs over many years, 

during which time the facility configuration is 

constantly changing.

Hydraulic systems must be designed so that they can be 

expanded or decommissioned and replaced as the facility 

grows.

The growth rate of the facility is estimated based 

upon projected quantities of refuse production.

Actual quantities must be evaluated periodically to determine if 

the rate of construction is adequate.

Water passing over or through the coal refuse can 

be destructive or environmentally unacceptable.

Proper design requires that potentially adverse environmental 

effects (e.g., corrosion of construction materials), and the 

cost of water collection and treatment, be considered in the 

evaluation of alternative hydraulic systems.

When placement of refuse is completed, the facility 

typically has no continuing utility, and the hydraulic 

systems are decommissioned, the impounding 

capability is eliminated, and the site is abandoned in 

accordance with mine reclamation requirements.

The sequence of constructing hydraulic systems must provide 

an arrangement that will function until decommissioning at a 

specified future date. Planning must allow for the possibility 

that decommissioning, elimination of impounding capability, 

and abandonment may be required for a configuration either 

larger or smaller than originally anticipated.
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The site conditions described in the following sections may affect decisions related to the selection 

and design of facility hydraulic structures.

9.3.2.1 Topography

The importance of topography on the geotechnical aspects of site selection and disposal facility 

configuration is discussed in detail in Section 6.2.2.1. As discussed in this section, the significance of 

topography is generally limited to the planning, design and construction of hydraulic conveyance 

structures.

9.3.2.1.1 Steep Terrain

In areas of steep and rugged terrain, many disposal facilities must necessarily be located in valleys 

formed by small streams. Two very significant problems may be encountered with respect to placing 

diversion ditches, spillways and conveyance channels in these areas:

• Channels cut into the side of the valley will require the excavation of large amounts 

of material, as illustrated in Figure 9.7a. With increased channel width, the cut 

becomes more extensive and the slope of the cut must often be decreased to achieve 

stability. These conditions combine to increase excavation quantities and costs disproportionately to the flow capacity gained.

• The potential for sloughing of overburden soil or weathered rock into the channel, 

thus restricting its flow capacity, is increased, as illustrated in Figure 9.7b. Major 

sloughing will often occur during a heavy rainstorm when a large flow capacity is 

desired. The possibility of main spillway channels becoming obstructed by sloughing must be considered in the geotechnical analysis and design of the cut slope.

Diversion ditches for non-impounding coal refuse embankments are designed based on the design 

storm (100-year-recurrence-interval storm). For impounding coal refuse facilities with more extreme 

design storms such as the Probable Maximum Precipitation (PMP), it is usually not feasible to design 

perimeter diversion ditches large enough to pass the maximum flow. While diversion ditches for 

impounding facilities still perform an important function, the hydraulic design of the impoundment 

generally is based on the assumption that during large floods the diversion ditches will be overtopped and the resulting overflow will enter the impoundment. However, such overtopping should 

not be permitted to occur if flows in excess of the diversion ditch design storm could cause erosion 

of the dam and spillway.

Although there are undesirable aspects to cutting channels into steep slopes, there may also be significant advantages. Bedrock is normally found near the surface in rugged terrain. Thus, channels cut 

in such areas will often be resistant to erosion without special protection. A channel should be located 

where its base will be on the most resistant material. If possible, channels should be constructed in 

sound rock, particularly where flow velocities will be erosive and where failure of the channel would 

create an unsafe general condition or large repair costs. Another advantage that may be realized from 

cutting channels into such slopes is the concurrent production of borrow materials suitable for use 

as resistant drainage material.

9.3.2.1.2 Gently Sloping Terrain

In gently sloping terrain, the disadvantages associated with hillside channel excavation are not as 

pronounced as in steep terrain. As shown in Figure 9.8a, the volume of excavation is a nearly linear 

function of channel width. In addition, achieving stability of the uphill cut slope is not as difficult 

as for channels cut into steep hillsides. However, as illustrated in Figure 9.8b, these areas often do 

not have rock near the surface. Therefore, the channels are more susceptible to erosion unless flow 
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Hydrology and Hydraulics

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velocity can be kept low or some type of stabilization system (e.g., channel lining) is provided. A key 

to economical design in this case is to minimize the length of channel sections where flow velocities 

exceed the natural erosion resistance of the channel.

9.3.2.1.3 Effects of Slope on Facility Staging

The combination of site topography and the constantly increasing size of coal refuse disposal facilities often creates design problems not normally encountered with other water-impounding facilities. 

For example, the previously mentioned difficulties involved in excavating wide auxiliary spillway 

channels in steep slopes are multiplied when the design requires that multiple auxiliary spillway 

channels be excavated as the height of the embankment is increased in subsequent stages. Problems VOLUME OF EXCAVATION

CHANNEL WIDTH

W1 W2

9.7a EXCAVATION VOLUME VS. CHANNEL WIDTH

FIGURE 9.7 CHANNEL CONSTRUCTION IN MODERATELY AND 

 STEEPLY SLOPING TERRAIN

W1

W2

POTENTIAL OVERFLOW

FAILURE

RESISTANT ROCK

RETARDS EROSION

SOIL OR WEATHERED

ROCK CAN SLOUGH AND

BLOCK CHANNEL

9.7b CHANNEL EROSION AND OVERBURDEN SLOUGHING

FIGURE 9.7 CHANNEL CONSTRUCTION IN MODERATELY AND STEEPLY SLOPING TERRAIN
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can also occur when it becomes necessary to tie an embankment into an excavated rock face, as 

opposed to the original soil cover on the natural slope. Tying the embankment material into the steep 

and broken rock increases the potential for future problems related to seepage, leakage and embankment stability.

For cases where multiple auxiliary spillways are required with construction of succeeding stages, 

the designer may wish to consider a series of cascading spillways. A new embankment stage with its 

associated spillway channel can be configured to extend a sufficient distance downstream to allow 

the outflow to drop into the spillway channel of the preceding stage with the addition of a plunge 

pool. For such configurations, the hydraulic design of the channels and plunge pool and the erosion 

resistance of the rock must be carefully evaluated.

In some cases, the topography may permit an open-channel spillway to be located away from the 

embankment, such as through a saddle in a ridgeline, so that the flow is discharged into an adjacent watershed, as shown in Figure 9.6. This arrangement can be beneficial in that potential issues 

associated with flow escaping from the spillway channel and adversely affecting the downstream 

face of the embankment are avoided.

9.8a EXCAVATION VOLUME VS. CHANNEL WIDTH VOLUME OF EXCAVATION

CHANNEL WIDTH

W1 W2

FIGURE 9.8 CHANNEL CONSTRUCTION IN GENTLY SLOPING TERRAIN

9.8b CHANNEL EROSION

ORIGINAL CHANNEL

BOTTOM

ERODED CHANNEL

BOTTOM

W1

W2

FIGURE 9.8 CHANNEL CONSTRUCTION IN GENTLY SLOPING TERRAIN
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9.3.2.2 Weather and Climate

Weather and climatic conditions should be considered as part of the planning associated with the 

design and construction of hydraulic structures. Specific examples include:

• In most coal regions of the United States, construction of channels, ditches, concrete spillways and decant systems should be scheduled to the extent possible 

during normal construction seasons and should be avoided in winter, when 

freezing conditions and snowfall may interrupt construction. Accordingly, the 

staging of any disposal facility should be planned so that there is adequate flexibility to allow extensions, replacement or modification to hydraulic systems 

during favorable weather even though coal refuse is handled and disposed on a 

year-round basis.

• Many western coal fields are in arid or semi-arid climates where the growth of vegetation is a very slow process. In these areas, using vegetation as a means of erosion 

protection in excavated channels may not be practical.

9.3.2.3 Geology

Normally, geologic conditions do not change drastically within a small geographic area, and thus 

they generally do not directly affect disposal facility site selection alternatives. However, soil and 

rock conditions at a site are always important to the design of hydraulic structures and often are the 

deciding factor in choosing among several hydraulic system alternatives with similar cost and utility 

characteristics. The following are important geologic and geotechnical factors that must be considered in design:

• If excavated channels in steep slopes are being considered, the designer should evaluate the stability of the cut slopes. If excessive costs will be required to achieve stability, either by benching or by constructing retaining systems, an alternative system 

may be more cost effective.

• If an excavated channel is to be located along a hillside, it should have a sufficient 

capacity that overtopping or discharge that could cause cascading water to flow 

onto a critical portion of the embankment does not occur. If a bend in the channel is 

required, the effects of flow, erosion, and water superelevation caused by the change 

of direction should be carefully evaluated. The outlet end of a spillway channel 

should be located sufficiently far downstream that the discharge will not erode the 

downstream face of the embankment.

• Channels should be designed to resist potential erosion effects, so that post-construction stabilization is not required.

• If hydraulic structures are to be constructed in or over soft soils or soft coal refuse, 

the amount of settlement that could occur should be estimated in order to determine 

whether special construction will be required. Similar considerations may arise in 

situations where differential settlement may occur, such as where hydraulic structures are constructed across rock abutments and onto fill materials. This is especially 

important for conduits through an embankment. Where possible, such conduits 

should be founded on and properly bedded in firm materials that will not settle 

significantly. Where settlement is unavoidable, the initial slope, camber, joints and 

conduit material should be selected such that anticipated settlements can be accommodated without damage to the system. 

The effect of geologic conditions on runoff during storms is discussed in Section 9.6.1.
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9.3.3 Construction Materials

The selection of construction materials for hydraulic structures should account for the following:

• The potential for corrosion of construction material is high at many coal refuse 

disposal facilities because of the chemical characteristics of water seeping through 

refuse materials. Choosing corrosion-resistant materials with higher initial cost 

may be far less expensive over the long term than repairing a deteriorated structure 

several years after its original installation, especially if the structure will be buried 

under many feet of refuse.

• Any conduit or structure beneath or within an embankment should be designed for 

the external pressure of the maximum potential height of the embankment above it 

and for deformations that may result from embankment construction.

• Channel lining material should be selected to be resistant to the maximum anticipated flow velocities with provisions for drainage and resistance to uplift pressures.

• Filter criteria for all materials used in the embankment and appurtenant structure 

construction should be evaluated so that the potential for erosion and piping within 

the embankment or loss of structural support and/or failure of the hydraulic conveyance structures is minimized.

9.4 DESIGN CONSIDERATIONS FOR DISPOSAL FACILITY EMBANKMENT TYPES

In addition to the general design considerations discussed in Chapter 5, there are specific hydrologic 

and hydraulic design considerations for each type of coal refuse disposal facility embankment. The 

following discussion of facility-dependent hydrologic and hydraulic design considerations is a general summary of the most common considerations for each type of disposal facility.

Some of the primary hydraulic system functions common to all refuse disposal facilities are listed 

below. The type and configuration of the coal refuse disposal facility determines the significance of 

each function.

• Collection of runoff from the watershed above the embankment and from the surface 

of the embankment.

• Control, conveyance and discharge of collected water to a downstream location.

• Control of the embankment slope utilizing benches at 50-foot or lower vertical intervals to reduce potential erosion.

• Erosion protection of the embankment surface during initial, interim and reclamation stages, especially along the embankment face.

• Protection of streams or wetlands from encroachment or other potential environmental impacts that may require mitigation.

• Protection of downstream water quality from sediment-laden runoff, leachate from 

internal drain collection systems, or collected seepage.

The specific impact of the above hydrologic and hydraulic design considerations is discussed in the 

following sections. While typical figures are presented to assist in recognizing specific conditions, 

they do not depict all design situations.

9.4.1 Non-Impounding Embankments

Non-impounding embankments are used for the disposal of coarse, combined, and dewatered fine 

coal refuse. A non-impounding coal refuse disposal facility is designed such that no fine coal refuse 

slurry, process water or direct or indirect runoff can accumulate within or upstream of the disposal 
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facility limits. General types of non-impounding coal refuse embankments include valley, ridge, sidehill and heaped fills.

9.4.1.1 Valley-Fill Embankments

Valley-fill refuse embankments, as illustrated in Figure 9.9, are often constructed by starting disposal at the upper end of a valley and extending the embankment in stages down the valley in such 

a manner that an impoundment is never created. Often these types of embankments are located in 

large valleys so that large refuse disposal volumes can be placed. The potential runoff in such valleys 

during a large storm event can be high, and to prevent excessive erosion large diversion channels 

may be needed. A key design objective associated with the collection of watershed and embankment 

surface runoff and the discharge of the collected water at a downstream point is to provide the optimum balance between channel cross section and slope, thereby minimizing the cost associated with 

channel erosion protection.

The most difficult portion of the channel design is along the embankment face at the interface of the 

coal refuse and the natural ground surface where the steep slope typically results in high velocities. If 

practical, the channels should be extended along the valley wall, within natural soil and rock, beyond 

the limits of the coal refuse embankment to discharge beyond the embankment toe. If such an extension is not practical, it is normally necessary to construct a lined or otherwise protected channel at 

the interface of the refuse embankment and valley wall to carry the runoff safely to the valley floor. 

The long diversion/collection ditches along the crest of the disposal facility should be designed with 

a base width and slope that allows use of grass-lined channel sections, if possible. FIGURE 9.9 VALLEY-FILL, NON-IMPOUNDING EMBANKMENT

 DRAINAGE CONTROL

SLOPE TO

DRAIN

DIVERSION CHANNEL

PERIMETER COLLECTION

CHANNEL

SEDIMENT POND

FIGURE 9.9 DRAINAGE CONTROL FOR VALLEY-FILL, NON-IMPOUNDING EMBANKMENT
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The mixture of runoff, leachate and seepage may require treatment prior to discharge to the receiving waterway. Such treatment could entail construction of sedimentation ponds and also ponds for 

chemical treatment. Sufficient area for construction of the sedimentation/treatment ponds should 

be allocated. However, these facilities should be located above the level of the 100-year floodplain 

associated with the receiving stream and not in a position where they could be affected by normal 

stream flows.

9.4.1.2 Side-Hill, Ridge and Heaped Embankments

Side-hill, ridge and heaped non-impounding embankments have design configuration considerations 

similar to those for valley-fill embankments. The upstream and perimeter watersheds are generally 

smaller than for a valley-fill embankment, but the steepness of the final embankment slopes and the 

water quality of the runoff and seepage result in similar hydrologic and hydraulic design considerations as for a non-impounding, valley-fill embankment. Figure 9.10 shows drainage control for sidehill and heaped embankments.

Side-hill embankments are usually constructed in stages that extend progressively higher on a natural slope. Therefore temporary diversion ditches are needed for collecting and diverting runoff at 

intermediate stages when the embankment is not at full elevation. The channel dimensions, slope 

and required erosion protection should be designed to meet the final conveyance requirements and 

to provide economical erosion protection. The location of the toe of the embankment should lie outside the 100-year floodplain limits of nearby streams to minimize any potential encroachments, and 

sufficient area should be available for sediment/treatment pond construction.

Ridge embankments are generally in the upper reaches of a watershed and may resemble a side-hill 

embankment extending above and over a ridge line. The collection and conveyance of precipitation 

falling directly onto the embankment is the primary issue since there is typically little if any upstream 

watershed. This type of facility generally has a limited downstream area available for sedimentation 

control and chemical treatment, and the natural ground surface may slope away from the disposal 

facility in several directions and potentially into other watersheds. Therefore, multiple sedimentation 

ponds and pumping to a common point for treatment may be required.

Heaped embankments are generally located on flat terrain. The collection and conveyance of runoff is 

primarily related to conveying precipitation that falls directly onto the facility and diverting adjacent 

area runoff away from the facility. Collection ditches on benches and at the crest are typically gently 

sloping, and grass-lining can normally be used as the channel erosion control. Ditches conveying 

runoff from the crest or benches to the toe of the embankment are steeper than the collection channels 

and typically require a more durable lining material such as riprap, concrete or manufactured erosion 

protection material. Also, the outlet structure must be sufficiently oriented and properly designed to 

prevent erosion of the embankment toe. For high embankments, special consideration is required 

at the discharge points so that the energy of the high velocity flow is dissipated and/or the flow is 

directed away from the embankment in a manner that prevents erosion of the toe.

9.4.2 Slurry Cell Embankments

The hydrologic and hydraulic aspects of slurry cell embankment design must accommodate the volumetric sequencing of the slurry cells as well as the collection and conveyance of both runoff around 

the cells and direct runoff that accumulates within the slurry cells. Individual slurry cell design must 

meet structural and hydraulic design requirements, and construction must be controlled in such a 

manner that the slurry cells do not become a large interconnected impoundment. The slurry cell concept is based on limiting the total capacity of all open cells (and flowable material if present in closed 

cells) to a level that is consistent with a low-hazard-potential classification for the facility or does not 

meet the criteria for a regulated impoundment provided in 30 CFR § 77.216.
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FIGURE 9.10 DRAINAGE CONTROL FOR SIDE-HILL AND HEAPED EMBANKMENTS

DIVERSION CHANNEL

PERIMETER COLLECTION

CHANNEL

SEDIMENT POND

9.10a SIDE-HILL EMBANKMENT DRAINAGE CONTROL

9.10b HEAPED EMBANKMENT DRAINAGE CONTROL

PERIMETER COLLECTION

CHANNEL

SEDIMENT POND

SEDIMENT POND

SLOPE 

DRAIN

TO

CREST/SLOPE

COLLECTION

CHANNELS

FIGURE 9.10 DRAINAGE CONTROL FOR SIDE-HILL AND HEAPED EMBANKMENTS
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In order for a slurry cell embankment with multiple cells to not require an approved impoundment 

plan in accordance with the criteria in 30 CFR § 77.216, each individual cell must not exceed the 20-

acre-feet size criterion. Furthermore, where the failure of one cell can result in the failure of another, 

or where slope failure can result in the release of water or slurry from multiple cells, the cumulative 

storage capacity of the affected cells must not exceed 20 acre-feet. In situations where multiple cells 

are operated or arranged such that they may interact and exceed the 20-acre-feet limit, the embankment should be classified as impounding and should be designed for the appropriate design storm 

based on its hazard classification. A critical consideration in determining the hazard classification for 

an impounding embankment is the flowability of the fine coal refuse. Generally, slurry cells work 

most effectively when the depth of fines in the cells is kept relatively shallow, preferably to five feet 

or less, such that after dewatering and capping the material is unlikely to be flowable. In instances 

where there is concern for draining of the fine coal refuse, the following guidance for assessing the 

flowability of fine coal refuse is suggested:

• Fine refuse is generally considered flowable for: (1) operating cells with active fine 

refuse disposal, (2) non-operating cells containing predominantly saturated, fine 

refuse deposits that have not been covered, and (3) covered cells with predominantly saturated fine refuse deposits characterized as very loose sand or very soft 

silt or clay.

• Fine refuse is generally considered non-flowable for: (1) non-operating cells with 

predominantly unsaturated, fine refuse deposits that have been covered and (2) 

covered cells with predominantly saturated fine refuse deposits characterized as 

medium dense sand or medium stiff silt or clay.

• Fine refuse should generally be considered flowable, unless additional testing and 

analysis demonstrates that it is non-flowable, for non-operating cells with predominantly saturated fine refuse deposits characterized as loose sand or soft silt or clay.

Michael et al. (2005) in an OSM report prepared a review of the flowability of impounded fine coal 

refuse that discusses recent work and ideas in the engineering profession.

The major hydrologic and hydraulic considerations for slurry cells are the collection, conveyance 

and discharge of runoff within the main diversion and perimeter ditches plus the discharge of 

direct runoff from individual slurry cells. As ditches are relocated and new cells are constructed 

at higher elevations, care should be taken so that the embankment is not advanced vertically to 

the extent that its impounding capacity exceeds the disposal plan criteria and affects hazard classification. Special consideration is required at the discharge points to control flow and prevent 

erosion of the embankment. The location of the toe of the embankment should lie above the 100-

year floodplain limits of nearby streams in order to minimize the potential for encroachments, and 

sufficient area should be available for sediment/treatment pond construction. Figure 9.11 shows 

drainage control measures for a typical slurry cell facility.

9.4.3 Slurry Impoundments

The primary hydrologic/hydraulic issue associated with slurry impoundment design is the continuous balancing of coarse coal refuse disposal, fine coal refuse slurry disposal and maintenance of storm 

water runoff storage/routing capacity. Direct runoff at a slurry impoundment is typically controlled 

by a decant system or principal spillway, although some disposal facilities also employ an auxiliary 

(or emergency) spillway. The operation and performance of these outlet works is integral to fine and 

coarse coal refuse disposal and the safe operation of the impoundment. To protect the impounding 

embankment from erosion, perimeter runoff control structures must also be incorporated into the 

design. The location of the toe of the embankment should lie outside the 100-year floodplain limits of 
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nearby streams to minimize any potential encroachments, and sufficient area should be available for 

sediment/treatment pond construction.

The type of coal refuse disposal facility configuration (e.g., cross-valley, diked or incised impoundment) is typically a function of topographic conditions in the vicinity of the coal mine. Frequently, 

a decant system and storage are used to control runoff and thus minimize costs associated with 

other types of outlet structures. However, this requires sufficient embankment materials to achieve 

the required storage and may not be feasible for large watersheds. Therefore, some impoundments 

with large watersheds have auxiliary (or emergency) spillways in combination with planned storage 

capacity and a decant system to control runoff from the design storm.

Regardless of the outlet structures chosen for various impoundment development stages, special 

consideration must also be given to the conditions that will exist when the site is no longer maintained as an impoundment. At that point, the impounding capacity must be eliminated by: (1) 

backfilling the impoundment (typically with coarse coal refuse), (2) excavating a channel through 

the embankment to the level of the backfilled stabilized fines, or (3) a combination of these methods, which is typically the most effective approach. The approach taken must include measures to 

prevent significant erosion.

9.4.3.1 Cross-Valley Impoundment

A cross-valley impoundment typically consists of an embankment constructed primarily of coarse 

coal refuse that functions as a dam to impound a mixture of settled fine coal refuse, slurry, clarified 

water and runoff. The impoundment storage and outflow capacity determine the hydraulic structures needed for controlling runoff.

DIVERSION CHANNEL

PERIMETER COLLECTION

CHANNEL

SEDIMENT POND

SLOPE TO

DRAIN

FIGURE 9.11 DRAINAGE CONTROL FOR SLURRY-CELL FACILITY
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The most appropriate method for minimizing the spillway construction effort is to provide a very 

large surcharge capacity between the initial pond elevation and the initial embankment crest. A spillway can then be constructed at a significant height above the initial pond level, providing adequate 

surcharge capacity for a long operational period before the hydraulic system must be expanded. 

Coarse coal refuse typically provides the material for economically constructing this surcharge capacity. An extension of this approach would be to initially provide for total storage with no requirement 

for a spillway (although with this approach there must be provisions for drawing down the reservoir 

in response to consecutive or repeated storms).

Regardless of the percentage of runoff to be handled through reservoir storage, the design configuration must always accommodate the continual rise in the normal pool level due to the disposal of fine 

coal refuse slurry. Reduction in reservoir storage capacity due to upstream construction pushouts 

and stages must also be taken into account. A decant system allows the controlled discharge of surcharge runoff. It may also be used to evacuate clarified slurry water. Depending upon the configuration of the impoundment, an open-channel spillway may be needed to discharge runoff from larger 

storm events.

To protect the downstream face of the coal refuse embankment from erosion, perimeter runoff that 

is intercepted by embankment bench gutters, road gutters and collection and diversion ditches must 

be controlled and routed to a sediment/treatment pond. The conveyance structure configuration and 

erosion protection should be designed to be appropriate for all stages of development, including 

reclamation. Some typical drainage control measures for a cross-valley impoundment are illustrated 

in Figure 9.12.

9.4.3.2 Diked Impoundment

Diked impoundments have design constraints similar to those for cross-valley impoundments. If a 

facility is completely diked such that there is no upstream watershed, the required impoundment 

surcharge capacity is minimized, and the primary factor affecting the impoundment storage capacity 

is the production of fine coal refuse and clarification of slurry. Typically, a decant system and/or principal spillway are adequate for control of runoff. If an auxiliary spillway is employed, the channel 

section through the embankment requires erosion-resistant linings.

Perimeter ditches and bench gutters tend to be of substantial length and should be designed with 

sufficient slope to adequately convey runoff to sedimentation ponds and to drain effectively without 

low areas. Where ditches traverse embankment slopes, they should be provided with erosion-resistant linings. Figure 9.13 shows drainage control measures implemented for a typical diked impoundment.

9.4.3.3 Incised Impoundment

Incised impoundments, or ponds, are used for the disposal of fine coal refuse. They are typically small 

and often used for temporary or emergency disposal. The hydrologic and hydraulic considerations 

associated with cross-valley impoundments and diked impoundments are generally not major issues 

for incised ponds because of the reduced risk of catastrophic failure. There are three principal design 

considerations: (1) an outlet structure to decant or control the release of clarified process water, (2) 

diversion to convey adjacent area runoff around the incised pond, and (3) flooding potential, if the 

incised pond is located close to or within floodplain limits.

9.4.4 Other Impounding Structures

Coal mining operations generally include sedimentation, treatment and fresh water ponds. The 

capacity of each of these structures is a function of the intended use. Sedimentation or treatment 
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FIGURE 912 CROSSVALLEY IMPOUNDMENT DRAINAGE CONTROL

9.12b CROSS-VALLEY IMPOUNDMENT WITH DESIGN STORM STORAGE

 AND WITHOUT OPEN-CHANNEL SPILLWAY

CROSS SECTION

SURCHARGE CAPACITY

FOR DESIGN STORM

SEDIMENT POND

DECANT INLET

DECANT

OUTLET

PLAN

DECANT INLET DECANT

OUTLET

SEDIMENT

POND

9.12a CROSS-VALLEY IMPOUNDMENT WITH OPEN-CHANNEL SPILLWAY

CROSS SECTION

SEDIMENT POND

DECANT INLET

OPEN-CHANNEL

SPILLWAY

DECANT

OUTLET

PLAN

DECANT

INLET

DECANT

OUTLET

SEDIMENT

POND

OPEN-CHANNEL

SPILLWAY

FIGURE 9.12 DRAINAGE CONTROL FOR CROSS-VALLEY IMPOUNDMENT
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pond capacity is related to the ability of the structure to remove constituents such as suspended 

solids or metals that exceed effluent limitations. Fresh water ponds must have the reservoir capacity 

to meet the coal processing and other mining requirements. The size (height and reservoir storage 

capacity) and downstream impacts of failure of these structures determines the hazard potential and, 

as a consequence, the design criteria.

9.4.4.1 Sedimentation and Treatment Ponds

Sedimentation ponds and treatment ponds are typically located beyond the toe of coal refuse disposal facilities or below mining-disturbed land, so that they can receive gravity inflow. The sediment 

and settling capacity of these structures is typically specified in state erosion and sedimentation control guidelines and effluent limitations. Similarly, treatment pond size is dependent on the pond’s 

ability to treat/remove and discharge acceptable water quality. Pond principal and auxiliary spillway 

structures should be designed to discharge water at a rate consistent with design storm criteria and 

state regulatory requirements. A primary consideration is the maximum anticipated runoff associated with the embankment staging based on watershed size, hydrologic considerations, and the 

surcharge storage capacity, which is significantly less than the gross impoundment capacity. For sediment ponds, as storage capacity drops, the principal and auxiliary spillways must be able to handle 

increased discharges. For ponds located below coal refuse disposal facilities, pond size is a function 

of the size of and outflow from the upstream structure. The inflow may be only surface runoff from 

the face of a coal refuse embankment, but it more typically includes decant water discharges, internal 

drain system discharges from the coal refuse disposal facility, and other adjacent area runoff.

SECTION A - A

SURCHARGE CAPACITY

FOR DESIGN STORM

DECANT

PIPE

PLAN

A A

FIGURE 9.13 DRAINAGE CONTROL FOR DIKED IMPOUNDMENT
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9.4.4.2 Fresh Water Impoundments

Fresh water impoundment capacity is determined by the mine and mine processing plant requirements. Fresh water impoundment capacities are generally large, and these impoundments are often 

regulated as high-hazard-potential structures. Fresh water impoundments should be designed 

and constructed according to accepted criteria for conventional dams. Outlet structures for these 

impoundments generally include both principal and auxiliary spillways.

9.5 DESIGN STORM CRITERIA

The quantity and distribution of runoff during a design storm for a coal refuse disposal facility site 

largely controls the design of hydraulic appurtenant structures. This section discusses design storm 

criteria in terms of the recurrence interval of the precipitation and the magnitude of precipitation 

measured in inches of rainfall. Section 9.6 discusses methods for converting design precipitation to 

design runoff volume and peak flow rates.

The appropriate design storm for a coal refuse disposal facility depends primarily on the consequences of the uncontrolled release of impounded material due to failure or faulty operation of the 

facility. Other factors that may affect the design storm include the facility configuration and size, type 

of hydraulic systems and operational period. Portions of the total hydraulic system, such as drainage 

culverts, ditches and some diversion channels will not generally create potentially hazardous conditions, so other design criteria can be selected for these structures. This situation is most likely to occur 

at non-impounding disposal facilities and at the perimeter of and appurtenant structures associated 

with impounding facilities.

Criteria for selecting a design storm for the operational period of an impounding facility are presented 

in Section 9.5.1. Design storms that are applicable for short-term conditions are discussed in Section 9.5.2. 

Design storm criteria for minor site drainage conveyance structures are presented in Section 9.5.3.

9.5.1 Design Storms for Impoundments

9.5.1.1 General Considerations

Numerous design storm criteria are employed in hydrologic analyses for water retention and flood 

control dams. The common factor associated with practically all of these criteria is that differentiations 

are made based on the projected maximum size of the impoundment and the magnitude of potential 

downstream hazard in the event of failure. MSHA has developed guidelines for design storms for the 

impoundments and embankments that they regulate; however, state and local criteria must also be 

considered. For any impoundment, the most conservative of applicable criteria should be used.

As part of the identification of the design storm, the size of the dam and reservoir and the associated 

hazard potential is typically determined either by inspection or analysis. Table 9.4 indicates appropriate 

design storms as related to impoundment size and hazard potential. Coal refuse impoundments should 

be designed for the Probable Maximum Flood (PMF) event, unless a lesser criterion can be justified 

consistent with Table 9.4. For determining the impoundment size, the impoundment volume and depth 

should include all water, sediment, and slurry that can be impounded. For determining the hazard 

potential, both the water and flowable materials retained in the impoundment should be considered.

The PMF is defined as the maximum runoff condition resulting from the most severe combination of 

hydrologic and meteorological conditions that is considered reasonably possible for the watershed. A 

PMF consists of an antecedent storm, a principal storm and a subsequent storm. The current assumed 

conditions for a PMF design storm in the MSHA guidelines are the following (MSHA, 2007):

1. Antecedent storm – 100-year precipitation event, with antecedent moisture condition 

II (AMC II) occurring 5 days prior to principal storm.
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TABLE 9.4 RECOMMENDED MINIMUM DESIGN STORM CRITERIA FOR 

COAL REFUSE DISPOSAL IMPOUNDMENTS

A. Impoundment Size Classification

Category

Impoundment Size

Maximum Volume of Stored Water 

During Design Storm

(acre-ft)

Maximum Depth of Water

During Design Storm

(ft)

Small to Intermediate < 1,000 or < 40

Large ≥ 1,000 or ≥ 40

B. Hazard Potential Classification

Category Description

Low Hazard 

Potential

Facilities where failure results in no probable loss of human life and low economic and/or environmental 

losses. Such facilities would be located in rural or agricultural areas where losses would be limited 

principally to the owner’s property, or failure would cause only slight damage, such as to farm 

buildings, forest, and agricultural land, or minor roads.

Significant 

Hazard

Potential

Facilities where failure results in no probable loss of human life but can cause economic loss, 

environmental damage, or disruption of lifeline facilities. Such facilities would often be located in 

predominantly rural areas, but could be located in areas with population and significant infrastructures, 

and where failure may damage isolated homes, main highways, minor railroads or disrupt the use of 

service of public utilities.

High Hazard 

Potential

Facilities where failure will probably cause loss of life. Such facilities would be located where 

failure could be reasonably expected to cause loss of life, serious damage to homes, industrial and 

commercial buildings, important utilities, highways and railroads.

C. Recommended Design Storm for Long-Term and Short-Term Conditions(1)

Impoundment 

Size

Hazard 

Potential

Minimum 

Design Storm 

for Long Term

Minimum Design 

Storm for Short 

Term(1)

 Additional Criterion

Small to 

Intermediate

Low

Significant

High

100-Year

1⁄2-PMF

PMF

100-Year

100-Year

1⁄2-PMF

The indicated storm is appropriate only 

if the combination of spillways and 

decants for the facility can evacuate 90 

percent of the incremental volume of 

Large stored storm water within 10 days.

Low

Significant

High

1⁄2-PMF

PMF

PMF

100 Year

1⁄2-PMF

1⁄2-PMF

 Note: 1. Situations where short-term criteria may apply include:

 a. Initial construction. A new impoundment should be capable of accommodating the runoff from

 the short-term storm within one year and the long-term storm within two years.

 b. Changing from an open-channel spillway to handle the design storm by storage. The time period

 when the long-term design storm cannot be accommodated should be kept as short as possible

 with detailed planning of the process.

 c. Abandonment by elimination of impounding capacity. The impounding capability should be

 eliminated within two years after the impoundment can no longer accommodate the long-term

 design storm, and the work should be phased so that the facility is capable of accommodating

 less than the short-term storm for no more than one year.
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2. Principal storm – Probable Maximum Precipitation (PMP) with AMC III. The principal storm rainfall must be distributed spatially and temporally to produce the most 

severe conditions with respect to impoundment freeboard and spillway discharge.

3. Subsequent storm – The subsequent storm criterion can be considered to be met if, 

within 10 days of the peak impoundment level associated with the principal storm, 

at least 90 percent of the volume of water stored above the normal operating level 

can be discharged from the impoundment. Alternatively, for facilities designed with 

sufficient storage but limited discharge capabilities that do not meet this criterion, 

the subsequent storm may be a second PMP storm with the same hydrologic and 

meteorological parameters as the principal storm, provided that the storage from 

both storms is drawn down at a rate sufficient to evacuate 90 percent of one storm 

from the impoundment within 30 days.

The antecedent storm precipitation can be obtained from National Weather Service publications. The 

most current definition of PMP (NWS, 1988) is “theoretically, the greatest depth of precipitation for 

a given duration that is physically possible over a given size storm area at a particular geographical 

location at a certain time of the year.” The PMP can be determined from the National Weather Service 

publications discussed in the following paragraphs.

In the Western U.S., determination of the PMF may be based upon either: (1) the PMP and (2) the 

Probable Maximum Thunderstorm (PMTS). The PMTS is a very high-intensity, short-duration storm 

with intense precipitation occurring during a one-hour period. When designing a coal refuse disposal facility in this region, the more critical of these two criteria should be used. In this Manual, the 

term PMP represents the more severe of the PMP and PMTS for areas of the U.S. west of the 105th

meridian.

Dams or impoundments used for fine coal refuse disposal, fresh water retention, erosion and sediment control or other mine-related operations may need to have PMF storage/routing capacity. Less 

critical impoundments may have reduced design storm criteria based on embankment size and the 

potential downstream hazard. For such structures, both the 6-hour and 24-hour precipitation intensity (unless criteria are specified by state regulations) should be evaluated and the more conservative 

used for design.

As with water-impounding dams, basic design storm criteria apply to the long-term operation of coal 

refuse disposal facilities. However, short-term criteria, as summarized in Table 9.4, may be used for 

construction periods that typically extend from several months to two years for impounding structures subject to PMF design storm criteria. The designer of coal refuse disposal facilities must take 

into account that the configuration of the impounding embankment will be continually changing 

as additional refuse is placed and that the time associated with any one phase or the time between 

phases may be quite short. This can be accounted for by additional or modified design storm criteria 

presented in Section 9.6. These modified criteria should only be used for “unavoidable” situations 

that occur: (1) during short-term operations associated with initial construction of a disposal facility, 

(2) when a major modification is being made to an existing disposal facility, and (3) when a refuse 

disposal facility is being prepared for abandonment.

For water-retaining impoundments, different design storms are sometimes used for individual portions of the total hydraulic system such as the principal spillway and auxiliary spillway (NRCS, 

2005b). This practice is generally not followed in the design of coal refuse disposal facilities provided 

the overall hazard criteria are satisfied because of the operational characteristics of a disposal facility, 

the dynamic nature of facility growth and the limited operational period. This practice may be applicable to other impounding facilities that support the mining operations (e.g., fresh water impoundment, sediment ponds, etc.).
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In the design of coal refuse disposal facilities, it is important to differentiate between the functions of 

spillways and decants. The main function of a decant system is to discharge clarified water from the 

impoundment after the fine refuse has settled. Under normal precipitation conditions, the elevation 

of the decant inlet controls the normal operational water level in the impoundment. The capacity of 

a decant is limited and is typically too small to significantly affect the peak outflow during a large 

storm. Therefore, the storm runoff is almost totally controlled by impoundment storage or a combination of impoundment storage and auxiliary spillway capacity.

Even though an impoundment decant system does not have a significant impact on the outflow during 

the design storm, its capacity must be considered in other analyses related to storms. If the auxiliary 

spillway level is above the normal impoundment level (the typical condition) or if the hydraulic 

system design relies entirely on storage (no auxiliary spillway), the excess storm runoff must either 

be discharged totally through the decant system, or the decant system must serve as the primary 

outlet until the spillway level is reached. As indicated in Table 9.4, within ten days, the combined 

capacity of the spillway and decant systems must be capable of removing 90 percent of the maximum 

volume of water stored above the allowable normal operating water level during the design storm. 

The 10-day drawdown criterion begins at the time the water surface reaches the maximum elevation 

associated with the design storm. Alternatively, if there is sufficient impoundment capacity to store 

the runoff from two design storms (specifically, the antecedent storm and two principal storms), an 

extension of the 10-day criterion is reasonable, provided that an effective means for discharging the 

storage from both storms is available. Generally, an evacuation rate that will remove 90 percent of the 

stored runoff from one design storm within 30 days is considered to be reasonable.

9.5.1.2 Recommended Design Storm Criteria

Table 9.4 provides recommended minimum design storm criteria for coal-mining-related impounding facilities for both long-term and short-term conditions. Selection of the appropriate storm for a 

specific impounding structure is based on the impoundment size and hazard-potential classification. 

The selected criteria for the storage and routing of the design storm and hydraulic structure design 

should also reflect any other applicable regulatory reviewing agency criteria.

Dams and impoundments that are small to intermediate in size (less than 40 feet in height or 1,000 

acre feet in storage volume) with low hazard potential should be designed for a long-term storm 

event with no less than a 100-year recurrence interval. For coal refuse impoundments equal to or 

greater than 40 feet in height or 1,000 acre feet in storage volume with low or significant hazard 

potential, the minimum long-term design storm should be either the 1⁄2 PMF or full PMF, respectively. 

The 1⁄2 PMF design storm should have one-half of the inflow rate and runoff volume of the full PMF. 

For coal refuse impoundments with high hazard potential, the minimum long-term design storm 

should be the full PMF. In cases where the design storm for long-term conditions is less than the full 

PMF, it may be prudent to adopt minimum design storm criteria greater than those provided in Table 

9.4 and thus achieve greater protection from flood events and related damage.

The following paragraphs discuss the basis and/or justifications for criteria and information presented in Table 9.4. Procedures for quantitatively determining the magnitude of precipitation to be 

used in the calculation of runoff are discussed in Section 9.6.

9.5.1.3 Size and Hazard-Potential Classification

The rationale for relating the design storm to the size and hazard potential of the disposal facility 

impoundment is evident. Impoundment size is defined by the maximum depth and total volume 

of retained water, sediment and slurry; however, determining the hazard-potential classification 

requires judgment and, unless otherwise obvious, should be based upon hydraulic analyses. The 

bases for the criteria listed in Table 9.4 are discussed in the following subsections.
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9.5.1.3.1 Impoundment-Size Classification

The size classification presented in Table 9.4 is based on the total volume and depth of all water, sediment and slurry impounded during the design storm. As indicated in the table, the recommended 

design storms for small and intermediate size impoundments are the same.

9.5.1.3.2 Hazard-Potential Classification

The hazard-potential classification presented in Table 9.4 is the same as that presented in Chapter 3 

and used in the overall classification system for coal refuse disposal facilities. Dams that are located 

where loss of life is probable in the event of failure are classified as having high hazard potential. 

In applying these criteria, it is important to recognize the difficulty of determining whether minor 

or major damage or the loss of life will result from the failure of a refuse disposal facility. For most 

coal refuse disposal facilities, this determination is based upon: (1) the configuration and location 

of the facility and (2) the downstream conditions (both existing and planned) including population, topography and the size of streams that would receive flood flow resulting from an embankment failure or a breakthrough-type release from the impoundment. Downstream conditions are 

typically evaluated by reviewing USGS topographic quadrangle maps and by field verification. 

The manner that MSHA addresses the hazard associated with a breakthrough-type release is discussed in Section 3.1.

Generally, unless it is otherwise evident, the determination of hazard potential is based upon a dam 

or impoundment breach analysis and inundation mapping. Section 9.9” presents dam-breach-analysis methods. A dam-breach analysis should provide inundation levels for two conditions: (1) postulated failure of the dam under design-storm conditions and (2) postulated failure of the dam during 

normal operations (sunny day or fair weather breach failure). If doubt exists as to the possible effects 

of an impoundment failure on downstream areas, the more conservative hazard classification should 

be selected. However, it may also be useful to evaluate the downstream inundation and damage that 

could result from a major storm in the refuse disposal facility watershed, but without failure of the 

impoundment. This inundation level and related damage can then be compared to the incremental 

inundation and damage that would be caused by failure of the disposal facility under design-storm 

conditions. If the additional damage can be reasonably predicted as small, then a less conservative 

design storm may be appropriate (FEMA, 2004a), or the hazard-potential classification may be governed by the fair weather breach.

For most large dams and impoundments where downstream residential, commercial or industrial 

development is present adjacent to streams, a high-hazard-potential classification is selected based 

on probable loss of human life. Other situations can arise where the threat is less evident or where 

the distinction between significant and low hazard potential is important. FEMA (2004a) provides 

guidance for interpreting the probable loss of life by clarifying that “postulating every conceivable 

circumstance that might remotely place a person in the inundation zone should not be the basis 

for determining the appropriate classification level.” In the definition of high hazard potential, the 

probable loss of human life is clarified to exclude consideration of the casual user of downstream 

or upstream areas. However, personnel who routinely or frequently work or occupy locations or 

structures in the downstream area should be considered in the assessment of hazard-potential classification.

USBR (1988) provides guidance based upon the number of lives in jeopardy (all individuals within the 

inundation boundaries who, if they took no action to evacuate, would be subject to danger) to aid in 

assessing the potential for probable loss of life. In cases where a dam-breach analysis indicates limited 

inundation at occupied structures in relatively undeveloped areas, such guidance in assigning hazard 

potential may be useful. USBR (1988) provides guidelines for interpreting the significance of predicted 

inundation depth and velocity at downstream residences, roadways, and pedestrian routes.
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There have been a limited number of mining situations, primarily in the western United States, where 

high-hazard-potential dams have been designed using hydrologic design criteria associated with a 

lower hazard-potential-classification and incorporating a warning system. An example is a dam constructed across a watercourse for prevention or mitigation of flooding damage to a surface mine pit. 

To design a flood-control structure to totally accommodate the design event would necessitate the 

construction of a very large dam that would function only temporarily. Failure of this dam could possibly result in a higher hazard potential due to the additional storage. In such cases, some designers 

have proposed dams using low- or significant-hazard-potential criteria and incorporating warning 

systems. The warning systems are designed to notify the minimg operation when the water behind 

the dam reaches a specified level. At that time, all potentially affected personnel are withdrawn from 

the downstream area. Allowance for warning time must not be a substitute for appropriate dam 

design and construction. MSHA (Fredland, 2008) has indicated that this approach may be acceptable 

on a case-by-case basis for temporary mining operations. Conditions associated with warning systems for this approach are discussed in Section 3.7.

Hazard-potential classification is also dependent on the potential for economic, environmental or 

lifeline losses. If a dam or impoundment is not classified as having high hazard potential because 

there is no probable loss of human life, generally it reflects a situation where there are few downstream structures and thus limited potential for associated economic damages. FEMA (2004a) clarifies that for classification of a dam as having low hazard potential (as opposed to significant hazard 

potential), the economic, environmental or lifeline losses must be low and generally limited to the 

owner of the structure. While economic damages to downstream development may be determined 

to be low and thus could support classification of a dam as having low hazard potential, the possibility of environmental damages may warrant consideration of higher hazard classification levels. 

9.5.1.4 Determination of Design Storm Precipitation

Once the size and hazard-potential classification of a disposal facility impoundment are established, 

the recommended design storm can be determined from Table 9.4. The procedure for determining the 

magnitude of the precipitation for the design storm is discussed in the following paragraphs, while 

the procedure for computing the resulting runoff is presented in Section 9.6.

9.5.1.4.1 Prediction of the PMP and PMTS

Predictions of the PMP (inches of rainfall) for a watershed of 10 square miles and durations of 6 to 

72 hours are presented in reports prepared by the National Weather Service’s Hydrometeorological 

Design Studies Center. Figure 9.14 identifies applicable Hydrometeorological Reports (HMRs) for 

various regions of the U.S. For areas east of the 105th meridian, HMR 51 (NWS, 1978) should be used 

for determining the PMP magnitude and for extending the PMP to longer durations. The only exception is an area in the Tennessee River Valley that is addressed in HMR 56 (NWS, 1986). Procedures for 

determining critical rainfall spatial and temporal distribution for areas east of the 105th meridian are 

provided in HMR 52 (NWS, 1982); however, the document may not be applicable to all watersheds, 

particularly watersheds with areas less than 10 square miles. Seasonal variation of PMP for areas east 

of the 105th meridian is addressed in HMR 53 (NWS, 1980). For the region between the 103rd and 105th

meridian, HMR 55A (NWS, 1988) and HMR 52 should be used. For the area between the 103rd meridian and the continental divide, HMR 55A is applicable. For areas west of the continental divide, HMR 

49 (NWS, 1977), HMR 57 (NWS, 1994), HMR 58 (NWS, 1998), or HMR 59 (NWS, 1999) should be used, 

as indicated by the shaded areas in Figure 9.14.

As indicated above, HMR 56 (NWS, 1986) was developed for the Tennessee Valley. While HMR 56 is 

recommended for projects in that region, the study indicates that in non-orographic areas numerous 

comparisons were made between the results from HMR 56 and the results from HMR 51 and HMR 

52, indicating that minor differences in results can be expected depending upon the size of the study 
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region. In more mountainous orographic areas, HMR 56 provides guidance for determining the areal 

distribution of storm-averaged depths with reference to HMR 52.

The extension of the PMP for watersheds exceeding 10 square miles and other durations are discussed in relation to analyses for determining runoff in Section 9.6. For coal refuse disposal impoundments, the applicable watershed is typically much smaller than 10 square miles, resulting in no or 

only limited adjustments for spatial distribution using HMR 52 (NWS, 1982). Because impoundments 

are designed with considerable storage capacity, and in many cases the ability to store the runoff from 

the entire design storm, determination of adjustments that affect the peak inflow rate may not be nec

essary. However, in such cases the PMP must generally be extended to 72 hours. For impoundments 

with watershed areas as small as one square mile that rely on open-channel spillways for routing 

the PMF, HMR 52 (NWS, 1982) provides a means for estimating the adjusted PMP distribution using 

depth-duration ratios and 1-hour PMP values.

9.5.1.4.2 Prediction of the 100-year and Lesser Design Storms

The 100-year-recurrence-interval design storm and lesser design storm precipitation data can be obtained 

from several sources. NOAA Atlas 14 Volume 1 (NWS, 2006a) and Volume 2 (NWS, 2006b) provide rain

(ADAPTED FROM FERC, 2001)

 NOTE: 1. HMR 57 REPLACED HMR 43 IN 1994.

 2. HMR 58 AND 59 REPLACED HMR 36 AND 49 FOR CALIFORNIA IN 1999.

 3. HMR 51, 52, 53 MAY APPLY FOR BASINS UNDER 24,000 SQUARE MILES.

 4. HMR 33 (SEASONAL PMP VARIATIONS FOR AREAS NOT SUPERCEDED

 BY SITE-SPECIFIC SEASONAL PMP REPORTS)

HMR 57(1)

HMR 49

HMR 55A

HMR 51, 52, 53(4)

103RD

MERIDIAN

105TH

MERIDIAN

HMR 48 (SNOWMELT CRITERIA)

HMR 40(3)

HMR 42, 54

TP 47

HMR 58, 59(2)

CONTINENTAL

DIVIDE

EPRI, 1993

PMP WI/MI

HMR 56

TENNESSEE

VALLEY REGION

HMR 52, 55A

FIGURE 9.14 U.S. REGIONS COVERED BY GENERALIZED PMP STUDIES
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fall frequency values for much of the U.S., and NOAA Atlas 2 (NWS, 1973) provides data for some areas 

of the western U.S not covered in Atlas 14 Volume 2. Other areas of the country are addressed in various 

technical publications available from NOAAor in other sources listed in Table 9.5. Access to precipitation, 

frequency, and intensity data for specific locations is available from the NOAA web site.

The current practice of precipitation frequency analysis is based upon the implicit assumption that 

past experience can be used to predict future events and that the climate will not change. In its current studies, the NWS is assuming that the full period of the available historical record is suitable 

for use, as current climate change forecasts do not reliably define future changes in precipitation 

frequency distribution.

9.5.2 Special Considerations for Short-Term Conditions

Although coal refuse disposal facilities are typically dynamic or constantly changing entities, careful 

consideration of growth characteristics and proper planning of modifications will result in compliance with long-term design storm criteria over the facility’s entire service life. Occasionally, however, 

it may be impossible to meet long-term requirements during brief periods when significant physical 

changes to the facility are occurring.

Appropriate design storms for short-term conditions are provided in Table 9.4. Short-term conditions for 

periods of significant physical change are more related to general construction practices, and therefore the 

criteria for temporary (stream) diversions are generally dictated by state guidelines. The upper limit for a 

short-term condition is two years. The short-term design storms provided in Table 9.4 are more conservative (higher) than those normally used for dam construction (USBR 1987a). The more stringent criteria are 

recommended because planning and implementation of modifications at coal refuse disposal facilities are 

dependent upon day-to-day coal and refuse generation unlike other types of embankments.

It is stressed that these short-term criteria are not intended as less costly design alternatives based on 

the rationale that a short-term condition is always appropriate for a given site because it is continually changing in configuration. If such an approach is followed, it should be expected that regulatory 

acceptance of a lesser storm will not be granted. The temporary use of design storms of lesser magnitude than those required for long-term facility operation will likely be accepted only if the following 

conditions are met:

• The facility will be designed to satisfactorily meet the requirements for such interim 

use, including, but not limited, to safe control of the short-term design storm.

• As part of the overall design and planning process, interim periods of short-term 

use are unavoidable and are identified and their duration realistically scheduled. As 

these periods are approached during construction, the scheduling of these transitional periods should be adjusted as required and thereafter strictly followed. Such 

preplanning and scheduling should be done in a manner that minimizes the duration of the short-term condition and facilitates the speedy transition to either a longterm operating status or abandonment.

Periods during the service life of a refuse disposal facility when even careful planning may occasionally be insufficient to achieve compliance with long-term design criteria include:

• Initial construction of a new impounding structure. The impoundment should be 

capable of accommodating the runoff from the short-term storm within one year and 

the long-term storm within two years.

• Transitioning from a lower open-channel spillway to a higher open-channel spill-way 

as part of raising the embankment stage crest or changing from an open-channel spill-
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way to handling the design storm by storage. The time period when the long-term 

design storm cannot be accommodated should be kept as short as possible, and a comprehensive plan and schedule for the sequence of the change should be provided.

• Abandonment by elimination of impounding capability. The impounding capability 

of the facility should be eliminated within 2 years after the time that the impoundment can no longer accommodate the long-term design storm. Additionally, abandonment should be phased such that the time period when the facility is capable of 

handling less than the short-term storm is no more than one year.

TABLE 9.5 NWS PRECIPITATION FREQUENCY PUBLICATIONS

 Location

Design Storm Duration

5 to 60 min 1 to 24 hrs 2 to 10 days

DE, IL, IN, KY, MD, NJ, 

NC, OH, PA, SC, TN, VA, 

DC

NOAA Atlas 14

Volume 2 (NWS, 2006b)

NOAA Atlas 14

Volume 2 (NWS, 2006b)

NOAA Atlas 14

Volume 2 (NWS, 2006b)

Remainder of Eastern 

United States

Technical Memorandum 

NWS HYDRO-35

(Frederick et al.,1977)

Technical Paper 40

(Hershfield, 1961)

Technical Paper 49

(NWS, 1964)

AZ, NV, NM, UT, 

Southeast CA

NOAA Atlas 14 

Volume 1 (NWS, 2006a)

NOAA Atlas 14

Volume 1 (NWS, 2006a)

NOAA Atlas 14

Volume 1 (NWS, 2006a)

Remainder of Western 

United States

Arkell and Richards (1986); 

Frederick and Miller (1979)

NOAA Atlas 2

(NWS, 1973)

Technical Paper 49

(NWS, 1964)

Alaska Technical Paper 47

(NWS, 1963)

Technical Paper 47

(NWS, 1963)

Technical Paper 52

(NWS, 1965)

(ADAPTED FROM NRCS, 1986)

9.5.3 Hydraulic Design Criteria for Drainage Conveyance Installations

Hydraulic structures for both non-impounding and impounding coal refuse disposal facilities fall 

into three general categories:

1. Those structures that by failure, overtopping and/or blockage could threaten the 

overall stability of the disposal facility.

2. Those structures that by failure, overtopping and/or blockage would not threaten the 

overall stability of the disposal facility, but could lead to localized instability.

3. Those structures that, even if non-functional, would not endanger the overall stability of the facility and would not greatly affect day-to-day operation of the facility.

Hydraulic structures that are critical to the overall safety of coal refuse disposal facilities must be 

designed to adequately control the facility design storm. Although this most commonly applies 

to the hydraulic structures associated with impounding facilities, the requirement applies equally 

to hydraulic structures at non-impounding facilities. Permanent hydraulic structures (other than 

impoundment spillways) at coal refuse disposal embankments should be designed to handle the 

100-year storm.

The purpose of many refuse disposal facility permanent hydraulic structures is to limit erosion or 

other types of localized instability rather than to provide total hydraulic control during major storms. 

Whether or not a facility is impounding, non-impounding, active or abandoned, these structures are 
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important to the development and operation of a disposal facility and should generally be designed 

for the 100-year storm. Design criteria for these structures may also be governed by state or local 

regulations. Table 9.6 provides a summary of typical design criteria for minor hydraulic structures at 

locations that are not part of the coal refuse disposal facility. These structures typically include storm 

sewers, culverts, drainage ditches and gutters.

9.6 DETERMINATION OF RUNOFF QUANTITIES

The most important aspects of hydrologic analyses related to refuse disposal facility performance 

during and after storm rainfalls are the determination of the peak runoff rate and the total runoff 

volume at the point of interest. Four methods for determining these parameters that are available to 

the designer are presented in Table 9.7. The first three methods presented in the table are discussed 

in this section following a general discussion of basic hydrology parameters.

9.6.1 Basic Hydrology Parameters

There are three basic factors that must be considered when predicting runoff rates and quantities. 

These are: (1) precipitation (intensity and duration), (2) watershed (size and time of concentration), 

and (3) soil types and land use conditions. These factors are further explained in the following 

subsections.

9.6.1.1 Precipitation Intensity-Duration and Distribution

Storms are defined by their precipitation intensity-duration relationships. Storms can range from 

high-intensity, short-duration thunderstorms to low-intensity, long-duration rainfalls lasting several 

days. The intensity-duration relationship that should be used for hydrologic analyses and channel 

design is that which produces the maximum peak runoff rate. This is particularly true for the small 

watersheds common to coal refuse facilities where the time of concentration (time required for rainfall to travel from the most hydrologically distant point in the watershed to the point of interest) is 

TABLE 9.6 TYPICAL DESIGN CRITERIA FOR MINOR HYDRAULIC STRUCTURES(1)

Structure Type and Condition Design Criteria

Storm Sewers 10-year rainfall

Diversion Systems

Temporary (1-year life or less and watershed > 5 acres)

Construction areas, roads, pipelines 2-year rainfall

Permanent

Sediment Retention Structures (watershed <100 acres and 

height < 15 feet):

10-year rainfall

Emergency spillway capacity 25-year, 24-hour rainfall

Principal spillway capacity 10-year, 24-hour rainfall

Culverts:

Access Roads and Drainage Swales 10-year, 24-hour rainfall

Local and Urban Roads 25-year, 24-hour rainfall

Highways and Streams 100-year, 24-hour rainfall

Drainage Ditches and Gutters 10-year rainfall

Note: 1. These criteria do not apply to minor structures on coal refuse disposal facilities. Permanent

 perimeter ditches and bench gutters on coal refuse disposal facilities should be designed for

 the 100-year storm.
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small. In larger watersheds, the averaging effects of short and long times of concentration tend to 

compensate for small errors in the predicted intensity-duration relationship.

Figure 9.15 presents the dimensionless design storm distribution frequently used to evaluate the 

intensity-duration relationship (NRCS, 2005b) based upon a 6-hour-duration storm. The 24-hour 

storm can be constructed by critically stacking incremental rainfall amounts for successive 6-, 12-, 

and 24-hour durations, as discussed in HMR 52. Runoff determinations for mining facilities are typically based upon either 6-hour- or 24-hour-duration precipitation events (with extension to periods 

up to 72 hours for impoundments that rely on storage for flood routing) except for states west of the 

105th Meridian where PMTS runoff must also be considered.

As indicated in Table 9.4, development of specific-frequency flood hydrographs may be required for 

the design of structures with low- or significant-hazard-potential classification. High-hazard-potential structures require design for the PMF, which for coal refuse disposal facilities is typically derived 

from the PMP for the watershed. The PMP is a 6- to 72-hour duration precipitation distribution that 

results in a peak intensity occurring during the third quadrant of precipitation. This distribution 

curve is recommended for most coal refuse impoundment hydrologic design and analysis applications. For smaller impounding structures, it is recommended that both short-duration (6-hour) and 

long-duration (24-hour) storms be utilized to determine the peak runoff for sizing of the outflow 

structures.

In addition to knowing how the intensity of precipitation may be distributed within a six-hour storm, 

it is also important to recognize that storms may continue for longer periods of time at decreasing 

intensities. Such storms may be critical for disposal facilities that rely primarily on reservoir storage 

to control runoff, since the amount of runoff occurring after the first six hours may represent a significant portion of the runoff volume of the total storm. HMR 51 and HMR 52 can be used to extend 

the predicted six-hour PMP.

Figure 9.14 shows applicable HMRs for determining magnitudes and temporal distributions for 

probable maximum storms based upon regionalized criteria. Charts for PMP values are presented 

in HMR 51 for most areas east of the 105th Meridian, and procedures are provided in HMR 52 

TABLE 9.7 METHODS FOR DETERMINING RUNOFF RATE AND VOLUME

Method Applicable Conditions

Hydrograph Method (Section 9.6.2) Applicable to all runoff analyses, but normally used when a timerelated runoff distribution is required or when less exact methods 

for estimating runoff are not sufficiently accurate for design of an 

economical drainage system.

Peak Runoff Determination (Section 9.6.3) For (1) determining estimates of peak runoff rate and runoff 

volume for system sizing when time-related runoff distribution is 

not required for final design or (2) preliminary system sizing prior 

to generating a runoff hydrograph for flood routing.

Rational Method (Section 9.6.4) For designing drainage conveyance structures such as diversion 

and collection ditches and road culverts for small watersheds.

Stream Gage Data Analysis (USDA, 1972;

Chow, 1964)

For predicting runoff by statistical analysis of measured stream 

flow records when a long history of data is available for the stream 

or for a nearby similar stream and watershed. Since these data are 

not generally available for the types of streams passing through or 

adjacent to coal refuse disposal facilities, methods using stream 

flow records are not presented herein.
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that translate these values to a spatially and temporally distributed estimate of the site PMP. The 

computer program HMR 52 developed by the U.S. Army Corps of Engineers (USACE, 1984b) 

determines the most severe storm conditions considering basin characteristics and regional conditions that are critical for watersheds with areas greater than 10 square miles. For many coal refuse 

impoundments, watersheds are small (typically less than 1 square mile) and the procedures in the 

computer program HMR 52 may need to be adjusted for these smaller watersheds using methods 

presented in NWS (1982).

HMR 56 is applicable in the Tennessee Valley region. For the region between the 105th Meridian and 

the Continental Divide, HMR 55A should be used. Probable maximum storm estimates for areas west 

of the Continental Divide may be developed using HMR 49, HMR 57, and HMR 59, which account for 

orographic effects and include procedures for evaluating local (thunderstorm) PMP storms. 

Short-term design storm criteria and low-hazard-potential dam design criteria require precipitation 

frequency information that is available from NOAA, as indicated in Table 9.5.

9.6.1.2 Unit Hydrographs and Time of Concentration

Unit hydrograph theory is the basis for computing inflow hydrographs for design storms. A unit 

hydrograph can be derived from observed hydrographs recorded on gauged streams, although for 

most coal refuse disposal facilities located in small watersheds, they are synthesized using relationships between rainfall and runoff that are dependent on watershed conditions. Empirical equations 

are typically employed to estimate parameters for synthetic unit hydrographs, although some government agencies can provide parameters for ungaged stream basins, including:

FIGURE 9.15 DIMENSIONLESS DESIGN STORM DISTRIBUTION

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

DURATION

ACCUMULATION

(NRCS, 2005b)

FIGURE 9.15 DIMENSIONLESS DESIGN STORM DISTRIBUTION
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Hydrology and Hydraulics

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• USACE has developed coefficients for use in computing Snyder and Clark unit 

hydrographs for some areas of the U.S. USACE district offices can provide information on the results of studies in their districts.

• The USBR has developed a set of lag-time equations, dimensionless unit hydrographs, and S-graphs for different parts of the western U.S. (Cudworth, 1989).

• The USGS has performed regional studies for development of unit hydrographs 

in cooperation with state departments of transportation. These are published as 

USGS water resources investigation reports. Some of these are applicable to the 

states of Illinois, Tennessee, and Alabama (Graf et al., 1982; Robbins, 1986; Olin 

and Akins, 1988).

Before applying published parameters for watersheds in a region, the possible effects of differences 

in drainage area, cover, soil type, orientation, or geology should be evaluated. Additionally, the terminology used to define the various hydrographs and basin parameters in a regional study should be 

carefully reviewed so that application to ungaged watersheds is consistent (e.g., lag time and channel 

slope may be defined differently in the various methodologies).

If published parameters for watersheds in the region are not available, and the drainage basin is 

larger than about 100 square miles, a regional analysis may be prepared by analyzing rainfall and 

streamflow records at gauged watersheds to relate the peak flow rate and lag time to the drainage 

area. Procedures are described in FERC (2001).

The most common method available to designers for the small watersheds typically associated with 

coal refuse disposal facilities is based on empirically derived coefficients for synthetic unit hydrographs. The common methods for developing parameters from empirical equations include the Clark, 

Snyder and SCS unit hydrograph procedures. These methods are incorporated into the widely used 

computer programs for development of inflow design floods (e.g., HEC-1 and HEC-HMS developed 

by the U.S. Army Corps of Engineers Hydraulic Engineering Center (HEC) and similar privately 

marketed programs).

9.6.1.2.1 Snyder Unit Hydrograph

The equations used for the Snyder unit hydrograph are (USACE, 1990b):

tp = Ct (L Lca) 0.3 (9-1)

Cp = (Qp tp) / (640A) (9-2)

tp = time lag measured from the centroid of precipitation excess to the time of peak

flow at the point of interest (hr)

Lca = length along the main watercourse measured from the outlet upstream to a

point nearest the basin centroid (mi)

L = length of the main watercourse (mi)

Qp = peak flow rate of the unit hydrograph (cfs)

A = drainage area (mi2

)

The coefficients Ct and Cp are empirical values applicable to specific regions that account for 

watershed storage and slope and flood-wave velocity and channel storage, respectively. These 

parameters are obtained from regional studies and, if they are representative of conditions of the 
9-42

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watershed being analyzed, are entered into HEC hydrologic software for a Snyder unit-hydrograph analysis.

9.6.1.2.2 Clark Unit Hydrograph

The Clark unit hydrograph uses a time-area curve to represent the watershed and uses a computed time of concentration (Tc ) that can be calculated based on SCS procedures unless more 

reliable regional data are available. Additionally, the Clark unit hydrograph also uses a coefficient 

that reflects the effect of storage within the watershed. HEC hydrologic software (e.g., HEC-1) can 

be used to calculate the value of this coefficient through its optimization routine, but the result 

obtained should be evaluated and compared to published or available regional data. The Clark 

method is usually not employed for the small watersheds that are typically associated with coal 

refuse disposal facilities.

9.6.1.2.3 SCS Dimensionless Unit Hydrograph

The SCS method is the most commonly used approach for small watersheds and is frequently used 

for coal refuse disposal facility design. The primary analytical requirement for this method to be 

applied in a HEC-1 analysis is the estimation of the lag time for the basin, which is generally assumed 

to be equal to 0.6 Tc.

Time of Concentration and Lag Time

The time of concentration Tc is the time required for runoff to travel from the most hydrologically 

remote point in the watershed to the point of interest (Figure 9.2). The hydrologically most distant 

path within a watershed may not necessarily be along the longest water course; therefore, various 

watershed length and slope combinations should be evaluated.

Empirical equations have been developed by the SCS, the USBR, and others for estimation of Tc as a 

function of the length, surface texture and vegetation, and watershed slopes. Additionally, Tc may be 

computed by analysis of the overland and channel flow travel time using surface drainage software. 

A common method is to use the computer program TR-55 to determine flow velocity and associated 

time of concentration for subbasins within a watershed and thus estimate Tc. Empirical equations for 

determination of Tc are presented below.

USBR Method

The USBR (1973) determined Tc from the following equation that has historically been applied to 

small watersheds for design of small dams and coal refuse disposal facilities:

Tc = [(11.9L3)/H ] 0.385 (9-3)

where:

Tc = time of concentration (hr)

L = length of longest watercourse in watershed (mi)

H = elevation difference between the highest and lowest points in the watershed (ft)

For watersheds west of the 105th meridian and forested mountain watersheds east of the 105th meridian, Table 9.8 lists correction factors that should be applied to Tc, as predicted by Equation 9-3. The 

Modified Snyder Method developed by USBR (1987a), which is discussed in subsequent paragraphs, 

is now more commonly used.
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TABLE 9.8 RECOMMENDED CORRECTION FACTORS FOR TC FOR WATERSHEDS 

WEST OF THE 105TH MERIDIAN

CN T′c / Tc

80 1.0

70 1.4

60 1.8

50 2.2

(USBR, 1973)

SCS Methods

The lag method developed by the Natural Resources Conservation Service (NRCS, 1986), also referred 

to as the curve-number method, applies to areas less than 2,000 acres:

Tc = 5⁄

3 [(L0.8 (S+1)0.7)/1900Y0.5 ] (9-4)

where:

S = (1000/CN) – 10

L = hydraulic length of watershed (ft)

Y = average watershed land slope (percent)

CN = curve number (dim)

The NRCS (1986) also developed an approach to determining time of concentration by computing the 

travel time (Tt) for runoff to traverse the watershed, considering three components of flow: (1) sheet 

flow in upland areas (generally applied to distances of 300 feet or less), (2) shallow concentrated flow 

as runoff concentrates beyond the sheet flow areas, and (3) open-channel flow as runoff is conveyed 

downstream. By summing the travel times, an estimate of Tc can be determined from the following 

relationship:

Tc = Tt1 + Tt2 + Tt3 (9-5)

Sheet Flow

Tt1 = [0.007 (nL1)

0.8] / (P20.5 s 0.4) (9-6)

where:

Tt1 = sheet flow travel time (hr)

n = Manning’s roughness coefficient for sheet flow (Table 9.9)

L1 = flow length (ft)

P2 = 2-year, 24-hour rainfall (in)

s = average watershed land slope (ft/ft)
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TABLE 9.9 ROUGHNESS COEFFICIENTS (MANNING’S N) FOR SHEET FLOW

 Surface Description n

Smooth surfaces (concrete, asphalt, gravel, or bare soil) 0.011

Fallow (no residue) 0.05

Cultivated soils

Residue cover ≤ 20% 0.06

Residue cover ≥ 20% 0.17

Grass

Short grass prairie 0.15

Dense grasses(1) 0.24

Bermuda grass 0.41

Range (natural) 0.13

Woods(2)

Light underbrush 0.40

Dense underbrush 0.80

 Note: 1. Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama

 grass and native grass mixtures.

 2. When selecting n, cover should be assumed to have a height of 0.1 foot. This is

 the only portion of the plant cover that will obstruct flow.

(NRCS, 1986)

Shallow Concentrated Flow

Tt2 = L2/(3600V2) (9-7)

where:

Tt2 = shallow concentrated flow travel time (hr)

L2 = flow length (ft)

V2 = average velocity (ft/sec) from Figure 9.16 

Open-Channel Flow

 Tt3 = L3/(3600V3) (9-8)

 V3 = (1.49 R0.67s0.5)/n

where:

Tt3 = open-channel-flow travel time (hr)

L3 = channel flow length (ft)

R = hydraulic radius = cross-sectional-flow area/wetted perimeter (ft)

s = slope of the channel (ft/ft)

n = Manning’s roughness coefficient (Section 9.7.2.2)

V3 = average channel flow velocity (ft/sec)
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FIGURE 9.16 AVERAGE VELOCITIES FOR ESTIMATING TRAVEL

 TIME FOR SHALLOW CONCENTRATED FLOW

UNPAVED

PAVED

AVERAGE VELOCITY (FT/SEC)

1 2 4 6 10 20

0.005

0.01

0.02

0.04

0.06

0.10

0.20

0.50

WATERCOURSE SLOPE (FT/FT)

(NRCS, 1986)

FIGURE 9.16 AVERAGE VELOCITIES FOR ESTIMATING TRAVEL TIME FOR 

SHALLOW, CONCENTRATED FLOW
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USBR (Modified Snyder) Method

The USBR (1987a) provides guidance for unit hydrograph development based on lag time using the Modified Snyder Equation, including charts for regions of the country to assist in estimating the lag time:

Lg = C [(L Lca)/ S0.5)]0.33 (9-9)

where:

Lg = unit hydrograph lag time (hr)

C = constant (estimated as 26 times the average Manning’s n value for in-channel

flows with higher values for overbank flow conditions)

L = length of main watercourse (mi)

Lca = length along the main water course measured from the outlet upstream to a

point nearest the basin centroid (mi)

S = overall slope for longest water course (ft/mi)

9.6.1.3 Precipitation-Runoff Relationship

Generally, not all of the precipitation that falls on a watershed during a design storm becomes runoff; 

a portion is retained in the soil and on vegetation. Chow (1964), the USDA (1972), the NRCS (2004a), 

and the USBR (1973, 1987a) and other references on hydrology discuss the watershed characteristics 

that determine the amount of precipitation that becomes runoff. These characteristics include: (1) the 

types of soil and their effect on the amount of water seeping into the ground; (2) the conditions (wet, 

dry, snow covered or frozen) of the ground surface immediately prior to the precipitation (known as 

the antecedent moisture condition or AMC); (3) the type and density of vegetation; (4) the types of 

development, such as paved surfaces, channeling and storm sewers; and (5) dams, lakes, ponds, or 

swamps upstream of the site that could store water on a permanent basis, release water at a slow rate, 

or fail, thereby suddenly releasing large volumes of water.

9.6.1.3.1 General Rainfall Conditions

The NRCS (updating previous work when it was known as the SCS) has quantified precipitation 

runoff conditions for a wide range of soil, moisture and soil-cover conditions. The procedure utilizes 

a runoff curve number (CN) and the following equations:

Q = (P – Ia)

2

/ (P + 0.8S) (9-10)

S = (1000/CN) – 10 (9-11)

where:

Q = direct runoff (in)

P = rainfall (in)

Ia = initial abstraction = 0.2 S

S = maximum potential difference between P and Q at beginning of storm

Figures 9.17 and 9.18 present charts for estimating runoff from precipitation based on the above equations. Discussion of the derivation and use of these charts is presented in the NRCS (2004b) National 

Engineering Handbook (NEH) updating previous work by the USDA (1972). To determine the char-
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acteristic CN, it is necessary to know or to estimate four watershed conditions: (1) the hydrologic soils 

classification, (2) the land use and surface status or treatment, (3) the hydrologic effect of the land use 

and status, and (4) the antecedent moisture condition (AMC).

Traditionally, the hydrologic soil classification has been determined from county soil surveys. The soils 

identified in the soil surveys are categorized into four hydrologic soil groups (HSGs), as described in 

NEH Chapter 7 (NRCS, 2007a). The HSGs for soils of the United States are presented in Appendix A 

of TR55 (NRCS, 1986; Appendix A updated 1999). Groups A through D for natural soils are described 

below. The disturbed area soil profile for each group for watershed areas that have been affected by 

mining related impacts and/or urbanization is also provided.

A. Low runoff potential – Soils having high infiltration rates even when thoroughly 

wetted – consisting chiefly of well to excessively drained deep sands or gravels, typically with less than 10 percent clay. These soils have a high rate of water transmission.

 Soil description: Sand, loamy sand, or sandy loam.

B. Moderately low runoff potential – Soils having moderately low infiltration rates when 

thoroughly wetted, consisting chiefly of moderately deep to deep soils with moder

FIGURE 9.17 DIRECT RUNOFF FOR RAINFALL LESS

 THAN OR EQUAL TO 8 INCHES

RAINFALL, P (IN)

0

1

2

3

4

5

6

7

8

DIRECT RUNOFF, Q (IN)

 (USDA, 1972)

100

95

90

85

80

75

70

65

60

55

50

45

40

35

30

0 1 2 3 4 5 6 7 8

NOTE: S = POTENTIAL MAXIMUM RETENTION

 AFTER RAINFALL BEGINS

CURVE NUMBER (CN) = 

1000

10 + S

Q = (P - 0.2S)

P + 0.8S

2

FIGURE 9.17 DIRECT RUNOFF FOR RAINFALL LESS THAN OR EQUAL TO 8 INCHES
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ately fine to moderately coarse textures, moderately well to well drained, typically 

with 10 to 20 percent clay. These soils have a moderate rate of water transmission.

 Soil Description: Silt loam or loam.

C. Moderately high runoff potential – Soils having slow infiltration rates when thoroughly wetted, consisting chiefly of soils with fine texture, or of soils with a layer that 

impedes downward movement of water, typically with 20 to 40 percent clay. These 

soils have a slow rate of water transmission.

 Soil Description: Sandy clay loam.

D. High runoff potential – Soils having very slow infiltration rates when thoroughly 

wetted. These are typically clay soils with high swelling potential, soils with a claypan or clay layer at or near the surface, shallow soils over nearly impervious material, and soils with a permanent high water table. These soils typically have greater 

than 40 percent clay and a very slow rate of water transmission.

 Soil Description: Clay loam, silty clay loam, sandy clay, silty clay, or clay

0 4 8 12 16 20 24 28 32 36 40

0

4

8

12

16

20

24

28

32

36

40

RAINFALL, P (IN)

DIRECT RUNOFF, Q (IN)

 (USDA,1972)

Q = (P - 0.2S)

P + 0.8S

CURVE NUMBER CN = 

1000

10 + S

30

20

15

25

35

100

10

40

45

50

80

60

70

90

2

NOTE: S = POTENTIAL MAXIMUM RETENTION

 AFTER RAINFALL BEGINS

FIGURE 9.18 DIRECT RUNOFF FOR RAINFALL GREATER THAN OR EQUAL TO 8 INCHES
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The second and third watershed conditions are simply descriptions of watershed land use and 

surface status or treatment, such as woods or straight row, small grain crops, and the hydrologic 

density or impact of the land use. A general description of the hydrologic grading for soil-cover 

conditions typically encountered is described in conjunction with selection of the curve number 

(Table 9.10).

The fourth watershed condition is an index of watershed wetness referred to as the antecedent moisture condition (AMC). Antecedent moisture conditions are categorized into three groups:

AMC-I Optimum soil conditions – soils are dry but not to the point of wilting

 vegetation (not recommended for design).

AMC-II Average conditions and average value for annual floods.

AMC-III Wet or saturated conditions associated with heavy rainfall or light rainfall

 and low temperatures within 5 days prior to the given storm.

The antecedent moisture condition that should be used for design of impoundment structures is 

either AMC-II or AMC-III. The NRCS provides procedures for estimating the antecedent moisture 

condition based on the 5 days of antecedent rainfall. Typically AMC-II is used for the design of drainage channels, and AMC-III is used when determining the PMF for impoundment design.

The curve number can be estimated based on the hydrologic soils group, land use, and soil-cover 

conditions, as presented in Table 9.10 for AMC-II. Adjustments in the estimated CN value can be 

made for AMC-I and AMC-III using Table 9.11. The NRCS (2004b) provides methods for developing 

composite CN values when there are multiple antecedent moisture conditions.

Typically, basin or subbasin averaging is performed (as in the HEC-1 model) to represent watershed 

areas and compute runoff hydrographs using SCS or Snyder parameters. However, distributed calculations based on mapped soil and land use conditions yield more representative runoff estimates 

than basin-averaged parameters, and the availability of programs such as HEC-GeoHMS that utilize 

GIS terrain and spatial information will efficiently facilitate this approach when digitized soils surveys are available. An overview of procedures that can be employed is presented in FERC (2001).

When calculating runoff, it is important to consider actual conditions that may be present in the 

watershed, including the potential effects of any impoundments. The following guidelines are applicable to refuse disposal facilities:

• For watersheds having varying runoff characteristics, subbasins should be developed 

or a weighted average CN should be computed.

• The actual impoundment is generally considered impervious and contributes 100 

percent to the runoff.

• Upstream impoundments require special consideration when determining runoff. 

Runoff hydrographs should be routed through such impoundments to confirm their 

operation during the design storm and the potential impact should the structures 

overtop and fail.

9.6.1.3.2 Thunderstorm Rainfall Conditions

Hydrometeorological reports for the western regions of the U.S. provide guidance for analysis of 

thunderstorm rainfall. For the Rocky Mountain region, the USBR (1987a) identifies a local, highintensity thunderstorm event that should be considered along with the general storm event. Charts 

provide guidance for selection of lag time and development of unit hydrographs.
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 TABLE 9.10 RUNOFF CURVE NUMBERS FOR WATERSHED COMPLEXES AND AMC-II

Cover Description Curve Numbers for

Hydrologic Soil Group

Cover Type Hydrologic 

Condition A B C D

Cultivated Agricultural Lands(1, 2, 3, 4)

Fallow

Bare soil – 77 86 91 94

Crop residue cover (CR) Poor 76 85 90 93

Good 74 83 88 90

Row crops

Straight row (SR) Poor 72 81 88 91

Good 67 78 85 89

SR + CR Poor 71 80 87 90

Good 64 75 82 85

Contoured (C) Poor 70 79 84 88

Good 65 75 82 86

C + CR Poor 69 78 83 87

Good 64 74 81 85

Contoured & terraced (C&T) Poor 66 74 80 82

Good 62 71 78 81

C&T + CR Poor 65 73 79 81

Good 61 70 77 80

Small grain

SR Poor 65 76 84 88

Good 63 75 83 87

SR + CR Poor 64 75 83 86

Good 60 72 80 84

C Poor 63 74 82 85

Good 61 73 81 84

C + CR Poor 62 73 81 84

Good 60 72 80 83

C&T Poor 61 72 79 82

Good 59 70 78 81

C&T + CR Poor 60 71 78 81

Good 58 69 77 80

Close-seeded or 

broadcast legumes 

or rotation meadow

SR Poor 66 77 85 89

Good 58 72 81 85

C Poor 64 75 83 85

Good 55 69 78 83

C&T Poor 63 73 80 83

Good 51 67 76 80

Pasture, grassland, or range – continuous 

forage for grazing(5)

Poor 68 79 86 89

Fair 49 69 79 84

Good 39 61 74 80

Other Agricultural Lands(1)

Meadow – continuous grass, protected from 

grazing and generally mowed for hay – 30 58 71 78

Brush – brush-weed-grass mixture with brush 

the major element(6)

Poor 48 67 77 83

Fair 35 56 70 77

Good 30(7) 48 65 73

Woods – grass combination (orchard or tree 

farm)(8)

Poor 57 73 82 86

Fair 43 65 76 82

Good 32 58 72 79
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Cover Description Curve Numbers for

Hydrologic Soil Group

Cover Type Hydrologic 

Condition A B C D

Woods(9)

Poor 45 66 77 83

Fair 36 60 73 79

Good 30(7) 55 70 77

Farmsteads – buildings, lanes, driveways, and 

surrounding lots – 59 74 82 86

Arid and Semiarid Range Lands(1, 10)

Herbaceous – mixture of grass, weeds, and lowgrowing brush, with brush the minor element

Poor 80 87 93

Fair 71 81 89

Good 62 74 85

Oak-aspen – mountain brush mixture of oak 

brush, aspen, mountain mahogany, bitter brush, 

maple, and other brush

Poor 66 74 79

Fair 48 57 63

Good 30 41 48

Pinyon-juniper – pinyon, juniper, or both; grass 

understory

Poor 75 85 89

Fair 58 73 80

Good 41 61 71

Sagebrush with grass understory Poor 67 80 85

Fair 51 63 70

Good 35 47 55

Desert shrub – major plants include saltbush, 

greasewood, creosotebush, blackbrush, 

bursage, palo verde, mesquite, and cactus

Poor 63 77 85 88

Fair 55 72 81 86

Good 49 68 79 84

 Note: 1. Average runoff conditions and Ia = 0.2S.

2. Crop residue cover applies only if residue is on at least 5 percent of the surface throughout the year.

3. Hydrologic condition is based on a combination of factors that affect infiltration and runoff, including:

 (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or

close-seeded legumes, (d) percent of residue cover on the land surface (good ≥ 20 percent), and

 (e) degree of surface roughness.

4. Poor: Factors impair infiltration and tend to increase runoff.

Good: Factors encourage average and better than average infiltration and tend to decrease runoff.

5. Poor: < 50 percent ground cover or heavily grazed with no mulch.

 Fair: 50 to 75 percent ground cover and not heavily grazed.

 Good: > 75 percent ground cover and lightly or only occasionally grazed

6. Poor: < 50 percent ground cover

 Fair: 50 to 75 percent ground cover

 Good: > 75 percent ground cover

7. Actual curve number is less than 30; use CN = 30 for runoff computations.

8. CN’s shown were computed for areas with 50 percent woods and 50 percent grass (pasture) cover.

 Other combinations may be computed from the CN’s for woods and pasture.

9. Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.

 Fair: Woods are grazed, but not burned, and some forest litter covers the soil.

 Good: Woods are protected from grazing, and litter and brush adequately cover the soil.

10. Poor: < 30 percent ground cover (litter, grass, and brush overstory)

 Fair: 30 to 70 percent ground cover

 Good: > 70 percent ground cover

(ADAPTED FROM NRCS, 1986)

TABLE 9.10 RUNOFF CURVE NUMBERS FOR WATERSHED COMPLEXES AND AMC-II

(Continued)
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9.6.1.4 Channel and Impoundment Storage Characteristics

For non-impounding coal refuse disposal facilities, storage normally does not enter into the hydrologic and hydraulic analyses. Channels are generally designed for the peak runoff rate, and storage 

beyond the channel capacity is not a consideration. Channel and flood plain storage can be a consideration when evaluating some natural drainage systems, and hydraulic analysis software can incorporate the associated storage into the model through cross sections determined from topographic 

maps. For impounding disposal facilities, reservoir storage provisions are an important component 

of the hydraulic design and can vary between:

• The condition where the impoundment does not have reservoir storage capacity to 

handle any significant portion of the runoff during a storm, requiring the outflow 

to essentially equal the inflow rate. This condition usually requires a relatively large 

spillway to pass the storm inflow.

• The condition where the impoundment does have storage capacity to temporarily 

handle all runoff during a storm, allowing the immediate outflow discharge to be 

essentially zero. This condition requires an embankment high enough to safely store 

the storm inflow.

Most impounding disposal facilities during some period of their life fall between these extremes, and 

part of the runoff becomes reservoir storage and the remainder is passed as outflow. An important 

factor that differentiates coal refuse disposal facility impoundments from other types of impoundments is that the embankment and impoundment configurations continually change with both the 

TABLE 9.11 RUNOFF CURVE NUMBERS (CN) FOR ANTECEDENT MOISTURE CONDITIONS

AMC II

AMC I

(Not Recommended for 

Hydrologic Design)

AMC III

100 100 100

95 87 98

90 78 96

85 70 94

80 63 91

75 57 88

70 51 85

65 45 82

60 40 78

55 35 74

50 31 70

40 22 60

30 15 50

20 9 37

10 4 22

0 0 0

(USDA, 1972)
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disposal of coarse refuse on the embankment and the disposal of fine refuse slurry into the impoundment. This variation must be taken into account in the hydraulic design of a disposal facility.

Several terms commonly used to describe conditions associated with an impoundment are defined below:

• Normal pool elevation – The surface elevation of water or slurry impounded by an 

embankment during normal disposal operations. The normal pool elevation is usually 

established by the decant inlet level or pump control level. With slurry impoundments, 

the normal pool elevation changes with time as fine refuse slurry is disposed and the 

decant level or pump control level is raised to successively higher elevations.

• Minimum pool elevation – The lowest surface elevation that can normally be 

attained. This is often the same as the normal pool elevation, but can be lower if 

drainage is provided by siphoning, pumping, or modifying the outlet system.

• Useful storage – The storage capacity between the normal and minimum pool elevations.

• Dead storage – The storage capacity below the minimum pool elevation that is usually filled primarily by settled slurry.

• Surcharge storage – The storage capacity between the normal pool elevation and the 

maximum permissible pool elevation. This capacity is intended primarily for temporary storage of runoff during storms.

• Freeboard – The difference in elevation between the dike or embankment crest (i.e., 

lowest portion of impoundment perimeter except for an open-channel spillway) 

and the impoundment water surface. Normal freeboard is the distance between the 

minimum embankment crest elevation and the normal pool level, as established by 

the lowest outlet structure used for flood routing purposes. Design-storm freeboard 

is the distance between the embankment crest and the maximum pool level during 

the design storm. The design-storm freeboard should be such that the embankment 

is not overtopped.

Reservoir storage volumes can be determined in several ways, all based on the topographic configuration of the impoundment area. Topographic data, generally obtained by aerial photography with 

2- to 5-foot contour intervals, are suitable for most analyses. USGS topographic quadrangle maps 

may be used for preliminary calculations for evaluation of initial feasibility.

The normally preferred procedure for calculating the elevation-volume relationship for an impoundment is described in Section 9.2.3 and is illustrated in Figure 9.4. The impoundment surface area for 

each successive elevation contour is first determined, from which an area-elevation curve can be plotted. The storage is then computed as the area beneath the area-elevation curve at any elevation, from 

which the volume-elevation curve can be plotted. CADD software can be used to generate area- and 

volume-elevation data. Typically, both area and volume curves are presented as part of an impoundment design report or plan.

Design of impoundments for upstream or centerline construction involves placement of subsequent 

embankment stages within the impoundment area, which impacts reservoir storage associated with 

that stage of construction. Accordingly, the impoundment area-elevation and volume-elevation data 

for each stage of construction must allow for the effect of upstream or centerline construction.

Surcharge storage and freeboard to accommodate the design-storm runoff is required for most 

impoundments. In determining the surcharge storage, the volume of the settled fines above the pool 

level (delta deposits) must be taken into account, particularly for diked configurations where slurry is 

discharged over a substantial perimeter of the reservoir. The volume of the delta deposits is generally 

estimated based upon the position and elevation of the slurry discharge and an assumed slope of the

deposit (typically between 1 and 3 percent).
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Freeboard at an impoundment is provided in order to account for such factors as uncertainties 

in the hydrologic analyses, settlement of the embankment crest, and extreme wind effects such 

as wave runup. The design-storm freeboard for any impounding embankment is a function of 

the wave height and the wave runup conditions at the upstream face of the embankment. Guidance for the evaluation of freeboard for reservoirs is provided in USBR (1987a). Other factors that 

should be considered in determining freeboard requirements include: (1) frequency of the design 

storm, (2) duration of high water level, (3) ability to resist erosion, (4) and potential for settlement or mine subsidence. A reference for wave runup analysis is the Coastal Engineering Manual

developed by the USACE (2002). Coal refuse impounding embankments are typically required 

to have a design-storm freeboard of 3 feet, which is consistent with a wind fetch of generally less 

than one mile.

9.6.2 Runoff Determination: Hydrograph Method

When a time history of runoff is required for final reservoir routing (Section 9.8), a runoff hydrograph 

must be developed. As illustrated in Figure 9.3, a hydrograph is a plot of flow rate at the point of 

interest versus time following storm initiation. Development of a runoff hydrograph requires the 

watershed runoff data discussed in the preceding section. The data are used to create unit value 

runoff hydrographs for small time increments within the total storm duration. A unit hydrograph 

models the time history of runoff flowing from the watershed at the point of interest for one time 

increment of precipitation. A composite runoff hydrograph can then be constructed by superimposing unit hydrographs for all increments of precipitation.

Computer programs developed by the USACE Hydrologic Engineering Center (HEC) have often 

been used for the hydrologic and hydraulic design of coal refuse disposal facilities. The HEC-1 Flood 

Hydrograph package calculates runoff hydrographs and has several optional capabilities. Use of 

HEC-1 for precipitation-runoff modeling requires subbasin boundary delineation, precipitation data, 

and runoff and routing parameters. Typically, synthetic unit hydrographs based on the SCS, Snyder 

or Clark methods are employed. The HEC-Hydrologic Modeling System (HEC-HMS) computer program provides advancements in several areas over the HEC-1 program, including the optional use of 

distributed analysis of runoff through interfacing with GIS terrain models.

The following capabilities of HEC-1 and HEC-HMS are frequently utilized during the design of coal 

refuse disposal facilities:

• Distributed runoff analysis using kinematic wave and Muskingum-Cunge routing, 

which can provide a more refined analysis of peak flows for impoundment designs 

incorporating open-channel spillways.

• Modeling of base flow, which may be important for large watersheds and inundation 

analysis.

• Channel routing using a variety of methods (e.g., Muskingum, Modified Puls, etc.) to 

perform inundation analysis and evaluation of impoundments in series.

• Reservoir routing (level pool routing) based on storage, elevation, and outflow rating 

parameters, which is a basic requirement for impounding facility design.

• Dam breach and inundation analysis for determination of the hazard-potential 

classification for an impounding facility and for preparation of an Emergency 

Action Plan.

User’s Manuals for HEC-1 (USACE, 1998b) and HEC-HMS (USACE, 2000) provide documentation of their capabilities and applications. Several private companies market these programs with 

enhanced input and output features and also have developed similar programs with enhanced 

capabilities.
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Regardless of the method of hydrograph development and analysis, the following computation and 

data validation issues should be considered:

• The computational time increment should be selected based on the lag time and 

precipitation data such that a smooth hydrograph is obtained. The maximum time 

increment of rainfall to be used in the hydrograph analysis should be the lag time 

divided by 5 (Lg/5) rounded to the next lower even number (FERC, 2001). The 

USACE (1998b) indicates that the time increment should be no larger than 0.29 Lg. 

It can be problematic to apply this limitation to small watersheds with small lag 

times because some software programs limit the time increment; however HECHMS does not have this limitation. For impoundments that are designed to store 

the design storm with release through a decant pipe, the volume of runoff and 

peak impoundment pool level is relatively insensitive to the computation time 

increment.

• Determination of the lag time and time of concentration using multiple methods is 

a useful check of the parameter validity prior to use in hydrograph simulation. If a 

regional analysis is performed to estimate these parameters, performing a check on 

the time of concentration using the TR-55 computer program should be considered.

• The applicability of any hydrograph development method is subject to uncertainty, 

and verification is sometimes accomplished by investigating multiple methods (and 

sources of watershed parameters such as the USACE) or by reproducing a large 

historical flood of record from a watershed with similar characteristics. This consideration is most important for impoundments where the flood hydrograph is routed 

through an open-channel spillway.

9.6.3 Peak Runoff Determination – Key Parameters Method

When a complete time history of runoff is not required, estimates of the key runoff parameters can be 

used to determine the total runoff volume and peak runoff rate developed during a design storm. The 

key parameters method is useful for final design when: (1) the entire volume of runoff will be temporarily stored so that the rate of runoff does not need to be calculated or (2) the storage capacity is so 

small that the peak outflow rate is essentially the same as the peak runoff rate and reservoir routing 

is not necessary. The key parameters method is also useful for estimating approximate storage and 

outflow requirements during feasibility planning for various facility configurations and preliminary 

sizing for various hydraulic structures prior to undertaking more detailed procedures using hydrographs and reservoir routing analyses.

The USDA (1973b) published charts for the determination of total runoff volume and peak runoff 

rates for small volumes of runoff that can be used to estimate key parameters for design purposes. 

These charts provide runoff values based on watershed size, slope, and runoff curve number (CN).

Figures 9.19 and 9.20 present total runoff volume as a function of total precipitation and watershed 

size for runoff CN = 70 and CN = 80, respectively. The curves were derived directly from Figures 9.17

and 9.18 and do not include the minimum retentions due to soil infiltration discussed in Section 

9.6.1.3. Through interpolation and extrapolation, these curves may be used to estimate total runoff 

volume for other CN values.

Figures 9.21 and 9.22 may be used for estimating the peak runoff rate for a range of six-hour design 

storms for CN = 70 and CN = 80. These figures were prepared from hydrograph analyses utilizing the recommended SCS rainfall distribution presented in Figure 9.15. Through interpolation 

or extrapolation, these curves can be used to estimate the peak runoff rate for other CN values 

between 70 and 80.
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FIGURE 9.19 ESTIMATED TOTAL RUNOFF VOLUME FOR CN = 70

TOTAL VOLUME OF RUNOFF (ACRE-FEET)

1

10

100

1,000

10,000

50,000

TOTAL RAINFALL (IN)

WATERSHED (ACRES)

60

40

50

35

30

25

20

15

10

5

5 10 100 1,000 10,000

FIGURE 9.19 ESTIMATED TOTAL RUNOFF VOLUME FOR CN = 70
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FIGURE 9.20 ESTIMATED TOTAL RUNOFF VOLUME FOR CN = 80

TOTAL VOLUME OF RUNOFF (ACRE-FEET)

1

10

100

1,000

10,000

50,000

TOTAL RAINFALL (IN)

60

40

50

35

30

25

20

15

10

5

WATERSHED (ACRES)

5 10 100 1,000 10,000

FIGURE 9.20 ESTIMATED TOTAL RUNOFF VOLUME FOR CN = 80
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9.6.4 Runoff Determination – Rational Method

The rational method is the simplest procedure for estimating peak runoff rates and is typically used 

for developing design flows for minor drainage features. Originally developed by the U.S. Bureau 

of Public Roads and subsequently updated by the FHWA (2001), the method is usually restricted to 

watersheds of less than 200 acres and to storm recurrence intervals of less than 100 years. The USGS 

(2005) presents a review and comparison of the method with observed runoff events. Using the rational method, the peak rate of runoff is determined from the following relationship:

Q = C i A (9-12)

Q = peak rate of runoff (cfs)

C = weighted average runoff coefficient

i = average precipitation intensity for a duration equal to the watershed time of

concentration and the selected storm recurrence interval (in/hr)

A = watershed area tributary to the point of interest (acres)

TIME OF CONCENTRATION (HR)

PEAK RATE OF RUNOFF (CFS/ACRE)

6-HOUR RAINFALL (IN)

40.0

20.0

0.05

0.1

0.2

0.5

1.0

2.0

5.0

10.0

0.1 0.2 0.5 1.0 2.0 4.0

34

2

3

4

6

10

14

18

22

26

30

FIGURE 9.21 ESTIMATED PEAK RUNOFF RATE FOR 6-HOUR DESIGN STORM AND CN = 70
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The runoff coefficient C can be further defined as the ratio of the rate of runoff to the rate of precipitation during the storm when all the drainage area is contributing to the runoff. The value of C can be 

estimated from the data provided in Table 9.12. The range of the coefficients for rural areas permits 

some allowance for differences in slope and ground cover conditions. The lower values in the table 

should only be used when the watershed is flat and the surface is permeable. When the watershed 

exhibits multiple slope and ground cover conditions, the runoff coefficient should be determined as 

a weighted average based on the relative area of each of the conditions present.

The time period is equal to the time of concentration. This is the time required for the entire drainage 

area to be contributing to the runoff. The average precipitation intensity (i) can be determined from 

precipitation frequency tables presented by the National Weather Service Precipitation Frequency 

Data Server (web site) as an update for several states covered by Technical Paper 40. Table 9.5 presents publications for precipitation frequency data.

It is emphasized that use of the rational method for determining runoff should be limited to the 

design of minor drainage appurtenances at a coal refuse disposal facility. An example of application 

of the rational method is presented in Figure 9.23.

TIME OF CONCENTRATION (HR)

PEAK RATE OF RUNOFF (CFS/ACRE)

6-HOUR RAINFALL (IN)

2

3

4

6

10

14

18

22

34

30

26

0.1 0.2 0.5 1.0 2.0 4.0

0.05

0.1

0.2

0.5

1.0

2.0

5.0

10.0

20.0

40.0

FIGURE 9.22 ESTIMATED PEAK RUNOFF RATE FOR 6-HOUR DESIGN STORM AND CN = 80
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9.7 DESIGN OF OUTFLOW SYSTEMS

This section presents the planning and design requirements for hydraulic systems to safely transport 

watershed runoff through, around and beyond a coal refuse disposal facility. Because the discussion 

is general in scope, frequent references for specific applications are made to texts and publications on 

hydraulic design and engineering, including: Chow (1959), USBR (1987a), Brater et al. (1996), Henderson (1966), and the USACE (1990b).

The analytical steps necessary for hydraulic design are discussed with emphasis on the important 

relationship of each system component to the overall requirements of the disposal facility. The discussion is presented in the following order:

• The basic considerations that determine the types of analyses to be performed.

• Introduction to basic hydraulic system components including the inlet, transport and 

outlet components for both open-channel and closed-conduit flow.

TABLE 9.12 RUNOFF COEFFICIENTS FOR USE IN THE RATIONAL METHOD

Rural Areas (Haan and Barfield, 1978)

Cover Type Terrain

Soil Texture Runoff Coefficient

Open Sandy 

Loam

Clay and Silt 

Loam Tight Clay

Woodland

Flat (0-5% slope) 0.10 0.30 0.40

Rolling (5-10% slope) 0.25 0.35 0.50

Hilly (10-30% slope) 0.30 0.50 0.60

Pasture

Flat 0.10 0.30 0.40

Rolling 0.16 0.36 0.55

Hilly 0.22 0.42 0.60

Cultivated

Flat 0.30 0.50 0.60

Rolling 0.40 0.60 0.70

Hilly 0.52 0.72 0.82

Rural Areas (FHWA, 2001)

Cover Description Runoff Coefficient(1)

Concrete or asphalt pavement 0.8 to 0.9

Asphalt macadam pavement 0.6 to 0.8

Gravel roadways or shoulders 0.4 to 0.6

Bare earth 0.2 to 0.9

Steep, grassed areas (2:1 slope) 0.5 to 0.7

Turf meadows 0.1 to 0.4

Forested Areas 0.1 to 0.3

Cultivated fields 0.2 to 0.4

Note: 1. For flat slopes or soils with high hydraulic conductivity, the lower values should be used; for steep slopes or lowhydraulic-conductivity soils, the higher values should be used.
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• Identification of special design considerations for hydraulic structures including erosion protection, effects of direction changes, cavitation and materials 

selection.

• Discussion of the types of principal hydraulic systems commonly encountered at 

coal refuse disposal facilities including spillways, decants, diversion ditches, culverts 

and natural streams.

9.7.1 Basic Considerations

Geometric design constraints, the coarse and fine coal production rates and the estimated runoff rate 

and volume influence the selection of the outflow structure(s) that can be utilized for a coal refuse 

disposal facility embankment or impoundment. Basic considerations in the design of hydraulic systems are noted for the following four situations:

1. No runoff storage – In this case, the hydraulic system must be designed to handle the 

maximum rate of runoff (i.e., outflow capacity is equal to the peak runoff rate).

2. Maximum reservoir storage – Reservoir storage is maximized with limited spillway 

outflow during the design storm event (Section 9.8). In this case, the designer must 

provide outflow capacity for all runoff in excess of the available storage. Under some 

conditions the optimal solution may involve total storage of the design storm runoff 

with delayed release through a decant system.

3. Defined surcharge storage capacity – Surcharge reservoir storage is available, 

but the available volume is generally limited. In this case, an iterative procedure 

must be employed to establish the optimum balance between reservoir storage 

and outflow, after which the calculated maximum outflow is used to design the 

hydraulic system.

4. Fixed hydraulic structure capacity – For this case, the hydraulic system capacities 

are fixed and the runoff that will cause failure must be determined. This situation is most often encountered when evaluating an existing facility or the flow in a 

natural stream.

The types of impounding and non-impounding disposal facilities and their unique characteristics 

have been discussed in Sections 9.3 and 9.4. The major points of these previous discussions are summarized below:

• To accommodate the progressive development and operation of coal refuse disposal 

facilities, intermediate-stage hydraulic drainage systems may be needed.

• All operational periods of the impoundment hydraulic systems will need to be evaluated if the progressive development of a coal refuse disposal facility impacts available reservoir storage and/or outflow capacity.

• Coal operators may be performing site development construction and, as a consequence, have certain types of equipment always available on site; the design of 

hydraulic system components should reflect the equipment available and operator 

capabilities.

• The reclaimed configuration of a coal refuse disposal facility may impact hydraulic 

structure design at intermediate facility stages. The final facility abandonment configuration must be taken into account in the design of intermediate stage hydraulic 

structures.

• The design of hydraulic structures should take into account corrosion, abrasion, erosion, weathering and maintenance requirements.
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FIGURE 9.23 EXAMPLE OF RATIONAL METHOD OF INFLOW CALCULATION

STORM DURATION (MIN)

0

2

4

6

8

10

STORM INTENSITY (IN/HR)

1 2 5 10 20 50 100

PROBLEM DETERMINE THE RUNOFF FROM A 100-YEAR RECURRENCE INTERVAL STORM FOR A STEEPLY-SLOPED, 85-ACRE 

WATERSHED IN ALLEGHENY COUNTY, PENNSYLVANIA. THE WATERSHED HAS THE FOLLOWING CHARACTERISTICS:

LONGEST WATERCOURSE = 1900 FEET

ELEVATION DIFFERENCE = 100 FEET

TOPOGRAPHY = 60% WOODED, 30% PASTURE, 10% BARE EARTH

SOLUTION 1. DETERMINE A WEIGHTED-AVERAGE RUNOFF COEFFICIENT FROM TABLE 9.12

TOPOGRAPHY RUNOFF COEFFICIENT

WOODED-STEEP 0.30

PASTURE-STEEP 0.70

BARE EARTH-STEEP 0.90

 THEN C = 0.60 (0.30) + 0.30 (0.70) + 0.10 (0.90) = 0.48

 2. DETERMINE 100-YEAR INTENSITY-DURATION RELATIONSHIP FROM TP-40 (HERSHFIELD, 1961)

 

DURATION PORTION OF 30-MIN

RAINFALL

RAINFALL INTENSITY

HR MIN IN IN/HR

1 60 — 2.35 2.35

1⁄2 30 1.00 1.85 3.70

1⁄4 15(1) 0.72 1.33 5.32

— 10(1) 0.57 1.05 6.30

— 5(1) 0.37 0.68 8.16

 NOTE: 1. FROM PAGE 5 OF TP-40.

 3. DETERMINE 100-YEAR RAINFALL INTENSITY (i) FOR SITE

 FROM FIGURE 30 OF USBR (1973), FOR L = 1,900 FEET AND H = 100 FEET, TC = 8 MINUTES

 FROM INTENSITY-DURATION PLOT ABOVE, i = 6.9 INCHES/HOUR

 4. CALCULATE 100-YEAR PEAK RUNOFF FOR SITE:

 Q = C i A = 0.48 (6.9) (85) = 281.5 CFS

FIGURE 9.23 EXAMPLE OF RATIONAL METHOD OF INFLOW CALCULATION
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9.7.2 Hydraulic System Components

Hydraulic systems that transport runoff through, around and beyond coal refuse disposal facilities 

have three basic components:

1. The inlet, where flow enters the system.

2. The transport or conveyance section that carries flow between the inlet and outlet.

3. The outlet, where water is discharged in an acceptable manner.

The rate of flow can be controlled by any portion of the hydraulic system by varying the size, location, elevation, slope, shape or configuration of these three components. Interrelationships between 

the inlet, the transport section and the outlet must be evaluated as part of hydraulic system design. 

An important design consideration when optimizing the balance between storage and outflow (Section 9.8) is that the maximum head of water needed to develop the desired rate of outflow does not 

exceed the maximum permissible impoundment pool elevation at any time during the life of the 

structure.

The following sections detail important aspects of each hydraulic system component, identifying 

appropriate design procedures and discussing the role of each component in various types of hydraulic systems.

9.7.2.1 Inlets

The inlet to a hydraulic system can: (1) simply direct flow to the transport section or (2) regulate the 

rate of flow (volume and/or velocity) to the transport section.

In the first case, the inlet must have a larger flow capacity than the transport section so that the inlet 

does not restrict the flow, and the inlet must be arranged such that water cannot bypass the transport 

section. Often the design requirement for regulating the amount of flow passing into the transport 

section is associated with the entrance to a decant system (Section 9.7.4.1) or to a spillway (Section 

9.7.4.2). In these cases, a primary design criterion is the relationship between the height of water 

above the controlling elevation of the inlet, referred to as head H and the discharge Q. Typical headdischarge curves for decant-conduit and spillway systems are presented in Figure 9.24.

As shown in Figures 9.25 and 9.26, most inlets have a form of weir control, and the basic equation for 

the associated flow is:

Q = CLH 3/2 (9-13)

where:

Q = discharge or rate of flow (cfs)

C = a coefficient that depends on the shape of the channel entrance and

the head on the structure

L = the effective length of the entrance crest (ft)

H = the total head on the entrance crest (ft)

Equation 9-13 can be used to determine flow rates over both broad- and sharp-crested weirs. Broadcrested weirs are characterized by small H/B ratios, where H and B are as defined in Figure 9.25a. An 

unrestricted channel entrance, such as shown in Figure 9.25c, can sometimes be described as a broadcrested weir. The discharge coefficient C for broad-crested weirs is a function of the weir geometry and 
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approach conditions, as discussed in texts on hydraulics such as Brater et al. (1996), Chow (1959), and 

Henderson (1969). The value of the weir coefficient is generally about 3.0, but varies based on the weir 

configuration. Hydraulic analysis of the discharge and depth of flow through a broad-crested weir, 

including charts for determining flow profiles, is presented in USDA Technical Release 39 (1968).

Discharge coefficients for sharp-crested weirs, such as the decant inlets shown in Figure 9.26, are 

related to the weir height and the hydraulic head. The value of C can be determined from the revised 

Rehbock formula (Brater et al., 1996), which has been verified in tests performed by the USBR:

C = 3.22 + 0.44 (H/P) (9-14)

where:

H = hydraulic head (ft)

P = height of weir (ft)

A range of weir coefficients is presented in Figure 9.25b for an ogee crest. The designer is referred 

to the USBR (1987a) for a more detailed presentation of discharge coefficients for ogee crests. The 

USACE (1990b) presents design methods and discharge coefficients for elliptical (ogee) spillways and 

spillways with free outfall.

With increasing head on a decant or spillway conduit, the weir will become submerged and orifice 

flow conditions may govern the head-discharge relationship, or pressure flow may prevail depending on the transport section. Under orifice flow, the control section is located within the conduit or 

throat of the transition between the inlet and transport section, below the crest of the vertical intake 

shown in Figure 9.26a. USBR (1987a) presents procedures for determining the head-discharge rela

FIGURE 9.24 TYPICAL HEAD-DISCHARGE CURVES FOR HYDRAULIC SYSTEMS

DECANT - CONDUIT

 SYSTEM

SPILLWAY SYSTEM

DISCHARGE OR RATE OF FLOW, Q

HEAD OR HEIGHT OF WATER 

ABOVE CONTROLLING ELEVATION, H

FIGURE 9.24 TYPICAL HEAD-DISCHARGE CURVES FOR HYDRAULIC SYSTEMS
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FIGURE 9.25 SPILLWAY APPROACH CHANNELS

B

H

A

A

9.25c UNRESTRICTED CHANNEL ENTRANCE

FOR STEEP CHANNEL SLOPE Q = C L H1.5 WHERE C = 3.0

FOR MILD CHANNEL SLOPE Q DEPENDS ON CHANNEL GEOMETRY AND

 MAY BE CONTROLLED BY DOWNSTREAM

 CONDITIONS

9.25a RECTANGULAR WEIR

RECTANGULAR FLOW SECTION

SECTION A - A

L

TRAPEZOIDAL FLOW SECTION

~ L

SECTION A - A

9.25b OGEE CREST DAM

H

Q = C L H1.5

C = WEIR COEFFICIENT

L = LENGTH OF CREST

B = BREADTH OF CREST

P = HEIGHT OF WEIR

H = HEAD ON CREST

Q = C L H1.5

C = 3.80 TO 3.95

L = LENGTH OF CREST

H = HEAD ON CREST

H

P

B

FIGURE 9.25 SPILLWAY APPROACH CHANNELS
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FIGURE 9.26 DROP-INLET DECANT SYSTEMS MAY 2009

9.26a CIRCULAR DROP INLET PIPE

Q = CDH1.5

C = WEIR COEFFICIENT

D = DIAMETER OF DROP-INLET PIPE

H = HEAD AT TOP OF DROP-INLET PIPE

9.26b RECTANGULAR DROP INLET

W

L

Q = 2C(W+L)H1.5

C = WEIR COEFFICIENT

W = WIDTH OF DROP INLET

L = LENGTH OF DROP INLET

H = HEAD AT TOP OF DROP INLET

NOTE: 1. FLOW IS SHOWN ON FOUR SIDES OF DROP

 INLET. IF PARTIALLY OBSTRUCTED, THE

 ACTUAL FLOW LENGTH SHOULD BE USED.

2. THE INLET CONTROL FLOWS ARE ONLY

 APPLICABLE PRIOR TO PRESSURIZED

 PIPE FLOW.

D

TOP OF DROP INLET PIPE

H

TWO-DIMENSIONAL CROSS SECTION

OF CIRCULAR DROP-INLET PIPE

tionship for conduit spillways. For small-diameter conduits, as typically found with many decant 

structures, the head-discharge relationship under orifice flow control can be estimated as follows:

Q = CA(2g H)

0.5 (9-15)

where:

C = coefficient of discharge (dependent on configuration of conduit opening)

A = cross-sectional area of conduit (ft2)

g = gravitational acceleration (ft/sec2)

H = total head on the conduit entrance (ft)

FIGURE 9.26 DROP-INLET DECANT SYSTEMS
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For orifice flow, the coefficient of discharge C for vertical, circular decant systems is typically 

about 0.6 for square-edged conditions. Brater et al. (1996) and Haan and Barfield (1978) present 

the results of studies of varying orifice conditions and associated coefficient of discharges for 

various inlet shapes.

9.7.2.2 Transport Sections

The transport or conveyance section of a hydraulic system conveys flow from the inlet to the outlet. 

The two basic types of flow in transport sections are:

1. Open-channel flow – Gravity flow of water through an open channel where the 

flow depth and velocity depend upon the cross section, surface material and channel slope, and possibly also upon interrelationships with the inlet and outlet. Openchannel flow is typical for spillway channels, diversion ditches and natural streams, 

but may also apply to a less-than-full culvert or a decant conduit.

2. Pressure flow – Pressure flow through a closed conduit or pipe where the flow 

capacity depends upon the inlet and outlet conditions, the pressure head on the conduit and the conduit size and material. This type of transport section is common to 

decant systems.

Flow within a spillway system, as illustrated in Figure 9.25a and 9.25b, where the transport section 

is simply the free-fall flow over a weir or high-velocity flow over an ogee crest, are not generally 

encountered at coal refuse disposal facilities. The designer is referred to the USBR (1987a) and the 

USACE (1990b) for this type of hydraulic design 

9.7.2.2.1 Open-Channel Flow

Figure 9.27 illustrates several types of open-channel cross sections. The flow rate Q for each channel 

is the product of the average flow velocity V and flow area A:

Q = VA (9-16)

where:

Q = flow rate (cfs)

V = average velocity of flow (ft/sec)

A = cross-sectional area of flow (ft2)

The flow area is a function of the flow depth which, along with the required freeboard, establishes 

the required height of the channel. Velocity is important because it is directly related to the potential 

for erosion, cavitation and energy dissipation. Normally, the required flow area is determined first, 

because the channel geometry is often controlled by the refuse disposal facility and site configurations and the desire to have uniform channel cross sections to simplify construction.

Methods for the analysis of open-channel flow and procedures for calculating depth of flow at any 

point along a channel are available in many texts, including Brater et al. (1996), Henderson (1969), 

and Chow (1959). The FHWA (1961) presents tables and charts for determination of open-channel 

flow parameters in prismatic channels. Computer software for determining steady-state and transient flow conditions in open channels is also readily available. Accordingly, the following discussion 

of design and analysis considerations relates to flow regulation and the identification of specific conditions or features encountered at coal refuse disposal facilities.
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Four possible flow (and depth) conditions must be considered in the design of an open channel. 

These are: (1) critical depth, (2) depth at subcritical flow, (3) depth at supercritical flow, and (4) normal 

depth. The point at which open-channel flow passes through the critical depth is a regulating condition of flow. Hereafter, the point where this occurs is referred to as a control point or location regulating the flow rate. The depth of flow at a control point may also be referred to as the control depth. 

Several examples illustrating these basic flow conditions are presented in Figure 9.28. Additional 

explanation of these conditions is provided in the following:

1. Critical depth – The critical depth of flow in an open channel is the depth at which 

flow occurs with the minimum specific energy, defined as the minimum depth y

plus the velocity head (V2/2g) for the channel. At the critical state of flow, the velocity head is equal to half the hydraulic depth D, as defined in Figure 9.27. Chow 

(1959) and FHWA (1961) provide detailed discussion of the critical depth and minimum specific energy. For design, the critical depth is normally calculated for comparison to the actual depth to determine if the actual flow condition is subcritical 

or supercritical. Figure 9.29 presents curves for determining the critical depth for 

trapezoidal and circular channels. Except at flow control points, design at or close to 

critical depth should be avoided in order to prevent the occurrence of unstable flow 

regimes.

2. Depth at subcritical flow – For subcritical flow the channel slope is milder than that 

associated with critical flow, and the flow velocity is relatively low. Channel slopes 

with subcritical flow are called subcritical slopes. For subcritical flow, the control 

point will be downstream.

FIGURE 9.27 OPEN-CHANNEL CROSS SECTIONS

A

WP

CONDUIT

A

WP

TRIANGULAR

A

WP

RECTANGULAR

A

WP

TRAPEZOIDAL

HYDRAULIC RADIUS (R) = AREA (A)

WETTED PERIMETER (WP)

 HYDRAULIC DEPTH (D) = AREA (A)

 TOPWIDTH (T)

T T

T

T

FIGURE 9.27 OPEN-CHANNEL CROSS SECTIONS
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3. Depth at supercritical flow – For supercritical flow the channel slope is steeper than 

that associated with critical flow and the flow velocity is relatively high. Channel 

slopes with supercritical flow are called supercritical slopes. For supercritical flow, 

the control point will be upstream. If it is important that water be carried away 

from the inlet section without a possible back up, it may be desirable to design for 

a supercritical flow condition for a portion or all of the length of the channel. However, this may result in high flow velocities, and special attention will have to be 

given to factors such as erosion, flow at direction changes and energy dissipation at 

the discharge point. Most coal refuse disposal facilities have a large elevation difference between the embankment crest and downstream toe, and this generally results 

in some section of the channel between these two points having a supercritical flow 

condition.

4. Normal depth – Normal flow depth occurs when the energy increase during elevation drop is exactly balanced by friction losses along a channel. When this occurs, the 

flow depth and velocity in a channel of constant cross section and slope will remain 

constant. Open-channel flow naturally tends toward the normal depth condition, 

but for short channel lengths it often does not reach the normal condition. Because 

normal depth is easily determined, it is the flow condition most often used during 

preliminary hydraulic analyses to establish whether or not the selected channel slope 

is feasible (as limited by geotechnical or structural conditions).

The Manning equations are used to estimate the velocity and depth of flow for a given flow rate and 

channel configuration. These equations are:

Q = 

1.49 A R0.67S0.5 (9-17)

 n

V = 

1.49 R0.67S0.5 (9-18)

 n

where:

Q = flow rate (cfs)

V = average velocity of flow (ft/sec)

A = cross-sectional area of flow (ft2)

R = hydraulic radius (ft)

S = slope of the channel (ft/ft)

n = Manning’s coefficient of channel roughness

The depth determined from the Manning formula is normal depth. The term R is the hydraulic radius 

of the flow and is equal to the area of flow A divided by the wetted perimeter WP. The wetted perimeter for several types of cross sections is shown in Figure 9.27. Values of Manning’s coefficient of channel roughness n are listed in Table 9.13. An extensive presentation of n values for a wide variety of 

channels is provided in Chow (1959) and FHWA (1961). Depending upon the effective roughness of 

the surface, n can range from as high as 0.2 for channels with dense brush growth and many obstructions to as low as 0.01 for smooth-finish, concrete-lined channels.

Since the cross-sectional area A and hydraulic radius R are both functions of flow depth, an iterative 

procedure is required to determine a normal depth that satisfies the Manning equations. Commercially available software can be used to determine flow depth, velocity, critical depth and slope, and 

various open-channel design parameters (e.g., HEC-RAS and privately marketed similar programs).
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9.28a STEEP SLOPE (SUPERCRITICAL FLOW)

D2

D1

FIGURE 9.28 CHANNEL FLOW CONDITIONS

9.28b MILD SLOPE (SUBCRITICAL FLOW)

1. D2 IS CONTROLLED BY DOWNSTREAM CONDITIONS

2. D1 IS CONTROLLED BY D2

3. D1 APPROACHES NORMAL DEPTH

D1

D2

CRITICAL DEPTH

D1 D2

9.28c MILD SLOPE WITH STEEP DROP

1. CONTROL IS AT THE POINT WHERE CRITICAL DEPTH (Dc) OCCURS

2. Dc IS A FUNCTION OF CHANNEL SHAPE AND FLOW (Q)

3. D2 AND D1 ARE CONTROLLED BY Dc 

Dc

1. D1 IS CONTROLLED BY INLET CONDITIONS

2. D2 IS CONTROLLED BY D1

3. D2 APPROACHES NORMAL DEPTH

FIGURE 9.28 CHANNEL FLOW CONDITIONS
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Many commercially manufactured erosion control products have been developed to resist potential 

erosion. These products include vegetation control mats, interlocking concrete blocks, concrete mat 

systems, cellular confinement mats, etc. The Manning’s n value for these erosion control products is 

typically provided by the manufacturer.

An experienced designer can determine if the normal depth of flow from the Manning equations is sufficient for determining flow conditions for final design. Detailed analyses of water surface profiles may 

be necessary, particularly when depths associated with changes in channel configuration, erosion protection and slope may be required. In these situations, the change in water surface elevation (water surface profile) due to velocity head and channel losses should be determined. The normal depth does not 

adequately describe conditions at the ends or transitional zones in the transport section. For example, 

where a channel with supercritical slope changes to a short length of channel with subcritical slope (e.g., 

benches on an embankment face), the designer must determine if the depth in the subcritical section 

will create a “hydraulic jump,” and calculation of the flow profile along the length of the open channel 

often must be made. Another instance where flow profiles must be determined is when establishing a 

spillway rating curve (stage-discharge curve) for an approach channel between the impoundment and 

an open-channel spillway weir (control section) or when subcritical flow is present downstream of or 

within the spillway channel. Procedures for analyzing such channel sections are presented in many 

references on open-channel flow, including Brater et al. (1996), Henderson (1969) and Chow (1959). 

The FHWA (2006), USBR (1987a) and the USDA (1956) present methods and charts for determining the 

depths and other parameters associated with hydraulic jumps in open channels. If a hydraulic jump 

occurs, the channel depth must be sufficient to contain the sequent flow depth that will develop.

FIGURE 9.29 CRITICAL DEPTH FOR TRAPEZOIDAL AND CIRCULAR SECTIONS

CIRCULAR

0.0001 0.001 0.01 0.1 1 10

0.001 0.01 0.1 1 10 100

0.01

0.02

0.03

0.04

0.06

0.1

0.2

0.3

0.4

0.6

1

2

3

4

6

10

1

z

b

y

y do

z = 0.5

z = 1.0

z = 1.5

z = 2.0

z = 2.5

z = 3.0

z = 4.0

FOR CIRCULAR SECTIONS

z = 0 (RECTANGULAR)

CIRCULAR

 y/b AND y/d

Q

do

2.5 g

FOR TRAPEZOIDAL SECTIONS Q

g b2.5

o

FIGURE 9.29 CRITICAL DEPTH FOR TRAPEZOIDAL AND CIRCULAR SECTIONS
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TABLE 9.13 MANNING’S COEFFICIENTS OF CHANNEL ROUGHNESS

Constructed Channel Condition

Values of n

Minimum Maximum Average

Earth channels, straight and uniform 0.017 0.025 0.022

Dredged earth channels 0.025 0.033 0.028

Rock channels, straight and uniform 0.025 0.035 0.033

Rock channels, jagged and irregular 0.035 0.045 0.045

Concrete lined, regular finish 0.012 0.018 0.014

Neat cement lined 0.010 0.013 0.011

Grouted rubble paving 0.017 0.030 0.025

Corrugated metal 0.023 0.025 0.024

Natural Channel Condition Value of n

Smoothest natural earth channels, free from growth with straight alignment 0.017

Smooth natural earth channels, free from growth, little curvature 0.020

Average, well-constructed, moderate-sized earth channels 0.0225

Small earth channels in good condition or large earth channels with some growth on banks or 

scattered cobbles in bed

0.025

Earth channels with considerable growth, natural streams with good alignment and fairly constant 

section, or large floodway channels well maintained

0.030

Earth channels considerably covered with small growth or cleared but not continuously maintained 

floodways

0.035

Mountain streams in clean loose cobbles, rivers with variable cross section and some vegetation 

growing in banks, or earth channels with thick aquatic growths

0.050

Rivers with fairly straight alignment and cross section, badly obstructed by small trees, very little 

underbrush or aquatic growth

0.075

Rivers with irregular alignment and cross section, moderately obstructed by small trees and 

underbrush

0.100

Rivers with fairly regular alignment and cross section, heavily obstructed by small trees and 

underbrush

0.100

Rivers with irregular alignment and cross section, covered with growth of virgin timber and 

occasional dense patches of bushes and small trees, some logs and dead fallen trees

0.125

Rivers with very irregular alignment and cross section, many roots, trees, large logs, and other 

drift on bottom, trees continually falling into channel due to bank caving

0.200

(USBR, 1987a)

9.7.2.2.2 Pressure Flow

Closed conduits are used as the transport section for two types of hydraulic systems. When the flow 

enters the conduit with a nearly horizontal approach, the system is called a culvert. When the flow 

enters the conduit through a steeply inclined or vertical drop inlet, the system is called a decant. 

These two types of hydraulic systems are illustrated in Figure 9.30.

When flowing full, both types of closed-conduit systems behave in essentially the same manner, 

and their capacity can be determined from pipe flow analyses. However, the systems differ significantly in behavior during conditions leading up to the full-flow condition. The following 

paragraphs discuss the hydraulics of culvert systems in detail. The hydraulic behavior of decant 

systems is then presented with particular emphasis on the differences between decant and culvert systems.
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Culvert Conduits

A culvert is a pipe placed beneath an embankment with an entrance that is horizontal or slightly 

inclined to the direction of flow. A culvert can be prefabricated or cast-in-place and the cross section 

can be round, square, rectangular, oval or arched. Factors that control the flow in a culvert are slope, 

size, shape, length and roughness (material) and inlet and outlet configuration. The combined effects 

of these factors determine the hydraulic control location, which in turn determines whether the culvert flows partly or completely full and establishes the head-discharge relationship. As with openchannel flow, the slope of the culvert may be mild or steep, resulting in subcritical or supercritical 

flow conditions. For either slope condition, the control point may be either at the inlet or the outlet, 

depending on the system geometry and the upstream and downstream head conditions. Figures 9.31

and 9.32 illustrate the various factors that affect flow in culverts.

The control point for a culvert on a mild slope flowing partly full will usually be at the outlet if the 

inlet is not submerged. The flow ordinarily will be subcritical, and the discharge may be predicted 

according to the open-channel flow procedures discussed above. If the outlet discharges freely, the 

flow at that point will pass through critical depth (Figure 9.31a). As the inlet becomes submerged, 

the control point moves downstream within the culvert. The flow could be supercritical just inside 

the culvert if limited submergence is sustained (H/D ~ 1.2). As submergence of the entrance increases 

(H/D > 1.2, Figure 9.31b), or the culvert is sufficiently long, or the elevation of the downstream backwater is sufficiently high, full pipe flow occurs with control at the downstream end of the conduit. 

FIGURE 9.30 CLOSED-CONDUIT HYDRAULIC SYSTEMS

SEEPAGE DIAPHRAGM (OUTLET NOT SHOWN)

CONDUIT

INLET WITH NEAR

HORIZONTAL FLOW

9.30a CULVERT SYSTEM

CONDUIT

SEEPAGE DIAPHRAGM (OUTLET NOT SHOWN)

DROP INLET WITH

NEAR VERTICAL FLOW

9.30b DECANT SYSTEM

FIGURE 9.30 CLOSED-CONDUIT HYDRAULIC SYSTEMS
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Most coal refuse disposal facility decant systems are less that 36 inches in diameter with a length of a 

few hundred to more than 1,000 feet and are thus sufficiently long (Chow, 1959) that there is full pipe 

flow in the conduit and free discharge from the outlet. If the elevation of the downstream backwater 

is above the critical depth elevation, the depth associated with the backwater may control the flow in 

the culvert. One of these conditions typically occurs, such that culverts flow full for their entire length 

(Figure 9.31b).

When a culvert is on a steep slope and the inlet is not submerged, the flow is controlled by critical 

depth at the inlet (Figure 9.32a), and open-channel flow at supercritical velocity will occur throughout the culvert. After the inlet has been submerged (H exceeds about 1.2D), it is still possible to have 

open-channel flow at supercritical velocities in the culvert if the control point remains at the inlet 

(Figure 9.32b). In this case, discharge is governed by orifice flow at the inlet, leading to formation of 

a flow contraction at the top of the culvert entrance and aeration over the remaining culvert length.

As the head at a submerged inlet increases, friction or local disturbances may reduce the flow velocity, causing the culvert to flow full near the outlet. This may seal the downstream end of the pipe, 

even though an orifice flow contraction tends to occur at the inlet. The associated high-velocity flow 

will tend to carry away the air trapped at the top of the culvert, thus reducing the internal pressure 

to less than atmospheric pressure. This can lead to damage to the culvert from cavitation (Section 

9.7.3.3). However, if the entrance is shaped to eliminate the inlet flow contraction, the culvert will 

start to flow full near the inlet, after which the full flow zone will extend rapidly toward the outlet. 

The effect of the full flow condition will be a draft tube action (similar to siphonic action) that will 

FIGURE 9.31 TYPICAL FLOW CONDITIONS - CULVERT CONDUITS ON MILD SLOPES

PART FULL FLOW - INLET NOT SUBMERGED

9.31a MILD SLOPE - SUBCRITICAL FLOW: CONTROL AT

 CRITICAL DEPTH AT OUTLET

H

D < 1.2

S < CRITICAL

H dc

D

FULL FLOW - INLET SUBMERGED

9.31b MILD SLOPE - CONTROL AT OUTLET: EFFECTIVE HEAD = Ht -  LOSSES

S < CRITICAL

Ht FOR DISCHARGE

AFFECTED BY DOWNSTREAM BACKWATER

Ht FOR FREE

DISCHARGE

SHARP-EDGED OR

ROUNDED INLET

H

D > 1.2

H D

FIGURE 9.31 TYPICAL FLOW CONDITIONS – CULVERT CONDUITS ON MILD SLOPES
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FIGURE 9.32 TYPICAL FLOW CONDITIONS - CULVERT CONDUITS ON STEEP SLOPES

PART FULL FLOW-INLET NOT SUBMERGED

PART FULL FLOW-INLET SUBMERGED

FULL FLOW-INLET SUBMERGED

9.32b STEEP SLOPE - SUPERCRITICAL FLOW: ORIFICE FLOW CONTROL AT INLET

SHARP-EDGED INLET

H

D ~ 1.2

S > CRITICAL

D

dc

9.32a STEEP SLOPE - SUPERCRITICAL FLOW: CONTROL AT CRITICAL DEPTH AT INLET

H

D < 1.2

S > CRITICAL

D

dc

H

ROUNDED INLET

H

D = 1.2 TO 1.5

9.32c STEEP SLOPE - SUPERCRITICAL PULSATING SLUG FLOW: CONTROL

 SWITCHING BETWEEN INLET AND SECTION WITHIN THE CONDUIT

S > CRITICAL

D

H

9.32d STEEP SLOPE CONTROL AT OUTLET EFFECTIVE HEAD = Ht -  LOSSES

H

D > 1.5

S > CRITICAL

Ht FOR DISCHARGE

AFFECTED BY

DOWNSTREAM

BACKWATER

Ht

ROUNDED INLET

H D

H

FIGURE 9.32 TYPICAL FLOW CONDITIONS – CULVERT CONDUITS ON STEEP SLOPES
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increase the discharge. The increased discharge will cause a deep drawdown just upstream from the 

inlet, and a vortex will form, allowing air to enter the culvert, breaking the draft tube action. This 

immediately reduces the discharge and causes a return to orifice control at the inlet. The full flow 

action will begin again and the cycle will be repeated. This alternating action will cause a pulsating 

slug flow phenomenon (Figure 9.32c). When the storage elevation and culvert dimensions are such 

that the H/D ratio exceeds about 1.5, the inlet drawdown will be insufficient to allow air to enter the 

culvert and steady full flow will prevail (Figure 9.32d).

The flow velocity during the full-flow conditions illustrated by Figures 9.31b and 9.32d can be determined from the Bernoulli equation:

V 2

 = Ht – ∑ losses (9-19)

 2g

where:

V = flow velocity (ft/sec)

g = acceleration of gravity (ft/sec2)

Ht = head (elevation difference) between the impoundment surface and the

point of discharge (ft)

The term “∑ losses” encompasses all flow-reducing conditions associated with the inlet, culvert 

geometry and culvert friction. These losses can be related to the velocity by:

∑ losses = 

V 2

( f L + ∑ KL ) (9-20)

 2g D

where:

f = friction factor for the culvert material and flow condition in the culvert

L = length of the culvert (ft)

D = diameter of the culvert (ft)

∑KL = sum of head losses associated with the inlet, valves, constrictions

and directional changes (ft)

For most flow conditions and circular culvert materials, the friction factor ( f ) can be determined from 

the following equation:

f = 185 n2/d 1/3 (9-21)

where:

n = Manning’s roughness coefficient for culvert

d = pipe diameter (ft)

By substituting terms from Equation 9-20 into Equation 9-19, the relationship between Ht and V becomes:

Ht = 

V 2 

(f L + ∑KL + 1) (9-22)

 2g D
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Typical head loss coefficients KL are tabulated in Table 9.14. After all of the loss parameters have been 

determined, Equation 9-22 can easily be solved.

However, Equation 9-22 is only appropriate when dealing with water at normal temperatures. The 

friction factor approach is superior when working with smooth pipes and/or large values of Reynolds number. For materials and pipe sizes commonly used, the above equation will give acceptable 

results. When unusual surfaces or very large pipe sizes are involved or when very long conduits with 

large energy losses are used, an experienced hydraulic engineer should be consulted.

The geometry of an inlet is important to achieving discharge efficiency. Until the headwater surface is 

well above the culvert inlet, a square-edged inlet produces an inlet flow contraction without greatly 

reducing the discharge capacity. Flow contractions can also be formed (but at reduced discharge 

capacity) by a projecting inlet, a mitered inlet, an inlet orifice ring, or a top curtain wall closure. These 

inlet configurations are shown in Figure 9.33.

TABLE 9.14 HEAD LOSS COEFFICIENTS FOR CONDUITS

 KL for Inlets (USBR, 1987a)

Inlet Type KL

Fully rounded entrances (r / D ≥ 0.15) flush with vertical walls 0.10

Square edge entrances flush with vertical walls 0.50

Socket-ended concrete pipe flush with vertical walls 0.15

Projecting concrete pipe with socket ends 0.20

Projecting smooth-wall or corrugated pipe 0.85

Gate in thin wall – unsuppressed contraction 1.50

Gate in thin wall – corners rounded 0.50

 KL for Directional Changes (Linsley and Franzini, 1972)

Radius of Bend/Pipe Diameter Angle of Bend

 90° 45° 22.5°

1 KL 0.50 0.37 0.25

2 KL 0.30 0.22 0.15

4 KL 0.25 0.19 0.12

6 KL 0.15 0.11 0.08

8 KL 0.15 0.11 0.08

 KL for Valves and Fittings (Linsley and Franzini, 1972)

Type of Valve or Fitting KL

Butterfly valve (wide open) 0.2

Gate valve (wide open) 0.2

Gate valve (half open) 5.6

Return bend 2.2

Standard tee 1.8

Standard 90° elbow 0.9
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If it is desired that a culvert flow full at increased headwater elevations, the control point during 

maximum flow will be at the outlet and the geometry of the inlet will be less significant. For this case, 

the inlet should be shaped so as to minimize the inlet flow contraction, thereby increasing the tendency for full flow for all conditions except when the inlet is not submerged. Streamlining the shape 

of the inlet will streamline the flow and reduce inlet head losses. The tendency to develop cavitation 

pressures will also be reduced. Rounded entrances or a gradually tapering transition in size to the 

basic culvert dimension will generally provide the desired streamlining.

9.33b PROJECTING INLET

9.33c MITERED INLET

FIGURE 9.33 CULVERT INLET CONFIGURATIONS

B

B

9.33e TOP CURTAIN WALL INLET

SECTION B - B

A

A

9.33d INLET ORIFICE RING

SECTION A - A

9.33a SQUARE-EDGED INLET

FIGURE 9.33 CULVERT INLET CONFIGURATIONS
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It is obvious from the previous discussions and related tables and figures that culvert inlets may 

vary with respect to approach conditions, entrance arrangements, and cross-sectional shapes. For 

example: (1) the approach to the inlet may or may not be a well defined channel with or without constructed wing walls, (2) the culvert may be flush with or protrude past the upstream embankment 

surface or constructed headwall, (3) the face may be square, beveled or rounded, and (4) the cross 

section may be round, square, rectangular or arched. All such variations have a marked influence on 

culvert performance as they affect weir or orifice discharge, inlet flow contractions and head losses 

during flow entrance to the culvert.

For coal refuse disposal facilities, circular culverts with vertical square-edged headwalls and flared 

wingwalls are sometimes encountered. The hydraulic design for these installations is discussed in 

detail by the USBR (1987a). Also, procedures and charts for design of these types of culverts are presented in FHWA (2005b).

The purpose of hydraulic analysis of a closed-conduit system is similar to that for an open-channel 

spillway, which is to determine the relationship between the elevation of the impounded storage 

(headwater) and the rate of outflow or discharge from the system. The head-discharge relationship 

for culvert-type conduit systems may be controlled by weir, orifice and pressure flow conditions 

depending on the upstream head, as illustrated in Figures 9.31 and 9.32.

Drop Inlet Conditions

Closed-conduit flow in drop-inlet decant systems is similar to that for culvert systems, as discussed 

above. The relationship between the elevation of the impounded storage and the rate of discharge 

during these types of flow is indicated by the curves shown in Figure 9.34. For the drop-inlet systems 

illustrated in Figure 9.34, curve A results from crest-controlled (also referred to as weir-controlled) 

flow, as affected by the inlet geometry and water level conditions. The inlet geometry can range from 

a sharp-edged, vertically placed circular pipe to an elaborately formed concrete ogee called a morning glory spillway.

Curve B results from orifice-controlled flow. The flow control point for curve A is at the crest, as illustrated in Figure 9.34a. The flow control point for curve B is at the entrance to the transport section of 

the conduit, as illustrated in Figure 9.34b. The curve B flow condition occurs when the open-channel 

capacity of the transport section and the discharge capacity of the outlet are both greater than the 

capacity of the inlet orifice. This results in a backup of water in the drop tube and drowning of the 

inlet weir. 

The curve C flow condition is associated with full flow, generally controlled by the transport section. Analysis for the curve C condition is the same as for a culvert system under full flow (Equation 

9-22) except that an additional head loss term related to the drop inlet should be included. The loss 

coefficient KL for the transition from the drop inlet to the transport section can range from 0.2 for an 

unobstructed, specially-formed structure to 2.0 for a debris-clogged, square-edged turn. A value of 

1.0 is normally satisfactory for design.

To maximize the discharge capacity of decant systems under low heads, conical-shaped inlets that 

extend the crest control flow (curve A) have been employed. Other inlet shapes that incorporate an 

orifice ring have been used to maintain orifice flow (curve B) and to prevent the transport pipe section 

from flowing full or under pressure.

As a guide for assisting the designer in the evaluation of important secondary characteristics affecting closed-conduit flow, a list of Manual sections and supplemental references for secondary hydraulic design issues is provided in Table 9.15.
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FIGURE 9.34 FLOW AND DISCHARGE CHARACTERISTICS OF DECANT-INLET,

 CLOSED-CONDUIT SYSTEM

H

WEIR CONTROL AT CREST

TRANSPORT SECTION OF CONDUIT

OPEN CHANNEL FLOW

9.34a CURVE A - CREST CONTROL

OUTFLOW, Q

A

B

C

POINT OF CHANGE FROM ORIFICE

TO FULL FLOW CONTROL

POINT OF CHANGE FROM

CREST TO ORIFICE CONTROL

HEAD ON INLET, H

H

CREST

FOR DOWNSTREAM

BACKWATER CONDITION

FOR FREE

DISCHARGE

CONDITION

9.34c CURVE C - FULL FLOW CONTROL

FULL FLOW

IN CONDUIT

H

ORIFICE CONTROL AT ENTRANCE

TO TRANSPORT SECTION

OPEN CHANNEL FLOW

9.34b CURVE B - ORIFICE CONTROL

CREST

FIGURE 9.34 FLOW AND DISCHARGE CHARACTERISTICS OF DECANT-INLET, 

CLOSED-CONDUIT SYSTEM
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9.7.2.3 Outlets

There are two basic requirements that typically dictate the size, configuration and location of the 

outlet section of the hydraulic system:

1. If the outlet is intended to control the relationship between headwater storage and 

outflow for the entire hydraulic system, the outlet size and configuration must be 

designed specifically for this purpose.

2. The configuration and location of the outlet must be such that discharged water will 

not create a hazardous condition (e.g., erosion of the channel or embankment toe) 

and the energy of the outflow will be dissipated. The potential for localized damage 

must be controlled to a level acceptable to the owner and to regulatory agencies.

Whether the first requirement applies depends upon the design choices for the inlet and transport 

sections of the system, as previously discussed. If applicable, the outlet size and configuration must 

be designed to control the required discharge, generally by weir or orifice control. Evaluation of this 

discharge was discussed in Section 9.7.2.1 and is not discussed further herein.

The second requirement has more general significance and necessarily requires considerable planning at the earliest stages of design so that the outlet of the hydraulic system will not be incompatible 

with any other portion of the coal refuse disposal facility. Criteria to be considered in determining the 

degree of effort required for construction of an outlet system normally include:

• Under no circumstance should the outlet location or configuration allow flow to discharge beyond the outlet in a manner that could contribute to embankment failure, 

endangering lives or major downstream property.

• Situations where significant localized damage could occur during moderate storms 

should be avoided. Exceptions are when the cost of constructing a damage-safe 

outlet far exceeds the cost of occasional maintenance. Often the 100-year storm (Section 9.5) is considered as the basis for design when it is unlikely that lives or major 

downstream property could be endangered.

• For very large storms, such as those approaching the PMF (Section 9.5), it is generally 

not required that the outlet totally prevent downstream damage due to discharge, if 

the first criterion above is satisfied. Under such severe storms, significant downstream 

damage (such as erosion and roadway overtopping) can be expected to occur even if 

the hydraulic system of the disposal facility satisfies all requirements of good design.

TABLE 9.15 REFERENCES FOR SECONDARY HYDRAULIC DESIGN ISSUES

Item Manual Section Supplemental References

External Embankment Pressure 6.6.6.2 FEMA (2005a)

Internal Water Pressure 9.7.2.2 Brater et al. (1996)

Thrust Forces 9.7.3.5 Brater et al. (1996)

Trash Racks 9.7.4.1 USBR (1987a)

Vortex Control 9.7.4.1 USBR (1987a)

Flow at Bends 9.7.3.4 Brater et al. (1996), USACE (1980)

Cavitation 9.7.3.3 USACE (1980), USBR (1990)

Materials Selection 6.6.6.1 FEMA (2005a)
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Several types of outlet conditions are illustrated in Figure 9.35. If the discharge point can be located 

an adequate distance away from the overall facility, the most reliable and least expensive outlets are 

those that allow the flow to discharge over erosion resistant rock or large riprap. This type of outlet 

condition is most often used when the channel or spillway will function only infrequently because of 

the limited recurrence of very large storms.

The energy dissipating structures shown in Figure 9.35 are commonly used for water-impounding earth 

dams, but are normally not necessary for coal refuse disposal facilities if sufficient access is available 

that periodic maintenance and repair can be performed. However, such outlets should be considered for 

moderate but regular discharges or when future access could be difficult. Also, if extending the transport 

section a significant distance beyond the downstream limits of the facility will be costly, coupling a short 

transport section with an energy-dissipating structure may be a more cost-effective solution.

State and local agencies may prefer specific methods or structures for energy dissipation at outlet 

structures, and applicable regulatory guidance should be reviewed as part of design. Hydraulic 

FIGURE 9.35 EXAMPLES OF CHANNEL AND CONDUIT OUTLETS

DISCHARGE WITHOUT ENERGY

DISSIPATING STRUCTURES

BOULDERS, RIPRAP OR

OTHER PROTECTION

SYSTEM

9.35b RIPRAP PROTECTION

DISCHARGE WITH ENERGY

DISSIPATING STRUCTURES

OUTFLOW CHANNEL

OR CONDUIT

9.35d PLUNGE POOL STILLING BASIN

IMPACT BLOCK OR

ROCK HILLSIDE

9.35e IMPACT BLOCK STILLING MECHANISM

OUTFLOW CHANNEL

OR CONDUIT

9.35c HYDRAULIC JUMP STILLING BASIN

SUBCRITICAL

FLOW SUPERCRITICAL

FLOW

OUTFLOW CHANNEL

OR CONDUIT

COAL REFUSE EMBANKMENT

OUTFLOW CHUTE,

EXCAVATED SPILLWAY

CHANNEL OR CONDUIT

STREAM

9.35a FREE OUTFALL

EROSION

RESISTANT

ROCK 

FIGURE 9.35 EXAMPLES OF CHANNEL AND CONDUIT OUTLETS
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design requirements for structures similar to those shown in Figure 9.35 are discussed by FHWA 

(2006), USACE (1990b), USACE (1980), and USBR (1987a).

9.7.3 Special Design Considerations

The following discussion briefly emphasizes some important special considerations of hydraulic 

system design and illustrates situations where special conditions must be evaluated.

9.7.3.1 Channel Freeboard

Freeboard is needed in open channels to account for minor channel irregularities, air entrainment and wave action. The required freeboard is a function of the design flow velocity and flow 

depth and typically varies from less than 5 percent to greater than 30 percent of the depth of 

flow (Chow, 1959). Low freeboards may be appropriate for smooth, uniform channels with flow 

velocities less than 8 feet per second. The following is an empirical relationship that provides the 

desirable spillway channel freeboard based upon surface roughness, wave action, air bulking, 

splash, and spray under supercritical conditions (USBR, 1987a):

Freeboard (ft) = Cf + 0.025 V D1/3 (9-23)

where:

Cf = freeboard coefficient

V = design flow velocity (ft/sec)

D = flow depth (ft)

The USBR (1987a) has indicated that a value of Cf equal to 2 is desirable for spillway channels 

for dams. While Equation 9-23 has been applied to spillways with the cited flow conditions, this 

formula has been used by some state agencies for the design of uniform channels ranging from 

perimeter and diversion ditches to small spillway channels by adopting values for Cf ranging from 

0 (with not less than 0.3 feet of freeboard) to 1. MSHA (2007) has indicated that a value of Cf equal 

to 1 is prudent for design of perimeter ditches. Channel bends, convergence, and other geometric 

changes may require greater freeboard to address variations in the flow profile. Additional discussion is presented in Section 9.7.3.4.

9.7.3.2 Erosion Protection

Two methods are typically employed in the design of erosion protection linings for channels: (1) the 

permissible velocity method or (2) the tractive (shear stress) force method. Under the permissible 

velocity approach, the channel is assumed to be stable if the mean velocity is lower than the maximum permissible velocity for the channel materials. The tractive force approach focuses on stresses at 

the channel boundary. The factors that normally cause erosion are velocity of flow (e.g., in steep channels and at intakes) and water impact at points where direction changes occur (e.g., at sharp bends in 

channels, at the intersection of channels, and where free-falling water discharges from a channel or 

conduit). If unchecked, erosion can result in degradation of water quality and frequent maintenance. 

Excessive or uncontrolled erosion on or near an impounding embankment can result in catastrophic 

failure during large storms.

The shear resistance of the soil/rock in an unlined channel or the shear resistance provided by the 

channel lining will determine the stability of the channel relative to erosion. The shear force generated by water flowing through a channel section is given by:

τ = γ R S (9-24)
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where: 

τ = shear stress along the wetted perimeter of flow (lb/ft2)

γ = unit weight of water (lb/ft3

)

R = hydraulic radius = A/WP (ft)

S = energy grade line slope (equal to the channel slope for uniform flow) (ft/ft)

The maximum shear stress on the channel bottom and sides in a straight channel depends on the 

channel shape. For trapezoidal channels with a ratio of bottom width to depth greater than 4, the 

maximum shear stress on the channel bottom frequently governs the lining selection and can be estimated using the relationship (FHWA, 2005a):

τd = γ d S (9-25)

where:

τd = shear stress in channel at maximum depth (lb/ft2)

d = maximum depth of flow in channel (ft)

For bottom width to depth ratios less than 4, the above equation conservatively overestimates the 

shear stress for straight uniform channels. If a more refined analysis is desired, Equation 9-24 should 

be applied.

The shear stress on the wetted perimeter (τ) is then compared with the permissible shear stress (τp) 

for the channel bottom or lining. If the permissible shear stress is greater than or equal to the computed shear stress, including a safety factor (SF), the channel and lining are considered stable. If this 

condition is not met, a different channel configuration or lining with a higher permissible shear stress 

is selected. The concept is expressed as:

τp ≥ SF τ (9-26)

The safety factor provides for a measure of uncertainty and conservatism for the designer. A safety 

factor of 1.5 is generally recommended for the following conditions:

• Critical or supercritical flows

• Climatic regions where vegetation may be uneven or slow to establish

• There is significant uncertainty regarding the design discharge

• The consequences of failure are high

Permissible velocity and tractive shear force values for channel linings are presented in Table 9.16. 

Determination of allowable tractive shear forces for commercially available erosion control products 

should be obtained from the manufacturers. Determination of tractive shear forces for grass- and 

riprap-lined channel sections is a function of the vegetation retardance class and the median riprap 

size for grass- and riprap-lined channels, respectively. More detailed explanations of the design procedures for grass- and riprap-lined channels are presented below. It should be noted that the use of 

grass and riprap erosion protection will generally be limited by the channel slope. For steep embankments with slopes greater than 10 percent, grass or riprap channel lining has limited application. 

Grouted riprap and concrete should be considered when tractive forces exceed the allowable limits 

for grass or riprap channel linings. Also, commercially manufactured products have been developed 
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TABLE 9.16 PERMISSIBLE VELOCITY AND TRACTIVE FORCE FOR CHANNEL LININGS

Channel Lining Permissible Velocity(1)

(ft/sec)

Permissible Tractive 

Force(2)

(lb/ft2)

Temporary Lining

Jute netting 0.45

Straw with net 1.45

Coir-double net 2.25

Coconut fiber-double net 2.25

Curled wood mat 1.55

Curled wood-double net 1.75

Curled wood-high velocity 2.00

Synthetic net 2.00

Vegetative Lining

Class A 3.70

Class B 2.10

Class C 1.00

Class D 0.60

Class E 0.35

Kentucky Bluegrass, Tall Fescue(3) 5-7

Grass Mixture, Red Canarygrass(3) 4-5

Lespedeza Sericea, Weeping Lovegrass 2.5-3.5

 Redtop, Red Fescue, Annuals(3)

Riprap(4)

R-1 2.5 0.25

R-2 4.5 0.50

R-3 6.5 1.00

R-4 9 2.00

R-5 11.5 3.00

R-6 13 4.00

R-7 14.5 5.00

R-8 17 8.00

Gabions 22 8.35

Reno Mattress

6-10 inches thick 12 8.35

 10-12 inches thick 15 8.35

 12-18 inches thick 18 8.35

 Note: 1. USACE (1994), PADEP (2000); permissible velocity should only be applied in cases of straight, uniform

 steady flow (e.g., bank protection)

2. FHWA (2005a), PADEP (2000)

3. Permissible velocity range for easily eroded to erosion resistant soils (clayey fine-grained soils and

 coarse-grained soils) at slopes less than 5 percent. Use velocity exceeding 5 ft/sec only where

 good cover and proper maintenance can be provided.

4. Based on rock with unit weight of 165 lb/ft3.
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to provide greater tractive shear resistance for steeper slopes, but product limitations and the importance of quality construction practices must be clearly understood, if these products are used.

9.7.3.2.1 Grass-Lined Channels

Design of stable grass-lined channels is related to the type of vegetation selected, as reflected by the 

vegetation retardance class and the tractive stress associated with peak flow. The FHWA (2005a) presents design procedures and data for the design of vegetative linings. Grass linings control erosion by 

dissipating shear force within the grass stems before it reaches the soil surface, and the root and stem 

stabilize the soil against turbulent water forces. As indicated in Table 9.17, vegetation retardance for 

grasses is divided into five classes – A through E. In general, taller and denser grass species have a higher 

resistance to flow (Class A), while short flexible grass has a low resistance to flow (Class E). The design 

procedure presented in FHWA (2005a) includes initially estimating the flow depth and effective shear 

stress on the grass lining based on the grass retardance class, roughness, stiffness, and cover conditions. 

Manning’s n is determined from the effective shear stress, and the discharge is computed and compared 

to the original estimate. Through a series of iterations, a solution is obtained that balances the flow depth, 

shear stress, and discharge. The permissible shear stress for the soil type is then estimated from soil properties such as grain size, plasticity, and void ratio for comparison to the effective shear stress.

Haan and Barfield (1978) present a simplified procedure in which permissible velocities for vegetated 

channels are used with the relationship between channel conveyance and Manning’s n for the retardance classes shown in Figure 9.36. Table 9.17 presents the vegetative cover and condition for various 

retardance classes. The design procedure is to: (1) select the vegetation and an initial estimate of n, 

(2) determine retardance class and permissible velocity based on Table 9.17 and an initial estimate 

of VR (velocity V times hydraulic radius R) from the curves in Figure 9.26, (3) obtain the permissible 

velocity for the vegetation based on Table 9.16 (or state regulatory guidance manual values) and (4) 

compute the hydraulic radius R from Equation 9-18 using the permissible velocity. The product of the 

permissile velocity and R from Step 4 is compared with the initial estimate of VR to determine a new 

value of VR until convergence.

Commercially available erosion control products can be used to reinforce natural vegetation, including non-degradable synthetic fibers, filaments, netting and/or wire mesh. These materials can be 

integrated into the vegetation and soil lining or applied over the surface, affecting performance 

and altering the design procedure. The FHWA (2005a) presents design methods for incorporating 

such products into the design of channel linings, and manufacturers have developed software for 

analysis of unreinforced and reinforced vegetated channel sections (e.g., North American Green’s 

Erosion Control Materials Design software). When such software is used, knowledge of the grass 

retardance class, vegetation type, soil type and duration of peak flow are essential requirements in 

the determination of the tractive shear resistance of the channel linings.

9.7.3.2.2 Riprap-Lined Channels

Riprap linings consist of a layer of rock or stone with a characteristic size, generally designated by 

D50, the median grain size of the lining material. As with grass linings, the flow conditions and channel shear stress are a function of the Manning’s n, and an iterative procedure is required for evaluating lining stability. Values for permissible shear stress for riprap linings are based on laboratory and 

field research. More turbulent flow conditions are more likely to cause lining failure, and a higher 

safety factor is recommended for such conditions. Typically, for situations where riprap is needed, 

a safety factor of 1.5 should be used, although a lower value may be justifiable with mild slopes and 

low velocities. The FHWA (2005a) presents a design procedure for riprap-lined channels.

Barfield et al. (1981) present a simplified approach for riprap lining design based on USACE procedures and the critical tractive shear stress. The force on median-size riprap at the threshold of 
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TABLE 9.17 VEGETAL RETARDANCE CLASSES

Retardance Class Cover Condition

A

Reed canarygrass Excellent stand, tall (average 36 inches)

Yellow bluestem ischaemum Excellent stand, tall (average 36 inches)

B

Smooth bromegrass Good stand, mowed (average 12 to 15 inches)

Bermudagrass Good stand, tall (average 12 inches)

Native grass mixture (little bluestem, blue 

grama, and other long and short mid-west 

grasses)

Good stand, unmowed

Tall fescue Good stand, unmowed (average 18 inches)

Sericea lespedeza Good stand, not woody, tall (average 19 inches)

Grass-legume mixture – Timothy, smooth 

bromegrass, or orchardgrass

Good stand, uncut (average 20 inches)

Reed canarygrass Good stand, mowed (average 12 to 15 inches)

Tall fescue, with bird’s foot trefoil or iodino Good stand, uncut (average 18 inches)

Blue grama Good stand, uncut (average 13 inches)

C

Bahia Good stand, uncut (6 to 8 inches)

Bermudagrass Good stand, mowed (average 6 inches)

Redtop Good stand, headed (15 to 20 inches)

Grass-legume mixture – summer 

(orchardgrass, redtop, Italian ryegrass, 

and common lespedeza)

Good stand, uncut (6 to 8 inches)

Centipedegrass Very dense cover (average 6 inches)

Kentucky bluegrass Good stand, headed (6 to 12 inches)

D

Bermudagrass Good stand, cut to 2.5-inch height

Red fescue Good stand, headed (12 to 18 inches)

Buffalograss Good stand, uncut (3 to 6 inches)

Grass-legume mixture – fall, spring 

(orchardgrass, red-top, Italian ryegrass, 

and common lespedeza)

Good stand, uncut (4 to 5 inches)

Sericea lespedeza After cutting to 2-inch height, very good stand 

before cutting

E

Bermudagrass Good stand, cut to 1.5-inch height

Bermudagrass Burned stubble

(HAAN AND BARFIELD, 1978)

motion is considered to be the critical tractive resisting force per unit area (τc) for riprap-lined 

channel sections. Since riprap is a layer of discrete, individual rocks, the movement of an individual rock can initiate further erosion, weakening the overall long-term erosion resistance of the 

riprap layer. For most situations at coal refuse disposal facilities, the general practice is to consider 

application of a safety factor for the channel base (SFbase) of 1.5 for the design of riprap channel sections. The critical tractive resisting force at the channel base can be estimated from the following 

equation:

SFbase = (cos θ tan φ)/(sin θ + ηb tan φ) (9-27)
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where:

ηb = τ/τc

τ = shear force per unit area on channel bed = γRS (lb/ft2)

τc = critical tractive resisting force per unit area = 0.047 γ(SG – 1)D50 (lb/ft2)

γ = unit weight of water (lb/ft3

)

R = hydraulic radius of channel (ft)

SG = specific gravity = 2.65 for durable limestone and sandstone

D50 = median riprap diameter (ft) (Table 9.18)

S = energy grade line slope (equal to the channel slope for uniform flow) (ft/ft)

θ = channel bed slope (deg)

φ = angle of repose of riprap (deg)

FIGURE 9.36 MANNING’S n VERSUS VR FOR VARIOUS

RETARDANCE CLASSES

0.02

0.03

0.04

0.05

0.06

0.08

0.10

0.20

0.30

0.40

0.50

MANNING'S n

A

B

C

D

E

0.10 0.20 0.30 0.40 0.60 0.80 1 2 3 4 6 8 10 20

VELOCITY TIMES HYDRAULIC RADIUS (FT2 /SEC)

(HAAN AND BARFIELD, 1978)

Table 9.18 presents guidance for estimating D50 for various classes of riprap based on data from the 

National Stone, Sand and Gravel Association (NSSGA, 1989). The FHWA (1989) presents guidance for 

estimating the angle of repose of riprap based on size and angularity; common values are typically 

in the range of 35 to 40 degrees. The tractive force on the channel bank is less than that applied to the 

channel bottom and can be estimated separately (Barfield, et al., 1981), although due to constructability issues, the riprap used for the channel base is also generally used for the channel banks. The 

FHWA (2005a) also provides a method for checking bank riprap stability.

FIGURE 9.36 MANNING’S n VERSUS VR FOR VARIOUS RETARDANCE CLASSES
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At channel bends, a correction factor K3 is normally applied for estimating the shear force:

τ = K3γ R S (9-28)

where:

K3 = 4V2/Rd (dimensionless)

V = straight channel velocity (ft/sec)

Rd = channel radius of curvature measured at the outside channel bank (ft)

The FHWA (2005a) presents details and discussion of design procedures at bends, including the 

length over which increased shear stresses are experienced and the superelevation of the water surface at and downstream of bends.

Geotextiles have been used increasingly as the filter medium between the riprap and underlying material. However, riprap placed on a geotextile has a tendency to creep over time. Gravitational and water 

forces have a tendency to re-orient riprap toward the base of the channel resulting in exposed geotextile 

at the top of the channel slope. The use of an aggregate filter layer between the subbase soil and the 

TABLE 9.18 RIPRAP SIZE DESIGNATION

Graded Riprap Stone

NSSGA No.(1) Sieve or Square-Opening Size (in)

Recommended Filter 

Stone

NSSGA Size No.(1) Maximum D50 Minimum(2)

R-1 11⁄2 3⁄4 No. 8 FS-1

R-2 3 11⁄2 1 FS-1

R-3 6 3 2 FS-2

R-4 12 6 3 FS-2

R-5 18 9 5 FS-2

R-6 24 12 7 FS-3

R-7 30 15 12 FS-3

R-8 48 24 15 FS-3

Filter Stone

NSSGA No.(1) Sieve or Square-Opening Size (in) Recommended 

Placement Thickness

Maximum D50 Minimum (in) (2)

FS-1 3⁄8 No. 30 No. 100 3

FS-2 2 No.4 No. 100 4 to 6

FS-3 61⁄2 21⁄2 No. 16 8 to 10

Note: 1. National Stone, Sand & Gravel Association (formerly National Stone Association) designation. The

 table is based on a stone dry density of 165 lb/ft3.

2. Pieces smaller than the minimum size shown should not exceed 15 percent of the tonnage shipped.

(ADAPTED FROM NSSGA, 1989)
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riprap is preferred to geotextile due to the increased frictional resistance between the aggregate filter 

material and the riprap. If geotextile is used, extra riprap should be placed on the channel slopes.

The recommended thickness and gradation of the filter stone in relation to the riprap size is presented in Table 9.18. The recommended riprap layer thickness is greater than or equal to 1.5 times the 

D50 stone size or greater than or equal to the D100 size, whichever is greater (FHWA, 1989).

The publication Practical Riprap Design (Maynord, 1978) based on model testing provides riprap 

size as a function of channel flow depth and Froude number (function of velocity and depth). Additionally, the USACE (1994) provides a generalized procedure for determining riprap size and distribution, including guidance for steep slope conditions (up to 20 percent). The model studies and 

guidance have proved beneficial for determining riprap requirements in steep slope conditions where 

the tractive force determined from Equation 9-27 may result in large riprap size relative to flow depth 

such that uniform flow conditions may no longer be valid. In such steep slope conditions, grouted 

riprap is often employed to reduce the riprap size. Software is available for riprap design that utilizes 

the USACE and other procedures.

9.7.3.2.3 Grouted-Riprap Channels

Grouted-riprap channel lining consists of riprap with a cement grout filling the voids to create a 

monolithic erosion protection mat. The use of grouted riprap can reduce the size and quantity of rock 

required in a channel lining by creating a greater material mass to resist hydraulic forces. Because 

grouted riprap linings are rigid, they are susceptible to cracking and damage due to subbase movement or freeze-thaw action. The tractive force resistance of cracked, unconfined grout sections is 

dependent on intimate surface contact between adjacent sections, section mass, and flow characteristics. In situations of severe cracking with displacement, the primary resistance to tractive forces is the 

grout and rock fragment pieces, which may be only slightly greater in size and mass than the riprap 

alone. Thus, to limit the potential for displacement, subbase design is very important.

Design procedures for grouted-rock linings installed on channel banks are provided in FHWA (1989) 

and address aspects such as rock size and lining thickness, rock grading, filter design, and pressure 

relief. Rock size and lining thickness are a function of the flow velocity; the median rock size should 

not exceed two-thirds of the lining thickness, and the largest rock size should not exceed the lining 

thickness. Table 9.19 and Figure 9.37 can be used to estimate the riprap thickness based on velocity, 

riprap size and the recommended depth of the grout penetration as follows:

• Based on the average flow velocity, Figure 9.37 can be used to estimate the riprap thickness. For the case where the ratio of bottom width to depth is greater than 2, the flow velocity should be increased by 25 percent when determining the riprap thickness.

• The gradation should be selected based on available riprap class (e.g., cobbles), considering that the median rock size should not exceed two-thirds of the lining thickness 

and the largest rock size should not exceed the lining thickness. Table 9.19 presents 

AASHTO class designations based on weight, which is approximately equivalent to 

the effective diameter shown.

• Grout penetration recommendations for each class or size of riprap are also presented 

in Table 9.19.

This procedure is applicable to trapezoidal channels where the ratio of bottom width to depth is less 

than 2. For wider channels, the velocity and shear stress on the channel bottom may be greater, and 

a corresponding adjustment in velocity or shear stress may be necessary. Typically, the design velocity will increase by 25 percent because the tractive shear for channel side slopes is approximately 75 

percent of the maximum shear at the base of the channel.
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0 5 10 15 20 

0

1

2

3

4

5

VELOCITY IN VICINITY OF BANK (FT/SEC)

RIPRAP THICKNESS (FT)

25

(FHWA, 1989)

TABLE 9.19 RECOMMENDED GRADING OF GROUTED ROCK RIPRAP LINING

Rock Sizes Classes (Percent Larger Than Given Rock Size)

Equivalent Diameter

(ft) 2.75 2.25 1.75 1.25 1.0 0.5

3.5 0-5

2.75 50-100 0-5

2.25 50-100 0-5

1.75 95-100 50-100 0-5

1.25 95-100 50-100 0-5

1.0 95-100 95-100 50-100 0-5

0.5 95-100 95-100

Minimum Penetration of Grout 

(ft) 2 1.5 1.3 0.8 0.67 0.5

(FHWA, 1989)

Similar to riprap channel lining, the use of an aggregate filter layer between the natural ground surface and the riprap is recommended, but geotextiles can also be used in this application provided the 

riprap does not shift on channel slopes during construction or under subsequent flow conditions.

FIGURE 9.37 GROUTED RIPRAP LINING THICKNESS AS A FUNCTION OF FLOW VELOCITY
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Foundation preparation is a critical factor in the performance of a grouted riprap lining. Grouted 

riprap is not flexible and foundation conditions, the potential for development of hydrostatic pressure beneath the rigid mat, and end treatment conditions need to be evaluated as part of the design 

and construction process. Damage to any section of grouted riprap lining can result in complete 

failure of the system.

The foundation for grouted-riprap channel lining should be a contoured, firm stable surface. Small 

surface irregularities can be accommodated within the filter stone layer. The slopes should be graded 

at no steeper than 1.5H:1V.

Weep holes for hydrostatic pressure relief should be installed in a grouted riprap channel lining. The 

weep holes should extend below the grouted surface to the gravel zone between the grout and the 

filter stone. It is recommended that 3-inch-diameter weep holes, constructed from PVC pipe and with

protective end screening, be installed at maximum 10-foot vertical spacing (FHWA, 1989).

The head, toe and terminal ends of grouted riprap linings require special treatment such as extension 

of the lining to rock or to a depth below potential scour to prevent undermining. Additional riprap 

can provide extra protection against undercutting at a bank toe. Figure 56 of Hydraulic Engineering 

Circular 11 (FHWA, 1989) presents recommended construction details for these treatments. Guidance 

is also provided for rock grading and quality, grout strength and penetration, and filter design and 

pressure relief measures. The grout mix should typically be designed for a strength of at least 3,000 

psi, with maximum aggregate size of 3⁄4 inch and a slump between 3 and 4 inches. Sand mixes may 

be used where roughness of the grout surface is undesirable. The finished grout should leave the 

face rock exposed for approximately one-quarter of their depth, and the surface of the grout should 

expose the matrix of coarse aggregate. The following construction details should be addressed in the 

specifications for effective grouted riprap lining:

• The prescribed method of riprap placement should prevent segregation of rock 

sizes, and the riprap should be wet immediately prior to commencing grouting 

operations.

• The prescribed method of grout placement should control segregation and uniformity.

• The prescribed method of grout placement should facilitate grout penetration by use 

of vibrating, spading and/or rodding.

• Quality control requirements during construction should be followed, including all 

material placement methods, grout mix design and strength testing, and recording 

of quantities of all materials. The volume of grout used should be compared to that 

required to meet penetration requirements.

9.7.3.2.4 Concrete-Lined Channels

Concrete can provide a continuous, rigid channel lining. Similar to grouted riprap, foundation conditions, hydrostatic pressure development and end treatments are design considerations that must be 

addressed. Offsets at joints may create additional hydrodynamic uplift forces. Reinforcing steel (No. 

6 gage wire mesh or No. 4 reinforcing bars at 6-inch spacing are typical minimum recommendations 

for 4-inch and 6-inch lining thicknesses, respectively) to reduce the development of thermal stresses, 

shrinkage and flexural stresses within the Class A (AASHTO classification) concrete is normally provided. The FHWA (1989) provides guidance for design of concrete linings.

Filter layers should be placed below the concrete pavement, and weep holes should be employed 

to prevent the development of hydrostatic uplift pressures. The weep-hole configuration described 

above for grouted-riprap revetments should be the maximum spacing considered. Hydrodynamic 

uplift pressure may also be a consideration where vertical offsets or changes in channel slope occur, 
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particularly at transitions from a steep slope to a flatter slope. The USBR (2007b) presents results of 

testing for a range of flow velocity, joint widths, and offset dimensions and provides empirical estimates of hydrodynamic uplift as a function of velocity head. The USACE (1990b) presents guidance 

for analysis of vertical curve transitions and recommends that construction joints be excluded from 

sections transitioning from steep to flatter slopes. Other measures that may be considered include 

anchor systems grouted into rock or secured to deadmen.

Edge treatment at the toe, head, and terminal ends should be utilized to prevent undermining. Stub 

walls or cutoff walls are recommended at expansion joints.

9.7.3.2.5 Commercially Available Composite Erosion Control Products

Commercially available composite erosion control products include permanent reinforced vegetation mats; interlocking concrete blocks; grout-filled nylon mats (unanchored or anchored); soil, 

rock or concrete filled cellular confinement mats; gabion mats; cabled concrete or other similar 

products. The allowable tractive shear for these products will generally be specified by the manufacturer or can be determined by computer software developed by the manufacturer. The effectiveness of a particular type of channel lining relative to the intended use should be verified with the 

manufacturer prior to design. The FHWA (1989, 2005a) provides guidance for some commercially 

available products.

Applications of interlocking concrete blocks, grout-filled nylon mats, cellular confinement mats, 

and cabled concrete at coal refuse disposal facilities have included emergency spillway linings, 

groin ditches, principal spillway outlet channels, and diversion ditches where high velocities are 

present and excavation into rock is not possible. Design procedures are generally available from 

the manufacturers and these procedures should include methods for assessing tractive forces and 

uplift. For grout and concrete systems, additives such as steel or polyester fibers are sometimes 

employed to increase the strength of the concrete, decrease cracking, and to improve resistance to 

hydraulic wear.

Some design and construction issues for concrete and grouted riprap linings may also apply to composite systems, including:

• The foundation should be graded smooth and compacted to maintain support and 

prevent detrimental movement. Some composite systems require a filter and/or 

drainage layer beneath the lining.

• Side and end anchorages should follow manufacturers’ recommendations, and when 

unusual site conditions are encountered, the manufacturer should be contacted for 

input.

• Weep holes must be provided where necessary, consistent with manufacturers’ recommendations.

• Access to allow inspection and maintenance must be planned. If vehicles and equipment have to cross the channel lining, a reinforced section should be designed.

9.7.3.3 Cavitation

Cavitation of a hydraulic system occurs when flow separates from a containment surface. The result 

is the formation of a region of subatmospheric pressure that can lead to deterioration of the confining 

surface. Cavitation can occur in either open-channel or closed-conduit flow. As a rule of thumb, cavitation should be investigated whenever flow velocities exceed 35 feet per second (USACE, 1990b). 

The potential for damage is a function of flow duration and geometry and the abrasion resistance of 

the material.
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In open-channel flow, cavitation can occur at steepening grade changes. At constructed ogees and 

paved channels, formation of a cavity of subatmospheric pressure can lead to damaging vibrations; 

under extreme conditions, these vibrations may actually displace the structure. In unpaved channels, 

cavitation can produce an upstream propagating erosion of the channel bottom that will worsen and 

possibly cause failure of the entire hydraulic system if uncorrected.

In closed conduits, cavitation most frequently occurs at sharp turns. For hydraulic systems common 

to coal refuse disposal facilities, this is particularly likely to occur at the transition from a drop inlet 

to the entrance of the transport conduit. Should the frequency and duration of high-velocity flow 

and conduit susceptibility to cavitation damage warrant, methods to prevent cavitation in closed 

conduits (USBR, 1987a) and USACE (1980, 1990b) may be needed.

9.7.3.4 Directional Changes in Open-Channel Flow

Methods for predicting the superelevation of the surface of curving flow are available, but the 

superelevation is usually small for subcritical flow velocities. When the flow at a curve is supercritical, a directional change is an exceptionally complicated problem because of the formation, 

propagation and combination of channel cross waves. These factors must be considered in design 

if channel overtopping is to be prevented. A simplified determination of the additional flow depth 

associated with superelevated flow at a rectangular channel bend is provided by the following 

equation (Chow, 1959):

Δh = {Vmax2/g} {(20⁄3)(rc /b) – 16(rc

3

/b3) + ((4rc

2

/b2) – 1)2 ln[(2rc + b)/(2rc – b)]} (9-29)

where:

Δh = change in depth associated with the superelevation (ft)

Vmax = velocity from Manning’s equation in straight channel section

approaching bend (ft/sec)

rc = radius of curvature measured to the centerline of the channel (ft)

b = width of the channel (ft)

g = gravitational acceleration = 32.2 ft/sec2

The FHWA (2005b) presents a method for estimating the superelevation of flow at bends in trapezoidal channels based on the velocity head. Additional guidance is provided in USBR (1987a) and 

USACE (1980, 1990b).

9.7.3.5 Directional Changes in Conduit Flow

Sharp directional changes and intersections in closed conduits cause static and dynamic thrusts 

that must be considered in design. Depending on the conduit size, the internal pressure and the 

flow velocity, thrust forces can cause the backfill surrounding the conduit to compress, and joints 

in the conduit can open or the deflection of the conduit can exceed the allowable pipe deflection 

due to the resulting movement. This is particularly a problem when the conduit location is shallow 

(or the conduit is supported above ground) and soil resistance to movement is low. Where thrust 

movements are identified as a potential problem, a concrete thrust block is often poured around 

the conduit to add mass and to distribute pressure over a larger area of soil. Discussions for evaluating the necessity of special construction measures at directional changes or intersections and 

procedures for analyzing requirements of thrust blocks are provided by Brater et al. (1996) and in 

engineering manuals from conduit manufacturers. The need for thrust blocks should be evaluated 
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for fittings such as tees and prefabricated bends and at directional changes where conduit joints 

could separate.

9.7.3.6 Materials Selection

The construction of any type of hydraulic system, and particularly a closed-conduit system, should 

not be undertaken without investigating the hydraulic and structural suitability of the proposed 

construction materials. This topic is discussed in Section 9.3.3 and Section 6.6.6.1 of this Manual. 

The discussion herein is for open-channel systems and culvert-conduit systems with emphasis 

on the importance of selecting materials that will function as intended and avoid costly repair 

or replacement. A detailed discussion of decant system material selection is presented in Section 

9.7.4.1 with specific attention to the structural integrity of the decant system due to embankment 

loading conditions.

9.7.3.6.1 Open-Channel Lining Systems

As part of the hydraulic design process, the suitability of the selected channel lining material should 

be determined. Material selection should be based on factors such as resistance to abrasion, the type 

of flow in the channel (continuous or intermittent), the acidic nature of the coal refuse, the impact 

that water quality characteristics may have on lining system integrity, the availability of the lining 

material, foundation conditions (particularly for more rigid lining systems), the constructability of 

the system (e.g., site topography and site access conditions), cost, and maintenance.

9.7.3.6.2 Conduit Materials

Culvert pipes can be corrugated metal (CMP); concrete; corrugated plastic (CPP), both smooth-wall 

or corrugated interior; high density polyethylene (HDPE); polyvinyl chloride (PVC); aluminum; steel 

or other materials. The type of material recommended is generally a function of intended use of the 

culvert (temporary or permanent), loading conditions, foundation conditions, construction limitations, and cost. Uncoated corrugated metal and steel are generally not recommended for long-term 

use in a mining environment because of the corrosion potential of mine water. Limited usage or protective coatings can make these material alternatives more acceptable.

Culverts constructed of concrete, CMP and aluminum are manufactured in various shapes (box, 

oval, arch, etc.) for installation in areas with limited height and clearance. Minimum and maximum cover height limitations are associated with all culvert installations. CPP and HDPE pipe 

are generally more flexible and structurally stable in conditions where minor settlements may 

occur.

In all applications, installation is critical to the structural integrity and hydraulic conveyance capabilities of conduits. The thickness of and installation procedures for bedding and backfill materials 

are critical to successful culvert construction. Joint connections in most culvert installations should 

be minimal. If joints are present, they should be soil-tight and in most applications watertight. 

Fusion-welded HDPE, gasketed joints for some concrete and CPP, glued PVC, and welded steel 

pipe provide the most watertight applications. Pressure testing can be performed to verify that 

joints are watertight.

9.7.4 Types of Hydraulic Systems

Sections 9.7.1, 9.7.2 and 9.7.3 have identified: (1) basic considerations for planning hydraulic systems 

at coal refuse disposal facilities, (2) the primary components of all hydraulic systems and techniques 

appropriate for their analyses, and (3) special design considerations associated with hydraulic systems. This section integrates this basic information into the following discussions of specific types of 

hydraulic systems most common to coal refuse disposal facilities.
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9.7.4.1 Decant Systems

Decant systems at impounding disposal facilities serve one of the following three purposes or a combination thereof:

1. To remove clarified water during normal disposal of fine refuse.

2. To provide outflow during low-precipitation storms so that storage volume will be 

available if a large storm occurs.

3. To drain (possibly in conjunction with a spillway) the stored volume of inflow due to 

a large storm within a reasonable period after occurrence of the storm.

A decant system typically consists of: (1) an inlet section located in the impoundment at the elevation 

required for controlling or limiting the normal water surface level, (2) a transport section consisting 

of a closed conduit beneath or through the embankment, and (3) a discharge section located downstream from the embankment so as to minimize erosion of the embankment toe. It is the responsibility of the designer to select the optimum location for the transport section conduit and the discharge 

point, based on foundation conditions, conduit size and material, and discharge rates.

The optimal selection of the decant inlet type, configuration and location is primarily a function of 

the overall facility configuration, the required discharge capacity, the method for disposing of the fine 

refuse, the rate at which the impoundment level will rise during operations, and eventual abandonment or post-mining land use requirements. Design considerations include the height of the decant 

inlet invert above the settled fine coal refuse, the method of evacuation of clarified water between the 

decant inlet invert and the settled fine coal refuse level, the trash rack system for preventing debris 

from entering the decant pipeline, and the potential buoyancy of inlet and conduit.

Often the most challenging design consideration is selecting a conduit that will withstand the weight 

of the overlying refuse embankment and that can deform as the foundation and embankment materials settle vertically or displace laterally. Generally, only specially designed concrete, high density 

polyethylene (HDPE), or steel pipes are capable of withstanding the high pressures beneath a refuse 

embankment. Section 6.6.6 provides guidance for the selection of decant materials and designing for 

embankment loading, including design of the decant pipe bedding and backfill. The transition from 

the riser to the transport section of the decant must be designed to handle the impact loads associated with directional change in flow. This is particularly important for rigid pipe systems, which are 

sensitive to movement. Control of seepage along and adjacent to the transport section of the decant, 

where it extends through the embankment, must also be addressed (Section 6.6.2.3).

To prevent erosion, the decant outlet must be able to accommodate the design flow rate and velocity. 

The rate and velocity of outflow must also be considered in the design of downstream conveyance 

and/or storage structures.

In the following sections, guidelines for the design of the hydraulic conveyance components of a 

decant system are presented. The inlet, transport and outlet sections of a decant system will be discussed separately. The discussion is based on the use of concrete, welded steel, and HDPE, since these 

are the most commonly used materials for impoundment decant systems.

9.7.4.1.1 Inlet

Location within the Impoundment

Location of the decant inlet within the impoundment is a function of the following factors: (1) 

site terrain and foundation conditions, (2) limitations of the transport section of the decant such 

as length and foundation conditions, (3) type of embankment construction such as upstream or 
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downstream construction, and (4) limitations in positioning the slurry discharge. Often, the inlet 

is positioned in the upstream portion of the impoundment, so that fine refuse can be deposited to 

form a delta at the upstream slope of the embankment and clarified water accumulates in the farthest upstream portion of the impoundment.

Trash Rack

The entrance to decant inlets should be protected by a trash rack. Even a partially obstructed pipe 

will have a substantially reduced capacity, thus increasing the potential for dam overtopping during 

a large storm event. Trash racks can become plugged if the openings are too small, and openings that 

are too large can result in the obstruction of the pipe due to the intake of large debris. The connection of the trash rack to the decant structure must be strong enough to withstand the hydrostatic and 

dynamic forces exerted on the trash rack during periods of high flow.

It is recommended that trash rack openings be sized so that they are a maximum of one-half the 

nominal dimension of the outlet conduit. The minimum opening size should be 6 inches by 6 inches 

or greater. The USBR (1987a) recommends that the area of trash rack openings be established based 

on the flow velocity through the rack. Where trash racks are inaccessible for cleaning, this velocity 

should not exceed 2 feet per second. A velocity of up to approximately 5 feet per second can be tolerated for racks that are accessible for cleaning. An anti-vortex device should be incorporated into the 

trash rack design to prevent the formation of a flow-inhibiting vortex during periods of high flow. 

The USBR (1987a) recommends that the anti-vortex device extend at least two diameters in front of 

and to each side of the inlet. In practice, these devices are usually part of the trash rack assembly and 

consist of a steel plate with width equal to the width (or diameter) of the rack. 

Evacuation of Water below Riser Invert

As part of the design process, a sufficient minimum depth of water and associated height of riser above 

the settled fine coal refuse should be provided in order to prevent short circuiting of the impoundment and release of fine coal refuse slurry. This depth is frequently estimated based on experience 

and judgment, usually varying between 5 and 10 feet to accommodate settling of fine refuse in the 

slurry. Settling tests can be performed in the laboratory to aid in determination of the rate of settlement and the required impoundment retention time. The settling velocity can be determined from 

Stoke’s Law based on the particle size and specific gravity of the fine coal refuse:

Vs = [ g (S-1)D2] /18 ν (9-30)

where:

Vs = settling velocity (cm/sec)

g = acceleration of gravity (981 cm/sec2)

D = diameter of particle (cm)

ν = kinematic viscosity of fluid (cm2/sec)

S = specific gravity of particle

The minimum height of the riser above the average settled fine coal refuse level can be calculated by 

determining the approximate detention time based on the outflow rate (slurry discharge rate plus 

watershed base flow) and impoundment geometry. To establish a balance between the detention time 

and period for settling, an iterative process is required. Because of the broad range in refuse particle 

sizes and varying impoundment geometry, experience and engineering judgment are generally used 

for determining the depth of water to be retained over the fine refuse.
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Pumps are generally used to remove the clarified water below the riser invert. Such pumps should be 

capable of meeting the discharge requirement without increasing the flow rate through the settling 

zone sufficiently to cause removal of fine coal refuse.

Inlet Riser Pipe Buoyancy

Inlet riser pipe sections may be susceptible to the buoyancy forces sufficient to cause uplift, if the pipe 

weight and backfill height is not adequate. This is particularly a concern for steel and HDPE decant 

pipe inlets. Inlets to an HDPE decant pipe are generally installed in a trench extending up a natural 

hillside with the inlets located at specified elevation intervals. Sufficient fill or anchorage must be 

provided to overcome buoyant forces, with a recommended factor of safety of 1.5.

Extension of Inlet

The decant riser inlet elevation is generally established based on storm routing such that adequate 

impoundment freeboard and surcharge storage is provided. As a refuse embankment dam is raised 

to increase the capacity of the impoundment, the riser inlet is correspondingly raised to provide 

additional slurry disposal capacity. If the decant system has multiple risers, the lower riser is sealed 

and abandoned and the next upper riser is put into service. Extension of an inlet will require that 

the connection, extension, and new inlet section be designed to accommodate the flows and related 

forces associated with continuing operation. If the decant system is designed with multiple risers, 

sealing (such as by employing a bolt-on plate) and abandonment (such as by embedment in concrete) 

of the riser must be part of the design. To assess the potential for additional stress and deflection at 

the base of the sealed riser, the loads associated with abandonment (along with future embankment 

construction) must be evaluated.

In some instances where high embankments are required and there are concerns about pipe loading, 

designers have limited the service life of a decant inlet and transport pipe section by abandoning the 

entire system and installing a second system at a higher elevation. In such situations, the original inlet 

and transport section should be sealed and abandoned upon completion of the replacement system.

9.7.4.1.2 Transport Section

The transport section of the decant pipe must typically be watertight and must include seepage control structures to intercept the flow of seepage along and adjacent to the conduit where it extends 

through the refuse embankment. For these reasons, structural considerations based on the external loading generally govern the design of this portion of the decant once the diameter has been 

established. Determination of external loading conditions and structural design of a decant pipe are 

addressed in Section 6.6.6.

Rigid pipe used within a refuse embankment is typically concrete pressure pipe because of the large 

external loads and the requirement that the pipe be watertight. Concrete pipe is rigid and sensitive 

to differential settlements, particularly at the inlet-riser transition, resulting from directional flow 

impacts, foundation conditions, or imposed embankment loading conditions.

Infiltration and exfiltration leakage problems can develop within the transport section of the decant 

pipe. Irreparable damage of the pipe and possibly failure of the refuse embankment can occur if such 

leakage is undetected. As part of the installation process, project specifications should require that 

pipes installed within the limits of the impounding refuse embankment be pressure tested prior to 

backfilling so that detected leaks can be immediately repaired.

Joint tightness is also a concern in non-pressure pipe installations, such as in the upstream inlet section of the decant pipe or in a concrete riser extension. Infiltration or exfiltration at joints could impact 

the pipe backfill if the material is erodible. Testing of non-pressure pipe joints is recommended.
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Flexibility, watertightness and relatively easy installation procedures have led to the use of 

HDPE for decant pipes. Structural evaluation procedures associated with the flexible conduit 

under large embankment loads are presented in Section 6.6.6. For flexible pipe, both structural 

and hydraulic design considerations may control the decant pipe size. Thus, evaluation of the 

embankment loading and required wall thickness of the pipe should be performed in parallel 

with the hydraulic design.

Schematic examples of decant inlets that are most adaptable to coal refuse disposal facilities are 

illustrated in Figure 9.38, while illustrations of actual decant systems that have been constructed 

are shown in Figures 9.39 and 9.40. The primary advantage of the inlet types shown in Figure 9.38a 

through d is their access for expansion as the level of settled fine refuse increases. The primary disadvantage is that the length of conduit required upstream of the embankment, beneath the settled 

slurry, is relatively great. The length of conduit in the Figure 9.38a and c decant systems is particularly significant because it must be as long as the entire impoundment. However, an advantage of 

these systems is that the inlet is located at the point where the water depth is normally the greatest 

(when the slurry is discharged at the embankment end of the impoundment), allowing the decant 

system to drain the water without pumping.

The inlet shown in Figure 9.38d appears to be the simplest arrangement because it offers the shortest 

conduit length and the inlet can be extended in height by adding subsequent sections as the settled 

fine refuse level rises. Major difficulties with this type of decant system are: (1) its location within the 

impoundment, which makes access very difficult, and (2) large impact forces at the point of directional change from the inlet to the transport section. With regard to the latter, the effect of forces due 

to falling water must be accounted for by providing a curved section at the base of the vertical riser 

and/or constructing a concrete pad to distribute impact loads to the underlying soils.

Figure 9.39 presents an example of a decant system installed at a diked-impoundment facility. In 

this example, a flexible pipe serves as the decant with the riser extending vertically up into the 

impoundment. A water return line to the coal preparation plant is installed parallel to the decant 

line, and the impoundment water level is controlled by a pumping system. The decant inlet is positioned so as to provide for storage of the design storm runoff, which for a diked impoundment is 

predominantly the precipitation falling upon the impoundment surface. Note that the water return 

line, as would the case for any conduit passing through the embankment, must be designed with 

seepage control measures.

Figure 9.40 shows an example of a decant system installed at a cross-valley impoundment, where 

multiple risers, or inlets, extend from the transport section of the flexible pipe. In this example, a 

pumping system is used to maintain the impoundment water level, and the decant serves to rout significant storm runoff through the facility. As the settled slurry accumulates within the impoundment, 

the lower riser inlet is sealed and the next upper riser inlet is fitted with a trash rack for operation. In 

the example system shown in Figure 9.40, a seepage interception zone has been installed within the 

backfill for the decant system.

9.7.4.2 Overflow Spillway Systems

When the design storm is not stored in the impoundment at coal refuse disposal facilities, overflow spillways that protect downstream life and property by safely discharging the accumulated 

runoff from large storms are the most crucial hydraulic systems. They frequently must be designed 

to convey substantial flows over steep terrain. The vital importance and design complexity of these 

structures combined with the complications of a constantly changing embankment configuration, 

dictates that spillways for coal refuse disposal facilities be viewed differently than spillways for other 

types of impoundments.
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FIGURE 9.38 EXAMPLES OF DECANTS

COAL REFUSE

EMBANKMENT

SETTLED 

SLURRY

SETTLED

COAL REFUSE SLURRY

EMBANKMENT

BLOCKED EXTENSION FOR

FUTURE DECANT INLET

SEALED DECANT INLETS

FROM PREVIOUS EMBANKMENT STAGES

9.38a DECANT INLET ON SLOPE AT SIDE OF IMPOUNDMENT

COAL REFUSE

EMBANKMENT

SETTLED SLURRY

9.38c DECANT INLET AT UPSTREAM END OF IMPOUNDMENT

COAL REFUSE

EMBANKMENT

SETTLED

SLURRY COAL REFUSE

EMBANKMENT

9.38d VERTICAL DECANT INLET

9.38b DECANT INLET ON EMBANKMENT SLOPE

FIGURE 9.38 EXAMPLES OF DECANTS
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Hydrology and Hydraulics

MAY 2009FIGURE 9.39 EXAMPLE DECANT SYSTEM AT DIKED IMPOUNDMENT

28"  DECANT

RISER (INLET EL. 650)

9.39a PLAN

STAGE VIIIA 

CREST 

EL. 658

PUMP FOR PLANT WATER

USE AND FLEXIBLE LINE

ON FLOATING PLATFORM

IMPOUNDMENT

FILTER/DRAINAGE

DIAPHRAGMS

DEPOSITED

FINE REFUSE

8"  SDR 15.5 - CLARIFIED WATER DISCHARGE

22"  SDR 15.5 - STORM WATER DISCHARGE

DECANT OUTLET

INVERT EL. 585.5

DEPOSITED

FINE REFUSE

9.39b DECANT RISER DETAIL 9.39c TRENCH DETAIL

TRASH RACK

CONCRETE FOUNDATION

CONCRETE SAND

22"  SDR 15.5 POLYETHYLENE PIPE

28" FLANGE ADAPTER (SDR 15.5)

24" TEE

(SDR 15.5)

Bd

6"

COMPACTED COARSE

COAL REFUSE

6"

CONCRETE SAND IN

IMPOUNDMENT AREA

MINIMUM Bd = PIPE O.D. + 24”

Most transport sections for open-channel spillway systems at impounding disposal facilities are channels excavated into an embankment abutment. Erosion protection should be provided if the abutment materials in the excavated channel are not durable and could erode, creating stability concerns. 

The spillway channel should extend to a point downstream of the embankment and be appropriately 

lined and have sufficient freeboard to protect the embankment. Examples of spillways that have been 

constructed at impounding disposal facilities are illustrated in Figures 9.41 through 9.43.

Figure 9.41 illustrates a condition where a spillway channel was constructed in an existing valley fill 

and side hill embankment, necessitating a variety of channel configurations and linings. Concrete 

and grouted riprap materials were used in steep channel segments and at changes in flow direction.

FIGURE 9.39 EXAMPLE DECANT SYSTEM AT DIKED IMPOUNDMENT
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FIGURE 9.40 EXAMPLE DECANT SYSTEM AT CROSS-VALLEY IMPOUNDMENT

9.40a PROFILE

SEEPAGE

INTERCEPTION

ZONE

OPERATING

DECANT RISER

SEALED

DECANT RISER

INITIAL

STAGE

DECANT PIPE

PROTECTIVE COVER

COMPACTED TYPE A

CONCRETE SAND

AASHTO NO. 8

COARSE AGGREGATE

9.40c SECTION B - B

PROTECTIVE COVER

COMPACTED TYPE A

CONCRETE SAND

PERIODIC PIPE SUPPORT

FLOWABLE FILL

CRADLE

9.40c SECTION A - A

9.40b PLAN

C

C

A

A 4"  PVC COLLECTION 

PIPE (SLOTTED)

4"  PVC COLLECTION

DRAIN (SOLID)

END CAP (TYP.) AASHTO NO. 8 COARSE AGGREGATE TRENCH

SEEPAGE

INTERCEPTION ZONE

UPSTREAM

IMPOUNDMENT EMBANKMENT ZONE

D D

FABRIFORM LINING

TRENCH

2"  WEEP HOLES

DECANT RISER

CONCRETE

ENDWALL

26"  HDPE PIPE, DR11 FLOW

COMPACTED TYPE A

CONCRETE SAND

B

B

9.40f SECTION D - D

PROTECTIVE

COVER

26" HDPE PIPE, DR 11 O

COMPACTED TYPE A

CONCRETE SAND

BOLT ON TRASH RACK

OR TEMPORARY STEEL

PLATE WITH VENT

FLOWABLE

BACKFILL

DECANT 

RISER 26"  HDPE

PIPE, DR 11

TERRACE EXCAVATION

FOR FLOWABLE FILL

AT RISER

PROTECTIVE COVER

PERIODIC PIPE SUPPORT

9.40e SECTION C - C

FLOWABLE

BACKFILL

FLOWABLE FILL

CRADLE

FIGURE 9.40 EXAMPLE DECANT SYSTEM AT CROSS-VALLEY IMPOUNDMENT
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Figure 9.42 illustrates a condition where the height of the natural abutment for a new refuse embankment was only slightly higher than the planned embankment crest. A spillway was constructed by 

making an excavation through the abutment so that storm flows would discharge into an adjacent 

valley and the refuse embankment would not be endangered.

Figure 9.43 illustrates a cascade spillway system. A new spillway at the higher elevation was excavated into a hillside to direct flow to a point above the beginning of the lower spillway. The plunge 

pool in the lower spillway dissipates the energy of the cascading water and turns the direction of flow 

downhill and away from the embankment.

The design capacity for spillway systems is usually determined by the reservoir routing procedures 

presented in Section 9.8. Spillway design involves selecting the control location and method to achieve 

the required flow capacity while fully utilizing the available storage capacity.

Figures 9.44 and 9.45 show typical inlet and outlet controls for spillway systems. To determine the 

most appropriate control for a particular situation, the following factors should be considered:

Inlet Control – Inlet control for spillway systems is desirable when it is important to minimize 

the size of the transport channel, including the following situations:

• The length of the channel downstream from the inlet that can be economically 

constructed is limited (flow will be supercritical downstream from the inlet).

• The area for construction of a downstream transport section is limited due to terrain instability (hilly and steep terrain).

• Substantial storage capacity is available in the impoundment and economies can 

be realized by reducing the size of the transport section or channel (encountered 

at some large facilities sited in long valleys).

• Competent, erosion-resistant material is available at the inlet of the system but 

not at the outlet (inlet control is more easily accomplished).

Outlet Control – Outlet control for spillway systems is desirable when it is important to minimize 

the velocity of flow or the grade of the transport section, including:

• The upstream flow in the transport section must be maintained at subcritical 

velocities to minimize erosion of soft channel materials. An overflow weir located 

at the downstream end of the transport section or channel can be used to create 

this condition.

• Competent materials that provide erosion resistance are present at the outlet.

• A natural overfall occurs at the outlet, where the water discharges by free fall 

without causing significant damaging erosion.

Regardless of the point of control, the primary requirement of spillway system design is that the discharged flow not adversely affect the safety of the overall coal refuse disposal facility. Section 9.7.2.3

discusses the types of outlets available for either safely discharging the flow away from the facility or 

for dissipating the flow energy with a stilling basin. Generally, the spillway discharge point is located 

a sufficient distance away from the embankment so that a special hydraulic structure is not required. 

The examples illustrated in Figures 9.41 and 9.43 show excavated channels discharging onto steep 

slopes. The major design issue for this approach concerns the frequency and severity of damage that 

might result from such discharge. Discharges should be rare occurrences, and damages should be 

limited to surface erosion of the steep slope without adverse impact to the disposal facility embankment. Also, the discharge point and slope should be within mine property ownership and not be 
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FINE COAL REFUSE

IMPOUNDMENT

1600

1650

1700

1750

1200

1300

1450

1500

1450

1400

1550

CREST OF

EMBANKMENT STREAM

HAUL ROAD

BENCH @ EL. 1375

BENCH @ EL.1250

SPILLWAY

BENCH @ EL. 1500

SPILLWAY

SEE DETAIL 1

1250

1200 1150

1300

1350

1400

1450

1500

1550 1600 B

B

A

A

9.41a PLAN

FIGURE 941 EXAMPLE SPILLWAY SYSTEM A

9.41c SECTION A - A

6' 1

1.5

37'

1 3

COARSE COAL

REFUSE

10'

TOP OF ROCK

2' THICK RIPRAP

EXISTING GROUND

SURFACE

TOP OF ROCK

1

1 5.5' 1

1

21' 3

1

COARSE COAL

REFUSE

9.41d SECTION B - B

1100

1150

1200

1250

1300

1350

1400

1450

SLOPE 10%

BOTTOM EL. 1425 SLOPE 0.5%

BOTTOM EL. 1435

ELEVATION (FT)

9.41b SPILLWAY PROFILE (NTS)

SLOPE 60%

9.41e DETAIL 1 9.41f SECTION C - C

15'

EXISTING

GROUND SURFACE

1 3

3' KEY INTO ROCK

8" CONCRETE SLAB

SET ON ROCK

BOTTOM OF

CHANNEL

20' 9'

1' REINFORCED

CONCRETE WALL

12'

11' 120'

3:1

SLOPE

21'

REINFORCED

CONCRETE WALL

9'

60% SLOPE

10% SLOPE

C C

FIGURE 9.41 EXAMPLE SPILLWAY SYSTEM – A
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Hydrology and Hydraulics

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2 1

2 1

50'

EMERGENCY

SPILLWAY

CL

9.42c SECTION A - A

FIGURE 9.42 EXAMPLE SPILLWAY SYSTEM - B

9.42b SPILLWAY PROFILE

ELEVATION (FT)

800

810

820

830

840

850

860

870

880

890

900

1% SLOPE 2.2% SLOPE

20' CREST AT

EL. 825

700

675

675 700

725

775

750

800

825

850

875

900

850

875

900

925

925

900

875

850

825

800

750

775

800

825 850

875

900

BENCH AT

EL. 725.0

2:1 SLOPE

EMBANKMENT

CREST

EL. 830.0

2:1 SIDE

SLOPE

EMERGENCY

SPILLWAY

A

A

CREST AT

EL. 830.0

20' CREST AT

EL. 825

2:1 SLOPE

9.42a PLAN

FIGURE 9.42 EXAMPLE SPILLWAY SYSTEM – B
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9.43c SECTION A - A

25'

EXISTING GROUND

SURFACE

CL SPILLWAY

1

2

1 2

2

1

1.5

1

TOP OF ROCK

40' CREST 

@ EL. 1660.0

EXISTING SLURRY

IMPOUNDMENT AVERAGE

SLOPE 2.8:1

1800

1675

1650

1625

1575

1800

1775

1750

1725

1700

1675

A

SPILLWAY

SPILLWAY

9.43a PLAN

1775

1500

1750

1725

PLUNGE POOL

1525

1700

1550

1600

A

9.43b SPILLWAY PROFILE

300'

EL. 1647

SPILLWAY

ENTRANCE

375'

EL. 1644

40'

EL. 1570

EL. 1600 EL. 1605

PLUNGE POOL

40'

FIGURE 9.43 EXAMPLE SPILLWAY SYSTEM – C
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IMPOUNDMENT

CULVERT

9.44a CULVERT INLET CONTROL

FIGURE 9.44 SPILLWAY INLET CONTROL

SECTION B - B SECTION A - A

IMPOUNDMENT

WEIR

9.44d WEIR INLET CONTROL

IMPOUNDMENT

9.44c NARROW INLET CONTROL

B

B

A

A

IMPOUNDMENT

9.44b INLET TO STEEP CHANNEL CONTROL

FIGURE 9.44 SPILLWAY INLET CONTROL
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upgradient from structures or public transportation facilities. More difficult to evaluate are property 

and/or environmental damages that might occur below the discharge point due to vegetation loss, 

surface erosion and even local landslides. The following criteria are suggested as design guides for 

preventing impacts to a disposal facility embankment when the discharge of open-channel spillways 

onto slopes is contemplated:

• If the spillway will only be activated during very large storms (i.e., on the order of 

the PMF), special provisions are seldom required at the outflow point because the 

resulting erosion damage typically will not be significantly greater than the erosion 

FIGURE 9.45 SPILLWAY OUTLET CONTROL

IMPOUNDMENT

TRANSPORT SECTION

9.45a WEIR OUTLET CONTROL

SECTION A - A SECTION B - B

IMPOUNDMENT

TRANSPORT SECTION

9.45b OUTLET TO STEEP CHANNEL CONTROL

IMPOUNDMENT

B

B

9.45c NARROW OUTLET CONTROL

TRANSPORT SECTION

A

A

FIGURE 9.45 SPILLWAY OUTLET CONTROL
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Hydrology and Hydraulics

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damage that would have occurred anyway, and the probability of the spillway functioning many times during the operational period of the facility is low.

• If the spillway is expected to be activated as often as every 25 years or more frequently, special provisions should be made to transport the flow without excessive

erosion, or the flow should be discharged through a separate system.

High velocity flow in open channels represents a significant source of energy that must be controlled for safe 

water conveyance. Special considerations for the design of excavated channel spillway systems include:

• Initial planning and design should account for the manner in which the spillway 

system may need to be extended or modified as the disposal facility increases in size.

• The channel materials (natural or constructed) must be resistant to erosion

 (Section 9.7.3.1).

• The stability of the excavated slopes forming the channel must be evaluated

 (Section 6.6.5).

• Potential erosion at directional changes, particularly during supercritical flow, must 

be evaluated and accounted for in the channel design (Section 9.7.3.3).

• Rigid spillway linings must be designed to resist the development of hydrostatic and 

flow-induced uplift pressures.

• Sufficient freeboard to contain the discharge within the spillway channel must be 

provided.

An important consideration in the design of rigid spillway linings is uplift pressures. For spillways 

with rigid lining, uplift is typically estimated based on the hydrostatic head associated with the 

normal pool level applied at the upstream end, varying linearly to the hydrostatic head associated 

with the downstream flow depth (tailwater) level. Where there is a potential for open, offset joints 

in steep spillway chutes, the velocity may be converted to dynamic pressure. While the theoretical 

maximum dynamic pressure should be calculated (e.g., the stagnation condition where all velocity is 

converted to pressure), surface effects will limit the dynamic pressure. The USBR (2007b) presents the 

results of testing for a range of flow velocities, joint widths, and offset dimensions, which is intended 

to be a refinement of the estimation of associated potential uplift forces where open joints are a 

concern. However, the uplift associated with the impoundment level must be estimated separately 

based upon site-specific conditions. For example, the uplift pressure for dynamic forces is typically 

50 to 75 percent of the stagnation pressure for joint offsets ranging between 1⁄8 and 3⁄4 inch (e.g., at an 

average flow velocity of 35 feet per second, measured pressure was approximately 10 feet of head as 

compared to a 14-foot stagnation value for a 1⁄2-inch offset and 1⁄4-inch joint). Such uplift forces should 

be considered in situations where lining failure would trigger impoundment failure. Measures for 

reducing or controlling the potential development of uplift pressures include:

• Grouting to control foundation seepage, where applicable.

• Installation of an underdrain system consisting of perforated pipe within a graded 

sand and gravel filter to control seepage.

• Use of rigid foam insulation between the concrete spillway and underdrain system 

to prevent freezing in cold climates.

• Installation of embedded waterstops in floor joints to prevent the flow of water 

through the joints to the foundation.

• Use of longitudinal reinforcement and transverse cutoffs at joints to prevent relative 

displacement.

• Increasing the weight of the channel lining.
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For channels on rock foundations where the above measures may not be effective, structural 

design of the channel lining with anchors to resist uplift pressure should be employed.

9.7.4.3 Diversion and Collection Ditches

Diversion and collection ditches are important because the collection and control of runoff from a 

refuse embankment surface, from the slopes of an embankment, and from hillsides draining to and 

away from the embankment should minimize environmental damage to downstream waters, prevent 

damage to the embankment, and reduce maintenance and repair efforts related to site erosion. This is 

particularly true when a refuse disposal facility is reclaimed and the drainage facilities must function 

with limited repair for a long period of time. Basic procedures for designing diversion and collection 

ditches are the same as those for other types of channels, as discussed in Sections 9.7.1 and 9.7.3.

The following are the most important considerations in the design of drainage channels:

• Diversion ditches should be designed to reduce the amount of water reaching the 

disposal facility during moderate storms when it can be shown that they will appropriately reduce operating or environmental concerns. It is seldom practical to design 

diversion ditches that will not fail during very large storms such as the PMF.

• The constantly changing size and configuration of refuse disposal facilities often 

makes it necessary to provide short-term runoff diversion and collection ditches at 

intermediate development stages. Erosion protection criteria for these structures 

must be established on a case-by-case basis and may differ from the erosion protection procedures for more permanent ditches and channels.

• When a diversion or collection ditch will later serve as a permanent channel, the 

capacity and type of erosion protection material should meet the requirements for 

the permanent installation.

• When a collection ditch will also serve to route the design storm discharge from an 

impoundment outlet (by receiving the discharge from a decant or open-channel spillway), its capacity and durability should be based on the impoundment design storm. 

In this instance, the concern is that the collection ditch must be able to function without damage to the integrity of the embankment for storms up to the impoundment 

design storm.

9.7.4.4 Culverts

Culverts are typically used beneath access roads that permit runoff from minor storms to pass without loss of road use. The appropriate design storm for such culverts depends on the importance of 

the roadway to the overall operation and the effort and cost of repairing the road if it is overtopped 

or washed out. An exception might be a roadway to an impounding embankment that must remain 

open during or immediately following a large storm. However, the design criteria established for 

impoundments in Section 9.5 are based on the assumption that it is not practical to maintain access to 

the impoundment during that time period.

The basic design requirements for culverts were previously discussed in Sections 9.7.1 and 9.7.2.

9.7.4.5 Natural Streams

A natural stream flowing adjacent to a coal refuse embankment can cause significant damage if 

the water at flood elevation can reach the refuse embankment toe and cause erosion. Flow in 

natural streams can be determined based on the principles of open-channel flow (Section 9.7.2.2), 

although the analysis to determine maximum flow depth may be more involved due to the irregu-
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lar cross sections of natural streams. Computer software for estimating the flood levels along natural streams is readily available.

As with constructed channels, the flow in a stream can be controlled by upstream or downstream 

conditions, depending on the stream slope, configuration and discharge. Normally, the flow may be 

assumed to be uniform and Manning’s equation (Equation 9-17 or 9-18) can be used to approximate 

the flow depth and velocity, except immediately upstream of bridges, road embankment crossings, 

natural channel constrictions, etc. that cause backwater effects requiring the use of open-channel 

profile analysis for determination of flow depth and velocity. The roughness coefficient n for natural 

streams may be in the higher ranges shown in Table 9.13 due to vegetation including trees, variations in alignment, and the irregularity of cross sections. The hydraulic radius can be calculated in a 

manner similar to that for a constructed channel, except that the areas and wetted perimeter must be 

determined from topographic maps or cross-section drawings.

Based on the peak runoff to a natural stream, the approximate water depth can be calculated. If the 

computed flow depth indicates that the water in the stream could encroach upon the refuse embankment toe, provisions to prevent embankment erosion or disruption of the facility hydraulic system 

should be employed.

9.8 RESERVOIR ROUTING

As part of the design of the hydraulic system for an impounding refuse disposal facility, reservoir 

routing analyses are typically performed to determine the outlet spillway discharge and impoundment storage requirements. This is critical for open-channel spillway systems, but is also important 

for sites that rely on storage of the design storm and discharge of the runoff through the decant, 

because these facilities have 10 days to discharge the storm inflow in accordance with the criteria presented in Section 9.5. The methodology and key parameters for routing analyses are described herein, 

and references to the computer software typically employed are provided. For specific applications, 

frequent reference is made to texts and publications on hydraulic design and engineering such as 

Chow (1959), USBR (1987a), and the USDA National Engineering Handbook (1956).

9.8.1 Basic Routing Methodology

Reservoir routing is performed by analyzing the inflow hydrograph (Section 9.6), the storage capacity of the impoundment (Section 9.7), and the discharge-head relationship for the spillway outlet 

(Section 9.7) to determine the reservoir level and spillway outflow hydrograph. The spillway outlet 

may consist of a conduit system (decant or principal spillway), emergency open-channel spillway, or 

a combination of conduit and open-channel spillway. Flood routing analyses should be based upon 

an initial impoundment water level no lower than the lowest functional decant inlet. For impoundments that rely solely on a conduit system, the majority of the runoff from the design storm must be 

stored within the impoundment because of the limited discharge capacity of the conduit. However, 

the conduit must be capable of discharging 90 percent of the runoff within a 10-day period following 

the design storm.

Impoundments with an emergency open-channel spillway generally have significant discharge capacity, and thus less of the design storm runoff must be stored within the reservoir. The open-channel 

spillway inlet will be above the normal pool level, such that some initial storage or accumulation of 

runoff from the inflow hydrograph occurs before outflow through the open-channel spillway is initiated. Subsequently, the spillway discharges at a rate dependent on the reservoir level, which in turn 

is a function of the inflow hydrograph and storage capacity. After the peak of the inflow hydrograph 

passes, the reservoir level will continue to rise until the inflow rate and spillway outflow capacity are 

equal. Thereafter, the reservoir level will decline as spillway discharge becomes predominant. This 

relationship is shown in the hydrographs presented in Figure 9.3. The development of the reservoir-
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storage relationship is discussed in Section 9.6.1.4, and the spillway-discharge relationship is discussed in Section 9.7.2.

USBR (1987a) and the USDA National Engineering Handbook (1956) present mathematical procedures 

for computation of flood routing, using an iterative process to arrive at the outflow hydrograph. 

Computer programs such as HEC-1 and HEC-HMS are frequently employed to perform the routing 

analysis USACE (1998b, 2000).

9.8.2 Basic Routing Parameters

An important factor that differentiates coal refuse disposal facility impoundments from other types 

of impoundments is that the embankment and impoundment configurations continually change 

with both the disposal of coarse refuse on the embankment and the disposal of fine refuse slurry into 

the impoundment. These effects must be accounted for in the hydraulic design and reservoir routing 

for a refuse disposal facility.

Figure 9.46 presents a sectional view of an impoundment facility illustrating features that impact the 

routing of floods and the development of design parameters, including normal pool elevation, minimum pool level, surcharge storage, normal freeboard and design-storm freeboard.

The design-storm freeboard for an impounding embankment is a function of the wave height and 

the wave run-up conditions at the upstream face of the embankment. Guidance for the evaluation 

of design storm freeboard for reservoirs is presented in USBR (1987a). Coal refuse impounding 

embankments are typically required to have a minimum design storm freeboard of 3 feet above 

the maximum reservoir pool level associated with the design storm, consistent with a fetch of less 

than 1 mile.

Since the crest elevation of a slurry impoundment can change frequently, the facility plans and 

specifications should include a graph or table that shows the maximum allowable normal pool 

level and allowable spillway and decant levels for each stage of operation. To ensure that adequate 

freeboard is available to handle the design storm, the normal pool level and spillway inlets must 

not be raised until the appropriate crest elevation has been reached. The disposal of fine coal refuse 

within an impoundment affects the reservoir storage capacity. While most of the accumulation 

occurs below the impoundment operational water level, the slurry discharge results in the buildup of deposits forming “deltas” or “beaches” above the pool level. While this impact is frequently 

insignificant for many valley-fill type impoundments, it can have an impact on routing and freeboard particularly at diked-type facilities. This loss of storage capacity is generally estimated based 

DECANT

INLET

DESIGN-STORM FREEBOARD

NORMAL POOL

ELEVATION

NORMAL

FREEBOARD

SURCHARGE

SETTLED FINE

REFUSE 

EARTH OR

COARSE REFUSE

MINIMUM POOL LEVEL

CONTROLLED BY PUMPING

REFUSE

EMBANKMENT

FIGURE 9.46 SECTIONAL VIEW THROUGH IMPOUNDING FACILITY
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upon the position and elevation of the planned slurry discharge and an assumed slope of the 

deposit (typically between 1 and 3 percent).

9.9 DAM BREACH ANALYSIS AND INUNDATION MAPPING

9.9.1 Background

Impoundments are assigned a hazard-potential classification based on the consequences to downstream workers, residents and property in the event the dams were to fail. FEMA (2004a) classifies 

dams as “high hazard potential,” “significant hazard potential,” or “low hazard potential,” and the 

states typically have comparable classification systems. A high-hazard-potential dam or embankment is one whose catastrophic failure would likely result in loss of human life. A significant-hazard-potential dam or embankment is one whose failure would not be expected to result in the loss 

of human life, but could cause substantial property damage. Minimal property damage would be 

expected from the failure of a low-hazard-potential dam or embankment. As discussed in Section 3.1, 

MSHA requires evaluation of the hazard potential for other impoundment breach pathways, such as 

breakthrough into underground mines.

In practice the hazard-potential classification for a dam may be apparent from site conditions; for 

example, a large impoundment located upstream from a populated area will likely be classified as 

having a high hazard potential. To aid in determining the hazard-potential classification and to assist 

in preparation of an Emergency Action Plan, a dam-breach analysis is performed to determine the 

downstream consequences of a dam failure.

An EAP should be prepared for high- and significant-hazard-potential dams and embankments, so that 

procedures are in place for responding to an emergency at the dam and to conditions in the potentiallyinundated area downstream (Chapter 14.0). This is a requirement in several states, and MSHA encourages EAP preparation for high- and significant-hazard-potential dams in order to protect the public that 

would potentially be affected by a dam failure (MSHA, 2007). An important step in the preparation of 

an EAP is to perform a dam-breach analysis so that the potentially-inundated area downstream from 

the dam or refuse embankment can be defined. Scenarios for dam failure, methods of analysis, software 

used for analysis, data requirements, and other aspects of the dam-failure flow release and determination of the resulting inundated area are discussed in the following paragraphs.

9.9.2 Failure Scenarios

Dam failures can occur in a number of ways, but most result from: (1) overtopping of the dam due 

to inadequate spillway capacity, (2) failure of the dam structure as the result of an earthquake, or (3) 

the flow of water through the embankment leading to development of a breach in the embankment 

(i.e., “piping”).

The first scenario is the most common, as there are many existing dams in the U.S. that do not have 

a spillway capacity adequate to handle the most extreme rainfall events. Consequently, states are 

requiring upgrades at these facilities to meet current requirements. These upgrades typically include 

such remedial measures as raising the dam crest and increasing spillway capacity. In some cases, 

measures such as armoring the dam with roller-compacted concrete are employed to allow overtopping to occur while the structure remains intact.

Piping failures result from pathways through a dam embankment where seepage gradually increases 

transport of fine materials until a point is reached where pore pressures are high over a relatively 

large area and a breach initiates. Spillway/decant pipes extending through a dam embankment are 

vulnerable locations for this phenomenon to occur, and care must be taken to minimize the potential 

for seepage flow along these structures.
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A major earthquake can result in an increase in soil pore pressure and sliding failure, usually in the 

upstream portion of the embankment. The crest of the dam drops during the embankment failure, 

resulting in a breach. Upgrades to prevent this type of failure typically require major and very costly 

repairs.

Catastrophic dam failures can also result from causes such as landslides, foundation failure, sabotage 

or damage to operational equipment.

While some technical studies of breach formation have been carried out (Wahl, 1998), the composition of earth embankment dams is highly variable and there is little actual data available for calibrating model results. Thus, breach development has not been accurately related to a specific dam failure 

scenario.

At the present time, modeling of potential dam failures usually involves two basic scenarios. The 

first is overtopping of the dam during high flow (Inflow Design Flood) conditions and the second is 

catastrophic failure of the dam on a day with normal (sunny day or fair weather) flow conditions. 

These two scenarios provide a reasonable representation of the range of conditions resulting from the 

possible failure modes.

Coal refuse disposal facility embankments are developed in stages over several years, ultimately 

resulting in a massive structure sometimes 1,000 feet or more in width and several hundred feet high. 

Dam breach analyses should consider intermediate stages (which may have a narrower embankment 

cross section) as well as the final facility configuration, because the failure of an intermediate stage 

could occur more rapidly and could result in a greater breach flow and thus more significant downstream inundation than breach of the facility in its final configuration.

An important consideration for coal refuse slurry impoundments is the volume of fine coal refuse 

that could be released during a hypothetical dam failure. Based upon data from a wide range of tailings dam failures, Vick (2000) estimates that, while on average about 25 percent of the impounded 

contents are released, the release of impounded contents can approach 100 percent. Rico et al. 

(2008) evaluated historical records of tailings dam failures and releases to identify factors affecting the runout distance and peak discharge, finding that, on average, one-third of the tailings and 

water (to the post-failure level) was released. Overtopping from floods appears to mobilize and 

release more tailings.

While considerable uncertainty exists, the above estimate appears to be consistent with observations 

reported by Owens (2008) for incidents at coal refuse impoundments. The settled fine coal refuse 

frequently remains in a soft or loose condition until sufficiently consolidated and thus may be in a 

flowable state. Michael et al. (2005) performed a literature review to evaluate the ability of fine coal 

refuse to flow. While no specific test data are available for fine coal refuse, evaluation of other tailings 

materials led them to conclude that saturated refuse may be susceptible to high pore pressure and 

static liquefaction when containment is breached. In order to conservatively estimate the amount of 

fine coal refuse that could be released from a dam breach, some states prescribe consideration of all 

water, slurry and settled fine coal refuse contained within the impoundment from the breach invert 

to the crest, when computing downstream flows. Adoption of a reduced volume of settled fine coal 

refuse will generally require site-specific information concerning the consistency and resistance to 

flow of the material.

In addition to failure of the dam, breakthrough of the impoundment into an underground mine may 

represent another type of release pathway. This pathway could lead to significant discharges in streams 

in other watersheds, depending on the alignment and extent of extraction of the underground mine.
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9.9.3 Analytical Methods

The failure of a dam results in a condition referred to as rapidly varied unsteady flow. This is a very 

complex flow condition that can be modeled with computer software. However, programs that provide the most sophisticated modeling of rapidly varied unsteady flow can be difficult to use. Thus, 

the popular programs used for dam breach analysis represent simplifications to some degree of rapidly varied unsteady flow analysis. A listing of software frequently used for dam breach analysis and 

the analytical approach employed is provided in Table 9.20.

HEC-1 is frequently used for dam breach analysis for mine impoundments, and the other software 

listed in the table are less commonly employed. In terms of sophistication, HEC-RAS, DAMBRK and 

FLDWAV are the most technically advanced and should provide more accurate results than HEC‐1. 

HEC-1 and HEC-RAS were developed by the USACE Hydrologic Engineering Center, while the latter 

three programs in the preceding table are National Weather Service programs.

HEC-RAS (River Analysis System) is a second generation program from the USACE Hydrologic 

Engineering Center and is the successor program to HEC-2. It was first released in 1995 and gained 

unsteady flow analysis capabilities in 2000. The unsteady flow portion of the program was adapted 

from UNET. The program has the capability of modeling mixed flow regimes and can account for 

channel constrictions and off-channel storage.

DAMBRK was developed by Fread (1988) for modeling unsteady flow associated with dam breaches. 

FLDWAV, which was introduced in 1998, is the successor to DAMBRK and DWOPER and provides 

advanced capabilities over both programs. SMPDBK is based upon an approximate methodology, and 

under some circumstances can provide results within 10 percent of the results provided by DAMBRK.

In terms of modeling accuracy, both HEC-1 and SMPDBK have clear limitations. The accuracy of 

these programs diminishes in situations involving channel constrictions and resulting backwater. 

The final version of HEC-1 was released in 1998, and the program has been replaced by HEC-HMS, 

which offers one-dimensional kinematic wave routing for dam breach analyses. This methodology 

does not account for inertial and pressure forces, and the energy slope is assumed to be equal to the 

channel slope. Thus, HEC-HMS is best suited to relatively steep channels and urban areas where 

natural channels have been modified to regular shapes and constant slopes.

FLDWAV is the most sophisticated program currently (2009) available. It utilizes finite-difference 

approximations to solve the Saint-Venant equations for one-dimensional unsteady flow and can 

account for natural features such as off-channel storage and channel constrictions. The program is 

capable of handling a wide range of channel configurations and data input. However, FLDWAV 

requires some calibration for optimum accuracy. Other programs such as HEC-RAS and predecesTABLE 9.20 INUNDATION ANALYSIS SOFTWARE

Program Method of Analysis

HEC-1 Muskingum-Cunge

Modified Puls

HEC-RAS One-Dimensional Unsteady State Flow

SMPDBK Approximate Method

DAMBRK One-Dimensional Unsteady State Flow

FLDWAV One-Dimensional Unsteady State Flow
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sors to both HEC-RAS and FLDWAV are capable of providing adequate results depending upon the 

nature of the breach, outflow hydrograph and downstream channel configuration.

GIS-based software is gaining in popularity in hydrology and hydraulics applications and has been 

used in combination with unsteady flow analysis software. WMS (a GIS-based hydrologic model) can 

be used in conjunction with software such as HEC-RAS and SMPDBK. BREACH and FLDWAV have 

reportedly been used in combination with GIS-based software to assess dam breaches and inundation mapping. Also ESRI, the developer of ArcGIS, and the USACE Hydrologic Engineering Center 

have worked together to create HEC-GeoRAS, which allows the results of flood routing analyses to 

be displayed in a GIS environment. Eventually, GIS-based models, either packaged with or used in 

combination with sophisticated unsteady flow analysis software, will be the accepted approach for 

dam breach modeling and presentation of results.

Another issue related to software selection is the available topographic data for the analyses, the level of 

accuracy required, and the user’s familiarity with the software. It is common to obtain topographic and 

cross-section information from USGS quadrangle maps with some field observation and verification. 

Generally, a high degree of accuracy is not required for defining inundation limits and for identifying 

potential evacuation requirements, particularly in remote areas. Thus selection of less sophisticated 

software for EAP development is quite often adequate. However, use of a breach analysis to support 

a hazard-potential classification other than high hazard potential may require careful evaluation of 

the assumptions incorporated into the software. As discussed previously, communication should be 

maintained with dam safety regulators (both state and federal) relative to software usage.

9.9.4 Input Data

9.9.4.1 Breach Parameters

A breach in an earth embankment dam is generally assumed to be trapezoidal in shape and can be 

defined by depth, width, side slopes and development time. A comprehensive study of breach development was carried out by Wahl (1998) for earth dams. While an accurate depiction of breach development is desirable, the process is highly variable and difficult to predict, and conservative assumptions 

based on published guidelines are normally accepted. Discussions with agency personnel should 

prove valuable in this regard. In situations where the downstream channel is relatively broad in comparison to the volume of the release, the use of conservative breach development geometry and time 

may not matter, as discussed in Section 9.9.6. Some state agencies require that the release include the 

entire volume of tailings. When a site-specific evaluation is performed, it should be compared with 

the following guidance for estimating the minimum volume of release for fine refuse:

• An equal volume of fine refuse (as stored) and flood water

• One-third of the fine refuse stored

The volume of fine refuse released can then be used with other site-specific factors to estimate the 

breach depth and configuration and duration of outflow. Some guidelines for determination of 

breach parameters (FERC, 1993) are presented in Figure 9.47. Physical limitations such as the width 

and depth of the valley should be considered when applying the guidelines.

Programs used for routing dam breach flows (HEC-1 or HEC-RAS, DAMBRK, FLDWAV, etc.) typically have input parameters for defining the breach geometry and development time. These programs expand the size of the breach from zero to the full dimensions in the specified development 

time using an internal algorithm.

One software program for breach development is the National Weather Service (NWS) program 

BREACH, which was developed by Fread (1988). This program is a physically-based breach simula-
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tion model, but concerns regarding the model have been raised (Wahl, 1998). The hydrograph calculated by BREACH can be used as the dam breach hydrograph by unsteady flow modeling software. 

Coal refuse impoundments can include massive embankment stages, such that an intermediate 

embankment stage configuration may represent a more critical breach geometry and time of failure 

than the final development configuration. Modeling a partial failure of a coal refuse dam using programs such as SMPDBK may result in a peak outflow discharge that is higher than that from breaching 

the full height of the dam all the way down to the natural valley bottom. Since slurry impoundments, 

especially upstream-construction dams, often contain a considerable amount of consolidated slurry 

and have a relatively small water storage volume as compared to conventional water supply, flood 

control, or multipurpose dams, this may be more representative of the actual consequences of a 

slurry dam failure. However, some state regulatory agencies require that all of the saturated fine coal 

DAM CREST INITIAL BREACH

APPROXIMATE

FINAL BREACH

CONFIGURATION

BR

1

Z Z

1

FIGURE 9.47 BREACH PARAMETERS

HD

(ADAPTED FROM FERC, 1993)

FOR NON-ENGINEERED DAMS OF SLAG, REFUSE MATERIALS:

BR 0.8 x CREST > LENGTH

1 Z 2 > >

0.1 >TFH 0.3 HOUR >

FOR ENGINEERED EARTHEN DAMS:

>

0.25 Z 1 > >

0.1 >TFH 1.0 HOUR >

HD BR 5HD >

WHERE:

BR = AVERAGE BREACH WIDTH

TFH = TIME TO FULLY FORM BREACH

Z = HORIZONTAL COMPONENT OF BREACH SIDE SLOPE

HD = HEIGHT OF DAM

FIGURE 9.47 BREACH PARAMETERS
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refuse be treated as flowable and that a breach analysis be based upon a breach extending the full 

height of the embankment (from the final crest to the foundation).

9.9.4.2 Initial Conditions

Generally, the flow into an impoundment and in the channel downstream from the dam is assumed 

to be constant and equal to the design storm condition immediately prior to dam breach. Frequently 

the design storm is the PMF, which represents an extreme upper-bound inflow to the impoundment 

that will only rarely be approached (Section 9.5). Sometimes the design storm will be less than the 

PMF, but as long as it exceeds the spillway capacity, the dam could be overtopped and catastrophic 

failure could occur. The impoundment water surface elevation used in the dam breach model should 

be the minimum required for breach initiation.

For a sunny day breach, normal steady-state stream flows into the impoundment and in the downstream channel should be assumed. These can usually be obtained from published stream gage 

data or on-site records. Estimates of flow can be developed based upon channel dimensions and 

slope, estimated roughness and calculated normal flow velocity in the absence of recorded flow 

data. The water surface in the impoundment is typically assumed at normal pool elevation for a 

sunny day breach.

9.9.4.3 Flow Channel Geometry and Roughness

All dam-breach flow models require a description of the downstream channel and floodplain 

geometry (i.e., cross sections) and roughness. Roughness is usually defined in terms of Manning’s 

n, and values for Manning’s n are available in the literature for a wide range of conditions (Chow, 

1959). Typically, out-of-channel flow encounters substantial resistance from brush, trees, debris 

and even dwellings, so that the Manning’s n for the floodplain is much higher than for normal 

channel (USGS, 1989).

A key factor that can cause elevated flood levels is the presence of constrictions such as bridge and 

railroad embankment crossings or severe natural channel narrowings in the reach downstream from 

the dam. These can cause temporary backwater elevation and localized increased flooding. Additional cross section data (i.e., closer cross section spacing) may be needed in these constricted areas.

The presence of an existing downstream impoundment or impoundments may result in the need to 

extend the flow model farther downstream.

Typically for a dam breach analysis under design storm conditions, the area of interest along the 

channel terminates when the flow reaches a certain increment above the flow elevation without the 

dam breach (typically 1 to 2 feet). For a sunny day breach, the area of interest along the channel 

normally terminates when the flow returns from the floodplain to the natural channel or to a level 

associated with a specified recurrence interval flood. Thus, downstream channel data should extend 

past the points where these control points are anticipated to occur.

In addition to natural channels, another pathway for an impoundment breach is via breakthrough to 

an underground mine, with discharge through the mine and out of associated mine openings. This 

can lead to potential inundation in watersheds other than that in which the impoundment is located, 

depending on the size and extent of the mine.

9.9.5 Results of Analysis/Inundation Mapping

The output from flow model software will be water surface elevations and flow velocity at each 

channel cross section in the model. Additionally, the model software can provide the time of 
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arrival of the flood wave from the breach, which is useful for the EAP development. The maximum water surface elevation at each cross section following the dam breach will define the extent 

of inundation. It is important to note that the maximum inundation elevation for an unsteady 

flow analysis does not occur at the same time at each cross section. Therefore care must be taken 

to record the highest level at each cross section and to use these values in determining the limits 

of inundation.

The flow velocity is also computed for each channel cross section, which may be helpful in assessing 

the potential threat to occupied structures or roadway travelers, as discussed in Section 9.5.1.3.2. It 

is important to note that while the average velocity in the floodplain may be provided as part of the 

model output, it is advisable to perform an independent computation of the flood velocity for the 

structure location, considering the maximum water surface elevation and energy grade line.

As discussed in the previous section, the inundation map should extend downstream from the dam 

to the farther of the termination points associated with a breach occurring under design storm and 

sunny day conditions.

Plotting the extent of inundation can be tedious if done manually because the water surface elevation is falling relative to a fixed datum and thus the extent of inundation will not match or be 

parallel to any ground surface elevation contours. GIS-based software can provide plots of the 

inundation limits as part of the normal output and thus eliminate the need for manual plotting.

9.9.6 Sensitivity Analyses

It may be useful to perform multiple dam breach analyses with variations in selected input data to 

evaluate the effect of the variation on the analysis results. For dam-breach analyses, the parameters 

associated with the breach development (i.e., dimensions and development time) are likely to be the 

most controversial. Since a breach analysis can directly affect the safety of downstream residents, it 

is prudent that conservative dam-breach analyses be performed. If, for example, substantially reducing the breach development time does not significantly alter the results in terms of inundation levels 

and extent of inundation, then conservative breach assumptions can be used in the analysis and may 

expedite regulatory review of the EAP.

9.9.7 Hazard Classification

Most commonly, a dam-breach analysis is performed as part of preparing an EAP for a dam that has 

already been classified as having high or significant hazard potential. Another purpose for a dambreach analysis could be to establish the hazard classification in the first place. In this event, the FEMA 

(2004a) or applicable state criteria should be followed. The FEMA criteria are listed in Table 9.21.

TABLE 9.21 HAZARD POTENTIAL CLASSIFICATIONS

Hazard-Potential 

Classification Loss of Human Life Economic, Environmental,Lifeline Losses

Low None expected Low and generally

limited to owner

Significant None expected Yes

High Probable – One or more Yes

(FEMA, 2004a)
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If populated areas are impacted, particularly areas located close to the dam, a high-hazard-potential 

classification should normally be assigned. As discussed in Section 9.5.1.3.2, other criteria, such as 

depth or velocity of flow, may also be considered by designers and accepted by regulatory agencies 

for determining the potential significance of the inundation level and to assign the associated hazard 

classification. Other classifications should be consistent with Table 9.21 and the limits of inundation.
Chapter 10

ENVIRONMENTAL

CONSIDERATIONS

MAY 2009 10-1

Environmental considerations for coal refuse disposal facilities generally involve potential impacts to 

streams and wetlands, air quality, and water quality. Impacts to streams and wetlands originate with 

facility siting. Air quality issues arise from dust and burning associated with coal refuse embankments. Water quality issues are typically related to the generation of acid leachates by coal refuse or 

to erosion and sedimentation at refuse surfaces or disturbed areas under development. Liner systems have been used to provide protection of groundwater, and reclamation of coal refuse disposal 

embankments can mitigate air and water impacts.

Federal and state air and water quality regulatory programs govern site discharges and must be 

considered in coal refuse disposal facility design. Thus, review of applicable regulatory programs 

and permit requirements should precede the design of coal refuse disposal facilities. Similarly, liner 

systems are generally regulated by states.

In light of the above, this chapter provides a general discussion of environmental issues associated 

with coal refuse disposal facility design, construction, and reclamation.

10.1 STREAMS AND WETLANDS

Coal refuse disposal facilities often impact streams and wetlands regulated by the Clean Water Act 

(CWA). This legislation was originally enacted in 1972 and was subsequently amended in 1977. When 

a planned coal refuse disposal facility will impact streams and wetlands, several types of permits 

and certifications may be required by CWA regulations. Although the U.S. Environmental Protection 

Agency (USEPA or EPA) has regulatory authority over the CWA, the permits and certifications may 

be administered and enforced by other federal, as well as state or local agencies. These agencies may 

include the USEPA, the U.S. Army Corps of Engineers (USACE), the U.S. Fish and Wildlife Service 

and state Departments of Environmental Protection (state DEPs).

The CWA was enacted with the intent of restoring and maintaining the chemical, physical and biological integrity of the waters of the United States. The term “waters of the United States” includes 

the following:

1. All waters which are currently used, or were used in the past, or may be susceptible 

to use in interstate or foreign commerce, including all waters which are subject to the 

ebb and flow of the tide;
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2. All interstate waters including interstate wetlands;

3. All other waters such as intrastate lakes, rivers, streams (including intermittent 

streams), mudflats, sandflats, wetlands, sloughs, prairie potholes, wet meadows, 

playa lakes, or natural ponds, the use, degradation or destruction of which could 

affect interstate or foreign commerce including any such waters:

i. Which are or could be used by interstate or foreign travelers for recreational or other purposes; or

ii. From which fish or shellfish are or could be taken and sold in interstate 

or foreign commerce; or

iii. Which are used or could be used for industrial purpose by industries in 

interstate commerce;

4. All impoundments of waters otherwise defined as waters of the United States under 

the definition;

5. Tributaries of waters identified in paragraphs 1-4 of this section;

6. The territorial seas;

7. Wetlands adjacent to waters (other than waters that are themselves wetlands) identified in paragraphs (a) (1)-(6) of this section.

Waste treatment systems, including treatment ponds or lagoons designed to meet 

the requirements of CWA (other than cooling ponds as defined in 40 CFR § 123.11(m) 

which also meet the criteria of this definition) are not waters of the United States.

8. Waters of the United States do not include prior converted cropland. Notwithstanding the determination of an area’s status as prior converted cropland by any other 

federal agency, for the purposes of the Clean Water Act, the final authority regarding 

Clean Water Act jurisdiction remains with the EPA.

Coal refuse disposal facilities that impact waters of the United States must be permitted and certified 

under the federal regulations outlined in the CWA. The various sections of the CWA regulate the 

activities described below:

• Section 401 – Water Quality Certification

This section of the CWA requires that any applicant for a federal permit to construct 

and operate a coal refuse disposal facility that may result in the discharge of any pollutant must obtain certifications for those activities from the state in which the discharge originates. This certification is referred to as the Water Quality Certification 

for the project.

• Section 402 – NPDES Regulations

The 1972 amendments to the CWA established the National Pollutant Discharge 

Elimination System (NPDES) permit program to control discharges of pollutants 

from point sources. The NPDES permit may be administered and enforced by a local 

USEPA branch or state DEPs. Some states have additional requirements for stormwater discharges that may impact planned coal refuse disposal facilities and are not 

covered by the CWA.

• Section 404 – Dredge/Fill Permitting

This section of the CWA established a permit program to regulate the discharge 

of dredged or fill material into waters of the United States. This permit program 

is administered by the USACE under a memorandum of agreement between the 
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Department of the Army and the USEPA. Under Section 404 of the CWA an individual or general permit may be needed based on the proposed activities.

In addition to the Clean Water Act, other statutes and regulations such as the Surface Mining and 

Reclamation Act (SMCRA) of 1977 and the Safe Water Drinking Act (1974) may be applicable to coal 

refuse disposal facilities with respect to streams and wetlands. These regulations may result in additional permitting not covered by the CWA. The Office of Surface Mining (OSM), U.S. Department 

of the Interior, is responsible for the national program to regulate the surface effects of coal mining 

activities, although each state may take on primary responsibility if the state’s regulatory program is 

approved by the OSM.

Consideration should be given early in the design process to the permits and certifications required 

for coal refuse disposal facilities as they relate to streams and wetlands. The time involved in the 

permitting process is typically lengthy and must be accounted for in coal refusal disposal facility 

design. Agencies such as the USEPA, USACE, U.S. Fish and Wildlife Service, state DEP, and local 

municipalities should be contacted prior to permit preparation to determine what permits and certifications will be required and which agencies will administer and enforce them. Once the required 

permits are determined, it may be beneficial to hold a pre-submittal meeting with the appropriate 

agencies. After the meeting, the permit applications should be submitted in a timely manner, allowing for responses to permit application comments. Some states have moved to a combined application process, although generally permit applications are submitted separately and at various times 

during the design process.

10.2 AIR QUALITY

Coal refuse disposal can create two types of air quality problems: (1) fugitive dust and particulate 

matter and (2) noxious gases originating from burning refuse embankments. Fugitive dust becomes 

airborne due to wind and coal refuse handling and placement. Sources may include: emissions from 

haul roads; wind erosion from exposed surfaces, storage piles and spoil piles; reclamation operations; and other material or earth disturbance activities. Fugitive dust can be ingested by humans 

and animals and can also be harmful to vegetation. High concentrations of sulfur dioxide associated with the combustion of coal refuse are toxic to nearby vegetation. Also, sulfur dioxide, organics 

(polynuclear aromatic hydrocarbons such as benzo(a)pyrene), and metals (mercury and arsenic) are 

harmful if inhaled in significant volumes by humans.

Dust is regulated as an air emission by state DEPs or, if no approved state program exists, by the 

USEPA. If amendments are being considered or co-disposal with combustion waste is planned, dust 

control requirements can take on greater significance than with normal construction. If accidental 

combustion occurs at coal refuse disposal facilities, air emissions can become a significant health and 

safety concern, and methods to address burning may need to be developed and implemented as part 

of a remedial action.

The following sections discuss measures for controlling dust and for reducing the potential for combustion or controlling burning should it occur.

10.2.1 Dust Control

The transportation and placement of coal refuse can create a considerable amount of fine particulate matter that is susceptible to wind erosion. Coal refuse is compacted and crushed by machinery 

during placement and further deteriorates through physical weathering and chemical decomposition. When refuse-related dust problems occur, they can be mitigated by stabilizing the surface layer 

of the refuse. This can be accomplished by applying water or a dust suppressant solution over disturbed areas, establishing windbreaks of trees or hedgerows that alter both the direction and the 
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velocity of wind over the refuse material, or performing reclamation by covering and vegetating the 

disturbed surface (Coalgate et al., 1973).

In situations where a relatively quick dust control procedure is needed or where vegetation is for 

some reason impractical, stabilization has been achieved using various commercially available chemical agents. Chemical seals have been accomplished through application of: (1) a lime chip-sodium or 

potassium silicate topdressing over the refuse material, (2) a resinous or bituminous-base adhesive, 

(3) calcium, ammonium and sodium lignin sulfonates and bark extracts, (4) resin and wax emulsions 

or neoprene, and (5) elastomeric organic polymers (Coalgate et al., 1973; Dean and Havens, 1972; 

and Eigenbrod, 1971). When applying such products to areas such as haul roads that will experience 

truck or heavy-equipment traffic, the effect on traction should be considered.

Erosion control mats that have plant seeds incorporated within the binding material have been successfully used to vegetate disturbed construction areas and to control dust.

10.2.2 Combustion Control

Current practices in the mining industry have virtually eliminated coal refuse fires. The reason is 

two-fold. First, the amount of coal in coal refuse has been greatly reduced because of more efficient 

removal of coal during mining and processing. Secondly, current embankment construction practices 

involve thorough compaction of refuse material, thus restricting the flow of air and moisture that can 

create a favorable environment for heat generation. Thus, the discussion provided herein is mainly 

applicable to older existing embankments.

Components of air emissions from burning coal refuse may include carbon, nitrogen, sulfur compounds and metals such as arsenic and mercury. These emissions can impact human health and the 

environment. Air emissions along with elevated temperatures can degrade existing vegetation and 

make establishment of new vegetation impossible.

Coal refuse embankment fires have been caused by spontaneous combustion and in some instances 

from careless burning of trash or other debris. Coal refuse fires have also been intentionally started 

to obtain “red dog” material for use as a road construction base or have been accidentally ignited by 

natural causes such as lightning or forest fires. Historically, the most common cause of coal refuse 

fires has been spontaneous combustion resulting from the self-heating tendencies of coal. The potential for spontaneous combustion is greatly increased if oxidizing materials such as pyrites are present 

and if these oxidizing materials are wet (Coalgate et al., 1973; Mihok and Chamberlain, 1968; Nicholas and Hutnik, 1971).

Self-heating of coal refuse generally occurs due to exposure of organic and carbonaceous materials to 

moisture and oxygen, creating reactions that generate heat. When the generation rate of heat exceeds 

the rate of heat loss, temperatures within a refuse pile can reach the ignition temperature of the 

remaining coal and carbonaceous materials. The generation rate of heat is a function of the concentration of reactants (thermophillic bacteria, carbon and oxygen), surface area of the pile, particle sizes of 

the coal refuse and ambient air temperature (Kim and Chaiken, 1990). When coal refuse is exposed 

to water and oxygen, heat can be generated from the respiration of bacteria up to a temperature of 

about 120 to 170 degrees Fahrenheit (° F), when the bacteria die. Beyond this temperature range, oxidation of carbon and carbonaceous materials has to occur if the ignition temperature of coal (in the 

approximate range of 620 to 788° F for bituminous coal and 842 to 950° F for anthracite coal) is to be 

reached (Maneval, 1969).

In addition to creating air quality problems, burning refuse embankments can also create potentially 

dangerous working situations. The most common of these is the creation of burned-out voids or pockets within the interior of the refuse embankment that can lead to surface cave-ins and/or hazardous

slides. Attempts to extinguish smoldering refuse facilities with water can cause violent explosions if 
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these burned-out voids become filled with pressurized steam. Explosions can also occur in the vicinity 

of burning material as a result of airborne coal dust produced during the handling of coal refuse.

Under current disposal conditions, the likelihood of coal refuse igniting is extremely low because of 

low pyrite and/or coal content. When coal refuse is spread and compacted in lifts in stable embankments, fires rarely occur. Other standard construction practices that should be followed for mitigating combustion potential include:

• Prior to placement of any coal refuse material at a new site, all vegetation and other 

combustible materials should be removed from the area where refuse will be placed.

• All refuse materials with high pyritic and coal content should be compacted as the facility is constructed, and all large rocks should be crushed or removed to a separate location to prevent the creation of air pockets in the embankment (Coalgate et al., 1973).

• If present, waste materials with high pyritic and coal content should be allowed to 

weather separately prior to their placement at a refuse facility in order to lessen the 

chance of a thermal buildup due to oxidation.

• If oxidation is a potential problem, coal refuse facilities should be designed and constructed in a manner that minimizes the amount of exposed surface area in order to 

decrease the air infiltration (Coalgate et al., 1973).

Typically, detection of burning is based upon on-site visual observation (i.e., noting the presence or 

absence of smoke and/or sulfur dioxide fumes). However, there is no inexpensive means of detecting 

overheated refuse materials below the embankment surface prior to their combustion. Methods that 

have been used to detect combustion of conditions leading to combustion include:

• Gas Emission Monitoring – Carbon monoxide (CO) is a by-product of coal refuse 

oxidation and can be detected very early in the oxidation process. Surface monitoring of CO emissions can thus indicate the potential for spontaneous combustion. 

Hydrogen sulfide (H2S) is also a by-product of coal oxidation. Concentrations of this 

noxious gas will be present prior to combustion and can also be detected through 

monitoring (Chamberlain and Hall, 1973; Chamberlain et al., 1970; Guney, 1968).

• Direct Thermal Monitoring – The internal temperatures of refuse embankments can 

be monitored by inserting temperature probes into driven pipes or drilled holes. The 

temperature buildup associated with oxidizing refuse material can thus be profiled.

• Remote Sensing – Thermal and optical images from an airborne platform can be 

used to identify the location, depth, size and propagation of hot spots and fires 

(Zhang et al., 2004). Landsat TM imagery and airborne thermal scanner data have 

been employed in remote sensing studies for measuring ground surface temperatures. The surface temperature data can then be used for estimating the extent and 

depth of coal fires using thermodynamic models.

• Electrical Resistivity Geophysical Survey – Some researchers have employed surface 

DC electrical resistivity for distinguishing burnt sedimentary rock with relatively 

high resistivity from non-impacted sedimentary rock. The burnt rock has a higher 

porosity, more cracks and lower water content, which allows it to be distinguished 

from the non-impacted rock.

10.2.3 Refuse Fire Extinguishment

Extinguishing coal refuse fires is normally not a problem confronting engineers and designers of new 

coal refuse facilities. However, when an existing facility is being modified or added to, fire abatement 
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can be an important part of the engineering and design process. Fire extinguishment can also be a 

critical consideration when a refuse embankment is being prepared for abandonment.

Studies have determined that refuse embankment fires generally burn in a temperature range 

between 600° and 2000° F. It has also been found that once refuse materials have reached a temperature of approximately 200° F, either through spontaneous heat buildup or through heat transfer from 

adjacent areas, they will eventually self-ignite given favorable conditions such as an abundant supply 

of air and moisture (Magnuson and Baker, 1974).

Since the reactions that create heat are inherently variable, no single safe temperature has been 

identified below which heat buildup and refuse ignition will not occur. Ignition temperatures vary 

with each embankment and with location within the embankment and are largely a function of 

available air and the site-specific characteristics of the coal refuse. It is therefore not enough to 

extinguish the burning portion of a refuse embankment. Steps must also be taken to: (1) lower the 

temperature of the refuse below the point of re-ignition and (2) eliminate embankment conditions 

that could lead to temperature buildup and future re-ignition.

Temperatures in coal refuse embankments that are sufficient for combustion have been measured at 

depths of 100 feet or more. However, at that depth the amount of available oxygen is minimal and 

ignition will not occur. If, however, “hot spots” are exposed through the excavation of overburden 

or through some other embankment modification, the additional available oxygen may cause these 

areas to ignite. Critical extinguishment depths are therefore related to site-specific conditions and 

may be affected by future actions that may alter these conditions.

As indicated previously, the most critical concerns facing those attempting to extinguish a coal refuse 

fire are the unique dangers involved in using water and in excavating materials in ways that may 

cause airborne dust. Explosions that can result from such practices can hurl hot debris over nearby 

areas and can lead to failure of the refuse embankment. Similarly dangerous are smoldering internal 

voids created when a refuse embankment burns. These areas of potential cave-in can be extremely 

dangerous to workers and fire fighters alike. Carbon monoxide poisoning is also a danger.

Despite these potential dangers, a number of fire-fighting techniques have proven successful in certain situations. For purposes of discussion these techniques can be grouped into three general categories: (1) physical removal of the burning refuse, (2) quenching and/or sealing by surface treatment, 

and (3) quenching and/or sealing by injection into the burning refuse. These methods are briefly 

discussed in the following paragraphs and are also summarized in Table 10.1.

10.2.3.1 Excavation and Removal

Excavation and removal has historically been the predominant method for extinguishing refuse 

embankment fires (Kim and Chaiken, 1993). This approach has several variations, each generally 

involving the removal of burning materials from the refuse embankment. The removed materials 

may be extinguished by quenching, cooling, and suffocation, or they may simply be allowed to burn 

out. This method can be effectively used when the burning areas are relatively small and accessible 

and when removal activities do not adversely affect embankment stability. Extreme care must be 

taken to minimize airborne coal dust when handling burning refuse materials. This dust can ignite 

and cause violent explosions. Also, any time that equipment is working over burned-out areas, there 

is a danger that large voids created by the fire will collapse under the weight of the equipment. Variations of the excavation and removal approach include:

• Excavation – Small and readily-accessible burning areas can be extinguished by 

removing the burning refuse material from the embankment using construction 

equipment. The removed material can then be extinguished through quenching, or 
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TABLE 10.1 FIRE EXTINGUISHMENT TECHNIQUES

 Method Brief Description Limitations

Physical Removal

Excavation

Burning refuse excavated from embankment; 

extinguished or allowed to burn itself out; facility 

regraded and sealed

•	 Dust and noxious fumes

•	 Access to burning material

•	 Possible cave-ins

•	 Weakens refuse facility

Water cannons

Water cannons used to dislodge and quench 

burning refuse; quenched material replaced and 

recompacted on refuse facility

•	 Source of quenching water

•	 Weakens refuse facility

•	 Potential for dust explosion

Isolation 

Burning zone isolated by excavating trenches; 

burning zone quenched or buried with inert sealing 

material; trenches refilled with inert material 

•	 Access to burning material

Controlled 

burnout

Burning refuse is allowed to burn under monitored 

and controlled conditions

•	 Access to burning material

•	 Duration is uncertain

•	 Weakens refuse facility

Surface Treatment

Blanketing or 

sealing

Entire burning embankment covered with mantle of 

clay or soil; compacted; burning is smothered

•	 Limited to small facilities

•	 Maintaining seal’s integrity

•	 Possible cave-ins

•	 Source of clay or soil

Foam covering

Entire refuse facility is sealed with a commercial 

foam blanket; oxygen denied the refuse; burning is 

extinguished

•	 Facility size

•	 Maintaining a seal

•	 Can’t use where burning is near 

surface

Rice paddy 

technique

Suited for flat refuse areas; dikes constructed 

around perimeter and area flooded; water 

percolates into burning zone; fire quenched.

•	 Supply of water

•	 Possible cave-ins

•	 Slow

•	 Stability

Water sprinklers

Burning refuse facilities are “wet-down” or 

saturated by a system of sprinklers until burning is 

extinguished

•	 Water source

•	 Saturation weakens structure

•	 Reignition possible

Internal Treatment

Multiple wellpoint system

Horizontal insertion of perforated metal piping 

near base of embankment; water injected; pipes 

removed and reinserted in higher strata; process 

repeated for total structure

•	 Source of quenching water

•	 Slow

•	 Weakens refuse facility

Slurry injection

Vertical or angle holes drilled into burning 

embankment at various depths; liquid slurry 

injected into burning voids; steam vent pipes 

inserted; heat reduction monitored

•	 Slow

•	 Stability

it can be allowed to burn at a safe distance from the refuse embankment. Once the 

burning material has been removed from the embankment, the excavated portion 

should be backfilled, regraded, compacted and covered with a sealing material that 

will limit air flow (Coalgate et al., 1973; Jolley and Russell, 1959). A major drawback 

to this approach is that machinery operators may be exposed to large doses of noxious and toxic gases that are dangerous if exposure is prolonged. Health and safety 

monitoring, air monitoring and use of personal protective equipment are required 

for this activity.

• Water cannons – Water cannons similar to those used by fire departments have been 

used to dislodge and quench burning refuse materials when they are near embank-
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ment surfaces. Removal of the quenched material can be accomplished by: (1) 

hydraulic sluicing using a water cannon, (2) excavation by dragline, and (3) loading 

on trucks for dumping elsewhere. For all three alternatives the extinguished material 

should be re-spread and compacted in accordance with facility plans and specifications. The use of this technique is contingent upon the availability of water and the 

stability of the embankment during hydraulic excavation (McNay, 1971).

• Isolation – Burning materials can be isolated from the remainder of the refuse facility 

by cutting trenches around them. To eliminate heat transfer, such excavations should 

be at least 6 feet wide and should extend into the embankment foundation. Once the 

burning material is isolated, it can be extinguished with water, by applying a sealant, 

or by burying under a blanket of non-combustible material. The exposed trench faces 

should be sealed with clay or fine-grained soil to restrict air flow, or the trenches 

should be backfilled with non-combustible material such as soil. To prevent heat 

transfer from the burning portion of the embankment to non-burning areas, sand or 

other heat-conducting material should not be used as backfill (Coalgate et al., 1973; 

Jolley and Russell, 1959).

To mitigate the potential for explosions, excavations into refuse materials that are known or suspected 

to be burning must be performed with extreme care if hot or burning materials will be exposed to 

airborne coal dust and/or moisture. Through monitoring, areas of high material temperature can be 

mapped (if boreholes are used, they should be sealed to prevent airflow). Excavation should be performed in stages and monitored with the intent of avoiding opening up burning areas to moisture and 

coal dust in confined spaces. Work should proceed downwind (from upwind areas) using equipment 

that can operate from above and away from burning areas. Upon completion of the excavation, backfill 

materials should be placed in lifts and compacted, which will minimize the potential for rekindling.

10.2.3.2 Surface Treatment

The methods described in this section require that the embankment be relatively small and have 

accessible slope faces. Basically, surface treatment involves sealing of the entire surface of an embankment to restrict air flow to the fire. The primary problem with surface seals is maintaining them 

until sufficient cooling has occurred to prevent re-ignition. This maintenance period can exceed 20 

years (Kim and Kociban, 1994), which is greater than the effective life of many types of surface seals. 

Common surface treatment methods are described in the following:

• Blanketing or sealing – In some instances, it may be practical to extinguish burning 

refuse by blanketing the entire embankment with about 2 feet of non-combustible 

material such as fly ash, clay or other soil. This cover should be compacted as it is 

applied, thereby smothering the burning refuse. Breaks in the seal can occur through 

water erosion, heat cracks, cave-in of burned-out voids, or even wind erosion (Coalgate et al., 1973; Jolley and Russell, 1959; McNay, 1971; Myers et al., 1966). In extreme 

cases, where the need to extinguish an embankment fire exceeds normal economic 

constraints, commercial foam sprays (e.g., polyurethane) have been applied (Magnuson and Baker, 1974).

• Rice-paddy technique – This procedure is only suited for large, stable, flat-topped 

refuse facilities. Since minimal fumes and dust are created, it is ideal for sites located 

near residential areas. Dikes are constructed around the top perimeter of the burning 

refuse facility and at appropriate intermediate locations. Each diked area or pond is 

then flooded, and the impounded water percolates into the embankment. Draglines 

can be used periodically to stir the bottoms of the ponds to increase the rate of percolation. The use of this fire-abatement procedure is dependent upon an abundant 
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MAY 2009

supply of water and is further dependent upon the ability of the burning embankment to support earth-moving equipment during dike construction (Coalgate et 

al., 1973; McNay, 1971). The impact of dike construction and water irrigation on the 

stability of the coal refuse embankment must be evaluated prior to implementation 

of this method.

• Water sprinklers – In some instances, water sprinklers have been used to wet down 

burning embankments and to provide a continuous supply of water over and 

through the refuse material. The success of this procedure is largely dependent upon 

the hydraulic conductivity of the embankment, and vertical drilling may be required 

to increase percolation into the embankment interior. The saturation of an impounding embankment can be dangerous, as its stability may be greatly reduced (Coalgate 

et al., 1973; Myers et al., 1966).

Surface treatment methods should be implemented sequentially with monitoring of explosion and 

emission hazards, particularly if concurrent or subsequent excavation activities are planned, as previously discussed.

10.2.3.3 Water and Slurry Injection

This approach involves injection of water or slurry into the burning zones under pressure. The 

injected material quenches and smothers the burning material. The use of an injection method can 

offer one or more of the following advantages:

• While usually more expensive on a unit volume basis, injection is well suited to spot 

treatment of smaller burning areas within a larger embankment in contrast to excavation and removal or surface treatment, which require remedial work over a much 

larger area.

• Inaccessible areas on steep slopes can be treated. Pipes can be driven with air hammers while other equipment (mixers, pumps, etc.) can be placed at a nearby level 

location.

• Men and equipment do not have to work directly over burning areas.

There are basically two types of injection methods:

• Multiple well-point system – This procedure entails driving perforated pipes in a 

single horizontal plane near the toe of the embankment and pumping water into 

the pipes. The pipes are placed relatively close to each other (approximately 2 feet 

on center) so that the injected water thoroughly saturates the entire zone. Once the 

burning is extinguished in that zone, the pipes are withdrawn and then re-inserted 

a short distance above their previous location. Water is again introduced to extinguish the fire in this new area. This procedure is repeated until all the burning areas 

within the embankment are extinguished. It should be emphasized that in order to 

minimize the potential for re-ignition of the refuse material, this procedure should 

progress from the bottom of an embankment upward. Because the burning portion 

of the embankment becomes saturated, the use of this method is not recommended if 

stability is an issue.

• Slurry injection – When slurry is injected into an embankment, voids and air 

channels are blocked and air access is restricted (McNay, 1971). The slurries most 

commonly used are suspensions of fly ash, limestone dust, vermiculite, sodium 

bicarbonate or mine drainage sludge in water. Pipes are typically driven vertically 

into the burning zone on 10- to 15-foot centers. Slurry is injected under low pres-
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sure (usually 10 to 15 psi) to depths of 40 feet or more. When the slurry is no longer 

accepted, the pipes are raised and injection is resumed. The interior or deepest portion of the burning zone is treated first in order to prevent further penetration of the 

fire. Injection then progresses toward the surface of the embankment. Because of the 

danger of explosions, open pipes should be inserted next to the injection holes to 

vent steam. Use of cryogenic slurry consisting of liquid nitrogen and granular carbon 

dioxide to enable quick cooling of the burning material has been proposed. Some 

initial testing demonstrating the ability of this approach to lower temperatures over 

an extended period was conducted (Kim and Kociban, 1994).

10.3 WATER QUALITY

As indicated previously, coal refuse facilities can substantially degrade the quality of water in nearby 

drainage courses if they are improperly constructed. In addition to adversely affecting surface-water, 

drainage from refuse facilities can also affect the groundwater. Although a variety of water quality 

problems can be created by coal refuse drainage, the most common effects are: (1) increased turbidity 

and suspended solids and (2) water quality degradation due to acidic leachates (Martin, 1974).

Water pollution problems created by coal refuse can be substantial. Coal refuse leachates can be 

acidic, can contain elevated concentrations of metals such as iron, aluminum and manganese, and 

can also be corrosive. When leachates enter a stream, aquatic environments may be greatly altered 

and desirable organisms may be reduced or eliminated entirely. When refuse leachates percolate into 

the groundwater, aquifers can be significantly impacted. The following sections provide a discussion 

of mine refuse water quality issues and various procedures and techniques for controlling and/or 

mitigating their adverse effects.

10.3.1 Erosion and Sedimentation

Erosion and sedimentation control plans must be submitted to state and local regulatory authorities 

as part of refuse disposal facility designs. These plans typically include a variety of measures for 

diverting drainage from disturbed areas, for controlling erosion, and for removing sediment from 

runoff before release of surface water from the refuse disposal site. As part of these plans, effluent 

monitoring programs are typically established to verify that erosion and sedimentation control measures are effective.

10.3.1.1 Prevention

When coal refuse and earthen materials are exposed to weathering, erosion and sedimentation can 

occur. The following practices can be implemented to minimize erosion and sedimentation:

• Stripping of vegetation from a disposal site should be limited to only the area that 

is needed for construction. Future fill areas should be stripped immediately prior to 

construction.

• Topsoil that is removed from a construction area and stockpiled for future use 

should be stored in a manner that minimizes erosion and should be revegetated as 

soon as possible.

• During the construction process, care should be taken to preserve vegetation on 

areas surrounding the disturbed construction area.

• Collection ditches and sedimentation ponds should be constructed at the downstream end of the construction site.

• All fill material exposed during construction should be graded in a manner that minimizes the potential for runoff over the downstream face of the embankment. This is 

particularly important for the crest and downstream face of the refuse embankment.
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• Completed embankment surfaces should be reclaimed and vegetated as soon as 

practical, while accommodating seepage control measures such as extension of 

underdrains or installation of collection and discharge systems at the embankment toe.

10.3.1.2 CONTROL

Control procedures for reducing the amount of suspended material entering streams are presented 

in the following subsections.

10.3.1.2.1 SEDIMENTATION PONDS

Sedimentation ponds are structures designed to intercept and retain water-borne sediment and 

debris. They are primarily intended for use during construction prior to the establishment of effective vegetation on the disturbed area. Sedimentation ponds should be sized and constructed in accordance with criteria prescribed by state mining regulation agencies. These structures normally do not 

retain water for long periods and are usually maintained with low water surface levels except following rainfall. Engineering design criteria and standards for sedimentation ponds have evolved from 

requirements for surface mining operations. In most instances, these standards are also applicable to 

coal refuse (Davis, 1973).

OSM rules for sedimentation ponds under 30 CFR § 816.46 to 49 generally include the following:

• Sedimentation ponds can be used individually or in series.

• They should be located as near as possible to the disturbed area and not in perennial 

streams.

• They should provide adequate detention time to meet effluent standards and should 

contain or treat the runoff from the 10-year, 24-hour precipitation event.

• They should provide sediment storage capacity with periodic sediment removal sufficient to maintain adequate volume.

• Ponds with embankments that meet or exceed the impoundment size criteria 

or other conditions indicated in 30 CFR § 216 (20 acre-feet capacity or 20 feet in 

height) should have principal and emergency spillways designed to safely pass the 

runoff from a 100-year precipitation event or larger, depending upon the hazard 

potential classification. For ponds that do not meet or exceed the impoundment 

size criteria, the principal and emergency spillways should be designed to safely 

pass runoff from the 25-year precipitation event or greater, as specified by the state 

regulatory authority.

State agencies generally provide additional guidance regarding determination of the sediment storage capacity and may require specific design storm parameters or values for sizing the principal and 

emergency spillways.

In situations where very fine particulate material is suspended in the refuse drainage, the amount 

of time required for natural settlement or clarification in a settling basin can be long. If the drainage 

is carrying a significant volume of suspended solids, clarification can be accelerated through use 

of chemical flocculants. This practice may also be considered when the capacity of a sedimentation 

pond is relatively small.

Sediment/sludge removal is required in order to sustain sedimentation pond capacity. In the event 

that such removal is not practical, sedimentation ponds should be designed with a capacity large 

enough to accommodate sedimentation over the appropriate operating period.
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10.3.1.2.2 Sediment Traps and Check Dams

Sediment traps and check dams may be useful as intermediate structures between erosion sources and 

sedimentation ponds or can be employed where sedimentation ponds are prohibited or unfeasible. 

They should be located within site drainage structures and should not cause channel overflow under 

design flow conditions. Design and installation should be in accordance with state regulations.

10.3.1.2.3 Silt Fences

Silt fences are temporary structures for detaining sediment-laden overland (sheet) flow long enough 

that the larger-sized particles are deposited and silt-sized particles are filtered out. State regulatory 

publications provide design and construction guidance for silt fences, and manufacturers provide 

similar information for their products. The following are general guidelines for silt fences:

• The drainage area should not exceed 0.25 acres per 100 feet of silt fence length.

• For slopes between 50:1 and 5:1, the maximum allowable upstream flow path length 

to the silt fence should be 100 feet.

• The filter material should be able to retain at least 75 percent of the sediment.

• The bottom edge of the silt fence should be tied or anchored into the ground to prevent underflow.

• There should be no ponding behind silt fences.

• Silt fences should be regularly maintained.

Appropriate state guidelines should be reviewed prior to installation of silt fences.

10.3.1.2.4 Erosion Control Blankets and Reinforcement Mats

Erosion control blankets can be used to stabilize freshly seeded slopes and drainage or ditches until 

such time that a cover of vegetation is established. Typically, they are most effective on slopes up to 3:1 

and in drainage ditches with slopes up to 20:1. Erosion control blankets typically degrade within 6 to 24 

months of installation, depending on their composition (straw, fiber, and plastic systems). Design and 

installation guidance are available in state regulatory publications and manufacturers’ literature. 

Reinforcement mats are similar to erosion control blankets, but provide greater protection because 

of the use of synthetic fibers that reinforce vegetation and result in more erosion-resistant construction. Reinforcement mats are used for steep slopes (greater than 3:1) and channels with slopes in the 

range of 15:1 to 10:1. Design and installation guidance are available in state regulatory publications 

and manufacturers’ literature.

10.3.1.2.5 Vegetation

Erosion and stream turbidity are best minimized by establishing a protective layer of vegetation 

on embankment slopes and along exposed ditch surfaces. The establishment of grasses in drainage 

ditches reduces flow velocity and, consequently, erosion.

Vegetation covers on embankment slopes are not practical until construction has proceeded far enough 

that relatively stable slope conditions are achieved. Vegetation is further discussed in Section 10.5.5.

10.3.2 Acid Generation and Control

The potential for acid generation from coal refuse materials can be estimated, and measures can be 

implemented to control acid formation or migration. State regulatory programs vary in terms of prediction methodology and the measures required to control or contain acid mine drainage.
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10.3.2.1 Background

Acid generation is principally the result of pyrite oxidation. Pyrites are commonly associated 

with coal formations and surrounding strata. Several types of pyrites may be present, and the 

reactivity of different forms varies significantly (Kleinmann, 2000). Acidity is produced by the 

oxidation of pyrites (sulfide components and iron components), which leads to the dissolution 

of metals (ferric iron, manganese, and aluminum, and occasionally other metals such as copper, 

zinc, and nickel). Rock strata may contain carbonate materials that neutralize acidity; however, 

coal refuse is material segregated from coal and generally includes minimal overburden materials that will neutralize acidity.

Acid mine drainage is a major problem in the northern Appalachian Basin (particularly within the 

Allegheny Group stratigraphic section) and less significantly in the Midwest (Kleinmann, 2000; Appalachian Regional Commission, 1969; Wetzel and Hoffman, 1989). Kleinmann (2000) provides a discussion of geology, hydrology and prediction of acid generation, including acid-base accounting (static or 

whole rock analysis) and simulated weathering tests (kinetic testing such as leaching tests in various 

columns and chamber arrangements). Testing procedures associated with acid-base accounting can be 

applied to individual samples of overburden and spoil materials for predicting acid generation or reclamation performance. Table 10.2 presents a summary of suggested criteria for interpreting the results 

of acid-base accounting analysis. While simulated weathering tests are not routinely used for coal 

mine drainage prediction, they can provide data for estimating the relative concentrations of net acidity, metals and sulfate, and they can be useful for evaluating the effectiveness of various amendments 

for mitigating problem water quality conditions. Kleinmann (2000) provides a detailed discussion of 

criteria for determining whether to conduct kinetic testing as well as testing methods.

Mitigation of acid generation can also be accomplished by hydrologic controls that minimize water 

contact with air and refuse. This typically involves: (1) compaction of the refuse surface, (2) sealing 

of the refuse surface and diversion of runoff from active disposal areas, and (3) capping and covering 

of completed refuse disposal areas. The USEPA (2000) developed a best management practices guidance manual for remining of refuse disposal sites providing specific guidance related to erosion and 

sedimentation controls and mitigation of acid generation.

10.3.2.2 Grading, Compaction and Sealing

Grading, compaction and sealing of coal refuse embankment surface areas will minimize the potential for infiltrating water contacting pyrites and thus reduce the potential quantity of acid generation and groundwater migration. Grading facilitates control of surface water flows, and compaction 

reduces the hydraulic conductivity of the refuse material. Regular sealing of the refuse embankment 

surface using smooth-drum rolling equipment facilitates runoff and thus reduces infiltration and 

the generation of acid leachates. Before subsequent placement of additional lifts, the sealed surface 

should be scarified to enhance bonding between lifts and to minimize potential stratification.

10.3.2.3 Amendments

A number of amendments for neutralizing acidity have been used with coal refuse, including coal 

combustion waste (lime-containing materials), kiln dust, phosphate rock, lime and other products. The amount of amendment material required for neutralizing acidity is a function of several 

factors, as described by Kleinmann (2000), USEPA (2000), and Brady et al. (1998). Stewart et al. 

(1997, 2001) evaluated neutralization and leaching from various blends of combustion waste and 

acid-producing refuse based upon a series of multi-year unsaturated column experiments. With 

sufficient combustion ash (20 percent and greater for the cited ash and coal refuse), no evidence 

of acid conditions was detected and low levels of most metals were observed, although high 

concentrations of boron and sulfate were reported. In column tests where the combustion ash
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TABLE 10.2 SUMMARY OF SUGGESTED CRITERIA FOR INTERPRETING 

ACID-BASE ACCOUNTING

 Criteria Application References

Rocks with NNP less than -5 parts/1000 

considered potentially toxic.

Coal overburden rocks in northern 

Appalachian basin for root zone 

media in reclamation.

Smith et al., 1974, 1976; West 

Virginia Surface Mine Drainage Task 

Force, 1979; Skousen et al., 1987

Rocks with paste pH less than 4.0 

considered acid toxic.

Coal overburden rocks in northern 

Appalachian basin for root zone 

media.

Smith et al., 1974, 1976; Surface 

Mine Drainage Task Force, 1979

Rocks with greater than 0.5% sulfur 

may generate significant acidity.

Coal overburden rocks in northern 

Appalachian basin, mine drainage 

quality.

Brady and Hornberger, 1990

Rocks with NP greater than 30 

parts/1000 and “fizz” are significant 

sources of alkalinity.

Coal overburden rocks in northern 

Appalachian basin, mine drainage 

quality.

Brady and Hornberger, 1990

Rocks with NNP greater than 20 

parts/1000 produce alkaline drainage.

Coal overburden rocks in northern 

Appalachian basin. Base and 

precious metal mine waste rock and 

tailings in Canada.

Skousen et al., 1987;

British Columbia Acid Mine Drainage 

Task Force, 1989;

Ferguson and Morin, 1991

Rocks with NNP less than -20 

parts/1000 produce AMD.

Base and precious metal mine 

waste rock and tailings in Canada.

British Columbia Acid Mine Drainage 

Task Force, 1989; Ferguson and 

Morin, 1991

Rocks with NNP greater than 0 do not 

produce acid. Tailings with NNP less 

than 0 produce AMD.

Base and precious metal mine 

waste rock and tailings in Canada.

Patterson and Ferguson, 1994;

Ferguson and Morin, 1991

NP/MPA ratio less than 1 likely results 

in AMD.

Base and precious metal mine 

waste rock and tailings in Canada.

Patterson and Ferguson, 1994;

Ferguson and Morin, 1991

NP/MPA ratio classified as less than 

1 (likely AMD), between 1 and 2 

(possible AMD), and greater than 2 (low 

probability of AMD).

Base and precious metal mine 

waste rock and tailings in Canada.

Ferguson and Robertson, 1994

Price et al., 1997

Theoretical NP/MPA ratio of 2 needed 

for complete acid neutralization.

Coal overburden rocks in northern 

Appalachian basin, mine drainage 

quality.

Cravotta et al., 1990

NP/MPA ratio used with NP threshold 

to determine confidence levels for acid 

producing samples. 80% confidence of 

no acid production if NP/MPA ratio of 

6.5 and NP threshold of 3.3%.

Coal overburden samples from 4 

states: PA, WV, TN, and KY. Bradham and Caruccio, 1995

Use actual NP and MPA values as 

well as ratios to account for buffering 

capacity of the system.

Base metal mine waste rock, United 

States. Filipek et al., 1991

Note: NP = Neutralization potential

NNP = Net neutralization potential

MPA = Maximum potential acidity

(ADAPTED FROM KLEINMANN, 2000)
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was insufficiently alkaline or where insufficient ash was combined with the refuse, acid generation ultimately exceeded the neutralizing alkalinity of the ash, resulting in a decline in pH and 

increased concentrations of metals. Stewart et al. (2001) recommend that careful attention be paid 

to balancing the acid-generating potential of refuse with the alkalinity of combustion ash. Some 

practitioners recommend increasing the alkalinity by some factor in order to prevent acidic conditions (Daniels et al., 1996).

Daniels et al. (2002) evaluated various combustion ash and coal refuse mixing strategies (including 

layering and partial blending) to determine their effectiveness in reducing acidity; they demonstrated 

the value of blending in alkaline materials as close as possible to the area where acid generation is 

occurring. Rich and Hutchison (1990) discuss the use of kiln dust for neutralizing combined coal 

refuse. Use of limestone, oxides, phosphate rock and other materials for neutralization is addressed 

by Skousen et al. (1998).

If amendments are used, provisions should be included in the design plans to verify that proper 

placement and/or mixing are achieved. The effect of amendments on the geotechnical characteristics 

of the refuse materials, particularly the strength and hydraulic conductivity of materials placed in 

structural embankment zones, should be assessed, as discussed in Sections 5.1.5 and 6.2.3.5.

10.3.2.4 Reclamation and Vegetative Cover

Reclamation and vegetative cover following completion of disposal operations provides drainage 

control and limits contact of the coal refuse with infiltrating water. The USEPA (2000) provides a 

qualitative discussion of improvements in the control of acid generation associated with reclamation and vegetation and cites supporting quantitative studies. Gentile et al. (1997) describe a cover 

system for an Illinois refuse disposal facility consisting of a compacted clay liner and protective soil 

cover designed to reduce infiltration by 84 percent. Meek (1994) describes the use of a PVC liner that 

reduced acid loads from a spoil pile by 70 percent.

While placement of barriers to infiltration as part of reclamation can address acid generation, provisions such as drainage systems should also be incorporated, so that internal seepage can discharge 

from the toe of a refuse embankment without raising the phreatic surface.

10.3.3 Water Quality Control

10.3.3.1 Diversion of Runoff

Drainage from undisturbed portions of a watershed should be conveyed around coal refuse disposal 

facilities to the extent practical using diversions. Thus, the amount of drainage contacting coal refuse 

and potentially subject to water quality impacts will be minimized. State regulatory guidelines provide criteria for the design and construction of diversion systems for control of runoff from undisturbed areas. While use of diversion ditches for impoundments can assist with controlling runoff, 

their capacity can only be considered in the impoundment flood routing if they are designed and 

constructed to handle the associated impoundment design storm (e.g., the Probable Maximum Flood 

for a high-hazard potential impoundment).

10.3.3.2 Treatment

Treatment of acid mine drainage typically involves neutralization of acidity and precipitation of 

metal ions to meet applicable effluent standards (USEPA, 1983). To meet the required standards, a 

variety of treatment methods including active and passive treatment technologies can be employed.

Selection of an active treatment system involves evaluation of the flow rate, pH, total suspended 

solids, acidity/alkalinity, iron and manganese concentrations, the receiving stream’s flow rate and 
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use, availability of electric power, the distance from the point of chemical addition to the point where 

the water enters a settling pond, and the volume and configuration of the settling pond. Most active 

chemical treatment systems consist of an inflow pipe or channel (sometimes a raw water storage 

pond and aerator for large flows), a storage tank or bin for treatment chemicals, a chemical metering 

system, a settling pond for precipitated metal oxyhydroxides, and a discharge point for treated water. 

Table 10.3 presents a summary of chemical compounds used for acid mine drainage (AMD) treatment 

and an equation for estimating the quantity of chemicals required based on the stream flow and the 

acidity of the AMD. Aeration enhances oxidation of metals such that chemical treatment is more efficient. Oxidants and pH adjusters are also sometimes used in the oxidation process to enhance metal 

oxyhydroxide precipitation and reduce metal floc volume. Mechanical surface aerators are generally 

used for large flows where aeration is required; simpler aeration systems using gravity to cascade 

water over rocks or splash blocks may be useful in smaller applications. Chemicals for neutralizing 

acidity are generally selected based on technical and cost factors. Skousen et al. (1998) discuss active 

treatment system design and costs and provide case studies.

Passive treatment technologies that take advantage of naturally occurring chemical and biological processes to cleanse impacted water and do not require continuous chemical inputs have been 

developed. The primary passive technologies include constructed wetlands, anoxic limestone drains 

(ALD), vertical flow systems such as successive alkalinity producing systems (SAPS), limestone 

ponds, and open limestone channels (OLC). Table 10.4 presents design factors and references for passive treatment systems. Skousen et al. (1998) discuss passive treatment system design and costs and 

provide case studies.

10.3.4 Water Quality Impacts on Construction Materials

The corrosive nature of coal refuse and leachates from coal refuse makes construction material selection important if facility appurtenant structures are to function as intended for long periods of time 

TABLE 10.3 CHEMICAL COMPOUNDS USED IN AMD TREATMENT

Common Name Chemical Name Formula Conversion 

Factor(1)

Neutralization 

Efficiency(2)

Limestone Calcium Carbonate CaCO3 1.00 50%

Hydrated Lime Calcium Hydroxide Ca(OH)2 0.74 95%

Pebble Quicklime Calcium Oxide CaO 0.56 90%

Soda Ash Sodium Carbonate Na2CO3 1.06 60%

Solid Caustic Soda Sodium Hydroxide NaOH 0.80 100%

20% Liquid Caustic Soda Sodium Hydroxide NaOH 784 100%

50% Liquid Caustic Soda Sodium Hydroxide NaOH 256 100%

Ammonia Anhydrous Ammonia NH3 0.34 100%

 Note: 1. The conversion factor may be multiplied by the estimated tons of acid per year to get tons of chemical 

needed for neutralization per year. For liquid caustic, the conversion factor gives gallons needed for 

neutralization.

 2. Neutralization efficiency is an estimate of the relative effectiveness of a chemical in neutralizing AMD 

acidity. For example, if 100 tons of acid per year is the amount of acid to be neutralized, then 78 tons of 

hydrated lime would be needed to neutralize the acidity in the water (100 x 0.74 / 0.95).

(ADAPTED FROM SKOUSEN ET AL., 1998)
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Environmental Considerations

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including abandonment. Table 11.6 lists common construction materials used for facility appurtenant structures and corrosion or deterioration mechanisms. The potential for chemical reaction and 

for clogging of drainage materials are critical considerations in the design of drainage systems. For 

drainage structures that are in contact with coal refuse or leachate, measures such as sulfate-resistant 

cement and coatings applied to metal surfaces should be used, as appropriate.

10.3.5 Hydrogeology

Groundwater recharge, unsaturated groundwater flow and saturated groundwater flow are hydrogeologic mechanisms that can affect migration of coal refuse constituents from a refuse disposal 

site. Groundwater flow is the primary migration mechanism, as erosion and sedimentation control 

measures are generally capable of controlling overland flow processes. Table 10.5 presents an overview of the hydrogeologic process and significance of the saturated and unsaturated groundwater 

regimes. Some hydrogeologic features and their effect on the design of coal refuse disposal facilities include the following:

• Springs – To minimize the potential for contact of water with coal refuse, natural 

hillside spring flows should be collected and controlled. Spring collection drains 

provide a means to collect and convey spring water from the source to downstream locations. 

• Mine discharges and underground mine workings – In some instances, discharges 

to and from mines may be important hydrogeologic features, because mines collect 

and convey groundwater. Similar to springs, discharges from mine openings can be 

controlled by collection drains that convey the mine water from the source to downTABLE 10.4 DESIGN CONSIDERATIONS FOR AMD PASSIVE TREATMENT SYSTEMS

Treatment 

System

Raw Water 

Conditions Construction Design Factors to Size Treatment System References

Aerobic Wetland Net alkaline water

Overland flow, 

cattails planted in 

substrate

•	 10 to 20 g Fe/m2/d

•	 0.5 to1.0 g Mn/m2/d Hedin et al. (1993)

HorizontalFlow Anaerobic 

Wetland

Net acidic water, 

generally low flow 

rate

Horizontal flow 

above organic 

substrate

•	 3.5 g acidity/m2/d

•	 Hydraulic conductivity of 

substrate generally 103 to 104

cm/sec

•	 Rate of sulfate reduction

(~300 mmoles/m3/day)

•	 Hydraulic loading

Hedin et al. (1993), 

Eger (1994), 

Wildeman et al. 

(1993)

Anoxic Limestone 

Drain (ALD)

Net acidic water 

DO, Fe3+, Al < l.0 

mg/l 

Horizontal flow 

through buried 

limestone

•	 15 hours contact time

•	 6- to 15-cm-diameter 

limestone

•	 Lifetime limestone 

consumption

Hedin et al. (1994)

Successive 

Alkalinity 

Producing 

Systems (SAPS)

Net acidic water

Vertical flow 

through an 

organic layer 

overlying a 

limestone bed

•	 15- to 30-cm organic matter 

with adequate permeability

•	 15 hours contact time in 

limestone

•	 Lifetime limestone 

consumption

•	 6- to15-cm-diameter 

limestone

Kepler and 

McCleary (1994)

(SKOUSEN ET AL., 1998)
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stream locations. Impoundments may require construction of a barrier to control 

flow of slurry into the mine workings. Additionally, the underground workings may 

act as a sink for groundwater migration, including seepage from the impoundment.

• Groundwater – Groundwater flow beneath a disposal site may be affected by seepage from refuse materials. If adverse water quality impacts are anticipated, liner 

systems and amendments can be used to mitigate these concerns.

• Surface water – Surface-water bodies may be a recharge source or receiving body. 

Disposal sites located near surface water bodies or impounding facilities may require 

measures such as liners, cutoffs, and other barriers to protect the hydrogeologic regime.

In addition to provisions for protecting the hydrogeologic regime, state regulatory agencies will 

require monitoring systems for detecting potential impacts to groundwater quality. This requireTABLE 10.5 OVERVIEW OF THE HYDROGEOLOGIC PROCESS

Source and Flow Process Description References

Recharge

Recharge into the disposal facility may occur from:

•	 Infiltration of precipitation and runoff

•	 Seepage from impoundment waters

Kleinmann, 2000

Unsaturated flow in 

embankment materials

Unsaturated flow in embankment materials is 

influenced by the recharge rate and unsaturated 

hydraulic conductivity and generally migrates 

vertically toward saturated embankment zones.

Hutchison and Ellison, 

1992

Saturated flow in embankment 

materials

Saturated flow in the embankment materials is 

influenced by underlying barriers such as foundation 

materials and internal drainage structures designed 

to control phreatic levels. Saturated flow generally 

migrates horizontally along foundation surfaces, 

although a component of flow can be into foundation 

soils. A liner system may be employed to restrict this 

component of flow.

Hutchison and Ellison, 

1992

Groundwater flow in embankment foundation soils

Saturated groundwater flow in foundation soils is 

influenced by underlying aquicludes or bedrock 

barrier and generally migrates horizontally along such 

surface, although a component of flow can be into 

deeper horizons or bedrock. Monitoring well systems 

are typically employed to monitor groundwater quality 

conditions beyond the limits of disposal sites.

Hutchison and Ellison, 

1992

Groundwater flow in bedrock 

fracture system

Saturated groundwater flow in the bedrock is 

influenced by the fracture system (and, in some 

cases bedrock primary porosity) and generally 

migrates horizontally toward groundwater discharge 

zones. Monitoring well systems may be employed to 

monitor groundwater quality conditions beyond the 

limits of disposal sites.

Kleinmann, 2000

Groundwater interaction with 

underground mines

Saturated groundwater flow may interact with 

underground mines, which may act as a discharge 

zone. Flow may follow discharge gradients in 

response to coal seam dip or pressure head within 

the mine. Monitoring well systems may be employed 

to monitor groundwater quality conditions.
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Environmental Considerations

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ment is usually satisfied through installation of monitoring wells located upgradient and downgradient from the disposal facility. Guidance for monitoring programs is typically available from 

state regulatory agencies. General guidance for groundwater monitoring systems is provided by 

Hutchison and Ellison (1992).

10.4 LINER SYSTEMS

Site-specific factors that should be considered in liner system design are summarized in Table 10.6. 

Liner systems are generally used for containment in situations where acid generation from coal 

refuse may impact the groundwater. Liner systems are an option in addition to amendments that can 

be considered for neutralizing acid generation.

Liner systems for protection of groundwater are cited in some state regulatory guidance for coal 

refuse disposal. Generally the reference is to a single-component, low-hydraulic-conductivity layer. 

Liner systems employed for other waste containment systems such as combustion waste (DiGioia et 

al., 1995) generally comprise multiple layers. The layers from the bottom up typically include: (1) subTABLE 10.6 SITE-SPECIFIC FACTORS TO CONSIDER IN LINER SYSTEM DESIGN

Potential Waste Material Toxicity

•	 Chemical properties of refuse and coal preparation additives

•	 Net acid generation potential

•	 Soluble constituents for anticipated environmental conditions

•	 Special treatment or neutralization procedures utilized

•	 Total mass of soluble constituents

General Water Resource Values at Site

•	 Adequate quality for beneficial use

•	 Adequate quantity for beneficial use

•	 Existing or identified beneficial uses

•	 Probable locations of future beneficial uses

Leachate Availability to the Environment

•	 Waste material characteristics

•	 Thickness of waste

•	 Site climatic conditions

•	 Provisions at closure to restrict infiltration

Site Factors

•	 Topography

•	 Geology, including predictability of uniformity and/or potential for discontinuities

•	 Unsaturated zone thickness, continuity, hydraulic conductivity and natural water content

•	 Potential migration time for seepage to groundwater

•	 Effects of climatic conditions on long-term unsaturated zone mitigation characteristics

•	 Constituent attenuation potential

Waste Disposal Facility Management Practices

•	 Facility type

•	 Waste placement method

•	 Protection of liner system from environmental damage

•	 Controls on the hydraulic head

•	 Risk reduction practices such as placement of underdrains, sub-aerial deposition, limited time of operations

•	 Non-liner barriers such as cutoff walls

•	 Installation of special early warning monitoring systems

(HUTCHISON AND ELLISON, 1992)
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grade or cushion materials, (2) leak detection zone, (3) liners (primary, secondary and/or composite), 

and (4) leachate collection layer.

10.4.1 Design Requirements

Design and performance requirements for liner systems are generally determined by the following 

(Hutchison and Ellison, 1992):

• Waste material characteristics including chemical composition, grain-size distribution, hydraulic conductivity, and the presence of free liquids.

• Waste disposal facility characteristics including liner hydraulic conductivity, 

slope of the liner, depth and slope of waste placed on the liner, waste placement 

method, hydraulic head controls, and the duration of operation for all or portions 

of the facility.

• Site characteristics including location and depth of the water resource to be protected, unsaturated zone conditions, and climatic conditions.

The potential for release of leachate is a function of the magnitude of the hydraulic head above the 

liner, the thickness and effective hydraulic conductivity of the liner material (considering the frequency of discontinuities in the liner such as cracks or holes), and the length of time the hydraulic 

head is applied to the liner. Leachate from coal refuse is generally not reactive with liner materials, 

but, if organic chemicals or strong bases are used in the coal preparation process and remain present 

in the waste, the issue of liner material compatibility may need to be addressed.

A liner system generally consists of a single low-hydraulic-conductivity layer (clay soil or geosynthetic material). Clay soil liners may include an overlying protection layer to protect the liner from 

erosion and desiccation. Where a geosynthetic material is used, an underlying cushion layer and an 

overlying protection layer are usually employed to minimize the potential for penetrations. Additionally, single liner systems may include overlying hydraulic head controls such as a pervious layer 

above the liner. Such systems reduce the head on the liner and thus further limit potential migration 

of leachate from the disposal facility. Composite double liners and leachate collection and removal 

systems are used when redundant systems are needed, although such liner systems are not generally 

used at coal refuse disposal facilities. Table 10.7 summarizes materials and handling and construction 

procedures associated with individual components of liner systems.

Major considerations in choosing materials for soil liners are availability and composition. Soils 

must contain a sufficient portion of clay material such that the constructed liner has low hydraulic 

conductivity, high plasticity, and chemical stability. Suitable soils are usually classified CL, CH, or 

SC in the Unified Soil Classification System (USCS) with a liquid limit between 35 and 60 and a 

plasticity index of 10 or greater. Material for soil liners can consist of on-site or local borrow materials, imported bentonite, or mixtures thereof. To achieve the low hydraulic conductivity required 

for a containment layer, soils must have consistent properties and may need thorough mixing, 

preprocessing (e.g., removal of rocks, breakdown of soil clods, addition of bentonite), conditioning (e.g., adjustment of water content), placement in controlled lifts, and compaction. Imperfections such as gravel zones, organics and roots should be removed during construction. Protection 

against cracking from drying or shrinking may also be required.

The required thickness and hydraulic conductivity of the barrier layer are a function of the hydraulic 

heads, refuse material characteristics, and state policies or regulations. Typically, requirements for a 

soil liner or barrier layer are a minimum thickness of 2 feet and a hydraulic conductivity less than 5 x 

10-5 cm/sec (≈ 50 ft/yr) and in some applications less than 1 x 10-7 cm/sec (≈ 0.1 ft/yr). Variations from 

these criteria are generally dependent upon in-situ conditions.
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Environmental Considerations

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TABLE 10.7 AVAILABLE MATERIALS OR PROCEDURES FOR LINER SYSTEM COMPONENTS

A. Low-Hydraulic-Conductivity Liners

A.1 Low-Hydraulic-Conductivity Natural Soil or Rock – Natural soils or rock may be used as a low-hydraulicconductivity liner so long as it is possible to demonstrate by field investigations that the material is continuous 

and of sufficient thickness and properties over the entire area requiring the liner. This demonstration may be 

particularly difficult for rock because of jointing and fracture conditions.

A.2 Constructed Low-Hydraulic-Conductivity Liners – Low-hydraulic-conductivity liners that are constructed 

beneath a mine waste disposal facility may consist of any of the following materials:

•	 Compacted, low-hydraulic-conductivity soils (e.g., clayey-silt to clay depending upon the required 

hydraulic conductivity)

•	 Soil and bentonite or cement mixtures

•	 Pre-formed flexible geotextile impregnated with bentonite or pre-formed, granulated bentonite laminated 

to a geomembrane, referred to as geosynthetic clay liner (GCL)

•	 Pre-formed flexible membrane liners made from a variety of available polymeric material, generally 

referred to as geomembranes; varying in thickness from about 20 to 100 mils

•	 Field-applied liners, varying from about 80 mils of spray-on asphaltic materials to 6 inches of 

conventionally-placed asphaltic materials

•	 Composite liners, consisting of combinations of soil and geomembrane low-hydraulic-conductivity layers

A.3 Waste Material – Settled or mechanically placed tailings often have a low hydraulic conductivity and can be 

used as part of the long-term liner system, provided the tailings serve one of the low-hydraulic-conductivity liner 

functions.

B. Cushion or Liner Protection Materials

B.1 Geotextiles – Synthetic geotextile materials varying in weight from 4 to 20 ounces per square yard may be 

used above or below geomembranes to protect against penetrations from rock particles due to loads from 

construction activities or the weight of the waste material. The suitability of a geotextile to act as a cushioning 

layer varies and is defined by the method employed by its fabrication (needle-punch non-woven versus woven).

B.2 Fine-grained Soil for Geomembrane Protection – Soils varying from clay to sand can also be used to protect 

most geomembranes from equipment traffic or static loading of the waste material. Small gravel-size material 

has also been used to protect thick geomembranes. The protective soil must be relatively free of large rock 

particles that could cause stress concentrations on the liner.

B.3 Cover Material for Clay Liner Protection – Cover protection may also be required for a compacted soil liner if 

the liner could be subjected to extreme loads, such as construction equipment traffic, or exposed to drainage or 

desiccation.

C. Hydraulic Head Control Components

C.1 Free-Draining Gravel Layer – Several inches of free-draining gravel (including coarse sand) are usually 

adequate to rapidly remove small volumes of leakage. However, thicker layers (8 to 18 inches) are usually 

placed to facilitate construction and protect the liner layer from being damaged. The waste material itself may 

serve this purpose if the material is granular, durable and relatively free draining.

C.2 Perforated Pipes – Closely-spaced perforated pipes can be used to control hydraulic head above the liner. 

The required spacing is calculated based on the maximum desired head and the flow rate and hydraulic 

conductivity of the waste material between the pipes.

C.3 Geocomposite Systems – Composite systems consisting of synthetic drainage associated with geotextile filters 

have been developed for a wide range of drainage control functions. Performance of these systems under load 

must be confirmed.

D. Leachate Collection and Removal Systems

D.1 Synthetic Geonet Materials – Geonets are net-like polymer products designed to allow high rates of transverse 

flow. Typical thicknesses of these materials vary from 0.16 to 0.30 inches.

D.2 Free-draining Gravel Layer – (See Item C.1)

(HUTCHISON AND ELLISON, 1992)
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Geomembranes made from high-density polyethylene (HDPE), polyvinyl chloride (PVC) and very 

low density polyethylene (VLDPE) have been used as liners. Important considerations in the selection of geomembranes are thickness, strength, durability, chemical resistivity, cost, cover material needed for cushioning above and below the barrier, method of construction, and the method 

for seaming the liner (Hutchison and Ellison, 1992). Most geomembranes are manufactured with 

ultraviolet inhibitors (e.g., carbon black) and can be expected to last more than 50 years even when 

exposed to sunlight. Geosynthetic clay liners (GCLs) consisting of bentonite sandwiched between 

two geotextiles (woven or non-woven synthetic fabrics) that are glued or sewn together or bentonite laminated to an HDPE geomembrane have also been used. GCLs are resistant to damage 

due to handling during installation, but they lose shear strength as the bentonite is hydrated, thus 

decreasing stability.

Geotextiles and soil materials above and below the geomembrane layer may be needed for protection 

against penetrations by underlying rocks or sharp objects during construction. The protective soil 

layers should be relatively free of large rocks and roots that could cause concentrated stresses in the 

liner. State agencies can provide guidance on the use of geomembranes and may specify a minimum 

thicknes and requirements for compatibility with the refuse materials.

An effective QA/QC program is essential for installation of soil liners, geomembranes and GCLs. 

Past failures have been attributed to poor material placement, seaming, and protection (Daniel and 

Koerner, 1995). Composite systems that consist of a combination of soil and a geomembrane have less 

potential for quality control problems, but may only be economically feasible when suitable soils are 

available on site.

Figure 10.1 shows three examples of soil and geomembrane liner designs used at coal refuse disposal 

facilities. Compacted subgrade, as shown in Figure 10.1a, is acceptable in many situations for containment of coal refuse. Soil liners and synthetic liner systems, as shown in Figures 10.1b and 10.1c, 

respectively, may be attractive in some situations. Some soil and synthetic liner systems may require 

a prepared subgrade and protective cover materials.

Other layers can be added to a liner system if warranted, including a leachate collection layer 

and a leachate detection layer. The leachate collection layer should be positioned above the liner 

to collect and convey seepage from the refuse and to limit the buildup of hydraulic head on the 

liner. The thickness and hydraulic conductivity of the leachate collection layer should be designed 

based upon the potential leachate flow, liner configuration (slopes and other geometry that affect 

seepage), and any restrictions on hydraulic head associated with the liner. The leachate collection 

layer typically consists of sand and/or gravel designed to be more hydraulically conductive than 

the waste itself. Geotextiles may be used between this layer and the liner for cushioning and to 

improve stability.

A network of perforated pipes is sometimes provided within the leachate collection layer to increase 

capacity, and these pipes must be properly designed to withstand crushing under the embankment 

weight. The leachate collection layer typically drains to one or more central collector or header pipes. 

Solid-wall pipes convey the leachate from the disposal area to holding areas for eventual treatment (if 

required) and discharge. Manholes may be installed at bends and at regular intervals for pipe inspection and cleaning; cleanout fittings may also be used.

Geonets and geocomposite drainage products have been used in some applications for leachate collection (DiGioia et al., 1995) if chemical compatibility and flow capacity under the applied load is 

acceptable. These products are also sometimes used for leak detection zones beneath the primary 

liner when conditions warrant.
10-23

Environmental Considerations

MAY 2009

FIGURE 10.1 LINER SYSTEMS USED AT COAL REFUSE

 DISPOSAL FACILITIES

10.1a COMPACTED SUBGRADE

10.1c SYNTHETIC LINER

GEOMEMBRANE OR

GEOSYNTHETIC CLAY LINER

COAL REFUSE PROTECTIVE COVER MATERIAL

COMPACTED SUBGRADE

NATURAL SOILS OR ROCK

COMPACTED SUBGRADE

COAL REFUSE

NATURAL SOILS OR ROCK

10.1b SOIL LINER

PROTECTIVE COVER MATERIAL

COMPACTED SUBGRADE

NATURAL SOILS OR ROCK

COAL REFUSE

2-FOOT-THICK SOIL LAYER

COMPACTED TO 95 TO 100

PERCENT STANDARD PROCTOR

10.4.2 Stability

Structural stability of a liner system is a critical element in design. For embankments, the following types of waste stability issues could cause liner damage: (1) sloughing of loose uncompacted 

material from surficial zones, (2) block failure of the waste material moving laterally with shearing 

occurring predominantly within the liner system (particularly for sloping liner system), and (3) 

dynamic slope instability and permanent displacement related to earthquake or other dynamic 

loading. Liner damage in the vicinity of the impoundment can arise from erosion due to slurry 

discharge or from natural runoff, as well as instability of adjacent hillsides. A more detailed discusFIGURE 10.1 LINER SYSTEMS USED AT COAL REFUSE DISPOSAL FACILITIES
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Chapter 10

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TABLE 10.8 STABILITY CONSIDERATIONS IN LINER DESIGN

Problem Liner Stress Free Body Diagram

Required Properties Typical 

Factor of 

Geomembrane Landfill Safety

1. Liner self-weight Tensile

T

W

FL

T

W

FL G, t, σallow, δL β, H 10 to 100

2. Weight of filling Tensile

W

FL

T FU

t, σallow, δU, δL β, h, γ, H 0.5 to 10

3. Impact during

construction Impact I I d, W 0.1 to 5

4. Weight of landfill Compression

n

σallow γ, H 10 to 50

5. Puncture Puncture

P

I

σp γ, H, P, Ap 0.5 to 10

6. Anchorage Tensile

T

FU

FL  t, σallow, δU, δL β, γ, φ 0.7 to 5

7. Settlement of

landfill Shear



n

 τ, δU β, γ, H 10 to 100

8. Subsidence

under landfill Tensile 

z T

n

FU

FL

t, σallow, δU, δL, z α, γ, H 0.3 to 10

 Legend:

G = specific gravity β = slope angle

T = tensile force H = landfill height

t = thickness γ = unit weight

σallow = allowable strength h = lift height

τ = shear strength α = subsidence angle

I = impact resistance φ = friction angle

σp = puncture strength d = drop height

δU = friction coefficient with material above W = weight

δL = friction coefficient with material below P = puncture force

FU = friction force upper Ap = puncture area

FL = friction force lower z = mobilization distance

(ADAPTED FROM KOERNER, 2006)

sion of slope stability for situations involving liners is provided in Section 6.6.4. Koerner (2006) also 

presents procedures for analysis of liner stability. Table 10.8 summarizes stability issues associated 

with liner system design.

10.4.3 Performance Considerations

Liner system performance is measured in terms of the extent of control of leachate seepage from 

the refuse disposal facility. Liner system performance is related to the types of waste material present, the hydraulic head, and subsurface conditions, and these factors can mitigate or exacerbate the

potential hydrologic impacts. Table 10.9 provides a summary of guidance related to performance 
10-25

Environmental Considerations

MAY 2009 TABLE 10.9 LINER SYSTEM PERFORMANCE FACTORS AND CONSIDERATIONS Liner

Type

Thickness

(ft)

Hydraulic

Conductivity

(cm/sec)

Example 

Hydraulic

Head

Estimated Min.

Steady-State

Leakage Rate 

Q(gpad)

Travel Time 

through

Liner

(years)

Equations

Clay

2 1 x 10-6 10 3,500 0.12 1 x 10-7 10 510 1

Geomembrane

Membrane with single perforations of 0.03 cm2/acre(for liner evaluations)

10 7.9(1) NA

Membrane with single perforations of 1 cm2/acre(for leakage detectioncapacity calculations)

10 13.7(1) NA

 Note: 1. Subsoil hydraulic conductivity = 1 x 10-5 cm/sec.

Legend:

Q = seepage flow in gallons per acre per day (gpad) Dt = thickness of saturated waste (length)q = seepage per unit area (length/time) hf = soil pore-water pressure below liner (length)kl = hydraulic conductivity of liner (length/time) n = effective porosity (dimensionless)Dl = thickness of liner (length) φi = initial in-situ volumetric moisture content (dimensionless)kt = hydraulic conductivity of waste (length/time) hc = soil pore-water pressure (length)d = diameter of hole in geomembrane (length) ks = hydraulic conductivity of subsoil (length/time)

qr = reduced specific leakage (length) H = depth of ponded liquid (hydraulic head) above top of liner (length)

T = travel time

(ADAPTED FROM HUTCHISON AND ELLISON, 1992)

q = 

π ks

qr2 4

d = 4 qr exp [ –π(H – hf)] π2 qr

T = (n – φi) D[ l – (H-hc) ln ( 2Dl + H – hf )] kl Dl + H – hf

q = kl 

D[ t + Dl – hf ] Dl + Dt kl kt
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Chapter 10

MAY 2009

evaluation and provides some specific examples. Measuring the performance of a liner system 

typically involves monitoring of drain discharges and down gradient groundwater conditions.

10.5 RECLAMATION

Reclamation requirements vary according to land use, climate, and state regulations. The purpose of 

this section is to provide general guidelines for reclamation grading, impoundment elimination, soil 

and topsoil covering, and revegetation of coal refuse disposal facilities.

10.5.1 Design Considerations

The content of a reclamation plan is related to the planned post-mining land use, site terrain and 

disposal facility configuration, climate, and pre-mining and adjacent area conditions. Generally, postmining land use is open space and wildlife habitat and may be oriented to specific wildlife species 

and vegetation biodiversity (e.g., forest and grass land mix). Other land use possibilities, although 

rarely considered, include agriculture, recreation, and site development. All of these land uses typically require the establishment of persistent, low-maintenance vegetation for controlling erosion. 

Site access and topography significantly affect future land use, and the engineering properties of the 

embankment and the method of construction are important if structural foundations are planned. 

Table 10.10 presents a summary of potential final land uses and related key requirements and considerations.

In evaluating potential land uses, the availability of resources, and specifically soils for revegetation, 

must be determined. Other resources include water, access roads and existing site infrastructure. 

Preparation of an inventory of resources is an integral step in the development of a reclamation plan. 

Because soils are used for a variety of applications besides reclamation (e.g., starter dams, liners, etc.), 

an understanding of the quantity and quality of soils available at or near the site is essential. During 

the planning and design phases, geotechnical exploration should include field characterization of soil 

TABLE 10.10 POTENTIAL FINAL LAND USES

 Land Use Examples Key Requirements and Considerations

Wildlife Habitat and Open Space Adequate cover of appropriate vegetation for desired wildlife 

species.

Agriculture

•	 Pasture and Hay

•	 Fiber Crops 

•	 Tree Nursery

Agricultural land uses should include assessment for trace 

elements.

Recreation

•	 Active Recreation (sports fields, 

golf courses, ski/biking facilities) Access to site, topography, erosion and drainage control.

•	 Passive Recreation (hunting, 

hiking, nature study) Access to site, topography.

Site Development

•	 Commercial and Industrial 

(buildings, storage areas)

Access to site, topography, structural support, erosion and drainage 

•	 Residential (housing, parks) control.

•	 Infrastructure (highways, airports)

(DIGIOIA ET AL., 1995)
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Environmental Considerations

MAY 2009

properties such as thickness, texture, and color. Evaluation of soil pH and potential lime requirements 

during geotechnical laboratory testing of soils will enable improved planning of complex reclamation 

sites. Analyses related to soil and other material handling should be performed with the capabilities 

and limitations of the available excavation equipment in mind, so that costs associated with recovery 

and segregation of soils during excavation, stockpiling, and redistribution are realistic.

Soils in the eastern U.S. and the Midwest tend to be neutral to acidic and, with addition of appropriate amounts of lime and fertilizers, can support plant growth without irrigation if appropriate species are selected. For practical purposes, lime will neutralize soils only to the depth of incorporation 

(plow depth). If lime cannot be incorporated to a sufficient soil depth, plant species with tolerance to 

low pH should be selected. Soils from arid or semi-arid regions in the west tend to be high in soluble 

salts and/or sodium, and revegetation in this material can be challenging. All soils should be tested 

for nutrient availability before lime and fertilizer application is specified (Page et al., 1982). Table 

10.11 presents design considerations for reclamation soils.

10.5.2 Grading

Final grading plans for reclamation should include development of surfaces and slopes in order to 

achieve effective site drainage and to facilitate access for placement of soil and topsoil, vegetation, and 

maintenance. While plans for a refuse disposal facility provide an anticipated final configuration and 

slopes, the facility may not reach its planned capacity prior to reclamation. In such circumstances, a 

reclamation grading plan providing site drainage (eliminating impounding conditions as necessary) 

and minimizing erosion potential should be developed. The configuration of embankments, slopes, 

benches and drainage channels at abandonment is subject to state regulatory criteria, which generally include requirements for overall embankment slopes, benches, and top surface grades.

TABLE 10.11 DESIGN CONSIDERATIONS FOR RECLAMATION SOILS

Support for Plants Non-toxic for plants, capable of root penetration and storing sufficient 

amounts of plant-available water

Soil Type and Thickness

Typical regulatory requirements are up to 4 feet in total thickness, with 

equivalent topsoil placement to pre-mining condition. Alternate cover 

and growth medium may be considered.

Root Anchorage For large shrubs and trees, soil thickness of greater than 4 feet may be 

required for anchorage against wind and gravity.

Water Storage Field capacity and wilting point can be measured or estimated from 

texture or grain-size distribution.

Establishment of Vegetation Ability to add nutrients, pH adjustments, soil conditioners for 

acceptable growth medium

pH

Most plant species grow best at a pH between 6.0 and 7.5. Soils 

between pH of 3.5 and 6.0 can be limed; the cause of excessive pH 

should be determined before adjustments are made.

Salt Stress Sodium and salt soils can be evaluated using electrical conductivity 

and sodium adsorption ratio.

Nutrient/Trace Element Availability Nutrient and toxic element testing (Baker, 1988) can be used to identify 

fertilizer and amendment requirements.

Species Selection

In addition to soil conditions, species should be selected based on 

short- and long-term availability of irrigation water, short-term erosion 

control requirements, and maintenance intensity and methods.

Yield of Vegetation for Land Use Adequate balance of nutrients and trace metals for sustained yield

Engineering Properties Acceptable erosion resistance, hydraulic conductivity, load bearing 

capacity, resistance to traffic, etc.

(ADAPTED FROM DIGIOIA ET AL., 1995)
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MAY 2009

10.5.3 Impoundment Elimination

Elimination of slurry impoundments requires special measures for grading and reclamation. The 

impoundment surface may be wet, dry, dessicated, or vegetated, but the underlying materials typically remain soft and can exhibit sudden shearing under equipment operation. The impoundment 

elimination plan should address factors associated with: (1) fine refuse properties, (2) impoundment 

size and depth, (3) the presence of water, and (4) the availability of and access to borrow sources for 

regrading and covering materials. Preparation of an impoundment elimination plan may require 

characterization of the fine refuse materials (including drainage and consolidation properties), specification of the borrow material for covering the fine refuse, and specification of the equipment for 

implementing the work. Section 11.5.2 provides guidance for upstream construction that should be 

considered in developing and implementing plans for covering of an impoundment. The following 

are typical guidelines for impoundment elimination:

• Drainage toward the impoundment should be collected and routed away, and 

ponded water should be removed.

• Access into the impoundment area for delivery of borrow materials to cover the fine 

coal refuse should be developed. This may involve construction of access roads and 

designation of temporary stockpile areas in preparation for initial pushout of borrow 

materials over the fine refuse.

• An initial lift of borrow materials should be pushed out over the fine refuse using 

a bulldozer. This initial lift should typically be between 4 and 6 feet thick, with the 

lower end of this range more desirable for minimizing displacement of the fine 

refuse. The pushout should not be performed into standing water, and dewatering measures in the fine refuse (prolonged drying, drainage sumps, wick drains, 

etc.) may be needed to facilitate placement of the initial lift. The initial lift should 

be advanced from firm areas along the perimeter of the impoundment, creating a 

wide area of operation rather than a narrow one. The initial lift should generally be 

advanced a distance of at least 50 feet before additional lifts are placed or trucks or 

haulage equipment are allowed onto covered areas. This distance should be maintained until the impoundment surface has been covered, and trucks should generally 

not be allowed into this zone for delivery of borrow materials. Monitoring should be 

conducted throughout the initial pushout period.

• Subsequent lifts should be placed in accordance with geotechnical requirements for 

the disposal embankment and should not exceed a thickness of 2 feet. Generally, 

material for impoundment elimination is considered placed fill unless structural fill 

is required by final land use. Depending on the geotechnical design requirements, 

restrictions on the rate of fill placement may be warranted in order to limit loading 

and to allow consolidation of the fine coal refuse.

• Should displacement of fine refuse occur during pushout, the following measures 

should be considered: (1) slowing the advance of the pushout to allow dissipation 

of pore pressure in the fine refuse, (2) use of low-ground-pressure equipment, (3) 

improving drainage within the fine refuse (e.g., sumps, wick drains), and (4) stabilization of the fine refuse or reinforcement of the pushout lift using geotextiles or 

geogrids. If displacement is unavoidable, the impoundment elimination plan should 

include provisions for containment of the displaced material.

• The material used to cover the fines and eliminate the impounding capability 

should be cambered such that when settlement occurs due to consolidation of the 

underlying fines, the surface will always provide positive drainage off the site. The 

amount of long-term settlement should be estimated based on the consolidation 

characteristics of the fines.
10-29

Environmental Considerations

MAY 2009

In the development of an impoundment elimination plan, the safety of equipment operators covering slurry 

deposits should be addressed as well as monitoring of the work. The following general guidance applies:

• Initial and periodic review sessions covering the procedures and anticipated performance of the initial pushout, delivery of borrow material, and subsequent lift placement 

should be held by engineering personnel for equipment operators and supervisors.

• The impoundment should be maintained in a dewatered condition to the extent practical.

• Initial pushouts should be restricted to daylight hours or times when the work area 

is sufficiently illuminated to provide good visibility.

• Radio communication for pushout equipment operators and supervisors should be 

provided, and it is recommended that equipment operations be within sight of mine 

personnel during the initial pushout and that operators be provided with floatation 

devices (e.g., life jackets).

• The work area should be examined frequently for signs of instability such as cracking or sinking, and work should be suspended in areas exhibiting such indications.

• Monitoring of the work should be performed by engineering personnel with an understanding of technical issues such as slope stability, displacement, and deformation.

• Pore pressures within the fine refuse and deformations or displacements may be 

monitored with instrumentation, if warranted.

10.5.4 Soil and Topsoil Cover

Soil and topsoil cover materials with the properties that meet regulatory requirements for reclamation should be stockpiled and recovered from locations near the disposal facility. While OSM and 

state regulations typically require 4 feet of soil and topsoil cover, there are situations where a variance in cover thickness may be considered. Also, isolation of the refuse materials from infiltrating 

water may be necessary. In these circumstances, supplemental materials and/or modified placement 

procedures may be needed:

• In Appalachian regions, there may be insufficient soil and topsoil for reclamation, 

and reduced cover thicknesses may be necessary. Dove et al. (1987) and Daniels (2005) 

evaluated direct seeding and reduced topsoil thickness alternatives to determine the 

optimal combination of soil amendments and topsoil thicknesses for successful vegetation of refuse with varying levels of potential acidic leachate generation. Daniels (2005) 

indicates that for moderately acid producing refuse, acceptable vegetation can be 

established with less than 12 inches of soil cover if lime is added to the refuse surface.

• Alternatives such as bio-solids and combustion ash may be considered. Bio-solids 

(sewage sludge) can be plowed into refuse surfaces and used to establish an alternative growth medium. Combustion ash can also be applied or mixed into the refuse 

surface to establish an alternative growth medium.

• If isolation of the refuse materials requires a low-hydraulic-conductivity cap, use of 

clay or a geomembrane may be appropriate. The evaluation and design of caps generally follows the procedures for liner systems presented in Section 10.4.

Achieving good adhesion of soil placed on refuse surfaces may require special procedures. The refuse 

surface should be scarified by tracking up and down slopes with a bulldozer, by shallow tillage 

along contour lines, or by other methods that will loosen the surface. Soil should be placed in a relatively dry condition with low-ground-pressure equipment, avoiding excessive compaction (unless 

required for construction of a low hydraulic conductivity cap). If soils have a low pH (below 5) and 

require amendment with lime or gypsum, it may be appropriate to place and amend the soil in lifts, 

with incorporation (plowing in) of amendments through the entire lift prior to placement of the next 

lift. Table 10.12 presents a summary of reclamation guidelines (DiGioia et al., 1995).
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Chapter 10

MAY 2009

TABLE 10.12 SUMMARY OF RECLAMATION GUIDELINES

 Task Recommended Guidelines

Application of Lime Gypsum 

and Fertilizer

Lime and gypsum should be plowed into the entire lift thickness, and fertilizers and 

other plant nutrient sources should be applied evenly and plowed under within 24 

hours and to a depth of at least 2 inches. If seedbed preparation includes creation of 

furrows, seedbed preparation may be done in concert with fertilizer incorporation. If 

hydroseeding is utilized, apply no more than 40-80-40 pounds N-P2O5-K2O per acre 

with seed and do not leave seed and inoculants in contact with fertilizer-containing 

solutions for more than 1 hour.

Furrowing and LandImprinting

Where management of water or reduction in wind or salt stress is desired, deep 

furrowing (6 to 10 inches), land imprinting, or other methods should performed. 

Furrows should be oriented parallel to site contours or on flat surfaces, perpendicular 

to prevailing winds.

Seedbed Preparation

If the furrowing or land imprinting procedures are performed more than a few days 

before seeding, or a crust has formed on the soil surface, these procedures should be 

repeated just prior to seeding.

Seeding and Inoculating

Seeding depths using the drill seeding method should be set for the shallowest 

seeded species. To maximize the opportunity for biological nitrogen fixation, legume 

seed can be inoculated with Rhizobium strain specific to the species being sown. 

Broadcast and hydroseeding work best when the seeds are promptly covered with soil 

and mulch.

Selection of Planting of 

Woody Species

The emphasis is on establishment of herbaceous, not woody plants. Guidance on 

selection, planting, maintenance and specification of woody plants is presented in 

Vogel (1987) and Himelick (1981).

Mulching and Tacking

Mulch should be applied within a day of seeding and before rain. Straw and/or hay 

applied at a rate of 3,000 to 6,000 pounds per acre and wood cellulose fiber mulch 

applied at a rate between 1,200 and 2,500 pounds per acre are acceptable mulch for 

most purposes. Tacking can be performed by crimping mulch into soil with large disks 

set along the direction of travel or by application of wood cellulose fiber mulch over 

the straw/hay using a hydroseeder. Crimping techniques that leave some straw/hay 

standing up in the soil crease and in rows at right angle to the prevailing wind are 

desirable for dry, windy sites.

Watercourse Protection For watercourse protection (swales, ditches) wood excelsior, coconut fiber, nylon, and/

or jute blankets should be used according to manufacturers’ instructions.

Irrigation

Irrigation should be considered during the establishment year in arid and semi-arid 

regions and other areas where the gains from improvements in establishment rate 

and long-term survival outweigh the risk of failure.

(ADAPTED FROM DIGIOIA ET AL., 1995)

10.5.5 Vegetation

Species for vegetation and reclamation should be selected based on their adaptability and tolerance 

to site climate and soil (or alternative media) conditions and their suitability for the final land use and 

compatibility with regulatory provisions. Additional considerations include erosion and sedimentation control requirements, the need for irrigation water, and maintenance requirements. To the extent 

possible, local expertise should be sought for development of vegetation plans for specific land uses. 

Where available, state erosion and sedimentation publications and university agronomy studies can 

provide important guidance related to seeding/planting mixtures and cultivation practices. Potential 

vegetation species and their adaptability to various climates and soil conditions are summarized in 

Table 10.13 (DiGioia et al., 1995).

In humid regions, winter rye and redtop mixed with more slow-to-establish perennial species such 

as birdsfoot trefoil and deertongue grass are often used to provide cover. Too high a seeding rate of 
10-31

Environmental Considerations

MAY 2009 TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION Regions of U.S.(1) Common Name Scientific Name and Cultivar(2) Growth Habit/ Forms(3,4) Growth Season(5) Native Species(6) Drain- age Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

NC, SC, 

W

Thickspike 

Wheatgrass

Agropyron 

dasystachyum

peren./

grass

cool yes poor- well 10-17 5.0-8.5 H, A, R, D 4-6 Sod-forming bunchgrass.

NC, W Crested Wheatgrass Agropyron desertorum peren./ grass cool no well 10-17 5.5-8.5 R, D 4-6

Establishes slowly. Intolerant to flooding. Bunchgrass. Stratify seed.

NC, SW, 

SC

Tall 

Wheatgrass

Agropyron 

elongatum

annual/

grass

cool yes poor- well 12-20+ 6.0-8.0 H, A 4-6

Use adapted cultivars. Establishes easily. Sodforming bunchgrass. Boron intolerant.

NC, W Streambank Wheatgrass Agropyron riparium annual/ grass cool yes poor- well 11-19 6.5-8.5 R, D 2-4 Spreads rapidly. Sod- forming.

NC, SC, 

M, W, 

NE

Western 

Wheatgrass Agropyron smithii peren./ grass cool yes poor- well 5-20+ 4.5-9.0 H, A, R, D 4-6 Use adapter cultivars. Sod-forming.

NC, SC, 

W

Slender 

Wheatgrass

Agropyron 

trachycaulum

peren./

grass

cool yes med.- poor 15-20+ 6.5-8.5 H, A 2-4 Grows fast, short-lived.

SC, M, 

NE, S Redtop Agrostis alba peren./ grass cool no med.- poor 20+ 4.0-7.5 R, D 2-4 Sod-forming short-lived.

NC, SC, 

M Big Bluestem Andropogon gerardi peren./ grass warm yes med.- poor 16-20+ 5.5-7.5 H, A, R,D 4-8 Use adapted cultivars. Deep roots.

All Common Oats Avena sativa annual/ grass cool no well 10-5 5.5-7.0 H, A, R, D 30-50 Use adapted cultivars. Cover crop. Cold tolerant. Needs N.

S King Ranch Bluestem Bothriochloa ischaemum peren./ grass warm no well 20+ 5.0-7.0 A 2-6 Mix with Sercica Lespedeza.

NC, SC, 

M

Sideoats 

Grama

Bouteloua 

curtipendula 

‘Vaughn’ or 

‘Elreno’

peren./

grass

warm yes

med.-

well 12-20+ 6.0-7.5 H, A 2-4 Calcareous soils. Bunchgrass.

NC, SC, 

W Blue Grama Bouteloua gracilis ‘Lovington’ peren./ grass warm yes well 8-20 6.0-8.5 H, A, R, D 10-12 Use adapted cultivars. Sod-forming bunchgrass. Animal forage.
10-32

Chapter 10

MAY 2009

TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

All Smooth Brome Bromus inermus peren./ grass cool no med.- well 17-20+ 5.5-8.0 R, D 10-15 Use adapted cultivars. Establishes easily for quick cover. Sod-forming.

NC, SC Buffalo Grass

Buchloe 

dactyloides 

Sharps ‘Improved’ 

or ‘Texoka’

peren./

grass

warm yes

med.-

poor

12-20+ 6.5-8.0 H, A, R, D 5-10

Sod-forming, stoloniferous. Stratify seed. Monoecious.

NC, SC, 

W

Prairie 

Sandreed

Calamovilfa 

longifolia ‘Goshen’

peren./

grass

warm yes well 10-20 6.0-8.0 H, A, R, D 6-9 Sod-forming, rhizomatous.

S Rhodes Grass Chloris gayana peren./ grass warm no med. 20+ 4.0-7.0 H, A, R, D 8-12 Very tolerant to salinity. Stoloniferous.

NC, S, 

SC, NE

Bermuda 

Grass

Cynodon dactylon 

Quicksand 

‘Common’

peren./

grass

warm no

med.-

poor

16-25+ 4.5-7.5 A, R, D 3-5

Four-year cover. Rhizomatous and seed spreading.

NC, M, 

NE, S

Orchard 

Grass

Dactylus 

glomerata

peren./

grass

cool no med.- well 20+ 5.0-7.5 H, A, R, D 5-8 Use adapted cultivars. Bunchgrass.

M, NE, 

S

Deertongue 

Grass

Dichanthelium 

clandestinum 

‘Tioga’ (Panicum 

c.)

peren. 

/grass warm yes poor- well 20+ 3.8-8.0 H, A, R 6-20 Stratify seed. Wet and dry sites. Establishes slowly.

NC, SC, 

W Saltgrass Distichlis stricta peren./ grass warm yes med.- poor 14-20+ 6.0-8.5 H, R, D 4-9 Salt tolerant.

NC, W Basin Wildrye Elymus cinerius peren./ grass cool yes poor 14-20+ 6.0-8.0 H, A, R 4-8 Long-lived bunchgrass.NC, W Russian Wildrye Elymus junceus peren. /grass cool no med.- well 13-20 6.0-8.0 R, D 4-8 Drought tolerant.NC, W Beardless Wildrye Elymus triticoides peren./ grass cool yes poor 16-20+ 6.0-8.0 H, A, R, D 4-8 Establishes slowly.

W Boer Lovegrass Eragrostis chloromelas ‘A-84’ peren./ grass warm no med.- well 12-19 6.0-8.0 R, D 4-8 Hot, dry climates. Bunchgrass
10-33

Environmental Considerations

MAY 2009 TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

SC, M, 

NE, S

Weeping 

Lovegrass

Eragrostis curvula 

‘Ermelo’

peren. 

/grass warm no well 20+ 4.5-8.0 R, D 1-3 Bunchgrass, short-lived in the NE.

All Tall Fescue

Festuca 

arundinacea 

‘Kentucky-31’

peren./

grass

warm yes

med.-

poor

18-20+ 5.0-8.0 R, D 10-12

Use adapted cultivars Bunchgrass Use only endophyte-free varieties for agricultural land uses.

SC, M, 

NE, S Red Fescue Festuca rubra peren./ grass cool no poor- well 20+ 5.0-7.5 A, R, D 4-6 Use adapted cultivars. Establishes slowly. Sod- forming.

W Galleta Hilaria jamesii ‘Viva’ peren. /grass warm yes med.- well 6-12 6.0-8.0 H, A, R, D 6-10 Very drought tolerant, rhizomatous.W Big Galleta Hilaria rigida peren. /grass warm yes med. 9-14 6.0-8.0 H, A, R, D 6-10 Very drought tolerant, rhizomatous.

NC, SC 

M, NE, 

S

Barley Hordeum vulgare annual/ grass cool no med.- well 20+ 6.0-7.8 H, A, R, D 10-25 Winter cover crop.

NC, SC, 

M

Annual 

(Italian) 

Ryegrass

Lolium multiflorum annual/

grass

cool no med.- well 20+ 5.5-7.5 A, R, D 4-6 Quick cover crop.

SC Klein Grass

Panicum 

coloratum 

‘Selection 75’

peren. 

/grass warm yes med. 18-20+ 6.0-8.0 A, R, D 6-10 Texas.

NC, SC, 

M, NE, 

S

Switchgrass Panicum virgatum peren./ grass warm yes med.- poor 18-20+ 5.0-7.5 H, A, R 2-5

Use adapted cultivars and ecotypes. Blackwell variety is competitive. Rhizomatous. Good winter cover.

S Bahia Grass

Paspalum 

notatum 

‘Pensacola’

peren./

grass

warm no

med.-

well 20+ 4.5-7.5 H, A 25-35 Stratify seeds. Sod- forming semi-bunchgrass.

NC, NE, 

M

Reed Canary 

Grass

Phalaris 

arundinacea

peren. 

/grass cool no poor 18-20+ 5.0-7.5 H, A 5-8 Wet sites.
10-34

Chapter 10

MAY 2009

TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

M, NE, 

S

Timothy 

Grass Phleum pretense peren./ grass cool no poor- well 20+ 4.5-8.0 H, A, R 4-7 Use adapted cultivars Rhizomatous. Short-lived.

NC, SC, 

M Big Bluegrass Poa ampla peren. /grass cool yes well 12-20 6.0-8.0 H, A 6-10 Bunchgrass. Pheasant nesting habitat.

ALL Little Bluestem

Schizachyrium 

scoparium 

(Andropogon 

scoparius)

peren./

grass

warm yes well 12-20+ 6.0-8.0 H, A, R, D 4-8 Use adapted cultivars.

NC, SC, 

M, S, 

NE

Rye Secale cereale ‘Balbo’ annual/ grass cool no well 20+ 5.5-7.5 H, A, R, D 30-60 Cover crop.

NC, SC, 

M, S, 

NE

Indian Grass Sorghastrum nutans peren. /grass warm yes med.- poor 15-35 4.0-7.5 R, D 2-4 Sod-forming, short-lived.

SC, S Johnson Grass Sorghum Sudanese annual/ grass warm no med.- well 20+ 5.5-7.5 A, R, D 15-20 Use adapted cultivars. Cover crop.

NC, SC, 

W

Alkali 

Sacaton

Sporobolus 

airoides

peren./

grass

cool yes poor- well 6-18 7.0-8.5 H, A, R, D 6-10 Use local ecotypes. Bunchgrass. Needs initial Irrigation.

SC, W, 

M Dropseed Sporobolus spp. peren. /grass cool yes med.- well 10-20+ 6.0-8.5 H, A, R, D 4-8 Bunchgrass. Seeds prolific.

NC, SC Cicer Milkvetch Astragalus cicer ‘Lutana’ bien./ legume cool no poor- well 18-35 5.5-7.5 R, D 2-5 Drought tolerant, sod- forming. Establishes slowly. Scarify Seed.

NC, M, 

NE, S Crown Vetch Coronilla varia peren. /legume cool no well 23+ 5.0-7.5 R, D 5-8

Establishes slowly. Excludes tree seedings. Emerald variety for dry sites. Boron tolerant.

M, SC, 

S

Illinois 

Bundleflower

Desmanthus 

illinoensis

peren./

legume warm yes

med.-

well 15-20+ 5.0-7.5 H, A, D 2-5 Drought resistant.

NC, SC, 

M, NE, 

S

Soybean Glycine max annual/ legume warm no poor- well 25+ 5.0-7.0 H, A 30-50 Cash crop. Flooding intolerant.
10-35

Environmental Considerations

MAY 2009 TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

NC, SC, 

M, NE, 

S

Lathco 

Flatpea

Lathrus sylvestris 

‘Lathco’

peren. 

/legume cool no med.- well 23+ 4.5-7.0 R, D 20 Seeds, toxic, excludes tree seedlings. Establishes slowly.

SC, M, 

NE, S

Sericea 

Lespedeza

Lespedeza 

cuneata

peren./

legume warm no well 20+ 4.5-7.0 A, R, D 10-20

Use adapted cultivars. Scarify seeds. Woody stems.

SC, M, 

NE, S

Prostrate 

Lespedeza

Lespedeza 

daurica var. 

schimadai

peren./

legume warm no well 20+ 4.5-7.0 A, R, D 15-20 Prostrate form used with tree seedlings.

M, NE, 

S

Korean 

Lespedeza

Lespedeza 

stipulacea

annual 

/legume warm no

med.-

well 20+ 4.5-7.0 H, R 6-12 Persistent cover Iowa-6 variety for cold sites.

All Birdsfoot Trefoil Lotus corniculatus ‘Empire’ peren./ legume cool no poor- well 18-20+ 5.0-7.5 H, A, R, D 6-8 Use adapted cultivars. Boron tolerant. Does not cause livestock bloat.

NC, SC, 

W Alfalfa Medicago sativa peren./ legume cool no med.- well 17-20+ 6.5-7.5 H, A, R 4-12

Use adapted cultivars. Needs Ca and P. Disease susceptible, flooding intolerant.

All White Sweet Clover Melilotus alba bien./ legume cool no well 16-20+ 6.0-8.0 A, R 4-7 Invasive and competitive. Scarify seed. May cause livestock bloat.

M, NE, 

S

Yellow Sweet 

Clover

Melilotus

officinalis

bien./

legume cool no well 14-20+ 6.0-8.0 A, R 4-7

Invasive. More drought tolerant than M.alba. Scarify seed. May cause livestock bloat.

W, SC Sanfoin Onobrychis vicaefolia peren./ legume cool no well 10-16 6.0-7.5 H, A, R, D 10-20 Use adapted cultivars.

M, SC, 

NE, S

Purple Prairie 

Clover

Petalostemum 

purpureum 

‘Kaneb’

peren./

legume warm yes med. 12-18 6.0-8.0 H, A, R, D 6-8 Showy display of flowers.

NC, W Strawberry Clover Trifolium fragiferum peren./ legume cool no poor 18-20+ 6.0-7.0 A, R, D 2-3 Scarify seed. Provides pasture.
10-36

Chapter 10

MAY 2009

TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

NC, W Alsike Clover Trifolium hybridum peren./ legume cool no poor- well 18-20+ 5.0-7.5 H, A, R 3-5 Attracts bees. Short-lived.

SC, S Crimson Clover Trifolium incarnatum bien./ legume cool no med. 20+ 5.5-7.0 A, R, D 15-25 Cover crop, but persistent with adapted cultivars.

NC, SC, 

W White Clover Trifolium repens peren./ legume cool no med. 18-2-+ 6.0-7.0 H, A, R, D 2-4 Sod forming. Attracts bees.

S, SC Arrowleaf Clover Trifolium vesiculosum annual/ legume cool no med. 20+ 5.0-7.0 H, A 10-15 Use adapted cultivars. Persistent cover.

NC, SC, 

W Hairy vetch Vicia villosa annual/ legume cool no well 18-20+ 5.0-7.5 R, D 20-30 Sod-forming cover corp.

All Common Yarrow Achillea millefolium peren./ forb. warm yes med.- well 10-20+ 5.0-6.0 H, R, D 1-2 Forms ground cover. Drought resistant.

W Western Yarrow Achillea millefolium landulosa peren./ forb. warm yes med.- well 12-18 6.0-8.0 H, A, R, D 1-2 Drought resistant.

NE, S Dwarf-eared Coreopsis Coreopsis auriculata peren./ forb. cool yes well 20+ 5.0-7.0 R, D 2-3 Showy display of flowers. Stoloniferous.

SC, W Rattlesnake Weed Euphorbia albomarginata peren./ forb. warm yes well 14-20+ 6.0-8.0 H, R, D 1-2 Cold tolerant.

All Annual Sunflower Helianthus annauas annual/ forb. warm yes well 15+ 5.0-7.0 H, A, D 6-8 Showy display.

NC, W Annual Sunflower Helianthus annuas ‘Jaegeri’ annual/ forb. warm yes poor- well 6-14+ 6.0-8.0 H, A, D 6-10 Showy display.

SC, NC, 

M, NE, 

S

Gayfeather

Liatris 

pycnostachya 

‘Eureka’

peren./

forb. warm yes poor- well 10-20+ 6.0-7.5 H, A, R, D 2-4 Showy display. Drought resistant.

M, SC, 

NE, S Smartfeed Polygonum pennsyl-vanicum annual/ forb. warm yes poor 15+ 5.0-7.0 H, A 10-12 Wet sites. Roots at nodes. Scarify seed.

NC, SC, 

M, NE, 

S

Coneflower

Ratibida 

columnifera 

‘Sunglow’

peren./

forb. warm yes poor- well 16+ 6.0-7.5 H, A, R, D 2-4 Showy display of flowers.
10-37

Environmental Considerations

MAY 2009 TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

NC, SC, 

W

Four-wing 

Saltbush

Atriplex 

canescens

peren. / 

shrub cool yes pool- well 5-15 7.0-8.0 H, A, R, D 8-12

Use adapted ecotypes. Evergreen. Seeds prolific. Drought tolerant. Very salt tolerant.

SC, W Saltbush species Atriplex spp. peren./ shrub cool yes poor- well 6-12 7.0-8.5 H, A, R, D 8-12 Some species deciduous.

NC, SC, 

W

Siberian Pea 

Shrub

Caragana 

arborescens

peren./

shrub warm no well 12-20+ 4.0-8.5 H, A, R, D 1-2 Aboreal, deciduous.

W Alkali Rabbitbrush Chrysothamnus nauseosus consimilus peren./ shrub cool yes well 8-18+ 6.0-8.0 H, R, D 1-2 Deciduous. Deep roots.

NC, SC, 

W Russian Olive Eleagnus angustifolia peren./ tree warm no poor- well 10-20+ 6.0-8.0 A, R, D 1-2 Invasive, deciduous, shrubby. N-fixer.

NC, SC, 

W Winterfat Eurotia lanata peren./ shrub cool yes well 6-15+ 6.0-8.0 H, A 1-2 Evergreen. Not for Montana.W Gray Moll Kochia Americana ‘Vestita’ peren./ shrub warm yes poor- well 4-20 6.0-9.0 H, A, R, D 1-2 Alkaline sites.

NC, SC, 

W

Summer 

Cypress Kochia prostrata peren./ shrub warm no well 12-18 6.0-8.0 R, D. 1-2 Long-lived very drought tolerant. Prostrate form.

NC, SC, 

W

Weeping 

Mulberry

Morus alba 

‘Kingan’

peren./

tree warm no poor- well 12-20+ 5.0-6.5 H, R, D 1-2 Deciduous.

NE, M, 

S, W

Norway 

Spruce Picea abies peren./ tree warm no med.- poor 16-20+ 5.0-6.0 R, D Evergreen. Cold sites. Plant seedlings.

All Lombardy Poplar Poplus nigra ‘Italica’ peren./ tree warm no well 12-20 5.0-7.0 R, D Deciduous, short-lived. Plant seedlings.

SC, S Live Oak Quercus virginiana peren./ tree warm yes med. 15-20+ 6.0-7.0 H, R, D 1-2 Evergreen. Slow growing.

NC, SC, 

M Bur Oak Quercus macrocarpa peren./ tree warm yes med. 10-20 4.0-7.0 H, R, D 1-2 Deciduous. Plant acorns or seedlings.

M, NE, 

S Bristly Locust Robina fertilis “Arnot’ peren./ shrub warm yes well 15+ 3.5-7.5 H, R, D 2-5 Deciduous. Forms thickets. N-fixer. Scarify seed.
10-38

Chapter 10

MAY 2009

TABLE 10.13 CHARACTERISTICS OF COMMON SPECIES POTENTIALLY SUITABLE FOR RECLAMATION

(Continued)

Regions

of U.S.(1)

Common 

Name

Scientific Name 

and Cultivar(2)

Growth 

Habit/ 

Forms(3,4)

Growth 

Season(5)

Native 

Species(6)

Drainage Precip. Range(7) (in) pH Range(7) Land Uses(8) Seeding Rate(9) (lb/ac) Miscellaneous Notes

All Black Locust Robinia pseudoacacia peren./ tree warm yes poor- well 15+ 4.0-7.5 R, D 1-3 Deciduous. N-fixer. Scarify seed. Most common reclamation tree.

SC, W Tamarisk species Tamarix spp. peren./ shrub warm no poor- well 8-20 6.5-7.5 H, R 1-2 Aboreal, evergreen.

Note: 1. Regions of the U.S.:

M = mid-western states (IA, IL, IN, KY, MI, MN, MO, OH, WI)NC = north-central states (ND, NE, SD)NE = northeastern states (CT, DE, MA, MD, ME, NH, NJ, NY, PA, RI, WT, WV)S = southern states (AR, AL, FL, GA, LA, MS, NC, SC, TN, VA)SC = south-central states (KS, OK, TX)W = western states (AZ, CA, CO, ID, MT, NM, NV, OR, UT, WA, WY)

2. Species and cultivars may vary for site-specific conditions. Consult local agronomy extension agents.3. Annual species are best suited as cover crops. Perennial species are best for persistent cover.4. Grasses should be planted in combination with legumes and other forbs. Legumes should be inoculated with appropriate rhizobium at five times the recommended rate.

Many additional forbs, shrubs, and trees are available and suitable for reclamation.

5. Warm season grasses generally are more tolerant of drought and water stress.6. Native distribution noted as occurs in the particular region.7. Ranges are indicated for optimal growth conditions and may be wider for some species.8. Post-closure land uses. Much overlap is possible:

H = wildlife habitat and open spaceA = agriculturalR = recreationalD = site development

9. Rate indicated for seed mixtures. Higher rates apply for monotypic stands. Seeding rates and mixes should be determined by site-specific conditions. Consult local

agronomy agents.

(DIGIOIA ET AL., 1995)
10-39

Environmental Considerations

MAY 2009

the quick-cover species may choke out and prevent successful long-term establishment of perennial 

species. A balance between short-term erosion control from quick-cover annuals and long-term selfsustaining perennials can be achieved by two-step seeding. This involves an initial dense planting of 

quick-cover annuals, allowing them to be winter-killed, and then seeding perennials into the stubble 

remaining from the annuals the following spring. Plants and recommended cultivation practices for 

humid regions are discussed in Vogel (1987) and Bennett et al. (1978).

In arid and semi-arid areas, exceptionally drought- and salt-tolerant species should be selected (Packer 

and Aldon, 1978). Even if adaptable species are used, high seeding and planting densities without 

supplementary irrigation can lead to excessive water stress and failure. Supplementary irrigation 

may be necessary until root systems are developed. Furrowing along contour lines and planting in 

the furrows will generally result in efficient use of irrigation water and natural precipitation.

Selection of vegetation for impounding embankments should take into account potential impacts 

on dam safety inspection and performance. Inspection of vegetated surfaces of dams and adjacent 

areas, particularly the crest, downstream slope, toe, and adjacent foundation areas is important, as 

discussed in Section 12.3. Trees and woody vegetation are detrimental to both inspection and the 

long-term durability of the embankment. Grasses and shallow rooted native vegetation are the most 

desirable surface cover for an active impounding embankment and dam. Guidance on this issue is 

presented in Marks and Tschantz (2002).
Chapter 11

CONSTRUCTION AND

DISPOSAL OPERATIONS

MAY 2009 11-1

The preparation of clear and concise construction drawings, specifications and an operation and 

maintenance plan to guide contractors and mine personnel is an essential part of coal refuse facility 

development and operation. Property procurement, selection of hauling and transport equipment, 

determination of haul road locations, determination of refuse disposal procedures, and scheduling 

of construction for the various facility components are among the first steps of coal refuse disposal 

facility planning. These planning steps must reflect long-term operational needs as well as short-term 

start-up requirements.

Communication of this information to the personnel responsible for implementing the plans is essential for effective operations. A basic understanding of the general design concepts and potential safety 

hazards associated with a disposal facility will increase the likelihood that a program of monitoring 

and construction control will be conscientiously followed. Regular monitoring and on-site testing 

will facilitate construction and refuse disposal. Monitoring data related to disposal facility development will aid in compliance with regulatory agency requirements and will facilitate modifications 

to facility development should changes occur in mine production. Table 11.1 presents a summary of 

refuse facility operational considerations and related sections in this Manual, as well as references by 

other authors.

11.1 OPERATIONAL PLANS AND CONSTRUCTION DOCUMENTS

Constructing and operating a coal refuse disposal facility in accordance with regulatory requirements 

while meeting the mine’s operational needs requires careful development of construction plans, specifications and an operation and maintenance plan. Development of these construction documents 

requires that the designer collect, interpret and analyze data related to design requirements, regulatory requirements, site conditions and the mine’s operational needs. Input from the mine operator is 

critical throughout the design and permitting process so that mine refuse production, particularly 

start-up rates, are accommodated. Important aspects of refuse disposal facility design should be documented in a design report accompanying the construction documents. This report should present the 

rationale for facility design, important assumptions and constraints, and engineering calculations supporting facility design.

The plans and specifications should provide essential information and should be organized and presented in such a way that they can be easily interpreted by mine personnel, contractors, regulatory 

agencies, and monitoring personnel. The operation and maintenance plan should provide important 
11-2

Chapter 11

MAY 2009

TABLE 11.1 REFUSE DISPOSAL OPERATIONS

Operational Considerations Manual 

Sections Supplemental References

I. Operational Plans and Scheduling

•	 Determine the scope of activities 11.1 to 11.2

•	 Review equipment capabilities 11.3 to11.5 O’Brien et al. (1996), Hartman (1992)

•	 Schedule development 11.2 Clough et al. (2008), Day and Benjamin (1991)

II. Equipment Selection and Use

•	 For transport of refuse from preparation 

plant to disposal area

11.3 O’Brien et al. (1996), Day and Benjamin (1991), 

Hartmann (1992)

•	 For spreading and compacting refuse 11.4 O’Brien et al. (1996), Day and Benjamin (1991), 

Hartmann (1992)

•	 For multiple use in related earth work 

activities

11.4 USBR (1998), Sherard et al. (1963), Church (1981)

III. Refuse Disposal

•	 Disposal as structural fill 11.5 USBR (1991, 1998), Sherard et al. (1963),

Leonards (1962), MSHA (2007)

•	 Fine refuse disposal 11.5 Hartmann (1992), MSHA (2007)

•	 Combined refuse disposal 11.5 Hartmann (1992), MSHA (2007)

IV. Related Activities

•	 Haul road construction and maintenance 11.6 Leonards (1962), Oglesby and Hicks (1982)

•	 Liner systems (if required) 11.6 USBR (1998)

•	 Embankment foundation preparation 11.6 Sherard et al. (1963), USBR (1998), Leonards (1962)

•	 Control of water 11.6 USBR (1998)

•	 General excavation 11.6 Sherard et al. (1963), Day and Benjamin (1991),

O’Brien et al. (1996)

•	 Placement of earth borrow as structural fill 11.6 Sherard et al. (1963), USBR (1998)

V. Materials Selection for Facility 

Appurtenances

•	 Filters and drains 11.7 MSHA (2007)

•	 Culverts and decants 11.7 MSHA (2007), FEMA (2005a)

•	 Concrete structures 11.7 USBR (1987a)

•	 Gates, valves and other metal works 11.7 USBR (1987a)

•	 Mine opening and auger-hole seals and 

drains

11.7 MSHA (2007)

VI. Quality Control and Field Testing

•	 Testing of compacted earth and refuse fill 11.8 USBR (1998), Leonards (1962), Sherard et al. (1963)

•	 Materials testing 11.8 USBR (1998), USBR (1991)
11-3

Construction and Disposal Operations

MAY 2009

information in a concise and easily understandable format so that it can quickly be used as an efficient reference guide for mine personnel performing construction and conducting construction and 

performance monitoring activities.

Ambiguous details, terms and statements in the specifications and drawings should be avoided. The 

extent of information and details provided in the plans and specifications will vary depending upon 

the size and complexity of the refuse disposal facility. The following sections of this chapter present 

guidance as to the content that should be provided in plan drawings, specifications and operation 

and maintenance plans for refuse disposal facilities.

An expansion or modification plan is sometimes prepared for an existing coal refuse disposal facility 

to address changes in facility construction associated with revisions to production rates or changed 

design criteria. Such plans typically include construction drawings, specifications, and a design report 

with supporting engineering calculations. For expansion or modification plans, it is important that an 

index of drawings citing all remaining original and new plan drawings be provided. Revised drawings should clearly reference appropriate specifications, and a revised set of specifications should be 

included in the associated submittals. If facility drawings are modified or revised as part of the new 

submission, it is recommended that a current index and cover sheet be prepared with a complete set 

of drawings to prevent confusion during subsequent review and construction.

11.1.1 Design Report

The design report should present the background data upon which the design is based, along with 

critical aspects of the design related to construction, operation, and abandonment. The design report 

generally includes a discussion of the following:

• Project requirements (disposal facility requirements such as refuse generation rates 

and anticipated life of mine)

• Existing conditions and history of the site including general geologic setting, environmental setting (wetlands, streams or other sensitive areas) previous mining 

activities, and climate/weather

• Field investigations such as surficial reconnaissance and geotechnical and environmental explorations

• Laboratory testing of refuse, water, soil, rock, geosynthetic materials, and structural 

materials

• Geologic and geotechnical site conditions, including assessment of impact of previous and planned mining activities

• Site limitations based upon reconnaissance, explorations and testing results

• Facility development and staging

• Special considerations (e.g., amendments, liner systems, mine bulkheads and barriers, construction issues such as dewatering)

• Equipment considerations (e.g., pumping equipment if needed for meeting 

impoundment drawdown requirements, equipment for spreading and compaction 

consistent with coal refuse production when conveyor systems are employed for 

hauling)

• Geotechnical engineering analyses including slope stability, settlement and seepage 

analyses (and pillar stability and subsidence analyses if underground mining is a 

factor)

• Hydrologic and hydraulic analyses (including design storm, dam breach and inundation mapping) and hydraulic structure design
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• Structural design and detailed monitoring provisions for buried pipelines and inlet 

structures

• Environmental issues

• Abandonment requirements

• Monitoring and maintenance requirements (schedules for implementation and maximum recommended readings for instrumentation)

• Design recommendations and limitations

11.1.2 Construction Drawings

Construction drawings for a coal refuse disposal facility are probably the most important component of 

the construction documents. Construction drawings will likely be referred to more often by mine personnel, inspectors, engineers and regulatory agencies than any other construction documents. Accurate 

and detailed depiction of refuse facility design will clearly indicate the intent of the design and will 

facilitate construction of the facility. Construction drawings must provide information as required in 

30 CFR § 77.215 and 77.216 and must satisfy applicable state regulatory agency requirements. 30 CFR 

§ 77.216 also indicates specific information that must be presented, including the locations of surface 

and underground coal mine workings and the depth and extent of such workings within a distance of 

500 feet from the perimeter of the facility. Construction drawings generally show initial site development and intermediate construction steps with enlargements or details of critical construction features 

and delineation of dimensions and materials. The following sections provide guidance as to the type of 

information and level of detail that should be provided on construction drawings.

11.1.2.1 Title Sheet and Existing Conditions Plans

Construction drawings should include a Title Sheet listing drawings and other project and site reference information. A plan showing existing conditions at the site should be prepared, and each refuse 

facility structure should be identified on an USGS 7.5-minute or 15-minute topographic quadrangle 

map as a general location reference. This is a requirement for all impounding facilities per 30 CFR § 

77.216. Plan drawings depicting existing conditions at a proposed refuse disposal site should generally be prepared at a scale of 1 inch = 100 feet (state regulatory requirements may be more stringent). 

Alternate scales (either smaller or larger) may be appropriate provided that existing features can be 

accurately located and represented and regulatory requirements are satisfied. Existing conditions 

plans should provide coverage of the proposed refuse disposal area and support areas and an additional 500-foot-wide area around the perimeter of the site.

In general, existing features that impact site development and construction of the refuse disposal 

facility should be shown on existing conditions plans. The following guideline describes the general 

level and type of information that should be included:

• Topographic contours of existing ground surface at 5-foot intervals or less (contours 

at 10-foot intervals may be appropriate for steep slopes)

• Footprint of proposed refuse facility for all proposed embankment stages

• Existing and proposed new underground mine workings within 500 feet from the 

perimeter of the proposed facility

• Extent and location of spoil piles and surface, auger, or highwall mining beneath the 

dam and impoundment areas

• Location of coal seam outcrops

• Reported geologic features or observed structures (e.g., joints, hillseams, etc.)

• Surface and subsurface utilities

• Existing refuse disposal facilities
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• Property boundaries

• Prospective borrow areas

• Existing dams and embankments

• Delineation of forests/woods and heavy vegetation

• Existing buildings and structures

• Oil, gas and water wells (active and abandoned)

• Watershed limits, streams and wetlands

• Springs, seeps and mine discharges

• Landslide areas, mine subsidence features, and other ground disturbance

• Exploratory boring and test pit locations

• Public roads and mine access/haul roads

• Mine shafts, boreholes, vent holes and other mine openings

• Other identifiable natural or man-made features (e.g., cemeteries, buildings, etc.) that 

could affect the operation of the refuse facility

11.1.2.2 Initial Site Development Plans

As explained throughout this Manual, extensive site development may be required for preparing a 

site to receive coal refuse material. To provide a stable embankment foundation and safe working 

area for the placement of refuse and efficient operation of heavy equipment, site development construction may be necessary. The following items should be considered for presentation on initial site 

development plans at the same scale or larger than the existing conditions plans:

• Areas requiring clearing, grubbing and topsoil stripping

• Topsoil and excess material stockpile areas

• Utility line modifications or relocations

• Oil, gas and water well modifications or abandonment details

• Areas requiring subsurface drainage such as spring/seep collection systems and 

other underdrain systems

• Prospective borrow areas

• Stream diversions

• Access/haul road alignments and proposed grading contours

• Location(s) of survey control points

• Foundation and abutment areas requiring special subgrade preparations or treatment

• Barriers or backfill areas for treatment of mine openings (shafts, boreholes, auger 

holes, drift mine openings) or underground mine workings

• Initial erosion and sedimentation control measures

• Diversion and collection ditch alignments

• Environmental barriers or buffer zones

• Cutoff and key trench location (for impounding facilities)

• Seepage barrier locations and requirements

• General subgrade preparations including benching (“keying”) requirements

• Subsurface drainage (spring, seep or stream drains)

• Limits of liner installation (clay or synthetic) if applicable per state regulations

• Decant pipe (with riser locations and elevations), principal spillway, and emergency 

spillway alignments
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Some of the items listed above will probably require depiction with additional plans and details. 

To meet both operational and environmental requirements, some larger coal refuse disposal sites 

may need to have phased initial site development plans. These plans should be organized such that 

site development requirements are depicted sequentially. Sections 11.1.2.5 and 11.1.2.6 address additional drawings that may be required and the level of detail that should be provided to supplement 

plan view drawings.

11.1.2.3 Refuse Embankment Construction Plan Views

Proper presentation of refuse embankment designs, whether for an impounding or non-impounding facility, will normally require more than one plan view drawing. As indicated in Chapter 5, 

coal refuse embankments are typically constructed in stages. Each stage of a coal refuse embankment should be depicted by at least one plan drawing at the same scale or smaller than used in the 

existing conditions or initial site development plan drawings. Detailed plan views, cross sections, 

profiles and larger-scale details should be used to depict the design of refuse embankments and 

appurtenances. The required drawings for impounding facilities (including slurry cell facilities) 

will generally be greater in number and detail than for non-impounding facilities. Detailed drawings of impounding facilities should normally be prepared at a scale of 1 inch = 100 feet, although a 

smaller scale may be appropriate for larger sites if sufficient detail is provided in cross sections and 

enlargements. As a guideline, the following items should be presented on embankment construction drawings:

• Footprint and limits of each embankment stage and associated haul road or conveyor 

belt system

• Delineation of the embankment and material zones

• Proposed topographic contours of each stage at intervals at 5 feet or less

• Footprint, grades and elevations, and flow direction of internal drains

• Collection and diversion ditch alignments, grades and elevations

• Decant pipe alignment and riser locations 

• Decant riser schedule with allowable inlet elevations for each embankment stage

• Spillway location and alignment

• Drainage requirements for the working surface

• Piezometer and other instrument locations

• Tabulation of allowable phreatic surface levels measured at piezometers

• Tabulation of maximum pool level for each stage under design storm conditions

• Anticipated fine refuse level at end of each stage, especially for impounding facilities 

with upstream construction 

• Work area sequence within each stage of construction where multiple zones of placement are specified (upstream, downstream or centerline)

• Construction items and sequence for each stage, including features such as seepage 

barriers, mine barriers and seals, etc.

• Haul road and conveyor belt locations and planned extensions

The plan view drawings for each construction stage should include references to cross sections and 

details for complex features of the design.

11.1.2.4 Cross Sections and Profiles

Cross sections and profiles are normally prepared to provide details not depicted on plan views. 

Cross sections are very useful for delineating the various stages of facility development. Cross sec-
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tions are also useful for illustrating complex features of refuse facility design such as cutoff trenches, 

subsurface drainage/underdrains, decant pipe installations, and subgrade preparations.

Consistent with 30 CFR § 77.216, at least two cross sections including longitudinal and lateral cross 

sections through the highest and lowest elevations of a refuse embankment must be provided. Additional cross sections may be needed for complex refuse embankments, especially impounding embankments. The number of additional cross sections is dependent on the complexity of the facility 

design and the construction required for tying into previously constructed features. The following 

items should be considered for inclusion on refuse embankment cross sections:

• Soil and rock units with meaningful descriptions below and at foundation grade

• Elevations and locations of any underground mines (this may require a separate 

cross section, as the depth to the mine workings may require too large a scale to 

depict other items)

• Original ground surface (and existing ground surface if different from original 

ground surface)

• Foundation improvements and special subgrade preparations

• General subgrade preparations

• Embankment surfaces and slopes depicting material zones, if applicable

• Upstream and downstream slope and abutment protection measures

• Delineation of each embankment stage

• Maximum and minimum crest elevations for each embankment stage

• Anticipated fine refuse level at the end of construction for each embankment stage

• For upstream stages, the approximate mixing zone of coarse and fine refuse associated with pushout and development of the embankment stage

• Maximum normal and design storm pool level for each stage

• Final surface grade upon reclamation including provisions for drainage control

• Underdrains/subsurface drainage features and internal drainage systems

• Decant pipeline and risers

• Spillways, including dimension, peak water surface elevation and erosion protection

• Terraces/benches on embankments

• Piezometer locations with sensing zone elevations

• Proposed support equipment or structures (conveyor belts, load out bins, haul roads, etc.)

• Liner systems

• Design phreatic surface

• Stability analysis results depicting subsurface conditions/properties, critical failure 

surfaces (circular, block and wedge surfaces) and minimum factors of safety

Profiles are useful when presenting the design of haul/access roads, decant pipe alignments, ditch 

alignments, and underdrains and other components that extend a great distance laterally. As a guideline, profiles should include the following information:

• Original ground surface

• Proposed/final ground surface

• Grade breaks with corresponding slope designations

• Geometry and layout data (such as radii, point of vertical intersection, etc.)

• Bedding and backfill details for pipes and spillways
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• Erosion protection measures for ditches and spillways (if not shown on detailed 

cross sections)

• Critical subsurface soil and rock conditions and site development requirements

The scale for cross sections and profiles should be the same for both the vertical and horizontal directions, 

although exaggerated cross section segments may be appropriate at large sites where there is significant 

relief. The scale should be sufficient for clear and accurate representation of design features. If necessary, 

more detailed, larger-scaled cross sections should be prepared for critical facility components.

11.1.2.5 Detailed Plan Views and Cross Sections

For instances when the plan view and cross-section drawings for individual stages of a facility do not 

convey the level of detail for critical design features desired by the designers, larger-scale plan views 

and cross sections should be prepared. The following are facility components or features for which 

detailed plan views and cross sections may be needed:

• Cutoff trench/keyway cuts, buttresses and other stabilization structures

• Subsurface drains and underdrain systems including spring/seep drains

• Soil liners and graded filters

• Collection and diversion ditches

• Terraces/benches on the refuse embankment

• Subgrade and foundation preparations

• Treatment provisions (backfill, barrier, drainage, etc.) for mine workings and openings

• Spillways

• Decant pipes and risers, including bedding/backfill zones, thrust blocks, and trashracks

• Stilling basins

• Filter diaphragms

• Haul/access roads

• Piezometers, weirs and other instrumentation

• Embankment and impoundment capping details

• Stream diversions

• Erosion and sedimentation controls

• Culverts and other piping systems

11.1.2.6 Miscellaneous Details and Information

Documentation for impounding refuse disposal facilities should include a stage-area curve and a 

stage-volume curve for the impoundment and a stage-volume curve for the embankment. For facilities that rely on storage of all or part of the design storm runoff, notation of the maximum decant 

level inlet, open channel spillway inlet (if present), and the design storm volume (or portion thereof 

that must be stored) should be included on the appropriate curves to demonstrate that sufficient storage is available without overtopping the embankment. Additionally, the head-discharge curve for the 

decant and spillway should be presented, if applicable. These plotted data should be provided in the 

operation and maintenance plan, as discussed in Section 11.1.4.

If special construction methods or items are required, they should be detailed in the drawings. 

Facility components/details such as berms, pipe beddings, piezometers, V-notched weirs, staff and 

rain gauges, clear water cells, and sealing of mine openings may require additional drawing details 

and information related to their installation or construction.
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11.1.3 Technical Specifications

A complete set of technical specifications that corresponds directly to the construction drawings 

should be prepared for the construction of a refuse disposal facility. Similar to the construction drawings, the level of detail required in the specifications is a function of the type and complexity of the 

refuse facility being constructed. At a minimum, the critical construction requirements that impact 

dam safety and facility operation, as cited in Section 11.2.2, should be clearly addressed in the specifications. Any information related to construction sequencing or methods that are recommendations, 

but not requirements, should be cited as such in appropriate locations in the plan.

There are several standardized specification systems ranging from basic to sophisticated. Specifications 

for refuse disposal facilities must provide sufficient detail that contractors and/or mine personnel can 

easily and clearly understand the facility design requirements. It is recommended that specifications 

follow a consistent format. The following are industry accepted standard specification systems that 

may be applicable and are regularly used for other civil engineering projects:

• CSI by the Construction Specifications Institute

• SPECTEXT® by the Construction Sciences Research Foundation, Inc.

• SpecsIntact by the National Aeronautics and Space Administration (NASA)

These specifications are organized in groups and/or divisions that address categories of work such as 

site development, earth fill, and concrete construction. Each individual specification section normally 

has three parts: (1) General/Scope of Work, (2) Products and (3) Execution. Appropriate specification 

sections to suit the needs and requirements of facility design should be chosen from the standardized 

specifications by the designer. Because of the specialized nature of coal refuse disposal facilities, the 

organization of the standardized specifications may need to be modified to suit specific construction 

activities and to reflect construction staging and chronological sequence requirements.

If the designer chooses not to use a standardized specification system, the format of the specifications 

should include at a minimum the following three parts: (1) General/Scope of Work, (2) Materials/

Products, and (3) Construction Requirements/Execution. If necessary for clarification of design 

requirements, a fourth part titled Method of Measurements may be included. It is ultimately at 

the discretion of the designer as to the format of the specifications and the level of detail provided. 

Typically, project specifications will consist of a number of individual specifications, each of which 

follows the format just described. The following describes the content of each portion of a specification in the above format:

• General/Scope of Work should provide an overall description of the work and 

construction items covered by the specification. Details including the general construction methods and suggested or required sequence of work progression should 

be included. Specific construction drawings should be identified to clarify the work 

covered by the specification. Information concerning related specifications, applicable references, and required submittals should also be provided. It is often useful 

for this portion of the specification to include a technical description of the outcome 

or function desired by the designer for the particular section. Administrative and 

procedural requirements applicable to the work should also be included.

• Materials/Products should specify the requirements of the materials, products and 

accessories to be utilized for construction. Information such as strength, gradation, 

composition, and tolerances of the materials should be provided. The desired testing 

requirements and material certifications to be supplied by the manufacturer should 

also be provided in this section along with a list of submittals where the engineer’s 

approval is required prior to construction. A listing of prospective manufacturers of 
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some materials may be helpful. Where possible, standardized material designations 

from American Association of State Highway and Transportation Officials (AASHTO), 

American Society for Testing Materials (ASTM), or state and federal agencies should 

be provided. If substitutions are allowed, it is important that sufficient information is 

provided to allow for equivalent substitution. Submittals to verify that the appropriate 

materials/products are being used by the contractor should be specified in this section.

• Construction Requirements/Execution should be detailed and should include specifics 

as to construction methods and, if appropriate, equipment that meets the designer’s 

requirements. A detailed construction sequence and schedule can be provided along 

with a description of the final work product desired. Plan view, detail and cross-section drawings should be referenced, along with specifications for related work. When 

applicable, dimensions and tolerances of construction items should be provided. Oversight, inspections, quality control testing, and reporting requirements should be indicated. Lastly, requirements for inspection of critical aspects of the construction by the 

designer or other engineers familiar with the design requirements should be included.

• Method of Measurements can be included in a specification to facilitate work progress and to establish benchmarks for payment and scheduling. For most the construction items, methods of measurements can be established using standard measurement units such as length, area and volume. Methods and required accuracy for 

field measurements should be prescribed. Where unit measurements are not feasible, 

other performance criteria should be defined.

For a refuse facility, detailed specifications that are carefully linked to construction drawings normally provide the most complete depiction of the design and construction requirements. Each individual specification should complement and be consistent with other individual specifications that 

make up the total specification package for facility construction. Responsibility for permits for certain 

activities (e.g., burning or blasting) should be cited in the associated specification. The number of 

specifications required for a particular refuse facility depends upon the type and complexity of the 

facility design. It is the responsibility of the designer to determine the number of individual specifications needed to convey the design intent. The following is a list of specification topics that may be 

appropriate for a refuse disposal facility:

• Survey control and construction documentation

• Clearing, grubbing, stump removal and demolition

• Utility relocation and protection

• Well abandonment (gas and water) and mine opening sealing (boreholes, shafts, vent 

holes, auger holes, drifts, and slopes as well as subsidence features)

• Control of water and stream diversion

• Temporary erosion and sedimentation control

• Sediment/treatment ponds

• Excavation and earth embankment construction

• Foundation preparation

• Exploration for mine openings and sealing of coal seams (exposing coal seams to 

locate openings and covering coal seams with non-combustible material)

• Backfilling of underlying mine workings

• Internal drains and underdrains

• Liner system

• Culverts
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• Decant pipe and principal and emergency spillways

• Conduit thrust blocks and filter diaphragms

• Stilling basins

• Collection and diversion ditches

• Haul and access roads

• Refuse disposal and construction (including placement, compaction, and testing)

• Concrete structures

• Instrumentation

• Reclamation, seeding, soil supplements and mulching

It is recommended that each set of specifications have a summary or administrative section that 

identifies and defines the entities (e.g., Mine Owner/Operator, Engineer, and Contractor) involved 

in the design, construction and operation of the refuse disposal facility and that clearly identifies 

the roles and responsibilities of each party throughout the construction of the facility. Items such as 

basic contract descriptions, work to be provided by the owner and use of the site should be included. 

Additionally, this section should address site safety responsibility. It is also recommended that each 

set of specifications be accompanied by a copy of the design report along with guidance related to 

construction methods and sequence of construction.

Generally, an individual familiar with the design and engineering requirements, such as the design 

engineer, is designated as the Engineer. Also, a representative of the Mine Owner/Operator (Operator) 

is also identified. Both the Engineer and Operator typically have responsibilities for monitoring, 

inspection, and reporting.

11.1.4 Operation and Maintenance Plan

An operation and maintenance plan is a guidance document recommended for use by construction 

and inspection/monitoring personnel during construction and operation of the facility. The intent 

of an operation and maintenance plan is to provide information routinely needed on a daily basis 

by mine personnel, contractors and inspectors/monitors for construction and operation of the facility after disposal operations have commenced. The plan should clearly identify the roles of various parties in monitoring and inspection activities and should cite the involvement of a registered 

Professional Engineer who is familiar with the design (including dam safety criteria) for the structure. Additionally, the representative of the Mine Owner/Operator (Operator) who is responsible for 

the facility and work with the Engineer should also be identified.

An operation and maintenance plan should also provide guidance regarding evaluation of conformance with the approved disposal plan and actions to be taken when observations, testing or instrumentation data identify suspected adverse conditions or obvious hazardous, unsafe or unacceptable 

operating conditions. It should be noted that the operation and maintenance plan is not a substitute 

for construction plans and specifications. A complete set of construction plans and specifications 

should always be readily available during construction and operation of the facility.

An operation and maintenance plan generally does not contain information related to initial site 

preparations and development activities, but may provide provisions for the safety of operating personnel and the facility, including the following information:

Embankment Construction

• Construction method (upstream, downstream or centerline) and upstream construction implementation procedure, if applicable
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• Summary table of crest elevations for each stage

• Schedule of decant riser inlet elevations applicable for activation during each stage 

with the corresponding minimum required crest elevation

• Specific equipment, methods and lift thicknesses for refuse placement

• Procedures for upstream construction and allowable pore pressure readings during 

upstream pushouts

• General grading requirements and slopes for embankment outslopes, working surface, benches and haul/access roads

• Reclamation schedule with general capping information

• Monitoring frequency for refuse generation rates for comparison to rates used for 

facility design

• Survey control and monitoring frequencies

• Decant pipe deflection monitoring

Handling, Spreading and Compaction Equipment Criteria

• Recommendations and criteria for hauling equipment based upon refuse generation 

rates as provided by the operator or owner

• Recommendations and criteria for spreading and compaction equipment to achieve 

optimum lift thicknesses and compaction

• Testing and monitoring requirements (criteria and frequency) for evaluating the effectiveness of handling, spreading and compaction equipment

Instrumentation

• Table listing all instrumentation and site plan showing locations of instruments 

• Installation procedures, details, and schedule

• Frequency/schedule of monitoring and measurements with explanation if other than 

7-day frequency

• Identification of action levels for instrumentation reading, if appropriate (e.g., design 

basis phreatic levels at each piezometer location)

• Routine maintenance schedule and procedures

• Responsibility for plotting and evaluating data

Surface Drainage Controls

• Construction sequence for spillways and decants

• Construction sequence for diversion and collection ditches 

• Schedule of ditch geometries and lining materials

• Acceptable repair techniques

• Cleanout and maintenance frequencies

Impoundment Pool Monitoring

• Schedule of required freeboard for each embankment stage 

• Schedule of decant risers to be activated for each stage with corresponding minimum 

required crest elevation

• Schedule for relocating slurry discharge point to develop delta above normal pool

• Pumping equipment and operating levels, if appropriate

• Stage-storage and head-discharge curves

• Survey control requirements (pool and delta levels)

• Frequency of measurements
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Routine Maintenance

• Vegetation

• Reclamation cover

• Ditches

• Decant pipe risers and emergency and principal spillways

• Outlet areas for decant pipes

• Outlets for internal drains

• Minor erosion

• Instrumentation

General Observations and Data Collection

• Frequency of surficial reconnaissance

• List of specific areas requiring inspection

• Description of unusual conditions or features that should be noted and reported to 

the Engineer

• Reference to schedule and frequency of instruments to be measured with explanation if other than 7-day frequency

• Blank inspection forms and/or data collection forms tailored for site use

Data Review and Reporting Requirements

• Frequency of data review (including plots of pool, piezometers, and seepage data) by 

the Engineer and Operator

• Evaluation of conformance factors (embankment alignments, moisture/density test results, surface drainage channel geometries, impoundment freeboard, decant levels and 

serviceability) or adverse condition indicators (seepage flows, piezometer readings, 

erosion, slope movement, settlement and subsidence) by the Engineer and Operator

• Appropriate document for noting non-conformance factors and adverse condition 

indicators (weekly reports, monthly progress reports, annual reports) with recommended courses of action for such situations by the Engineer and Operator

• Summary of regulatory reporting requirements

Emergency Management

• Description of items that constitute a hazardous condition, including MSHA’s hazardous conditions program (30 CFR § 77.216-3(e))

• Reporting requirements and contact information

• Emergency Action Plan updating requirements (for significant or high hazard potential impounding embankments)

11.1.5 Calculation Brief

The term “calculation brief,” as used herein, refers to an organized compilation of engineering analyses and calculations associated with facility design prepared for documentation and for facilitation 

of regulatory review. The engineering parameters used in the analyses should be substantiated and 

assumptions should be supported. In addition to the design report, plans, and specifications, the 

calculation brief is an important part of the documentation of coal refuse disposal facility design. A 

calculation brief should include the following:

• Coal refuse production rates (by weight and volume) and anticipated facility life

• Starter embankment and refuse disposal staging (impoundment and embankment 

volumes by stage)
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• Hydrology and hydraulic analyses (sedimentation control, design storm routing, 

decant design, spillway channel and drainage ditch designs, dam breach analysis)

• Stability analyses (including seismic hazard assessment)

• Settlement analyses

• Seepage analysis and internal drain design (including filter design)

• Surface drainage channel lining design

• Buried conduit structural and durability analysis

• Design of appurtenant structures (trashracks, thrust blocks, stilling basins, etc.)

• Subsidence analysis, pillar stability analysis, breakthrough potential evaluation, and 

mine barrier design

• Environmental analyses

11.2 PLANNING AND SCHEDULING

Planning and scheduling of operations for coal refuse disposal includes the following:

• Time required for site selection, geotechnical investigation and testing, environmental assessments, preliminary and final design, and for obtaining regulatory permits/approvals

• Stipulations associated with current or future mining, such as anticipated life of mine 

and mining within or near the embankment and impoundment area

• Current and predicted future rates of production of coal refuse furnished by the 

Operator

• Anticipated physical and chemical properties of the material to be disposed

• The rate and method of transport or hauling of material to the disposal area

• The capability of the materials handling equipment to place, spread and/or compact refuse

• The planned embankment configuration, crest elevation and impoundment capacity 

at any given time

• The anticipated pool level and settled fine refuse level at any given time

• Construction timing of facility appurtenances related to disposal operations

• Potential delays or special procedural requirements related to weather, work suspensions, regulatory agency stipulations or other circumstances beyond the control of 

site personnel.

Construction of some features, such as drainage control structures, which are constructed or extended 

during the facility operation, may require considerable planning and scheduling if they are to be 

completed by the mine operator’s forces. Otherwise, the services of a construction contractor may be 

needed for completion of the work in a timely manner.

The above items should be re-evaluated periodically as changes in mining operations occur. Lesser 

considerations, including those related to specific mine and site locations, are discussed throughout 

this Manual. Through proper planning and scheduling of the various development aspects of the coal 

refuse disposal facility, operations can proceed in an economical and efficient manner. Early identification of production changes and their impact on the disposal plan and timely preparation of design 

modifications will allow more flexibility in meeting long-term disposal requirements.

11.2.1 Planning of Coal Refuse Disposal Operations

Plans for coal refuse disposal operations differ from plans for typical construction projects because 

they must account for the following:
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• The disposal facility will usually be in operation for many years necessitating longterm and somewhat speculative planning and timely modifications as conditions 

change.

• Disposal operations will occur under a variety of weather conditions. The construction of facility appurtenances must be properly timed to avoid delaying or interfering with disposal operations.

• The configuration and characteristics of a refuse embankment during facility development must suit changing conditions frequently unrelated to the final abandonment condition. For example, at impounding facilities, the impoundment storage 

plus the spillway outflow must be sufficient in combination to handle the design 

storm hydrograph at all times.

• Over the entire period of disposal operations, changes in equipment, technology and 

regulatory requirements may necessitate changes in schedule and operations. On-going reviewing and updating of implementation procedures are required.

Equipment requirements should be evaluated as part of the planning of coal refuse disposal operations. 

The equipment used for haulage, spreading and compaction should be determined based on production forecasts, and all aspects of the process should be balanced such that the specifications (lift thickness, density, moisture content, grading) for material placement can be met. When conveyors are used 

to provide haulage to the disposal surface, the equipment must be capable of spreading and compacting the refuse such that lift thickness, moisture content and density specifications are met.

11.2.1.1 Long-Term Planning

Long-term planning for refuse disposal operations is concerned with decisions and actions that affect 

implementation of the overall general plans discussed in Chapter 4. Items that should be considered 

in the long-term planning include:

• Anticipated mine life

• Property acquisition

• Proximity to the preparation plant

• Time periods associated with obtaining permits/approvals

• Equipment selection and procurement

• Infrastructure requirements

• Refuse disposal techniques

• Timing for construction of facility appurtenances

Property acquisition often entails lengthy transactions associated with price negotiations and legal 

documentation. The acquisition of properties for a refuse disposal facility should be based upon the 

ultimate refuse disposal capacity that will be required over the life of the mine. Acquisition of all 

necessary property at the initiation of refuse disposal operations may not be required because the 

facility will not occupy the full disposal area for many years. Thus, an agreement to purchase or lease 

additional property at a later date may be satisfactory.

The equipment and support structures in place at the start of disposal operations should not only 

meet short-term needs, but should continue to be fully functional as the disposal area is developed. 

Selection of hauling equipment based only upon initial disposal requirements can lead to inefficient equipment use before the equipment is fully depreciated. Factors such as haul distance, haul 

road alignment and grade, and refuse production affect the optimization of equipment selection. 

Loss of equipment efficiency due to age and wear must also be considered in long-term planning. 
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Some types of equipment have significantly shorter useful life expectancies than others, and it may 

be possible to have the replacement and upgrading of site equipment coincide with major changes 

in the facility.

A key aspect of long-term planning of refuse handling at the point of disposal involves analysis of 

the spreading and compaction of coal refuse that will be required for construction of the embankment. Achievement of specified compaction requires planning for, acquiring and using suitable 

equipment.

Planning for construction of facility appurtenances, infrastructure, and related items is particularly 

important if the disposal of refuse is not to be interrupted during long-term operations. With proper 

long-term planning, construction can be accomplished with minimum interference with disposal 

operations.

11.2.1.2 Short-Term Planning

Short-term planning is mainly concerned with current and near-term disposal operations and associated construction activities. Short-term planning should result in:

• Facility development in a manner that does not delay refuse disposal operations

• Completed construction conforming to design and regulatory requirements

• Maximum utilization of on-site equipment and materials

• Control of costs during the completion of all tasks

It is ultimately the facility operator’s decision as to what specific planning and scheduling techniques 

are employed. It should be recognized that short-term planning for the development of coal refuse 

disposal facilities may differ from long-term planning in several respects. Some short-term construction may be accomplished by the operator using on-site labor and equipment. Therefore, manpower 

and equipment must be available at the site to accomplish needed construction without interference 

to mining, coal preparation or refuse disposal activities. Utilizing existing staff and equipment for 

multiple purposes is an essential part of short-term planning.

Many aspects of operational planning are governed by technical design and/or regulatory requirements. Short-term design storm criteria should be established for facility start-up and abandonment periods, as discussed in Section 9.5. A key aspect of operational planning is maintenance of the 

embankment configuration and spillway capacity during disposal operations, when long-term design 

storm criteria must be met. Spillways, decants, drains and other hydraulic appurtenances must be 

constructed without loss of facility function and with expansions sequenced to maintain safety and 

environmental control. Periodic expansions of hydraulic structure capacity should be planned such 

that long-term design storm criteria are met. Any temporary periods during such expansions when 

flood routing capacity is unavoidably reduced or limited must be minimized. Planning for such 

expansions should include: (1) construction procedures, (2) evaluation of the impact of the construction on design storm management and the capacity of impoundment and hydraulic structures, (3) 

provisions for a monitoring program, (4) a schedule for the expansion work, and (5) potential contingent actions in the event of major storms.

Under emergency conditions, it may be necessary to accomplish some tasks in the shortest possible 

time. Providing increased spillway capacity for an existing impoundment, stopping embankment 

leakage, or stabilizing a sloughing embankment are examples of such emergency conditions. Using 

on-site labor and equipment may not provide a satisfactory solution in such circumstances, as the 

needed manpower and equipment may not be available. Thus, contracting of work, rental of additional equipment, employment of contractors with special skills, or working during adverse weather 
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conditions may be required, thus increasing costs. Effective short-term planning is important in these 

situations and can minimize unanticipated expenses.

11.2.2 Scheduling Methods and Application

Scheduling refers to the selection of dates for starting each identified task and the assignment of a 

period for completion. In other words, scheduling represents the time sequence and duration aspects 

of planning. The following are industry-accepted techniques that can be used for both long-term and 

short-term scheduling:

• Intuitive judgment/experience

• Bar chart

• Critical Path Method (CPM)

• Program Evaluation and Review Technique (PERT)

11.2.2.1 Scheduling Data

With any scheduling technique, assumptions and input data greatly influence the outcome. Therefore, 

it is important that the assumptions and data used to generate the schedule are evaluated and agreed 

upon by key operational personnel. Otherwise, the results of the scheduling exercise many not be 

valid and may possibly be misleading. Input data used in short- and long-term scheduling associated 

with development, construction and operation of a refuse disposal facility may include:

• Mine development and coal preparation plant construction times

• Refuse generation rates (present and future)

• Design and permitting times

• Specialized material lead times

• Equipment delivery lead times

• Contractor lead times

• Site development and support facility construction times

• Start date of refuse generation or date when refuse facility is required to be operational

• Refuse construction milestone dates

• Regulatory requirements for facilities to be capable of handling design storm runoff, 

especially at start-up and decommissioning

Acquiring the above data requires a thorough understanding of the construction and operation of 

refuse disposal facilities, and knowledgeable personnel should be consulted in the very early stages 

of planning for a refuse facility. During the initial phases of planning for a new refuse facility, available data regarding the coal seam to be mined and the proposed preparation process to be employed 

may not be well known. As the planning process moves forward, refinement of the input data should 

be performed as new information becomes available.

The operator should utilize as many resources and information sources as practical when performing initial planning and scheduling for a new refuse facility. Initial data for refuse generation rates 

can generally be obtained from process flow diagrams for the proposed preparation plant or from 

nearby active preparation plants processing coal from the same coal seam. Permitting requirements 

and associated approval periods can normally be obtained by contacting the applicable regulatory 

agencies and design consultants working in the area. Data regarding site development and related 

construction activities can sometimes be obtained from engineers/consultants and contractors. Once 

the refuse facility becomes operational and refuse is being actively placed, more detailed site data 

will be available.
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Accurately quantifying refuse generation rates for both coarse and fine refuse is important for both 

long-term and short-term planning. Refuse generation rates may change gradually or abruptly over 

the life of a refuse disposal facility. The rate of refuse generation and the ratio of coarse to fine refuse 

are the governing factors that dictate the construction sequencing and ultimate life of a refuse disposal facility. Accurately knowing the rate of refuse production will allow for better forecasting and 

budgeting of required site development and support facility construction activities. As mentioned 

earlier, impounding facilities must be designed such that the design storm hydrograph can be safely 

retained and/or passed through the spillway. Typically, the embankment crest level must be maintained at a minimum height above the level of the impoundment. Knowledge of the refuse generation 

rate will facilitate maintaining the required crest level.

An effective method for establishing accurate refuse generation rates is through analysis of periodically collected site topographic data. This method requires at least two sets of topographic data 

obtained at known dates. The time period between data collection should be sufficient to allow for 

the placement of a considerable volume of refuse. The two sets of topographic data can then be 

used to determine the net in-situ refuse volume. The rate of refuse generation is then the net volume 

divided by the elapsed time. This method is applicable to both slurried fine refuse and coarse refuse. 

In the case of slurried fine refuse, the receiving impoundment must be sounded to determine the top 

of settled fine refuse. The frequency for performing such analyses depends upon the mining operations and level of accuracy desired. Annual analysis is consistent with MSHA’s annual reporting 

requirements. It is not unreasonable for refuse rates to be evaluated semi-annually, especially during 

early phases of facility development. Of course, any time that mining conditions or plant operations 

change, when the preparation plant accepts coal from different sources, or when the plant is modified, changes in refuse characteristics and production rates must be accommodated. This may entail 

re-evaluation of engineering properties of the refuse, including the refuse grain-size and compaction 

testing, in order to evaluate impacts on the strength and hydraulic conductivity of the fill and enable 

tracking of material placement and embankment construction.

11.2.2.2 Applicability of Scheduling Techniques

Some disposal facility operations are relatively simple and consist of only a few activities or tasks. 

In these cases, scheduling by managers based upon their past experiences and intuitive judgments 

may be adequate for efficient implementation of operation plans. As site operations become more 

extensive and complex, more formal scheduling methods become increasingly valuable for evaluating the economic, technical and time implications of task completion. Bar chart scheduling is the least 

complicated formal scheduling method and is applicable to most small refuse disposal operations. 

Large disposal facilities often have many interrelated activities, and for these sites, use of the CPM or 

PERT scheduling methods may be more suitable.

As the sophistication of the scheduling process increases, management time and costs also increase. 

Generally, this investment has an early and valuable benefit through reduction of errors and omissions and lessening the degree of management control required during operation. However, it is 

possible to over-refine a schedule by subdividing the required tasks into too many activities or by 

projecting the schedule beyond the point where the data provide dependable information.

Often the best approach to scheduling incorporates two or more scheduling methods. For example, 

short-term more well-defined plans can be scheduled with considerable detail using CPM or PERT, 

and long-term, more broadly-defined plans can be conceptually scheduled using a bar chart. Intuitive 

judgment may be used for updating or modifying the more complex schedules.

It is emphasized that regardless of the approach, schedules must be periodically updated. This is 

especially important for long-term coal refuse disposal, where technology, regulations and equipment capabilities may change with time. Through updating of the schedule, operations can be regu-
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larly re-evaluated. Thus, the schedule becomes an effective management tool for decision making 

and operations planning throughout the entire operation period of the coal refuse disposal facility.

Because each mine operator has unique scheduling requirements and employs different scheduling 

techniques, it is not feasible to specify scheduling techniques for use at specific types of refuse disposal sites. The most important considerations are that the scheduling techniques employed meet the 

needs of the mining operation and that the design capacity of the disposal facility and compliance 

with critical parameters (e.g., freeboard, decant level, crest level) are maintained.

11.3 REFUSE TRANSPORT

Refuse transport systems are used for conveying coal refuse from the preparation plant to the disposal area. The purchase, operation and maintenance of these systems can represent a significant 

portion of the total cost of refuse disposal. The refuse transport system has impacts upon the construction and layout of the refuse disposal facility and in many cases dictates the type of handling, 

placement and compaction equipment required. As discussed previously, methods for placing and 

compacting refuse control the geotechnical characteristics and particularly the strength properties of 

the in-place refuse. Therefore, it is important that the refuse transport system be carefully evaluated 

as part of refuse disposal facility planning.

This section discusses effects that the selection of the refuse transport system may have on the geotechnical design and construction of a refuse disposal facility. Other sources should be consulted for 

the design and selection of refuse transport equipment.

Transport systems for coal refuse disposal operations typically consist of:

• Individual motorized hauling units (on- or off-road haul trucks or scrapers/pans) 

and associated access and haul roads

• Continuous mechanical arrangements such as conveyor belts

• Continuous hydraulic assemblies and/or pipelines for pumping slurried fine refuse

For both of the first two systems listed above, transport of the refuse typically represents the most 

significant portion of the disposal costs. However, handling of the refuse at either end of the transport system is also a consideration in the design of a refuse disposal facility. Sections 11.4 and 11.5 

discuss equipment for handling, placement and compaction of refuse after it is transported to the 

refuse disposal site. Typically, loading at the preparation plant is accomplished by dumping from 

bins, and this is essentially the same for either transport system. Handling of refuse at the point of 

disposal can vary depending upon the system selected, refuse characteristics, and disposal requirements. Individual hauling units such as trucks can usually place refuse near the final disposal location and thus limit further handling. Conversely, continuous mechanical systems generally require 

additional refuse movement using hauling units and spreaders at the disposal area. Combinations 

of continuous mechanical equipment and individual hauling units are usually required to meet the 

needs of a disposal facility.

The anticipated refuse generation rates and the distance from the preparation plant to the refuse disposal site are the most important considerations in choosing a coal refuse transport system. For relatively low refuse generation rates, individual hauling units (on- or off-road haul trucks) are usually 

feasible and cost effective for transporting coarse coal refuse. As refuse generation rates increase, the 

economics begin to favor use of a continuous conveyor belt system for coarse refuse. In either case, 

slurried fine refuse will require a hydraulic system/pipeline for transport. At mines with low refuse 

generation rates, slurried fine refuse is sometimes pumped to small cells near the preparation plant or 

mechanically dewatered. Once the fine refuse reaches a state where it can be handled with excavating 
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equipment, it is placed in haul trucks and transported to the refuse site for disposal either separately or 

combined with coarse refuse. With higher refuse generation rates, it may be cumbersome to dewater 

slurried fine refuse prior to disposal. For this case, slurried fine refuse is normally pumped directly 

from the preparation plant to the refuse disposal facility. Alternatively, the fine coal refuse may be 

mechanically dewatered at the preparation plant and transported with the coarse refuse. In any case, it 

is recommended that the final selection of a refuse transport system or combination of systems be based 

upon detailed information relative to refuse material characteristics and in-situ refuse compaction and 

strength requirements.

Although rarely pursued, crushing of coarse refuse and pumping with fine coal refuse has been tried 

in a few cases. This total refuse pumping system adds complexity to the staged construction of the 

impounding embankments (recovery of hydraulically placed materials for subsequent construction) 

and may affect the durability and life of the conveyance piping.

11.3.1 Trucks and Scrapers

Individual hauling units and the haul roads upon which they operate can be considered as an integrated transport system and should generally be addressed in the facility plans and specifications. 

This section discusses the types of hauling units available, the relative advantages and disadvantages 

of each, and their applicability to specific site conditions. Haul road characteristics must be considered as part of equipment evaluation. Additional information for such evaluations is provided in 

Section 11.6.1 and by MSHA (1999) in The MSHA Haul Road Inspection Handbook.

Typical hauling units include: rear-dump trucks (conventional type), bottom-dump tractor-trailers, 

side-dump tractor-trailers and rear-dump tractor-trailers. Hauling units are available for both onhighway use and for more rugged off-highway use. Off-highway scrapers may also be used as hauling units and have the added advantage of being able to self-load and spread the hauled materials. 

Each type of hauling unit has advantages and disadvantages that must be evaluated with respect to 

the specific requirements of disposal operations.

Individual hauling units are standard construction equipment. Thus, their capability and flexibility 

are frequently utilized in embankment construction. They provide a means to effectively deliver fill 

materials throughout the work area and to provide some compaction if their routing is distributed 

evenly across the disposal area.

11.3.2 Conveyor Belt Systems

Conveyor belts can be used to transport coal refuse from the preparation plant directly to the disposal area for controlled constructed fill or placement or to intermediate storage areas for subsequent 

transport by individual hauling units. Conveyor belts are particularly useful in mountainous regions 

where haul road construction is difficult or where steep grades decrease the efficiency or safety of 

operating individual hauling units. The evaluation of any transport system is primarily based on a 

comparative economic analysis of the cost for conveying refuse, but the handling requirements at 

both ends of the transport system must also be considered.

When conveyor belt systems are used for direct transport of refuse to the working surface of a constructed fill, handling, spreading and compaction equipment must be employed for placement and 

compaction of refuse. Direct placement of refuse with a conveyor belt and inadequate handling, 

spreading and compaction equipment (e.g., a dozer with insufficient capacity for spreading given the 

delivery rate of the conveyor) can result in refuse lifts that are either not horizontal, are too thick, or 

are otherwise not adequately compacted. Other sources should be consulted concerning the selection 

and design of conveyor systems.
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11.4 HANDLING EQUIPMENT

Handling equipment is usually required at each end of a coal refuse transport system to: (1) load 

refuse at the preparation plant or point of intermediate storage, (2) haul refuse from a conveyor belt 

bin to the point of disposal, and (3) spread and compact refuse at the point of disposal. The principal 

function of the handling equipment is to spread and compact refuse in accordance with the design 

requirements of the refuse disposal facility so that a stable embankment is constructed. Specifically, 

handling equipment is used to place and spread refuse uniformly over the embankment’s working 

surface in horizontal lifts with thicknesses and densities consistent with design specifications. The 

purpose of the compaction specifications is to ensure that the material placed in the embankment has 

the engineering properties (e.g., strength and hydraulic conductivity) used in the design. 

The selection and use of handling equipment to distribute and spread refuse at the point of disposal 

has a considerable impact on the capability for meeting geotechnical design requirements. If the 

capability of the distribution and spreading equipment does not at least match the arrival rate of 

the refuse, layers will be too thick for proper compaction or will not receive an adequate number of 

passes, and as a result the compacted refuse will have lower strength and be more permeable. As 

discussed in Section 5.1 and Section 11.5, embankments may be designed with zones having different 

properties and requirements for different levels of refuse handling, spreading and compaction. For 

any embankment zone, it is desirable to place refuse in lifts so that the working surface can accommodate heavy equipment during adverse weather conditions and the potential for spontaneous ignition is addressed.

Table 11.2 summarizes the major types of handling equipment that serve various transport systems, 

excluding loading equipment. For a conveyor belt system, loading at the preparation plant will generally be continuous from relatively small hopper bins. If the transport system comprises individual hauling units (on- or off-highway haul trucks or scrapers), loading at the preparation plant may also be from 

hopper bins, but in this case, the hopper bins should be sized to accommodate surge storage, particularly if the hauling units operate intermittently. A limited capacity for intermediate stockpiled storage 

can be provided adjacent to the preparation plant, although this should generally be avoided. Loading 

of the coal refuse into the hauling units would then normally be performed by front-end loaders.

For conveyor belt systems, refuse must be moved from the downstream end of the transport system 

to the point of final disposal using hauling and/or spreading equipment. For most transport systems, 

an off-road haul truck and tractor dozer can be used for hauling and spreading the refuse at the 

disposal area. Compaction of the refuse can be accomplished with the hauling and spreading equipment and/or with separate compaction equipment. Routing of the hauling and spreading equipment 

over the refuse can achieve partial compaction if performed systematically. Bulldozers typically will 

need to make more passes over thinner lifts to achieve the same level of compaction as equipment 

specifically designed for compaction, and a bulldozer alone can only achieve suitable compaction 

by many passes over each lift (with overlapping tread coverage) if it is not interrupted by spreading 

requirements. Thus, matching equipment capacity to the refuse delivery rate, with suitable backup 

to accommodate downtime, is important and can impact the ability to use only spreading or hauling 

equipment for compaction.

11.4.1 Loading and Hauling Equipment

Although coal refuse is typically loaded from hopper bins directly into hauling units, loading from 

a stockpile may be required for transport systems composed of individual hauling units. For general 

construction or earth-moving operations, the capacity of the loading equipment should be closely 

matched to that of hauling units to minimize loader slack time and hauler spotting and loading time. 

To minimize hauler cycle time, the loader should be sized so that it can quickly fill a hauler, even 

though slack time for the loader may result.
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Even though loaders may not be required for typical refuse disposal operations, especially those that 

utilize conveyor belts and bins for refuse transport, some loading capacity should be available at the 

disposal facility site. It is probable that loading equipment will periodically be needed for construction of other facility structures. Also, a loader is likely to be used for some unplanned aspect of refuse 

disposal. For example, a needed repair at some location on the embankment may make it desirable 

to quickly load and haul disposed refuse from another location. Loading equipment used for such 

general purposes should be selected on the basis of versatility.

Hauling units are usually employed for transporting refuse from conveyor belts and bins to the point 

of refuse disposal. These units are normally built for off-road use and can supplement compaction 

efforts, if used correctly. Individual hauling units are also recommended for facilities where conveyor 

belts transport refuse directly to the point of disposal. In this situation, if spreading equipment is unable 

to keep pace with the refuse generation rate, the hauling units can be used to transport refuse to other 

areas of the embankment working surface to prevent accumulation of refuse at the conveyor belt.

11.4.2 Spreading Equipment

As indicated in Table 11.2, tractor dozers are the commonly-used materials handling equipment serving the point of refuse disposal, regardless of the type of transport equipment used. If the proper 

number of units are available and they are sized and operated correctly, they can efficiently spread 

refuse in relatively horizontal layers of controlled thickness, and they can economically move refuse 

a significant distance. For a tractor dozer to be efficient and economical, an analysis should be performed to size it specifically to site conditions. Equipment manufacturers and experienced construction personnel should have input to the selection process. It may be useful to experiment by trying 

various sized tractor dozers and dozer blades at the refuse facility under normal operating conditions 

to determine the most economic and efficient tractor dozer/blade combination.

For coal refuse disposal operations, track-type tractor dozers are generally advantageous because 

of their greater ability to grip inclined surfaces at refuse embankments and their ability to perform 

TABLE 11.2 TRANSPORT SYSTEMS AND ASSOCIATED HANDLING EQUIPMENT 

AT THE POINT OF DISPOSAL

 Transport System Handling Equipment and Use

Off-Highway and On-Highway

Haulers

1. Tractor dozer(s) to spread dumped refuse and to construct and 

maintain haul roads.

2. Compaction equipment for structural fill disposal of refuse – may be 

supplemented by routing of hauling units.

Scrapers/Pans

1. Compaction equipment for structural fill disposal of refuse – may be 

supplemented by routing of hauling units.

2. Tractor dozer to shape embankment and assist with construction and 

maintenance of haul roads.

Conveyor Belt System

1. Tractor dozer to distribute and spread refuse dumped from end of 

system and to construct and maintain access roads.

2. Short-haul equipment if distance from end of system to point of final 

disposal is large with tractor dozer to distribute and spread refuse.

3. Compaction equipment for structural fill disposal of refuse.

 Note: This list of equipment excludes handling equipment requirements at the preparation plant or

other equipment requirements for construction of facility appurtenances other than access and

haul roads.
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excavation operations as part of the construction of other facility structures. Low-ground-pressure, 

track-type tractor dozers are also desirable for situations where low ground pressures are required 

for coarse refuse pushouts over settled fine refuse at impounding embankments using the upstream 

construction method. If the disposal surface is relatively flat and well maintained and upstream 

construction over fine refuse is not required, a rubber-tired tractor dozer will be attractive because 

of its greater speed and mobility. Detailed information regarding the sizing and capabilities of tractor dozers is provided in Church (1981) and is also available from equipment manufacturers.

Scrapers can be used to transport and spread refuse at disposal sites, minimizing the need for a tractor dozer. However, fine grading of slopes and other site construction activities frequently require 

the use of dozers.

11.4.3 Compaction Equipment

Compaction that increases density and strength and reduces the hydraulic conductivity of disposed 

coal refuse is essential for construction of stable embankments. Compaction is intended to achieve a 

homogeneous fill with relatively uniform properties over the depth of each layer or lift. Most designs 

and construction specifications for refuse embankments define compaction in terms of density and 

moisture content and leave equipment selection to the owner or contractor. However, the type of 

compaction equipment used is directly related to the efficiency and uniformity achieved and can 

also affect material breakdown. Particles of coal refuse are often susceptible to breakdown, such that 

following compaction, scarifying measures may be required for achieving lifts that are well kneaded 

together and have uniform properties.

Embankments for slurry impoundments and the downstream shell used to confine combined refuse 

or slurry cells typically are designed with a well-compacted, refuse-material zone placed in lifts with 

specified density and moisture criteria (sometimes referred to as the structural zone) to provide for 

stability and seepage control. Other zones of these embankments may be designed with less stringent 

lift thickness or compaction criteria provided that embankment stability is acceptable and the potential for spontaneous combustion is addressed.

Depending upon the specified compaction requirements, the compaction of coal refuse can normally 

be achieved by the systematic and consistent travel of transport and spreading equipment over refuse 

that has been placed in controlled lifts generally equal to or less than one foot in thickness. This 

method of compaction will almost always satisfy compaction requirements associated with control of 

burning even if the lifts are relatively thick (up to 2 feet as allowed under 30 CFR § 77.215). However, 

routing of hauling units over the working surface may be insufficient to consistently compact the 

refuse to meet typical criteria, and periodic test pad demonstrations and compaction testing should 

be employed to verify effective results if only hauling units are being used. For well-compacted 

embankments or embankment zones, specifically-chosen compaction equipment and/or procedures 

should generally be employed in order to achieve consistent and effective compaction.

Table 11.3 presents a general comparison of the compaction capabilities of various types of equipment moving over various refuse lift thicknesses. In all cases, the wheel or track of the equipment 

must pass over each area several times. Figure 11.1 illustrates this procedure. The number of passes 

and lift thicknesses required for obtaining various degrees of compaction is further discussed in 

Section 11.5.1. The resulting effect of compaction should be verified through field monitoring.

The basic types of compaction equipment generally employed at coal refuse disposal sites are:

• Rubber-tired, off-highway hauling unit(s) (preferably loaded)

• Sheepsfoot/segmented pad self-propelled roller (vibratory and non-vibratory)

• Smooth-drum, self-propelled or towed vibratory roller
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Loaded rubber-tired off-highway hauling units are effective for compacting both cohesive and granular materials due to the heavy weight of the loaded equipment concentrated at the relatively small 

footprint of the tires. Sheepsfoot, self-propelled rollers are generally most effective on cohesive materials with a high percentage of silt- or clay-sized particles. However, if the sheepsfoot roller has a 

vibratory capability, it will also be effective on granular materials such as coarse coal refuse. Smoothdrum vibratory compactors are most effective on granular materials and are useful for surface sealing to minimize infiltration.

Vibratory rollers, either smooth-drum or sheepsfoot, are better suited for coarser materials containing 

small amounts of clay-sized particles. According to a study performed on test pads of coarse refuse, 

Saxena et al. (1984) found that either a vibratory smooth-drum roller or a vibratory sheepsfoot roller 

were effective for compaction of lift thicknesses up to 1 foot thick. The vibratory action of these types 

of compactors, which can usually be adjusted, is obviously important for achieving compaction. The 

vibration associated with the operation of track-type tractor dozers is likely a key component that 

facilitates the compaction of granular materials, as indicated in Table 11.3. Manufacturers’ literature, 

information in periodicals, and Church (1981) provide information on the compaction force associated with specific types of equipment.

Because the characteristics of coal refuse vary considerably, selection of the most appropriate compaction equipment should be based upon both material and site-specific factors, and the selected equipment should be capable of achieving specified strength and hydraulic conductivity requirements. 

Vibratory-type compaction equipment has been found to be effective for compacting coal refuse. 

Sheepsfoot rollers (vibratory or non-vibratory types) have the advantage of crushing and compacting weathered or soft rock in coarse refuse. However, larger pieces of resistant rock may make such 

rollers ineffective because the feet may strike the rock and lift the roller, preventing compaction of 

adjacent material. The same effect can occur with smooth-drum rollers. If larger, resistant rocks are 

prevalent at a site, loaded off-highway haul trucks should be used.

TABLE 11.3 COMPACTION CAPABILITIES OF TRANSPORT AND SPREADING EQUIPMENT

Equipment Relative Lift Thickness Effect of Compaction

On-Highway Hauling Units

Very Thin Fair to Good

depending on size of unit

Medium Poor

Off-Highway Hauling Units

Thin Good

Depending on size of unit

Medium Fair to Good

depending on size of unit

Scrapers:

Double-Axle Mover Thin Fair

Track-Type Mover Very Thin Generally Poor – Good for thin lifts

if material is clean and granular

Tractors:

Rubber-Tired Tractor Thin Fair

Track-Type Tractor Very Thin Generally Poor – Good for thin lifts

if material is clean and granular

Note: Proper overlap of wheel or track and multiple passes over each area is required in all cases (Figure 11.1) 

 The resulting effect of compaction should be field monitored and verified.
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FIGURE 111 COMPACTION BY HAULING UNITS USING 

 2. TO ACHIEVE REQUIRED

 COMPACTION, MULTIPLE

 PASSES OF HAULING

 EQUIPMENT OVER THE

 SAME AREA MAY BE

 NEEDED.

 

NOTE: 1. EDGES OF PASSES

 SHOULD OVERLAP.

THIRD PASS

TIRES SECOND PASS

FIRST PASS

SECOND PASS

CL

CL

CL

TIRES FIRST 

PASS

FIGURE 11.1 COMPACTION BY HAULING UNITS USING SUCCESSIVE OFFSET PASSES
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Both vibratory and non-vibratory compactors are generally manufactured in self-propelled, towed and 

pushed models. Similar to the selection of other handling equipment, compactors should be capable of 

handling the refuse production rate and providing adequate compaction. Evaluation of the performance 

of compaction equipment should be conducted for the expected refuse placement rates. The effectiveness of any compactor on coal refuse generated at a particular site will not be fully known until field 

performance has been evaluated through measurement of the in-situ density of the compacted refuse.

The use of spreading equipment such as tractor dozers for compaction is less effective than equipment designed for compaction because less ground pressure is exerted and generally thinner lifts are 

needed in order to consistently meet specified criteria. Additionally, use of spreading equipment for 

compaction can result in inconsistent compaction, particularly if multiple dozers are not available to 

accomplish spreading and compaction as separate operations. If compaction of structural fill with 

tractor dozers is desired, demonstration test pads should be periodically utilized in conjunction with 

field compaction testing to establish and document procedures for attaining consistent and acceptable results throughout the structural zone.

11.4.4 Use of Handling Equipment for Related Activities

The adaptability of handling equipment for performing other tasks at a mining site is an important 

consideration in equipment selection. However, this consideration should be secondary to the capability of the equipment for placing, spreading and compacting refuse. If the handling equipment is 

suitable for refuse embankment construction and can also be used to construct haul or access roads, 

clear disposal areas, prepare the embankment foundations, and construct spillways and drainage 

ditches, there will be a savings in equipment cost. Such equipment will also allow work crews that 

are not fully utilized on refuse disposal to work on other site construction activities.

Effective refuse disposal should be the predominant handling equipment selection consideration. 

However, at some facilities, the generated refuse may not be of sufficient quantity to keep handling 

equipment busy on a full-time basis. For these sites, multiple-use capabilities for the spreading equipment are important.

11.5 CONSTRUCTION AND PLACEMENT OF EMBANKMENT MATERIALS

As discussed in Chapters 5 and 6, the structural portions of a coal refuse disposal facility must be 

constructed in a controlled manner meeting technical design requirements. This is particularly true 

for the refuse embankment and its associated drainage controls. Coal refuse disposed in these portions of the facility, as previously discussed, is referred to as constructed or structural fill refuse. Fill 

at other portions of the facility, where less stringent compaction requirements may apply, is referred 

to as placed refuse.

Earthen fill or refuse that is placed as structural fill should be spread at a specified lift thickness and 

compacted to specified density and moisture content criteria such that material properties are consistent with facility design requirements. Placement of structural fill should be performed in generally horizontal lifts. Figure 11.2 shows several correct and incorrect methods of refuse placement. 

As discussed in Chapters 4 and 5, those portions of the facility that will require structural fill refuse 

disposal should be identified during site selection and facility planning. After placed refuse has been 

built up through many lifts, little can be done to densify it to structural fill specifications. Therefore, 

the impact on future site development of placed refuse fill with lower compaction standards must be 

fully understood.

11.5.1 Embankment Fill

Embankment fill at a coal refuse disposal facility generally consists of coarse refuse or earth and 

rockfill borrow materials placed in specified stages or zones. For instance, many impounding facili-
11-27

Construction and Disposal Operations

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INCORRECT

CORRECT

COMPACTING SPREADING DUMPING FROM TRUCK

CORRECT

CUTTING AND SPREADING

COMPACTING IN LAYERS

INCORRECT

CUTTING AND DUMPING WITHOUT

SPREADING AND COMPACTION

NON-UNIFORM DUMPING FROM TRUCK OR CONVEYOR

WITH INADEQUATE SPREADING AND COMPACTION

INCORRECT

FREE DUMP FROM

CONVEYOR

CORRECT

SPREADING COMPACTING

FIGURE 11.2 PROCEDURES FOR CONTROLLED PLACEMENT OF REFUSE
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ties consist of an initial stage embankment constructed of soil and rockfill, with subsequent coarse 

refuse embankment stages that increase the height and capacity of the impounding facility. In many 

respects, placement and control at the facility embankment are not unlike that for earth and rockfill 

dams. However, there are major differences in timing because construction extends over the total 

operational period of the facility, which is usually many years. Differences also occur in materials 

selection because refuse embankment construction is based on maximum use of refuse and minimum 

use of borrow materials.

The placement and compaction effort for embankment fill refuse at an impounding facility is generally more stringent than that for a non-impounding facility. In both cases, the refuse should be placed 

in controlled lifts and compacted as required by the facility plans. Lifts are generally horizontal and 

extend over a large portion of the embankment.

Compaction of coarse refuse or soils in an impounding facility is intended to produce a well-compacted zone consisting of uniform, consistent and dense lifts. This is accomplished by limiting the 

lift thickness of the refuse or soil and by compacting the lifts to achieve specified strengths. Guidance 

for compaction of soil and rockfill materials is available from a number of sources, including Church 

(1981), DOD (2005), and USBR (1998). Table 11.4 presents published guidance (Church, 1981) for 

compaction of residual soil, weathered rock, durable rock, and alluvium. As indicated in the table, 

the compactive effort may be applied by a range of actions including pressure, kneading, impact and 

vibration. Various compactive actions can achieve compaction, but the efficiency and effectiveness 

for overcoming problem moisture conditions varies. Table 11.5 presents published guidance (DOD, 

2005) for various types of equipment and materials with recommended base lift thickness, typical 

passes or coverages, and equipment characteristics. This table provides initial guidance only; actual 

specification of the lift thickness or selection of equipment and passes is a function of the design 

requirements or construction conditions.

Experience and studies (Saxena et al., 1984) have shown that typical coarse refuse spread in lift thicknesses of one foot or less can usually be compacted to densities sufficient for embankment stability at impoundments. Normally, refuse for these applications is placed in one-foot-thick lifts and 

compacted to at least 95 percent of the maximum dry density at minus two to plus three percent of 

optimum moisture as determined from the Standard Proctor test, which is described in ASTM D 698, 

TABLE 11.4 COMPACTION GUIDE FOR MATERIAL TYPES

Compactor 

Type

Soil Weathered Rock-Earth, Ripped Semisolid and 

Solid Rock, Blasted

Clays Silts Sands Maximum

Weathering

Average

Weathering

Minimum

Weathering

WellBlastedPoorlyBlasted

Sheepsfoot Residual Soil

Tamping Foot Residual Soil

Vibratory:

Footed 

Drum

Smooth Alluvial Soil

Drum

Pneumatic Alluvial Soil

Tires

 (ADAPTED FROM CHURCH, 1981) 
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“Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort 

(12,400 ft-lbf/ft3 (600 kN-m/m3)).” In some cases, a more stringent compaction requirement may be 

necessary for impounding facilities depending upon the geotechnical characteristics of the embankment material. For instance, soils used as embankment fill will likely require a lift thickness of less 

than one foot to achieve the desired density.

The oversize correction provision of ASTM D 698 for material retained on the 3⁄4-inch sieve should 

be followed, including the density and moisture content correction as per ASTM D 4718. If the correction is applicable, but is not applied, then the measured field density could overestimate the true 

compaction being achieved. As a result, while it might appear that the compaction specifications are 

being met, the actual degree of compaction could be less than specified, leading to problems with 

stability and seepage. In situations where there is a large percentage of oversize durable rock, such as 

sandstone, there can be a significant difference in the specific gravity, and laboratory measurements 

should be made of both the oversize and remaining material specific gravity (rather than adopting 

assumed values) when applying the correction. Application of ASTM D 698 may be difficult or may 

even be precluded when coarse refuse contains more than 25 to 30 percent of plus-3⁄4-inch particles, 

based on application of the oversize correction provisions of the procedure. Sometimes the oversize 

correction yields unreasonable target densities (i.e., higher than achievable). In such cases other compaction test methods or field procedures should be considered. While use of the relative density test 

in accordance with ASTM D 4253, “Standard Test Methods for Maximum Index Density and Unit 

Weight of Soils Using a Vibratory Table,” and ASTM D 4254, “Standard Test Methods for Maximum 

Index Density and Unit Weight of Soils Using a Vibratory Table,” may be a possibility, this is generally not recommended because of the fine content in coal refuse.

Alternative material testing methods may be employed when more than 25 or 30 percent of plus-3⁄4-

inch particles are encountered. One such testing method is specified in Appendix VIA of the USACE 

Publication EM 1110-2-1906 titled “Laboratory Soils Testing” (USACE, 1986). This method employs 

a 12-inch-diameter mold that accommodates larger particle sizes. Disadvantages to this method are 

that it is more time consuming and costly than the ASTM D698 method, and many geotechnical 

laboratories are not equipped to perform the test. However, if refuse with a significant proportion of 

oversize particles is routinely generated, it may be desirable to use the USACE test to supplement or 

replace other methods.

Another possible method for compaction control is using a method specification such as the roller 

pass test, as described in WVDOT (1999) Standard MP-700.00.24. This procedure consists of preparing a roller pass test strip with fresh coarse refuse at the optimum moisture content. Compaction of 

fresh refuse is performed with the equipment that will be used during embankment construction 

(e.g., vibratory smooth-drum roller or similar or with loaded haul equipment), using sufficient passes 

(usually not more than 12) to compact the material without rutting or noticeable working surface 

deflection. Initial density tests are then performed. Additional compaction using two passes of the 

equipment is performed, and additional density testing is conducted to confirm that the maximum 

density has been achieved (i.e., the incremental difference in the average wet densities is less than one 

pound per cubic foot). In this case, the target maximum density and moisture content for compaction 

would be based on the roller pass maximum dry density.

To achieve the desired compaction and the desired geotechnical characteristics for a structural zone 

or embankment, the water content of the refuse must be controlled. This can be difficult during 

winter and inclement weather periods when there may not be sufficient space or time for drying or 

thawing wet or frozen materials. Embankment zones that may be designated for such conditions are 

discussed in Section 11.5.3.

Typically, refuse piles and other non-impounding facilities can be designed with embankment zones 

placed in greater than one-foot lift thicknesses and/or can be compacted to a density lower than 95 
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TABLE 11.5 SUGGESTED COMPACTION EQUIPMENT AND METHODS

Equipment Type Applicability

Typical Requirements for Compaction of 95 to 100 Percent

 of Standard Proctor Maximum Density

Possible Variations in 

Equipment Compacted Lift 

Thickness

(in)

Passes or 

Coverages Dimensions and Weight of Equipment

Soil Type

Foot 

Contact 

Area

(ft2

)

Foot 

Contact 

Pressures

(psi)

Sheepsfoot Rollers

For fine-grained soils or dirty coarsegrained soils with more than 20 percent passing No. 200 sieve. Not suitable for clean, coarse-grained soils. Particularly appropriate for compaction of impervious zone for earth dam or linings where bonding of lifts is important.

6

4 to 6 passes 

for fine-grained 

soil

6 to 8 passes 

for coarsegrained soil

Fine-grained 

soil (PI > 30) 5 to 12 250 to 500

For earth dam and related embankment work, articulated self- propelled rollers are commonly used. For smaller projects, towed 40- to 60-inch drums are used. Foot contact pressure should be regulated so as to avoid shearing the soil on the third or fourth pass.

Fine-grained soil (PI < 30) 7 to 14 200 to 400Coarsegrained soil 10 to 14 150 to 250Efficient compaction of soils wet of optimum requires less contact pressure than the same soils at lower moisture contents.

RubberTireRoller

For clean, coarse-grained soils with 4 to 8 percent passing the No. 200 sieve.For fine-grained soils or well graded, dirty, coarse-grained soils with more than 8 percent passing the No. 200 sieve.

10

6 to 8

3 to 5 

coverages

4 to 6 

coverages

Tire inflation pressures of 35 to 130 psi for clean granular material for base course and subgrade compaction. Wheel load 18,000 to 25,000 lbs.Tire inflation pressure in excess of 65 psi for fine-grained soils of high plasticity. For uniform clean sands or silty fine sands, use large-size tires with pressures of 40 to 50 psi.

Wide variety of rubber tire compaction equipment is available. For cohesive soils, light wheel loads, such as provided by wobble-wheel equipment, may be substituted for heavy wheel load if lift thickness is decreased. For granular soils, large-size tires are desirable to prevent shear and rutting.

Smooth Wheel Rollers

Appropriate for subgrade or base course compaction of well-graded sand-gravel mixtures.

8 to 12 4 coverages

Tandem-type rollers for base course or subgrade compaction; 10- to 15-ton weight; 300 to 500 lb per lineal inch of rear roller width.

3-wheel rollers are obtainable in a wide range of sizes; 2-wheel tandem rollers are available in the range of 1 to 20 tons.
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Equipment Type Applicability

Typical Requirements for Compaction of 95 to 100 Percent

 of Standard Proctor Maximum Density

Possible Variations in 

Equipment Compacted Lift 

Thickness

(in)

Passes or 

Coverages Dimensions and Weight of Equipment

SmoothWheelRollers

May be used for fine-grained soils other than in earth dams. Not suitable for clean, well-graded sands or silty, uniform sands.

6 to 8 6 coverages

3-wheel roller for compaction of finegrained soil; weights from 5 to 6 tons for materials of low plasticity to 10 tons for materials of high plasticity.

3-axle tandem rollers are generally used in the range of 10 to 20 tons weight. Very heavy rollers are used for proof rolling of subgrade or base course.

Vibrating Sheepsfoot Rollers

For coarse-grained soils: sandgravel mixtures 8 to 12 3 to 5 1 to 20 tons ballasted weight. Dynamic force up to 20 tons. May have either fixed or variable cyclic frequency.

Vibrating Smooth Drum Rollers

For coarse-grained soils: sandgravel mixtures – rock fills 6 to 12 (soil) to 36 (rock) 3 to 5 4 to 6

Vibrating Baseplate Compactors

For coarse-grained soils with less than about 12 percent passing No. 200 sieve. Best suited for materials with 4 to 8 percent passing No. 200 sieve, placed thoroughly wet.

8 to 10 3 coverages

Single pads or plates should weigh no less than 200 lb. May be used in tandem where working space is available. For clean coarse-grained soil, vibration frequency should be no less than 1,600 cycles per minute.

Vibrating pads or plates are available, hand-propelled, single or in gangs with width of coverage from 1-1⁄2 to 15 ft. Various types of vibrating-drum equipment should be considered for compaction in large areas.

Crawler Tractor

Best suited for coarse-grained soils with less than 4 to 8 percent passing No. 200 sieve, placed thoroughly wet.

6 to 10 3 to 4 

coverages

Vehicle with “standard” tracks having contact pressure not less than 10 psi. Tractor weight up to 85 tons.

Power Tamper or Rammer

For difficult access and trench backfill. Suitable for all inorganic soils.

4 to 6 

for silt or 

clay; 6 for 

coarsegrained 

soils

2 coverages

Minimum weight 30 lb. Considerable range is tolerable, depending on materials and conditions.

Weights up to 250 lbs, foot diameter 4 to 10 in.

(ADAPTED FROM DOD, 2005)

TABLE 11.5 SUGGESTED COMPACTION EQUIPMENT AND METHODS

(Continued)
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percent of the maximum dry density. For a refuse pile, MSHA regulations do not permit lift thicknesses in excess of 2 feet unless an adequate factor of safety has been demonstrated. However, lift 

thicknesses of two feet (or greater) may preclude uniform compaction. From a technical standpoint, 

coarse refuse at non-impounding facilities that contains predominantly large particles similar to rock 

fill can be placed and compacted in larger lifts, if adequately-sized compaction equipment is used. 

Since coarse coal refuse typically consists of both rock and soil particles, one to two-foot lifts can typically be used if suitable compaction equipment is employed.

Appropriately-sized equipment does not guarantee that placement and compaction of refuse will be 

as desired. The coordination of equipment operations and sequencing of refuse placement greatly 

affects the level and quality of compaction achieved. Refuse should be placed in piles on the working 

surface and spaced so as to allow spreading equipment to achieve the specified lift thickness with 

minimal effort prior to compaction. Refuse should be spread away from a conveyor discharge point 

to the specified lift thickness. A sequence or progression of refuse placement should be established. 

When handling equipment of sufficient number and size is employed using an efficient operating 

system, refuse should be evenly distributed on the working surface and can be spread in relatively 

thin horizontal lifts, allowing compaction equipment to perform under favorable conditions.

To achieve consistent and uniform compaction, successive lifts should be knitted together by scarifying smooth compacted surfaces prior to placement of subsequent lifts. This is particularly important where concentrated haul traffic has resulted in additional breakdown of the material or where 

smooth-drum rollers are used. Generally, little scarifying is necessary where padded or sheepsfoot 

rollers are employed unless the working surface has been dormant for a long period of time or where 

concentrated haul traffic has occurred. A rock ripping attachment for a tractor dozer may be required 

to scarify the working surface properly.

Where an intermediate crest elevation has existed for a period of time, the surface may become highly 

compacted from the combination of traffic and breakdown of the surface material due to weathering. 

Additionally, freezing conditions can result in frost heave and formation of ice lenses. If this condition is not addressed when the next lift is placed, then a layer of differing hydraulic conductivity may 

cause seepage to run horizontally and exit at the face of the downstream slope. Such a condition, 

which will lead to concerns about seepage and stability, can be addressed by scarifying or removing 

the top surface material prior to placing the next lift.

Special care must be taken to achieve adequate compaction at the sloped edges of each lift where, 

due to the lack of natural confinement, the refuse tends to move away from the equipment without 

densification. Although the total stability of an embankment is not significantly affected by refuse 

density at the slope face, loose material is susceptible to erosion and creep. To achieve adequate surface density, compaction should extend several feet down the slope. If this is not possible, the loose 

material should be shaved off with a dozer or excavator and pushed back onto the working surface 

for compaction.

When structural fill is placed against a hillside or against an existing embankment, the existing material should be keyed or benched. This can be accomplished by using a dozer to cut a sufficient distance into the slope (e.g., approximately three or four feet horizontally where the terrain is steep) as 

the new refuse embankment is advanced in height. Figure 11.3 illustrates the process of benching to 

tie structural fill into a natural slope. This process removes surface material that may not be at the 

required density, permits compaction at the construction interface, and reduces the tendency for a 

natural slip surface to develop at this critical location.

Compaction tests, as discussed in Section 11.8, should be performed relatively often during initial 

structural fill refuse construction when material characteristics and equipment efficiency are being 
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Construction and Disposal Operations

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evaluated. Thereafter, the testing frequency can be reduced, provided that a history of successful 

equipment performance and density testing indicates that the desired compaction is being accomplished. A typical criterion for the frequency of compaction testing for embankment fill is: at least 

one density test per 2,000 cubic yards of material placed or one test for every lift placed, whichever is 

greater (USBR, 1998). Tests should also be performed when it is suspected that adequate compaction 

is not being achieved.

11.5.2 Upstream Construction Implementation Procedure

Incidents have occurred while establishing pushouts where bulldozers have sunk into the underlying fines and not been recovered. Even in situations where the fine refuse surface is dry, dessicated 

and vegetated, the underlying materials may remain soft and can exhibit sudden shear failure under 

equipment operation.

As indicated previously, upstream construction of an impounding embankment poses technical and 

construction challenges. Since upstream construction involves the development of an embankment over 

saturated, unconsolidated fine coal refuse, the stability of the embankment under both static and dynamic 

loading must be carefully evaluated. The potential for sudden failure of the pushout surface and the 

underlying fine refuse during upstream construction requires that special techniques and equipment be 

used and that the work be performed by experienced and properly trained equipment operators.

Placement of material on top of saturated, hydraulically-placed fine coal refuse results in the compression of fine materials. Since the material is initially loose and saturated, as the particles move to 

a closer packing, the water in the pores is placed under pressure. This excess pore-water pressure 

reduces the effective stress and can make the material unstable. Fill material must be placed on top of 

the fines at a slow enough rate that the pore-water pressure can dissipate without causing significant 

instability. If too thick a layer is placed too quickly, instability will occur that can adversely affect both 

the immediate safety of the equipment operators and the overall safety of the embankment.

It is important that equipment operators understand the potential for instability during upstream 

construction and the general concept that the rate of material placement during upstream construction must be controlled. Placing material thicker and/or faster on hydraulically-placed fines can be 

detrimental. When excess pore-water pressures are created in one area, construction activity should 

be moved to another area to allow pore-water pressures to dissipate.

Task-specific training should be provided to the equipment operators that will be performing 

upstream pushout embankment construction and to workers who will be in the vicinity of the 

3-4 FT

REFUSE EMBANKMENT

DOZER CUT FOR BENCHING

FILL INTO NATURAL SLOPE NATURAL SLOPE

FIGURE 11.3 BENCHING OF EMBANKMENT FILL INTO NATURAL SLOPE

FIGURE 11.3 BENCHING OF EMBANKMENT FILL INTO NATURAL SLOPE
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upstream construction. The training should familiarize operators and workers with the risks associated with upstream construction and should include specific instructions for developing access 

to pushout areas, along with specific construction methods for performing upstream construction. 

Information describing the risks associated with upstream construction and features that are indicative of unstable working surfaces should be provided as part of the training program. Records documenting the training should be kept.

Certain precautions are essential for minimizing the potential for failure of coarse coal refuse placed 

over fine refuse. These precautions will also help to minimize the potential for occurrence of accidents associated with the upstream construction activities. These precautions include the following:

• Impoundment construction and discharge of fine refuse should be managed such 

that a sufficient fine refuse delta on which to initiate upstream construction is created. The delta should be as uniform as is practical, which can be facilitated by routinely moving the slurry discharge point along the upstream slope of the embankment.

• The normal pool elevation should be lowered via pumping or other means (if practical) to the lowest practical level and away from the fines delta.

• A buffer should be established from the edge of proposed pushout where highground-pressure vehicles such as haul trucks are excluded from travel. The width 

of the buffer should be established based on site-specific conditions and equipment 

(e.g., 50 feet has been satisfactorily used).

• Only low-ground-pressure equipment should be used to perform upstream construction within the buffer area.

• Two-way radio communication for equipment operators and mine personnel should 

be provided during upstream construction activities, and it is recommended that 

equipment operations during the initial pushout be within sight of mine personnel 

and that operators be provided with floatation devices (e.g., life jackets).

• Work should only be performed during daylight hours until a stable working surface 

is established.

• The placement of coarse refuse for upstream construction should initiate with advancement of a thick layer (typically 4 to 6 feet thick) of coarse coal refuse onto the 

exposed fine refuse delta. Placement of the initial lift of coarse refuse should begin 

along the embankment upstream slope and gradually advance upstream over the 

fine refuse. A portion of the advancing lift may sink into the fine refuse in soft areas 

or areas where the surface is saturated. It may be possible to minimize this effect by 

reducing the lift thickness or lowering the impoundment water level.

• Equipment working near the upstream edge of the pushout should be oriented perpendicular to the face of the active edge (i.e., no equipment should travel near and 

parallel to the upstream edge of the pushout).

• Pushout construction should be sequenced so that haul trucks do not travel adjacent 

to pushout areas until a stable working surface is established.

• Pushouts should be constructed utilizing a buffer consisting of at least one pile of 

coarse refuse. The buffer pile of coarse refuse should remain between the dozer and 

impoundment as the refuse is pushed onto the fine refuse delta. Use of this method 

will always keep the dozer in a safer position away from the edge of the fine refuse.

• Pushouts should be performed perpendicular to the upstream face of the embankment and/or impoundment and should be limited to a prescribed length onto the 

delta (e.g., 25 feet measured from the upstream edge of the embankment or stable 

working surface). It is recommended that the initial lift for upstream construction be
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spread to a width of at least two times the push out length (e.g., 2 times 25 feet or 50 

feet) before further advance of the lift upstream over the delta.

• The surface of the upstream pushout embankment should be graded to drain toward 

the impoundment.

Monitoring and inspection of the upstream construction area should be performed by a qualified 

person who is familiar with upstream construction methods and risks. Prior to and during initial 

pushout construction, this person should inspect the refuse embankment and the area of the upstream 

construction. The inspection should focus on identifying conditions that could affect the safety of the 

equipment operators as well as conditions that could affect the safety of the embankment. These 

include the following conditions:

• Development of cracks with vertical displacement or scarps in the vicinity of the 

pushout (the orientation and shape of the cracks may indicate shearing rather than 

differential settlement)

• Excessive pumping of the pushout surface

• Excessive bulging of the fine refuse delta (e.g., bulge or displacement height in excess 

of the pushout lift level) where work is being performed

• A situation posing a threat that the embankment could be overtopped by water or slurry

• Sudden or major subsidence of the embankment crest

• Longitudinal or transverse cracking of the embankment crest

• Major sliding/failure of upstream or downstream embankment slopes or abutment 

slopes adjacent to the embankments

• Unusual seepage from areas of the downstream face or from the toe of the embankment

• Unusual conditions on the embankment downstream slopes that develop during 

upstream construction

• Significant landslides within the impoundment area

In addition to the above conditions, embankment piezometers should be monitored before and during 

the upstream construction. Where development of significant pore pressures are or remain a concern for 

the initial pushout, new piezometers can be installed within the fine refuse to aid in monitoring of the 

upstream construction process. The location for piezometers should reflect the potential interference 

from construction activities and the likely displacements associated with upstream construction.

If any of the above listed conditions are observed, or if piezometers indicate unacceptable levels of 

pore pressure, the information should be reported to the Engineer and Operator, and equipment and 

personnel should be moved to another work area until the cause of the problem is identified and corrected. The results of the inspection should be documented.

11.5.3 Excess Coarse Refuse/Inclement Weather Disposal

Some coal preparation plants may generate more coarse refuse than required for impounding 

embankment construction (excess coarse refuse), with the result that specific embankment zones 

or separate embankments are designed with different material placement and compaction criteria (e.g., thicker lifts). This situation may also result in the designation of a location for inclement 

weather disposal, when compaction to the normal embankment specifications is precluded due to 

moisture or freezing conditions.

To mitigate concerns for combustion, excess coarse refuse should generally be spread in layers or 

lifts less than 2 feet thick. Additionally, lift placement should result in a working surface capable 
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of supporting equipment traffic associated with subsequent disposal operations. Other geotechnical factors that must be considered before constructing embankment zones with thick lifts include 

settlement and hydraulic conductivity.

Placement of refuse in thick lifts may also be acceptable at non-impounding embankments and in 

surface mine backstack areas where such materials do not influence the stability and drainage control 

of the site. Note that 30 CFR § 77.215 addresses construction requirements for refuse piles. Section 

77.215(h) requires that refuse piles be constructed in compacted layers not exceeding 2 feet in thickness except that the MSHA District Manager may approve construction of a refuse pile in compacted 

layers exceeding 2 feet in thickness where engineering data substantiate that a minimum static safety 

factor of 1.5 will be attained.

Sealing of any disposal surface that will be exposed for a long period of time is always important. 

Sealing requires use of smooth-wheeled vehicles (hauling units or smooth-drummed compaction 

equipment) rather than tamping-foot rollers, with appropriate grades provided for drainage. For 

final surfaces, preparation for abandonment and revegetation should follow procedures discussed 

in Chapter 10.

11.5.4 Disposal of Fine Refuse Slurry

The fine coal refuse slurry disposal system is a distinct part of a coal refuse disposal facility. Slurry 

pipe size, composition, anchor/bracing details, discharge point locations and construction methods 

should be included in facility plans and specifications for implementation as part of overall refuse 

disposal operations. Disposal of fine coal refuse slurry is discussed here with respect to site-related 

planning, scheduling and the relationship to other construction and disposal operations. If possible, 

the disposal of fine refuse slurry should result in a fine coal refuse delta above the impoundment pool 

to: (1) provide the best possible foundation for upstream advancement of the embankment whether or 

not the facility is initially planned to be developed by the upstream construction method, (2) protect 

the embankment face from wave erosion by minimizing the impounded water depth at the upstream 

embankment slope, and (3) reduce seepage into the embankment and keep the phreatic surface associated with embankment seepage as low as possible (assuming the fine coal refuse hydraulic conductivity is lower than the embankment hydraulic conductivity). At some facilities clarified water from 

the impoundment is pumped to a separate cell, typically at the upstream end of the reservoir, further 

isolating the pool from the embankment face.

As illustrated in Figure 11.4, slurry disposal functions most effectively if the discharge point of the 

transport pipe is located at a relatively low elevation on the impoundment side (upstream slope) 

of the embankment and is periodically moved to adjacent locations along the upstream slope. The 

coarsest material in the slurry will be deposited in the immediate vicinity of the discharge point, 

leading to maximum material strength near the embankment and maximum water depth near 

the upstream end of the impoundment. Periodic movement of the discharge point back and forth 

along the upstream edge of the embankment will create a more uniform delta deposit. The slurry 

may be discharged at other locations to achieve slurry deposition at specific features such as mine 

barriers or to more fully utilize available impoundment capacity. The refuse disposal plan should 

discuss the type and size of equipment required to move the slurry discharge pipe around the 

impoundment.

Pipes transporting the slurry to the impoundment should not be placed through the embankment 

unless seepage and structural provisions are provided. Pipes through embankments tend to be natural paths for seepage and could result in embankment failure from internal erosion. Also, pipes 

installed within the embankment or along the downstream face of the embankment could cause erosion and are an environmental hazard if they leak or fail (Figure 11.4a).
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11.4a INCORRECT SLURRY PIPE INSTALLATION

SLURRY

IMPOUNDMENT

NOTE: DISCHARGE SOLELY FROM THE UPPER

 END IS GENERALLY NOT RECOMMENDED

 UNLESS THE FACILITY IS DESIGNED TO

 ACCOMMODATE IT.

11.4b CORRECT SLURRY PIPE INSTALLATION

SLURRY

IMPOUNDMENT

NOTE: DISCHARGE FROM THE UPPER END OF THE

 IMPOUNDMENT SHOULD GENERALLY ONLY

 BE IN CONJUNCTION WITH OTHER DISCHARGES

 UNLESS THE FACILITY IS DESIGNED TO

 ACCOMMODATE IT.

FIGURE 11.4 SLURRY DISPOSAL
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11.5.5 Disposal of Combined Refuse

As discussed in Chapter 2, fine coal refuse slurry can be processed by dewatering with a vacuum 

filter, belt filter press or centrifuge. A filter cake of fines that in some instances is at a water content 

too high for easy transport and disposal as a solid waste material can result, particularly during 

inclement weather periods. Under such adverse conditions, the filter cake may be difficult to confine 

in haulage units. To minimize spillage and resulting degradation of haul roads, conventional trucks 

fitted with sealing tailgates may be necessary. As a separate waste material, filter cake typically can’t 

be spread in lifts and compacted and will not support construction equipment.

To resolve this materials transport and disposal problem, the filter cake can be mixed with coarse 

refuse at the preparation plant, resulting in a combined refuse that normally can be transported in 

conventional trucks or by conveyor. However, watertightness of transport unit bodies is desirable, 

because the material is typically very wet.

Combined refuse disposal facilities are typically designed as non-impounding embankments or 

refuse piles. In some instances where the combined refuse is very wet, the material may not be suitable for normal embankment construction. A large area may be needed for spreading and drying the 

material. During wet or winter periods, movement of spreading or compacting equipment over this 

area may be difficult or impossible. This can lead to operational inefficiencies and larger disposal 

areas than normally required. If confined disposal is necessary, a structural zone or embankment 

may be needed for retaining the combined refuse. Such embankments must be constructed from borrowed soil and rock materials, or some of the coarse refuse must be diverted for this purpose.

The mine operator or designer should carefully evaluate the effects of combined refuse disposal 

prior to implementation of this type of total disposal system so that all relevant requirements and 

limitations are factored into the planning. The procedure for disposing of coarse refuse and dewatered fine refuse filter cake must be planned and designed specifically for conditions found at the 

disposal site.

11.5.6 Use of Amendments

Coal refuse may require amendments for neutralization and stabilization. The relative quantities 

and methods for combining refuse and materials such as lime, combustion ash, and kiln dust should 

be detailed in the facility plans, specifications and the operation and maintenance plan. Rich and 

Hutchinson (1990) discuss the operational implications of using lime kiln dust to neutralize and 

stabilize combined refuse at a West Virginia mine site using the following practices: (1) mixing the 

kiln dust with filter cake at the preparation plant to absorb moisture, (2) refuse cell development 

with elevated roadways constructed with rock for equipment access for dumping of the combined 

refuse, and (3) methods to shed excess water by separating the dumped piles to allow drainage 

prior to grading, and (4) maintaining drainage control within the refuse cell. For the cited case, an 

application of 2 percent by weight of kiln dust provided sufficient improvement in the combined 

refuse to enable effective disposal. Section 10.3.2.3 provides additional discussion of amendments.

11.6 RELATED ACTIVITIES

Many activities besides refuse disposal are involved in the development of a disposal facility. Activities 

associated with coal refuse disposal include:

• Construction and maintenance of haul or access roads

• Construction of liner systems prior to refuse placement

• Development of underdrain and spring collection systems
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• Foundation preparation prior to disposal of coal refuse

• Placement of earth borrow material as structural fill and impervious cores and/or 

seepage control blankets

• Control of water while constructing facility appurtenances and developing the embankment foundation

• Construction of internal drainage systems within the embankment

• Excavation for spillways, ditches, drainage installations, repair work, etc.

• Construction of embankments for mine seals and barriers (e.g., fills for protecting 

impoundments from the influence of potential underground mine subsidence)

In general, construction procedures for these related activities are the same in mining as for heavy 

construction. Therefore, discussion of these topics herein is limited and is related specifically to coal 

refuse disposal operations.

The activities listed above and the construction of appurtenant structures subsequently discussed 

in Section 11.7 are similar in the sense that neither pertain directly to the primary facility purpose 

of refuse disposal, yet both are needed for refuse disposal to proceed. The distinction made herein 

is: (1) the above activities will generally be performed with the same equipment used for refuse disposal, and the materials required are basically available at the site, and (2) the structures discussed 

in Section 11.7 generally require special equipment and procedures, and the materials involved must 

be purchased.

11.6.1 Haul and Access Road Construction and Maintenance

Haul and access roads are an integral part of a site refuse transport system and have been a source of 

safety concerns. Surfacing, width, signage, runaway vehicle provisions, drainage control and berms 

are important design issues. Roadway layout, design and maintenance impacts the operations of 

the refuse disposal facility. Haul and access road locations must be determined in the initial stages 

of planning and design of the refuse disposal facility. Consideration must also be given to temporary haul roads required during various stages of facility construction for safe access to the working 

surface of the embankment. Because of the importance of disposal facility haul roads, MSHA (1999) 

published The Haul Road Inspection Handbook, which focuses on the safety aspects of haul road design. 

Guidance for the layout and design of haul and access roads is also provided in the U.S. Bureau of 

Mines publication, Design of Surface Mine Haulage Roads – A Manual (Kaufman and Ault, 1977), and 

other references.

11.6.2 Liner Systems

State regulations require protection of the groundwater. This may in some states necessitate the 

installation of a liner system to reduce the potential for impacts to the environment. The extent 

and type of liner system required for groundwater protection generally depends upon the environmental setting of the refuse disposal site and the potential acidity of the refuse. Liner systems 

are normally constructed of clay soils from on-site borrow areas or from geosynthetic materials. 

Geosynthetic liners may consist of a geomembrane or a clay-impregnated geotextile (geosynthetic 

clay liner or GCL). Clay soil liners are used when the acid potential of the refuse is low to moderate, and geosynthetic clay liners may be considered for higher acid potential refuse or when clay 

is unavailable at the site. Liner systems are normally installed after foundation preparations have 

taken place and prior to the placement of refuse. Extensive underdrain and spring collection systems are sometimes installed beneath liner systems to prevent sloughing and/or slides that could 

compromise liner integrity. Because of the potential for a liner to introduce a slip plane, the effect of 

a liner system must be evaluated in the stability analyses. Section 10.4 provides additional discussion of liner systems.
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11.6.3 Embankment Foundation Preparation

As discussed in Section 6.3, the foundation of a coal refuse embankment will affect embankment 

stability, settlement and seepage. To achieve stability or to minimize potential settlement, removal of 

soft cohesive materials may be desirable. However, for seepage and leachate control, these materials can provide an effective seal if left in place. Thus, cost-effective foundation design for a disposal 

facility embankment requires that important decisions be made relative to removal or use of on-site 

materials.

11.6.3.1 Clearing and Topsoil Stockpiling

Cutting and removal of trees, brush and other vegetation within the footprint of the embankment 

should be specified. Vegetative matter, if not removed, can decay and cause settlement and the formation of slip planes. Vegetative matter may also contribute to spontaneous combustion and burning. 

Additionally, floating trees and branches can plug hydraulic structures, particularly culverts and 

decants. Cleared material should be removed from the construction area or burned.

Topsoil present in areas where refuse will be placed should be removed and stockpiled for reclamation. Stockpiling should be performed in a controlled fashion in areas away from natural drainage 

courses and areas of planned future development. State mining regulations typically provide guidance for topsoil removal and stockpile requirements.

Within impoundment areas, trees and heavy brush should be cleared and removed so that floating 

debris that could impact operation of spillways and decants is not present. Soil and topsoil should be 

stockpiled for future use, as required. Clearing and removal of soils within the impoundment area 

should be sequenced so as to minimize excessive disturbance to hillsides that could cause erosion 

and lead to potential slope instabilities. Typically, clearing should be limited to areas that will potentially be affected by the impoundment within one or two years.

11.6.3.2 Soft Soil Foundations

An effort should be made during the geotechnical subsurface exploration program to identify soft 

soil foundation areas. If soft soils are present where a refuse embankment is planned, removal of the 

soft materials may be necessary. Such removal can generally be accomplished with normal excavating 

and hauling equipment. Temporary access roads for construction equipment should be constructed 

as needed to facilitate safe operations. Depressions associated with removal of soft soils can be backfilled with either on-site borrow material or coarse refuse, as available. Some types of soft soils may 

be suitable for clay liner construction or for use as final reclamation cover, in which case they should 

be stockpiled on the site and reused.

11.6.3.3 Competent Soil Foundations

If generally competent foundation materials are present in the area where embankment construction 

is planned, it may be possible to utilize these in-situ materials. Some over-excavation and recompaction of the material may be required if the soil type is acceptable but the in-situ density is too low. Prior 

to such work, the area should be cleared of vegetation and organic topsoil. Clearing and excavation/

recompaction should not be performed very far in advance of refuse disposal since prolonged exposure of the foundation soil could lead to weathering and deterioration of important physical properties and/or cause environmental damage. Construction of temporary access roads and stockpiling of 

topsoil or other recoverable material should be performed in the manner previously described.

11.6.3.4 Rock Foundations

From the standpoint of strength, rock is usually an adequate foundation material for a coal refuse 

embankment. However, an evaluation of foundation rock conditions should be performed, since trou-
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blesome layering or discontinuities and localized weaknesses may be present. Seepage and uncontrolled discharge of leachates through fractures in foundation or abutment rock can be a significant 

problem, especially for impounding facilities. Sections 6.6.5 and 8.9 discuss methods for preparing 

rock foundations and abutments for construction of embankments.

Not all rock characteristics may be apparent following the geotechnical exploratory drilling and laboratory testing performed as part of facility design. Thus, it is highly desirable that the foundation 

surface be inspected by a geologist or geotechnical engineer after the foundation has been exposed 

by excavation. This will allow final confirmation of the embankment design prior to construction. If 

special provisions associated with rock foundations are needed, they will typically involve control 

features such as impervious soil blankets, localized grouting, shotcrete placement, or preparation of 

a planned contact between clayey soils and rock. Section 11.6.6.1 discusses the placement of impervious blankets. Foundation grouting of rock formations to cut off or minimize seepage through fractures is discussed in Fell et al. (2005) and USACE (1984a).

11.6.3.5 Mine Spoil

Mine spoil may be present in the foundation of the area planned for coal refuse disposal, either as 

remnant materials from past mine operations at the site or as a result of planned surface mining and 

refuse disposal. Evaluation of the mine spoil characteristics and associated foundation preparation 

requirements should be addressed in the refuse disposal facility design. Characterization of the mine 

spoil properties and the potential for use of mine spoil in construction along with the possible need 

for removal, densification, and seepage/internal erosion control should also be addressed. Section 8.8 

presents design and construction considerations associated with mine spoil. State regulations may 

limit the use of mine spoil at a refuse disposal facility.

If unanticipated spoil materials are encountered, they should be thoroughly characterized to determine if 

they are suitable for incorporation into the foundation at an impounding coal refuse disposal facility.

11.6.4 Water Control

Control of water in relation to the development of coal refuse disposal facilities falls into four basic 

categories:

1. Control of flood water by storage capacity combined with spillway and decant 

structures for impoundments and by diversion ditches for non-impounding embankments

2. Control of seepage through and under the refuse embankment (from springs/

groundwater and from impounded water)

3. Control of natural stream flow and storm runoff during construction when permanent facility hydraulic structures are not completed

4. Control of site drainage from storm runoff

The first category of water control structures refers to hydraulic structures associated with conveying watershed runoff through or around the coal refuse disposal facility. Design requirements for 

these structures are presented in Section 9.7, and construction requirements for these features are 

discussed in Section 11.7.

The second category of water control structures refers to internal drains for collection and control of 

springs in the foundation area, seepage from an impoundment, interception of leachates, and control 

of the phreatic level in the embankment. Design requirements for filters and drains are presented in 

Section 6.6.2.3.1, and construction requirements are discussed in Section 11.7.
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The third category of water control is often referred to as a “diversion” and may be required during 

facility construction. A diversion can be as simple as installing a small drainage culvert under a 

haul road or as complex as construction for passing a stream around a site prior to completion of 

permanent facility hydraulic structures. For coal refuse disposal, diversions may be required on 

several occasions during the operational period of the facility as, for example, the perimeter ditch 

system at a valley fill is upgraded to match the advancing refuse embankment. Temporary diversions should be designed and constructed consistent with the risk associated with failure during 

the time of use (i.e., the shorter the period of time required for diversion, the smaller the design 

storm requirement, consistent with regulatory requirements). Permanent diversion ditches should 

be designed as long-term drainage channels. Typical design criteria for stream diversions are presented in Chapter 9.

Diversion requirements discussed in Chapter 9 include:

• Inlet and outlet sections sized to handle expected flows and designed without obstructions or sharp angles

• Transport section alignment and grade

• Freeboard design for preventing overflow to areas that must be protected

• Design for prevention of erosive damage through material selection and energy dissipation control

Planning, scheduling and construction of diversions should reflect requirements for continued refuse 

disposal and other site related activities. Access across open diversions may be difficult, especially 

where permanent stream flows are being conveyed. Light-gauge, temporary culverts may be capable of passing the required flow. However, heavy equipment passing over such culverts can cause 

damage if they are inadequately designed and constructed.

Although material selection for temporary diversions is not as important as for permanent hydraulic 

structures, strength, erosion and corrosion issues must be considered. Further discussion of these 

topics is provided in Section 9.7.3 and Section 11.7.

The fourth category associated with water control is related to on-site drainage for handling storm 

runoff. This type of water control generally includes grading of embankment surfaces and construction of conveyance and collection structures. Temporary and final embankment surfaces, 

including the active work area for refuse placement, must be graded such that runoff is directed 

toward conveyance channels and that ponding and saturation of the fill is minimized. This is typically accomplished by specifying surface grades of at least 1 percent and controlling the placement 

of lifts so as to maintain a positive gradient toward the perimeter of the embankment where the 

runoff is collected and conveyed from the area in channels. The channels then convey site runoff 

to sedimentation traps or ponds for clarification before release. These drainage control features 

should be incorporated into the facility plans and specifications. The design of site drainage structures is addressed in Chapter 9.

A critical element in control of on-site drainage is the actual process of coarse refuse placement. Lift 

placement (typically end-dumped piles placed by trucks) and spreading to a uniform thickness for 

compaction is dependent upon a working surface that can support construction traffic and is not 

excessively wet or does not have standing water that would interfere with the compaction effort. If 

each lift is advanced uniformly across the entire active working surface, the surface gradient to drainage channels is maintained. Also, when periods of sustained inclement weather occur that adversely 

impact the working surface, it is useful to grade and seal the working surface so that it sheds water 

and to have other non-critical areas for short-term disposal.
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Mobile conveyor systems are sometimes employed to deliver coarse refuse directly to the working 

surface, with dozers, loaders and trucks used to spread the lift materials. Mobile conveyor locations 

and equipment selection should reflect the need for advancing lifts uniformly across the working 

surface to maintain the drainage gradient.

11.6.5 General Excavation

Excavation is required for construction of diversions, minor drainage ditches and channels, trenches 

for internal drains and control of natural springs, major decants or spillways, embankment foundation development, liner construction, haul and access road construction, and other site operations. 

Many of these earth-moving activities have already been discussed. However, certain precautions 

should be taken for excavations at coal refuse disposal sites.

Many coal refuse disposal facilities are located in mountainous areas. It follows that some excavations associated with facility development at these sites will result in steep, side-hill cuts. In 

making such excavations, care should be taken to disturb as little vegetation as possible, particularly on the uphill side of drainage ditches. This is especially important if the cut being made is 

for the construction of a permanent drainage channel. The stability of slopes above drainage channels is crucial to their performance during storms. Vegetation, especially in the form of trees with 

extensive root systems, helps to maintain slope stability of steep natural hillsides and also aids in 

minimizing the potential for erosive movement of soil into the channel and consequent blocking 

or reduction of flow capacity.

Stability of excavated slopes, whether the excavation is permanent or temporary, can be achieved by 

decreasing the angle of the cut or by bracing the exposed surface. Only if excavations are in sound, 

competent rock should vertical cuts be made. Steeply-sloped excavations in clayey soils may appear 

stable at the time of excavation, but sudden collapse, especially during rainy weather, can occur. 

Safety regulations such as those developed by OSHA for providing safe conditions in and around 

open excavations should be strictly followed. Slope stability issues are discussed in more detail in 

Section 6.6.4.

11.6.6 Embankment Fill Constructed from On-Site Borrow Materials

Starter embankments for impounding facilities are frequently constructed from on-site borrow materials. At some coal refuse disposal facilities, coarse coal refuse may not be available in sufficient 

quantities or with adequate properties for all embankment fill requirements. Combined coarse and 

fine refuse may be difficult to compact to the density and strength required for structural fill. Also, as 

discussed in Chapter 6, an impervious earth core or cutoff may be needed within the refuse embankment or as an upstream blanket for controlling seepage and water pressure. These considerations may 

necessitate construction of a zoned embankment with design implications as discussed in Sections 

5.1 and 6.3.1.2. Borrow materials may also be needed for fills and embankments associated with mine 

seals and barriers. For any of these situations, use of on-site sources may be necessary. While borrow 

materials generally can be characterized as “clayey and silty” or “rock and granular,” mine spoil 

materials may exhibit both types of characteristics.

11.6.6.1 Clayey and Silty Borrow Materials

If fine-grained borrow materials such as clays or silts are to be placed in zones as part of embankments or for seepage control (impervious blankets and cores), major considerations during construction include:

• Material properties such as grain-size distribution, water content, densities, strength, 

hydraulic conductivity, etc.
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• Material availability in sufficient quantities, preferably located near the construction site

• Equipment required for loading, transporting and placing the material

• Lift thickness during placement

• Equipment required for compaction and appropriate compaction procedures

Detailed discussions of earth-fill operations can be found in Sherard et al. (1963), Church (1981), 

USACE (1995b) and USBR (1998). Some general comments regarding the distribution and placement 

of clayey and silty borrow materials (for structural fill and impervious blankets and cores) are:

• The material may be difficult to remove from the borrow areas. If scrapers are used 

to excavate earth borrow of this type, an additional tractor may be required.

• The material should be placed in relatively thin lifts. Sherard et al. (1963) recommended approximately 6 to 9 inches. If thicker lifts are used, the required density 

may not be achieved throughout the full depth of the lift.

• Compaction equipment used for clayey or silty soils is typically of the segmentedpad or tamping-foot type, commonly referred to as a sheepsfoot roller. Vibration 

does not significantly improve the compaction of these soils.

• The compactability of clayey and silty soil is very sensitive to moisture conditions. 

In most regions, borrow areas should be graded to shed surface water, and fill areas 

should be sloped to drain. In dry regions, moisture may have to be added to the

borrow material. Sealing of fill areas by rolling with smooth-wheeled vehicles or 

non-vibrating, smooth-drum rollers is recommended if precipitation is anticipated. 

USBR (1998) and Church (1981) provide discussions of embankment construction 

and moisture control.

• If the borrow material water content is too high to permit adequate compaction, drying may be required. Drying can sometimes be accomplished by disking and allowing the sun and wind to remove the moisture.

• Placement and compaction of fill for impervious blankets and cores in winter 

weather is always difficult and is essentially impossible if the soil is frozen. Operations should be scheduled to avoid fill placement during periods of adverse weather 

conditions.

The above discussion relates primarily to embankment zones where strength is a major factor. If 

borrow material is being used for seepage control such as an impervious core within the embankment or an imperious blanket in the valley bottom, it is essential to obtain a uniform, non-layered 

clayey or silty soil consistency. In such applications, the borrow material should be placed a few percentage points wetter than optimum water content if the material can meet the associated strength 

requirements and is in accordance with the construction specifications. This should result in a homogeneous mass flexible enough to accept deformation without cracking. Further discussion of this 

subject is provided in Section 6.5.3.

11.6.6.2 Rock and Granular Borrow Materials

Many considerations for use of rock or granular borrow material for structural fill are similar to those 

mentioned above for clayey or silty borrow material. However, the handling and compaction equipment needed, acceptable lift thicknesses, and material properties are different.

Rock-fill material is generally handled by large excavators and haul trucks, and granular borrow 

material can be handled by scrapers if large boulders are not present. Quarrying hard rock may 
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require explosives and blasting, but removing soft weathered rock may only require a powerful 

dozer with ripper teeth. Church (1981) discusses the various dozer attachments available for ripping 

and their applications.

The wear on equipment used for handling rock and granular materials is significant. Steel tracks on 

dozers require frequent replacement. Rubber tires are easily cut by sharp rock edges and require frequent replacement, although replaceable chain guards can be used to reduce damage. Special armor 

plating is needed for the bodies of trucks that haul rock and coarse granular materials.

Acceptable lift thicknesses for rock and granular material are much greater than for fine-grained soils, 

provided that increased compactive effort is applied. USBR (1998), USBR (1987a) and Sherard et al. 

(1963) recommend a lift thickness of one to two times the maximum rock diameter for rock fills that 

contain fines, while USACE (1995b) recommends that lift thicknesses for rockfill dams be no greater 

that 24 inches unless test fills show that adequate compaction can be obtained using thicker lifts. This 

guidance may require modification for mine spoil conditions where large rock on the order of the lift 

thickness may inhibit densification of surrounding spoil or where uniform gradation or maximum 

hydraulic conductivity characteristics are critical. In situations where the largest dimension of rock 

in the fill approaches the desired lift thickness, the interstices around the rock should be filled with 

finer materials and compacted until there is no visible evidence of consolidation of the material being 

compacted. The recommended lift thickness for gravel is about one foot, but this can be increased 

to two feet or more if the gravel is coarse. Vibratory compactors are very effective on rock and clean 

granular fill and may allow even higher lift thicknesses. The type and size of vibratory compactors 

required depends on the borrow material characteristics and lift thickness.

The water content for rock and granular fill is relatively unimportant. However, wetting of rock and 

clean gravels used for compacted fill will minimize friction between contact points of adjacent particles, allowing them to approach maximum density with reduced compactive effort.

Density controls for rock and granular fills are not as precisely defined as for fine-grained soils, and 

use of a method specification (i.e., minimum number of passes by construction equipment) based on 

site-specific observations may be appropriate. It is relatively difficult to take accurate in-place density 

tests for rock fills, as discussed in Section 11.8. Careful observation of the action of compaction equipment is an important part of evaluation of the effectiveness of the placement and compaction process. 

Key points to watch for include the following:

• Compaction efforts should continue until no further decrease in the lift thickness is 

observed during a pass of the compaction equipment.

• When compaction is complete, loose fine particles on the surface near the compaction equipment bounce due to vibrations transmitted through dense fill.

• When compaction is complete, the edges of adjacent rock particles will be crushed 

from being wedged into a tight position. Further compactive effort will only increase 

the amount of crushing rather than significantly increase the density. If the presence 

of crushed rock particles reduces desired drainage, compaction should be closely 

observed and terminated when crushing begins to occur.

11.6.6.3 Use of Mine Spoil

Mine spoil may be an acceptable borrow material for use in refuse disposal embankments, including 

structural zones, provided that the material characteristics are identified and accommodated in the 

design and construction. Mine spoil may include substantial portions of fine silt and clay materials, 

but typically is predominantly granular materials and large rock. Accordingly, in a zoned embankment, more pervious mine spoil would be placed downstream of less pervious embankment mate-
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rials such as coarse refuse. Key issues are the consistency of the spoil borrow source material and 

measures that need to be implemented so that it meets design requirements. Internal erosion of finer 

material into mine spoil is a particular concern. Characterization, design, and construction considerations of mine spoil are presented in Section 8.8.

Characterization of mine spoil can be accomplished by: (1) review of the mine overburden geology, 

(2) use of exploration test pits and borings to obtain samples for testing, and (3) performance of geophysical surveys to determine density and shear-wave velocity. The durability of the spoil material 

should be evaluated; in particular drainage and grain-size characteristics are important. Use of larger 

specimens for laboratory testing to obtain a representative sample may be needed. Measures that 

may be necessary during construction include segregation of large rock (using screens or in some 

instances during lift placement using dozers and graders) and rigorous quality control testing to 

verify material properties. Additionally, it may be desirable during mining operations that will provide borrow material, to strip and segregate soil and some rock strata (e.g., shales) from more durable 

rock strata for subsequent use.

11.7 FACILITY APPURTENANT STRUCTURES

Facility appurtenant structures generally require materials and construction procedures that are 

different from those for the disposal of refuse. These structures generally include, but are not limited to:

• Internal drainage systems in the refuse embankment such as internal drains, drainage trenches, etc.

• Spillways, decant structures and other major hydraulic control structures

• Minor surface drainage structures such as collection ditches, erosion control devices 

and culverts

• Haul and access road bridges and miscellaneous buildings associated with refuse 

disposal operations

• Mine-opening/auger-hole seals and drains

Discussion of material requirements and construction procedures for various types of appurtenant 

structures is provided in the following pages.

11.7.1 Material Requirements

The potentially corrosive nature of some coal refuse or leachates from the coal refuse makes material 

selection important if the appurtenant structures are to function as intended for long periods and into 

abandonment. Table 11.6 presents the most common construction materials used for facility appurtenant structures, shows the corrosive or deterioration mechanism for each material, and indicates 

protective measures that may be taken.

Coal refuse, and particularly leachate water therefrom, should be chemically tested for corrosive 

components. Typically, leachates will be acidic (low pH value) and will contain sulfates that cause 

corrosion of many materials. Coal refuse varies significantly, and testing is advisable prior to identifying appropriate protection measures at a particular disposal facility.

11.7.2 Filters and Drains

Control of water flowing across a disposal site, or seeping through an embankment or its foundation, is an important factor in satisfying both safety and environmental requirements. Design 

requirements for limiting, controlling and collecting these flows are presented in Section 6.6.2.3. 
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TABLE 11.6 EFFECTS OF CORROSION AND COUNTERMEASURES

Facility

Component

Materials and their Corrosive or 

Deterioration Mechanism Protective Measures

I. Internal Drainage Systems

A. Bank run gravels 

and sands or 

crushed rock

Sandstones: If the bonding agent between 

sand grains is silt particles or clay flakes 

consisting of calcite (CaCO2), sandstones will 

be attacked by acidic leachates.

Use only corrosive resistant sandstone with 

silica bond (SiO2).

Limestones: Mineral calcite (CaCO3) will be 

dissolved by acidic leachates. Do not use in internal drainage system.

Shale: Silt and clay composition breaks down 

when subjected to weathering or moisture. Do not use in internal drainage system.

B. Geosysnthetics Nylon, Polypropylene, Polyester, HDPE, PVC: 

Good corrosive resistance.

Most materials are sensitive to excessive 

heat. Protection from ultraviolet light is 

necessary prior to installation.

Steel and Corrugated Metal: See

discussion of metal products,

Item III below.

Galvanizing and/or coating with asphaltic 

or epoxy protective material or coal 

tar. Asphaltic and coal tar coatings are 

susceptible to damage. Asphalt and coal 

tar pipes are not recommended in critical 

situations. Properly designed epoxy-coated 

steel pipes can be considered for critical 

situations.

Use thick-walled pipe to account for 

corrosion; applicable if rate of corrosion is 

relatively low.

Cathodic protection in vulnerable areas.

C. Drainage and

 transport pipes

Use special alloy steels, stainless or other, 

(See discussion of Metal Products under 

Item III).

Cast Iron: Same as steel, but corrosion may 

form protective coating on metal under ideal 

conditions.

Coating with asphaltic protective material 

or coal tar. Coating is susceptible to 

damage and is not recommended in critical 

situations.

Use thick-walled pipe to account for corrosion; applicable if rate of corrosion is 

relatively low.

Cathodic protection is required in 

vulnerable areas.

Plastic (General): Excellent corrosion 

resistance. Consideration should be given to the 

compatibility of plastic pipe materials with 

bedding preparations and backfill.
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Facility

Component

Materials and their Corrosive or 

Deterioration Mechanism Protective Measures

Polyethylene (PE): Excellent for corrosive 

liquid transport and backfill.

Check characteristics and strength for 

the density PE pipe considered. Outside 

diameter controlled HDPE, when properly 

designed, can be used in critical situations.

ABS: Good corrosion resistance. Check characteristics and strength of

ABS pipe.

C. Drainage and

 transport pipes

 (Continued) PVC: Good for use with corrosive solutions 

below 140°F, brittle at low temperatures.

Do not use when temperature above 

140°F may be contacted. Check allowable 

application of specific PVC type used.

Concrete: See discussion on Concrete Structures, Item II below.

Thermosetting: Epoxy and polyester resin 

reinforced (fiberglass and filament wound) 

and non-reinforced pipe.

Check properties of specific resin used for 

strength, temperature effects and corrosion 

resistance.

D. Gates and 

miscellaneous 

metal work

See discussion of Metal Products under Item III.

II. Concrete Structures (spillways, decant structures, channels, outflow conduits, etc.)

A. Cement

The bonding agent in ordinary concrete 

(hydrated Portland cement) leaches when 

exposed to acidic waters.

Use Type II or IIa cement for contact with 

water containing (100 to 150 ppm) sulfates 

as SO4.

Sulfates in water or refuse react with 

tricalcium aluminate in cement causing 

swelling and disintegration of concrete.

Use Type V cement for contact with water 

containing greater than 1000 ppm of 

sulfates as SO4.

Provide coatings for concrete structures as 

discussed in ACI (2005) Committee 515 

report.

Low water-cement mix ratios result in less 

permeable concrete thus reducing penetration of water-containing, deleterious 

compounds. Air entrainment and other 

additives are also recommended for 

increasing resistance to water penetration 

and improving workability of low-water 

cement mixes.

Replace concrete materials with materials 

that are less susceptible to deterioration 

(e. g., coated cast-iron pipe in place of 

concrete pipe).

TABLE 11.6 EFFECTS OF CORROSION AND COUNTERMEASURES

(Continued)
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Facility

Component

Materials and their Corrosive or 

Deterioration Mechanism Protective Measures

B. Aggregate

See discussion of bank-run gravels or crushed rock, Item I.A.

See also ACI (2005), ACI Committee 201 for reactive aggregates.

C. Reinforcing

steel

See also discussion of metal products under Item III.

Porous concrete or inadequate cover 

permits rapid attack on reinforcing steel. 

Corroding steel causes expansion, 

disintegrating the concrete.

Typical concrete cover may protect reinforcing 

steel if corrosive environment is not severe. 

For severe environments and long service life, 

epoxy-coated rebar may be considered.

Provide coatings for concrete structures as 

discussed in ACI (2005).

Low water-cement mix ratios result in less 

permeable concrete and thus provide greater 

protection against corrosion. Air-entrained 

concrete is recommended as an aid to 

placement and for increased resistance to 

water penetration.

III. Metal Products (gates, valves, outflow conduits, decant pipes, etc.)

A. Steel

Electrochemical corrosion caused by 

oxidation in an ionizing medium (water). 

Rate of corrosion is governed by environmental factors such as hydrogen ion 

concentration, oxygen concentration, and 

temperature. For pH between 6.5 and 

8.0, corrosion protection is probably not 

required.

Use internal and external coatings such 

as asphalt, coal tar, epoxies or other nonreactive materials.

Provide cathodic protection. Use sacrificial 

anodes to reduce corrosion.

Use thicker material in construction to account 

for deterioration; applicable if rate of corrosion 

as determined from bench or field studies is 

relatively low.

Use special-alloy steels, stainless or other.

Corrosion caused by reaction with 

dissolved, ionized or colloidal substances in 

the corroding medium (water) such as: H+, 

02, C02, NH3 (bacteria), S04

-2, S-2, Cl.

Use of internal and external coatings such 

as epoxy, asphalt, coal tar, epoxies or other 

nonreactive materials.

Provide cathodic protection. Use sacrificial 

anodes to reduce corrosion of metal product 

to be protected.

Use thicker material in construction to account 

for deterioration; applicable if rate of corrosion 

as determined from bench or field studies is 

relatively low.

Use special-alloy steels, stainless or other.

TABLE 11.6 EFFECTS OF CORROSION AND COUNTERMEASURES

(Continued)
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The material and construction issues associated with design implementation are discussed in the 

following subsections.

11.7.2.1 Granular and Geotextile Filters

Filters consist of granular materials placed against disposed coal refuse or earth fill where seepage is 

likely to exit, or between materials of greatly different particle size (e.g., between clayey silt and coarse 

coal refuse) where seepage flows from the smaller-particle-sized material to the larger-particle-sized 

material. The primary purpose of a filter is to allow movement of water without allowing fine particles to exit or to move into the coarser material void spaces. Such movement could lead to internal 

erosion and weaken the fine material. Internal erosion can create voids in an embankment, increase 

rates of seepage and cause eventual failure. Movement of finer particles can also obstruct the flow 

within the coarse material. Granular filters may consist of multiple layers. The gradation of each layer 

should be sized such that finer granular material is unable to enter coarser layers and cause clogging. 

Geosynthetic materials such as geotextiles can be used in single layer applications or sometimes in 

combination with granular materials to create multiple layers. Layered granular and/or geotextile 

systems are commonly used in filter diaphragms and internal drains in slurry impoundment embankments. The National Dam Safety Review Board (NDSRB), which comprises federal agencies that deal 

with dam safety, recommends that a geotextile not be used as a filter for a critical internal drain in a 

water impounding dam unless it is accessible in the event that it does not perform as intended.

Granular filters should be composed of durable free-draining granular materials. Section 6.6.2.3

discusses design requirements for grain-size distribution. Filter materials typically should not 

contain more than five percent clay- and silt-size particles, either before or after placement. 

Similarly, geotextiles should be designed based on the grain-size distribution of the soil or refuse 

and apparent opening size of the geosynthetic material. Design requirements and limitations for 

geotextiles, including guidance for evaluating the potential for chemical and biological clogging, 

are presented in Section 6.6.2.3.2. Construction-related guidance for installation of geotextiles for 

filters includes:

• In preparing surfaces for geotextile placement, depressions, holes, and voids should 

be filled so that the geotextile sheet is continuously supported. The geotextile should 

be placed to loosely drape the surface with no sagging between surface contact 

points. Continuous contact between the geotextile and the support material is considered critical in addressing clogging potential (Talbot et al., 2000). Geotextile 

should not be placed over sharp or angular rocks that could tear or puncture it; an 

intermediate layer of compatible finer material should be placed over such rock as a 

bedding layer or protection buffer.

• Geotextiles should be secured by sewing, pins, staples, or weights so that specified 

overlaps are maintained during construction.

• Adjacent geotextile sheets should generally be placed with upstream layers overlapping downstream layers.

• In placing material on a geotextile, care must be taken to avoid punctures or tears. 

The construction specifications should limit the size and drop height of rocks to be 

placed on the geotextile. Generally, stones weighing more than 250 pounds should 

be placed with no free fall. Field trials should be made to verify that no damage will 

occur due to the rock placement procedures, or a cushion layer of finer material may 

be utilized to protect the geotextile.

• Geotextiles should be covered or protected as soon as possible to prevent degradation 

or damage from exposure or equipment traffic and to prevent fines from accumulating 

on the fabric. Manufacturer recommendations relative to exposure should be followed.
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Geosynthetic materials are commonly used as filter layers for drainage systems at a refuse embankment and as separating and reinforcing layers for channel linings and haul roads. Similar to granular filters, a geosynthetic material will allow the movement of water while preventing fine particles 

from entering the voids of a coarser drainage medium. Properties of filtering geosynthetics such as 

permittivity, hydraulic conductivity, percent open area and apparent opening size must be evaluated as part of drainage system design. Where geosynthetics are employed at internal drainage 

structures for impounding embankments, performance tests (e.g., gradient ratio test) should be 

conducted using actual embankment materials. Separating and reinforcing geosynthetics are normally chosen based upon their strength properties such as puncture strength, grab tensile strength, 

wide width tensile strength, grab tensile elongation, and trapezoid tear strength. Manufacturers’ 

construction guidance for geotextile filters should also be followed when geotextiles are used as a 

separation layer.

Care should be taken during construction to prevent runoff from washing fine particles into an unprotected filter. Filters should be constructed to an elevation slightly above that of adjacent materials. If 

this is not practical, diversion ditches should be constructed or temporary coverings should be placed 

over the filter.

Filter materials should be stockpiled, handled and placed in a manner similar to material used for 

underdrains, as discussed in the following paragraphs.

11.7.2.2 Granular Underdrains

Underdrains typically consist of trenches filled with granular material, granular blankets, or granular 

collection zones along stream bottoms in valleys or at isolated seeps or springs. These drains can be 

constructed with or without perforated pipes for conveying seepage from the collection zone to a safe 

exit location. The granular zones are generally thin, and typically there are no compaction specifications for the granular material; however, where more extensive granular zones are present, such as 

with chimney drains, provisions for placement should be provided.

Granular drains or collection zones must remain free-draining. Therefore, drain materials should 

be selected to: (1) allow flow and act as filter protection for adjacent material (Section 6.6.2.3), (2) 

resist deterioration over time, and (3) resist deterioration due to chemical attack. Table 11.6 lists 

possible effects of chemical attack or weathering on gravels and crushed rock that might be used 

as granular drainage materials. Granular aggregate used for underdrains and filters should be 

obtained from reputable suppliers or verified borrow sources. To minimize the potential for degradation, rock used in granular drains should have high durability; applicable rock testing for 

durability is discussed in Section 6.5.9.4. It is particularly important to avoid the use of most limestone because of the possibility for generation of acidic leachates that can react chemically causing 

dissolution or degradation. As a quick field test, the presence of limestone in granular material can 

be determined by placing a dilute solution of hydrochloric acid on the material. If an effervescent 

or fizzing reaction occurs, the material should not be used. Laboratory test data for both the refuse 

and the potential drain material should be carefully reviewed prior to the purchase or use of any 

drain material.

Granular material must not be allowed to segregate prior to placement. Figure 11.5 shows correct and incorrect ways of stockpiling and handling granular materials. The stockpile location 

and materials handling procedures should be such as to prevent siltation contamination by the 

loading or unloading equipment or by wind- or water-carried particles. Well-graded materials 

should not be dropped or allowed to roll down a slope for any significant distance, because the 

larger particles will separate from other materials by moving to the outside of the pile as they 

roll down the slope.
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CRANE OR OTHER MEANS OF PLACING

MATERIAL IN PILE IN UNITS (NOT LARGER

THAN A TRUCK LOAD) THAT REMAIN WHERE

PLACED AND DO NOT RUN DOWN SLOPES.

PREFERABLE OBJECTIONABLE

METHODS THAT PERMIT THE MATERIAL TO ROLL

DOWN THE SLOPE AS IT IS ADDED TO THE PILE

OR PERMIT THE HAULING EQUIPMENT TO OPERATE OVER THE SAME SURFACE REPEATEDLY.

FIGURE 11.5 STOCKPILING AND HANDLING OF GRANULAR MATERIALS

INCORRECT

FREE FALL OF MATERIAL FROM END OF

STACKER PERMITTING WIND TO SEPARATE

FINE FROM COARSE MATERIAL.

CORRECT

CHIMNEY SURROUNDING MATERIAL FALLING

FROM END OF CONVEYOR BELT TO PREVENT

WIND FROM SEPARATING FINE AND COARSE

MATERIALS. OPENINGS PROVIDED AS REQUIRED

TO DISCHARGE MATERIALS AT VARIOUS

ELEVATIONS ON PILE.

WIND

SEGREGRATION

LIMITED ACCEPTABILITY

PILE BUILT RADIALLY IN HORIZONTAL LAYERS

BY BULLDOZER WORKING WITH MATERIALS

DROPPED FROM A CONVEYOR BELT.

GENERALLY OBJECTIONABLE

UNLESS MATERIALS STRONGLY RESIST

BREAKAGE, STACKING PROGRESSIVE

LAYERS ON SLOPE NOT FLATTER THAN 3:1.

FIGURE 11.5 STOCKPILING AND HANDLING OF GRANULAR MATERIALS
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If granular drains must be placed in thick layers, or are critical to the stability of the embankment because 

of their position, the guidance in Section 11.6.6.2 relative to the placement and compaction of rock and 

granular fill should be reviewed prior to granular drain construction. Should compaction be necessary, 

the construction specification should account for potential breakdown of the granular material.

11.7.2.3 Drainage Pipe

Drainage pipes are often placed in portions of the refuse embankment to convey water from the 

granular drains (internal or surface drains) to an acceptable exit point. Within the collection zone, 

drainage pipes are normally perforated to facilitate inflow of water that collects in the drain. The 

remainder of the drainage pipe may or may not be perforated. Drainage pipes should be constructed 

from a material resistant to chemical attack by refuse leachates and should be sized according to flow 

capacity and structural requirements. They should generally be designed and installed such that they 

are self-cleaning and the potential for accumulation of fine material in the pipe is minimized.

Although corrugated metal pipe (CMP) can often be protected by coatings, the potential for coating 

damage during installation is a potential problem. Since plastic pipe is not normally susceptible to 

corrosion from acids, it is often more suitable than CMP for drainage pipes in coal refuse.

When perforated pipe is placed, the perforations should normally be placed facing downward. This 

causes the water to flow up into the pipe, minimizing the chance for transport of fines into the pipe 

and consequent reduction in flow capacity. Instructions from the pipe manufacturer are often explicit 

in this respect and should be followed. For most perforated pipe, several rows of perforations are 

normally provided around the circumference of the pipe. For this type of pipe it will not be possible 

to orient all of the perforations facing downward. For installation in very fine materials, a double 

layer filter and granular drain may be needed to prevent fine material from entering the perforations. 

Such requirements should be detailed in the facility plans and specifications.

Drainage pipes beneath embankments should be designed to withstand the loading of the embankment, with specification of pipe support and backfill that prevents point loading on the pipe. Design 

and construction of conduits for dams was published by FEMA (2005a), and additional guidance by 

FEMA related to plastic pipe has also been published (FEMA, 2007). During construction, sufficient 

fill must be placed over the pipe before traversing it with heavy equipment. Manufacturers’ recommendations should be checked.

11.7.3 Culverts and Decants

Conduits are often used for transporting decanted impoundment water and flood water beneath 

embankments and also for culvert-type spillways. Conduit materials should be selected on the basis 

of strength and resistance to corrosion. Potential corrosion and deterioration mechanisms for conduit 

materials are discussed under Item I-C (Drainage and transport pipes) in Table 11.6. Progressive 

deterioration of large conduits that are critical to refuse facility hydraulic systems is not acceptable, 

and effort should be made to prevent loss of conduit flow capacity, leakage that can lead to material 

infiltration or exfiltration, and structural distress.

Conduits require special bedding or thrust blocks at turns. Uniform contact around and under the 

conduit is essential, particularly at thrust- block areas. When conduits associated with decant systems and culvert spillways are installed in critical areas, such as through impounding embankments, 

special provisions are required for performing compaction beneath and around the conduit to minimize the potential for seepage along the exterior of the pipe. Compaction beneath the haunches of 

the pipe is difficult and normally can not be accomplished unless special materials and designs are 

employed. Concrete cradles and sand bedding have been used to improve the support beneath and 

around conduits.
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Other more specialized materials and methods such as use of flowable backfill or the cut-earth 

cradle method can be employed for conduit installation. Flowable fill, or controlled low-strength 

material (CLSM), is a low-strength, fine-aggregate concrete with unconfined compression strengths 

between 50 and 200 psi. The use of flowable fill to backfill a conduit trench and serve as bedding for 

the conduit provides firm support extending from the spring line around the base of the conduit. 

Flowable fill has the added benefit of having low hydraulic conductivity. The mix must be designed 

to prevent shrinkage and to have satisfactory strength/deformation characteristics, as discussed in 

Sections 6.5.10.2 and 6.6.6.3.3. Either shrinkage or cracking of flowable fill will allow a flow path for 

seepage.

The cut-earth cradle method involves compaction of select material at the base of the conduit location 

and subsequent excavation using a specialized attachment on an excavator resulting in a semi-circular cradle excavation. The cradle excavation is sized and shaped so that it closely fits the conduit. 

A material such as bentonite powder has been used to compensate for small, discontinuous gaps 

between the cradle and the pipe. The depth of the cradle should be approximately one-half the diameter of the pipe. This method requires a high level of quality control in order to prevent a flow path 

from occurring along the pipe. 

To collect and discharge any possible seepage along the conduit in a controlled manner, a filter diaphragm is recommended regardless of the method employed for conduit backfill and support.

11.7.4 Concrete Structures

Coal refuse disposal facility appurtenant structures that may be totally or partially constructed of 

concrete include spillways, decant structures, culverts and drainage channels, as well as foundations 

for various small structures. Such concrete work is not unique to coal refuse disposal facilities, and 

most applications are extensively discussed by the ACI (2007), the USBR (1992b) and in numerous 

concrete design textbooks.

Chemical attack and deterioration and related preventative measures are key considerations in the 

use of concrete for any facility structure that may be in contact with coal refuse or leachates from coal 

refuse. Table 11.6 presents a summary of major considerations. Although acidic leachates are an obvious corrosive environment, it is emphasized that for concrete durability it is also important to guard 

against the sulfates common to coal refuse and coal refuse leachates. The USBR (1987a) shows typical 

examples of sulfate attack and discusses methods for retarding it.

Before specific protective measures for concrete exposed to coal refuse and leachates are selected, the 

acidic potential and sulfate contents of the refuse should be evaluated. Chemical testing is recommended. Concrete structures exposed to natural ground adjacent to coal refuse disposal facilities 

should also be protected from deterioration by chemical attack. Groundwater in and around coal 

refuse may contain corrosive elements and can cause concrete deterioration even if the concrete is not 

in direct contact with coal refuse.

Minimum practices and protective measures for concrete used in or near coal refuse disposal facilities should include:

• Use of sulfate-resistant cement (preferably Type V, but in some mine environments 

Type II cement will suffice). Depending on cement availability, Type II or Type I cement in combination with pozzolans (e.g., fly ash, slag cement) may provide adequate sulfate resistance.

• Low water-cement ratios and relatively rich mixes should be used so that dense, 

high-strength concrete results.
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• Air entrainment should be used to increase the workability of low water-cement 

ratio mixes and to decrease the hydraulic conductivity of the hardened concrete. 

The use of other water reducing chemical additives to allow further reduction of the 

water-cement ratio and to provide increased strength should also be considered.

• Extra cover concrete should generally be provided over all reinforcing steel.

• Fresh concrete should be thoroughly vibrated to maximize densification and to 

eliminate honeycombing.

• Concrete should be thoroughly cured to maximize strength gain.

Many protective coatings discussed in the following section are also applicable to concrete structures, 

although application and maintenance may be more difficult than for application to metals.

11.7.5 Gates, Valves, Pipelines and Other Metal Work

Metal work in and around a coal refuse disposal facility must be protected against corrosion from 

chemical attack. Special alloy steels may offer good protection against corrosion, but their selection 

should be made with care because some types of alloy steels are actually more susceptible to attack 

from certain chemicals than regular construction grade carbon steels. The cost of many alloy steels 

is similar to carbon steel, but cost must be evaluated on a case-by-case basis. Table 11.6 summarizes 

various types of chemical attack and corresponding measures for protection.

Installation of metal products with protective coatings is common, but installation should be accomplished 

in strict accordance with the recommendations of the coating manufacturer. Various coatings are available 

that will provide adequate protection if the integrity of the coatings is maintained during construction 

and operation. Coatings should be: (1) continuous, (2) not support bacterial growths or absorb water, and 

(3) not deteriorate with time. Epoxy coatings are very durable and provide excellent protection against 

corrosion and bacterial growth. However, field application of epoxy coatings is difficult and requires special surface preparation per the manufacturer’s recommendations. Coal tars meet all of the coating criteria 

and are one of the best bituminous materials available for protection of steel surfaces. Asphalt coatings 

are an alternate low-cost bituminous coating. However, if bituminous coatings are scratched or damaged 

during installation, they must be repaired prior to completion of the installation.

Special vinyl chloride or chlorinated rubber coatings are also available for corrosive protection. 

Application of these coatings must be made in strict accordance with recommendations of the manufacturer, as there are significant differences in methods of application.

Imperfect coating of metal may actually lead to more rapid deterioration than no coating at all. This 

can occur where galvanic action is very strong. If the predominant portion of the metal is protected 

by materials that resist electrical currents, galvanic action will concentrate at points of imperfection, 

leading to very rapid removal of the metal at these locations. Protection against this type of corrosion 

requires installation of a sacrificial anode designed by a professional experienced in corrosion protection. USACE (1985) and USACE (2004) provide background information and guidance on testing and 

design for cathodic protection.

11.7.6 Support Structures

Offices, maintenance buildings, loading bins, etc. should be built in accordance with applicable 

building codes and generally accepted construction practices. Coal refuse can often be used effectively for structural fills on which such structures are constructed. The same precautions against 

material deterioration due to acid or sulfate attack should be taken. Drainage and utility pipes, 

foundations and electrical conduits are all subject to corrosive action if exposed to coal refuse or 

coal refuse leachates.
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11.7.7 Mine-Opening and Auger-Hole Seals and Drains

Depending upon their location within an embankment or impoundment area, the presence of 

mine openings and auger holes in coal seams should be addressed with backfilling, barriers, 

bulkheads, seals or drains. The construction materials for these structures can range from structural materials such as cast-in-place concrete or block to aggregate materials for construction of 

filters and drains. On-site borrow materials may be specified for backfill or for compacted fill as 

part of a barrier.

11.8 QUALITY CONTROL AND FIELD TESTING

Quality control and field testing are required for verification that a refuse facility is constructed 

according to plans, specifications and the operation and maintenance plan. The principal concerns 

for construction of a coal refuse disposal facility include:

• Compaction control where refuse is to be used as structural fill.

• Compaction control where borrow materials are to be used as structural fill.

• Verification that all areas that require structural fill are so constructed.

• Verification that material properties that affect construction operations, such as 

grain size, water content and compaction characteristics are in accordance with 

specifications.

• Verification that facility appurtenant structures, especially conduits and open channel spillways, are being constructed in accordance with design requirements, manufacturers' recommendations and good construction practice.

Activities that require quality control can be identified through review of the design report, plans, 

specifications, operation and maintenance plan for the facility, and technical chapters of this Manual, 

particularly Chapter 6.

11.8.1 Compaction Control

The monitoring of compaction in the field is essential for verification that structural fill refuse is 

compacted to a density meeting design requirements. The density of fill material should be tested 

in order to verify that the pertinent engineering properties of the fill that are a function of density, 

such as shear strength and hydraulic conductivity, are consistent with the values used in the design. 

Compaction control entails field testing during the construction of embankments and at other areas 

requiring structural fill refuse. Compaction test locations should be selected so as to include areas 

where routine compaction activities are performed, as well as areas that are more difficult for compaction equipment to access with repeated passes or where successful compaction is doubtful. At 

some sites, a grid system has been employed to aid in test location selection, and for each lift random 

grid points are chosen for testing. USBR publications (1987a, 1992a, 1998) discuss methods for testing 

compacted fill materials. 

The in-situ density testing of structural fill has become much easier and more efficient with advances 

in nuclear testing methods. Unless special conditions arise during fill construction, most structural 

fill refuse, fine-grained soils, and fine gravels can be density tested using a nuclear density-moisture 

gauge. USACE (1995b) provides guidance on the use of these types of gauges, which should be operated in the direct transmission mode. Nuclear density-moisture gauges provide direct readings of insitu density and the moisture content of fill. The instrument operates by projecting radiation (gamma 

rays) into the fill and measuring returns with a Geiger-Mueller detector. The instrument is easy to 

use and requires only observation of a digital read-out. However, the presence of varying amounts 

of hydrocarbons in coal refuse can cause the gauge to indicate higher than actual moisture contents.
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Therefore, samples should be collected from the tested fill for laboratory moisture analysis so that 

accurate calculation of dry density can be performed.

For evaluation of the degree of compaction, measured field densities can be compared to Proctor density curves (Chapter 6) developed from laboratory testing. All field test results should be recorded. If 

test results indicate a lack of compliance with fill density requirements, compaction procedures should 

be modified. Areas of low density should receive additional compaction. If necessary, unacceptable fill 

should be removed and replaced, and recompaction should be performed. If it is determined that the 

fill is being compacted to densities greater than the maximum value from the Proctor test, an evaluation of the material (grain size and specific gravity) and the associated Proctor density curves may be 

necessary. In some cases, the presence of oversized particles, that is particles too large to be included 

in the Proctor mold (ASTM D 698), may be causing overestimation of density. If this is suspected, a 

sieve analysis should be performed and a rock-correction factor should be developed in accordance 

with ASTM D 4718, “Standard Practice for Correction of Unit Weight and Water Content for Soils 

Containing Oversize Particles,” and used to recalculate the maximum dry density of the refuse.

Application of the ASTM D 4718 may be difficult or precluded when coarse refuse contains more than 

25 to 30 percent of plus-3⁄4-inch particles, even after the oversize correction provisions using laboratory determined specific gravities are employed. Sometimes the oversize correction yields unreasonable target densities (usually higher than achievable), and in this case other compaction tests or field 

procedures should be considered. While determination of relative density in accordance with ASTM 

D 4253 and ASTM D 4254 is a possibility, this is generally not recommended because of the fine content in coal refuse. Another possible method for compaction control is the roller pass test (WVDOT, 

1999). This method is discussed in Section 11.5.1.

Alternative material testing methods may be employed when more than 25 to 30 percent of plus-

3⁄4-inch particles are encountered. One such testing method is specified in Appendix VIA of the 

USACE Publication EM 1110-2-1906 titled, “Laboratory Soils Testing” (USACE, 1986). This method 

employs a 12-inch-diameter mold that accommodates larger particle sizes. Disadvantages to this 

method are that it is more costly than the ASTM D698 method, and many geotechnical laboratories 

are not equipped to perform the test. However, if refuse with a significant proportion of oversize 

particles is routinely generated, it may be effective to use the USACE test to supplement or replace 

the roller pass method.

In addition to periodic field density tests, visual observation of refuse placement and compaction 

should be performed regularly. If compacted material tests as having an “acceptable” density, but the 

surface is not firm, then the compaction specification should be re-evaluated. Regular observations 

by field personnel can often lead to acceptable modification of compaction procedures. Such observations are discussed in Section 11.6. Key observations for control of earth-fill operations are discussed 

by Church (1981) and USBR (1998). These observations, which typically apply to refuse disposal 

operations, include:

• Changes in lift thickness with subsequent passes of compaction equipment (including controlled routing of haul units) should be noted.

• Compaction equipment should not excessively “weave” or “rut” the fill. If this occurs, the material is too wet to be properly compacted.

• The compacted fill surface exhibits “pumping” under construction equipment traffic, 

which may indicate that lifts below the surface were compacted at too high a moisture content, and need to be uncovered and reworked.

• The feet of a sheepsfoot or tamper-type roller will first penetrate the loose material 

of a new lift, but should then begin to ride up on the material with each successive 

pass, as compaction is performed.
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A consistent and reliable compaction procedure based upon a sufficient number of passes with 

appropriate equipment should be established. The USACE (1995b) provides general guidance relative to the number of passes required for various earthfill materials. Site-specific conditions must be 

considered in establishing compaction parameters.

It is recommended that structural fill refuse be compacted to at least 95 percent of the maximum dry 

density determined by the ASTM D 698. The refuse should be placed at a moisture content in the 

range indicated in the plans and specifications, typically in a range of –2 to +3 percent of optimum. 

The moisture content should be uniform throughout each lift. At least one field density test should 

be conducted for every 2,000 cubic yards of compacted structural fill with at least one test per lift. A 

common specification for compaction of pipe backfill and around structures is: at least one density 

test for every 200 cubic yards with at least one test per lift. Because of the importance of achieving 

adequate and consistent compaction of pipe backfill, more frequent density testing should be conducted. It is recommended that multiple tests be performed so that the testing frequency is sigmificantly lower than every 200 cubic yards. Testing should also be performed whenever it is suspected 

that adequate compaction is not being achieved.

If failing density tests occur, it is the responsibility of field personnel to determine the cause of the 

failed tests. Generally, failed density tests are a result of either insufficient compaction or a change of 

material from that tested in accordance with ASTM D 698. If it suspected that the fill was not placed 

and compacted adequately, the limits of the lift and/or area should be marked, and mine personnel 

should be notified that the lift/area requires reworking and additional compaction. No additional 

refuse should be placed in the area with failed density tests until it has been adequately compacted. 

Additional density tests should be taken after the lift/area is reworked to verify the effectiveness of 

the additional compaction effort. If it is believed that the lift/area has been placed and compacted 

adequately, and a variation in grain-size distribution of the refuse being placed is suspected, a new 

sample should be collected for the ASTM D 698 testing. The Proctor curve(s) should be verified on a 

regular basis, typically between quarterly and annually.

11.8.2 Material Testing

The material properties of coal refuse, borrow materials, filter or drain gravels, and concrete can be 

determined by on-site field testing and/or by submittal of test samples to a qualified testing laboratory. Laboratories should be selected based on qualifications. The AASHTO Materials Reference 

Laboratory (AMRL) performs evaluation and accreditation programs for many construction materials, and the Geosynthetic Accreditation Institute – Laboratory Accreditation Program (GAI-LAP) 

performs similar evaluations for synthetic materials. Laboratories should be accredited through these 

programs for the type of testing being performed. Water contents and fill densities are best tested in 

the field for real-time assessment of compaction efficiency, but occasional test verification by an independent source may be appropriate. Strength testing of refuse, soils, concrete and other construction 

materials generally requires laboratory equipment and in some instances may be incorporated into 

quality control programs cited in the construction specifications.

Routine sampling and testing of construction materials is recommended during the development of 

a refuse disposal facility. The resulting records can be used for future evaluation of material performance and for submittals to regulatory agencies. Most importantly, material sampling and testing 

during construction increases confidence that the work is being performed in compliance with design 

requirements.
Chapter 12

MONITORING, INSPECTIONS

AND FACILITY MAINTENANCE

MAY 2009 12-1

Routine monitoring and inspections are essential to the successful construction and operation of a 

coal refuse disposal facility. Inspections, as described herein, include the components of observations, testing and instrumentation measurements, as performed by a representative of the Mine 

Owner/Operator (Operator) in accordance with 30 CFR § 77.216-3. Personnel responsible for monitoring and inspections require MSHA impoundment inspection training and/or qualifications and 

should work with a registered Professional Engineer who is familiar with the design of the disposal 

facility (Engineer). It is recommended that the Operator designate an Engineer to provide assistance 

with the implementation of design plans and specifications, assess the performance of the facility, 

report on the progress of the construction work, and render professional opinions regarding the 

conformance of construction, operation, and maintenance activities with design plans and specifications. Note that the guidance in this chapter is not intended to address work-site safety issues or 

daily safety inspections.

Well executed monitoring and inspection programs during construction will facilitate development according to plans and specifications and aid in identifying modifications to the design that 

may need to be made. Routine inspections during operation of the facility allow for the identification of potential problems and resolution in a timely manner. Observations made during monitoring and inspections can provide a basis for either increasing or decreasing certain maintenance 

activities. This chapter is generally focused upon monitoring and inspection programs and their 

relationship to facility maintenance during development and operation of the refuse disposal facility. These programs may also be required during abandonment and reclamation of a coal refuse 

embankment.

Monitoring and inspection requirements should be established during the design of a coal refuse 

disposal facility and should be documented in the Operation and Maintenance Plan, which should 

clearly indicate the roles and responsibilities of various parties in monitoring and inspection activities. Sufficient controls should be in place so that, when monitoring and inspection is performed 

as specified, the intent of the design is met and satisfactory performance of the facility is achieved. 

Monitoring normally includes observation of work activities during facility construction, including 

those activities required to initially prepare the site. Monitoring may encompass such activities as 

visual observation, collection and analysis of quality control samples, performance of field and laboratory testing, evaluation of construction methods, collection and review of instrumentation readings 

and evaluation of survey data.
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Inspections for evaluating performance are typically conducted for the overall facility and at areas 

that have been recently constructed. These inspections normally include visual observations; collection of physical data, instrument readings and flow rate measurements (if applicable); and evaluation of the assimilated data. Photographing of site conditions, during construction and operation, is 

an excellent way to document site conditions and any changes in conditions and is a recommended 

practice. Information from previous inspections and monitoring activities should be reviewed and 

compared to current data and plotted instrumentation data so that an informed evaluation can 

be made. In accordance with 30 CFR § 77.216-3, facility inspections for impounding refuse facilities must be performed on a 7-day basis unless otherwise approved by the MSHA District Manager. Annual reporting and certification requirements for impoundments are provided in 30 CFR 

§ 77.216‐4. Additionally, some states have inspection, reporting, and certification requirements that 

differ from the federal requirements. Annual dam safety inspections may be required by some state 

regulatory agencies, but these can generally be combined with the MSHA annual reporting and 

certification requirements.

It is recommended that monitoring and inspections generally be performed under the guidance of 

a registered Professional Engineer who is familiar with the design (including safety criteria for the 

structure) and involved in the implementation of the construction plans and specifications. Thus, 

this individual can prepare required reports and certifications and can assess: (1) conformity with 

the approved design, (2) significance of the instrumentation records, (3) potential conditions that 

may warrant notification of agencies including MSHA, and (4) design modifications and remedial 

construction activities that may be needed. Typically, the design engineer performs or contributes to 

this activity. In describing the monitoring and inspection activities in this chapter, and particularly 

actions that may be in response to observed conditions, test results, or instrumentation readings, 

reference is made to the Engineer in situations that are typically important for such an individual to 

assess and determine subsequent action. The Engineer’s involvement in other activities may also be 

important, and the site-specific Operation and Maintenance Plan should cite monitoring and inspection requirements. By maintaining the involvement of a registered Professional Engineer who is thoroughly familiar with the design during construction and operation, the mine operator is in position 

to assess conformity with the approved plan and to respond to regulatory agencies regarding performance or compliance concerns.

If potentially hazardous conditions develop, MSHA requires under 30 CFR § 77.216-3 that the Operator take action to: (1) eliminate the condition, (2) notify the District Manager, (3) notify and prepare to 

evacuate miners who may be affected, and (4) perform inspections on an eight-hour or more frequent 

basis. The Engineer should be a source of technical advice in such situations. Potentially hazardous 

conditions can be identified based on criteria established for Emergency Action Plan preparation, as 

discussed in Chapter 14.

Facility maintenance includes routine recurring actions for establishing vegetation and controlling erosion, periodic actions after unusual events such as clearing debris, and long-term actions in response 

to deterioration of structures, such as repair of channel linings and maintenance of embankment 

slopes. Monitoring and inspections may identify conditions that are inconsistent with the Operation 

and Maintenance Plan because of differing site conditions, construction procedures, or performance. 

In such situations, new maintenance requirements may need to be developed.

12.1 MONITORING AND INSPECTIONS

There is an inherent degree of uncertainty in the engineering design of a refuse disposal facility that 

can normally be offset by applying factors of safety in analytical procedures and by using conservative estimates of material properties. Engineers, particularly in the geotechnical fields, have developed observational/monitoring procedures to deal with these uncertainties without being overly 
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conservative. The following is a brief listing of the purposes for monitoring and inspection and the 

basis for their incorporation into refuse disposal facility development:

• Construction quality can vary depending on the specific activity, site conditions, 

equipment and personnel, all of which can affect facility performance. Monitoring 

and inspection provide a means of controlling adherence to design requirements and 

documenting construction quality.

• Geologic conditions can vary significantly at a site. These variations can be accommodated by monitoring and inspection programs during construction and the use 

of reasonable factors of safety in the design. Monitoring during construction enables 

evaluation of the validity of design assumptions and analyses and provides an 

opportunity to make changes as needed.

• Because it is impractical to investigate all natural conditions at a site or anticipate 

every aspect of facility operation, confirmation of design assumptions through monitoring and inspection can offset limitations in site exploration.

• The long operating life of a refuse disposal facility can lead to many changes in site 

conditions. For example, changes in coal cleaning procedures and refuse characteristics 

may occur due to mining of different coal seams during the operational period. An 

effective monitoring and inspection program can identify when significant changes are 

occurring or performance problems are developing, so that appropriate modifications 

can be made quickly when implementation is least difficult and expensive.

• Monitoring and inspection programs form the basis for determining when routine maintenance is needed in order to limit the deterioration of important facility components.

Monitoring and inspection rely heavily on visual observation and are supported by quality control 

testing, instrumentation, and engineering evaluation. The complexity of a monitoring program can 

range from simple visual examination of surface conditions to sophisticated surveys using electronic 

equipment. The simplest means are usually the most effective, especially for the constantly changing 

conditions associated with coal refuse facilities. A more complicated monitoring and instrumentation 

program may be justifiable when the information obtained can result in an improved design, operation or significant savings in construction costs. Regardless of complexity, a monitoring program is 

successful only if the information obtained is pertinent to the design and operation and the data are 

accurate and measured, recorded and properly interpreted by qualified persons in a timely manner. 

Accordingly, any monitoring program must be based on knowledge of design requirements, and the 

observations and measurements must be obtained and interpreted by staff cognizant of the purpose 

for and limitations of the instrumentation.

12.1.1 Monitoring and Inspection Objectives

Important objectives for monitoring and inspection at coal refuse facility sites include the following:

• Validating the assumptions upon which the design was based – A valuable function 

of monitoring programs is to provide data for verifying that the assumptions upon 

which the facility design is based are valid. Therefore, it is critical that monitoring 

data be evaluated by an engineer who is thoroughly familiar with the design and 

associated engineering analyses and assumptions and who is responsible for certification of the work.

• Providing data for improving construction methods –As an example, the compaction equipment being used at a site may produce an undesirable particle compaction 

arrangement that, in turn, affects properties such as strength or hydraulic conduc-
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tivity. Through observation of embankment construction, changes can be made in 

equipment selection, traffic routing, drying or wetting, or other factors such that the 

desired results are achieved.

• Providing a means for early detection of potential problems or areas requiring maintenance or remedial improvement – A field monitoring and inspection program can 

identify problems before conditions deteriorate to the point that extensive repair is 

required or failure can occur. The natural variability of earthen and coal refuse materials may result in unexpected deviations from the original design. Changes in the 

rate of refuse placement could alter embankment elevations and pool levels, affecting embankment stability.

A project-specific failure modes analysis can be useful in establishing site-specific monitoring and 

inspection objectives, particularly for facilities with impounding embankments (Martin and Davies, 

2000). Through an objective review of potential modes of failure of the disposal facility, considering 

all loading conditions and design features, the most important monitoring, instrumentation, and inspection objectives can be identified. Martin and Davies suggest the following process:

• Identification of potential failure modes and corresponding warning signs.

• Evaluation of how quickly failure could occur and how potential problems could be 

detected before they develop into incidents.

• Critical assessment of how likely it is that warning signs will be detected, recognized, reported and acted upon.

• Identification of instrumentation types and locations that provide the most relevant 

monitoring data.

• Evaluation of the significance of data trends as opposed to single measurements/observations.

• Evaluation of the value of establishing acceptable parameter limits or other limiting 

criteria.

• Review of lines of responsibility and communication.

• Review of data management, interpretation, action plans, and reporting.

Figure 12.1, as modified from Martin and Davies (2000), illustrates the relationship of failure modes, 

warning signs, and surveillance measures for tailings dams.

12.1.2 Visual Observations

The simplest, but often most valuable, monitoring method is visual observation by experienced personnel of important facility components and the surrounding area on a schedule compatible with site 

activity and the rate at which the facility configuration is changing. The required frequency for observations and the level/knowledge of the individual performing the observations should be established 

by the designer and should be consistent with federal and state requirements. The following should 

be considered when developing a schedule for visual observations:

• The type of refuse facility (impounding or non-impounding) and stage of construction. 

• Specific critical construction items (e.g., internal drains, surface channels, decant 

pipe, cut-off trench) that should be observed.

• Specific operational areas or items (e.g., spillway operation) required for evaluating 

performance and potential design modification.

• Identification of components/areas that require only periodic monitoring and inspections and their frequencies.
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Components/items to be observed depend upon the facility configuration. Typical factors include:

• Facility configuration and type of refuse disposal

• Geotechnical design requirements

• Site mining and foundation requirements

• Appurtenant hydraulic facilities (spillways or decant pipes)

• Environmental control provisions

• Construction procedures

FIGURE 12.1 FAILURE MODES, WARNING SIGNS

 AND SURVEILLANCE MEASURES

LOSS OF DIVERSIONS/

 SPILLWAY FLOW

 CAPACITY

LOSS OF CREST

 ELEVATION

INTERNAL EROSION

 OF DAMFILLS

INTERNAL EROSION

OF FOUNDATION

MATERIALS OR

PIPE BACKFILL

SURFACE EROSION

 OF DAM SLOPES

SLIDE THROUGH

DAMFILL AND/OR

FOUNDATION

BREAKTHROUGH

 OF TAILINGS

 INTO MINE

BLOCKAGE/FAILURE OF

DIVERSIONS/SPILLWAY GRADUAL INFILLING

 OF DIVERSIONS

TAILINGS POND

 LEVEL SETTLEMENT

 CHANGE IN CREST

GEOMETRY/CRACKING

CHANGE IN SLOPE

GEOMETRY (US/DS)/

 CRACKING

 CHANGE IN SEEPAGE

CONDITIONS/LOCATIONS/

 VEGETATION

SINKHOLES, TURBID

 SEEPAGE

 DISCHARGE

 GRADUAL TREND TO

INCREASED TURBIDITY

 IN SEEPAGE

SEEPAGE INDUCED

SLOUGHING IN 

EMBANKMENT

RAISED SATURATION

 LEVELS, SOFT

GROUND CONDITIONS

 LARGE, SUDDEN

 CHANGES IN

PIEZOMETRIC LEVELS

 LARGE, SUDDEN

 CHANGES IN

SEEPAGE FLOWRATES

 LONG-TERM

PIEZOMETRIC

 CHANGES

 LONG-TERM

 INCREASE IN

SEEPAGE FLOWRATES

 LARGE, SUDDEN

MINE DISCHARGES

 INSPECTIONS OF

DIVERSION/SPILLWAYS

 TAILINGS POND

 LEVEL CHECKS AND

PERIODIC SOUNDINGS

REQUIRED CREST 

ELEVATION VERSUS

DECANT ELEVATION

SURFACE MONUMENT

 SURVEYS

PIEZOMETERS/

WELLS

 INCLINOMETERS/

SETTLEMENT GAGES/

 EXTENSOMETERS

SEEPAGE WEIRS

INSPECTION OF DAM

 SLOPES/CREST/

DOWNSTREAM TOE

 SEEPAGE WATER

QUALITY/TURBIDITY

CLOGGING OF TOE

DRAINS/OXIDATION

 PRODUCTS

INSPECTION OF

 ABUTMENTS

 INSPECTION OF

MINE DISCHARGE

OVERTOPPING

 MASS

MOVEMENT

INTERNAL

EROSION

SURFACE

EROSION

 MINE

SUBSIDENCE

FREEBOARD 

LESS THAN

DESIGN CRITERIA

TAILINGS DAM

FAILURE MODES

 ADVERSE

CONDITIONS

RAPID WARNING

 SIGNS

 GRADUAL

WARNING SIGNS

SURVEILLANCE

 MEASURES

DECANT PIPELINE

 INSPECTIONS

MINE DISCHARGE

DOESN’T CORRELATE

TO RAINFALL

TAILINGS LEVEL

RISING FASTER

THAN ANTICIPATED

LOSS OF UPSTREAM

 EROSION

 PROTECTION

(ADAPTED FROM MARTIN AND DAVIES, 2000)

FIGURE 12.1 FAILURE MODES, WARNING SIGNS AND SURVEILLANCE MEASURES
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Checklists should be developed for each refuse disposal facility and should be incorporated into 

the Operation and Maintenance Plan. These checklists should be used in conjunction with the 

facility plans and specifications. Figure 12.2 presents an example of a data collection form that 

is incorporated into a site as-built drawing with topographic contours. The combination of the 

data collection form and as-built drawing provides a good point of reference during monitoring 

activities and allows areas of concern to be marked without the need for cumbersome descriptions/text. Items such as density test locations, seeps, sloughs, erosion and slurry discharge location can easily be indicated on the drawing.

SCALE

400 0 400

FIGURE 12.2 EXAMPLE DATA COLLECTION FORM
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Table 12.1 is a generic list that covers most aspects of refuse disposal facility activity. This list can be 

used as the basis for development of a site-specific data collection form. Other available references 

include the MSHA Coal Mine Impoundment Inspection and Plan Review Handbook prepared by MSHA 

(2007) and the Guidance Document for Coal Waste Impoundment Facilities & Coal Waste Impoundment 

Inspection Form prepared by the West Virginia Water Research Institute (2005).

Extensive digital photography is recommended for documenting visual inspections of a coal refuse 

disposal facility. Photographs not only document the observations made during monitoring or 

inspection activities, but they can also be used later to track the construction progress of the facility. 

Photographs are also invaluable for comparing how conditions such as seepage areas change over 

time. The location and/or orientation of the camera for each photograph should be documented on 

the as-built drawing or data collection form.

Hand-held global positioning system (GPS) units are very useful for locating items of interest. Most 

hand-held GPS units can provide position location to within about 10 feet. For most refuse facilities, 

this accuracy is sufficient for locating sampling or testing locations, wet areas, minor seeps, or other 

non-critical items. The use of a hand held GPS unit or altimeter unit for elevation determinations is 

not currently recommended due to lack of accuracy. It should be noted that the GPS coordinates may 

not conform to the coordinate system used by the mine. Software for performing coordinate conversions is available form commercial vendors.

12.1.2.1 Schedule and Checklist for Visual (and Other) Observations

A schedule for observations should be provided in the Operational and Maintenance Plan. The schedule of observations must be compatible with federal and state regulatory agency inspection requirements. In accordance with 30 CFR § 77.216-3, inspections (not to exceed 7-day intervals) are required 

for all impounding facilities meeting the requirements of 30 CFR § 77.216(a). Section 12.3 provides 

additional details relative to embankment/impoundment safety inspections. The recording of data or 

collection of measurements from installed instrumentation should also be defined in the schedule of 

observations. Chapter 13 provides additional details concerning the types of instruments available 

and information related to their installation and use. While 7-day visual inspections are required, 

continuous monitoring may be appropriate for some critical construction activities (e.g., structures 

that are ultimately buried, such as decant pipe and internal drain installations, instrumentations, 

etc.). Alternate frequencies (more or less frequent than every 7 days) for instrumentation monitoring 

may be desirable.

Inspection of impounding embankments during and after significant precipitation events is recommended. This is a good time to observe the operation of decant and spillway facilities and the outlets 

to internal drains and to verify that they are functioning properly. Special attention should be paid to 

whether flow is unobstructed and erosion protection, especially at outlets, is functioning properly.

Another important time to observe the operation of a dam is during first filling. The first time the foundation and embankment are subjected to seepage pressures and the first time that flow occurs through, 

and possibly along, conduits, is an important time to verify that performance is acceptable. Slurry 

impoundments are constructed over a long period of time and are filled as they are constructed. Special 

attention should be paid when the saturation level reaches the level of new features, such as conduits.

A schedule for observations should be developed for each major aspect of construction of a refuse 

disposal facility, including:

• Initial site development

• Liner installation (if applicable)

• Starter dam/embankment construction

• Refuse embankment construction
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• Instrumentation installation and data measurement

• Appurtenant hydraulic facility construction

• Backfill and abandonment of impoundments

• Mine barrier, bulkhead and seal construction, if applicable

To be most useful, the checklist/data collection form presented in Table 12.1 should be tailored to 

site conditions. The completed data collection form/checklist (e.g., Figure 12.2) should be regularly 

reviewed by the Engineer. In addition to descriptions of the conditions observed, deviations from the 

plans and specifications should also be indicated on the form and should be reported to responsible 

facility personnel (Engineer, Contractor, and Operator).

12.1.2.2 Facility Development

12.1.2.2.1 Site Preparation

Site preparation includes activities such as clearing and grubbing, topsoil/subsoil stockpiling, foundation preparation, and liner installation. These items relate to both initial facility development and 

facility expansion and are typically performed as individual construction activities rather than as a 

part of ongoing operations. A brief summary of typical observations associated with site preparation 

is provided in the following:

• Clearing and grubbing – Clearing and grubbing typically includes removal of 

unsuitable materials such as trees, tree stumps and roots, brush and other vegetation. 

Important items related to clearing and grubbing that should be observed and noted 

include: location and extent, adequacy of vegetation removal, debris disposition, 

and identification of unanticipated and/or suspect conditions (e.g., mine openings, 

landslides).

• Topsoil/subsoil stockpiling – Important items associated with topsoil/subsoil 

stockpiling that should be noted include: location and extent, drainage control 

provisions, depths removed, location and configuration of stockpiles, identification of unanticipated conditions (e.g., landslides, springs, potential for additional 

soil removal). Areas where the planned embankment and impoundment are to be 

located should be observed, particularly where soils have been excavated, to determine potential natural seepage conditions that may not have been evident before 

disturbance.

• Foundation preparation – Important items associated with foundation preparation 

that should be noted include: location and extent of work areas; preparation work 

performed prior to placement of any refuse materials (removal of unsuitable material, proof-rolling, slope modification and removal of rock overhangs, spring collection and underdrain installation, etc.); installation of cutoffs, keys or benches in 

preparation for refuse placement; and construction of mine barriers or bulkheads.

• Liners – If liners are required, the important items to be noted include: materials 

(natural or synthetic including properties), thickness placed, compaction (equipment 

type, tests), performance observations (firm or pumping), and cover protection (soil, 

vegetation, refuse).

12.1.2.2.2 Starter Embankment and Internal Drains

Following site preparation, construction of a starter embankment using borrow material (soil and 

rock) or coarse refuse is the first step in preparing a disposal facility site for ongoing refuse disposal. Starter embankments may be zoned or homogeneous. Important items to be observed include 
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TABLE 12.1 TYPICAL OBSERVATION AND INSPECTION CHECKLIST

 Item General 

Condition

Changes 

from 

Previous 

Inspection

 Item General 

Condition

Changes 

from 

Previous 

Inspection

FACILITY DEVELOPMENT DISPOSAL OPERATIONS 

AND MAINTENANCE

1. Erosion and Sediment 

Controls

a. Diversion Ditches

b. Silt Fences

c. Sediment Traps

d. Sediment Ponds

1. Stage of Construction 

and Refuse Placement

 (lift thicknesses, compaction, scarification, 

benching)

2. Clearing & Grubbing 2. Working Surface and 

Area Maintenance 

(survey control, 

grading and 

drainage control). 

Refuse Placement 

(lifts, compaction, 

scarification, benching)

3. Topsoil/Subsoil 

Stockpiling

3. Embankment Crest 

and Benches

a. Alignment and 

Grade

b. Evidence of 

Movement (cracks, 

sloughs, scarps)

c. Abutments 

(benching, drainage 

control, erosion)

 d. Crest low point 

elevation

4. Foundation Preparation

a. Removal of 

Unsuitable Material

b. Collection of Springs

c. Underdrain 

Installation

d. Benching, Cut-off or 

Keyway Const.

e. Sealing of Mine 

Entry Bulkheads and 

Barriers

4. Embankment Slopes

a. Alignment and 

Slope

b. Seepage (flow 

rates, clarity)

c. Slope Movement 

(scarps, sloughs, 

bulging)

d. Erosion 

e. Vegetation

f. Burning or 

combustion

5. Liner Construction 

(material, thickness, 

compaction, cover)

a. Soil

b. Geomembrane

c. Geosynthetic Clay 

Liner

d. Other

5. Liner, Foundation and 

Abutment Conditions

a. Drainage Control

b. Erosion

c. Concentrated 

Seepage or Boils
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 Item General 

Condition

Changes 

from 

Previous 

Inspection

 Item General 

Condition

Changes 

from 

Previous 

Inspection

FACILITY DEVELOPMENT DISPOSAL OPERATIONS 

AND MAINTENANCE

6. Starter Dam 

Construction (material, 

lift thickness, compaction, scarification, 

benching and keying, 

6. Internal Drains 

(discharge flow rate 

and clarity, drainage 

away from outlet)

7. Internal Drains 

(material, filter, 

placement, covering)

7. Decant Pipe

a. Inlet (elevation, 

inflow depth, debris, 

clogging, alignment, 

anti-vortex plate)

b. Outlet (discharge 

flow rate and clarity, 

debris, erosion, 

drainage away from 

outlet)

c. Structural Concerns

8. Decant Pipe or

 Spillway

a. Installation (material, 

joint connections, 

trench preparation, 

bedding, backfill, 

reaction blocks, 

pressure testing, 

cover)

b. Inlet (riser material, 

joint connection, 

reaction block and 

foundation, backfill, 

trash rack and antivortex plate)

c. Outlet (bank 

stabilization, stilling 

basin, animal guard) 

8. Channel Conditions

a. Alignment and 

Dimensions

b. Debris and Erosion

c. Lining System

d. Receiving Stream 

Condition (erosion) 

9. Channel Construction

a. Alignment and Grade

b. Dimensions

c. Lining (material, subbase filter, thickness)

d. Transition and 

Superelevation 

Sections (stationing, 

dimensions)

9. Instrumentation 

Monitoring 

(piezometers, weirs, 

 other)

TABLE 12.1 TYPICAL OBSERVATION AND INSPECTION CHECKLIST

(Continued)
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 Item General 

Condition

Changes 

from 

Previous 

Inspection

 Item General 

Condition

Changes 

from 

Previous 

Inspection

FACILITY DEVELOPMENT DISPOSAL OPERATIONS 

AND MAINTENANCE

10. Instrumentation 

(survey monuments, 

piezometers, weirs, 

other)

10. Reclaimed Slopes 

(vegetation 

development and 

control, drainage 

control, erosion, 

seepage)

11. Reclamation

a. Grading and 

Stabilization

b. Soil and Topsoil 

Cover

c. Revegetation 

(fertilizing, seeding, 

mulching)

d. Seepage Control 

(collection, 

conveyance, 

treatment)

11. Impoundment and 

Slurry Deposition

a. Abutment Slopes 

(erosion, instability)

b. Water Level and 

Control (decant 

inlet, pumping, 

other) 

c. Slurry Line 

Alignment and 

Discharge Location

c. Fines Deposition 

(location, distribution, delta areas, 

percent exposed/

above pool level)

ADJOINING AREAS

1. Foundation and 

Abutment Conditions

2. Upstream Watershed

3. Downstream 

Inundation Area

OTHER DEVELOPMENT (Work unrelated to the disposal facility)

1. Within Facility 

Watershed 

(potential for 

increased runoff or 

landslides)

2. Downstream of 

Facility (potential 

to change hazard 

classification)

3. Within Facility

 Note: Reference should be made to a design or as-built plan drawing.

TABLE 12.1 TYPICAL OBSERVATION AND INSPECTION CHECKLIST

(Continued)
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materials placed, lift thickness and compaction (including density and moisture testing), preparation 

(scarification) of compacted surfaces before placement of subsequent lifts, and benching of new lifts 

into adjoining abutments or embankment slopes, etc.

Internal drains are frequently part of starter embankments, and typical observations include: materials used for the drain and filter system, installation conditions (trenched into embankment or 

constructed on completed lift), placement of drain materials (susceptibility to segregation and contamination during placement and spreading) including geotextiles, drain dimensions and grade, and 

details for covering the completed installation to protect the drain from erosion.

12.1.2.2.3 Decant Pipes

During installation of decant pipes, the following observations should be recorded: alignment and 

grade, pipe material and joint connections, corrosion protection (where applicable), installation 

details (trench configuration), bedding and backfill (materials used and compaction, including 

density and moisture testing results), seepage control measures (location, dimensions and materials), location and configuration of reaction blocks at changes in alignment and grade, results 

of pressure testing, and details related to covering of the completed installation and protecting 

the conduit from damage by heavy equipment traffic. The adequacy of placement of backfill in 

the haunch area should be given special attention. Details related to the following should be 

observed for decant inlets: riser configuration and materials, joint connections, foundation and 

reaction block, riser backfill and compaction, inlet trash rack and anti-vortex plate. For decant 

outlets details related to the following should be noted: receiving channel bank stabilization or 

stilling basin, drainage control to preclude backup of water into pipe, and animal guard.

12.1.2.2.4 Channel Construction

Channels may include emergency spillways, decant outlets, diversion ditches, groin or perimeter 

ditches, haul and access road ditches, and bench gutters. Prior to construction, the upland areas 

should be observed for evidence of instability that may adversely impact planned excavations. Important observations to be recorded for channels during construction include: alignment and grade, 

dimensions (width, depth, side slopes), erosion-protection lining (sub-base, filter, lining thickness 

and material), and details of transition and superelevation conditions at changes in alignment, slope 

or section (stationing and dimensioning).

12.1.2.2.5 Instrumentation

Instrumentation is sometimes installed during starter embankment construction. Additional 

instrumentation is also generally installed during facility expansion. This instrumentation may 

include survey monuments, piezometers, weirs, and other devices, as discussed in Chapter 13. 

Important observations to be recorded may include: instrument type, material, make and model; 

installation details (materials, dimensions); the results of initial calibration and verification of 

function; and the results of periodic testing to verify continued performance. Maintenance should 

also be regularly performed as required by the type of instrument (Section 13.5). For example, periodic flushing of piezometers, if subject to fines accumulation, and regular cleaning of weir boxes 

are important.

Instruments need to be read in a consistent fashion such that the results are not affected by the individual taking the reading and are directly comparable. If necessary, specific instructions should be 

provided relative to collecting and recording instrumentation data.

12.1.2.2.6 Reclamation

Any areas that are covered with soil and/or vegetated should be noted by location and include 

a description of the work undertaken. Where areas are covered, the source of the soil or topsoil, 
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Monitoring, Inspections and Facility Maintenance

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the thickness of the cover, and the method used to place and/or compact the material should be 

noted. Where areas are vegetated, the type of seeding used, the method of placing the seeding, any 

amendments added, and procedures used to protect the vegetation, such as mulch or straw should 

be noted.

12.1.2.3 Adjoining Area Conditions

Areas adjoining a refuse disposal embankment include downstream areas, embankment abutments 

and upstream areas. Each of these areas should be carefully observed for the presence of conditions 

that could impact embankment stability, such as concentrated seepage, and other aspects of facility 

operation and safety, as discussed in the following subsections.

12.1.2.3.1 Sloughing or Sliding

Stability of an embankment is dependent not only upon conditions within the embankment itself, but 

also upon the embankment foundation materials. Therefore, areas immediately upstream and downstream from any embankment should be inspected for bulging conditions, open cracks with vertical 

displacement or other signs, such as the orientation of cracks, that could indicate movement of the 

soil or rock materials is occurring. In wooded areas, a possible means of identifying the presence of 

such conditions is to observe the trunks of the trees to determine if a group of trees has an unusual 

inclination. If such signs are evident immediately adjacent to an embankment, the embankment surface should be examined very carefully to see if the movement has extended into the embankment.

12.1.2.3.2 Signs of Subsidence

Evidence of mine subsidence can include surface cracks (particularly when oriented with mine workings and/or vertical displacement), depressions, and sinkholes. Often evidence of subsidence due to 

underground mining is more readily apparent on natural ground surfaces than on an embankment 

surface, because the embankment can arch over a subsided area for some period of time or the subsidence features can be hidden by disposal operation. Subsidence that occurs on the natural landform 

can cause rapid changes in contours where bedrock is shallow or sinkhole development where water 

washes material into the mine. Also, in areas where subsidence is not uniform, the inclinations of tree 

trunks can vary significantly over short distances. If possible signs of subsidence are observed, mine 

maps should be reviewed to determine if there are mine works present that could be causing the 

problem. Subsidence may also occur over mines that are unmapped or incompletely or inaccurately 

mapped. In many cases, small punch mines have been developed for residential coal consumption 

and may not be mapped, as is done for commercial coal mines. The lack of availability of maps 

should not be considered as evidence of the absence of underground mines.

12.1.2.3.3 Evidence of Erosion

Conditions where erosion of the natural ground surface occurs downstream or downslope from 

an embankment, or erosion of the embankment itself, can be critical and includes cases where: (1) 

an adjacent stream rises during a flood causing erosion of the embankment toe or (2) the drainage system from areas upstream of the embankment discharges onto or immediately adjacent to 

the embankment causing downcutting and eventual instability. The locations of any areas of significant increase in erosion of the embankment or the adjacent ground surface should be noted.

12.1.2.3.4 Springs or Seepage Areas

Instability of an embankment slope due to high pore-water pressures or piping of soil materials 

can occur in the embankment foundation and abutments as well as on the downstream face of the 

embankment. Therefore, it is important to observe the downstream slope and area downstream 

from the abutments and toe of an embankment and to note development of new seepage areas 

or changes in areas where seepage was previously occurring. In particular, it should be deter-
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mined whether existing seepage zones have increased in flow volume since the last observation 

and whether the seepage is carrying fines, which is an indication of internal erosion.

The presence of “boils” (points where a concentrated upwelling of seepage is creating a deposit of 

fine material) is an especially important indicator of internal erosion. Records of spring and seepage area flow volume should be kept for comparison purposes. Significant changes in seepage or 

evidence of internal erosion should be brought to the attention of the Engineer, because corrective 

measures to prevent instability, progressive internal erosion, or piping may be warranted. Seepage 

and water quality monitoring (e.g., temperature, turbidity, and total dissolved solids) and interpretation of trends are discussed in Pelton (2000).

12.1.2.3.5 Changes in Vegetation

As noted previously, changes in the inclination of tree trunks can indicate sliding and sloughing of 

the natural ground materials or subsidence due to underground mining. Also, areas that have either 

an abnormal amount of dying vegetation or unusually dense vegetation may indicate the presence of 

seepage zones that are not immediately evident at the ground surface.

12.1.2.4 Other Development Work

12.1.2.4.1 Upstream Development

Any changes in the watershed area above the facility, such as tree cutting, strip mining, and residential or industrial development should be noted, since these activities could affect runoff flows to the 

site. Also, these activities may affect the quality of surface water passing through the site, which could 

mistakenly be attributed to the refuse disposal facility. The location and extent of upstream changes 

and the nature of such changes should be documented and brought to the attention of the Engineer.

12.1.2.4.2 Downstream Development

Inundation mapping associated with dam breach analysis for establishing the hazard potential classification is based upon topography and stream conditions downstream from the refuse impoundment. 

Future development in the downstream floodplain could increase the hazard potential rating. Also, a 

new mine opening or coal preparation plant constructed downstream from the facility spillway might 

be susceptible to flooding during large storms. Any such change in downstream land use, including 

the location and description of the change, should be brought to the attention of the Engineer.

12.1.2.4.3 Disposal of Materials Other than Coal Refuse

The design of critical portions of refuse disposal facilities is typically based on the assumption that 

uniform materials will be used. The use of materials from other sources (e.g., power plant waste) or 

from site development work unrelated to the disposal facility, such as placement of coarse-grained 

rock from slope excavations in the disposal facility embankment, could cause undesirable behavior 

and should not be permitted unless specifically allowed in the approved design plan. Material that 

could lead to combustion in the coal refuse embankment is not permitted.

12.1.2.5 Refuse Disposal Operations and Maintenance

12.1.2.5.1 Refuse Placement and Compaction

Disposal operations comprise the placement of refuse materials within embankments in accordance 

with the plans and specifications. Important observations to be documented include: materials (gradation and moisture), delivery to site and spreading (location and equipment), lift thickness, compaction (equipment, number of passes), surface conditions (firm, pumping), surface preparation or 

scarifying (method or equipment, extent and frequency), and benching of new lifts into embankment and abutment materials (dimensions and extent). During inclement weather, it can be difficult 
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to meet construction specifications for refuse placement and compaction, particularly in structural 

zones, and documentation of potential problem areas is important to the planning of future actions. 

Where feasible and appropriate, other zones or areas that can accommodate coal refuse where placement standards are lower, such as during inclement weather, may be provided as part of the design. 

Monitoring and inspection of these areas typically include observation of lift thickness, grading, 

access, and drainage control.

12.1.2.5.2 Working Surface Maintenance

The working surface where active refuse disposal is taking place should be maintained consistent 

with the plans and specifications and with careful control of drainage. Observations should include: 

(1) survey control such as stakes or markers to define placement limits and grades, (2) lift placement 

and compaction, (3) scarification of the surface of layers that are too smooth to properly bond to 

the next layer, and (4) drainage control measures for directing water away from active placement 

areas and toward collection ditches. Density and moisture content testing must be fully documented 

including the location of the tests (which should generally be random or focused on areas of suspect 

compaction), the lift thickness, and the test results. Compaction conditions should be further evaluated if the test results show that the compaction specifications are being met, but the lift surfaces 

after compaction remain soft or spongy. If the compaction specifications are not met, then additional 

compaction will be required and, if this situation occurs frequently, the cause should be determined.

Adequate compaction requires the proper lift thickness and number of passes of the compaction 

equipment be used. The type and number of passes required for achieving the minimum compaction 

specifications will depend on the compaction equipment and lift thickness. When new or different 

compaction equipment is used, the number of passes needed to meet the compaction specifications 

needs to be established by a coordinated program of field density testing. The compaction operation 

should be observed to verify that lifts are being systematically compacted at the proper thickness 

with overlapping passes of the compaction equipment.

12.1.2.5.3 Embankment Crest Conditions

The crest of a refuse embankment should be constructed and maintained in conformance with 

the facility plans and specifications and should be generally level (or with a slight slope towards 

the impoundment) to provide adequate freeboard and drainage control. Important observations 

include: (1) alignment and level, (2) indications of movement (e.g., cracks, sloughs, scarps), and 

(3) abutment interface (including benching to tie refuse materials into natural hillsides, control of 

drainage from hillsides, and control of erosion).

Coal refuse tends to break down when exposed to the elements and to equipment traffic. When a 

crest elevation is maintained for a period of time, a layer of highly compacted and weathered material may be present at the surface. Prior to raising the crest, this material should be disturbed by 

thorough scarification or should be removed so that it does not create a continuous horizontal layer 

of low hydraulic conductivity within the embankment. Such a layer can cause elevated seepage 

levels in the embankment and defeat the purpose of internal drainage measures.

12.1.2.5.4 Embankment Slope Conditions

The slopes of a refuse embankment should be developed and maintained in conformance with the 

facility plans and specifications. Temporary slopes should be consistent with the geometry and overall development shown in the design report. Documentation should include alignment and steepness, seepage, movement, and erosion. The following conditions should be noted, as applicable:

• Sloughing and sliding – All embankment slopes should be carefully inspected for the 

possible presence of sloughing and/or sliding. Such conditions are usually accom-
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panied by bulging at or along the bottom of the distressed area and possibly the 

formation of a vertical displacement (scarp) and cracking at or along the top of the 

distressed area. Special care is required in areas of heavy vegetation to avoid overlooking these areas of distress. Heavy vegetation that hampers inspection activities 

should be cut or removed, as should any trees that begin to grow on the embankment. Particular attention should be paid to the downstream slope of an impounding 

embankment. Any signs of sliding or sloughing should be reported immediately to 

the Engineer and Operator.

• Subsidence – Subsidence of an embankment can occur as a result of three conditions: 

(1) the collapse of voids in the embankment or foundation that form due to piping, 

when seepage flows remove particles of refuse or soil, (2) consolidation of soft 

foundation or embankment materials, and (3) collapse of mine workings. Subsidence 

due to piping can usually be recognized by the presence of localized bowl-shaped 

depressions at the surface. Subsidence due to settlement of soft foundation materials usually results in low areas on the embankment surface and occasionally cracks. 

Subsidence due to underground mining can be identified by cracking along straight 

lines, typically parallel to the primary direction of mining, or as deep circular sink 

holes. When vegetation is tall, care should be taken to avoid overlooking such signs 

of distress. If subsidence is discovered, the size, shape and extent of the distress 

should be determined and recorded. Any evidence of subsidence should be reported 

immediately to the Engineer and Operator.

• Erosion – Erosion is loss of ground cover caused by a lack of proper drainage 

control. Sheet erosion is a surficial loss of soil or refuse in a more or less uniform 

pattern extending over wide areas. Gully erosion is the result of concentrated 

overland flow and leads to the creation of narrow and possibly deep channels in 

the ground surface. Any occurrence of erosion should be documented as to type, 

location, and extent. Erosion should be corrected when it starts to develop and 

before it becomes significant. Erosion that has progressed to deep gullies can affect 

seepage and stability and is much more difficult to correct. If erosion is found to 

concentrate or recur in particular areas, consideration should be given to improving surface drainage in those areas.

• Moisture conditions, springs, seeps and wet areas – Slopes should be monitored 

for the presence of springs, seeps and wet or boggy areas. Evidence of such conditions includes the presence of unusually wet and/or soft areas and areas where 

snow melts rapidly in the winter. Also, vegetation may be thriving due to the moist 

conditions, leading to unusually dense and taller growth as compared to surrounding vegetation, or vegetation may be adversely affected by acidic seepage and may 

be discolored or dying. The location and the extent of such conditions should be 

documented. If actual flow is observed, the rate of flow should be measured or 

estimated, and the quantity and character of any suspended solids (silt, sand, etc.) 

carried by flow should be noted. The Engineer and Operator should be notified 

immediately. The flow should be measured regularly, and the records should be 

maintained for comparison purposes. Particular attention should be paid to the 

downstream face of impounding embankments, as springs, seeps and wet areas can 

lead to sloughing and sliding.

• Condition of vegetation – All seeded areas should have a well-developed vegetation 

cover. The vegetation should be uniform and continuous. Any irregularities such as 

difference in color, density, rate of growth, type of growth, or a difference in the character of the vegetation should be noted. For example, cattails are a common indicator 
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of wet areas. Such irregularities could be evidence of other problems. The location 

and nature of any irregularities in vegetation should be recorded.

• Trees – The roots of trees can provide paths for seepage and, if the roots decay, 

voids may be created. Trees that are blown over during a storm can also damage an 

embankment. For these reasons, trees should not be permitted to become established 

on the crest or slopes of a refuse embankment. Guidance on maintenance of vegetation and embankment repair following removal of trees is presented in Marks and 

Tschantz (2002).

12.1.2.5.5 Liner, Foundation and Abutment Conditions

Liner, foundation and abutment conditions should be observed for signs of deterioration, inadequate 

drainage control, erosion or instability prior to placement of refuse. Along rock abutments, rock overhangs should be removed so that fill can be adequately compacted against the abutment. The treatment of open discontinuities in the foundation/abutments should be addressed in the specifications. 

If such foundation features are observed and are not addressed in the specifications, they should 

immediately be brought to the attention of the Engineer.

12.1.2.5.6 Internal Drains

The outlets of internal drains should be observed to: (1) determine discharge rate and clarity, (2) 

verify that no blockage restricting discharge is present, and (3) verify that drainage is directed away 

from the outlet and not backing up into the drain. In addition to this monitoring, flow rates should 

be measured, recorded, plotted, and regularly reviewed by the Engineer and should be compared 

to other facility data such as impoundment pool level, rainfall, and the presence or absence of a fine 

refuse delta.

12.1.2.5.7 Decant Pipe

The inlet of the decant pipe should be observed relative to inflow depth and rate, alignment, presence and condition of a trash rack and anti-vortex plate, and verification that there is no blockage by 

debris. The outlet of the decant pipe should be observed relative to discharge rate and clarity, presence of debris, erosion conditions, seepage along the outside of the pipe, and verification that the 

discharge is directed away from the outlet without backup into the pipe. Where exposed, the pipe 

wall should be observed for distress. The first time that decants flow and when they are subject to 

significant increases in head are important times to observe performance for possible indications of 

problems. During decant pipe observations, or as part of scheduled inspections, the following should 

be evaluated:

• Clogging – A common mode of failure of decant systems that is very difficult to correct is clogging of the intake or pipe with miscellaneous debris. Most designs include 

a trash rack around the intake to block material that cannot easily be carried through 

the pipe. The buildup of debris on and adjacent to the trash rack should be cleared, 

and the removal should be documented. Proper equipment and access should be 

provided so that any debris can be safely removed. Any damage to the trash rack 

or the presence of debris that cannot easily he removed from the intake should be 

reported.

• Characteristics of discharge – Where possible, an evaluation should be made of the 

character and volume of water flowing into the decant inlet and out of the decant 

discharge. If the appearance of the water at these two locations differs, it is possible 

that cracks or open joints in the decant pipe are allowing seepage flow to enter the

pipe or decant flow to exit the pipe. If this situation is observed, it should be reported 
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immediately to the Engineer. The presence of seepage flowing along the outside of 

the conduit at the downstream end of the decant should also be noted.

• Corrosion, cracking or crushing – Loss of any portion of the decant system can affect 

continued operation of a refuse disposal facility. For this reason, areas of distress 

to the decant system due to corrosion of materials, cracking or crushing should 

be reported immediately so that appropriate repairs can be made. Special inspection procedures and equipment may be needed for observation of the condition of 

a buried decant pipe. Typically, a decant should be treated as a confined space and 

only entered with appropriate precautions, including:

− Initially checking and then regularly monitoring of the atmosphere 

inside the conduit for safe conditions and air movement (e.g., a respirable, non-explosive atmosphere).

− Providing for continuous communications with personnel inside the 

conduit.

− Ensuring that personnel entering the conduit have appropriate personal safety equipment.

− Providing for safe ingress and egress, including harnesses and lifelines where appropriate.

− Providing workers with training with respect to potential hazards 

and required safety measures.

Track-mounted video cameras that can travel inside the decant pipe are commonly 

employed to avoid entry of personnel and in situations where pipe diameters are 

small. Inside-diameter measurements can be obtained using optical/laser equipment 

coupled to a track-mounted video camera.

• Erosion conditions – Erosion at the decant intake is not generally important to the 

integrity of the decant system, but is an indication that fine refuse particles are 

being transported into the decant pipe. These materials may settle out at low points 

or bends and eventually reduce the decant pipe capacity. Erosion at the discharge 

point can be more critical if it eventually undercuts the outlet causing a break in the 

pipe or undercutting of the toe of the adjacent embankment creating local stability 

problems. Seepage discharging along the exterior of the decant should be noted and 

observed for evidence of fines carried with the flow (an indication of internal erosion 

along the conduit). Any of these conditions should be reported to the Engineer and 

Operator.

12.1.2.5.8 Channel Conditions

Observation of channels associated with spillways, ditches and gutters should include: (1) alignment 

and dimensions (depth, width and side slopes), (2) evidence of debris, (3) erosion and condition of 

erosion protection, and (4) the condition of receiving channels, ponds, or streams. The following 

observations should be documented:

• Erosion – The banks and the beds of all channels should be examined to determine 

if erosion or siltation is occurring. Channels should have a uniform configuration 

consistent with construction plans. Erosion of banks is evidenced by side slopes that 

are steeper than shown on the plans or by sloughing or localized irregularities in 

configuration. The location of any eroded areas should be recorded. The location of 

the spillway discharge should be checked to see that it is far enough downstream 

that the embankment toe is not being eroded.
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• Undergrowth – The channel cross section should be clear of undergrowth such as 

brush and small trees from the top of one bank to the top of the opposite bank. Such

undergrowth decreases the efficiency of the channel and traps floating debris during 

high flows. Areas of excessive undergrowth should be cleared.

• Flow obstructions – Any obstructions to flow in a channel such as fallen trees, 

sloughed-in soil or rock from adjacent excavated slopes, or other foreign objects 

or excessive vegetative growth should be noted and removed at first opportunity. 

Where significant sloughing of a channel slope is occurring, engineering personnel 

should be notified immediately.

• Structure conditions – The condition of all structures that are part of, or appurtenant 

to, the channels should be noted, including culverts, retaining walls, wing walls, rock 

cuts, pavements, slabs, etc. Culverts should be maintained in good condition, free 

of debris and with unobstructed inlets and outlets. Walls, slabs, rock cuts and pavements should be checked for evidence of distress such as settlement or other movements, cracks, scour and erosion. All areas requiring improvement or maintenance 

should be documented.

Riprap is commonly used in channels and elsewhere for erosion protection. Observations of riprap 

should include the following:

• Irregularities and displacements – The surface of riprap areas should be examined 

for abrupt changes in slope or alignment and for gaps or cracks in the surface. Areas 

where the riprap is missing or has otherwise been disturbed should be noted. Localized changes in the character of the riprap material should also be recorded. If the 

deterioration of riprap is extensive, the Engineer and Operator should be notified.

• Sizes – The maximum and minimum sizes of the riprap should be examined to verify 

that they are generally in accordance with the plans and specifications. The distribution of riprap sizes should be uniform. Any observed segregation of riprap with 

respect to size should be noted. Irregularities in the size of the riprap could indicate 

that deterioration due to weathering or scouring is occurring.

• Weathering – Weathering is the deterioration of the riprap due to freezing and thawing cycles or due to chemical action in the presence of water, air, sunlight, etc. Weathering of riprap is usually indicated by a lack of larger-sized pieces, since weathering 

acts to break the larger pieces into smaller ones. If weathering of the riprap has 

occurred, the location and extent of the weathering and the largest remaining size 

should be recorded.

• Scouring – Scouring of riprap is caused by the effects of flowing water or wave 

action. The mechanical action of the water acts to segregate the riprap and carry 

smaller-sized pieces away. Scouring is usually indicated by a lack of smaller pieces. 

If scouring has occurred, the location and extent of the scouring and the size of the 

smallest remaining pieces should be recorded.

• Grouted riprap – At some locations, the riprap surface may be grouted to a certain 

depth with fine aggregate concrete. Disturbances to the riprap, such as settlement, 

undermining, erosion and scour can cause deterioration of the concrete grout. Any 

areas where the concrete grout has deteriorated, or areas where it is missing entirely, 

should be recorded.

Concrete is frequently used in the construction of spillways, channels and other hydraulic appurtenances. Distress to concrete structures can result from many factors ranging from simple deteriora-
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tion with time to movements of the structure foundation. Since the type of repair is dependent upon 

the cause, observed conditions should be recorded so that appropriate repair and maintenance procedures can be implemented.

• Cracking – The surface of all concrete structures and other concrete appurtenances 

should be inspected for the presence of cracks. Care should be taken to not overlook hairline crack patterns. The size, location and configuration of observed cracks 

should be recorded. If cracks are noted on a particular structure, other similar parts 

of the same structure should be carefully examined.

• Spalling – The surface of all concrete structures and other concrete appurtenances 

should be examined for the presence of spalling. Spalling is indicated by the removal 

of the concrete matrix at the surface. The remaining surface texture will be rough, and 

the aggregate will be exposed. In general, two types of spalling can occur. The first is 

a surficial or uniform deterioration of the concrete surface. The second type is a deep 

spalling that exposes the reinforcing bars. If spalling is observed, the type should be 

noted, and the location and the extent of all spalled surfaces should be recorded.

• Condition of joints – All construction joints, expansion joints and contraction joints 

should be examined to determine if relative movement at the joints has occurred. If 

there is evidence of movement at any joint, the size and uniformity of the gap should 

be noted. Variation in size and width of the gap may indicate the relative movement 

of the structure, and observation of changes in the condition with time will indicate 

if further movement can he expected.

• Leaks and seepage – All drains and weep holes in concrete structures should be 

examined to verify that they are free from debris and are functional. Often water 

flowing through improperly functioning drains and weep holes will wash backfill 

material or filter material from behind a wall and deposit it in front of a weep hole. If 

this condition is observed, it should be noted. All concrete surfaces should be examined for the presence of damp or wet surface areas. If damp or wet surface areas 

are found, their size, extent and location should be noted. Any water seeping from 

cracks or joints in any concrete structure should be noted.

• Settlement or heave – The vertical alignment of all concrete structures should be 

checked for irregularities. This can best be accomplished by sighting along horizontal 

lines of the structure. All structural lines should be straight. Settlement is indicated by 

a dip or depression. Heave is indicated by an elevated section or hump. If settlement 

or heave occurs over a substantial distance, it may be difficult to detect. Such largescale movements are normally associated with relative movement at one or more of 

the joints in the structure. If it is expected that settlement or heave is occurring over 

an extended distance, the condition of the joints in the structure should be carefully 

observed to determine where the movement is occurring. The location of and an estimate of the magnitude of any settlement or heave should be recorded.

• Deflection and lateral movements – The horizontal alignment of all concrete structures should be checked for irregularities. This can best be accomplished by sighting 

along vertical lines of the structure. All vertical lines should be plumb. Localized 

lateral movements will be indicated by bulges in the walls and/or misalignment of 

the structural elements. Relative displacement at the vertical joints may also occur. If 

such displacements are observed, the location and an estimate of the magnitude of 

displacement should be recorded.

• Tilting – Tilting is indicated by general rotation of a structural element. Tilting may 

often be combined with settlement or lateral movement. If tilting occurs, vertical ele-
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ments such as walls will be out of plumb and horizontal elements such as slabs will be 

out of level. Such a condition is probably most easily detected by viewing the structure 

from a distance. Tilting of water conveying channels is indicated by a tendency for the 

depth of flow to be greater at one side of a channel. Tilting of structures such as retaining walls and chute walls may be accompanied by a gap between the structure and 

backfill. Periodic surveying should be performed when tilt or movement is suspected.

• Undermining – Concentration of rainfall runoff along, around and behind concrete structures can cause erosion that can lead to undermining. This is particularly 

relevant for paved gutters and channels. Major seepage or leaks under or behind 

concrete structures can also cause undermining. Undermining may be indicated 

by holes or voids under or behind the structures or by localized settlements or 

depressed areas on the surface of the backfill around them. Care should be taken not 

to miss evidence of undermining that may be obscured by vegetation.

• Condition of backfill – The condition of the backfill around and behind all concrete 

structures should be examined for evidence of erosion, settlement and/or lateral 

movement. If the backfill was seeded, the condition of the vegetation should be 

noted. The surface of the backfill should be examined for the presence of depressions or small holes. Lateral movement of the backfill may be accompanied by a 

gap between the concrete structure and the soil, cracks in the surface of the backfill 

running generally parallel to the main lines of the concrete structure, or vertical steps 

or scarps in the surface of the backfill. Particular attention should be paid to backfill 

along and behind spillway chute walls.

12.1.2.5.9 Instrumentation

The overall condition of instrumentation such as flow monitoring weirs, survey monuments and 

piezometers should be determined by visual inspection. For example, weirs should be inspected to 

verify that they are in good condition and that erosion has not created bypass channels; survey monuments should be observed to verify that they are still in place and in good condition; and piezometers should be checked to verify that they are in good physical condition and operable. Observed 

damage to instrumentation should be immediately reported to the Engineer and Operator and the 

instrumentation should be repaired or replaced as necessary. Malfunctioning or damaged instrumentation needs to be repaired or replaced, unless there is a technical basis for its complete elimination. 

Instrumentation that is included in the construction plans and specifications should be maintained 

unless removal is approved by the appropriate regulatory agencies. Should supplemental instrumentation be recommended by the Engineer or implemented by the Operator, the purpose and planned 

frequency and duration of monitoring should be documented and submitted to MSHA.

12.1.2.5.10 Reclaimed Slopes

Slopes and other completed surfaces that have been reclaimed should be observed with respect to 

movement, vegetation development, drainage control, erosion and seepage. In addition to safety of 

the refuse disposal facility, they are important with respect to abandonment of the facility and environmental control. A primary reason for noting conditions in these areas is to verify that the type of 

covering and vegetation procedures being used are adequate and will result in the desired finished 

embankment. Observations should include: (1) notation of the uniformity of vegetation, (2) areas 

where the vegetation cover is apparently not surviving or is very sparse, and (3) areas where cracking or erosion of the soil cover is affecting its function of protecting the embankment or other surface.

12.1.3 Quality Control Sampling and Testing

In addition to visual observation, field quality control sampling and testing provides a means to 

verify that construction of refuse disposal facility components is being performed in conformance 
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with the plans and specifications. Table 12.2 presents a summary of typical quality control tests and 

guidelines for testing frequency associated with refuse disposal facility construction. Designers may 

wish to incorporate alternate testing requirements and/or frequencies to reflect site- or design-specific facility conditions. For example, more tests may be performed initially to establish the variability 

of properties.

12.2 MONITORING INSTRUMENTATION

Measurements should be routinely recorded as part of the monitoring and inspection program for 

each coal refuse disposal facility. Most instruments that require monitoring are specified and installed 

as part of the normal design and operation of the refuse disposal facility. Supplemental instrumentation may be installed in response to unexpected conditions. The exact types and location of instruments should be specified in the plans and specifications and the Operation and Maintenance Plan 

for the facility. For most facilities, monitoring instruments typically include survey monuments, 

piezometers, monitoring wells, and weirs or flumes. A method for monitoring the pool/slurry level is 

usually also provided. More complex instruments are sometimes required during construction and 

operation of a coal refuse disposal facility for monitoring and more closely observing selected areas 

or conditions. A thorough discussion of coal refuse facility instrumentation is provided in Chapter 

13. A brief summary of typical instrumentation at coal refuse disposal facilities is provided in the following paragraphs:

• Monument surveys– The use of monument surveys along with as-built topographic 

surveys based on aerial photography is an effective method for monitoring both 

displacements and configuration of the facility. Monument surveys can detect both 

horizontal and vertical movement of an embankment, and it is recommended that an 

active facility should be surveyed by a licensed surveyor at least annually. Annual asbuilt topographic surveys provide an effective basis for verifying that construction is 

progressing in accordance with the design plans and for preparing reports to regulatory agencies concerning the amount and extent of construction. They also provide 

a means of confirming refuse production rates and progress on individual embankment stages. More frequent as-built surveys for facilities during initial development 

may be important for verification that critical embankment heights and geometries 

are being achieved. Both monument surveys and as-built topographic surveys are 

limited in their accuracy, and smaller movements may not be detected. If displacements are suspected, additional instrumentation may be needed.

• Pool-level gauge – For slurry impoundments, a method is needed to monitor the 

level of the pool and the slurry delta. This information is needed in order to verify 

that the design provides adequate freeboard and slurry disposal capacity. The pool 

level can be determined based on visual observations at a staff gauge and delta 

monitoring conducted by periodic elevation surveys relative to established benchmarks near the upstream slope of the refuse embankment. Also, soundings can be 

performed to determine settled fine refuse levels within the pool area.

• Rainfall gauge – A rainfall gauge should be located close to an impounding refuse 

embankment so that daily precipitation can be monitored and compared to piezometer records or seepage rates observed at weirs. This is particularly important where 

underground mine workings are close to an impoundment and monitoring of mine 

discharge is being performed. In such cases, rain gauge data allow increases in mine 

discharge due to rainfall to be distinguished from increases caused by impoundment 

seepage or leakage.

• Piezometers – Piezometers are normally installed at impounding facilities for monitoring the phreatic level or pore pressures in a refuse embankment for comparison 
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to projected design levels used in stability analyses. Piezometers are also used for 

monitoring pore pressures during upstream embankment construction. Tolerable 

excess pore pressures predicted in the design can then be compared with monitored 

pore pressures. The three most commonly installed types of piezometers are standpipe, pneumatic and vibrating-wire. Chapter 13 provides a detailed discussion of 

these types of piezometers, along with others less commonly used. Consistent with 

CFR § 77.216-3, instruments should be measured/read every seven days and the data 

should be recorded. The actual frequency of measurement should also be based on 

the historical or expected rate of change in pore pressure and the disposal facility 

design features. More frequent piezometer readings are generally obtained during 

upstream embankment construction. Automated data acquisition systems coupled 

with vibrating wire piezometers have been used at refuse facilities to obtain more 

frequent pore pressure measurements; such systems are also employed because of 

their cost savings.

• Monitoring wells – Monitoring wells, and to a lesser extent piezometers, are used 

for monitoring the groundwater quality and levels at and adjacent to refuse disposal 

facilities. The monitoring or sampling frequency varies depending upon environmental conditions and state regulations.

• Weirs and flumes – Weirs and flumes are used to measure water flow rates in channels 

and from seeps and internal drains. The monitoring and measurement of flow rates 

from seeps and internal drains is an important means for evaluating the performance 

of the internal drainage system of an impounding refuse embankment. At sites with 

underground mining, flow from mine openings may also be measured as a means for 

monitoring changes in the amount of seepage into the mine, and these data may provide an indication of the performance of the impoundment with respect to potential 

breakthrough, provided that surface runoff is diverted away from the weir. Accurate 

measurement of weir flow rates is important, and the frequency and methods of measurement should be specified in the plans and specifications and the Operation and 

Maintenance Plan. Flow rates are typically measured on a 7-day basis and recorded, 

or less frequently if records indicate consistent measurements and conditions permit 

(consistent with the regulations and approved plan). If flow rates are relatively low, 

they can be measured using a standard bucket or calibrated container and stop watch. 

The time required for the water flow to fill the container is recorded, and a flow 

rate is calculated. Evaluation of flow data to determine whether they are normal or 

unusual will typically require correlations with pool level and rainfall. Flow-related 

data should be plotted on a common time-line, along with pool and piezometer data, 

so that trends in the data can be identified and flow trends can be evaluated to verify 

that they are consistent with expectations. To accurately correlate seepage flows with 

rainfall, measurements may need to be performed on a daily basis.

• Blast/vibration monitoring – Drilling and blasting activities can cause shock vibrations that can affect a structural component of a refuse disposal facility or cause 

displacements due to liquefaction of embankment foundation materials. Common 

instruments for monitoring shock vibrations are commonly referred to as accelerometers or velocity transducers. The determination of the equipment needed for vibration measurement requires the input of an experienced engineer who is familiar with 

the field data needed for analysis of the possible effects of vibration and dynamic 

displacement on facility components.

• Conduit monitoring – Deeply buried conduits may require monitoring during the life 

of the facility. Small video cameras mounted on a mobile unit that can travel through 

the conduits are commonly used to inspect the interior condition. Measurements of 
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deflection can be obtained with various measuring devices such as laser or optical 

equipment using the laser ring method. This method employs a non-contact laser that 

is projected onto the interior of the pipe and viewed by a video camera. Software is 

then used to calculate the interior dimensions of the pipe. The need and frequency of 

conduit monitoring is a function of the pipe design (including tolerable deformations) 

and site-specific parameters such as foundation conditions, depth of cover, observed 

performance, conduit extension schedule, and other aspects of the facility.

• Extensometers/settlement gauges/inclinometers – These instruments, as described in 

Chapter 13, may be used where there is potential for deformation of the foundation

or abutment, particularly where mine subsidence is present. The need and frequency 

of monitoring of these instruments should be determined on a site-specific basis, 

and often is related to performance monitoring of other instruments such as surface 

monuments.

• Temperatures – If necessary, instrumentation can be installed in a coal refuse 

embankment to monitor the internal temperature. Temperature monitoring instruments vary from conventional thermometers to telethermometers. Telethermometers 

are sometimes coupled with a vibrating wire piezometer in automated data collection systems.

Instrumentation data and measurements discussed above should be regularly reviewed and should 

become part of the annual dam safety inspection report required by some states and the Annual 

Report and Certification required by MSHA.

12.3 FACILITY AND DAM SAFETY INSPECTIONS

Facility and dam safety inspections provide a basis for identifying maintenance needs, for evaluating 

conformance of refuse disposal facilities with the plans and specifications developed for their construction, and for detecting signs of instability or other safety-related problems with the performance 

of the facility. It is recommended that impounding facilities have in place dam safety inspection programs including: (1) routine dam surveillance by the facility operator, (2) dam safety inspection by 

the Engineer, and (3) dam safety review (including evaluation of continued disposal facility development requirements) by an engineering team (Szymanski, 1999). Routine embankment surveillance 

should be consistent with the 7-day impoundment inspection requirement by MSHA. Dam safety 

inspections should be conducted by, or under the guidance of, the Engineer and should include at 

least annual inspections for evaluation of the condition of the facility and may also include routine 

inspections that supplement dam surveillance. Dam safety review includes a more comprehensive 

inspection and evaluation of future refuse production and disposal plans that typically is performed 

as part of stage completion and initiation of new disposal facility stages.

12.3.1 Routine Inspections

The qualifications of inspection personnel and the frequency of facility inspections are related to the 

type and design of the refuse disposal facility and federal and state regulatory requirements. Inspection records should be maintained for future reference. Each inspection should involve the review 

of data collected from previous inspections and from construction monitoring activities since the 

previous inspection. Routine inspections are typically performed on a weekly to quarterly basis or as 

specified in the Operation and Maintenance Plan.

12.3.1.1 Impoundments and Dams

MSHA requires inspections (at intervals not to exceed 7 days or as otherwise approved by the MSHA 

District Manager) of impoundments in accordance with CFR § 77.216-3 and provides guidance for 

these inspections in the MSHA Coal Mine Impoundment Inspection and Plan Review Handbook (MSHA, 
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TABLE 12.2 FIELD QUALITY CONTROL TESTING SUMMARY

 Item Test Suggested Frequency Reference

Topsoil/Subsoil for Reclamation

Material Composition Nutrients, organic matter Each area or as required by state 

regulatory agency

Foundation Preparation

Foundation Soils

USBR, 1998

Suitable Material 

Proof-rolling and/or field density 

and moisture (strength and 

gradation on suspect material)

> 1 test per foundation material

 type

Cutoff/Key Backfill

Suitable Material

Index properties, hydrometer, 

standard proctor, (hydraulic 

conductivity, if part of liner)

> 1 test per 20,000 cy

Compaction Compaction > 1 test per 2,000 cy

Underdrain

Suitable Material Gradation, durability > 1 test per material source

Liner Construction

Soil

USBR, 1998

Suitable Material

Sieve analysis, index properties, hydrometer, standard proctor, hydraulic conductivity> 1 test per lift per acre and as

 material varies or as required

 by state regulatory agencies

Compaction Compaction > 1 test per 2,000 cy or each lift

Geosynthetic

Haxo, 1986

Daniel and 

Koerner, 

1995

GRI, 2005

Geomembrane Strength at seams

6 per 3,300 feet of seam, if

sampled randomly, or 1 every

500 feet of seam, if sampled

on a uniform basis

Geosynthetic Clay Clay mass, strength, index flux > 1 test per material source

Structural Fill Placement

Material Index properties, sieve 

analysis

Each material and > 1 test per

 20,000 cy

MSHA, 2007

Compaction

Compaction (embankment fill)

Compaction (backfill around 

structures)

≥ 1 test per 2,000 cy

 (at least 1 test per lift)

≥ 1 test per 200 cy

 (at least 1 test per lift)

Proctor Each material and > 1 test per 20

 field compaction density tests
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 Item Test Suggested Frequency Reference

Placed Refuse Fill(1)

Material Index properties and sieve 

analysis

As material varies

Compaction Compaction Dependent on facility design

Internal Drain

Aggregate

Suitable Material Gradation and durability,

calcium carbonate > 1 test per material source

Geotextile

Material Permittivity, strength > 1 test per material type

Decant Pipe

Material Connections Leak (pressure) testing Completed sections or entire

length

Bedding and Backfill 

Material

 1. CLSM or Concrete

2. Soil

Strength, air content, temperature, slump, and potentially hydraulic conductivity.

Index properties, hydraulic 

conductivity (if penetrating 

liner), proctor compaction

 1. Each pour

 2. Each material; >> 1 compaction 

 test per 200 cy (min. 1 test per 

 lift)

Operational Performance 

or Long-Term Performance

Camera survey with deflection 

measurements

With major increase in embankment height for flexible pipe, 

depending on pipe design for 

deflection and strength(2)

Channels

Riprap Material(3) Gradation and durability Each material

Grout Material Strength, slump Each pour

Concrete(3) Strength, air content, 

temperature and slump Each pour

Soil Backfill Material Index properties, proctor 

compaction, compaction

Each material > 1 test per backfill

 segment

Reclaimed Slope

Soil and Topsoil

Material Thickness, nutrients Each material or as required by appropriate state regulatory agency

 Note: 1. “Placed refuse fill” may be designed zones where material placement and compaction standards are typically

 lower than for structural fill.

 2. A baseline camera and deflection survey may need to be performed upon completion of backfill and

 cover for flexible pipe installations, depending on the pipe design.

 3. Aggregate subbase and/or geotextile filter may be subject to similar testing as for an internal drain.

TABLE 12.2 FIELD QUALITY CONTROL TESTING SUMMARY

(Continued)
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2007). The 7-day inspection requirement normally applies to impoundments that are classified as 

having high or significant hazard potential; sites with low-hazard-potential impoundments may 

be inspected on a less frequent schedule, if approved by the MSHA District Manager. Personnel 

who perform these inspections are required to: (1) have current MSHA training, (2) complete inspection forms for documentation, and (3) maintain a record of the inspections. It is recommended that 

7-day inspections be supplemented by periodic inspections by engineering professionals (engineers 

or experienced technicians familiar with the design and construction requirements of the disposal 

facility and the purpose and function of the instrumentation). The periodic inspections should be 

documented in reports that are retained for reference. These engineering inspections and associated 

reports may also be used for periodic certification of stage construction by a registered Professional 

Engineer, as required by federal and state regulatory agencies. Monitoring data, such as seepage 

rates and piezometer levels, which are collected during 7-day inspections, need to be routinely 

reviewed by engineering personnel familiar with the design of the facility to verify that the facility 

is performing as designed. If monitoring data suggest that the facility is not performing adequately, 

the observed conditions should be investigated, and the Engineer should be advised and consulted.

12.3.1.2 Non-Impounding Facilities

Non-impounding coal refuse disposal facilities should be inspected periodically in compliance with 

the Operation and Maintenance Plan and as required by state permits (e.g., monthly and quarterly). 

In addition to specific permit requirements, these inspections should cover diversion and perimeter 

ditches, haul roads and gutters, benches and gutters, disposal work surfaces and embankment slopes, 

refuse compaction, reclaimed surfaces and slopes, instrumentation, etc.

12.3.2 Event-Related Inspections

Inspections of impounding facilities should be performed after events that may have negatively 

impacted the integrity of the impounding embankment. Examples of such events include mining 

subsidence, severe storm events, prolonged periods of cold weather, nearby blasting, earthquakes, 

sudden increase in pore pressures in the dam, sudden increase or decrease of internal drain flows, 

and landslides or major sloughing near or on the embankment. The scope of such inspections is 

dependent upon the nature of the event.

12.3.3 Annual Dam Safety Inspections

It is recommended that a dam safety inspection be performed at least annually by an engineer 

familiar with the design and construction requirements of the facility (preferably the design engineer). Annual dam safety inspections and a formal report by a registered Professional Engineer are 

required by some state agencies. Sometimes these inspections can be combined with other site data 

to satisfy the MSHA annual report and certification requirement. The annual inspection should 

include a review of critical aspects of the facility (such as the results of a potential failure modes 

analysis, as discussed in Section 12.1.1, or other issues documented in the Operation and Maintenance Plan) and thorough inspection of the facility including embankments and associated abutments. Plotted instrumentation records and construction data should be reviewed. Preparation of 

completed embankment cross sections reflecting instrumentation data may be an effective way to 

evaluate large or complicated geometries and the significance of piezometric levels. A tabulation of 

other monitoring data collected during the year should be prepared and submitted in the inspection report. Photographs should be taken at specific locations during the inspection and compared 

to earlier photographs taken from the same locations for evaluation of changes over time. Photographs should also be taken of areas that require repair or other special attention. The following 

data should be compiled in graphs or drawings:

• Piezometric levels with corresponding pool level elevations

• Internal drain flow rates with corresponding pool level elevations
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• Seepage flow rates with corresponding pool level elevations

• Refuse embankment density test results

• As-built conditions based upon most recent survey data

• As-built data and drawings for critical items such internal drains, piezometer, decant 

pipes and spillways

• Site rainfall and temperature records, if appropriate

A review of upstream and downstream areas should be conducted, and changed conditions that 

could impact the hydrology and hydraulics of the embankment should be identified. The review of 

the downstream area should concentrate on identification of new development and/or fills that could 

impact downstream flood levels in the event of embankment failure (Chapter 14) and the appropriate 

hazard-potential classification for the site. For instance, a bridge or fill constructed in the downstream 

floodplain could require a modification to the dam breach analysis to account for the potential channel constrictions.

Upstream areas should be inspected to determine if watershed conditions remain consistent with 

the design assumptions. If the watershed area has effectively increased or disturbance has increased 

runoff potential, the storm routing analysis may have to be modified and the embankment configuration possibly adjusted to provide adequate freeboard.

An evaluation based upon the visual observations and review of operating records should be performed with respect to evaluation of the overall condition of the embankment. Should areas require 

remedial attention, they should be identified along with recommended repairs. Similarly, the Operation and Maintenance Plan should be reviewed and modified as needed.

The annual inspection report is a good place to provide information related to the anticipated life of 

a refuse disposal facility and to provide an evaluation of whether changes may be needed to meet 

freeboard requirements. Thus, an analysis of observed coal refuse generation rates and the remaining 

capacity of the facility should be performed and included in the annual inspection report.

12.4 CERTIFICATION AND CONFORMANCE REPORTING

An annual report that includes a description of changes in the configuration of the impoundment is 

required under 30 CFR § 77.216-4. As part of the annual report, MSHA requires a certification by a 

registered Professional Engineer that all construction, operation and maintenance of an impoundment was performed in accordance with the approved plan for the facility. Some state agencies also 

require certification of construction of refuse disposal facilities for specific periods, structures or 

stages. The registered Professional Engineer should be familiar with design, construction, operation 

and maintenance of the structure, including dam safety criteria. The design engineer, if involved in 

the monitoring and inspection of the refuse disposal facility, may provide support or fulfill the certification responsibility.

Certification is generally performed concurrently with reporting of construction monitoring or inspection and is an expression of the Engineer’s professional judgment with respect to the consistency of 

the completed work relative to the approved design plan. It is recommended that the certification 

be supported by the results of the monitoring and inspection activities performed in response to 

the designer’s specifications and the Professional Engineer’s ongoing guidance, which may include 

visual observation, testing, and instrumentation measurements. The certification is not a guarantee 

or warranty of performance of the facility and does not relieve any responsible party’s obligation to 

monitor, inspect, construct or operate the facility in accordance with the design plans or to respond 

to conditions or situations that may represent a hazard to site personnel, public safety, or the envi-
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ronment. It is recommended that the annual report cite the specific scope of activities to which the 

certification applies and reference (or provide documentation of) the observations, testing, and instrumentation measurements providing support for professional judgments.

When observations, test results or instrumentation readings identify possible deviations from the 

approved plan, the Engineer should evaluate the situation to determine if there are non-conformances 

and should implement the actions necessary to address such non-conformances, which should be 

documented in a report available to the regulatory agencies (e.g., operator’s 7-day inspection documentation, other regular inspection report such as monthly or quarterly documentation, or the 

annual impoundment inspection report). MSHA should be notified immediately in accordance with 

regulatory requirements (30 CFR § 77.216-3) when a potentially hazardous condition to site personnel or the public develops, or in particular circumstances where the approved disposal plan requires 

that MSHA or state agencies be notified immediately. The Operation and Maintenance Plan should 

provide guidance regarding what actions need to be taken when observations, testing or instrumentation measurements identify suspected adverse, potentially hazardous, or unsafe or unacceptable 

operating conditions. When changes need to be made in the design plans or construction specifications, the modifications should be submitted to regulatory agencies for approval.

12.5 MAINTENANCE

Maintenance of a refuse disposal facility throughout its entire operating life is important for many 

reasons, including:

• Exposure to weather for long time periods results in the natural deterioration of 

some components that require periodic repair and maintenance.

• Neglecting to perform minor repair work can result in future major repairs or undiscovered deficiencies leading to major distress.

• A primary aim of design is to eliminate maintenance after abandonment. Regular routine maintenance during the operating life will facilitate abandonment of the facility.

The broad range of conditions that can be found at refuse disposal facilities makes it impractical to 

present a detailed set of instructions for maintenance and repair of each condition that might arise. 

Maintenance issues should be addressed in the facility Operation and Maintenance Plan. The checklist presented in Table 12.1 and the related discussion in Section 12.1 can serve as a guide for developing a site-specific Operation and Maintenance Plan. Some general maintenance requirements are 

discussed briefly in the following paragraphs.

12.5.1 Routine Maintenance

Routine maintenance should be performed on a continuous basis in accordance with the Operation and Maintenance Plan. This typically includes reseeding of disturbed vegetated areas, removal 

of debris and patching and repair of collection and diversion channels, removal of debris from the 

critical areas of the facility, and repair and reseeding of eroded areas. Such maintenance should be 

documented.

Routine maintenance should also include control of vegetation during facility operation. Periodic 

cutting of vegetation on slopes and toe areas of impoundments is necessary to facilitate dam safety 

inspections and will aid in preventing the development of heavy brush and trees. Trees should not 

be allowed to develop on an impounding embankment. Should trees develop and mature, their 

roots can provide seepage pathways leading to voids upon decay, and in some instances extensive 

slope repair measures may eventually be required. Guidance related to this issue is presented in 

Marks and Tschantz (2002).
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12.5.2 Maintenance after Unusual Events

Some portions of a coal refuse disposal facility and its appurtenant structures may be disturbed by 

unusual events such as heavy rainfalls, extremely severe frost periods, or periods of very high winds. 

If heavy rain is forecast, decants and spillways should be checked to ensure that they are clear of 

debris. Normally, the bulk of maintenance work after an unusual event is associated with repair of 

diversion and collection channels, eroded zones, and vegetation. The degree of disturbance can be 

minimized by a conscientious routine maintenance program that eliminates minor deficiencies that 

could become weak points during severe weather.

12.5.3 Long-Term Maintenance

The natural deterioration of materials exposed to weather requires that repair work beyond routine 

maintenance be undertaken from time to time. Such work items can include major repairs to riprap 

and replacement of concrete linings, valves, trash racks, monitoring instruments, etc. The frequency 

of this work will depend upon the care taken during the site routine maintenance program and the 

economics associated with performing numerous smaller maintenance efforts versus less frequent 

major maintenance. The Operation and Maintenance Plan should provide schedules for all levels of 

maintenance.
Chapter 13

INSTRUMENTATION

AND PERFORMANCE

MONITORING

MAY 2009 13-1

The design of coal refuse disposal facilities is governed by criteria for achieving desired levels of 

structural, geotechnical and hydraulic performance from initial startup through facility construction 

and operation to abandonment. To verify that the desired performance levels are being achieved, 

instrumentation should be installed and regularly monitored. Timely collection and review of instrumentation data can allow performance problems to be detected and addressed before unsafe conditions develop. This chapter discusses the factors that should be considered in planning a site-specific 

instrumentation program and the types of devices used for monitoring. Supporting discussion 

regarding the uncertainty associated with instrument measurements and types of instrument transducers and data acquisition systems is presented in Appendix 13A. Data typically monitored and the 

types of instrumentation most commonly used to monitor embankment performance at coal refuse 

facilities include:

• Vertical and lateral displacements at the ground surface using surface monuments

• Impoundment pool levels using staff gauges

• Piezometric levels and pore-water pressures in embankments and foundations using 

standpipe and vibrating-wire piezometers

• Surface water flow primarily from seepage and mine discharges using weirs

• Subsurface vertical and lateral displacements in situations of adverse conditions 

(e.g., settlement and slope deformation) using extensometers and inclinometers

• Meteorological conditions at or near the facility using weather stations (primarily 

rainfall gauges)

These and other instrumentation are discussed herein along with the ability to remotely monitor and 

efficiently process data from instruments.

13.1 INSTRUMENTATION PROGRAM PLANNING

Similar to instrumentation programs for other civil works projects, instrumentation program planning for a coal refuse disposal facility should follow a systematic approach with defined objectives. 

The process should follow a logical series of steps leading to preparation of construction plans and 

specifications that prescribe: (1) instrument types and installation methods, (2) performance and 

maintenance requirements, (3) data acquisition methods, (4) sampling intervals and reporting, and 
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(5) expected measurement ranges including appropriate action and hazard warning levels. This systematic approach, as defined by Dunnicliff (1997), should follow the steps presented in Table 13.1. 

The full benefit of an instrumentation program can best be realized if these steps are considered and 

carefully implemented. In doing so, the following adage by Ralph Peck (1972) can be realized,

“We need to carry out a vast amount of observational work, but what we do

 should be done for a purpose and done well.”

Monitoring the performance of coal refuse disposal facilities typically involves the measurement of 

deformations, piezometric levels, impoundment levels, seepage flows, and other site-specific parameters. The steps presented in Table 13.1 provide a rationale for determining the instrumentation and 

staffing requirements, establishing provisions for instrument maintenance, and identifying the use 

and benefit of monitoring results. Table 13.1 can be used as a design aid to focus attention on the 

important parameters and locations to be monitored and to avoid the use of instrumentation that 

may yield little value. Among the critical steps is No. 2 – defining performance questions that need to 

be answered. Performance questions arise from project or geotechnical conditions that could lead to 

instability or non-functionality of portions of the structure. Often, critical performance features such 

as an internal drain or suspect conditions such as a weak subsurface foundation layer beneath the 

facility or an observed seepage condition, may play a role. These critical or suspect conditions will 

then be the focus of performance questions related to instrumentation.

Some performance questions that are typically relevant for coal refuse disposal impoundments are 

summarized in Table 13.2. While this list includes common questions, each site will undoubtedly have 

unique features or conditions that may introduce other performance questions. At new facilities, the 

designer must rely on project information and engineering analyses and judgment to identify performance questions and select instrumentation. At existing facilities, where some performance data are 

available, the designer may identify suspect conditions such as observed seepage or deformation that 

may lead to more refined answers to the performance questions.

While impoundments are generally facilities where the consequences of non-performance are great, 

instrumentation should also be employed at non-impounding embankments and slurry cell facilities. 

Similar performance questions and suspect conditions should be evaluated as part of the identification of the need for and the types of instrumentation to be employed.

In selecting an instrument to measure a desired parameter within the project setting, consideration 

should be given to: (1) instrument error and uncertainty, (2) instrument type reliability and survivability relative to its expected application, and (3) possible integration of the instrument into a 

data acquisition system to facilitate data collection, processing, management and decision making. 

Discussion of the uncertainty associated with instrument measurements and the types of instrument 

transducers and data acquisition systems is presented in Appendix 13A.

13.2 MEASUREMENT TECHNIQUES

Instrumentation is typically installed at coal refuse disposal facilities to monitor movements and displacements, piezometric levels and pore-water pressures, and surface- and seepage-water flow and 

the hydrologic and operational factors that influence these flows. Instruments may also be installed 

to monitor miscellaneous factors such as soil pressure, vibration and shock, and internal temperatures. The following subsections discuss the significance of these measurements and instrumentation 

that can be used to provide the required data.

Selection of appropriate instrumentation depends on a variety of factors discussed in the previous 

sections of this chapter. In addition, instrument selection relates to whether performance monitoring
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TABLE 13.1 INSTRUMENTATION PLANNING AND DESIGN STEPS

Step 1

Define project conditions and mechanisms that control expected behavior including subsurface conditions and stratigraphy, engineering properties of subsurface soil, rock and groundwater, 

environmental conditions and other factors that may affect the planned construction.

Step 2

Define the performance questions that need to be answered. For a coal refuse facility these 

might include: (1) What are the initial or current site conditions? (2) Is performance satisfactory 

during construction and facility operations? and (3) Is performance satisfactory during special loading conditions such as rapid drawdown or upstream construction?

Step 3

Select the most important parameters to be monitored, magnitudes of change, and each 

type of instrumentation considering parameter variations resulting from both cause and effect. 

For example, lateral slope displacements may result from elevated groundwater levels, and specific 

hazard warning levels can be determined. If a clear purpose cannot be defined, the need for instrumentation cannot be defended.

Step 4

Identify staffing responsibilities for monitoring, interpreting and devising remedial action(s)

in terms of required labor and materials needed to respond to problem situations, including 

resources and reporting requirements.

Step 5

Select instruments and locations considering reliability for measurement of the desired parameter 

within the project site setting and the appropriateness of the location for measuring predicted behavior, compatibility with methods of analysis to be used, and device survivability. Other features that 

should be considered include in-service calibration, operator skill requirements, potential interference during construction, and location access during installation and reading.

Step 6

Develop record of factors that may affect the measured data and establish procedures for 

controlling data quality such as geologic conditions in the vicinity of the instrument, use of redundant instruments, regular examination of data for consistency, and in-service calibration checks.

Step 7

Prepare instrumentation system report, budget and procurement specifications summarizing 

Steps 1 to 6 and including sections on the contracting method and basis for instrument procurement 

and field instrumentation services.

Step 8 Integrate instrumentation system report into the Operation and Maintenance Plan, including 

data collection, processing, presentation, interpretation, reporting, calibration and maintenance.

(ADAPTED FROM DUNNICLIFF, 1993)

is required for a relatively short period of time up to a few years, or for a much longer time period up 

to and including the expected service life of the refuse facility.

13.2.1 Movements and Displacements

The performance of embankments can generally be evaluated by monitoring the movement of the 

embankment surface or configuration changes within the embankment and its foundation. For static 

loading conditions (i.e., no earthquake or blasting loads), movement can be determined by: (1) visual 

observation, (2) measurement of surface movements, and (3) measurement of internal movements. 

Table 13.3 provides an evaluation of each approach relative to complexity and cost, applicability, and 

limitations. Additional information is provided by Dunnicliff (1981), Hanna (1985), Bartholomew et 

al. (1987), Dunnicliff (1993) and USACE (1995c).

13.2.1.1 General Observations

Periodic surveying of monuments and instruments provides an effective means for observing the 

magnitude and rate of change of deformations at a coal refuse disposal facility during:

• Initial facility development and initial disposal operations

• Normal disposal operations
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TABLE 13.2 PERFORMANCE QUESTIONS FOR SLURRY IMPOUNDMENTS

 Questions Typical Instruments 

for Monitoring 

Other Typical 

Measures for 

Monitoring 

Suspect 

Conditions

Embankment Conditions

1. Where is the phreatic surface:

a. in the refuse embankment?

b. in the fine refuse deposit (e.g., when upstream construction is 

employed)?

Piezometers

Closed-System 

Piezometers

2. What is the seepage rate? Weirs

3. What is the discharge rate from an internal drain? Weirs

4. Are embankment grades and alignments conforming to plans? Survey Monument Survey Methods

5. Are unacceptable deformations occurring:

 a. at the embankment surface?

 b. within the embankment?

Survey Monument Survey Methods

Inclinometer

6. Could unacceptable pore-water pressure develop due to the 

effects of the rate of fill placement, rapid drawdown, or earthquake 

loading? 

Closed-System

Piezometers

7. Could unacceptable deformations develop due to the effects of 

the rate of fill placement, mine subsidence, rapid drawdown, or 

earthquake loading?

Survey Monuments

Extensometers

Survey Methods

Inclinometer

Foundation Conditions

1. Could unacceptable deformations develop due to the presence of 

compressible foundation materials, subsidence from mining below 

the embankment or impoundment, or earthquake loading?

Survey Monuments

Extensometers

Geophysical

Surveys

2. Could unacceptable pore-water pressures develop due to the 

effects of rapid embankment construction, upstream construction 

over soft fine refuse, or earthquake loading?

Closed-System

Piezometers

3. Could unacceptable pore-water pressures develop due to the 

effects of an elevated reservoir level or an undetected pervious 

stratum?

Closed-System

Piezometers

Abutment Conditions

1. Will excavation or natural slopes be stable:

 a. during construction?

 b. long term?

Survey Monuments Survey Methods
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Instrumentation and Performance Monitoring

MAY 2009

 Questions Typical Instruments 

for Monitoring 

Other Typical 

Measures for 

Monitoring 

Suspect 

Conditions

2. Could unacceptable deformations or cracking develop due to 

subsidence from mining beneath the abutment or impoundment?

Survey Monuments

Extensometers

3. Could unacceptable pore-water pressures develop due to the 

effects of an elevated reservoir level or an undetected pervious 

stratum?

Open-Standpipe

Piezometers

Reservoir Level

1. Is the water level in the impoundment at an acceptable level, and 

is there adequate freeboard? Staff Gage

Breakthrough Potential

1. Could an unusual increase in seepage quantity or change in 

seepage quality from underground mine workings indicate a 

potential problem related to impoundment leakage or possible 

breakthrough?

Weir

2. Could the level of water in underground mine workings have an 

effect on the impoundment or indicate possible breakthrough? Observation Well

Decant Works, Primary or Emergency Spillways

1. Is the flow rate from the decant acceptable? Weir Pipe Discharge 

Measurement

2. Is the strain within the flexible decant conduit acceptable 

considering embankment loading and backfill support?

Dial Gage

Deflectometer

Circumferential Survey

Camera

3. Is there movement affecting the primary conduit spillway joints (or 

cracks) that could lead to separation? Dial Gage

(Crack Monitor)

4. Is there movement affecting the lined emergency spillway joints (or 

cracks) that could lead to separation? Dial Gage

(Crack Monitor)

5. Are the excavation slopes for the emergency spillway stable? Survey Methods

Survey Monuments

Weather Conditions

1. How do changes in measured facility performance (e.g., phreatic 

surface, flow from internal drains) relate to changes in weather 

conditions such as precipitation?

Precipitation Gage Weather Station

Note: Instrumentation shown in italics is generally considered when adverse conditions are observed or suspected

 (e.g., seepage or movement).

TABLE 13.2 PERFORMANCE QUESTIONS FOR SLURRY IMPOUNDMENTS

(Continued)
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MAY 2009

• Changes in the rate of disposal, in the physical characteristics of the fine and coarse 

coal refuse components, or in the phreatic or piezometric levels in the embankment

• Facility abandonment

Excessive movement or progressively increasing rates of movement can provide a warning of a 

potential failure and may indicate that modification of a facility operation is necessary.

Long-term periodic monitoring of surface movements provides an inexpensive record of embankment behavior. If changes due to natural phenomena or operations result in a reduction in stability 

not anticipated in the design, long-term records can provide a forewarning of impending distress. 

The potential value of surveyed records in relation to their cost justifies the installation of benchmarks and monuments for most types of refuse facilities, even if the instrumentation must be regularly monitored during the facility life and to abandonment.

TABLE 13.3 SUMMARY OF GENERAL METHODS FOR DETECTING EMBANKMENT 

MOVEMENTS AND DISPLACEMENTS

 Technique Complexity 

and Cost Applicability Limitations

Visual Observations Low to 

Moderate

Normally used for monitoring 

general conditions to identify areas 

of potential distress where more 

detailed evaluation is required.

Surface cracking indicates the 

occurrence of displacements, but 

does not provide quantitative data.

Measurements of 

Surface Movements Low to High

Reasonable costs and high accuracy provide desirable monitoring 

procedure for practically any condition. Should be routine practice for 

all facilities.

Surface movements can be very 

local without being significant to 

overall safety of facility.

Measurement of 

Internal Movements

Moderate to 

High

Necessary when cause of 

movements is important and 

surface measurements are not 

sufficient for interpretation.

Programs requiring complex 

instrument installation, monitoring 

and interpretation must be 

performed by an expert.

If a surface movement monitoring program is implemented primarily to develop long-term records 

of embankment behavior, the measurement points are usually located along representative cross 

sections perpendicular to the longitudinal axis of the embankment (e.g., at the toe, on benches, and 

at the top of slope along each section) and at readily accessible points on other facility structures. A 

limitation of conventional surveying techniques is that only movements at the surface are monitored. 

Observations at the surface are not always sufficient for determining if the movements are shallow 

and relatively unimportant or if they are associated with deeper, more significant conditions occurring within the embankment or its foundation. In the latter case, it may be necessary to install more 

sophisticated subsurface instrumentation such as inclinometers to better define the mechanism causing the movement and its location.

Optical level surveys (land surveys) are often the best method for monitoring vertical movements 

due to:

• Settlement resulting from consolidation of an embankment foundation or constituent 

materials

• Impending stability failure due to embankment movement or foundation deformations

• Surface subsidence from underground mining
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The order of accuracy required for land surveys depends on the total potential magnitude of movement, while the accuracy obtained is a function of the equipment and personnel employed and the 

reliability of benchmark datums used for control.

Monitoring of horizontal surface movements is useful for detecting conditions that may indicate 

impending instability or for verifying that such movements are not occurring. Preferably, identical reference points or monuments should be used for measuring both vertical and horizontal 

movements.

Instruments for measuring vertical displacements within an embankment or its foundation are useful when:

• Accurate settlement data are required for comparison to predicted embankment or 

foundation settlement.

• The integrity of an impervious core or internal drainage system is dependent on 

limiting the amount of settlement.

• Large settlements could affect drainage slopes and ditches after abandonment.

• A contractor is being paid for construction on a unit-price basis and the volume of 

material placed could significantly change based on the magnitude of settlement.

• A rigid structure is to be placed on an embankment at the completion of construction.

• The embankment was constructed over settled fine refuse or the embankment foundation is soft clay and potentially large settlements are a possibility.

The installation of internal settlement instrumentation is not generally recommended for routine 

refuse disposal embankment construction. However, several of the techniques discussed in the following section are relatively simple and inexpensive, and they may be considered when embankment 

settlement is important. The installation, monitoring and interpretation of data from these systems 

should only be undertaken under the supervision of a knowledgeable engineer.

Durability of the installed instrumentation is particularly important for systems placed in an embankment. Care in placement is critical, because instruments can often be irreparably damaged due to 

improper installation techniques. The high potential for corrosion at coal refuse facilities should 

always be considered, and plastic pipes are preferred over metal pipes unless it is known that high 

temperatures that might affect the long-term behavior of the plastic pipe could occur.

For all types of instrumentation, erroneous data are much more likely when vertical settlement is 

coupled with large horizontal movement. In such cases, instrumentation for measurement of both 

vertical and horizontal movements should be installed. The purpose of measurement of horizontal 

displacements within an embankment is normally to determine if there is instability and the depth 

at which movement is occurring or to verify that surface distortions due to creep movements do not 

have a deeper origin. Inclinometers are often used to determine the depth to the surface where movement is occurring prior to implementation of remedial measures so that the costly remedial effort is 

focused solely on the problem area.

Normally, sophisticated instrumentation is not required for newly-constructed refuse embankments 

designed in accordance with current engineering practice and constructed in conformance with 

detailed plans and specifications. Possible exceptions may include embankments:

• Located above populated areas.

• Constructed where site conditions do not permit access for the desired scope of 

exploration and testing, such as where steep slopes and/or dense ground cover limit 

access to drilling and sampling equipment.
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• Supported on a material that cannot be readily tested, such as settled fine refuse or 

soft, sensitive clay.

• Constructed over or adjacent to areas of past or active underground mining.

• Where dynamic loading is a critical consideration during design.

More sophisticated instrumentation is often used to investigate existing facilities where limited data 

relative to site conditions and construction practices (e.g., fill material characteristics) are available. 

The value of installing and monitoring the instrumentation must be carefully judged against the ability to interpret the data (i.e., the ability to recognize adverse conditions and distress). When internal 

movement instrumentation is required, the location and arrangement of instruments should be based 

on the judgment of an expert, and the installation and monitoring should be conducted under the 

direct supervision of an engineer or technician that is experienced with the equipment used.

Movements associated with dynamic loads (e.g., earthquake or nearby blasting) differ from those 

associated with static loads in that measurements must be taken during the occurrence of the 

dynamic condition. Generally, where earthquake considerations are important in the site selection 

process, potential dynamic displacements are estimated based upon seismic engineering analyses 

and no on-site measurements are involved. However, in the case of blasting, the amount of movement realized is dependent upon geologic conditions, the explosive material and blasting technique, 

and the location of the blast relative to the embankment. Often, blasting effects are minor and will not 

affect embankment stability. Exceptions are when blasting is very close to an embankment or other 

facility structure or rock abutment or the magnitude of the blast is unusually high. In these isolated 

cases, surface monitoring of resultant movements may be justified for verification that no significant 

damage to the refuse disposal facility has occurred. Instrumentation for this purpose is discussed in 

Section 13.2.4.2.

13.2.1.2 Movement Measurement Techniques

As listed in Table 13.4, Dunnicliff (1993) identifies the following as general categories of deformation 

measuring techniques:

• Surveying methods

• Surface extensometers

• Tiltmeters

• Probe extensometers

• Fixed embankment extensometers

• Fixed borehole extensometers

• Inclinometers

Descriptions of instrumentation associated with each of these deformation measurement techniques 

are presented in the following sections. Supplemental techniques for deformation measurement, 

including transverse-deformation gages, liquid-level gages, time-domain reflectometry, and fiberoptic gages are presented in Appendix 13A at the end of this chapter.

13.2.1.2.1 Surveying Methods

Surveying methods are generally used for monitoring the magnitude and rate of vertical and horizontal movement of the ground surface, structures, and accessible parts of subsurface instruments 

at construction sites (Dunnicliff, 1993). For many applications, these methods are adequate for performance monitoring, and instrumentation is used only if greater accuracy is needed or if subsurface 

movements need to be determined. When instrumentation is employed, surveying methods are often 
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Instrumentation and Performance Monitoring

MAY 2009 TABLE 13.4 INSTRUMENT CATEGORIES FOR MEASURING MOVEMENTS Category

H

V

A

R

S

U Conditions for Use Relative Complexity Relative Cost Applicability to Coal Refuse Disposal Facilities

Surveying Methods • • • • Surface movements Simple Low to Moderate Routine for all embankments.Surface Extensometers

•	 Crack gages•	 Convergence gages

• • • •

Deformation between fixed points Simple Low

Usually limited to structure and rock movements, including subsidence monitoring.

Tiltmeters • • • Rotations on a surface Simple Low to Moderate Usually limited to structure movements.Probe Extensometers

•	 Mechanical probe gages•	 Electrical probe gages•	 Probe extensometers with 

inclinometers

• • • •

Displacement (usually vertical) between two or more points along a single axis

Moderate to complex Moderate

Useful if settlement of soft foundation is critical; normally single settlement plates are adequate.

Fixed Embankment Extensometers

•	 Settlement platform • • • •

Settlement at single depth. Practical for pre- or postconstruction installation Simple Low Useful if settlement of soft foundation is critical.

Fixed Borehole Extensometers

•	 Single- and multi-point•	 Subsurface settlement points 

and rod gages

• • • •

Displacement between two or more points along a single axis

Moderate to complex Moderate Useful if settlement of soft foundation or rock slopes is critical.

Inclinometers • • • • •

Movement profiling in vertical, horizontal or inclined casings

Moderate Moderate

Most commonly used method, but limited to zones where deformation profiles are needed or are a concern.

Transverse-Deformation Gages

•	 In-place inclinometers • • • • Movement profiling Moderate to complex

Moderate to 

High

Useful only if automatic monitoring is planned.

Liquid-Level Gages

•	 Single- and multi-point gages•	 Full-profile gages

• •

Settlement of foundation or embankment fill Moderate to complex Moderate Useful if settlement of soft foundation is critical.

Time-Domain Reflectometry • • • • • Movement detection at locations along cable axis Moderate Low to Moderate Still evolving method, but shows excellent promise.Fiber-Optic Sensors • • • • • Local deformation and strain measurements Moderate Moderate to High Still evolving method, but shows ex- cellent promise, especially for long- term monitoring applications.

Legend: H = Horizontal deformation A = Axial deformation S = Surface deformation

V = Vertical deformation R = Rotational deformation U = Subsurface deformation

(ADAPTED FROM DUNNICLIFF, 1993)
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used for locating the instruments relative to a reference datum. Table 13.5 summarizes the advantages, limitations, approximate accuracy, relative complexity, cost, and applicability to coal refuse 

disposal facilities of the various surveying methods identified by Dunnicliff (1993).

Optical Leveling

Most settlement surveys at construction sites are conducted using engineer’s levels at second- or 

third-order accuracy. Second-order leveling surveys are generally confined to extending vertical control data over long distances. Second-order leveling involves limited sight distances (± 225 feet), balancing foresight and hindsight, careful plumbing of the level rod, and readings made on well-defined 

marks and stable turning points. Third order leveling surveys are used to establish vertical control 

and to maintain benchmarks for project control, construction survey control, topographic survey 

control and major structure points. Third-order leveling permits greater sight distances (± 300 feet), 

along with balancing foresight and hindsight, plumbing of the level rod, and readings made on welldefined marks and stable turning points. For the magnitude of elevation change typically of interest 

at coal refuse disposal facilities, a third-order survey is usually adequate.

An important requirement for measuring horizontal movements is the establishment of reference 

monuments away from the embankment being monitored that are known to be fixed against horizontal movement and are convenient for the method of measurement used. This requirement may 

be difficult in many coal mining areas where natural slopes are steep with resulting downhill creep 

of the surface soils and where surface strains from nearby mine subsidence may be present. Where 

possible, reference monuments should be placed on flat natural ground or on bedrock by excavating 

holes in the soil/rock and backfilling them with concrete. For monuments in soil, the depth should 

extend below the maximum depth of frost penetration. Where assurance of undisturbed location 

is not possible for the primary reference monuments used for routine monitoring, they should be 

closed into a larger survey traverse or triangulation network for occasional checking of their locations. The large traverse should preferably have at least two permanent monuments in an area minimally susceptible to movement, such as adjacent to a highway in a broad valley bottom that does not 

overlie coal seams. Additional discussion of monuments and benchmarks is provided at the end of 

this section.

Trigonometric Leveling

Trigonometric leveling uses electronic distance measuring (EDM) equipment to measure the slope 

distance from the survey instrument to a prism on the surveyed location and to calculate the elevation and horizontal position of the surveyed location. The angle between the horizontal and surveyed location can be measured using a semi-precise (6-arc-second) or a precise (1- or 2-arc-second) 

theodolite.

Distance Measurement by Taping

Distance measurement by taping is probably the easiest measurement technique for horizontal movements because it requires only the use of a steel tape. When a known fixed point can be located in the 

direction of anticipated movement, this procedure consists of measuring the distance between the 

fixed reference and the monuments of interest. Typical corrections for sag, temperature and slope 

should be applied in order to obtain accurate measurements. The limitations of tape measurement 

should not preclude its use, because it may be the only means for locating lost reference points covered by deep snow or inadvertently buried by refuse placement.

Electronic Distance Measurement

Except for distances less than about 200 feet, distance measurement by taping has been replaced by 

EDM equipment. EDMs are used to directly measure the distance change or lateral position change
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by triangulation. EDMs make use of electromagnetic radiation (or lasers) to measure the distance 

between the instrument and a reflector prism.

Theodolite and Scale

Offsets from baseline using a theodolite and scale are measurements at a right angle to a baseline. 

These surveys are typically used for grade control for embankments and roadways. Laser beam leveling and offsets provide a faster and more accurate alternative to optical leveling.

Total Station Survey

Total station instruments combine electronic distance measurement, digital theodolites and microprocessors to simultaneously measure slope length and angle, calculate horizontal and vertical distance, 

and display the results in real time. Typically, a reflector prism is manually positioned at locations of 

interest, and the data are recorded and processed by the total station. This technology can provide 

real-time monitoring of predetermined points through use of one or more automated total stations 

combined with multiple-prism targets, a data acquisition system, and software interfaces. For this 

application, prism targets are mounted on the surface of the features to be monitored and are programmed into the routine of the total station. The survey frequency can vary (typically every two to 

five minutes) depending on the number of targets programmed into the total station’s routine. Each 

automated total station can monitor up to about 100 prism targets.

Traverse Lines and Triangulation

Traverse lines and triangulation are conventional survey techniques that have been used for decades. 

Their improved accuracy in recent years is due to the use of more precise equipment used to measure distance (EDMs) and angles (theodolites) between reference control points. A traverse is used 

to determine change in lateral position through measurement of successive distances and angles. If 

a traverse returns to its starting point, the sum of the interior angles of the enclosed polygon can be 

calculated and adjusted for measurement errors. Triangulation can also be used to determine change 

in lateral position. This is accomplished by accurately measuring a baseline offset from the surveyed 

locations and the angles between the ends of baseline and the surveyed locations, as illustrated in 

Figure 13.1. Then by periodic re-sighting on the surveyed locations from the ends of the baseline, 

changes in lateral position can be calculated, assuming the baseline is located on stable ground.

Airborne Mapping Systems

Airborne methods include photogrammetric and LIDAR mapping systems. Photogrammetry is a 

remote-sensing technology in which geometric properties of objects are determined from photographic images. Photogrammetric methods can be used to record movement of hundreds of survey 

points at one time and thus provide an overall pattern of deformation, but the accuracy is affected by 

weather conditions, baseline measurements and interpreter skill.

Light Detection and Ranging (LIDAR) is a remote sensing system used to collect topographic data 

and develop topographic maps. LIDAR consists of a laser imaging device, an inertial navigation 

system, a GPS receiver and a computer. The technology can be used to map and determine coordinates of dense patterns of ground points, which can be used to develop an image of the ground surface. The data can be used to produce digital elevation models (DEMs) and subsequently topographic 

maps. When LIDAR is used, weather conditions must be monitored because the flights cannot be 

flown during times of rain or fog, as the water vapor in the air could cause the laser beams to scatter 

and give false readings.

Global Positioning System (GPS)

The GPS consists of a constellation of 24 satellites. Each satellite orbits the Earth twice a day at an 

altitude of about 12,500 miles and continuously transmits information on specific radio frequencies 
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TABLE 13.5 SURVEYING METHODS

 Method Advantages Limitations Approximate Accuracy Relative Complexity Cost Applicability to Coal Refuse Disposal FacilitiesElevations by optical leveling

Fast, particularly with self-leveling equipmentUses widely available equipment

First order leveling requires high-grade equipment and careful adherence to standard procedures

Third order:± 0.05 ft 

×

Second order:± 0.025 to 0.033 

×

First order:± 0.012 to 0.020 

×

Simple Low to Moderate Routine for all embankments

Trigonometric leveling

Long range; fast and convenient; can be done simultaneously with traversing

Accuracy is influenced by atmospheric conditions; requires a very accurate measurement of zenith angle

Third order:± 0.05 ft 

×

Second order:± 0.025 to 0.033 

×

Simple for 

3rd Order; 

Moderate 

for 2nd 

Order

Low for 

3rd Order; 

Moderate 

for 2nd 

Order

Routine for all embankments

Distance measuringby taping Direct measurements

Requires clear, relatively flat surface between measuring points and reference datum; movement can only be measured in one direction; monuments must be located along a straight line with ready access between points; tape should be checked frequently against standard; except for short measurements, taping has been replaced by EDM. 

Third order:± 0.0033 to 0.0016 of distance between instrument and surveyed locationSecond order:± 0.00005 – 0.00002 of distanceFirst order:± 0.000003 of distance

Simple Low to Moderate Routine for all embankments

Electronic distance measurement (EDM)

Long range; fast and convenient; very accurate.

Accuracy is influenced by atmospheric conditions

For distance:± 0.001 to 0.03 ftFor lateral position change by triangulation:± 0.005 to 0.03 ft

Simple to 

Moderate

Low to 

Moderate

Routine for all embankments

Offsets from baseline using theodolite and scale

Direct measurements Requires baseline unaffected by movement

± 0.001 to 0.005 ft Simple Low to Moderate Routine for all embankments

miles

miles

miles

miles

miles
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Instrumentation and Performance Monitoring

MAY 2009 TABLE 13.5 SURVEYING METHODS

(Continued)

 Method Advantages Limitations Approximate Accuracy Relative Complexity Cost Applicability to Coal Refuse Disposal FacilitiesLaser beam leveling and offsets

Faster than conventional optical methods; readings can be made by one person

Seriously affected by air turbulence, humidity, and temperature differential; requires curvature and refraction corrections beyond about 200 ft

± 0.01 to 0.03 ft Simple Low

Routine forsignificant embankments

Traverse lines

Useable where direct measurements are not possible

Accuracy decreases as number of legs in traverse increases; traverse should be closed if possible

± 0.00003 to 0.000007 of distance between instrument and surveyed location

Simple Low to Moderate Routine for significant embankments

Triangulation

Useable where direct measurements are not possible; baseline can be located to avoid active construction areas

Requires accurate measurement of angles and baseline length

± 0.00003 to 0.000001 of distance between instrument and surveyed location

Moderate Low to Moderate Routine for significant embankments

Photogrammetric methods

Can record movement of hundreds of potential points at one time for determination of overall deformation pattern

Weather conditions can limit use; interpretation requires specialist; for good accuracy the baseline should not be less than one-fifth of the sight distance

± 0.0002 to 0.00001 ft Complex Expensive

Non-routine for significant embankments

Global positioning system (GPS)

Operates with little attention from personnel; can be set to trigger a warning device; very accurate; does not require line of sight; measurements can be made in almost any weather condition

Requires open skyline of sight 

Static and Relative Positioning:± (0.016 ft + 1ppm)Requires 10 to 30 minutes observation per point depending on method of survey and whether single or dual frequency receiver is used.

Complex

Moderate 

to 

Expensive

Non-routinge for significant embankments

 (ADAPTED FROM DUNNICLIFF, 1993) Note: = square root of distance in miles miles
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to ground-based receivers. Using sophisticated receivers and data-analysis techniques, receiver positions can be determined. The GPS satellites continuously transmit an estimate of their position, digital codes, and a precise time signal. A GPS receiver uses an internal clock and the codes to determine 

the distances to at least 4 satellites. Distance is calculated by multiplying the time it takes the radio 

signals to reach the receiver times the speed of light. Knowing where the satellites are located when 

signals are transmitted, the receiver calculates its position. GPS equipment is becoming more reliable, 

cheaper, faster, and easier to use compared to conventional instruments. New hardware, field procedures and software have also been developed to assist users in data collection and processing. GPS 

equipment is now used for a wide range of monitoring applications.

A stable reference datum is required for all survey measurements pertaining to deformation. A 

benchmark is a reference datum for vertical movements and a horizontal control station or reference 

monument is a reference datum for horizontal movements. As a minimum, the datum benchmark 

should be placed in the ground, on a structure or on a rock outcrop in such a manner that the effects 

of weather (frost heave) or equipment travel will not alter its position. It should also be located well 

away from the embankment area so that increased earth loads or water pressures in the embankment do not significantly change the ground elevation at the benchmark. Where rock is shallow, an 

excavated hole to rock with concrete backfill and an embedded brass pin reference point is normally 

acceptable. When rock is deep, the most reliable benchmark consists of a steel rod installed inside a 

casing placed through overburden into bedrock. The rod should be isolated from the casing by nylon 

spacers, and the casing should be provided with a protective cover, as shown in Figure 13.2. Because 

the outer casing may be disturbed or compressed due to settlement of the surrounding material, the 

FIGURE 13.5 LOCATION OF MONUMENTS USING TRIANGULATION

LEGEND

MONUMENT

TRIANGULATION POINT FOR

INSTRUMENT SETUP

BENCH

CREST OF EMBANKMENT

TOE OF

EMBANKMENT

KNOWN FIXED BASELINE

MEASURED ANGLE FOR

TRIANGULATION (TYP.)

FIGURE 13.1 LOCATION OF MONUMENTS USING TRIANGULATION
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Instrumentation and Performance Monitoring

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13.2b ROCK BENCHMARK IN COMMON

 HOLE WITH PIEZOMETER

PIEZOMETER

PROTECTIVE PIPE

STEEL ROD

HINGED

LID THREADED CAP

ELASTIC MEMBRANE

TO KEEP HOLE CLEAN

PROTECTIVE

PIPE COVER

GREASED

SURFACE

STEEL ROD

OR PIPE GROUT

CASING

CEMENT GROUT

OR CONCRETE

GROUND

SURFACE

PADLOCK

13.2a TYPICAL ARRANGEMENT

BEDROCK

FIGURE 13.2 EXAMPLE OF BENCHMARK IN ROCK
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benchmark should be placed on the top of the inner rod, which is isolated from the movement of the 

casing. The protective cover will prevent damage due to material becoming wedged between the 

inner rod and the casing or due to vandalism.

In areas with nearby mining activity, associated movements of the ground surface may occur on 

adjacent hillsides (both surface heave and subsidence have been observed to occur for horizontal distances of more than several hundred feet from the nearest point of mining), so it may be necessary to 

construct a benchmark in a deep hole so that mining in the coal seam does not affect the benchmark. 

If the depth of mining is such that the cost of constructing a deep benchmark is prohibitive, a project 

benchmark should be established and should be periodically checked against other benchmarks in 

the region. In all cases, a minimum of three datum benchmarks should be established for periodic 

checking. These benchmarks should be located on opposite sides of the facility and in locations where 

they will not be affected by weather or subsidence conditions.

Reference points or monuments where surface settlement monitoring is desired can range from 

stakes or rods driven into the ground to carefully installed concrete cylinders. Examples are shown 

in Figure 13.3. In all cases, the point of measurement must be clearly identified so that the same location is used each time. Brass or steel rods or pipes set in concrete or rock or chiseled/painted crosses 

are suitable.

Except for rapidly established and very short-duration monitoring programs, driven rods should be 

avoided because this type of arrangement can be easily disturbed. If used, the minimum depth of 

embedment should be at least two feet, and the maximum extension above grade should be one foot 

or less to minimize tilting or bending of the rod. Also, because settlement points are often located in 

areas of high equipment activity, they should be well marked with stakes and flags to minimize loss 

and disturbance. Where loss of a monument would reduce the extent or accuracy of an important 

survey, up to twice the number believed necessary may be installed. The monument shown on Figure 

13.3c can be used where surface creep is significant and conditions at depth (e.g., below the frost line 

or in rock or stable strata below the ground surface) better represent slope performance.

13.2.1.2.2 Surface Extensometers

Surface extensometers are devices used to monitor changes in distance between two points at the 

ground surface or on a structure (Dunnicliff, 1993). Surface extensometers are usually referred to as 

crack gages or convergence gages. Crack gages are used to monitor tension cracks behind slopes, 

across a joint or fault in a rock mass, or in concrete structures. Convergence gages have been used 

to measure the displacement between two surface monuments as part of the monitoring of ground 

surface strain induced by longwall mining near an impoundment. Mechanical crack gages include 

pins and tape, pins and steel ruler or caliper, pins and mechanical extensometer, and grid crack monitors. For each of these devices, the pins or grid crack elements are anchored on opposite sides of the 

discontinuity, and the distance between the anchored elements is measured periodically to determine 

the change in separation. Each of these types of surface extensometer is inexpensive and has a precision ranging from ± 0.01 to ± 0.1 inches. The principal limitation of mechanical crack gages is the 

relatively short span length between the pins. Additional details related to surface extensometers are 

provided in Dunnicliff (1993).

13.2.1.2.3 Tiltmeters

Tiltmeters are used for monitoring changes in inclination or rotation at the ground surface or on a 

structure in either a horizontal or vertical plane (Dunnicliff, 1993). A tiltmeter consists of a gravitysensing device that is sealed in a housing. Some tiltmeters are fixed in place, and others are portable 

and can be used to monitor multiple locations by mounting on a fixture that is permanently attached 

to the monitoring location. The most common types of tiltmeters have an accelerometer transducer, 
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FIGURE 13.7 TYPICAL MONUMENT INSTALLATIONS

BELOW FROST LINE

OR PLACED IN ROCK

CONCRETE

BRASS PLATE

MONUMENT OR

STEEL DOWEL

13.3d

CL CONCRETE

GROUT

BOTTOM OF ROD

OR PIPE SHOULD

EXTEND BELOW

FROST LINE

STEEL ROD

OR PIPE

13.3a

CL

ELASTIC

MEMBRANE

TO KEEP

HOLE CLEAN

THIN WALL

PIPE SLEEVE

THREADED

CAP

STEEL ROD

OR PIPE

~ 5'

~ 3'

13.3b

BRASS PLATE

MONUMENT

PUNCH MARK

THIN WALL

PIPE SLEEVE

CONCRETE

BELOW FROST LINE

OR PLACED IN ROCK

13.3c

FIGURE 13.3 TYPICAL MONUMENT INSTALLATIONS
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but devices with mechanical, vibrating wire and electrolytic transducers are also available. The typical range of accelerometer-type tiltmeters is ± 30° from horizontal or vertical; the precision is typically 

± 50 arc-seconds; and the temperature sensitivity is typically in the range of 2 to 3 arc-seconds/°F 

(Dunnicliff, 1993).

13.2.1.2.4 Probe Extensometers

Probe extensometers are used to monitor changes in distance between two or more locations along a 

common axis by passing a probe through a pipe (Dunnicliff, 1993). As the probe passes through the 

pipe, it mechanically or electrically detects the measuring locations, and the distance between them 

is determined by physical measurement of the probe positions. If an exact measurement of probe 

location is required, one measuring point must be accessible so that its location relative to a reference 

datum can be determined by surveying methods. Depending on project requirements, the pipe may 

be vertical for measurement of settlement or heave, horizontal for measurement of lateral displacement, or inclined. Dunnicliff (1993) provides a complete summary of probe extensometer types, their 

advantages, limitations and approximate accuracy.

The most commonly used probe extensometers are:

• Gage with current-displacement induction coil

• Magnetic reed switch gage

• Combined probe extensometers and inclinometer casing

Induction-coil transducers are described in Appendix 13A.2.4.6. When used as a probe extensometer, 

the device consists of a series of steel rings attached to and surrounding a telescoping or corrugated 

plastic pipe and a reading device consisting of a primary coil housed in a probe that is attached to 

a signal cable and current indicator. The pipe is installed in a borehole and backfilled. When the 

probe is inserted into the pipe, the steel rings can be located because the current indicator displays 

a maximum value at each ring location. The measurements enable determination of the distance 

between each ring, and a series of measurements over time permit determination of rate of deformation between the rings. Information related to installation, measurement precision and typical applications is provided in Dunnicliff (1993).

The magnetic reed switch gage is described in Appendix 13A.2.4.5. When used as a probe extensometer, the device consists of a series of circular magnetic anchors surrounding a rigid or telescoping 

plastic pipe. The pipe is installed in a borehole and backfilled. When the probe is inserted into the 

pipe, the magnetic anchors are located and their position measured as the coupled oscillator in the 

probe is activated at each ring location.

The Sondex system consists of a probe, a signal cable, a cable reel with a built-in voltmeter, and a 

number of stainless steel sensing rings. A survey tape is typically connected to the probe. The sensing rings are fixed to a continuous length of corrugated plastic pipe that is installed coaxially with 

inclinometer casing or access pipe. The annulus between the corrugated pipe and the borehole wall 

is grouted. The corrugated pipe and its sensing rings settle with the surrounding ground. To obtain 

measurements, the operator draws the probe through the access pipe. A buzzer sounds when the 

probe is near a ring, and the voltmeter indicates peaks when the probe is aligned with a ring. The 

operator refers to the survey tape and records the depth of the ring. A sensitivity adjustment allows 

operation adjacent to steel pipes, piles, or other metal objects. Settlement and heave can be calculated 

by comparing the depth of each ring to its initial depth.

Several types of probe extensometers can be used in conjunction with inclinometer casing in a vertical 

borehole to permit measurement of both horizontal and vertical deformations in one installation. Increx 
13-19

Instrumentation and Performance Monitoring

MAY 2009

and Sondex are examples of this type of probe extensometer. The Increx system consists of a number

of brass rings that are positioned at one-meter intervals along an inclinometer casing. The probe is 

positioned to successively take readings between each pair of rings. Periodic surveys are compared 

to the initial survey to determine changes in the distance between rings. If the distance has increased, 

extension has occurred; if the distance has decreased, compression has occurred. Movements can be 

referenced to the deepest ring, if it is located in stable ground, or the top of the casing can be optically 

surveyed. The systems can measure changes as small as 0.001 mm with an accuracy of ± 0.01 mm/m. 

A schematic illustration of a probe extensometer is provided in Figure 13.4.

13.2.1.2.5 Fixed Embankment Extensometers

Fixed embankment extensometers are devices placed in an embankment fill during construction to 

monitor changes in distance between two or more points along a common axis without a movable 

probe. They are used to measure settlement, horizontal deformation or strain (Dunnicliff, 1993). The 

most common type of fixed embankment extensometer is a settlement platform. A settlement platform consists of a square plate or anchorage to which a riser is attached. If the fill height exceeds 

about 25 feet, the riser should be isolated from the surrounding fill by an outer pipe within which 

the riser can move freely. As the height of fill increases, the riser and outer pipe are raised by adding 

additional pipe lengths. Movements of the settlement platform can be monitored by optical survey of 

elevation of the top of the riser. A schematic illustration of a settlement platform is provided in Figure 

13.5. Settlement platforms are easy to construct, but they are easily damaged during construction as 

fill is placed and compacted near them. This problem can be avoided by using liquid- level gages, as 

described in Appendix 13A.3.2.

READOUT UNIT SURVEY TAPE

PROBE CONTAINING

REED SWITCH

BOREHOLE FILLED

WITH GROUT

FLUSH-COUPLED RIGID

PVC ACCESS PIPE

SPRING

CIRCULAR

MAGNET

MAGNET ANCHOR

(SPIDER MAGNET)

1345

1344590L5

(DUNNICLIFF, 1993)

FIGURE 13.4 PROBE EXTENSOMETER
13-20

Chapter 13

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Tensioned-wire gages and fixed embankment extensometers with Linear Variable Differential 

Transformers (LVDTs) have been used to monitor horizontal displacements in an embankment fill 

(Dunnicliff, 1993). These devices have been largely replaced by probe or in-place inclinometers 

(Section 13.2.1.2.7) installed in a vertical borehole.

13.2.1.2.6 Fixed Borehole Extensometers

Fixed borehole extensometers are devices without moveable probes installed in boreholes in soil or 

rock and used to monitor the change in distance between two or more points along the borehole axis 

(Dunnicliff, 1993). If measurement of the exact deformation at a specific location is required, one end 

of the extensometer must be fixed in stable ground or it must be accessible for optical survey. Fixed 

borehole extensometers can be used to monitor deformations behind rock slopes, foundation settlements, the progression of subsidence above underground mines, and heave at the base of open-cut 

excavations. A schematic illustration of a fixed borehole extensometer is provided in Figure 13.6.

The distance between the end of the rod and the face of the collar anchor can be measured using either 

a mechanical device (e.g., dial gage) or electrical transducer (e.g., LVDT). The device illustrated in 

Figure 13.6 is a single-point borehole extensometer (SPBX). A multiple-point borehole extensometer 

(MPBX) consists of several downhole anchors within a single borehole along with a rod and measurFIGURE 13.9 SETTLEMENT PLATFORM

SURFACE OF

EMBANKMENT

COUPLED

RISER PIPE

EMBANKMENT

SQUARE PLATE ORIGINAL

GROUND

SURFACE

SETTLEMENT OF PLATFORM

IS DETERMINED BY MEASURING

ELEVATION OF THE TOP OF THE

RISER PIPE, USING SURVEYING

METHODS

(DUNNICLIFF, 1993)

FIGURE 13.5 SETTLEMENT PLATFORM
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Instrumentation and Performance Monitoring

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ing device for each. MPBXs allow the measurement of deformation or strain patterns over the length 

of the borehole so that potential failure or large deformation zones can be identified. Fixed borehole 

extensometers can be manufactured to meet site-specific requirements. The principal feature differences include choice of rod or tensioned wire attached to the downhole anchor, downhole anchor 

type (i.e., mechanical, hydraulic or groutable), SPBX or MPBX, transducer type and extensometer 

head. Dunnicliff (1993) provides guidance for selecting among these options for particular applications. The accuracy of the device depends on the type of mechanical or electrical transducer used to 

measure change in the anchor location.

13.2.1.2.7 Inclinometers

Inclinometers are devices used for monitoring displacements normal to the axis of a pipe by passing a 

probe along the pipe (Dunnicliff, 1993). They can be used to determine the depth and profile of lateral 

displacement above and below a slide plane such as in a distressed slope. A gravity-sensing transducer 

in the probe measures inclination with respect to vertical. Normally the probe is inserted into a vertical 

or inclined borehole to measure lateral displacements in landslide zones or structures, but probes are 

available for use in horizontal pipe for obtaining profiles beneath embankments and structures.

Inclinometer systems have four primary components:

• A probe casing with tracking grooves that is permanently installed in a borehole 

(vertical or inclined application) or trench (horizontal application)

• A probe with a pair of wheel carriages housed in steel to confine the gravity-sensing 

transducer

• A portable readout for power supply and measurement of probe inclination

• A graduated electrical cable connecting the probe and readout unit

Figure 13.7 shows a schematic representation of the operation of an inclinometer for vertical and 

near- vertical applications. The inclinometer casing is installed and grouted along its full length into 

a borehole so that one pair of the four orthogonal grooves in the casing is normal to the displaceFIGURE 13.10 FIXED BOREHOLE EXTENSOMETER

L

SLEEVED

STEEL ROD

BOREHOLE

DOWNHOLE

ANCHOR

COLLAR

ANCHOR

MEASURE DISTANCE FROM FACE OF

COLLAR ANCHOR TO END OF ROD:

CHANGE IN DISTANCE = L

L MEASURED WITH

DIAL GAUGE OR LVDT

(DUNNICLIFF, 1993)

FIGURE 13.6 FIXED BOREHOLE EXTENSOMETER
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FIGURE 13.11 INCLINOMETER OPERATION

PROBE CONTAINING

GRAVITY-SENSING

TRANSDUCER

READOUT

UNIT

GRADUATED

ELECTRICAL

CABLE

BOREHOLE

COUPLING

BACKFILL

GROUT

1345

1344590L5

GUIDE

CASING

13.7a INSTRUMENT AND CASING ARRANGEMENT

ACTUAL ALIGNMENT 

OF GUIDE CASING 

(EXAGGERATED)

L sin 

L sin 

L

DISTANCE BETWEEN

SUCCESSIVE READINGS

TRUE

VERTICAL 

13.7b DISPLACEMENT COMPUTATION 13.7c LOCATION OF PLANE OF MOVEMENT

GUIDE

CASING

PROBE

GUIDE

WHEEL

ENLARGED BOREHOLE PLAN

SLOT FOR

GUIDE WHEEL

BACKFILL

GROUT

(DUNNICLIFF, 1993)

INCLINOMETER READINGS

0 1 2 3 4

MOVEMENT (INCHES)

DEPTH (FEET)

DAYS SINCE

INITIAL READING

35

30

25

20

15

13

19

28

35

42

FIGURE 13.7 INCLINOMETER OPERATION
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Instrumentation and Performance Monitoring

MAY 2009

ments being measured. Normally the casing is installed to a depth that provides fixity. Measurements 

are made by lowering the probe along the pair of grooves normal to the desired direction of movement observation to the bottom of the casing and then incrementally raising the probe and measuring probe inclination at each interval. Once the uppermost reading is made, the probe is removed 

from the casing, rotated 180 degrees to reverse the orientation of the probe in the casing, lowered to 

the bottom of the casing, and the measurement process is repeated at the previous depth intervals. 

Reversal of the inclinometer is performed in order to eliminate or minimize the effects of errors due 

to long-term drift of the gravity-sensing transducers. If a fixed base is not achieved, the top of the 

inclinometer casing must be surveyed each time measurements are taken so that measurements from 

one time interval can be compared to another. Dunnicliff (1993) provides additional details on inclinometer type; operation and calibration; casing types and installation; and data collection, processing and interpretation. The reported precision for inclinometers is: ± 0.05 to 0.5 inches per 100 feet 

for force-balance accelerometer transducers, ± 0.02 to 1 inch per 100 feet for bonded resistance strain 

gage transducers, and ± 0.1 to 0.5 inches per 100 feet for vibrating wire transducers (Dunnicliff, 1993).

13.2.2 Piezometric Levels and Pore-Water Pressures

Knowledge of the piezometric levels and pore-water pressures in foundations and embankment 

cross sections is important for safe operation of coal refuse disposal facilities. Elevated piezometric 

and pore-water pressure levels can be a pre-failure indicator of general and localized instabilities in 

foundation soils, downstream embankment slopes, upstream embankment slopes (due to failure of 

fine refuse following pushouts or upstream construction), and temporary excavations in soil or rock. 

Commonly used techniques for measuring piezometric levels and pore-water pressures are discussed 

in this section and are summarized in Table 13.6. Valuable additional information related to groundwater monitoring instrumentation is provided by Dunnicliff (1981), Hanna (1985), Bartholomew et al. 

(1987), Dunnicliff (1993) and USACE (1995c).

Pore-water pressure and its significance relative to embankment fill and coal refuse behavior are 

discussed in Section 6.5.7. Monitoring of pore-water pressures may be important for controlling construction and maintaining embankment or foundation stability. Devices for measuring pore-water 

pressure (piezometers) operate by admitting water through openings or a porous element, while 

restricting inflow of fine particles, such that the pore-water pressure can be determined from the 

elevation of water in the piezometer or by a pressure gage.

13.2.2.1 Piezometer Types

Piezometers generally fall into two categories: open system (e.g., observation wells and open standpipes) and closed system (e.g., pneumatic, vibrating-wire, and electrical-resistance). Both types have 

application at coal refuse disposal facilities.

Open observation wells are not selectively screened and thus provide limited information. For open 

standpipe piezometers, a screen or filter is placed in the zone of interest and sealed off so that only 

the sensing zone will be monitored. Water rises in the open piezometer to an elevation corresponding 

to the piezometric head in the zone being monitored. Measurement of piezometric head is typically 

accomplished using an electrical water level meter.

Closed-system installations such as vibrating-wire, pneumatic or electrical piezometers respond more 

quickly to changes in pore pressure because no significant flow of water is necessary. Also, these types of 

piezometers can be monitored remotely. While these devices have some reliability problems over long 

time periods, they can perform acceptably if they are properly selected for the applications required 

and environmental conditions encountered. Rapid response time is necessary when an embankment is 

constructed over a saturated and sensitive foundation material that could fail if excess pore pressures 

develop during construction and temporarily exceed those assumed in design. Table 13.6 summarizes 
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the advantages and limitations associated with various types of piezometers, and Figure 13.8 presents guidance regarding the approximate response times for various types of open- and closed-system 

piezometers as a function of the hydraulic conductivity of the soil surrounding the sensing zone. As 

shown in the figure, the response time to achieve 90 percent equilibration increases with decreasing 

hydraulic conductivity and with increasing volume of the sensing zone. Methods for estimating the 

response time are presented in Terzaghi and Peck (1967).

13.2.2.2 Observation Wells

Observation wells are not selectively screened in a specific formation. Thus, they will usually not provide useful information unless they are located in a single aquifer near the surface. While observing 

the water level in an open or cased borehole with an electrical water level meter, measuring tape and 

weight, or similar device is relatively easy, the measured water level may represent the contribution of 

many soil layers. Therefore, it is generally not possible to distinguish between pressures occurring at 

different elevations. Observation wells should only be used when relatively homogeneous embankment/foundation materials are present, and generally open-standpipe piezometers are preferred.

TABLE 13.6 INSTRUMENTS FOR MEASURING GROUNDWATER PRESSURE

Instrument Type Advantages Limitations

Observation well

Can be installed by drillers without 

full-time participation of geotechnical 

engineering personnel

Provides undesirable vertical 

connection between strata so results 

can be misleading; should rarely be 

used

Open-standpipe piezometer

Reliable; long successful performance record; self de-airing if inside 

diameter of standpipe is adequate; 

seal integrity can be checked after 

installation; can be converted to 

diaphragm piezometer; can be used 

for ground-water sampling and 

permeability measurements 

Long lag time; subject to damage 

during construction; extension of 

standpipe through embankment fill 

interrupts construction and leads to 

poor compaction; porous filter can 

plug due to repeated in- and out-flow; 

push-in versions subject to several 

potential errors

Pneumatic piezometer

Short lag time; calibrated part of 

system is accessible; minimum 

interference to construction; level of 

lead wires and readout independent 

of tip level; no freezing problems

Attention must be paid to many 

details during instrument selection; 

push-in versions subject to several 

potential errors

Vibrating-wire piezometer

Easy to read; short lag time; minimum 

interference to construction; level of 

lead wires and readout independent 

of tip level; lead wire effects minimal; 

can be used to read negative pore 

pressures; no freezing problems

Special manufacturing required to 

minimize zero shift; need for lightning 

protection should be evaluated; 

push-in versions subject to several 

potential errors

Electrical-resistance piezometer

Easy to read; short lag time; minimum 

interference to construction; level of 

lead wires and readout independent 

of tip level; suitable for dynamic 

measurements; can be used to read 

negative pore pressures; no problems 

with freezing in cold weather

Low electrical output; lead wire 

effects; errors caused by moisture, 

temperature and electrical connections are possible; long-term 

stability is uncertain; need for 

lightning protection should be 

evaluated; push-in versions subject

to several potential errors

(ADAPTED FROM DUNNICLIFF, 1993)
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Instrumentation and Performance Monitoring

MAY 2009

13.2.2.3 Open-Standpipe Piezometers

The disadvantages of observation wells can be overcome using open-standpipe piezometers with 

sensing elements that are sealed in the zones where pore-water pressures are needed. Open-standpipe 

piezometers are suitable for a wide range of hydraulic conductivities and may even be acceptable in 

low hydraulic conductivity conditions where there is little concern for rapid changes in excess pore 

pressure. It is the most common type of piezometer used in coarse refuse embankment stages.

This type of piezometer is normally installed in a borehole supported by temporary casing or a string 

of hollow-stem augers. The casing or internal diameter of hollow-stem augers should be of a size to 

permit placement of the backfill around the piezometer pipe, usually at least twice the diameter of 

the piezometer. When the desired depth is reached, small-diameter plastic pipe with slotted-pipe or 

other type of screen at the appropriate interval is inserted into the temporary casing or augers, and 

the casing or augers are extracted while the plastic pipe is held in place. Traditionally, as the casing 

or augers are extracted, the annulus between the pipe and borehole walls is backfilled with granular 

material around the sensing zone, a bentonite seal is placed above the sensing zone, and a cementbentonite grout is placed above the bentonite seal to the ground surface. Many designers perceive 

that a bentonite seal is needed to protect the sand from being disturbed during grouting. However, 

if the sand zone is saturated, a well-designed grout with a creamy consistency will only penetrate 

the sand a few inches (Mikkelsen, 2002). Therefore, an alternative and simpler procedure for backfilling open-standpipe piezometers is to grout the annulus above the sand zone with cement-bentonite 

grout to the ground surface. The grout should be placed using a tremie pipe to avoid bridging of 

FIGURE 13.13 APPROXIMATE RESPONSE TIMES FOR VARIOUS

 TYPES OF PIEZOMETERS

(ADAPTED FROM TERZAGHI AND PECK, 1967)

TIME FOR 90 PERCENT RESPONSE (DAYS)

HYDRAULIC CONDUCTIVITY (CM/SEC)

CLOSED-DIAPHRAGM

PIEZOMETER

3/8-INCH OPEN-STANDPIPE PIEZOMETER (CASAGRANDE)

2-INCH OPEN CASING CLEANED OUT TO BOTTOM

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

10-10

10-11

2-INCH OPEN-STANDPIPE PIEZOMETER (8-INCH BOREHOLE)

SAND SILT CLAY

0.01 0.1 1 10 100 1000

FIGURE 13.8 APPROXIMATE RESPONSE TIME FOR VARIOUS TYPES OF PIEZOMETERS
13-26

Chapter 13

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materials in small-diameter holes or for placement at a substantial depth below the water table. Table 

13.7 presents a typical mix design for cement-bentonite grout for medium-to-hard-soil and soft-soil 

applications.

The sensing zone where pore pressure measurements are desired is typically a section of slotted pipe 

or an attached porous element for keeping granular filter material and soil/refuse from entering the 

piezometer. If a slotted zone is used, the slots should be sized using the criterion:

Slot Width ≤ 0.5 D85 of surrounding material (13-1)

where:

D85 = the soil particle size on a grain-size plot for which 85 percent of the soil sample by

 weight is smaller.

A schematic illustration of an open standpipe piezometer is provided in Figure 13.9.

Open-standpipe piezometers are often constructed using 2-inch-inside-diameter, flush-coupled, 

Schedule 80 PVC or ABS pipe with either cemented or threaded couplings that permit easy passage 

of an electrical or mechanical reading device for measuring the water level. When cemented couplings are used, one end of each pipe length should be machined as male and the other as female so 

that the coupling can be connected using a solvent cement. When threaded couplings are used, they 

should be self-sealing so that a watertight connection is achieved without any sealer. Smaller-diameter pipe may be used (a minimum diameter of 1 inch is recommended for operation of water level 

measurement equipment), but that may limit the ability to flush the piezometer or to conduct other 

measurements such as falling-head hydraulic conductivity tests or to obtain water samples. As the 

annulus around the standpipe is backfilled, the pipe should be maintained as straight as possible to 

avoid difficulty in lowering measuring devices to determine the level of the water surface.

In addition to using slotted PVC pipe in sensing zones, manufactured porous piezometer screens that 

admit water but not particulate matter may be used. The two most frequently used types are metal 

well points commonly used for shallow dewatering and porous tips specially made for piezometers, 

such as the Casagrande-type piezometer tip. In fine-grained deposits where electrolytic action may 

bring about formation of gas that can increase the lag time of a system or where water chemistry may 

cause corrosive damage, the use of nonmetallic piezometer tips is advisable. The Casagrande-type

TABLE 13.7 CEMENT-BENTONITE GROUT MIX

Materials

Grout for Medium to Hard Soils(1) Grout for Soft Soils(2)

Weight Ratio by Weight Weight Ratio by Weight

Water 30 gal 2.5 75 gal 6.6

Portland Cement 94 lb (1 sack) 1 94 lb (1 sack) 1

Bentonite(3) 25 lb (as required) 0.3 39 lb (as required) 0.4

Note: 1. The 28-day compressive strength of this mix is about 50 psi, similar to very stiff to hard clay.

 The modulus is about 10,000 psi.

2. The 28-day compressive strength of this mix is about 4 psi, similar to very soft clay.

3. Water and cement should be mixed prior to the addition of bentonite; sufficient bentonite (as required)

 should result in a smooth, thick mixture like thick cream or pancake batter.

(MIKKELSEN, 2002)
13-27

Instrumentation and Performance Monitoring

MAY 2009

piezometer tip, for example, consists of a one- to two-foot-long porous ceramic tube that can be connected to various sizes of riser pipe.

Continuous experienced supervision should be provided in order to maximize the potential for 

successful piezometer installation. Because of continuing construction activities at coal refuse 

embankments and the long service life, it is common to encounter loss of functionality or damage 

to piezometers, leading to a need for repair or replacement. At critical locations, particularly where 

access may be a concern, multiple installations should be considered.

Following the installation of an open-standpipe piezometer, measurements should be taken to verify 

that the piezometer is functioning as expected, and the top should be protected against accidental 

damage or vandalism. The hydraulic response of open standpipe piezometers can be checked by:

• Bailing or evacuating water from within the piezometer riser pipe and letting 

groundwater re-enter through the sensing zone (rising-head test)

• Adding water to the piezometer riser pipe and letting water flow out from the piezometer sensing zone (falling-head test)

• Inserting a solid rod into the standpipe and forcing water flow out from the piezometer sensing zone (slug test)

Selection of a method to check piezometer response depends on the depth of the sensing zone below 

the ground surface and the availability of an adequate supply of clean water to conduct a rising- or 

falling-head test. In general, rising- or falling-head tests are conducted when the groundwater level is 

shallow and clean water is readily available. If the water level is deep (greater than 50 feet) and/or an 

adequate water supply is not available, a slug test is typically used. ASTM D 4043, “Standard Guide 

for Selection of Aquifer Test Method in Determining Hydraulic Properties by Well Techniques,” 

should be followed for selection of suitable methods for checking the hydraulic response of openFIGURE 13.14 OPEN STANDPIPE PIEZOMETER/OBSERVATION WELL

POROUS FILTER

OR SLOTTED SCREEN

FILTER SAND

SAND

BENTONITE SEAL

GROUT

(DUNNICLIFF, 1993)

FIGURE 13.9 OPEN-STANDPIPE PIEZOMETER/OBSERVATION WELL
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Chapter 13

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standpipe piezometers. Care should be taken in using the results of slug tests to determine hydraulic 

conductivity of coarse-grained soils. Herzog (1994) reports that four commonly used methods to 

determine hydraulic conductivity in coarse-grained soils can yield results that differ by up to two 

orders of magnitude. By contrast, in fine-grained soils, the analytical method used to interpret slug 

tests typically results in consistent values of hydraulic conductivity.

If the material in which the piezometer tip is located is relatively permeable, these tests can often 

be conducted shortly after installation, because the flow into or out of the piezometer should occur 

rapidly. The procedure simply consists of: (1) reading the water depth; (2) changing the depth by bailing, adding water or inserting a slug; and (3) periodically reading the depth to determine the time 

required to return to the original measured level. Much greater care is required when the piezometer 

tip is located in a low-hydraulic-conductivity material and the flow of water is slow. In this situation, 

the test should be delayed until the system stabilizes following installation. Then only slight changes 

in level should be made; otherwise, the time for stabilization may interfere with required readings 

for which the piezometer was installed in the first place. A piezometer is considered to be developed 

when repeated testing results in nominal sediment detection in the sensing zone and when rising or 

falling head tests produce consistent results. Once operational, response tests should be conducted 

whenever the accuracy of readings from a piezometer is in doubt (e.g., readings are inconsistent with 

readings from other piezometers).

Figure 13.8 provides guidance regarding the approximate response times for various types of openand closed-system piezometers as a function of the hydraulic conductivity of the soil surrounding 

the sensing zone of the piezometer. As shown in the figure, the response time to achieve 90 percent equilibration increases with decreasing hydraulic conductivity and with increasing volume of 

the sensing zone (i.e., open-system piezometers are less responsive than closed-system diaphragm 

piezometers with pneumatic or vibrating wire systems). Hydraulic systems introduce additional 

delay in response time, associated with the length of tubing from the sensor to the readout location. 

Methods for estimating the response time are presented in Terzaghi and Peck (1967).

Open standpipe piezometers have often been lost due to debris dropped into the standpipe that 

either clogs the tip or precludes lowering of the water level measuring device. A grouted-in steel pipe 

with a removable cap can be installed at the top of the piezometer to protect temporary or permanent 

installations from accidental loss due to debris.

When an open-standpipe piezometer is installed in an area where the embankment height periodically increases, the riser pipe will need to be raised as fill is placed. To minimize the potential for 

surface water ponding and infiltration along the riser pipe/soil interface, the ground surface around 

the riser pipe should be elevated such that surface water drains away from the pipe.

Water levels in standpipe piezometers can be measured by a variety of methods, but the most 

common approach is an electrical water level meter. These devices typically consist of a two-conductor cable with a cylindrical stainless steel weight at the lower end. The weight is divided electrically into two parts and separated by a plastic bushing, and a conductor is connected to each part. 

The upper end is connected to a battery and a buzzer, light or ammeter. When the weighted probe 

is lowered into the piezometer, an electric circuit is completed when the probe encounters the water 

surface. The water depth is measured using permanent depth markings on the cable. Cable lengths 

over 1000 feet are available, but a probe with a 300-foot-long cable is usually sufficient for most 

coal refuse facilities. Special versions of these probes include features to measure temperature and 

conductivity. Other options for water level depth measurement include a weighted tape measure 

(for shallow-depth piezometers only), pressure transducer (Appendix 13A.2.4.3), ultrasonic transducer, and float and recorder system. Transducer and float/recorder systems are typically used with 

remote monitoring systems.
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Instrumentation and Performance Monitoring

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13.2.2.4 Closed-System Piezometers

Closed-system (pressure-transducer) piezometers have application when:

• Fine-grained material is being monitored such that response times for open-system 

piezometers would be too large.

• Remote readout is desired in order to minimize potential interference with and damage from earthwork equipment.

• Remote readout is desired so that measured piezometric levels can be integrated 

with data from other remotely monitored instruments.

• The material being monitored is not completely saturated.

Figure 13.8 provides guidance regarding the approximate response times for closed-system piezometers as a function of the hydraulic conductivity of the soil surrounding the sensing zone of the 

piezometer. As shown in the figure, the response time to achieve 90 percent equilibration increases 

with decreasing hydraulic conductivity. Hydraulic systems introduce additional delay in response 

time, associated with the length of tubing from the sensor to the readout location. Methods for estimating the response time are presented in Terzaghi and Peck (1967).

While pore pressures in unsaturated materials are not normally monitored at coal refuse disposal 

facilities, measurements in fine-grained materials (e.g., fine coal refuse or fine-grained foundation 

soils) and remote monitoring are situations when closed-system piezometers should be considered. 

As with instruments used for deformation and movement measurements, pneumatic, vibrating wire 

and electrical resistance transducers are available for measuring pore-water pressures. These transducers are typically installed in a borehole and backfilled with granular soil around the sensing 

zone, a bentonite seal above the sensing zone, and a cement-bentonite backfill to the ground surface. 

Recently, installation of closed-system piezometers within fully grouted boreholes has gained attention, as subsequently discussed. This installation method eliminates the cumbersome installation of a 

sand pack and bentonite seal, facilitates the installation of several piezometers in a nested configuration, and has a lower cost and faster installation than the traditional approach.

Pneumatic piezometers have advantages where high thunderstorm activity is anticipated. However, if 

pneumatic piezometers are used, normally-closed transducers are preferred to normally-open transducers because less diaphragm displacement is required for measuring the pore-water pressure in the 

sensing zone (Dunnicliff, 1993). The transducers are typically housed in corrosion-resistant plastic or 

stainless steel. A normally-closed pneumatic piezometer installation is illustrated in Figure 13.10.

Vibrating-wire piezometers have a metallic diaphragm separating the pore water from the measuring 

system (Dunnicliff, 1993). These devices are typically housed in stainless steel. Most vibrating-wire 

piezometers have a dried and hermetically-sealed cavity around the sensor to minimize the potential 

for corrosion, a thermistor to adjust for temperature effects, and some have an in-place check feature 

to account for zero drift and to permit in-service calibration. A vibrating-wire piezometer installation 

is illustrated in Figure 13.11.

Electrical-resistance piezometers are housed in stainless steel to resist the effects of lateral stresses 

on the device and to minimize corrosion. These devices are manufactured using semiconductor, 

unbonded and bonded electrical-resistance gages. An electrical-resistance piezometer installation is 

illustrated in Figure 13.12. These piezometers are constructed with an intake filter that separates 

the formation pore fluid from the diaphragm and sensor. The filter must be strong enough to not be 

damaged during installation and to resist embedment stresses without undue deformation. For coal 

refuse facilities, piezometers should be constructed using a coarse, low-air-entry filter that readily 

allows the passage of both gas and water. Typical low-air-entry filters have a pore diameter of 20 to 
13-30

Chapter 13

MAY 2009

80 microns (0.0008 to 0.0031 in) and air entry values ranging from 3 to 30 kPa (0.4 to 4.0 psi). Intake 

filters should be saturated prior to installation.

Contreras et al. (2007) have demonstrated the viability of directly surrounding diaphragm piezometers 

with cement-bentonite grout. Through computer simulation, laboratory testing, and field demonstration, they illustrated that a properly designed cement-bentonite grout mix will allow transmission 

of a low volume of pore water over a short distance (between the borehole wall and piezometer), yet 

maintain an overall low hydraulic conductivity in the vertical direction, thus isolating the instrument. 

The cement-bentonite grout should have a hydraulic conductivity of not more than 1,000 times the 

formation hydraulic conductivity to be effective, and trial mixes of typical water-cement-bentonite 

mixtures resulted in hydraulic conductivities between 10-5 and 10-7 centimeters per second. Note that 

the piezometers shown in Figures 13.10, 13.11 and 13.12 could also be installed using full grouting as 

described by Contreras et al. (2007).

FIGURE 13.15 NORMALLY-CLOSED PNEUMATIC PIEZOMETER

SAND

BENTONITE SEAL

GROUT

FILTER

GAS SUPPLY

GAS FLOW

CONTROLLER

GAS FLOW METER

(OMITTED IN SOME VERSIONS)

PRESSURE

GAGE

INLET

TUBE

VENT TO

ATMOSPHERE OUTLET

TUBE

FLEXIBLE DIAPHRAGM

ATTACHED TO TRANSDUCER

BODY AROUND RIM

TRANSDUCER BODY

(DUNNICLIFF, 1993)

FIGURE 13.10 NORMALLY-CLOSED PNEUMATIC PIEZOMETER
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Instrumentation and Performance Monitoring

MAY 2009

13.2.3 Surface Water Flows and Hydrologic Parameters

Monitoring of surface flows at a coal refuse facility site has the following purposes:

• To develop an understanding of seepage flows, either through, beneath or around an 

embankment and to evaluate the significant changes as the embankment configuration changes with time.

• To detect an unusual change in seepage quantity that may be an indicator of a developing seepage/stability problem.

• To determine discharges associated with runoff from storms for hydrology/hydraulic 

analyses and for environmental purposes.

• To determine discharges from mines with workings in proximity to an impoundment.

Monitoring of flows from coal refuse embankments through visual observation should be routine for 

all coal refuse disposal facilities, and quantitative monitoring of seepage through instrumentation 

or direct measurement should be performed during construction and operation of impoundments. 

Monitoring of surface runoff occurs much less frequently and usually is performed only when there 

are concerns related to flow diversion.

13.2.3.1 Measurement of Impoundment Water Level

Frequent monitoring of the water level at a coal refuse facility impoundment is important because:

FIGURE 13.16 VIBRATING-WIRE PIEZOMETER

SIGNAL

CABLE FREQUENCY

COUNTER

P = K ( f 2 - f 2 )

DIAPHRAGM

ELECTRICAL COIL

TRANSDUCER BODY

VIBRATING WIRE

SAND

BENTONITE SEAL

GROUT

1345

1344590L5

FILTER

(DUNNICLIFF, 1993)

FIGURE 13.11 VIBRATING-WIRE PIEZOMETER
13-32

Chapter 13

MAY 2009

• The pool elevation can change rapidly with time due to runoff from rainfall, spring 

flow from the watershed, inflow from the processing plant, increases in the level 

of settled solids, outflows through the decant and spillway systems, and seepage 

through the embankment.

• The level of water in the impoundment directly affects the rate of seepage flow and 

the groundwater conditions (pore pressures, piezometric levels, moisture contents) 

within the embankment.

• Existing reservoir conditions should be regularly checked for comparison with the 

storage capacity and freeboard requirements in the approved design plan.

Several types of gages for measuring water level are available. The simplest instrumentation is a staff 

gage consisting of a calibrated rod driven or concreted into solid ground or attached to a structure 

such as a decant tower. A staff gage allows the reservoir water elevation to be quickly and accurately 

determined and recorded during regular impoundment inspections. The location of the calibrated 

rod must be chosen so as not to be affected by changes in facility configuration and deposition of 

fine refuse and to be easily accessible. Often, this can be best accomplished by installation of several 

short gage rods with overlapping elevation markings in a stepwise arrangement along the upstream 

reservoir slope or along the hillside above the reservoir.

Continuously recording gages use floats within a stilling well for measuring the water level. The 

sensitivity of these instruments to siltation and changes in the level of settled solids make their use 

unattractive at most facilities.

For any type of gage, an accurate datum must be established so that the monitored pool elevation 

can be related to the elevations of other facility components. Care should be taken to minimize the 

potential effects of settlement, tilting, sliding and/or sloughing on the gages.

13.2.3.2 Measurement of Seepage Flows and Other Surface Water Flows

13.2.3.2.1 Importance of Seepage Flows

Seepage flows normally occur in one or a combination of the following forms:

• Flows from internal drainage systems that collect water seeping through the embankment for controlled discharge at the downstream toe.

• Underdrain systems installed (normally at non-impounding facilities) for collecting and transporting groundwater flow from springs through a drain system to the 

downstream toe without infiltration into the embankment material.

• Very low volume seepages that may emerge on the downstream embankment face.

• Flows from concentrated spring areas along abutments or in the valley bottom 

downstream from the embankment including those connected to abandoned mines.

• General upwelling of water into a valley bottom over a wide area with no single 

concentrated location.

• Seepage into or out of mine workings in the vicinity of the impoundment area.

Seepage flow rates are an important indicator of the performance of a facility’s seepage collection and 

control system, which is a critical element in the safety of an impounding embankment. An unusual 

reduction in seepage flow may indicate clogging of an internal drain, while an unusual increase may 

be a sign of internal erosion. Unusual changes in seepage quantities must be detected and further 

investigated, as they could lead to stability problems or even failure of the embankment. For this 

reason, seepage flow rates should be routinely monitored, especially at high and significant hazard 

potential impoundments.
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Instrumentation and Performance Monitoring

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Where seepage flows are through a pipe system, the flow volume normally can be monitored by:

• Collecting water in a calibrated bucket for a measured period of time.

• Calibrating the rate of flow versus the water depth in the discharge pipe.

Small seeps are normally monitored either by visual observation or by construction of small containments with discharge through a pipe. The discharge from the pipe can be collected in a bucket or 

the depth of flow can be correlated to the rate of flow. Large discharges downstream from springs or 

underdrains are usually monitored by a weir installed across the flow path. Widespread downstream 

seepage should be monitored during periods of dry weather when runoff from rainfall is not occurring. Flows can be measured at downstream culverts or by installing a weir at a point where all of the 

seeping water passes.

If collected seepage water must be treated prior to discharge, the treatment system may include a 

flow measuring device (e.g., flow meter, flume, continuous recorder) to control the rate of chemical additions or the retention time. These flow measuring devices can also be used for seepage rate 

monitoring.

The following characteristics of seeping water are useful for evaluation:

• Quantity of flow

• Temperature at the discharge point

FIGURE 13.17 BONDED ELECTRICAL-RESISTANCE PIEZOMETER

BONDED ELECTRICAL

RESISTANCE STRAIN GAGE

PRESSURE TRANSDUCER

SIGNAL

CABLE

READOUT

UNIT

SAND

BENTONITE SEAL

GROUT

1345

1344590L5

FILTER

(DUNNICLIFF, 1993)

FIGURE 13.12 BONDED ELECTRICAL-RESISTANCE PIEZOMETER
13-34

Chapter 13

MAY 2009

• Quantity of suspended solids

• Water chemistry

Suspended solids in seepage can be an important indicator of piping, which could result in significant increases in seepage and even eventual impoundment failure. Determination of the presence of 

suspended solids is usually made subjectively by visual observation (e.g., the water is collected in a 

container and checked for fines), but if a developing problem is suspected, installation of sediment 

traps and/or quantitative testing may be necessary.

Evaluation of water chemistry can be an important indicator of the source of seeping water. For 

example, when seepage increases erratically with time, the source of the seepage may be associated 

with local groundwater conditions rather than the refuse facility impoundment. Comparison of the 

chemistry of the impounded water or groundwater samples to that of the seeping water can provide 

correlations for identification of the source of the seepage. However, determination of water chemistry represents a significant increase in the complexity of the monitoring program and generally is 

not performed unless other data indicate that it is needed. Wells completed in site geologic strata for 

monitoring the impact of the disposal facility on the natural groundwater can be useful for establishing local groundwater quality and evaluating seepage paths.

When required, pH, specific conductance, and dissolved oxygen can be measured with relatively 

simple field water quality testing equipment or continuous recording equipment. If other chemical 

analyses are required, water samples must generally be collected and subjected to laboratory analytical testing. Key indicators in areas where refuse disposal facilities are normally located include 

sulfate, total and ferrous iron, manganese, acidity/alkalinity, dissolved solids and suspended solids. 

The use of amendments with coal refuse disposal may suggest other key indicators (e.g., chloride for 

combustion waste).

13.2.3.2.2 Measurement of Seepage Flow Rates

The primary purpose for measuring seepage rates is to verify that the magnitude of seepage is reasonable in relation to the potential sources such as the upstream impoundment and local groundwater conditions that can vary with seasonal rainfall. The most important observed condition, and 

one that requires immediate attention, is when long-term relatively constant flows increase rapidly 

without a corresponding change in upstream conditions.

A number of methods for measuring flow from pipes have been developed and are described in the 

Water Measurement Manual (USBR, 2001) or other textbooks on hydraulics such as Chow (1959) and 

Brater et al. (1996). These methods include, for example:

• Direct volume measurement using a calibrated bucket, pan or tank

• Calibrated weirs

• Flow nozzles and orifices

• Venturimeters

• Current meters

• Commercial water meters

• Flumes

For small flows, the simplest procedures for measuring the rate of flow are: (1) by filling a calibrated 

bucket during a measured time (volume per time) and (2) calculating the flow based on the geometry 

of the discharging water surface, provided the pipe discharges freely. Figure 13.13 illustrates the following methods for measuring the flow rate at open-ended horizontal and vertical pipes:
13-35

Instrumentation and Performance Monitoring

MAY 2009

• The California method (Figure 13.13a) for pipes not flowing full, where the measured 

trajectory and depth to water at the end of the pipe is used to calculate flow rate.

• The Purdue method (Figure 13.13b) is preferred for pipes flowing full or more than 

50 percent full. For this method, the x and y coordinates of the discharge are measured, and empirical graphs provided by the USBR (2001) are used to determine the 

corresponding flow.

• The third method (Figure 13.13c) is used for estimating discharge from a vertical 

pipe. The rate of flow is determined from the curves presented on the figure based 

on the height of the jet and the inside diameter of the pipe.

Calibrated weirs are a simple and reliable means for monitoring flow. If a weir is being used to monitor seepage flow or discharge from a mine, the flow should not be comingled with surface runoff. A 

V-notch weir is frequently used for monitoring low flows because the decreasing width of the weir 

with decreasing flow maximizes the height of flow and enables an accurate determination. Situations 

where use of a V-notched weir is appropriate are presented in USBR (2001). If monitoring is required 

for only a short period of time, a V-notch weir can be as simple as a piece of exterior plywood (with 

the V-notch) installed into the ground to a sufficient depth and width that there is no seepage flow 

beneath or around the weir. The disadvantage of this type of weir is that its life is very limited because 

of deterioration of the wood when exposed to water and weather conditions. Another alternative for 

this type of weir is construction of a concrete head wall with a plastic or steel plate forming the weir. 

For facilities where the water quality may have corrosive characteristics, the V-notch plate can be 

made of fiberglass or stainless steel to extend the useful life.

Figure 13.14 shows typical construction details for a concrete weir with an attached plate, and Table 

13.8 summarizes necessary conditions for accurate measurement for sharp-crested weirs (USBR, 

2001). Two important construction details for V-notched weirs are: (1) sealing of the entire upstream 

side of the weir so that no water flows under or around the weir and (2) placing sufficient riprap or 

other channel protection downstream to prevent erosion in the area where the weir discharges.

To avoid flow restriction, a free-fall condition where the nappe does not cling to the downstream 

side of a weir should occur. However, under field conditions, it is sometimes difficult to locate a weir 

so that a free-fall condition will occur at all discharges. Figure 13.15 provides formulas and a series 

of curves for estimating flow through a V-notch weir under free-fall conditions (design conditions), 

as well as an approximate method for estimating flow with downstream submergence of up to 90 

percent of the depth of flow through the weir. If the weir will be frequently submerged and accurate 

measurements are necessary, modifications to the weir structure may be necessary. When tables or 

graphs are used to determine the flow rate over weirs, care must be taken that the water measurement location and weir type match the conditions for which the tables or graphs were developed.

A rectangular weir should be used when relatively large flows are anticipated and should be designed 

by an engineer with experience related to weirs. The larger potential flows make construction details 

for a rectangular weir very important if excessive erosion and undercutting that could destroy the weir 

during high flow periods are to be avoided. The rate of flow over a rectangular weir is a function of the 

weir length and width and the shape of the crest and sides. Calibration of a rectangular weir should be 

performed in the field. An approximation of flow rate is given by the following formula (USBR, 2001):

Q = Ce Le H 3/2 (13-2)

where:

Q = discharge (length3/time)

Ce = 3.22 + 0.4 (H/P) for L/B equal to 1 (weir constant)
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Chapter 13

MAY 2009

Le = equivalent length of weir, approximately equal to weir length L (length)

H = measured head on the weir (length)

P = crest height of weir (length)

B = average width of approach to weir (length)

Refinements in estimating the effective coefficient of discharge (Ce) for other values of L/B are presented in USBR (2001). Other types of weirs have been used and are appropriate if designed and 

installed in accordance with the intended use. These types of weirs are discussed in the standard 

references on hydraulics, such as USBR (2001), Chow (1959), and Brater et al. (1996).

13.2.3.3 Measurement of Rainfall and Snowfall

Local precipitation data are needed at impoundment sites where differentiation of reservoir seepage 

from other sources is important. This need may occur at impoundment sites where underground 

mines are located below or near an impoundment and where it may be critical to monitor the rate 

and volume of water that can infiltrate the mine workings and affect mine breakthrough potential. 

Rainfall data may also be useful in evaluating how groundwater flow contributes to abutment seepage. Often, data from nearby meteorological stations operated by the U.S. Weather Bureau can be 

used for estimating precipitation at a site. However in mountainous regions or where the density of 

existing stations is low, rainfall and snowfall at a refuse disposal facility can vary significantly from 

measurements at meteorological stations. At these sites, simple precipitation measuring gages can 

be installed for daily measurement and recording of precipitation. If more information is required 

or if site personnel are not available to record daily measurements, a more sophisticated monitoring system with continuous recording may be appropriate. Standard equipment and procedures for 

monitoring climatic conditions are discussed by Linsley et al. (1982).

13.2.3.4 Observation of Temperature and General Weather Conditions

Whether a precipitation station is established or not, monitoring of temperature and general weather 

conditions (e.g., cloudy, sunny) should be performed at a refuse disposal site. This information is 

often useful when reviewing periodic inspection reports, assessing requirements for maintenance or 

evaluating unexpected conditions that have occurred.

13.2.4 Miscellaneous Instrumentation and Monitoring

The most commonly used monitoring instruments are those for measuring static displacements, 

pore-water pressures and water flow, as discussed in the previous sections. Other, less often used but 

frequently necessary, instrumentation is discussed briefly in the following subsections.

13.2.4.1 Measurement of Soil Pressure

Soil pressures on structures such as conduits installed under and through embankments are important. Two types of instruments for measuring total stress in soil are available: embedment cells and 

contact earth pressure cells (Dunnicliff, 1993). Embedment earth pressure cells are used to measure 

total stress in a soil mass, but these instruments are beset with problems that render most measurements useless. Most of the problems are attributable to errors associated with conformance between 

the cell and the surrounding soil mass. Dunnicliff (1993) describes these problems in detail. If the 

total soil pressure at points in a refuse embankment is needed for verification of design assumptions, 

field measurements of the in-place density should be considered in lieu of measuring total pressures 

with embedment pressure cells. For situations where data on the performance of a conduit is desired, 

monitoring of the internal deformation of the conduit will likely produce more useful results. Section 

12.2 provides guidance for monitoring the performance of deeply buried conduits with respect to 

the effects of earth pressure. Conduit deflection measurements can be obtained with laser or optical

equipment using the laser ring method.
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Instrumentation and Performance Monitoring

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13.13c DISCHARGE CURVES FOR MEASUREMENT OF

 FLOW FROM VERTICAL STANDARD PIPES

FIGURE 13.18 MEASUREMENT OF FLOW FROM PIPES

13.13b PURDUE METHOD OF MEASURING

 FLOW FROM A HORIZONTAL PIPE

x

d

y

30 100 200 400 1000 2000 3000

1 1/2

2

3

4

5

6

8

10

15

20

30

40

50

60

HEIGHT OF JET, H (IN)

FLOW, Q (GPM)

INSIDE DIAMETER, d (IN)

2

3

4

5

6

7

8

10

12

FLOW

d

H

600

13.13a TYPICAL ARRANGEMENT FOR MEASURING

 FLOW BY THE CALIFORNIA PIPE METHOD

Q = DISCHARGE IN CFS

a = DISTANCE IN FEET MEASURED IN THE PLANE OF THE

 END OF THE PIPE, FROM THE TOP OF THE INSIDE

 SURFACE OF THE PIPE TO THE WATER SURFACE

d = INTERNAL DIAMETER OF THE PIPE IN FEET

d

> 6d

a

HORIZONTAL

DISCHARGE PIPE

FLOW

Q = 8.69 (1 - ) a

d d 1.88 2.48

WHERE:

FIGURE 13.13 MEASUREMENT OF FLOW FROM PIPES
13-38

Chapter 13

MAY 2009 FIGURE 13.19 TYPICAL V-NOTCH WEIR

13.14b ELEVATION

12"

3"

12"

3"

6" 6" 18" 18" 6" 6"

6"

CL WEIR

3"

SEE

DETAIL A

2' MINIMUM

EMBEDMENT

ANCHOR BOLTS

STAFF 

GAUGE

CHANNEL APPROACH WIDTH (B)

P

45

FLOW

1/8" THICK

FIBERGLASS

OR METAL

SHEET

DETAIL A

H

DETAIL B - FLOW AT WEIR NOTCH

13.14a PLAN

MASTIC 4"

SEAL

2' MINIMUM EMBEDMENT

INTO IMPERVIOUS MATERIAL

OR ROCK

CONCRETE

STAFF GAUGE

STAFF GAUGE

CL WEIR

RIPRAP OR GRAVEL

PROTECTION AGAINST

SCOURING

1/8" THICK FIBERGLASS

OR METAL SHEET

FLOW

~ 4H 

FIGURE 13.14 TYPICAL V-NOTCH WEIR
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Instrumentation and Performance Monitoring

MAY 2009

Contact earth pressure cells can be used to measure total soil stress against a buried structure such 

as a wall or conduit. These instruments do not experience the problems associated with embedment 

cells, and it may be possible to measure total stress at the face of a structure with reasonable accuracy if issues of cell stiffness and temperature are considered during the instrumentation planning 

process. Dunnicliff (1993) recommends that diaphragm cells used for this application have two active 

faces (i.e., pressure is measured both on the cell face against the structure and on the face against 

the soil fill) and that hydraulic-type cells be avoided because they cannot be mounted flush with the 

structure face. Even if these guidelines are followed, however, considerable attention must be given 

to the number of cells needed, laboratory calibrations, temperature effects, cell stiffness, irregularities 

on the structure face, and a number of installation details. Therefore, if project requirements demonstrate a need for earth pressure measurements, earth pressure cells should be used with caution and 

with modest expectations for the outcome.

13.2.4.2 Measurement of Blast Vibrations

Portable seismographs are generally used for isolated cases where vibrations associated with blasting 

could affect structures of interest. The seismograph sensors are either accelerometers or velocity transducers that produce a record of the motion with time, usually in the form of particle velocity, although 

particle displacement and particle acceleration measurement are also possible. Velocities are measured 

triaxially (i.e., with respect to vertical, longitudinal and transverse axes) at the location where the instrument is anchored. If multiple instruments are installed at a site, they should be aligned in the same 

direction (e.g., with the longitudinal axes parallel to the long axis of the impoundment). Field instruments can be set to trigger at a specified velocity and can record and store hundreds of events that can 

then be downloaded to a personal computer or transmitted to a remote location. The frequency content 

of blast vibrations is typically much higher than would affect a coal refuse or earth dam. In most cases, 

blast monitoring is undertaken at surface structures (i.e., near local buildings) to verify that the blast 

ground motions do not exceed permissible standards. Section 6.6.7 addresses blasting impacts.

TABLE 13.8 GUIDANCE FOR ACCURATE WEIR FLOW MEASUREMENT

1. The upstream face of the weir plates and bulkhead should be plumb, smooth, and normal to the axis of the 

channel. Weir plate fasteners should be located and installed in a manner that minimizes effects on the flow and 

water surface.

2. The entire crest should be level for rectangular shapes, and the bisector of V-notch angles should be plumb.

3. The edges of the weir opening should be located in one plane, and the corners should have the specified angles.

4. The top thickness of the edges of the crest and side plates should be between 0.03 and 0.08 in.

5. All weir plates should have the same thickness for the entire boundary of the overflow crest. If the plates are 

thicker than specified in Item 4 above, the plate edges should be reduced to the required thickness by chamfering the downstream edge of the crest and sides to an angle of at least 45 degrees; an angle of 60 degrees is 

highly recommended for a V-notch to help prevent water from clinging to the downstream face of the weir.

6. The overflow sheet or nappe should touch only the upstream faces of the crest and side plates. Low flow conditions (< 0.2 ft of head) may result in the nappe clinging to the downstream weir face. If such conditions are 

sustained and accurate measurements are required, a reduced-size weir may be needed.

7. The measurement of head on the weir is the difference in elevation between the invert of the weir and the 

water surface at a point located upstream from the weir a distance of at least four times the maximum head on 

the crest.

8. Approach flow conditions should be fully developed, mild in slope, and free of curves, projections and waves. 

The approach flow velocity should generally be less than 0.5 ft/sec.

(ADAPTED FROM USBR, 2001)
13-40

Chapter 13

MAY 2009

~ 4H1

SECTION A - A (DESIGN CONDITION)

FLOW

STAFF 

GAUGE

P

H1 0.2 FEET,

MINIMUM

FIGURE 13.20 FLOW DEPTH-DISCHARGE RELATIONSHIP

 FOR 90-DEGREE, V-NOTCH WEIR

EQUATIONS FOR DESIGN DISCHARGE

Q = 4.28 Ce H1

5/2

Ce = 0.58 (FULLY CONTRACTED)

0

0.5 1.0

0.5

1.0

SUBMERGENCE, H2

H1 EFFECT OF SUBMERGENCE, K

0

2.0

DISCHARGE, Q (CFS)

1.5

1.0

0

0.5

H1 (FT)

0 0.5 1.0 1.4

H2 (FT)

90

A

A

B

H1/P

Ce

Ce (PARTIALLY CONTRACTED)

P/B = 1.0

0.8 0.6 0.4 0.3

0.2

0.1

0.60

0.58

0.56 0 0.2 0.4 0.6 0.8 1.0 1.2

FLOW

EQUATIONS FOR SUBMERGED DISCHARGE

Q = KCH1

n

H1

K = 1-

H2 n 0.385

C ~ 2.52

n ~ 2.47

P

H1

> 1

( )

P

H1

H2

45

SECTION A - A (SUBMERGED CONDITION)

FIGURE 13.15 FLOW DEPTH–DISCHARGE RELATIONSHIP FOR 90-DEGREE, V-NOTCH WEIR
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Instrumentation and Performance Monitoring

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13.2.4.3 Measurement of Temperature

Evaluation of the extent of (or potential for) burning in a refuse embankment or in coal seams requires 

monitoring of temperatures at and below the ground surface. Internal temperature can be monitored 

by permanently-installed thermocouples on probes driven into an embankment. Also, temperatures 

in an embankment or in abutment coal seams can be measured by lowering conventional thermometers or telethermometers (remote sensing thermometers) down boreholes or piezometer standpipes.

Evaluation of surface temperatures over large areas can also be performed using remote sensing technology, including thermal imaging. Aircraft-mounted thermal imaging can cover very large areas 

and can detect minor variations in the ground surface temperature, which can assist in site characterization and on focusing subsurface exploration efforts.

13.3 INSTRUMENTATION MAINTENANCE

While a functional and reliable instrumentation system can be achieved with proper instrument design 

and installation, the system must be maintained so that it will continue to be functional and reliable 

in service. This section provides a discussion of general maintenance and recalibration practices.

13.3.1 Importance of Maintenance and Recalibration

Careful attention to factory calibration, pre-installation acceptance tests, installation, and post-installation acceptance tests should result in an instrumentation system that performs in accordance with 

expectations. To keep instruments operating satisfactorily during their service life, regular maintenance and recalibration are required. Lack of regular maintenance and recalibration can result in 

erroneous data that can lead to incorrect conclusions regarding facility performance. Lack of maintenance can also jeopardize the functioning of an instrumentation system leading to the loss of instruments. Malfunctioning or damaged instruments will normally need to be replaced or repaired.

Maintenance and recalibration activities should be conducted in accordance with a plan for regular 

instrument calibration and maintenance, as described in Step 8 in Table 13.1. Maintenance procedures 

should be based on the manufacturers’ instruction manuals and on-site conditions, as indicated in the 

design plans. These procedures should include preventative maintenance program schedules, troubleshooting, cleaning, drying, lubricating, battery servicing, and repair and replacement instructions 

for each type of instrument. Maintenance and recalibration should be the responsibility of personnel 

responsible for instrument monitoring, and it is essential that these individuals be knowledgeable 

about the types, expected and actual performance, and the maintenance and recalibration requirements of the instrumentation and data acquisition systems for the facility.

Personnel responsible for instrumentation maintenance and recalibration should be reliable, dedicated, and motivated individuals who pay attention to detail. They should have a background in the 

fundamentals of geotechnical engineering and should understand mechanical and electrical equipment. They should understand the purpose of instrumentation and how the instruments function. 

Maintenance and recalibration of instrumentation should be supervised by the data collection personnel, who are most likely to notice potential problems and to observe malfunctions, deterioration, 

or damage.

A service history record of maintenance, recalibration, repair, and replacement of the components of 

the instrumentation system should be maintained, and this record should be reviewed by the personnel responsible for evaluating facility performance. The service history should include dates, observations, problems that occurred, measures taken, and the personnel who were involved. A service 

history documents the general behavior of the instrumentation system and can serve as a guide to 

future maintenance activities. The service history also facilitates the transfer of responsibility associated with turnover in personnel.
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Chapter 13

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Spare parts and interchangeable components should be available for replacing failed or questionable instrument components without interrupting system operation. An inventory of spare parts and 

instruments should be available and should be updated as necessary. Spare readout units should be 

available in the event of malfunction of the primary readout units.

13.3.2 Recalibration and Maintenance during Service Life

When instruments are in service, they require regular recalibration and maintenance if their normal 

operational characteristics are to be maintained. Generally, recalibration should be performed on a 

defined schedule, as documented in the Operation and Maintenance Plan. Routine recalibration of 

instrumentation system components is best performed by personnel responsible for data collection 

with the knowledge of personnel responsible for data analysis.

Instrumentation data should be carefully examined for indications that recalibration is needed. If 

abrupt changes or unexplained long-term trends in data are observed without apparent reason, the 

affected instrument should be checked to determine whether or not the data reflect actual conditions 

or are the result of instrument malfunction. If a need for recalibration is indicated, procedures should 

follow the instrument manufacturer’s recommendations. General guidelines for the recalibration and 

maintenance of geotechnical and structural instrumentation systems and associated data acquisition 

equipment are provided in USACE (1995c).

In addition to regular recalibration, weirs, flumes, or other water measurement devices require regular maintenance so that: (1) water does not bypass the device, (2) leakage around or under the device 

is sealed, and (3) sediment deposition has not altered the approach conditions for the device.

13.4 AUTOMATED DATA ACQUISITION

Automated data acquisition systems have been demonstrated to be reliable and cost effective for 

monitoring the performance of constructed works and their surrounding environment, especially 

at sites where access is difficult or where regular survey and/or monitoring control are not practical or economical. With time, automated data acquisition systems will continue to improve with 

the likelihood that their use at coal refuse facilities becomes more common. Where deployed at coal 

refuse disposal facilities, automated data acquisition systems have been generally used for monitoring impoundment pool and piezometer levels.

Most automated data acquisition systems are programmed to retrieve data from multiple instruments on a prescribed schedule, process the readings to present the results in terms of engineering dimensions, and store or transmit the data. System components and design are discussed in 

Appendix 13A.4.

Figure 13.16 illustrates the application of an automated data acquisition system at a coal refuse disposal impoundment for monitoring piezometer levels and transmitting the data to a mine office more 

than a mile away. Features such as data collection from multiple piezometers, radio transmission, 

data storage and distribution of data by e-mail have been incorporated into monitoring programs. 

Quaranta et al. (2008) discuss an automated data acquisition program used for collecting the following data at a coal refuse disposal facility: water level, pH, specific conductance, and temperature at 

piezometers; water discharge; and the impoundment pool level. Weather station data (ambient air 

temperature, barometric pressure, rainfall, and wind speed) were also collected. This reference presents the system design, equipment, and operation issues encountered.

13.5 INSTRUMENTATION COSTS

Instrumentation systems are typically employed at refuse embankments and dams for monitoring 

water levels, pore-water pressures, deformations, and other important parameters depending upon 
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Instrumentation and Performance Monitoring

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site conditions. For below-grade instrumentation used on conventional geotechnical engineering 

projects, the initial purchase price of an instrument and the cost of installation represents 15 to 30 percent of the total instrument life-cycle costs, which also include periodic readings, maintenance, data 

analysis and interpretation, data management, and eventual decommissioning (McKenna, 2006). 

Based on experience from numerous instrumentation projects, McKenna (2006) concluded that the 

typical life-cycle cost for an instrument employed for conventional geotechnical engineering projects 

is about 15 times the initial purchase price. For coal refuse disposal facilities that can operate over 

very long periods, the initial price of an instrument may represent an even smaller percentage of the 

life-cycle costs. McKenna (2006) offers the following lessons learned:

Lesson 1 – The best way to save money is to install instruments that are suited to the sitespecific and performance conditions critical to the safe operation of the structure. This lesson reinforces several steps in the planning process that were discussed in Section 13.1.

Lesson 2 – The actual cost of the instrument is very small compared to the life-cycle costs,

so instruments should be purchased with reliability in mind.

Lesson 3 – Every instrument should be installed with care, should be maintained, and

should be read by a trained and diligent technician in accordance with project 

quality control procedures.

Some additional thoughts on instrumentation are provided in the following:

• The greatest cost of instrumentation is obtaining the readings, and the greatest risk is 

the additional expense associated with poor quality or inaccurate readings.

• Instrumentation suppliers should be viewed as a partner in the complete process,

especially for designers who have limited experience with instrumentation.

1000 FT.

SCALE

COMPUTER STATION

TO DISTRIBUTE

DATA VIA E-MAIL 

RADIO SIGNAL

REPEATER

(SOLAR POWERED)

DATA COLLECTION AND

RADIO TRANSMISSION

STATION (SOLAR POWERED)

SIGNAL

CABLE

VIBRATING WIRE

PIEZOMETERS

COAL REFUSE

DAM

DATA LOGGER AND

SOLAR PANEL

FIGURE 13.16 AUTOMATED DATA ACQUISITION SYSTEM AT COAL 

REFUSE DISPOSAL FACILITY
13-44

Chapter 13

MAY 2009

• Measures should be taken to see that data reading procedures are in place for each 

type of instrument, personnel are trained with respect to the procedures and equipment, readout equipment is properly maintained and regularly calibrated, and all 

raw data is recorded, plotted and evaluated according to project requirements.

• The same technician and equipment should be used to obtain all instrumentation 

readings to the extent possible. Automated data acquisition systems can improve the 

consistency and reliability of the data while providing more continuous monitoring.

• Instruments should be carefully protected because replacement can be difficult 

and costly.

• In areas where it would be particularly difficult to replace a failed instrument, redundant instruments should be considered.

• As time progresses, some opportunities for instrumentation system improvement 

may become apparent. Changes should be implemented, as needed, to improve the 

data gathering process and the quality of the data obtained.

This chapter has provided a discussion of the important aspects of implementing an instrumentation 

program at a coal refuse disposal facility including planning, instrument selection, installation, and 

data retrieval and processing. The benefits of well planned and properly installed and maintained 

instrumentation include:

• Enhancing the ability to compare the actual performance with the performance anticipated by the designer

• Enhancing the ability to recognize problem conditions before they become serious

• Aiding in the design and construction of expansion plans or remedial measures 

when they are needed

In summary, the cost of geotechnical instrumentation should be considered in context with the benefits that can be achieved, especially given the long in-service life of most coal refuse facilities.
Appendix 13A

MEASUREMENTS, TRANSDUCERS AND

DATA-ACQUISITION SYSTEMS

MAY 2009 13A–1

This appendix provides information regarding measurements and measurement uncertainty, 

transducers and data acquisition systems supplementing the general discussion of these topics 

in Chapter 13.

13A.1 MEASUREMENT AND MEASUREMENT UNCERTAINTY

Quantitative measurements obtained by field instrumentation provide a means for confirming 

a safe and efficient design and verifying safe and economical construction. Because all measurements involve error and uncertainty, the measures by which error and uncertainty are defined 

should be considered as part of the field instrumentation design. The standard measures used 

to define error and uncertainty in instrument measurements include:

• Conformance is the ability of an instrument to measure performance without affecting the measurement. Similar to in-situ structures, instruments that are stiffer than 

the surrounding media tend to attract load while instruments that are softer than the 

surrounding media tend to shed or arch load away. Therefore, instrument behavior 

should match the behavior of the medium in which it is installed as closely as possible.

• Accuracy is the closeness of a measurement to the true value and is determined during calibration by comparison to the true value.

• Precision is the closeness of several measurements to the mean value and is synonymous with reproducibility. The difference between accuracy and precision is illustrated in Figure 13A.1.

• Resolution is the smallest division on the instrument readout scale.

• Sensitivity is the amount of output response an instrument produces in response to 

an input quantity (e.g., 1000 millivolts per inch)

• Linearity refers to the proportionality between the indicated value and the actual 

value. Thus an instrument with a linearity of 1⁄2 percent full scale means that the 

maximum error resulting from a linear calibration will be 1⁄2 percent of the full-scale 

reading.

• Hysteresis is a property of systems that do not instantly respond to the forces applied to them, but react slowly, or do not return completely to their original state.

• Noise refers to random measurement variations caused by external factors that result 

in lack of precision or accuracy.

• Error is the deviation between the measured value and the true value. Types of errors include gross error, systematic error, sampling error and random error.

Measurement uncertainties must be considered as part of instrument selection and also in the 

evaluation of measurement data.
13A–2

Appendix 13A

MAY 2009

13A.2 INSTRUMENT TRANSDUCERS

While performance monitoring at coal refuse facilities is typically conducted using position 

surveys of surface monuments and other devices such as staff gauges, inclinometers, standpipe 

piezometers, weirs, and rainfall gauges, geotechnical and structural instrumentation employing transducers is becoming more prevalent because these devices facilitate remote monitoring, 

transmission and processing of performance data. For such applications, instruments frequently consist of a transducer, data acquisition system and a communication link between them. A 

transducer is a device that converts a physical response or change into a corresponding output 

signal. Transducer types can be classified as:

• Mechanical

• Hydraulic

• Pneumatic

• Electrical

Measurement of geotechnical and structural behavior is typically accomplished using instruments employing more than one transducer type. The selection of an instrument appropriate 

for a particular application depends on the type of measurement required, the project setting 

and related environmental factors, and the duration for which measurements are required. 

Data acquisition systems for these instruments range from simple portable readout units to 

complex, automated systems.

13A.2.1 Mechanical Instruments

The most commonly used mechanical instruments are dial gages and micrometers. A dial gage 

converts the linear movement of a spring-loaded plunger to movement of a pointer against a 

dial scale. Accuracies are usually ± 0.001 inches or ± 0.0001 inches with a range of movement 

of typically 1 to 2 inches, although instruments with ranges up to 12 inches are available. Most 

dial gages are inexpensive, but they are somewhat fragile and can be affected by environmental 

factors such as dust and dirt. For long-term application, sealed and waterproof versions are 

available.

A micrometer functions by the rotation of a finely threaded measuring rod that moves into 

or out of a sealed housing. Movement of the rod is measured by a scale on the housing that 

indicates the number of revolutions. Fractional revolutions are determined using gradations 

marked around the rod and a vernier on the housing. Accuracy for micrometers reading in 

FIGURE 13.1 GRAPHIC ILLUSTRATION OF ACCURACY AND PRECISION

PRECISE BUT

NOT ACCURATE

ACCURATE BUT

NOT PRECISE

ACCURATE AND

PRECISE

(DUNNICLIFF, 1993)

FIGURE 13A.1 GRAPHIC ILLUSTRATION OF ACCURACY AND PRECISION
13A–3

Measurements, Transducers and Data Acquisition Systems

MAY 2009

inches is ± 0.001 inches. The rod length can be changed to permit a measurement range up to 6 

inches, and digital outputs are available to overcome difficulties with reading vernier scales.

Dial gauges, micrometers and other simple gauges (e.g., crack monitors) have been used at 

dams and impoundments on mine sites where adverse conditions are suspected. Some applications have included monitoring crack apertures and relative displacements in or adjacent to 

rigid structures, such as concrete spillways or intakes and pipes.

13A.2.2 Hydraulic Instruments

Although not normally used for monitoring the performance at coal refuse facilities, the most 

commonly used hydraulic instruments are Bourdon-tube pressure gages and manometers. A 

Bourdon tube is a flattened tube that is coiled into a C-shape. When the tube is pressurized, the 

tube expands causing it to uncoil. The uncoiling motion is linked to a pointer that rotates over 

a circular scale. Bourdon tubes typically have an accuracy of ± 0.5 percent full scale for 4- and 

6-inch-diameter dial gages, but gages with higher accuracy (e.g., ± 0.1 percent full scale) may be 

required for meeting project requirements, especially for long-term applications. Bourdon-tube 

gages are used with hydraulic piezometers, hydraulic load cells, borehole pressure cells, and 

some readout units for pneumatic transducers (Dunnicliff, 1993).

A manometer is a liquid-filled U-tube. The pressure on one side of the U-tube is balanced by 

an equal pressure on the opposite side. Manometers are used for long-term monitoring of very 

small positive or negative pressures and are easily calibrated and have a greater longevity than 

Bourdon tubes. Manometers are used with twin-tube hydraulic piezometers and liquid-level 

settlement gages (Dunnicliff, 1993). In these devices, the pressure difference between the liquid 

surfaces at the opposite ends of the manometer has a known relationship to the elevation difference via an assumed or measured density of the liquid. Potential problems with a manometer 

are discontinuity in the liquid due to formation of gas bubbles, changes in the density of the 

liquid due to temperature effects, and surface tension effects. Dunnicliff (1993) describes each 

of these problems and provides recommendations to prevent their occurrence.

Use of these instruments requires physical access to the readout location. If remote data recording is necessary, an electrical pressure gage should be used (Dunnicliff, 1993).

13A.2.3 Pneumatic Instruments

Pneumatic instruments function by supplying a known pressure via tubing that reacts against a 

sealed diaphragm in the pneumatic transducer. When the pressure in the inlet line just balances 

the pressure against the transducer diaphragm, the diaphragm displaces slightly. This condition is determined by a return of gas flow from the outlet line (for a gas-flowing instrument), or 

the pressure in the outlet line is measured using a Bourdon tube or electrical pressure gage (for 

a non-gas-flowing instrument). For simple applications, manually-operated readout units for 

pneumatic instruments are available. Where a large number of pneumatic instruments must be 

monitored, readout units can be connected to large gas tanks, and data acquisition and control 

systems can be used to energize the systems and to scan and record the measurements.

Important characteristics of most commercially available pneumatic transducers include sensitivity to diaphragm displacement, sensitivity to gas flow rate, and the length of tubing. For 

reliable performance: (1) transducer components should not be prone to corrosion, (2) a dry gas 

such as carbon dioxide or nitrogen should be used, and (3) the tubing and fittings should be 

air tight and impermeable to moisture infiltration. Pneumatic transducers and data acquisition 

systems are used in pneumatic piezometers, earth pressure cells, load cells, and liquid settlement gages (Dunnicliff, 1993).
13A–4

Appendix 13A

MAY 2009

Pneumatic piezometers are routinely used at coal refuse disposal facilities, primarily to monitor 

pore pressure within low-hydraulic-conductivity deposits that may exhibit elevated pore pressure 

when loaded. Specifically, these types of piezometers have been used to monitor pore pressures 

within fine coal refuse subject to upstream construction loading. When instruments for such an 

application are selected, their reliability and recommended service period should be evaluated.

13A.2.4 Electrical Instruments

13A.2.4.1 Electrical Resistance Gages

Electrical resistance gages are used for many geotechnical and structural monitoring applications. These gages are precisely manufactured conductors that change resistance in proportion 

to the change in length of the gage. Most strain gages have a nominal resistance of 120 or 350 

ohms. The gage factor, which is a measure of the strain sensitivity of the gage, is typically about 

2 for bonded foil and wire gages and between about 50 and 200 for semiconductor gages.

The bonded foil strain gage is the most widely used type, and it has significant advantages over 

all other types of strain gages. Bonded foil gages consist of a metal foil pattern mounted on an 

insulating backing or carrier, constructed by bonding a sheet of thin-rolled metal foil 2- to 5-μm 

thick on a backing sheet that is 10- to 30-μm thick. The measuring grid pattern including the terminal tabs is produced by photo-etching. Bonded foil gages are used because of the quality of 

their manufacturing and because the temperature characteristics of the gage can be matched to 

the material being measured. Higher resistance gages are preferred for transducer applications 

because such devices permit a higher voltage input that results in a higher voltage output and 

minimizes the effects of extraneous resistance changes. A variant of the foil gage is the weldable 

resistance strain gage, which typically consists of a foil gage attached to a thin stainless steel 

mounting flange. Strain gages can be used for long-term monitoring applications provided that 

methods used for gage installation, sealing, and protection are appropriate for the application.

The output from electrical resistance gages is generally measured using a Wheatstone bridge 

circuit, which is described by Dunnicliff (1993). The circuit consists of four resistors arranged 

in a diamond orientation, as shown in Figure 13A.2. An input DC voltage (excitation voltage) 

is applied between the top and bottom of the diamond, and the output voltage is measured 

across the middle. When the output voltage is zero, the bridge is “balanced.” One or more of 

the legs of the bridge may be a resistive transducer such as a strain gage. If the circuit has one 

strain gage, it is referred to as a quarter-bridge circuit. Similarly, if the circuit has two or four 

strain gages, the circuits are referred to as half-bridge and full-bridge circuits, respectively. For 

quarter- and half-bridge circuits, the other legs of the bridge are electrical resistors with resistance equal to that of the strain gage(s). Although half-bridge and quarter-bridge circuits are 

often used, the full-bridge circuit is the optimal configuration for strain gage usage. It provides 

the highest sensitivity and the fewest error components, and because the full-bridge circuit 

produces the highest output, noise is a less significant factor in the measurements. For these 

reasons, the full-bridge circuit is recommended. A full-bridge configuration is typically used 

for load cells (Dunnicliff, 1993). In general, the primary application for resistance networks in 

geotechnical and structural instrumentation described in this Manual is for resistance strain 

gages used in piezometers to monitor dynamic pore water pressures (e.g., from rapid or cyclic 

loading) and also for measuring loads in structural elements such as rock bolts or permanent 

ground anchors for slope stabilization.

13A.2.4.2 Linear Variable Differential Transformer (LVDT) and Direct Current Differential

Transformer (DCDT)

LVDTs and DCDTs consist of a moveable magnetic core passing through a primary and two 

secondary coils. An excitation voltage is applied to the primary coil that induces a voltage in the 
13A–5

Measurements, Transducers and Data Acquisition Systems

MAY 2009

secondary coils that is dependent upon the proximity of the magnetic core to each secondary 

coil. The secondary coils are connected in series opposition so that the net output from the device is the difference between the two voltages. When the core is located at the midpoint of the 

device, the net output voltage is zero. When the core moves from the midpoint, the net output 

voltage varies linearly. These devices have no hysteresis, and they are well suited for measuring 

very small displacements and dynamic movements. The primary difference between the two 

devices is that LVDTs are powered by an AC voltage while DCDTs are powered by a DC voltage. Also, DCDTs are provided with onboard oscillator, carrier amplifier, and demodulator circuitry while LVDTs require these components externally. DCDTs are preferred for geotechnical 

applications where specialized signal conditioning is not needed for amplifying output from 

the device. LVDTs and DCDTs are used in fixed borehole extensometers and other instruments 

to measure deformation (Dunnicliff, 1993).

13A.2.4.3 Vibrating-Wire Transducer

The vibrating-wire transducer consists of a taut, ferromagnetic wire that is excited into transverse vibrations by a drive coil. These vibrations are detected using a pick-up coil. Both coils 

have permanent-magnet cores, and once the wire has been excited to its resonant frequency for a 

given tension, it is maintained at this frequency by connecting the two coils through an amplifier 

to form a self-oscillating system. Each resonant frequency is a measure of the tension in the wire 

and therefore the applied force. The advantage of the vibrating-wire transducer is that its direct 

frequency output can be handled by digital circuitry, eliminating the need for an analog-to-digital 

converter. Sources of error include wire corrosion, wire creep under permanent tension, and wire 

slippage at the anchoring locations, all of which usually result in a reduction in frequency.

The output from vibrating-wire devices is measured using the pluck and read method or the 

continuous excitation method. The pluck and read method involves applying one or more voltFIGURE 13.2 WHEATSTONE FULL-BRIDGE NETWORK CONFIGURATION

APPLIED

KNOWN

VOLTAGE

VOLTMETER

STRAIN GAUGE (TYP.)

FIGURE 13A.2 WHEATSTONE FULL-BRIDGE NETWORK CONFIGURATION
13A–6

Appendix 13A

MAY 2009

age pulses to the drive coil to cause the wire to vibrate. The coil then receives the signal from 

the vibrating wire and transmits the measured voltage to a frequency counter, which measures 

the time required for a selected number of vibration cycles. The continuous excitation method 

uses a similar procedure to initiate wire vibration, but uses a second coil to detect the frequency. 

The signal is fed back to the driving coil, which then applies a continuous pulsing voltage. As 

the wire frequency changes, the driving frequency also changes, and the new frequency is measured as described for the pluck and read method.

Vibrating-wire transducers are used in pressure sensors for piezometers, earth pressure cells, 

liquid level settlement gages, load cells, and directly as surface and embedment strain gages 

(Dunnicliff, 1993). Vibrating wire piezometers are effective instruments for monitoring rapid 

pore pressure changes in fine coal refuse deposits during upstream construction and have 

also been used in standpipe piezometers to facilitate automated monitoring and data acquisition systems.

13A.2.4.4 Force-Balance Accelerometer

A force-balance accelerometer consists of a mass suspended in the magnetic field of a position 

detector. When the mass is subjected to a gravitational force along its measuring axis, the mass 

attempts to move and the motion induces a current change in the position detector. This change 

in current is relayed to a servo-amplifier connected to a restoring coil that imparts an electromagnetic force to the mass to resist movement. The current through the restoring coil is measured by the voltage across a precision resistor. The measured voltage is directly proportional 

to the input force. Force-balance accelerometers have exhibited good performance when used 

in portable tiltmeters and in inclinometers where the position of the transducer can be reversed 

to eliminate errors caused by zero shift.

13A.2.4.5 Magnetic Reed Switch

The magnetic reed switch is an on/off position detector used to indicate when conductive reeds 

are in a certain position with respect to a ring magnet. When a magnetic reed switch enters a 

sufficiently strong magnetic field, the reed contacts close and remain closed while they are in 

the magnetic field. Closure of the contacts normally results in activation of a buzzer and/or indicator light in a portable readout unit. Repeatability of the device depends on the radial position 

of the reed switch within the magnet. If the reed switch remains within the middle third of a 

11⁄4-inch-diameter ring magnet, repeatability will be within ± 0.01 inches (Dunnicliff, 1993). The 

device is simple, reliable, precise, inexpensive and well suited to long-term applications. The 

magnetic reed switch is used in probe extensometers.

13A.2.4.6 Induction-Coil Transducers

Induction-coil transducers function by supplying an AC source to a primary coil and measuring 

the voltage induced in a secondary coil that is located within the magnetic field of the primary 

coil. For geotechnical applications, inductive-coil transducers have been used to measure strain 

in soils and displacement in probe, fixed embankment, and borehole extensometers (Dunnicliff, 

1993). These devices have excellent long-term stability provided that the steel components are 

protected from corrosion.

13A.2.4.7 Other Types of Electrical Transducers

Other types of electrical transducers include the potentiometer, variable-reluctance transducer, 

magnetostrictive transducer, and the electrolytic level. These devices and their geotechnical 

and structural instrumentation applications are discussed in Dunnicliff (1993). Information is 

also available from commercial geotechnical and structural instrumentation suppliers.
13A–7

Measurements, Transducers and Data Acquisition Systems

MAY 2009

13A.3 SUPPLEMENTAL MOVEMENT-MEASUREMENT TECHNIQUES

Section 13.2.1.2 discusses the more commonly applied techniques for measurement of movements. This section discusses supplemental techniques including transverse deformation gages, liquid-level gages, time-domain reflectrometry and fiber-optic gages.

13A.3.1 Transverse Deformation Gages

Transverse deformation gages are installed in boreholes or pipes for measuring deformations 

transverse to their length (Dunnicliff, 1993). Typical applications include locating the depth of a 

slide plane and measuring deformations within and below embankments. Types of transverse 

deformation gages include shear plane indicators, plumb lines, inverted pendulums, in-place 

inclinometers and deflectometers. Of these devices, only in-place inclinometers have potential 

application at coal refuse disposal facilities.

In-place inclinometers are usually designed to operate in a vertical or near-vertical borehole and 

to provide nearly the same information as a standard probe inclinometer. The device consists of 

a series of gravity-sensors connected by articulated rods. The sensors can be either uniaxial for 

measuring displacements in one plane or biaxial for measuring displacements in two planes. 

The sensors are positioned at intervals along the borehole and can be oriented to capture displacements at critical locations. In-place inclinometers are typically installed in the same casings used for conventional inclinometers, thus permitting the devices to be removed for recalibration (possibly interrupting data continuity) or reused elsewhere. Compared to conventional 

inclinometers, in-place devices offer advantages such as more rapid reading, improved precision, automatic data acquisition, and alarm triggers. Their comparative disadvantages include 

greater complexity and expense and inability to remove the effect of any long-term drift of the 

gravity-sensing transducer by reversing the orientation of the sensor as can be done for conventional inclinometers.

13A.3.2 Liquid-Level Gages

Liquid-level gages are instruments that use a liquid-filled tube or pipe for determination of 

relative vertical deformation. Relative elevation is determined with a manometer or pressure 

transducer. Liquid-level gages are used primarily to measure the settlement of foundations 

or embankment fills. Because the devices are buried below the structure or embankment fill, 

there are no interferences of the type that occur with fixed embankment extensometers (Section 

13.2.1.2.5.). Liquid-level gages can only measure relative movements between the measurement 

location and the reading station. If the exact settlement or heave at a specific point is required, 

the reading station elevation must be surveyed and referenced to a benchmark datum each time 

measurements are taken.

These devices are usually sensitive to changes in liquid density due to temperature effects, surface tension effects, and loss of continuity of the liquid in the tube. The greatest potential source 

of error is discontinuity of the liquid due to gas bubbles in the fluid. Single-point, multi-point 

and full-profile liquid-level gages are available. Single-point gage types include:

• Both ends at same elevation

• Readout unit higher than the measurement cell

• Readout unit lower than the measurement cell

The most common single-point gage types have the readout unit higher than the measurement 

cell with pressure readings accomplished using a transducer located either in the measurement 

cell or in the readout unit. Both of these gage types suffer from the limitation that only a single 
13A–8

Appendix 13A

MAY 2009

point is monitored, but the unit with the transducer in the readout is generally preferred because the transducer is accessible for calibration during the monitoring period, and the system 

has fewer limitations and requires less diligent oversight during installation and monitoring. 

Additional details related to gage types, advantages and limitations, and approximate precision of liquid-level gages are provided in Dunnicliff (1993).

Various multi-point gages have been developed, but users tend to prefer installing several 

single-point gages because loss of a single multi-point gage can result in the loss of the entire 

system (Dunnicliff, 1993). Full-profile gages consist of a near-horizontal plastic pipe and an 

instrument that can be pulled along the pipe. Readings are made incrementally along the 

length of the pipe so a full lateral profile is obtained. However, the devices have numerous 

limitations (e.g., temperature errors, lack of exact knowledge of the horizontal position in the 

pipe) that make a full-profile device such as a horizontal inclinometer (Section 13.2.1.2.7) a 

preferred choice.

13A.3.3 Time-Domain Reflectometry

Time-domain reflectometry (TDR) is radar along a coaxial cable. TDR involves measurement 

of the time it takes for an energy pulse to travel down a cable, encounter a known or unknown 

distortion in the cable, and reflect a signal back to a reading instrument. The time is converted 

to distance and the data are displayed as a waveform and/or distance reading. The ability to 

interpret TDR reflections anywhere along the cable allows activity to be monitored over large 

volumes or areas such that TDR monitoring can replace many single-point measurement instruments. The technique has been used for a variety of deformation measurements related to 

landslide and slope monitoring, ground subsidence due to mining, sinkholes, groundwater 

extraction, and scour of bridge foundations.

The use of TDR can complement inclinometer technology. TDR cable surveillance can detect 

very thin or localized shear zones. Use of TDR in combination with inclinometers or tiltmeters allows remote operation as well as sensing of both gradual tilt and localized deformation. 

O’Connor and Dowding (1999), Dowding (2002) and Dowding and O’Connor (2000) describe 

some of these applications.

Figure 13A.3 shows a TDR cable grouted into a borehole. The cement-bentonite grout backfill 

in the borehole must be designed to fracture easily, so that the cable is deformed as movement 

occurs within the surrounding medium. Appropriate grout strengths vary considerably, but 

grouts for boreholes in soil and rock are typically tremmied into place using grout pumps on 

drill rigs. In general, the unconfined compressive strength of the grout should be less than the 

strength of the material where the shear failure can occur. Blackburn and Dowding (2004) provide details regarding grout selection and design. Dowding (2002) reports that braided coaxial 

cable is sometimes strapped to the outside of inclinometer casing in larger holes to save money 

and drilling costs. Unfortunately this cost-saving method has not performed well, probably 

because of the low sensitivity of the braided cable and spreading out of the localized shear zone 

caused by the casing.

13A.3.4 Fiber-Optic Gages

Fiber-optic sensors make use of the ability of optical fibers to convey light from a source to a 

photosensitive detector (Dunnicliff, 1993). The sensors can be used to indicate the relative position between an object and the end of the fiber or the distance between two points along the 

fiber. The sensors can also monitor bending. Advantages of fiber-optic sensors include small 

size, reliability, insensitivity to temperature and humidity changes, immunity to electrical noise 

(e.g., lightning) low signal loss, and the ability to transmit light along curved paths.
13A–9

Measurements, Transducers and Data Acquisition Systems

MAY 2009

A fiber-optic sensor consists of two facing mirrors that are made of a semi-reflective coating deposited on the tips of optical fibers. The gap between the mirrors (i.e., Fabry-Perot cavity length) 

varies from almost zero to a few tens of microns when the gage is unloaded. The separating 

distance is the gage length and represents the actual measuring base of the strain gage. The 

gage functions by reflection and cross interference of light emitted along the fiber-optic cable 

at the mirrors. The return optical signal is processed using a Fizeau interferometer and a linear 

charge-coupled device to determine changes in separation between the mirrors. If the gage is 

bonded to a substrate (e.g., a metal sensing element), strain variation in the axial direction will 

produce a variation of the cavity length, and strain is then equal to the ratio of the change in 

cavity length to the gage length. Both displacement and strain gage sensors are available (Choquet et al., 2000), but most reported applications for these devices have been for monitoring 

structural components.

13A.4 AUTOMATED DATA ACQUISITION

Although not yet commonly used to monitor performance at coal refuse disposal facilities, 

automated data acquisition systems have been demonstrated to be reliable and cost effective 

for monitoring the performance of constructed works and their surrounding environment, 

especially at sites where access is difficult or where regular survey and/or monitoring control 

are not practical or economical. With time, automated data acquisition systems will continue 

to improve with the likelihood that their use at coal refuse disposal facilities becomes more 

common.

Most automated data acquisition systems:

• Are programmed to retrieve data on a prescribed schedule without human intervention.

• Are designed to accommodate more than one transducer.

• Incorporate signal conditioning to amplify the output and present it in terms of engineering dimensions.

• Record data or transmit it elsewhere for recording.

FIGURE 13.12 TIME-DOMAIN REFLECTOMETRY OPERATION

TDR

Df

13.13b CABLE IN

 SLOPE

VOLTAGE

DISTANCE/DEPTH

13.12c REFLECTED

SIGNAL 

13.12a CABLE/GROUT

DET AILS

COMPLIANT

GROUT

COMPLIANT

COAXIAL CABLE

SHEAR

ZONE

1345

1344590L5

(DOWDING, 2002)

FIGURE 13A.3 TIME-DOMAIN REFLECTOMETRY OPERATION
13A–10

Appendix 13A

MAY 2009

Figure 13A.4 presents a generalized block diagram for an automated data acquisition system. 

As described by Dunnicliff (1993), the power supply and signal conditioning convert the output of analog transducers into a signal that is measured and converted to numeric values by an 

analog-to-digital converter. In most systems the signals from the transducers are conditioned to 

produce a DC voltage or binary decimal signal. The electronics controlling the systems perform 

various functions such as: (1) controlling measurement frequency, (2) scaling and displaying 

the data, (3) converting output to engineering units, (4) averaging data, (5) checking for alarm 

limits, and (6) controlling external equipment such as alarms. The controlling electronics have 

some memory capacity, but generally data must be electronically stored for further analysis.

Use of automated data acquisition systems is relatively simple for transducers that produce a 

full-scale DC output voltage of 1 volt or more because these devices require minimal interfacing and signal conditioning. LVDTs, DCDTs, potentiometers, force-balance accelerometers, and 

high-output electrical resistance strain gage networks are included in this category. Thermocouples, thermistors, and resistance temperature devices used for temperature measurements 

require minor interfacing for use with automated data collection systems.

Other transducers such as low-output electrical strain gages and vibrating-wire, magnetostrictive and induction-coil transducers require special signal conditioning. This signal conditioning 

is usually provided by a plug-in module to the automatic data acquisition system. For these 

types of transducers, a dedicated data logger provided by the transducer manufacturer is usually the best choice (Dunnicliff, 1993).

Non-electrical sensors such as pneumatic transducers; plumb lines and double-fluid, full-profile 

settlement gages; and twin-tube hydraulic piezometers can be monitored using an automatic 

data acquisition system and methods described in Dunnicliff (1993). Electrical transducers that 

function by being manually passed through a pipe (e.g., magnetic reed switches and inductioncoil transducers used with probe extensometers) are not appropriate for use with automatic 

data acquisition systems.

A complete data acquisition system consists of data logger, data communications/retrieval system, and software components. The data logger should have proven reliability for geotechnical 

applications and be compatible with a wide range of sensors and data retrieval options. A single 

data logger can read a large number of sensors provided that they are confined to a relatively 

small area. Additional data loggers should be used if instruments are deployed over a wide 

area. This approach keeps signal cables short, reduces problems with signal noise, and minimizes the potential for damage from construction activities and electrical transients. The cost 

savings realized by reducing cable runs can sometimes pay for the additional data loggers.

Multiplexers increase the number of sensors that can be monitored by a data logger. Data loggers 

can control several multiplexers, each of which are capable of handling 32 two-wire sensors or 

16 four-wire sensors of the same type. In practice however, a data logger usually controls only 

one or two multiplexers, and additional data loggers and multiplexers are employed if there are 

more instruments. Certain types of sensors require additional interfaces. For example, vibrating wire sensors require a vibrating-wire interface to be connected between the data logger and 

a multiplexer. Power to operate data loggers, multiplexers and instruments is generally from a 

battery that is charged by an AC power line or a solar panel. Where feasible, the system should 

be hard wired or potential power problems, such as maintaining battery charges during cold 

weather, should be investigated. All data loggers, multiplexers and related equipment should 

be housed in weatherproof enclosures.

Data retrieval options include wired and wireless links. Wired links for data retrieval include 

direct connection to a personal computer, telephone modems, short-haul modems, and multi-
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Measurements, Transducers and Data Acquisition Systems

MAY 2009

FIGURE 13A.4 GENERALIZED BLOCK DIAGRAM FOR AUTOMATIC

 DATA ACQUISITION SYSTEM

(ADAPTED FROM DUNNICLIFF, 1993)

 ANALOG

TRANSDUCER

 ANALOG

TRANSDUCER

 ANALOG

TRANSDUCER

DIGITAL

TRANSDUCER

OFF-SITE

COMMUNICATIONS

AND SCHEDULED

DATA COLLECTION

PERMANENT

DATA

STORAGE

DATA LOGGER

OR DATA

CONTROLLER

OR COMPUTER

ELECTRICAL

 CABLE

DISPLAY

AND/OR

PRINTER

SIGNAL

CONDITIONING

POWER

SUPPLY

OR

EXCITATION

ANALOG-TO-

 DIGITAL

CONVERTER

CONTROLLING

ELECTRONICS

MEMORY AND

ARITHMETIC

FUNCTIONS

DATA LOGGER

PROGRAMMING

CHANNEL RELAY

MULTIPLEXER

INTERFACE/CONTROL

PERIPHERALS

FIGURE 13A.4 GENERALIZED BLOCK DIAGRAM FOR AUTOMATIC DATA 

ACQUISITION SYSTEM
13A–12

Appendix 13A

MAY 2009

drop networks. Wired links are usually less expensive, easier to set up, and better for real-time 

data retrieval than wireless links. Wireless links for data retrieval include cell modems, spreadspectrum radio modems, licensed-frequency radio modems and satellite modems. Wireless 

links are useful when distances or obstacles make wired links impractical. Also, to the degree 

that wireless links eliminate surface runs of cable, they also reduce problems caused by electrical transients. However, before an investment is made in wireless links, a check should be made 

for the presence of high-voltage power lines or radio transmission towers that could make the 

site too noisy for a wireless link. Additional details regarding data retrieval options are described by van der Veen (2002).

Data logger control software provides a fast, reliable and cost-effective means to collect, process 

and distribute data from the data logger to a local or remote personal computer. The control 

software is typically customized for each application to facilitate automatic processing of readings, alarm checks, graphic displays, and report generation. The logger control software can 

operate on a web server so that access is available from the Internet, a company intranet, or a 

stand-alone personal computer. Users can access the web through web browsers and click on 

links to access data and graphic presentations. Access controls can be established for identification of individuals that have rights to set up projects, graphs, reports, and alarms. Data that are 

not logged automatically can be entered manually using a web browser. The software can scan 

incoming readings for alarm conditions and then store the readings in a project database.

Automatic data acquisition and transmission systems can make instrumentation programs more 

susceptible to lightning damage and ground fault problems, as the number and density of cables and electrical components increases on a site. Instrumentation manufacturers can provide 

guidance on protecting these systems, which may include (Shoup, 1992): (1) diversion systems 

consisting of multiple lightning arresters (lightning rods, grounded to depths below the sensors) positioned outside the area being instrumented, (2) protective ground systems consisting 

of non-insulated copper ground conductor paralleling cables from the proximity of the sensor to 

the terminal box (separated from the cables by approximately 6 inches and extending to a depth 

of 5 feet below the sensor), and (3) primary lightning protection devices such as gas discharge 

tubes to protect cable connections to data acquisition modules (provided an appropriate protective ground system is installed). Other measures may be appropriate for surface data acquisition 

systems and equipment, although replacement of these components is less of a concern.

13A.5 INSTRUMENT AND SYSTEM RELIABILITY

The most important criterion for performance monitoring is reliability (Dunnicliff, 1993). Reliability is a function of instrument and system features and project personnel. Instrumentation 

system features that are associated with reliable performance include:

• Simplicity

• Self verification

• Durability in the installed environment

Simplicity of transducers should be a primary objective when instruments for a particular application are planned and selected.

Self verification means that instruments can be verified (i.e., calibrated) in place. Examples given by Dunnicliff (1993) include:

• Checking a fixed borehole extensometer with electrical transducers by inserting a 

dial gage at the head.
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Measurements, Transducers and Data Acquisition Systems

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• Changing the head in a standpipe piezometer by raising or lowering the water level 

and verifying that the hydraulic response is consistent with the hydraulic conductivity of the surrounding ground.

• Using duplicate transducers of the same or different type to confirm local readings.

• Confirming that check sums from inclinometer readings are reasonably constant for 

all depth intervals.

Durability in the installed environment refers to the longevity of sensors, the connections between sensors and the data acquisition system, and the operation of the data acquisition system

within the project setting. Factors such as pressure and temperature changes; duration of operation; chemical, biological and moisture environmental factors; and electrical grounding affect 

instrumentation system durability.

Sometimes it is not possible to achieve simplicity, self verification and durability in the installed 

environment, but these should be objectives in planning instrumentation systems, particularly 

for long-term applications. Equally and perhaps more important to the success of an instrumentation program are the people involved. The success of system planning, initial calibration 

and inspections, equipment installation, maintenance and recalibration, and data collection, 

processing and interpretation is directly related to the care and motivation of the people responsible for these activities.

The in-service functionality and reliability of an instrumentation system can be assessed by implementing an instrumentation maintenance program. The features of such a program include 

regular maintenance and recalibration, as described in Section 13.3.
Chapter 14

EMERGENCY

ACTION PLANNING

MAY 2009 14–1

14.1 PURPOSE

Emergency action planning for dams includes establishing procedures and identifying potential 

actions and resources for responding to a condition of impending or actual dam failure. Guidelines 

published by the Federal Emergency Management Agency (FEMA; 2004a, 2004b) provide general 

information on hazard-potential classification and emergency action planning for dams. The guidance presented in this chapter is divided into two parts: (1) Emergency Action Plan (EAP) preparation and (2) emergency remedial actions that a facility Owner/Operator can take independent of 

responsibilities defined in the EAP to prevent an embankment/impoundment failure or to reduce 

potential damage in the event that a failure does occur.

EAP preparation is addressed in Section 14.2. The intent is to provide an understanding of what an 

EAP is, when one should be prepared, and recommended practice for preparation. However, it should 

be understood that EAPs are generally submitted to state agencies for review and approval and must 

meet the requirements of the state (or states) in which the facility and potential inundation area are 

located. In addition to state guidance, FEMA (2004b) and the Natural Resources Conservation Service 

(NRCS, 2007b) have developed guidelines for EAP preparation, and some states may be using or 

may adopt these or similar guidelines. If a facility is not under state jurisdiction, then the aforementioned federal guidelines should be useful. The first step in EAP preparation for any site should be to 

determine what guidelines are applicable. While the guidelines presented herein are recommended 

practice, some of the information presented may not be applicable in all instances. Most states have a 

specific structure and format that should be followed.

EAP preparation is a prudent practice for significant- and high-hazard-potential dams and the Owner/

Operator typically has specific responsibilities related to notification of authorities in the event of 

development of an emergency situation. However, there may be additional actions that can be taken 

by the Owner/Operator that will possibly prevent or minimize damage due to a failure. Potential 

actions for several emergency scenarios are discussed in Section 14.3.

14.2 EMERGENCY ACTION PLAN PREPARATION

14.2.1 Background

Structures that impound large volumes of water or tailings slurries represent a potential danger to 

inhabitants of low-lying areas located downstream. Some notable dam failures that have occurred 
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in the U.S. and the resulting loss of life are presented in Table 14.1. It was the sudden failure of the 

Buffalo Creek Dam on February 26, 1972 during a period of heavy rainfall that elevated national 

awareness of the potential danger associated with coal refuse impoundments and led to a program

of tailings dam and impoundment inspections and hazard classification, as well as MSHA’s current 

coal refuse impoundment regulations.

TABLE 14.1 HISTORICAL U.S. DAM FAILURES

Year Dam Location Deaths

1889 South Fork Pennsylvania 2209

1972 Buffalo Creek West Virginia 125

1972 Canyon Lake South Dakota 139

1976 Teton Idaho 11

1977 Toccoa Falls Georgia 39

Today most states regulate non-federal dams within their boundaries. This regulation normally 

entails: (1) classification as to hazard potential level, (2) design and construction requirements, and 

(3) periodic inspections. Impounding coal refuse embankments and dams are classified as to hazard 

potential in accordance with a system comparable to that adopted by FEMA, as discussed in Section 

3.1. There may be some variation in the number of hazard level categories and the terminology from 

state to state, but all state classification systems are generally consistent with the FEMA system. It 

should be noted that the hazard potential level is strictly a function of the potential consequences 

of a dam failure and is not related to the construction of the dam, its condition, or its susceptibility 

to failure.

Dams or impounding embankments that have significant or high hazard potential in accordance 

with the FEMA (or state equivalent) hazard classification system should have an EAP in place. 

These are structures that, if they were to fail, would likely cause loss of life or significant property 

damage. An EAP is a document that establishes emergency procedures to be followed in the event of 

a catastrophic failure of the structure leading to rapid downstream flooding. While some significant 

and high hazard potential dams in the U.S. may not currently have EAPs in place, they are generally being required for coal refuse impoundments and dams by state dam safety regulatory agencies. FEMA (2004b) in the Federal Guidelines for Dam Safety: Emergency Action Planning for Dam 

Owners (FEMA 64) indicates that each high and significant hazard dam should have an EAP; MSHA 

has encouraged mine operators to prepare EAPs consistent with FEMA 64.

It should also be noted that 30 CFR § 50 requires mine operators to immediately notify MSHA of accidents. Conditions that constitute an accident are defined under 30 CFR § 50.2. One of the definitions 

of accident indicated in 30 CFR § 50.2(h) (10) is, “An unstable condition at an impoundment, refuse 

pile, or culm bank which requires emergency action in order to prevent failure, or which causes individuals to evacuate an area, or failure of an impoundment, refuse pile, or culm bank.”

While not equivalent to the requirements for an EAP, MSHA’s regulations under 30 CFR § 77.216-3 

do require inspections of coal company dams at specified intervals for hazardous conditions and that 

actions be taken when a potentially hazardous condition develops, including: (1) notification of the 

MSHA District Manager, (2) notification and preparation for evacuation, if necessary, of coal miners 

who may be affected from coal mine property, and (3) examination of the structure by a qualified 
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person at least every 8 hours. These regulations should be reviewed as part of the preparation of an 

EAP, particularly as related to identification of hazardous conditions and mine personnel responsible 

for notification. Since a dam failure can impact persons living well away from mine property, an EAP 

must take into account the entire downstream area that will potentially be affected.

A possible mode of failure for an impounding coal refuse embankment is breakthrough of the 

impoundment into underground mine workings unrelated to dam failure. This can result in flooding 

of the mine workings and release of flood flows at a mine opening relatively far from the impounding 

embankment. EAPs for postulated dam failures involve a postulated dam breach and downstream 

release. FEMA and state regulations and guidelines are tailored to this type of event. However, if a 

breakthrough is plausible, an evaluation of such a release and the potential consequences should be 

made. Such a study may involve many conservative assumptions related to the size and timing of the 

breakthrough and analyses of flow through the breakthrough to determine the location and extent 

of possible flooding. The result should be an inundation map for the area that would be affected by 

flooding. All other aspects of EAP preparation would remain essentially the same.

14.2.2 Federal Guidelines for Dam Safety

The Interagency Committee on Dam Safety (ICODS) was established in the late 1970s to address 

dam safety issues. ICODS is chaired by FEMA and comprises the federal departments and agencies 

(including MSHA) that have responsibility for dams and dam safety.

EAP guidelines and a suggested format for use by the ICODS agencies are provided in FEMA (2004b) 

cited above. Some states have adopted the ICODS guidelines for EAP preparation, while others 

have developed their own guidelines and plan formats. In any case, the FEMA publication is recommended reading for anyone that is preparing an EAP. Also the NRCS (2007b) has developed a sample 

EAP and an electronic “fillable form” template” for preparation of EAPs. This template and related 

documents are available on the NRCS web site.

In the discussion that follows, EAP preparation and content are discussed in general terms and no 

specific format guidelines are presented. The appropriate state regulatory agency for each dam should 

be contacted to determine specific EAP format, submittal procedures, and approval requirements.

14.2.3 Basic Elements of an EAP

The following list of basic EAP elements is an expansion and refinement of a list originally developed by FEMA (2004b) and reflects some typical state requirements:

• Physical description of dam – A physical description and accurate location of the 

dam and related hydraulic structures with a map should be provided.

• Potentially inundated area – The maximum extent and arrival times for inundation resulting from postulated dam breaches should be clearly indicated on a map that shows 

the mining facilities, dam, downstream channel, important highways and streets, 

homes, businesses and any other critical facilities (e.g., hospitals, nursing homes, daycare centers) that could be affected by a dam failure. Locations of reception centers and 

treatment facilities designated in the EAP and locations of road blocks/traffic control 

points and evacuation routes should also be shown on the inundation map.

• Implementation triggers and emergency responses – Conditions that initiate implementation of emergency response procedures should be clearly indicated in the EAP

(e.g., reservoir elevations, spillway flow depths, or visible conditions of distress). 

Emergency response activities associated with the implementation triggers should be 

delineated. The NRCS Fillable Form Template (NRCS, 2007b) provides useful information on trigger conditions.
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MAY 2009

• Notification requirements – The responsibilities for notification should be clearly 

delineated. The notification hierarchy is frequently presented in a flowchart format, 

but other formats may be used consistent with state requirements. The important 

factor is that each plan participant has a clear understanding of his or her notification 

responsibilities.

• Participant responsibilities – Responsibilities of each plan participant should be 

clearly indicated in the EAP. Typical key responsibilities include the owner’s responsibility to notify appropriate agencies when there is a threat of dam failure, responsibility of local emergency management officials for warning and evacuating affected 

persons, requirements for police/fire departments for installing road blocks and/or 

managing traffic at specific locations, EMS/ambulance services responsibilities for 

transporting the injured, and the responsibilities of emergency service providers 

relative to operation of reception and treatment centers.

• EAP public notices and copies – Notices indicating the location of publicly available 

EAP copies may be required to be posted in public places such as municipal buildings, fire departments (social halls), tax offices, and other locations accessible to the 

public.

• Exercises/training – Some states require that EAPs have provisions for exercises 

and/or training while others do not. Exercises and training sessions are an effective 

way to provide EAP participants with a clearer understanding of their responsibilities and required actions.

• Plan maintenance – EAPs are normally required to be updated on a regular basis 

– typically one to five years. All elements of an EAP should be periodically reviewed 

and updated as needed to reflect changes in regulatory requirements, mine facilities, 

the dam and related structures, inundation area, street and highway infrastructure, 

plan participants and telephone numbers, and other key information. Some states 

require periodic evaluation of downstream development to determine whether 

changes have occurred that could affect the dam’s hazard classification and EAP contents. Optimally, the names of key plan participants and contact information should 

be updated annually or as changes occur.

• EAP Appendices – Supporting documents should be provided in appendices to 

the EAP. The contents of the EAP appendices will vary depending on state requirements.

The submittal and approval process for an EAP will vary from state to state, but a typical scenario 

might involve submittal of a draft to the county EMA for review and approval followed by review 

by state environmental and emergency management agencies. During this period, plan participants 

are also made aware of the EAP preparation and have the opportunity to provide input. Comments 

based on the reviews are addressed and the final version, once final approval is obtained from the 

state, is distributed to the plan participants.

14.2.4 Dam Inspection and Inundation Area Reconnaissance

The first step in the preparation of an EAP normally involves a field inspection of the dam or impounding embankment and a reconnaissance of the downstream area that would potentially be flooded as 

the result of a catastrophic failure.

14.2.4.1 Dam Inspection

A dam inspection should be performed in sufficient detail to allow a reasonable determination of critical sections and parameters for dam breach modeling. If the inspection is being performed to satisfy 
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Emergency Action Planning

MAY 2009

state annual (or other inspection frequency) requirements, then it should be detailed enough that the 

applicable state inspection forms can be fully completed. In addition to the dam, reservoir areas that 

could be overtopped or subject to instability should also be evaluated. The purpose of the inspection 

is not only to identify areas of concern, but also to evaluate features or conditions that may warrant 

special consideration in the dam breach analysis (e.g., overtopping threat at a saddleback ridge forming the impoundment or potentially rapid failure associated with a seepage area).

14.2.4.2 Downstream Channel Reconnaissance

Prior to the performance of a dam breach analysis, the channel downstream from the dam should be 

explored and features that might affect flow conditions following a dam breach should be accurately 

documented. For example, channel constrictions or other conditions that could cause a backwater 

effect should be noted. Cross-section data should be obtained at such locations for incorporation into 

the dam breach analysis model (Section 9.9). Also, if adequate cross-section data cannot be obtained 

from topographic maps, field surveys of cross sections should be performed. Floodplain conditions 

such as heavy brush or the presence of flow retarding structures (e.g., natural valley contractions, 

bridges, railroad and highway fills) should be noted and should be appropriately reflected in the dam 

breach model channel roughness parameters.

The downstream channel reconnaissance should also include notation of critical facilities that would 

likely be affected by flooding both on and off the mine property. At the mine property, areas that are 

staffed or are critical access points for personnel (e.g., mine structures, shafts, pits, utility stations, 

etc.) should be noted. Additionally, public structures such as hospitals, schools, day-care centers, 

nursing homes, utility plants and substations, emergency service provider bases, and industrial firms 

with large numbers of employees should be noted. Other conditions such as street locations and 

intersections not shown or incorrectly indicated on available maps should be noted so that the inundation map reflects current conditions.

14.2.4.3 Physical Description

A description of the impounding structure and its construction, related hydraulic structures, and 

other information such as observations relative to the potential inundation area, should be provided 

in the EAP. This description should be based upon data from the dam inspection and downstream 

channel reconnaissance, as appropriate.

14.2.5 Inundation Map Preparation

14.2.5.1 Causes of Dam Failures

There are many possible causes for dam failure and/or sudden impoundment release. Possibilities 

include:

• Overtopping of the dam caused by reservoir inflows that exceed the total combined 

storage and spillway capacity.

• Piping and internal erosion of the foundation or embankment materials.

• Structural failure of the dam embankment resulting from an earthquake.

• Movement and/or failure of the foundation supporting the dam.

• Failure of an upstream dam or major landslide into the reservoir.

• Inadequate maintenance leading to loss of spillway capacity or structural weakness.

• Breakthrough of an impoundment into mine workings and subsequent release 

downstream.

The first two items in the list account for most actual dam failures.
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14.2.5.2 Dam Breach Analysis

Consistent with the above, there are two modes of dam breach that are most commonly analyzed for 

determination of downstream inundation due to dam failure. These are: (1) overtopping of the dam 

due to inadequate spillway capacity during the inflow design flood (IDF) and (2) dam failure under 

normal conditions (i.e., normal pool in the reservoir and normal inflow to the reservoir). The latter condition is sometimes referred to as a “sunny day” failure. Breach development and determination of the 

outflow and resulting inundation from a breached dam or embankment are discussed in Section 9.9.

14.2.5.3 Inundation Map

The inundation map facilitates notification and evacuation of persons in the flooded area through 

graphical presentation of the flood limits. The inundation limit for emergency planning purposes is 

the maximum extent of flooding based upon the most critical dam failure scenario. For the IDF condition, the downstream limit of inundation mapping is typically determined as the point at which 

the flow level returns to within a specified vertical distance of the flow without dam breach (e.g., one 

or two feet). For a “sunny day” failure, the downstream limit of inundation mapping should be the 

point at which flow is wholly contained within the stream channel banks.

The limits of inundation should be plotted on a topographic map that clearly indicates the locations 

of streets and other roadways and any critical facilities that would be flooded. Use of semi-transparent shading to show the extent of inundation is recommended. Typically, inundation maps are 

developed from U.S. Geological Survey (USGS) topographic maps. The scale of an inundation map 

typically varies depending upon the density of streets and associated structures. Urban areas normally require considerably more map detail than less populated areas.

The inundation map should also clearly show locations where roadblocks/traffic control will be 

needed. Responsibilities for operation of these roadblocks/traffic control points may be delineated 

in a separate table, depending upon state requirements. The inundation map should indicate the 

location of any reception centers and/or treatment facilities identified in the EAP. Evacuation routes 

(delineated by arrows indicating the direction of evacuation) should be clearly marked on the inundation map. It is also recommended that the arrival times (in relation to the time of breach) for the 

leading edge and peak discharge associated with the flood wave be shown on the inundation map. If 

this information cannot be presented on the inundation map, it may be provided in a separate table. 

A portion of a typical inundation map is shown in Figure 14.1.

14.2.6 Participant Responsibilities

An EAP must clearly define the responsibilities of all parties that would be involved in a dam failure 

scenario. These parties include the dam owner (or designated representative); state, county and local 

emergency management officials; emergency service providers (police, fire, EMS, Red Cross); elected 

officials; radio and television stations; school systems; state transportation agencies; public transit 

authorities; railroads; and parties such as nursing home operators and large businesses that would 

be inundated.

The EAP should indicate that the dam owner or designated representative is responsible for monitoring the dam in any situation where catastrophic failure is a possibility. The owner or designated 

representative should be responsible for initiating the notification process (e.g., notifying applicable 

state agencies, the MSHA district manager, the County Emergency Management Agency (EMA), 

the County 911 center and the National Weather Service (NWS)). Further discussion of notification 

responsibilities is provided in Section 14.2.8.

The responsibilities of each participant organization or individual should be clearly delineated. For 

example, a police department or sheriff’s office might set up a mobile communication station (squad 
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Emergency Action Planning

MAY 2009

car) at the dam, operate several traffic control points, and be responsible for evacuation. Local Emergency Management Coordinators (EMCs) are typically responsible for maintaining contact with the 

County EMA, notifying local elected officials and utility services, coordinating evacuation efforts, 

and developing damage assessments.

14.2.7 Implementation Triggers and Emergency Responses

As indicated in the previous section, the dam owner or designated representative is responsible for 

monitoring the dam in the event that certain conditions occur that could eventually lead to failure. The 

extent of the monitoring may vary from part-time to continuous depending upon the severity of the 

dam breach threat. The EAP should clearly indicate what event or events are associated with a specific 

monitoring level and what conditions will trigger implementation of plan emergency responses. Triggers for monitoring are frequently tied to the occurrence of large amounts of rainfall in a short period 

of time. Continuous monitoring should be implemented in the event of a condition such as observed 

piping, significant slope movement, or the failure of a structural component of the dam.

Once monitoring has been initiated, triggers that implement emergency actions will typically be in 

the form of a specified water surface elevation below the embankment crest, a specified flow depth in 

the emergency spillway or other easily measurable parameter. However, any observation of a condition that could lead to dam failure or other sudden release of water or slurry from the impoundment, 

such as a breakthrough into an underground mine, should also trigger the process. The dam owner 

or designated representative is responsible for assessing the situation and, if an emergency condition 

is identified, providing notification to implement the emergency procedures in the EAP. Others who 

might assist in the assessment of site conditions and the need for notification, such as state dam safety 

officials and MSHA, may or may not be available to provide input.

Table 14.2 presents guidance related to conditions that may be triggers for monitoring and for implementation of an EAP, as adapted from NRCS (2007b). Such conditions may include blockage of the

FIGURE 14.1 INUNDATION MAP FOR COAL REFUSE IMPOUNDMENT

LEGEND

INUNDATED AREA

COAL REFUSE

IMPOUNDMENT

TRAFFIC CONTROL POINT

FIGURE 14.1 INUNDATION MAP FOR COAL REFUSE IMPOUNDMENT
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TABLE 14.2 GUIDANCE FOR EAP EVENTS AND SITUATIONS

Event Situation Monitoring or 

Trigger Level

Overtopping

•	 Impoundment level is rising and approaching specified depth below 

embankment crest (minimum freeboard elevation)

1

•	 Impoundment water level above specified depth below embankment 

crest (e.g., 1 foot)

2

•	 Water is flowing over the dam 3

•	 Water is flowing over an abutment or saddleback rim of impoundment 3

Spillway Erosion or 

Blockage

•	 Water starting to flow through emergency spillway 1

•	 Emergency spillway flowing with bottom erosion advancing toward 

control section

2

•	 Emergency spillway blocked by significant debris or landslide material 

with impoundment level approaching minimum freeboard elevation

2

•	 Emergency spillway flowing with erosion at control section 3

Seepage

•	 New seepage areas or increased discharge from internal drain outlet 

within or near dam

1

•	 New seepage areas or internal drain discharge with cloudy flow and 

increasing flow rate

2

•	 Seepage at greater than a specified flow rate or causing erosion of the 

dam or foundation

3

Mine Discharge

•	 New mine discharge in the immediate vicinity of impoundment 1

•	 New mine discharge in the immediate vicinity of impoundment with 

cloudy flow and increasing flow rate

2

•	 Mine discharge at greater than specified flow rate 3

Sinkholes

•	 Observation of new sinkhole in impoundment area or on embankment 2

•	 Rapidly enlarging sinkhole 3

Embankment and 

Abutment Cracking

•	 New cracks in the embankment or abutments at greater than

 specified width

1

•	 New cracks in the embankment with associated seepage 2

•	 New cracks in the abutment with seepage and increasing flow rate 2

Embankment 

Movement

•	 Observed movement/slippage of embankment toe, slope, or crest 1

•	 Sudden or rapidly proceeding slides at embankment slope 3

•	 Sudden or rapidly proceeding subsidence at embankment crest 3

Instruments •	 Instrument readings beyond specified values 1

Flood •	 Rainfall or snowmelt predicted to be above specified level 1

Earthquake •	 Measurable earthquake felt or recorded within specified distance of dam 1

 Monitoring or Trigger Levels: 1. Non-emergency, unusual event – slowly developing

2. Potential dam failure situation – rapidly developing

3. Urgent situation, dam failure appears to be imminent or is in progress

(ADAPTED FROM NRCS, 2007b)
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principal or emergency spillway, significant sloughing or slope failure in the dam embankment, overtopping or severe erosion, structural failure of a dam component, occurrence of sinkholes, cracking in 

the embankment, or other event that could lead to catastrophic failure. At sites where underground 

mine workings are present, elevated mine pool levels or mine discharges may also be triggers.

NRCS (2007b) recommends establishing emergency levels for identifying monitoring and response 

actions ranging from an initial level associated with a slowly developing, unusual (non-emergency) 

event to an advanced level associated with potential imminent failure or a failure in progress. While 

establishment of such levels may be useful to dam owners and operators in responding to dam emergencies, some states do not currently require that this be done.

It is recommended that emergency responses be identified for each implementation trigger. In 

addition to notification requirements, as subsequently discussed, other actions ranging from continued monitoring to preventative measures may be appropriate. It may also be useful to identify 

in the EAP additional personnel, sources of construction materials, and contractors that can be 

utilized in an emergency situation. This information may also be provided in the Operation and 

Maintenance Plan.

14.2.8 Notification Requirements

Notification of plan participants should be carried out in a systematic manner such that the most 

critical notifications are made immediately following a triggering event. Each plan participant must 

know his or her responsibilities for contacting other participants, including the preferred order of 

contact. The contact information should allow for communication on a 24-hour-per-day, seven-dayper-week basis. Contact information should be kept up to date as personnel or telephone numbers 

change. FEMA recommends that a flowchart be prepared that illustrates the notification process 

and each individual’s notification responsibilities. FEMA’s suggested format places the notification 

flowchart at the very front of the EAP. Some states have adopted the FEMA-recommended format 

(referred to as the ICODS format), while other states do not employ an actual flowchart, but instead 

use a structured text format to convey the same information.

Regardless of the location of a dam and the applicable state regulations, the important factor is that 

each participant’s contact responsibilities, including the preferred order of contact, are clearly indicated in some manner. Typically the dam Owner or Owner’s representative is responsible for contacting applicable state agencies, the County EMA/911 Center, MSHA and possibly the NWS. The County 

EMA/911 Center typically is responsible for contacting local EMCs, police and fire departments, EMS 

and ambulance services, local media, and other institutions, as appropriate. The EAP notification chart 

should be posted in prominent locations at the mine site for ready access during an emergency.

14.2.9 EAP Public Copies

It may be a requirement that copies of an EAP be made available to the public at locations such as 

state and local emergency management agency offices, public libraries, the dam owner’s office or 

other locations with public access. These locations may be part of the specified content for the EAP. 

Also, local emergency response organizations are a good source of information. It is important that 

all versions of the EAP regardless of location or media be updated at regular intervals, as required by 

the appropriate regulatory agency.

14.2.10 Exercises/Training

States may or may not require that EAP exercises be performed on a regular basis or that participant 

training be performed. FEMA (2004b) defines five levels of exercise that may be employed to maintain an enhanced state of readiness for an actual emergency. The five exercise levels are:
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1. Orientation seminar – This lowest exercise level involves bringing together the 

primary entities with a role or interest in the EAP (typically the dam owner and 

state and local emergency management agencies) to discuss the EAP. The primary 

purpose of an orientation seminar is for the participants to become familiar with 

the EAP and their roles/responsibilities. The orientation seminar may be used for 

planning a higher level exercise. Mine operators are encouraged to hold face-to-face 

meetings with local emergency management personnel to develop a working relationship so that their first contact is not during an actual emergency.

2. Drill – A drill is the lowest exercise level that involves actual exercise components. A 

drill typically involves in-house training for EAP participants.

3. Tabletop exercise – A tabletop exercise is typically a meeting of the dam owner and 

state and local emergency management officials in a conference room environment. 

A tabletop exercise begins with a description of a simulated event followed by discussions among participants of the EAP response procedures. Tabletop exercises are 

normally conducted in an informal, relaxed environment.

4. Functional exercise – A functional exercise is the highest level exercise that does not 

involve full activation of key plan participants or actual evacuation of downstream 

inhabitants. A functional exercise is intended to allow an evaluation of the ability of 

the dam owner and emergency management officials to perform their EAP responsibilities. A secondary objective is to improve coordination among EAP participants.

5. Full-scale exercise – A full-scale exercise involves the most realistic simulation of an 

actual emergency event. A full-scale exercise is intended to evaluate the operational 

capability of all aspects of and participants in the EAP interactively in a stressful 

environment with actual mobilization of personnel and resources. The participants 

“play out” their roles in a dynamic environment that provides the highest degree of 

realism possible. A full-scale exercise may involve actual evacuation of residents that 

would be in the downstream inundation area.

Typical state requirements are equivalent to the tabletop exercise level or lower. Exercises at any level 

will enhance the ability of EAP participants to perform their emergency function in the event of a 

dam breach.

14.2.11 Plan Maintenance

An EAP should be updated at regular intervals. Ideally, the names of key plan participants and contact information should be updated annually or as changes occur. The interval for regular updating 

varies from state to state, generally ranging between one and five years. EAP updates should be 

prepared by or have appropriate input from an engineer familiar with the impoundment and EAP 

preparation and should reflect any changes to the state’s EAP content and format requirements since 

the previous submittal.

Any changes to the dam and related structures and any changes that will affect the inundation map 

should be identified. For mine facilities where impoundment capacity has increased as a result of 

disposal plan modifications and staged construction, dam breach analyses should be reviewed and 

updated and, if necessary, a new inundation map should be prepared. Also, if there are changes to 

design bases such as a state-mandated increase in the IDF, a new dam breach analysis should be performed and a new inundation map should be prepared.

The scope of work for an EAP update should include a reconnaissance of the inundation area to 

determine if any new facilities (e.g., mining operation, day care center, nursing home, large business) 

are located in the inundation area and will be significantly affected. Also new highways or streets and 
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changes to existing highways and streets should be noted and the appropriate changes made to the 

EAP. Plans for roadblocks and traffic control should be reviewed. Evacuation routes should also be 

reviewed to make certain that they are still appropriate.

Personnel or positions previously identified in an EAP may have changed and updates should be 

made as necessary. Agencies such as the Red Cross should be contacted to verify that reception and 

treatment center information is current. Any information provided in the EAP that is no longer applicable should be removed or an update provided.

14.2.12 EAP Appendices

Supporting documents should be provided in appendices to the EAP. Depending upon individual 

state requirements, these could include:

• Calculation brief for the dam breach analysis, determination of downstream inundation area, and flood wave arrival times.

• A map showing the location of the dam and highways and streets that must be traveled to reach the dam with accompanying written directions.

• An inundation map providing limits of the extent of flooding and key information 

such as roadblock locations, reception and treatment center locations, evacuation 

routes, and other important data such as flood wave travel times to critical areas.

• Tables indicating responsibility for roadblocks and/or traffic management points 

(e.g., state highway patrol station, local police department, local fire department) 

shown on the inundation map.

• Roster of participating agencies, personnel and phone numbers.

• Notification flowchart indicating each participant’s responsibilities relative to contacting other participants in the EAP.

• Check-off sheets to document when persons/agencies/responders were notified relative to plan implementation.

• Announcements to be read on radio and television when the EAP is implemented.

• Public notices for posting indicating where copies of the EAP are available to the 

public for review.

14.3 PREPARATION FOR POTENTIAL EMERGENCY REMEDIAL ACTIONS

When an emergency situation develops at an impounding embankment/dam, implementation of 

appropriate remedial actions could possibly delay, moderate or even prevent the structure from 

failing. Potential emergency management actions and responsibilities should be delineated in the 

Operation and Maintenance Plan (Section 11.1.4) so that personnel qualified and responsible for 

implementing theses remedial actions are better equipped to assess the situation and to implement 

appropriate responses. Emergency situations should be evaluated at the earliest indication of a possible problem by an engineer familiar with dam safety who can recommend an appropriate course 

of action.

A potential failure mode analysis (PFMA), as discussed in Section 12.1.1, can help in the identification of equipment, materials and potential remedial actions needed for responding to an emergency. 

Potential sources of equipment and materials should be identified and documented in the Operation 

and Maintenance Plan. In some situations, aggregate stockpiles (excess materials for scheduled drain 

or filter construction) may be available on site and may be useful in an emergency. Also, sources of 

emergency power for lighting or equipment operation can be identified. Potential emergency remedial actions that may be appropriate for emergency situations and should potentially be included 
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in the Emergency Management section of the Operation and Maintenance Plan are discussed in the 

following subsections. The designer or other dam safety engineer should assess whether these emergency remedial actions are applicable to their site.

14.3.1 Potential Overtopping During a Storm

The following actions should be considered in the event of a severe storm where the water level in the 

impoundment is increasing and is approaching the crest of the embankment:

• Placing additional material along the crest to increase freeboard. This will have the 

effect of temporarily allowing more water to be stored and will allow more water to 

flow through the spillway.

• Providing erosion-resistant protection for the downstream slope by placing riprap, 

rockfill, or other material in erosion-prone areas.

• Terminating pumping of water or slurry into the impoundment until the emergency 

situation abates.

• Lowering the water level by pumping or siphoning water out of the impoundment.

• Creating additional spillway capacity by making a controlled cut or breach in a 

saddle area or in a low embankment section where the foundation materials are 

erosion resistant and adding supplemental erosion protection where needed. The 

potential for the cut area to rapidly erode and release the reservoir should be considered before taking this action.

14.3.2 Movement or Sliding of the Embankment

If there is movement or sliding of an impounding embankment, the following actions should be considered:

• Placing stakes or pins and recording and evaluating the amount of movement versus 

time. Movement of a slope will generally accelerate prior to complete failure. Other 

monitoring could include piezometers and seepage weirs (record and evaluate).

• Lowering the water level at a rate and to an elevation considered to be safer given 

the slide condition.

• Stabilizing the slide by buttressing the toe area with additional rockfill, mine waste, 

spoil, or soil. If the buttress material does not have characteristics that will allow it to 

act as a filter and drain, available sources of other materials to perform the function 

in combination with the buttress material, as appropriate, should be identified.

• Lowering the phreatic surface by placing impermeable material on the upstream 

slope.

• Restoring lost freeboard, if necessary, by placing sandbags or by filling in the top of 

the slide. Note that this action will further load the sliding mass if it is not offset by 

buttressing of the toe area.

14.3.3 Internal Erosion or Piping through the Dam, Foundation or Abutments

If seepage through a dam or its foundation or abutments is detected and is found to be increasing at 

a rate that could threaten the stability of the dam, the following actions should be considered:

• Moving the slurry discharge point so that fines are deposited in the reservoir near 

the area where the problem seepage is most likely originating. For example, there 

may be a whirlpool in the reservoir, or seepage may be visible at an abutment.
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MAY 2009

• Lowering the impoundment water level until the flow decreases to a non-erosive 

velocity or until it ceases. The water level should remain lowered until corrective 

action is taken.

• Plugging the reservoir side of the flow area with material (e.g., hay bales, bentonite, 

or other materials) if the entrance to the leak is in the reservoir basin.

• Placing a weighted filter over the exit area to hold the materials in place. Preferably, several feet of a protective sand filter should be placed. The sand layer can be 

weighted with additional material. If sand is not readily available, a geotextile could 

also be used for the filter layer.

• Isolating the area where the seepage is originating, if possible, by constructing a 

perimeter dike around the area.

• Monitoring seepage relative to quantity and suspended solids.

14.3.4 Excessive Seepage and a High Level of Saturation in the Embankment

If wet areas are detected on the downstream face of an embankment along with a high phreatic surface indicating a high level of saturation and reduced embankment stability, the following actions 

should be considered:

• Monitoring frequently for signs of slides, cracking or concentrated seepage. Recording and evaluating piezometer readings and seepage weir flow measurements.

• Relocating the slurry discharge line to the extent possible to develop a delta and 

push free water back away from the upstream slope of the embankment.

• Lowering the water level in the reservoir to a safer level.

• Operating at a reduced level until the situation is evaluated.

• Lowering the phreatic surface by placing impermeable material on the upstream 

slope.

14.3.5 Spillway Failure or Erosion that Could Cause a Dam Breach

If an impoundment spillway fails or its capacity is reduced, or if there is erosion progressing such 

that a dam breach could result, the following actions should be considered:

• Providing temporary protection at the point of erosion by placing sandbags, rip-rap 

materials, or plastic sheets anchored with sandbags.

• Lowering the water level to a safe elevation. If an outlet is not available, pumping, 

siphoning, or a controlled breach may be required.

• Continuing operation at a lowered water level to minimize spillway flow.

14.3.6 Excessive Settlement or Subsidence of the Embankment 

If excessive settlements are observed in the embankment or actual subsidence is detected, the following actions should be considered:

• Lowering the impoundment water level until the situation can be evaluated by: (1) 

releasing it through the primary spillway or by pumping, (2) siphoning, or (3) a controlled breach.

• Temporarily restoring freeboard, if necessary, by placing additional material such as 

mine waste, spoil, sandbags, etc. along the crest.

• Watching for increased seepage and internal erosion due to cracks resulting from differential movements. If observed, the actions indicated above should be employed.
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• Checking for indications of damage to the decant conduit resulting from differential 

movement.

14.3.7 Formation of a Sinkhole on the Crest or Face of the Dam

In the event of observation of a sinkhole at the embankment crest or on one of the faces of the dam, 

the following actions should be considered:

• Lowering the impoundment water level.

• Checking the area downstream of the sinkhole to identify where material is being 

carried out. The measures indicated in Section 14.3.3 above for stopping or reducing 

internal erosion should be considered.

14.3.8 Earthquake

Subsequent to the occurrence of an earthquake that could potentially have damaged site structures, 

the following actions should be considered for each impounding embankment:

• Conducting an overall visual inspection of the dam and monitoring instrumentation.

• If the embankment has any upstream construction, the seismic shaking may have 

caused high pore-water pressures in the loose fines. Embankment faces should be 

checked for signs of sliding, and the crest should be observed for indications of 

settlement.

• Evaluating instrumentation data. Piezometer readings should be taken and compared to normal readings. The situation should be monitored until elevated porewater pressures have dissipated. The impoundment water level should be lowered, if 

necessary.

• If settlement has occurred, it should be determined if measures should be taken so 

that the design storm can still be accommodated.
References

R–1

(* – Available on Manual DVD)

Adler, L. and M.C. Sun, 1968, Ground Control in Bedded Formations, Bulletin 28, Virginia Polytechnic 

Institute Research Division, Blacksburg, VA.

Albuquerque, A.J.J., 1994, Geoenvironmental Aspects of Coal Refuse-Fly Ash Blends, M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA.

* Almes, R.G. and A. Butail, 1976, “Coal Refuse: Its Behavior Related to the Design and Construction of Coal Refuse Disposal Facilities,” Proceedings of the Seventh Ohio Valley Soils Seminar on Shales 

and Mine Wastes, Lexington, KY.

American Association of State Highway and Transportation Officials (AASHTO), 2008, Standard 

Specifications for Transportation Materials and Methods of Sampling and Testing, Part I – Specifications, 

28th Ed., and Provisional Standards, 2008 Ed., AASHTO, Washington, DC.

American Concrete Institute (ACI), 2008, Building Code Requirements for Masonry Structures (ACI 

530-95), Farmington Hills, MI.

American Concrete Institute (ACI), 2007, ACI Manual of Concrete Practice, Parts 1, 2, and 3, Farmington Hills, MI.

American Concrete Institute (ACI), 2005a, Guide to Mass Concrete, ACI 207.1R-05, Farmington Hills, MI.

American Concrete Institute (ACI), 2005b, Building Code Requirements for Structural Concrete (ACI 

318-05) and Commentary (ACI 318R-05), Farmington Hills, MI.

American Concrete Pipe Association (ACPA), 2007, Concrete Pipe Design Manual, ACPA, Irving, TX.

American Concrete Pipe Association (ACPA), 1987, Concrete Pipe Design Manual, ACPA, Vienna, VA.

American Water Works Association (AWWA), 2007, Design of Prestressed Concrete Cylinder Pipe, 

Standard C304-07, AWWA, Denver, CO.

American Water Works Association (AWWA), 2004, Steel Water Pipe, M11, AWWA, Denver, CO.

American Water Works Association (AWWA), 1995, Concrete Pressure Pipe, M9, 2nd Ed., AWWA, 

Denver, CO.

* Andrews, D.E., J.L. Withiam, E.F. Perry and H.L. Crouse, 1980, Environmental Effects of Slaking of 

Surface Mine Spoils – Eastern and Central United States, Final Report, Contract No. J0285024, U.S. 

Department of the Interior, Bureau of Mines, Denver, CO.

Appalachian Regional Commission, 1969, Acid Mine Drainage in Appalachia, Congressional House 

Document No. 91-180, Vols. I, II, and III, Washington, DC.

Arkell, R.E. and F. Richards, 1986, “Short Duration Rainfall Relations for the Western United 

States,” Preprint Volume, Conference on Climate and Water Management, American Meterological Society, Boston, MA, pp. 136-141.

* Arman, A., N. Samtani, R. Castelli and G. Munfakh, 1997, Training Course in Geotechnical and Foundation Engineering: Subsurface Investigations – Participants Manual, Contract No. FHWA-HI-97-021, 

Federal Highway Administration, National Highway Institute, Arlington, VA.
R–2

References

ASTM International (ASTM), 2008a, Annual Book of ASTM Standards, Section Five: Petroleum Products, Lubricants, and Fossil Fuels, Volume 05.06, Gaseous Fuels; Coal and Coke, ASTM, West Conshohocken, PA.

ASTM International (ASTM), 2008b, Annual Book of ASTM Standards, Section Four: Construction, 

Volume 04.08, Soil and Rock (I): D 420 – D 5611, ASTM, West Conshohocken, PA.

ASTM International (ASTM), 2008c, Annual Book of ASTM Standards, Section Four: Construction, 

Volume 04.09, Soil and Rock (II): D 5877 – latest, ASTM, West Conshohocken, PA.

ASTM International (ASTM), 2008d, Annual Book of ASTM Standards, Section Four: Construction, 

Volume 04.13, Geosynthetics, ASTM, West Conshohocken, PA.

Atkinson, G.M. and D. M. Boore, 2006, “Earthquake Ground-Motion Prediction Equations for 

Eastern North America, Bulletin of the Seismological Society of America, Vol. 96, No. 6, pp. 2181-2205.

Atkinson, G.M. and D.M. Boore, 1995, “Ground Motion Relations for Eastern North America,” 

Bulletin of the Seismological Society of America, Vol. 85, No. 1, pp. 17-30.

* Atkinson, G.M. and S.I. Kaka, 2006, Relationships between Felt Intensity and Instrumental Ground 

Motion for New Madrid Shake Maps, Report for Award 05HQGR0039, Department of Earth Sciences, 

Carleton University, Ottawa, QC, Canada.

Atkinson, G.M. and W. Silva, 1997, “An Empirical Study of Earthquake Source Spectra for California Earthquakes,” Bulletin of the Seismological Society of America, Vol. 87, No. 1, pp. 97-113.

* Babcock, C.O and V.E. Hooker, 1977, Results of Research to Develop Guidelines for Mining Near Surface 

and Underground Bodies of Water, Information Circular 8741, U.S. Department of the Interior, Bureau of 

Mines, Pittsburgh, PA.

Backer, R.R., R.A. Busch and L.A. Atkins, 1977, Physical Properties of Western Coal Waste Materials, 

Report of Investigations 8216, U.S. Department of the Interior, Bureau of Mines, Spokane, WA.

Baecher, G.B. and J.T. Christian, 2003, Reliability and Statistics in Geotechnical Engineering, John 

Wiley & Sons, Hoboken, NJ.

Bakun, W.H. and M.G. Hopper, 2004, “Magnitudes and Locations of the 1811-1812 New Madrid, 

Missouri, and the 1886 Charleston, South Carolina, Earthquakes,” Bulletin of the Seismological Society of America, Vol. 94, No. 1, pp. 64-75.

Bardet, J.P., 1997, Experimental Soil Mechanics, Prentice Hall, Upper Saddle River, NJ.

Barfield, B.J., R.C. Warner and C.T. Haan, 1981, Applied Hydrology and Sedimentology for Disturbed 

Areas, Oklahoma Technical Press, Stillwater, OK.

* Bartholomew, C.L., B.C. Murray and D.L. Goins, 1987, Embankment Dam Instrumentation Manual, 

U.S. Department of the Interior, Bureau of Reclamation, Denver, CO.

* Bazán-Arias, N.C., S.R. Michalski, P.E. Glogowski and F. Howard, 2002, “In-place Solidification of 

Coal Tailings for Expressway Subgrade,” Proceedings, Tailings Dams 2002, Las Vegas, Association of 

State Dam Safety Officials, Lexington, KY, pp. 129-140.

Baziar, M.H. and R. Dobry, R., 1995, “Residual Strength and Large Deformation Potential of Loose 

Silty Sands,” Journal of Geotechnical Engineering, ASCE, Vol. 121, No. 12, pp. 896-906.

Becker, E., C.K. Chan and H.B. Seed, 1972, Strength and Deformation Characteristics of Rockfill Materials in Plane Strain and Triaxial Compression Tests, Report TE-72-3 to the California Department of 

Water Resources, University of California, Berkeley, CA.

Bennett, O.L., E.L. Mathias, W.H. Armiger and J.N. Jones Jr., 1978, “Plant Materials and Their 

Requirements for Growth in Humid Regions,” Reclamation of Drastically Disturbed Land, F.W. 

Schaller and P. Sutton, eds., American Society of Agronomy, Madison, WI, pp. 285-306.
R–3

Engineering and Design Manual Coal Refuse Disposal Facilities

Benson, R.C., R.A. Glaccum and M.R. Noel, 1984, Geophysical Techniques for Sensing Buried Wastes and 

Waste Migration, Report, Contract No. 68-03-3050, U.S. Environmental Protection Agency, Office of 

Research and Development, Environmental Monitoring Systems Laboratory, Las Vegas, NV.

Beresnev, I.A. and G.M. Atkinson, 1997, “Modeling Finite-Fault Radiation from ωn Spectrum,” Bulletin of the Seismological Society of America, Vol. 87, No. 1, pp. 67-84.

Bernreuter, D.L., J.B. Savy, R.W. Mensing and J.C. Chen, 1989, Seismic Hazard Characterization of 69 

Nuclear Plant Sites East of the Rocky Mountains: Methodology, Input Data and Comparisons to Previous 

Results for Ten Test Sites, NUREG/CR-5250 (UCID-21517), U.S. Nuclear Regulatory Commission, 

Vols. 1-8.

Berube, A.P., 2004, Investigating the Streaming Potential Phenomenon Using Electric Measurements 

and Numerical Modelling with Special Reference to Seepage Monitoring in Embankment Dams, Doctoral 

Thesis, Lulea University of Technology, Department of Civil and Environmental Engineering, 

Division of Applied Geophysics, Lulea, Sweden.

Betoney, T.P., Jr., 1994, MSHA Accident Investigation Report (Surface Area of an Underground Coal 

Mine), Non-Injury Impoundment (Coal Refuse Pile ID No. 1211WV30054-00), Arkwright No. 1 Mine, 

Consolidation Coal Company, Monongalia County, WV.

Bhabdaru, S. and O.P. Arora, 1987, “Vibrations in an Underground Coal Mine Due to Adjacent 

Surface Mine Blasts,” Proceedings, 2nd International Symposium on Rock Fragmentation by Blasting, 

W.L. Fourney and R.D. Dick, eds., Society for Experimental Mechanics, Keystone, CO.

Biarez, J., J.L. Bordes and P. Londe, 1970, “La Fissuration des Noyaux des Barrages en Enrochments ou en Alluvion,” Proceedings, 10th International Congress on Large Dams, Montreal, Canada, 

Vol. I, pp. 27-47.

* Bieniawski, Z.T., 1992, “A Method Revisited: Coal Pillar Strength Formula Based on Field Investigations,” Proceedings, Workshop on Coal Pillar Mechanics and Design, Information Circular 9315, U.S. 

Department of the Interior, Bureau of Mines, Washington, DC, pp. 158-165.

Bieniawski, Z.T., 1989, Engineering Rock Mass Classifications, John Wiley & Sons, New York, NY.

Bieniawski, Z.T., 1987, Strata Control in Mineral Engineering, John Wiley & Sons, New York, NY.

Bieniawski, Z.T., 1968, “The Effect of Specimen Size on Compressive Strength of Coal,” International Journal of Rock Mechanics and Mining Sciences, Vol. 5, pp. 325-335.

* Bigatel, A.J., G.R. Camus, D.W. Caylor, A.D. Dalberto and W.W. Hellier, 1999, Engineering Manual 

for Mining Operations, Chapter 3: Sediment Control Impoundments and Coal Refuse Disposal Impoundments, Pennsylvania Department of Environmental Protection, Bureau of Mining and Reclamation, 

Harrisburg, PA.

Bignell, V., 1977, “Case Study 2 - The Aberfan Failure,” Catastrophic Failures, V. Bignell, G. Peters 

and C. Pym, eds., Open University Press.

Bishop, A.W., 1955, “The Use of the Slip Circle in the Stability Analyses of Slopes,” Géotechnique, 

Vol. 5, No. 1, pp. 7-17.

Bishop, A.W. and D.J. Henkel, 1962, The Measurements of Soil Properties in the Triaxial Test, Edward 

Arnold, Ltd., London, England.

* Biswas, K., C. Mark and S.S. Peng, 1999, “A Unique Approach to Determining the Time-Dependent 

In Situ Strength of Coal Pillars,” Proceedings, 2nd Workshop on Coal Pillar Mechanics and Design, 

NIOSH IC 9448, U.S. Department of Health and Human Services, National Institute for Occupational Safety and Health, Pittsburgh, PA, pp. 5-13.

* Biswas, K., S.S. Peng and P. Tsang, 1995, “Influence of Partings in a Coal Seam on Long Term Pillar 

Stability,” Proceedings, 14th International Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, pp. 80-93.
R–4

References

Bjerrum, L., 1972, “Embankments on Soft Ground,” Proceedings, Specialty Conference on Performance 

of Earth and Earth-Supported Structures, American Society of Civil Engineeers (ASCE), Reston, VA, 

Vol. 2, pp. 1-54.

Blackburn, J.T. and C.H. Dowding, 2004, “Finite-Element Analysis of Time Domain Reflectometry 

Cable-Grout-Soil Interaction,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 

130, No. 3, pp. 231-239.

Blight, G.E., 1968, “A Note on Field Vane Testing of Silty Soils,” Canadian Geotechnical Journal, 

Vol. 5, No. 3, pp. 142-149.

Bommer, J.J., N.A. Abrahamson, F.O. Strasser, A. Pecker, P-Y. Bard, H. Bungum, F. Cotton, D. Fäh, 

F. Sabetta, F. Scherbaum and J. Studer, 2004, “The Challenge of Defining Upper Bounds on Earthquake Ground Motions,” Seismological Research Letters, Vol. 75, No. 1, pp. 82-95.

* Boulanger, R.W. and I.M. Idriss, 2004, “Evaluating the Potential for Liquefaction or Cyclic Failure 

of Silts and Clays,” Report No. UCD/CGM-04/01, Center for Geotechnical Modeling, Department of 

Civil and Environmental Engineering, University of California, Davis, CA.

Bowles, J.E., 1996, Foundation Analysis and Design, McGraw-Hill, New York, NY.

Bradham, W.S. and F.T. Caruccio, 1995, Sensitivity Analysis of Laboratory Based Mine Overburden 

Analytical Techniques for the Prediction of Acidic Mine Drainage, U.S. Department of the Interior, 

Office of Surface Mining, Pittsburgh, PA.

Brady, B.H.G. and E.T. Brown, 2004, Rock Mechanics for Underground Mining, Springer Publishing 

Company, New York, NY.

Brady, K.B.C., M.W. Smith and J. Schueck, eds., 1998, Coal Mine Drainage Prediction and Pollution Prevention in Pennsylvania, Pennsylvania Department of Environmental Protection, Harrisburg, PA.

Brady, K.B.C. and R.H. Hornberger, 1990, “The Prediction of Mine Drainage Quality in Pennsylvania,” Water Pollution Control Association of Pennsylvania Magazine, Vol. 23, No. 5, pp. 8-15.

Brater, E.F., H.W. King, J.E. Lindell and C.Y. Wei, 1996, Handbook of Hydraulics, 7th Ed., McGrawHill, New York, NY.

British Columbia Acid Mine Drainage Task Force, 1989, Draft Acid Rock Drainage Guide, BiTech 

Publishers, Richmond, BC, Canada.

* Brown, B.S. and G. Greenaway, 1999, “Field Experience in the Use of Wick Drains to Consolidate 

Soft Tailings,” Proceedings, Workshop on Stabilization of Soft Tailings, Part 2: Practice and Experience, 

Chemnitz, Germany.

* Brunner, D.J. and J.J. Schwoebel, 1999, “Directional Drilling for Methane Drainage and Exploration in Advance of Mining: Recent Advances and Applications,” REI Drilling, Salt Lake City, UT.

* Busch, R.A., R.R. Backer and L.A. Atkins, 1975, “Physical Property Data on Fine Coal Refuse,” 

Report of Investigations 8062, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA.

* Busch, R.A., R.R. Backer and L.A. Atkins, 1974, “Physical Property Data on Coal Waste Embankment Materials,” Report of Investigations 7964, U.S. Department of the Interior, Bureau of Mines, 

Pittsburgh, PA.

Butler, D.K., J.L. Llopis, T.L. Dobecki, M.J. Wilt, R.F. Corwin and G. Olhoeft, 1990, “Comprehensive 

Geophysics Investigation of an Existing Dam Foundation: Engineering Geophysics Research and 

Development, Part 2,” Geophysics: The Leading Edge of Exploration, Vol. 9, No. 9, pp. 44–53.

Butler, D.K., J.L. Llopis and C.M. Deaver, 1989, “Comprehensive Geophysical Investigation of an 

Existing Dam Foundation,” Geophysics: The Leading Edge of Exploration, Vol. 8, No. 8, pp. 10-18.
R–5

Engineering and Design Manual Coal Refuse Disposal Facilities

Byrne, P.M., S.-S. Park, M. Beaty, M.K. Sharp, L. Gonzalez and T. Abdoun, 2004, “Numerical Modeling of Liquefaction and Comparison with Centrifuge Tests,” Canadian Geotechnical Journal, Vol. 

41, No. 2, pp. 193-211.

Cai, S., 1983, “A Simple and Convenient Method for Design of Strength of Cemented Hydraulic 

Fill,” Proceedings, International Symposium on Mining with Backfill, Lulea, Sweden, S. Granholm, ed., 

pp. 405-412.

Campbell, K.W., 2003, “Prediction of Strong Ground Motion Using the Hybrid Empirical Method 

and Its Use in the Development of Ground-Motion (Attenuation) Relations in Eastern North 

America,” Bulletin of the Seismological Society of America, Vol. 93, No. 3, pp. 1012-1033.

Cannon, W.C., 1981, MSHA Report of Investigation, Impoundment Accident (Failure of Refuse Pile), 

Preparation Plant, Island Creek Coal Company, Boone County, WV.

* Carlson, C.L. and D.C. Adriano, 1993, “Environmental Impacts of Coal Combustion Residues,” 

Journal of Environmental Quality, Vol. 22, pp. 227-247.

Carroll, R.G., Jr., 1983, “Geotextile Filter Criteria,” Engineering Fabrics in Transportation Construction, 

Transportation Research Record 916, Transportation Research Board, Washington, DC, pp. 46-53.

Carter, M. and S.P. Bentley, 1991, Correlations of Soil Properties, Pentech Press, London, U.K.

Casagrande, A., 1971, “On Liquefaction Phenomena,” Géotechnique, London, U.K., Vol. 21, No. 3, 

pp. 197-202.

Castro, G., 2003, “Evaluation of Seismic Stability of Tailings Dams,” Proceedings, 12th Pan American 

Conference on Soil Mechanics and Geotechnical Engineering and 39th U.S. Rock Mechanics Symposium, 

Cambridge, MA, pp. 2229-2234.

Castro, G., 1995, “Empirical Methods in Liquefaction Evaluation,” Proceedings, First International 

Leonardo Zeevaert Conference, Mexico City, Mexico, Vol. 1, pp. 1-41.

Castro, G., 1994, “Seismically Induced Triggering of Liquefaction Failures,” Performance of 

Ground and Soil Structures During Earthquakes, Proceedings, Thirteenth International Conference on 

Soil Mechanics and Foundation Engineering, New Delhi, India.

Castro, G., 1975, “Liquefaction and Cyclic Mobility of Saturated Sands,” Journal of the Geotechnical 

Engineering Division, ASCE, Vol. 101, No. GT6, pp. 551‐569.

Castro, G. and J.T. Christian, 1976, “Shear Strength of Soils and Cyclic Loading,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 102, No. GT9, pp. 887-894.

Castro, G., R.B. Seed, T.O. Keller and H.B. Seed, 1992, “Steady State Strength Analysis of Lower 

San Fernando Slide,” Journal of Geotechnical Engineering, ASCE, Vol. 118, No. 3, pp. 406-427.

Caterpillar Tractor Company (Caterpillar), 2008, Caterpillar Performance Handbook, 38th Ed., Peoria, IL.

Cedergren, H.R., 1989, Seepage, Drainage and Flow Nets, 3rd Ed., John Wiley & Sons, New York, NY.

Canadian Geotechnical Society (CGS), 2007, Canadian Foundation Engineering Manual, 4th Ed., 

BiTech Publishers, Richmond, BC, Canada.

Chamberlain, E.A.C. and D.A. Hall, 1973, “The Ambient Temperature Oxidation of Coal in Relation to the Early Detection of Spontaneous Heating – Part 2,” The Mining Engineer, Vol. 132, No. 

152, pp. 387-399.

Chamberlain, E.A.C., D.A. Hall and J.T. Thirlaway, 1970, “The Ambient Temperature Oxidation of 

Coal in Relation to the Early Detection of Spontaneous Heating – Part 1, The Mining Engineer, Vol. 

130, No. 121, pp. 1-16.

Chandler, R.J., 1988, “The In-situ Measurement of the Undrained Shear Strength of Clays Using 

the Field Vane,” Vane Shear Strength Testing in Soils: Field and Laboratory Studies, ASTM STP 1014, 

A.F. Richards, ed., American Society for Testing and Materials, West Conshohocken, PA, pp. 13-44.
R–6

References

Chapman, M.C., J.W. Munsey, S.C. Whisner and J. Whisner, 2002, “The Eastern Tennessee Seismic 

Zone: Small Earthquakes Illuminating Major Basement Faults?” Proceedings, GSA Joint Annual 

Meeting, North-Central Section (36th) and Southeastern Section (51st), Lexington, KY, Session 21, 

Paper 21-9.

* Chekan, G.J., 1985, Design of Bulkheads for Controlling Water in Underground Mines, Information Circular 9020, U.S. Department of the Interior, Bureau of Mines, Washington, DC.

Chen, C.Y., 1979, “Engineering Properties of Coal Refuse,” Proceedings of the First International Symposium on Stability in Coal Mining, C.O. Brawner and I.P.F. Dorling, eds., Miller Freeman Publications, San Francisco, CA, pp. 328-347.

Chen, Z.Y. and N.R. Morgenstern, 1983, “Extensions to the Generalized Method of Slices for Stability Analysis,” Canadian Geotechnical Journal, Vol. 20, No. 1, pp. 104-119.

Choquet, P., M. Quirion and F. Juneau, 2000, “Advances in Fabry-Perot Fiber Optic Sensors and 

Instruments for Geotechnical Monitoring,” Geotechnical News, BiTech Publishers, Richmond, BC, 

Canada, Vol. 18, No. 1, pp. 35-40.

Chow, V.T., ed., 1964, Handbook of Applied Hydrology, McGraw-Hill, New York, NY.

Chow, V.T., 1959, Open-Channel Hydraulics, McGraw-Hill, New York, NY.

* Chugh, Y.P., R. Nath and S. Shankar, 1987, “Time-Dependent Behavior of Weak Floor Strata from 

an Illinois Coal Mine,” Proceedings, 6th International Conference on Ground Control in Mining, West 

Virginia University, Morgantown, WV, pp. 204-218.

Church, H.K., 1981, Excavation Handbook, McGraw-Hill, New York, NY.

Clark, J.C., W.J. Johnson and W.A. Miller, 1994, “The Application of High Resolution Shear Wave 

Seismic Reflection Surveying to Hydrogeological and Geotechnical Investigations,” Proceedings, 

SAGEEP ‘94, Environmental & Engineering Geophysical Society, Boston, MA.

Clough, R.H., G.A. Sears and S.K. Sears, 2008, Construction Project Management, 5th Ed., John Wiley 

& Sons, New York, NY.

Coalgate, J.L., D.J. Akers and R.W. Frum, 1973, Gob Pile Stabilization, Reclamation, and Utilization, 

U.S. Office of Coal Research, Research and Development Report No. 75, Interim Report No. 1, 

West Virginia University, Morgantown, WV.

Coates, D., 1981, “Caving, Subsidence, and Ground Control,” Rock Mechanics Principles, Chapter 5, 

Monograph No. 874, Canadian Centre for Mineral and Energy Technology, (CANMET), Canadian 

Government Publishing Centre, Ottawa, QC, Canada.

Coates, D.F. and Y.S. Yu, 1977, Pit Slope Manual, Chapter 9 – Waste Embankments, Report No. 77-9, 

Canadian Centre for Mineral and Energy Technology (CANMET), Canadian Government Publishing Centre, Ottawa, QC, Canada.

* Colorado School of Mines (CSM), 2007, Void Detection Demonstrations at the Colorado School of Mines, 

Edgar Experimental Mine, prepared for MSHA by the Western Mining Resource Center, CSM, 

Golden, CO.

Contreras, I.A., A.T. Grosser and R.H. VerStrate, 2007, “The Use of the Fully-Grouted Method for 

Piezometer Installation,” Proceedings, Seventh International Symposium on Field Measurements in Geomechanics (FMGM 2007), Boston, J. DiMaggio and P. Osborn, eds., ASCE Special Publication 175, pp. 67-86. 

Cornell, C.A., 1968, “Engineering Seismic Risk Analysis,” Bulletin of the Seismological Society of 

America, Vol. 58, No. 5, pp. 1583-1606.

* Cowherd, D.C. and I.J. Corda, 1998, “Seismic Considerations for Upstream Construction of Coal 

Refuse Dams,” Proceedings, 1998 Annual Conference, Las Vegas, Association of State Dam Safety 

Officials, Lexington, KY., pp. 523-534.
R–7

Engineering and Design Manual Coal Refuse Disposal Facilities

* Cowherd, D.C., M.J. Carlson and M.T. Hardert, 2002, “A Method for Evaluation and Mitigation of 

the Potential for Mine Breakthrough in Fine Coal Refuse Tailings Dam Pool Areas,” Proceedings, Tailings Dams 2002, Las Vegas, Association of State Dam Safety Officials, Lexington, KY, pp. 277-287.

Cravotta, C.A. III, K.B.C. Brady, M.W. Smith, and R.L. Beam, 1990, “Effectiveness of Alkaline Addition at Surface Mines in Preventing or Abation Acid Mine Drainage: Part 1, Geochemical Considerations,” Proceedings, 1990 Mining and Reclamation Conference, West Virginia University, Morgantown, 

WV, Vol. 1, pp. 221-226.

* Crone, A.J. and R.L. Wheeler, 2000, Data for Quaternary Faults, Liquefaction Features, and Possible Tectonic Features in the Central and Eastern United States, East of the Rocky Mountain Front, USGS Open 

File Report 00-260, U.S. Department of the Interior, U.S. Geological Survey, Denver, CO.

Cudworth, A.G., 1989, Flood Hydrology Manual, U.S. Department of the Interior, Bureau of Reclamation, Denver, CO.

Daniel, D.E. and R.M. Koerner, 1995, Quality Assurance and Quality Control For Waste Containment 

Facilities, EPA/600/SR-93/182, U.S. Environmental Protection Agency, National Risk Management 

Research Laboratory, Cincinnati, OH.

Daniels, W.L., 2005, “Effects of Cover Soil Thickness on Revegetation of Acidic Appalachian Coal 

Refuse,” Proceedings, 2005 National Meeting of the American Society of Mining and Reclamation, Breckenridge, CO, ASMR, Lexington, KY, pp. 842-861.

Daniels, W.L. and B.R. Stewart, 1993, “Reclamation and Closure of Coal Refuse Disposal Facilities 

in the Appalachians,” SME Preprint 93-169, Society of Mining, Metallurgy, and Exploration (SME) 

1993 Annual Meeting, Denver, CO.

Daniels, W.L., B.R. Stewart, K. Haering and C. Zipper, 2002, “The Potential for Beneficial Reuse 

of Coal Fly Ash in Southwest Virginia Mining Environments,” Reclamation Guidelines for Surface 

Mined Land in Southwest Virginia, Publication 460-134, Virginia Cooperative Extension, Powell 

River Project, Blacksburg, VA.

Daniels, W.L., B.R. Stewart and D.C. Dove, 1996, Reclamation of Coal Refuse Disposal Areas, Publication 460-131, Virginia Cooperative Extension, Blacksburg, VA.

* D’Appolonia, 2006, Final Report, Demonstration Project – Mine Void Detection, DC Resistivity Surface Geophysical Method, Lots Branch Tailings Impoundment Site, Prenter, West Virginia, Contract No. 

J53R/011, Mine Safety and Health Administration (MSHA), Arlington, VA.

D’Appolonia, 1988, Minimization of Coal Refuse Disposal Facility Related Water Quality Deterioration, 

report prepared for U.S. Department of the Interior, Office of Surface Mining, Washington, DC.

D’Appolonia, 1982, Improved Resource Characterization for Coal Mining, Geophysics and Drilling 

Technology, report prepared for U.S. Department of Energy, Office of Coal Mining, Contract No. 

USDOE AC22-80PC30114, DOE/PC/3011-4 (DE82016931).

* D’Appolonia, 1980, Environmental Effects of Slaking of Surface Mine Spoils, Eastern and Central U.S., 

report prepared for the U.S. Bureau of Mines, Contract No. J0285024, Pittsburgh, PA.

D’Appolonia, 1973, State-of-the-Art Study, Coal Waste Embankments in the United States, Draft Report, 

Contract No. 50133042, U.S. Department of the Interior, Mining Enforcement and Safety Administration, Washington, DC. 

* Davies, M., E.C. McRoberts and T.I. Martin, 2002, “Static Liquefaction of Tailings – Fundamentals 

and Case Histories,” Proceedings, Tailings Dams 2002, Las Vegas, ASDSO/USCOLD, pp. 233-255.

Davies, W.E., J.F. Bailey and D.B. Kelly, 1972, West Virginia’s Buffalo Creek Flood: A Study of the 

Hydrology and Engineering Geology, Circular 667, U.S. Department of the Interior, Geological Survey, 

Reston, VA.
R–8

References

Davis, C.A. and J.P. Bardet, 1998, “Seismic Analysis of Large-Diameter Flexible Underground 

Pipes,” Journal of Geotechnical Engineering, ASCE, Vol. 104, No. 10, pp. 1001-1015.

Davis, G., 1973, “Comparison of Fall and Spring Plantings on Strip-Mine Spoils in the Bituminous 

Region of Pennsylvania,” Ecology and Reclamation of Devastated Land, R.J. Hutnik and G. Davis, eds., 

Cordon and Breach, New York, NY, Vol. 1, pp. 525-538.

Davis, A.P., G. Castro and S.J. Poulos, 1988, “Strengths Back-Figured from Liquefaction Case Histories,” Proceedings, Second International Conference on Case Histories in Geotechnical Engineering, University of Missouri-Rolla, Rolla, MO, Vol. 3, pp. 1693-1701.

Day, D.A. and N.B.H. Benjamin, 1991, Construction Equipment Guide, 2nd Ed., Wiley-Interscience, 

New York, NY.

Dean, K.C. and R. Havens, 1972, “Comparative Costs and Methods for Stabilization of Tailings,” 

Tailings Disposal Today, Proceedings of the 1st International Tailing Symposium, Tucson, AR, C.L. 

Aplin and G.O. George, eds., pp. 450-477.

Degoutte, G. and J.J. Fry, 2002, “Geotextiles in Dams, Feedback,” Proceedings, Technical Symposium 

on Geotextiles and Geomembranes in Dams, CFG and FCOLD.

DiGioia, A.M., Jr. and R.E. Gray, 1979, “Power Plant Solid Wastes, Geotechnical Aspects of Disposal Site Selection and Design,” Current Geotechnical Practice in Mine Waste Disposal, ASCE, 

pp. 113-180.

DiGioia, A.M., Jr., G.G. Brendel, D.J. Spaeder, N.J. Balsamo and L.C. Shumway, 1995, Coal Fly Ash 

Design Manual, 3rd Ed., EPRI-TR-104137, Electric Power Research Institute, Palo Alto, CA.

* Department of Defense (DOD), 2005, Unified Facilities Criteria: Geotechnical Engineering Procedures 

for Foundation Design of Buildings and Structures, UFC 3-220-01N, U.S. Government Printing Office, 

Washington, DC.

Dove, D.C., W.L. Daniels and J. Bell, 1987, “Reclamation of Coal Wastes with Reduced Soil Depth 

and Other Amendments,” Proceedings, 4th Biennial Symposium on Surface Mining and Reclamation on 

the Great Plains, Billings, MT, American Society of Mining and Reclamation, Lexington, KY, pp. 1-9.

* Dowding, C.H., 2002, “Geo-Measurements with Metallic TDR Cable Technology for Infrastructure 

Surveillance,” TRB 2002 Annual Meeting CD-ROM, Washington, DC.

Dowding, C.H. and K.M. O’Connor, 2000, “Comparison of TDR and Inclinometers for Slope Monitoring,” Geotechnical Measurements: Lab and Field, Geotechnical Special Publication No. 106, W.A. 

Marr, ed., ASCE, Reston, VA, pp. 80-90.

DuMontelle, P.B., S.C. Bradford, R.A. Bauer and M.M. Killey, 1981, “Mine Subsidence in Illinois: 

Facts for the Homeowner Considering Insurance,” Environmental Geology Notes, Illinois State Geological Survey, Champaign, IL.

Duncan, J.M., 1996, “State of the Art: Limit Equilibrium and Finite Element Analysis of Slopes,” 

Journal of Geotechnical Engineering, ASCE, Vol. 122, No. 7, pp. 577-596.

Duncan, J.M. and S.G. Wright, 2005, Soil Strength and Slope Stability, John Wiley & Sons, Hoboken, NJ.

Duncan, J.M. and S.G. Wright, 1980, “The Accuracy of Equilibrium Methods of Slope Stability 

Analysis,” Engineering Geology, Vol. 16, No. 1, pp. 5-17.

Dunnicliff, J., 1997, “Systematic Approach to Planning Monitoring Programs Using Geotechnical Instrumentation – An Update,” Geotechnical News, BiTech Publishers, Richmond, BC, Canada, 

Vol. 15, No. 3, pp. 36-46.

Dunnicliff, J., 1993, Geotechnical Instrumentation for Monitoring Field Performance, John Wiley & Sons, 

New York, NY.
R–9

Engineering and Design Manual Coal Refuse Disposal Facilities

Dunnicliff, J., 1981, “Long-Term Performance of Embankment Dam Instrumentation,” Recent Development in Geotechnical Engineering for Hydro Projects, F.H. Kulhawy, ed., ASCE, New York, NY, pp. 1-22.

* Dunrud, C. and F. Osterwald, 1980, Effects of Coal Mine Subsidence in the Sheridan, Wyoming, Area, 

Professional Paper No. 1164, U.S. Geological Survey, Denver, CO.

Eger, P., 1994, “Wetland Treatment for Trace Metal Removal from Mine Drainage: The Importance 

of Aerobic and Anaerobic Processes,” Water Science and Technology, Vol. 29, No. 4, pp. 249-256.

Eigenbrod, W.F., 1971, “Use of Stabilizers and Binders for Conditioning Surface-Mined Land and 

Coal Mine Refuse for Revegetation,” Proceedings, Revegetation and Economic Use of Surface-Mined 

Land and Mine Refuse Symposium, West Virginia University, Morgantown, WV, pp. 46-47.

* Ellefsen, K.J., G.J. Tuttle, J.M. Williams and J.E. Lucius, 2005, S-Wave Refraction Survey of Alluvial 

Aggregate, Scientific Investigations Report 2005-5012, U. S. Geological Survey, Denver, CO.

* Ellison, R.D. and Y.Y. Cho, 1976, “Liquefaction Considerations for Fine Coal Refuse,” Ohio River 

Valley Soils Seminar, Lexington, KY.

Electric Power Research Institue (EPRI), 1989, Seismic Hazard Methodology for the Central and Eastern 

United States, Report NP-4726.

El-Ramly, H., N.R. Morgenstern and D.M. Cruden, 2003, “Probabilistic Stability Analysis of a Tailings Dyke on Presheared Clay-Shale,” Canadian Geotechnical Journal, Vol. 40, pp. 192-208.

Federal Emergency Management Agency (FEMA), pending publication (2009), “Geotextiles in 

Embankment Dams, Status Report on the Use of Geotextiles in Embankment Dam Construction 

and Rehabilitation.”

* Federal Emergency Management Agency (FEMA), 2007, Technical Manual: Plastic Pipe Used in 

Embankment Dams (Best Practices for Design, Construction, Problem Identification, and Evaluation, 

Inspection, Maintenance, Renovation, and Repair), FEMA P-675, U.S. Department of Homeland Security, FEMA, Washington, DC.

* Federal Emergency Management Agency (FEMA), 2005a, Technical Manual: Conduits through 

Embankment Dams (Best Practices for Design, Construction, Problem Identification and Evaluation, 

Inspection, Maintenance, Renovation, and Repair), FEMA 484, U.S. Department of Homeland Security, 

FEMA, Washington, DC.

* Federal Emergency Management Agency (FEMA), 2005b, Federal Guidelines for Dam Safety, Earthquake Analyses and Design of Dams, FEMA 65, U.S. Department of Homeland Security, FEMA, 

Washington, DC.

* Federal Emergency Management Agency (FEMA), 2004a, Federal Guidelines for Dam Safety: Hazard 

Potential Classification System for Dams, FEMA 333, Interagency Committee on Dam Safety (ICODS), 

Washington, DC.

* Federal Emergency Management Agency (FEMA), 2004b, Federal Guidelines for Dam Safety: Emergency Action Planning for Dam Owners, FEMA 64, U.S. Department of Homeland Security, FEMA, 

Washington, DC.

* Federal Emergency Management Agency (FEMA), 2004c, Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for Dams, FEMA 94, U.S. Department of Homeland 

Security, FEMA, Washington, DC.

* Federal Highway Administration (FHWA), 2006, Hydraulic Design of Energy Dissipators for Culverts 

and Channels, Hydraulic Engineering Circular No. 14, 3rd Ed., FHWA-NHI-06-086, U.S. Department of Transportation, FHWA, National Highway Institute, Arlington, VA.

* Federal Highway Administration (FHWA), 2005a, Design of Roadside Channels with Flexible Linings, 

Hydraulic Engineering Circular No. 15 (HEC-15), FHWA-NHI-05-114, U.S. Department of Transportation, FHWA, National Highway Institute, Arlington, VA.
R–10

References

* Federal Highway Administration (FHWA), 2005b, Hydraulic Design of Highway Culverts, Hydraulic Design Series No. 5 (HDS-5), FHWA-NHI-01-020, U.S. Department of Transportation, FHWA, 

National Highway Institute, Arlington, VA.

* Federal Highway Administration (FHWA), 2002, Subsurface Investigations: Geotechnical Site Characterization, Reference Manual for NHI Course No. 132031, FHWA-NHI-01-031, U.S. Department of 

Transportation, FHWA, Washington, DC.

* Federal Highway Administration (FHWA), 2001, Introduction to Highway Hydraulics, Hydraulic 

Design Series No. 4 (HDS-4), FHWA-NHI-01-019, U.S. Department of Transportation, FHWA, 

Washington, DC.

* Federal Highway Administration (FHWA), 1992, The Cone Penetrometer Test, FHWA-SA-91-043, 

U.S. Department of Transportation, FHWA, Washington, DC.

* Federal Highway Administration (FHWA), 1989, Design of Riprap Revetment, Hydraulic Engineering 

Circular No. 11, FHWA-IP-89-016, U.S. Department of Transportation, FHWA, Washington, DC.

* Federal Highway Administration (FHWA), 1961, Design Charts for Open-Channel Flow, Hydraulic Design Series No. 3 (HDS-3), FHWA-EPD-86-102, U.S. Department of Transportation, FHWA, 

Washington, DC.

* Federal Energy Regulatory Commission (FERC), 2001, “Chapter VIII, Determination of the Probable Maximum Flood,” Engineering Guidelines for the Evaluation of Hydropower Projects, U.S. Department of Energy, FERC, Washington, DC.

* Federal Energy Regulatory Commission (FERC), 1993, “Chapter II, Selecting and Accommodating 

Inflow Design Floods for Dams,” Engineering Guidelines for the Evaluation of Hydropower Projects, 

U.S. Department of Energy, FERC, Washington, DC.

Fell, R., P. MacGregor, D. Stapledon and G. Bell, 2005, Geotechnical Engineering of Dams, A.A. 

Balkema Publishers, London, UK.

Fellenius, W., 1922, Bulletin 2, Swedish State Railway Geotechnical Commission, Stockholm, 

Sweden.

Ferguson, K.D. and K.A. Morin, 1991, “The Prediction of Acid Rock Drainage – Lessons from 

the Database,” Proceedings, 2nd International Conference on Abatement of Acidic Drainage, Montreal, 

Canada, Vol. 3, pp. 85-106.

Ferguson, K.D. and J.D. Robertson, 1994, “Assessing the Risk of ARD,” Proceedings, 3rd International 

Conference on Abatement of Acidic Drainage, Pittsburgh, PA, U.S. Bureau of Mines Special Publication 

SP 06A-94, Vol. 1, pp. 2-11.

Fernandez, L.M. and P.K. Van Der Heever, 1996, “Ground Movement and Damage Accompanying 

a Large Seismic Event in the Klerksdorp District,” Proceedings, 1st International Congress on Rockbursts and Seismicity in Mines, N.C. Gay and E.H. Wainwright, eds., Johannesburg, South Africa, 

pp. 193-198.

Filipek, L.H., J. T. Gormley, R. Ewing and D. Ellsworth, 1991, “Kinetic Acid-Prediction Studies 

as Aids to Waste Rock and Water Management during Advanced Exploration of a Massive Sulfide Deposit,” Proceedings, 2nd International Conference on Abatement of Acidic Drainage, Montreal, 

Canada, CANMET, Ottawa, Vol. 3, pp. 191-208.

Finn, W.D.L., K.W. Lee and G.R. Martin, 1977, “An Effective Stress Model for Liquefaction,” Journal 

of the Geotechnical Engineering Division, ASCE, Vol. 103, No. GT6, pp. 517-533.

Fiscor, S., 2002, “Design Alternatives for Refuse Disposal,” Coal Age, Vol. 107, No. 5, pp. 34-36.

Fourie, A.B. and R.W. Green, 1993, “Damage to Underground Coal Mines Caused by Surface Blasting,” International Journal of Surface Mining, Reclamation and Environment, Vol. 7, No. 1, pp. 11-16.
R–11

Engineering and Design Manual Coal Refuse Disposal Facilities

Forrester, P.J. and B.H. Whitaker, 1976, “Effects of Mining Subsidence on Colliery Spoil Heaps – I 

and II,” International Journal of Rock Mechanics, Mining Sciences and Geomechanics Abstracts, No. 13, 

pp. 113-133.

* France, J.W., 2004, “Seepage Collection and Control Systems: The Devil is in the Details,” Proceedings, 2004 ASDSO Annual Conference, Phoenix, Association of State Dam Safety Officials, Lexington, KY. 

Frankel, A., 1995, “Mapping Seismic Hazard in the Central and Eastern United States,” Seismological Research Letters, Vol. 66, No. 4, pp. 8-21.

* Frankel, A.D., M.D. Petersen, C.S. Mueller, K.M. Haller, R.L. Wheeler, E.V. Leyendecker, R.L. 

Wesson, S.C. Harmsen, C.H. Cramer, D.M. Perkins and K.S. Rukstales, 2002, Documentation for the 

2002 Update of the National Seismic Hazard Maps, USGS Open-File Report 02-420, Denver, CO.

Frankel, A.D., C. Mueller, T. Barnhard, D. Perkins, E.V. Leyendecker, N. Dickman, S. Hanson and 

M. Hopper, 1996, National Seismic-Hazard Maps: Documentation - June 1996, USGS Open-File Report 

96-532, U.S. Department of the Interior, Geological Survey, Denver, CO.

Franklin, J.A. and R. Chandra, 1972, “The Slake Durability Test,” International Journal of Rock 

Mechanics and Mining Sciences, Vol. 9, pp. 325-341.

Fread, D.L., 1988, The NWS DAMBRK Model: Theoretical Background and User Documentation, Report 

No. HRL-258, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 

National Weather Service, Hydrological Research Laboratory, Silver Spring, MD.

Frederick, R.H. and J.F. Miller, 1979, “Short Duration Rainfall Frequency Relations for California,” 

Preprint Volume, Third Conference on Hydrometeorology, American Meterological Society, Boston, MA, 

pp. 67-73.

* Frederick, R.H., V.A. Myers and E.P. Auciello, 1977, Five- to 60-Minute Precipitation Frequency for the 

Eastern and Central United States, NOAA Technical Memo NWS HYDRO-35, U.S. Department of 

Commerce, National Oceanic and Atmospheric Administration, NWS, Washington, DC.

Fredland, J.W., 2008, personal communication concerning MSHA’s position of hydrologic design 

criteria for temporary mining operations.

* Frischknecht, F.C. and P.V. Raab, 1984, Location of Abandoned Wells with Geophysical Methods, Report 

No. 600/4-84-085, U.S. Department of the Environment, USEPA, Washington, DC.

Frischknecht, F.C., R. Grette, P.V. Raab, and J. Meredith, 1985, Location of Abandoned Wells by Magnetic Surveys – Acquisition and Interpretation of Aeromagnetic Data for Five Test Areas, Open-File 

Report 85-614-A, U.S. Department of the Interior, Geological Survey, Denver, CO.

Gamble, J.C., 1971, Durability-Plasticity Classification of Shales and Other Argillaceous Rocks, Ph.D. 

Thesis, University of Illinois, Champaign, IL.

Gan, Y., 2005, RES3DINV, Rapid 3D Resistivity and IP Inversion Using the Least-Squares Method, 

Geotomo Software, available from Geotomo web site.

Gan, Y., 2004, RES2DINV, Rapid 2D Resistivity and IP Inversion Using the Least-Squares Method, 

Geotomo Software, available from Geotomo web site.

Ganow, H., 1975, A Geotechnical Study of the Squeeze Problem Associated with the Underground Mining 

of Coal, Doctoral Thesis, University of Illinois, Urbana-Champaign, IL.

* Gardner, G.H. and K.K. Wu, 2002, “Challenges Associated with Evaluating the Liquefaction Potential of Coal Waste Impoundments Subject to Seismic Loading,” Proceedings, Tailings Dams 2002, Las 

Vegas, Association of State Dam Safety Officials, Lexington, KY, pp. 207-218.

Garret, W.S. and L.T. Campbell Pitt, 1961, “Design and Construction of Underground Bulkheads 

and Water Barriers,” Transactions, 7th Commonwealth Mining and Metallurgical Congress, South African Institute of Mining and Metallurgy, pp. 1283-1301.
R–12

References

GEI Consultants, 2007, personal communication regarding experience with testing coal refuse.

* Genes, B.E., T.O. Keller, and J.P. Laird, 2000, “Steady State Liquefaction Susceptibility of High 

Hazard Upstream-Constructed Coal Refuse Disposal Facilities,” Proceedings, Tailings Dams 2000, 

Las Vegas, ASDSO/USCOLD, pp. 47-58.

Gentile, L.F., K.W. Cargill, and S.D. McGarvie, 1997, “Enhanced Cover Methods for Surface Coal 

Refuse Reclamation,” Proceedings, 14th Annual Meeting, American Society for Surface Mining and Reclamation, Austin, TX, pp. 299-308.

Giroud, J.P., 1997, “Geotextile Filters: Reliable Design and Installation,” Proceedings, Recontres 97, 

Reims, France, Vol.1, pp. 170-196.

Graf, J.B., G. Garklavs and K.A. Oberg, 1982, Time of Concentration and Storage Coefficient Values for 

Illinois Streams, Water Resources Investigation Report 82-13, U.S. Department of the Interior, U.S. 

Geological Survey, Water Resources Division, Urbana, IL.

Gray, D.H. and Y Lin, 1972, “Engineering Properties of Compacted Fly Ash,” Journal of the Soil 

Mechanics and Foundations Division, ASCE, Vol. 98, No. 4, pp. 361-380.

* Gray, R.E., R.W. Bruhn and R.J. Turka, 1977, Study and Analysis of Surface Subsidence over the Mined 

Pittsburgh Coalbed, Report No. PB-281511, Contract No. J0366047, U.S. Department of the Interior, 

Bureau of Mines, Denver, CO.

* Geosynthetic Research Institute (GRI), 2005, Test Methods, Required Properties, and Testing Frequencies of Geosynthetic Clay Liners (GCLs), Standard Specification GRI-GCL3, Folsom, PA.

Griffiths, D.V. and P.A. Lane, 1999, “Slope Stability Analysis by Finite Elements,” Géotechnique, Vol. 

49, No. 3, pp. 387-403.

* Gritto, R., 2003, “Subsurface Void Detection Using Seismic Tomographic Imaging,” presented at 

Geophysical Technologies for Detecting Underground Coal Mines Interactive Forum, Lexington, 

KY, Paper LBNL-53227, Lawrence Berkeley National Laboratory, University of California, Berkeley, 

CA.

Guang-Xu, T. and H. Mao-Yuan, 1983, “Assessment of Support Ability of the Fill Mass,” Proceedings, International Symposium on Mining and Backfill, Lulea, Sweden, S. Granholm, ed., pp. 433-442.

Guney, M., 1968, “Oxidation and Spontaneous Combustion of Coal: A Review of Individual Factors,” Colliery Guardian, Vol. 216, pp. 105-110.

Gupta, B.K., R. Gross and P. Bethell, 2008, “Replacement of Belt Presses with a Deep Cone Thickener at Lone Mountain Processing,” Proceedings, Coal Prep Annual Coal Processing Exhibition and 

Conference, Coal Preparation Society of America, Lexington, KY.

Haan, C.T. and B.J. Barfield, 1978, Hydrology and Sedimentology of Surface Mined Lands, Institute for 

Mining and Minerals Research, Kentucky Center for Energy Research Laboratory, College of Engineering, University of Kentucky Press, Lexington, KY.

* Haeni, F.P., 1988, “Application of Seismic-Refraction Techniques to Hydrologic Studies,” Techniques

of Water-Resources Investigations of the United States Geological Survey, TWRI 2-D2, U.S. Department 

of the Interior, Geological Survey, Denver, CO.

Hanks, T.C. and H. Kanamori, 1979, “A Moment Magnitude Scale,” Journal of Geophysical Research, 

Vol. 84, No. B5, pp. 2348-2350.

Hanks, T.C. and R.K. McGuire, 1981, “The Character of High Frequency Strong Ground Motion,” 

Bulletin of the Seismological Society of America, Vol. 71, No. 6, pp. 2071-2095.

Hanna, T.H., 1985, Field Instrumentation In Geotechnical Engineering, Trans Tech Publications, Rockford, MA.
R–13

Engineering and Design Manual Coal Refuse Disposal Facilities

Harder, L.F. Jr., 1991, “Performance of Earth Dams During the Loma Prieta Earthquake,” Proceedings of the Second International Conference on Recent Advances in Geotechnical Earthquake Engineering 

and Soil Dynamics, University of Missouri-Rolla, Vol. II, pp. 1613-1629.

Harder, L.F., Jr. and R.W. Boulanger, 1997, “Application of Kσ and Ka Correction Factors,” Proceedings, NCEER Workshop on Evaluation of Liquefaction Resistance of Soils, T.L. Youd and I.M. Idriss, eds., 

NCEER-97-0022, National Center for Earthquake Engineering Research, SUNY, Buffalo, pp. 167-190.

Harmsen, S., D. Perkins, and A. Frankel, 1999, “Deaggregation of Probabilistic Ground Motions in the 

Central and Eastern United States,” Bulletin of the Seismological Society of America, Vol. 89, No. 1, pp. 1-13.

Harr, M.E., 1962, Groundwater and Seepage, McGraw-Hill, New York, NY.

Harteis, S.P. and D.R. Dolinar, 2006, “Water and Slurry Bulkheads in Underground Coal Mines: 

Design, Monitoring and Safety Concerns,” Mining Engineering, Vol. 58, No. 12, pp. 41-47.

* Harteis, S.P., D.R. Dolinar and T.M. Taylor, 2008, Guidelines for Permitting Construction, and Monitoring of Retention Bulkheads for Underground Coal Mines, Information Circular 9506, U.S. Department 

of the Interior, National Institute for Occupational Health and Safety, Pittsburgh, PA.

Hartman, H.L., ed., 1992, Mining Engineering Handbook, 2nd Ed., Society for Mining, Metallurgy, 

and Exploration (SME), Littleton, CO.

Haxo, H.E., Jr., “Quality Assurance of Geomembranes Used as Linings for Hazardous Waste 

Containment,” Geotextiles and Geomembranes, International Geosynthetics Society, Vol. 3, No. 4, 

pp. 225-247.

Head, K.H., 1986, Manual of Soil Laboratory Testing, Volume 3: Effective Stress Tests, Pentech Press, 

London, UK.

Head, K.H., 1982, Manual of Soil Laboratory Testing, Volume 2: Permeability, Shear Strength and Compressibility Tests, Pentech Press, London, UK.

Head, K.H., 1980, Manual of Soil Laboratory Testing, Volume 1: Soil Classification and Compaction Tests, 

Pentech Press, London, UK.

* Hedin, R.S., G.R. Watzlaf and R.W. Naim, 1994, “Passive Treatment of Acid Mine Drainage with 

Limestone,” Journal of Environmental Quality, Vol. 23, No. 6, pp. 1338-1345.

* Hedin, R.S., R.W. Naim and R.L.P. Kleinmann, 1993, Passive Treatment of Coal Mine Drainage, 

Information Circular 9389, U.S. Department of the Interior, Bureau of Mines, Pittsburgh Research 

Center, Pittsburgh, PA.

* Hegazy, Y.A., A.G. Cushing and C.J. Lewis, 2004, “Physical, Mechanical, and Hydraulic Properties 

of Coal Refuse for Slurry Impoundment Design,” Proceedings, 2nd International Site Characterization 

Conference, Porto, Portugal.

Henderson, F.M., 1966, Open Channel Flow, The Macmillan Company, New York, NY.

Henry, C.D., 2007, “Fine Coal Recovery Plant Installation: Smith Branch Impoundment – Pinnacle 

Mining,” Proceedings, Coal Prep Annual Coal Processing Exhibition and Conference, Coal Preparation 

Society of America, Lexington, KY.

* Hershfield, D.M., 1961, Rainfall Frequency Atlas of the United States for Durations from 30 Minutes to 

24 Hours and Return Periods from 1 to 100 Years, Technical Paper No. 40, U.S. Department of Commerce, Weather Bureau, Washington, DC.

Herzog B.L., 1994, “Slug Tests for Determining Hydraulic Conductivity of Natural Geologic 

Deposits,” Hydraulic Conductivity and Waste Contaminant Transport In Soil: ASTM STP 1142, D.E. 

Daniel and S.J. Trautwein, eds., ASTM, Philadelphia, PA, pp. 95-110.

Himelick, E.B., 1981, Tree and Shrub Transplanting Manual, 2nd Ed., International Society of Arboriculture, Champaign, IL.
R–14

References

Hoek, E., 2000, Practical Rock Engineering, Rocscience, Toronto, Ontario, Canada.

Holtz, R.D. and W.D. Kovacs, 1981, Introduction to Geotechnical Engineering, Prentice-Hall, Englewood Cliffs, NJ.

* Holtz, R.D., B.R. Christopher and R.R. Berg, 1998, Geosynthetic Design and Construction Guidelines, 

DTFH61-93-C-00120, Federal Highway Administration (FHWA), National Highway Institute, 

Washington, DC.

Houlsby, A.C., 1990, Construction and Design of Cement Grouting – A Guide To Grouting In Rock Foundations, John Wiley & Sons, New York, NY.

Howard, A.K. and J.L. Hitch, eds., 1998, The Design and Application of Controlled Low-Strength 

Materials (Flowable Fill), ASTM STP 1331, American Society for Testing and Materials. West Conshohocken, PA.

* Huang, Y.H., J. Li and G. Weeratunga, 1987, “Strength and Consolidation Characteristic of Fine 

Coal Refuse,” University of Kentucky, Lexington, KY.

Hunt, R.E., 1986, Geotechnical Engineering Practices, McGraw-Hill, New York, NY. 

Hustrulid, W.A., 1976, “A Review of Coal Pillar Strength Formulas,” Rock Mechanics, Vol. 8, No. 2, 

pp. 115-145.

Hutchison, I.P.G. and R.D. Ellison, eds., 1992, Mine Waste Management: A Resource for Mining Industry Professionals, Regulators and Consulting Engineers, Lewis Publishers, Chelsea, MI.

Hvorslev, J., 1948, Subsurface Exploration and Sampling of Soils for Civil Engineering Purposes, U.S. 

Department of the Army, Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

* Hynes-Griffin, M.E. and A.G. Franklin, A.G., 1984, Rationalizing the Seismic Coefficient Method, Miscellaneous Paper GL-84-13, U.S. Department of the Army, Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

* Idriss, I.M. and R.W. Boulanger, 2007, “Residual Shear Strength of Liquified Soil,” Proceedings, 

Modernization and Optimization of Existing Dams and Reservoirs, 27th Annual USSD Conference, 

Denver, CO, pp. 621-634.

Indraratna, B. and S. Radampola, 2002, “Analysis of Critical Hydraulic Gradient for Particle Movement in Filtration,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 128, No. 4, 

pp. 337-350.

Jaeger, J.C., N.G.W. Cook and R. Zimmerman, 2007, Fundamentals of Rock Mechanics, 4th Ed., John 

Wiley & Sons (Blackwell Publishing), Hoboken, NJ.

Jáky, J., 1944, “The Coefficient of Earth Pressure at Rest,” Journal for Society of Hungarian Architects 

and Engineers, Budapest, Hungary, pp. 355-358.

Janbu, N., 1973, “Slope Stability Computations,” Embankment-Dam Engineering: Casagrande Volume, 

R.C. Hirschfeld and S.J. Poulos, eds., John Wiley & Sons, Hoboken, NJ, pp. 47-86.

* Johnson, W.J., 2003, “Applications of the Electrical Resistivity Method for Detection of Underground Mine Workings,” Proceedings, Workshop on Geophysical Technologies for Detecting Underground Coal Mine Voids, Lexington, KY.

* Johnson, W.J. and J.C. Clark, 1992, High Resolution Shear Wave Surveying for Hydrogeological Investigations, Report No. DOE/CH-9211 (Contract No. 02112405 with Argonne National Laboratory), 

U.S. Department of Energy, Washington, DC.

* Johnson, W.J., R.E. Snow and J.C. Clark, 2002, “Surface Geophysical Methods for the Detection of 

Underground Mine Workings,” Proceedings, Symposium on Geotechnical Methods for Mine Mapping 

Verifications, Charleston, WV.
R–15

Engineering and Design Manual Coal Refuse Disposal Facilities

* Jolley, T.R. and H.W. Russell, 1959, Control of Fires in Inactive Coal Deposits in Western U.S. Including 

Alaska, 1948-1958, Report No. BM-IC-7932, U.S. Department of the Interior, Bureau of Mines, Washington, DC.

Jones, C.L., J.D. Higgins and R.D. Andrew, 2000, Colorado Rockfall Simulation Program, Version 4.0, 

Colorado Department of Transportation, Colorado Geological Survey, Denver, CO.

* Kalamaras, G.S. and Z.T. Bieniawski, 1993, “A Rock Mass Strength Concept for Coal Seams,” Proceedings, 12th International Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, pp. 274-283.

Kalinski, M.E. and J.L. Phillips, 2008, “Development of Methods to Predict Dynamic Behavior of 

Fine Tailings: Preliminary Results from Two Sites in Appalachia,” Geotechnical Special Publication 

No. 181, Geotechnical Earthquake Engineering and Soil Dynamics IV, D. Zeng, M.T. Manzari and 

D.R. Hiltunen, eds., American Society of Civil Engineers (ASCE), Reston, VA.

Kanamori, H., 1977, “The Energy Release in Great Earthquakes,” Journal of Geophysical Research, 

Vol. 82, No. B20, pp. 2981-2987.

Karmis, M., 2003, “Milos Statement – Sustainable Development Conference Held in Greece,” 

Mining Engineering, Society of Mining, Metallurgy, and Exploration (SME), Littleton, CO, Vol. 55, 

No. 8, pp. 31-32.

Karmis, M. and Z Agioutantis, 2004, “A Risk Analysis Subsidence Approach for the Design of Coal 

Refuse Impoundments Overlying Mine Workings,” Proceedings, 8th International Symposium on 

Environmental Issues and Waste Management in Energy and Mineral Production (SWEMP), Antalya, 

Turkey, pp. 205-210.

Kashef, A.I., 1986, Groundwater Engineering, McGraw-Hill, New York, NY, 

* Kaufman, W.W. and J.C. Ault, 1977, Design of Surface Mine Haulage Roads – A Manual, BM-IC-8758, 

U.S. Department of the Interior, Bureau of Mines, Washington, DC.

* Kendorski, F. S., 2006, “Effect of Full-Extraction Underground Mining on Ground and Surface 

Waters: A 25-Year Retrospective,” Proceedings, 25th International Conference on Ground Control in 

Mining, West Virginia University, Morgantown, WV, pp. 425-430.

* Kendorski, F. S., 1993, “Effect of High-Extraction Coal Mining on Surface and Ground Water,” Proceedings, 12th International Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, pp. 412-425.

* Kendorski, F.S., I. Khosla and M.M. Singh, 1979, Criteria for Determining When a Body of Surface 

Water Constitutes a Hazard to Mining, Final Report, Contract J0285011, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA.

* Kepler, D.A. and E.C. McCleary, 1994, “Successive Alkalinity-Producing Systems (SAPS) for Treatment of Acid Mine Drainage,” Proceedings, International Land Reclamation and Mine Drainage Conference, SP06A-94, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA, pp. 196-204.

Keys, W.S., 1997, A Practical Guide to Borehole Geophysics in Environmental Investigations, CRC Press, 

Boca Raton, FL.

* Kim, A.G. and R.F. Chaiken, 1993, Fires in Abandoned Coal Mines and Waste Banks, Information Circular No. 9352, U.S. Department of the Interior, Bureau of Mines, Washington, DC.

Kim, A.G. and R.F. Chaiken, 1990, “Relative Self-Heating Tendencies of Coal, Carbonaceous Shales 

and Coal Refuse,” Proceedings, 1990 Mining and Reclamation Conference and Exhibition, J. Skousen 

and J. Sencindiver, eds., West Virginia University, Morgantown, WV.

* Kim, A.G. and A.M. Kociban, 1994, “Cryogenic Slurry Method to Extinguish Waste Bank Fires,” 

Proceedings, International Land Reclamation and Mine Drainage Conference, U.S. Department of the 

Interior, Bureau of Mines, Pittsburgh, PA, pp. 129-138.
R–16

References

Kirkwood, D.T. and K.K. Wu, 1995, “Technical Considerations for the Design and Construction of 

Mine Seals to Withstand Hydraulic Heads in Underground Mines,” SME Preprint No. 95-100, Society for Mining, Metallurgy, and Exploration (SME) Annual Meeting, Denver, CO.

* Kleinmann, R.L.P., ed., 2000, Prediction of Water Quality at Surface Coal Mines, National Mine Reclamation Center, West Virginia University, Morgantown, WV.

Koerner, R.M., 2006, Designing with Geosynthetics, 5th Ed., Prentice Hall, Upper Saddle River, NJ.

* Koerner, R.M. and G.R. Koerner, 2005, “GSI White Paper #4, Reduction Factors Used in Geosynthetic Design,” GFR Magazine, Vol. 23, No. 2 (February 2005). Revised version (2007) available from 

the Geosynthetic Institute, Folsom, PA.

* Kohli, K. and F. Block, 2007, Guidance Manual: Outcrop Barrier Design for Above Drainage Coal Mines, 

U.S. Department of the Interior, Office of Surface Mining, Washington, DC.

Kravits, S.J. and J.J. Schwoebel, 1994, Directional Drilling Techniques for Exploration in Advance 

of Mining,” World Mining Equipment, Vol. 18, No. 1, pp. 24-28.

Krinitzsky, E.L., 2002, “How to Obtain Earthquake Ground Motions for Engineering Design,” 

Engineering Geology, Vol. 65, No. 1, pp.1-16.

* Krinitzsky, E. L., 1995, State-of-the-Art for Assessing Earthquake Hazards in the United States: Selection 

of Earthquake Ground Motions for Engineering, Miscellaneous Paper S-73-1, Report 29, U.S. Army 

Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

Kulhawy, F.H. and P.W. Mayne, 1990, Manual on Estimating Soil Properties for Foundation Design, 

Report EL-6800, Research Project No. 1493-6, Electric Power Research Institute (EPRI), Palo Alto, CA.

Kumar, S., V.K. Puri, B.M. Das, B.C. Devkota, 2001, “Geotechnical Properties of Fly Ash and LimeFly Ash Stabilized Coal Mine Refuse,” Electronic Journal of Geotechnical Engineering, Vol. 6 (2001).

Ladd, C.C., 1991, “Stability Evaluation During Staged Construction,” Journal of Geotechnical Engineering, ASCE, Vol. 117, No. 4, pp. 540-615.

Ladd, C.C. and R. Foott, 1994, “A New Design Procedure for Stability of Soft Clay,” Journal of Geotechnical Engineering, ASCE, Vol. 100, No. 7, pp. 763-786.

Lambe, T.W., 1967, “The Stress Path Method” Journal of the Soil Mechanics and Foundation Division, 

ASCE, Vol. 93, No. 6, pp. 309-331.

Lambe, T.W., 1951, Soil Testing for Engineers, John Wiley & Sons, New York, NY.

Lambe, T.W. and A.M. Marr, 1979, “Stress Path Method: Second Edition,” Journal of Geotechnical 

Engineering, ASCE, Vol. 105, No. 6, pp. 727-738.

* LaVassar, J.M., 2000, “Two Decades of Dam Safety Office Experience in Applying a Risk Based 

Approach to Mine Impoundments,” Proceedings, Tailings Dams 2000, Las Vegas, Association of 

State Dam Safety Officials (ASDSO), Lexington, KY.

Legget, R.F., 1962, Geology and Engineering, McGraw-Hill, New York, NY.

Leonards, G.A., ed., 1962, Foundation Engineering, McGraw-Hill, New York, NY.

Leonards, G.A. and J. Narain, 1963, “Flexibility of Clay and Cracking of Dams,” Journal of the Soil 

Mechanics and Foundations Division, ASCE, Vol. 89, No. SM2, pp. 47-98.

Leps, T. M., 1973, “Flow Through Rockfill,” Embankment-Dam Engineering: Casagrande Volume, R.C. 

Hirschfeld and S.J. Poulos, eds., John Wiley & Sons, New York, NY, pp. 87-107.

Linsley, R.K. and J.B. Franzini, 1972, Water Resources Engineering, McGraw-Hill, New York, NY.

Linsley, R.K., M.A. Kohler and J.L. Paulhus, 1982, Hydrology for Engineers, 3rd Ed., McGraw-Hill, 

New York, NY.
R–17

Engineering and Design Manual Coal Refuse Disposal Facilities

Lo, K-Y and Y.N. Lee, 1990, “Hydraulic Fracture in Earth- and Rock-fill Dams,” Canadian Geotechnical Journal, Vol. 27, No. 4, pp. 496-506.

Londe, P., 1970, “La Fissuration des Noyaux,” Proceedings, 10th International Conference on Large 

Dams, Vol. VI, pp. 384-387.

Love, E. R., W.H. Hammack, J. Sams, G. Veloski and T. Ackman, 2005, “Case History – Using Airborne 

Thermal Infrared Imagery and Helicopter EM Conductivity to Locate Mine Pools and Discharges in 

the Kettle Creek Watershed, North-Central Pennsylvania,” Geophysics, Vol. 70, No. 6, pp. B73–B81.

Lowe, J. and L. Karafiath, 1960, “Effect of Anisotropic Consolidation on the Undrained Strength of 

Compacted Clays,” Proceedings of the ASCE Research Conference on Shear Strength on Cohesive Soils, 

New York, American Society of Civil Engineers, Reston, VA, pp. 837-858.

Luettich, S.M., J.P. Giroud and R.C. Bachus, 1992, “Geotextile Filter Design Guide,” Journal of Geotextiles and Geomembranes, Vol. 11, Nos. 4-6, pp. 355-370.

Lunne, T., P.K. Robertson and J.J.M. Powell, 1997, Cone Penetration Testing in Geotechnical Practice, 

Spon Press, London, UK.

Luo, Y., S.S. Peng and H.J. Chen, 1996, “Identification of Factors Affecting Horizontal Displacement in Subsidence Process,” Proceedings, 15th International Conference on Ground Control in Mining, 

Colorado School of Mines, Golden, CO, pp. 155-165.

Mackey, R.E. and G.R. Koerner, 1999, “Biological Clogging of Geotextile Filters – A Five Year 

Study,” Proceedings, Geosynthetics ’99, Boston, Geosynthetics Research Institute, Folsom, PA, Vol. 2, 

pp. 783-798.

* Magnuson, M.O. and E.C. Baker, 1974, “State-of-the-Art in Extinguishing Refuse Pile Fires,” Proceedings, First Symposium on Mine and Preparation Plant Refuse Disposal, Louisville, KY, National 

Coal Association, Washington, DC, pp. 165-182.

Mainali, G., 2006, Monitoring of Tailings Dams with Geophysical Methods, Licentiate Thesis, Lulea 

University of Technology, Department of Civil and Environmental Engineering, Division of 

Applied Geophysics, Lulea, Sweden. 

Makdisi, F.I. and H.B. Seed, 1978, “Simplified Procedure for Estimating Dam and Embankment 

Earthquake-Induced Deformations,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 104, 

No. GT7, pp. 849-867.

* Makdisi, F.I. and H.B. Seed, 1977, A Simplified Procedure for Computing Maximum Crest Acceleration and Natural Period for Embankments, Report No. UCB/EERC-77/19, Earthquake Engineering 

Research Center, University of California, Berkeley, CA.

Malvick, E.J., B.L. Kutter, R.W. Boulanger and R. Kulasingam, 2006, “Shear Localization Due to 

Liquefaction-Induced Void-Redistribution in a Layered Infinite Slope,” Journal of Geotechnical and 

Geoenvironmental Engineering, ASCE, Vol. 132, No. 10, pp. 1293-1303.

Maneval, D.R., 1969, “Recent Advances in Extinguishment of Burning Coal Refuse Banks for Air 

Pollution Reduction,” Proceedings, Symposium on Pollution Control in Fuel Combustion, Processing and 

Mining, Minneapolis, MN, pp. 27-41.

Marcuson, W.F., III, M.E. Hynes and A.G. Franklin, 1992, “Seismic Stability and Permanent Deformation Analyses: The Last Twenty-Five Years,” Proceedings, Stability Performance of Slopes and 

Embankments II, R.B. Seed and R.W. Boulanger, eds., University of California, Berkeley, American 

Society of Civil Engineers (ASCE), Reston, VA, pp. 552-592.

* Mark, C., 2006, “The Evolution of Intelligent Coal Pillar Design: 1981-2006,” Proceedings, 25th 

International Conference on Ground Control in Mining, University of West Virginia, Morgantown, 

WV, pp. 325-334.
R–18

References

* Marks, B.D. and B.A. Tschantz, 2002, A Technical Manual on the Effects of Tree and Woody Vegetation Root Penetrations on the Safety of Earthen Dams, ASDSO/FEMA Workshop, compiled by Marks 

Enterprises of NC, PLLC, Arden, NC.

* Marshall Miller & Associates (MM&A), 2006, An Analysis of In-Seam Seismic Reflection Techniques to 

Identify and Locate Abandoned Underground Mine Voids in Advance of Mining, Geophysical Void Detection Demonstration Project, prepared for MSHA by MM&A and Virginia Tech, Bluefield, WV.

Martin, J.F., 1974, “Quality of Effluents from Coal Refuse Piles,” Proceedings, First Symposium on 

Mine and Preparation Plant Refuse Disposal, Louisville, KY, National Coal Association, Washington, 

DC, pp. 26-37.

* Martin, T.E. and M. P. Davies, 2000, “Development and Review of Surveillance Programs for Tailings Dams,” Proceedings, Tailings Dams 2000, Las Vegas, Association of State Dam Safety Officials 

(ASDSO), Lexington, KY, pp. 367-380.

Mayne, P.W. and F.H. Kulhawy, 1982, “Ko-OCR Relationships in Soil,” Journal of Geotechnical Engineering, ASCE, Vol. 108, No. GT6, pp. 851-872.

* Mayne, P.W., B.R. Christopher and J. DeJong, 2002, Subsurface Investigations (Geotechnical Site Characterization), FHWA-NHI-01-031, U.S. Department of Transportation, Federal Highway Administration (FHWA), National Highway Institute, Washington, DC.

Maynord, S.T., 1978, Practical Riprap Design, Miscellaneous Paper H-78-7, U.S. Department of the 

Army, Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.

* McCook, D.K., 2006, “Pipes in Drain Systems – Are They a Necessary Evil?” 2006 Annual Conference Proceedings, Boston, Association of State Dam Safety Officials (ASDSO), Lexington, KY.

* McCook, D.K., 2002, “Things Your Mother Never Told You about Earth Dam Design,” 2002 Annual 

Conference Proceedings, Tampa, Association of State Dam Safety Officials (ASDSO), Lexington, KY.

* McCook, D.K., and J.R. Talbot, 2008, “A Comprehensive Sand and Gravel Filter Design Approach 

with Examples, Flow Charts, and Step by Step Process,” 2008 Annual Conference Proceedings, Indian 

Wells, Association of State Dam Safety Officials (ASDSO), Lexington, KY.

McCutcheon, H.P., 1983, Liquefaction and Cyclic Mobility of Coal Refuse Material, M.S. Thesis, Carnegie Mellon University, Pittsburgh, PA.

McGrath, T.J. and R.J. Hoopes, 1998, “Bedding Factors and E’ Values for Buried Pipe Installations 

Backfilled with Air-Modified CLSM,” The Design and Application of Controlled Low-Strength Materials 

(Flowable Fill), ASTM STP 1331-EB, A.K. Howard and J.L. Hitch, eds., American Society for Testing 

and Materials (ASTM), West Conshohocken, PA.

* McGuire, R.K., 1978, FRISK: Computer Program for Seismic Risk Analysis Using Faults as Earthquake 

Sources, Open File Report 78-1007, U.S. Department of the Interior, U.S. Geological Survey, Denver, CO.

* McGuire, R.K., 1976, FORTRAN Computer Program for Seismic Risk Analysis, Open-File Report 76-

67, U.S. Department of the Interior, U.S. Geological Survey, Denver, CO.

McGuire, R.K., 1974, Seismic Structural Response Risk Analysis Incorporating Peak Response Regressions 

on Earthquake Magnitude and Distance, Research Report R74-51, Massachusetts Institute of Technology, Department of Civil Engineering, Cambridge, MA. 

McKenna, G., 2006, “Rules of Thumb for Geotechnical Instrumentation Costs,” Geotechnical News, 

BiTech Publishers, Richmond, BC, Canada, Vol. 24, No. 2, pp. 46-47.

McLaren, R.J. and A.M. DiGioia, Jr., 1987, “The Typical Engineering Properties of Fly Ash,” Proceedings, Geotechnical Practice for Waste Disposal ’87, R.D. Woods, ed., New York, American Society 

of Civil Engineers (ASCE), Reston, VA, pp. 683–697.
R–19

Engineering and Design Manual Coal Refuse Disposal Facilities

* McNay, L.M., 1971, Coal Refuse Fires, an Environmental Hazard, Report No. BM-IC-8515, U.S. 

Department of the Interior, Bureau of Mines, Washington, DC.

McNeill, J.D., 1990, “Use of Electromagnetic Methods for Groundwater Studies,” Investigations in 

Geophysics No. 5: Geotechnical and Environmental Geophysics, S.H. Ward, ed., Society of Exploration 

Geophysicists, Tulsa, OK, Vol. I, pp. 191–218.

* Meek, F.A., 1994, “Evaluation of Acid Prevention Techniques Used in Surface Mining,” Proceedings, International Land Reclamation and Mine Drainage Conference, SP 06B-94, U.S. Department of the 

Interior, Bureau of Mines, Pittsburgh, PA, pp. 41-48.

Mesri, G. and M.E.M. Abdel-Ghaffar, 1993, “Cohesion Intercept in Effective Stress Stability Analysis,” Journal of Geotechnical Engineering, Vol. 119, No. 8, pp. 1229-1249.

Michael, P. and M. Superfesky, 2007, “Excess Spoil Minimization and Fill Stability,” Proceedings, 

2007 Annual Meeting, Society for Mining, Metallurgy and Exploration (SME), Denver, CO. 

*Michael, P., R. Murguia and L. Kosareo, 2005, The Flowability of Impounded Coal Refuse: A Review of 

Recent Work and Current Ideas in the Engineering Profession, OSM Final Report, U.S. Department of 

the Interior, Office of Surface Mining, Washington, DC.

Michalek, S.J., G.H. Gardner and K.K. Wu, 1996, Accidental Releases of Slurry and Water from Coal 

Impoundments through Abandoned Underground Mines, U.S. Department of Labor, MSHA, Safety and 

Health Technology Center, Pittsburgh, PA.

Mihok, E.A. and C.E. Chamberlain, 1968, “Factors in Neutralizing Acid Mine Water with Limestone,” Proceedings, 2nd Symposium on Coal Mine Drainage Research, Mellon Institute, Pittsburgh, PA, 

pp. 165-173.

Mikkelsen, P.E., 2002, “Cement-Bentonite Grout Backfill for Borehole Instruments,” Geotechnical 

News, Vol. 20, No. 4, pp. 38-42.

* Mine Safety and Health Administration (MSHA), 2008, Final Reports on Geophysical Void Detection 

Demonstration Projects, U.S. Department of Labor, MSHA, Arlington, VA.

* Mine Safety and Health Administration (MSHA), 2007, MSHA Coal Mine Impoundment Inspection and 

Plan Review Handbook, Handbook No. PH07-01, U.S. Department of Labor, MSHA, Arlington, VA.

* Mine Safety and Health Administration (MSHA), 2003, Report to Congress: Responses to Recommendations in the National Research Council’s Report, “Coal Waste Impoundments: Risks, Responses and 

Alternatives,” U.S. Department of Labor, MSHA, Arlington, VA.

* Mine Safety and Health Administration (MSHA), 1999, Haul Road Inspection Handbook, PH99-I-4, 

U.S. Department of Labor, MSHA, Arlington, VA.

Mine Safety and Health Administration (MSHA), 1993, Coal Impoundment Inspection Procedures 

Handbook, PH89-V-4, U.S. Department of Labor, MSHA, Arlington, VA.

Mine Safety and Health Administration (MSHA), 1983, Design Guidelines for Coal Refuse Piles and 

Water, Sediment, or Slurry Impoundments, and Impounding Structures, Amendment to Informational 

Report 1109, Technical Support Center, Denver, CO and Pittsburgh, PA.

* Mining Enforcement and Safety Administration (MESA), 1975, Engineering and Design Manual, 

Coal Refuse Disposal Facilities, U.S. Department of the Interior, MESA, Washington, DC.

Mishra, G.C. and A.K. Singh, 2005, “Seepage through a Levee,” International Journal of Geomechanics, ASCE, Vol. 5, No. 1, pp. 74-79.

Mitchell, R.J., 1983, Earth Structures Engineering, Allen and Unwin, Boston MA.

Molinda, G.M., C. Mark and D. Debasis, 2001, “Using the Coal Mine Roof Rating (CMRR) to Assess 

Roof Stability in U.S. Coal Mines,” Journal of Mines, Metals & Fuels, Vol. 49, No. 8-9, pp. 314-321.
R–20

References

Morgenstern, N.R. and V.E. Price, 1965, “The Analysis of the Stability of General Slip Surfaces,” 

Géotechnique, Vol. 15, No. 1, pp. 79-93.

Morris, P.H. and D.J. Williams, 2000, “A Revision of Blight’s Model of Field Vane Testing, Canadian 

Geotechnical Journal, Vol. 37, No. 5, 1089-1098.

Moser, A.P., 2001, Buried Pipe Design, 2nd Ed., McGraw-Hill, New York, NY.

* Myers, J.W., J.J. Pfeiffer, E.M. Murphy and F.E. Griffith, 1966, Ignition and Control of Burning of Coal 

Mine Refuse, Report No. BM-RI-6758, U.S. Department of the Interior, Bureau of Mines, Washington, DC.

Naesgaard, E., P.M. Byrne, M. Seid-Karbasi and S.S. Park, 2005, “Modeling Flow Liquefaction, Its 

Mitigation, and Comparison with Centrifuge Tests,” Proceedings, Geotechnical Earthquake Engineering Satellite Conference, Osaka, Japan.

National Academy of Sciences, 1975, Underground Disposal of Coal Mine Wastes, Washington, DC.

National Coal Board (NCB), 1975, Subsidence Engineers’ Handbook, Mining Department of the NCB, 

London, UK.

National Coal Board (NCB), 1972, Review of Research on Properties of Spoil Tip Materials, NCB, 

London, UK.

National Coal Board (NCB), 1970, “Spoil Heaps and Lagoons,” Technical Handbook, NCB, London, UK.

National Research Council (NRC), 2002, Coal Waste Impoundments: Risks, Responses, and Alternatives, 

Committee on Coal Waste Impoundments, Committee on Earth Resources, Board on Earth Sciences 

and Resources, Division on Earth and Life Studies, National Academy Press, Washington, DC.

National Stone, Sand & Gravel Association (NSSGA – formerly National Stone Association), 1989, 

Quarried Stone for Erosion and Sediment Control, Alexandria, VA.

National Weather Service (NWS), 2006a, NOAA Atlas 14, Precipitation Atlas of the United States, 

Volume 1, The Semi-Arid Southwest, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, NWS (available on-line only).

National Weather Service (NWS), 2006b, NOAA Atlas 14, Precipitation Atlas of the United States, 

Volume 2, The Ohio River Basin and Surrounding States, U.S. Department of Commerce, National 

Oceanic and Atmospheric Administration, NWS (available on-line only).

* National Weather Service (NWS), 1999, Hydrometeorolgical Report No. 59, Probable Maximum Precipitation for California, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1998, Hydrometeorolgical Report No. 58, Probable Maximum Precipitation for California – Calculation Procedures, U.S. Department of Commerce, National Oceanic and 

Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1994, Hydrometeorolgical Report No. 57, Probable Maximum Precipitation – Pacific Northwest States, U.S. Department of Commerce, National Oceanic and Atmospheric 

Administration, NWS, Washington, DC

* National Weather Service (NWS), 1988, Hydrometeorolgical Report No. 55A, Probable Maximum Precipitation Estimates, United States between the Continental Divide and the 105th Meridian, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1986, Hydrometeorolgical Report No. 56, Probable Maximum Precipitation Estimates with Areal Distribution for Tennessee River Drainages Less than 3000 Square Miles 

in Area, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, NWS, 

Washington, DC.

* National Weather Service (NWS), 1982, Hydrometeorolgical Report No. 52, Application of Probable 

Maximum Precipitation Estimates, United States East of the 105th Meridian, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, NWS, Washington, DC.
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Engineering and Design Manual Coal Refuse Disposal Facilities

* National Weather Service (NWS), 1980, Hydrometerological Report No. 53, Seasonal Variation of 10-

Square-Mile Probable Maximum Precipitation Estimates, United States East of the 105th Meridian, U.S. 

Department of Commerce, National Oceanic and Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1978, Hydrometeorolgical Report No. 51, Probable Maximum Precipitation Estimates, United States East of the 105th Meridian, U.S. Department of Commerce, National 

Oceanic and Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1977, Hydrometeorolgical Report No. 49, Probable Maximum Precipitation Estimates, Colorado River and Great Basin Drainages, U.S. Department of Commerce, 

National Oceanic and Atmospheric Administration, NWS, Washington, DC.

National Weather Service (NWS), 1973, NOAA Atlas 2, Precipitation Frequency Atlas of the Western 

United States, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, 

NWS, Washington, DC.

* National Weather Service (NWS), 1965, Technical Paper No. 52, Two- to 10-Day Precipitation for 

Return Periods from 1 to 100 Years in Alaska, U.S. Department of Commerce, National Oceanic and 

Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1964, Technical Paper No. 49, Two- to 10-Day Precipitation for 

Return Periods from 2 to 100 Years for the Contiguous United States, U.S. Department of Commerce, 

National Oceanic and Atmospheric Administration, NWS, Washington, DC.

* National Weather Service (NWS), 1963, Technical Paper No. 47, Probable Maximum Precipitation and 

Rainfall Frequency Data for Alaska for Areas to 400 Square Miles, Durations to 24 Hours, and Return Periods from 1 to 100 Years, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, NWS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2007a, National Engineering Handbook, Chapter 7: 

Hydrologic Soil Groups, 210-VI-NEH, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2007b, National Engineering Handbook, Chapter 45: 

Filter Diaphragms, 210-VI-NEH, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2007c, NRCS Fillable Form Template, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service, 2005a, National Engineering Handbook, Chapter 52: Structural 

Design of Flexible Conduits, 210-VI-NEH, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2005b, Earth Dams and Reservoirs, TR-60, 210-VITR60, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2004a, National Engineering Handbook, Chapter 9: 

Hydrologic Soil Cover Complexes, 210-VI-NEH, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2004b, National Engineering Handbook, Chapter 10: 

Estimation of Direct Runoff from Storm Rainfall, 210-VI-NEH, U.S. Department of Agriculture, NRCS, 

Washington, DC.

* Natural Resources Conservation Service (NRCS), 2004c, National Engineering Handbook, Chapter 11: 

Snowmelt, 210-VI-NEH, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 2002, Pond, Conservation Practice Standard No. 

378, U.S. Department of Agriculture, NRCS, Washington, DC.

* Natural Resources Conservation Service (NRCS), 1994, National Engineering Handbook, Chapter 

26: Gradation Design of Sand and Gravel Filters, U.S. Department of Agriculture, NRCS, Washington, DC.
R–22

References

* Natural Resources Conservation Service (NRCS), 1986, Urban Hydrology for Small Watersheds, TR55, 210-VI-TR55, U.S. Department of Agriculture, NRCS, Washington, DC.

* Newman, D.A., 2003, “Rock Mechanics and the Analysis of Underground Mine Stability Adjacent 

to Coal Refuse Impoundments,” Proceedings, 22nd International Conference on Ground Control in 

Mining, West Virginia University, Morgantown, WV.

Newmark, N.M., 1965, “Effects of Earthquakes on Dams and Embankments,” Géotechnique, Vol. 15, 

No. 2, pp. 139-160.

Nicholas, A.K. and R.J. Hutnik, 1971, Ectomycorrhizal Establishment and Seedling Response on Variously Treated Deep-Mine Coal Refuse, Special Report SR-89, School of Forest Resources, Pennsylvania 

State University, University Park, PA.

* Nicholls, H.R., C.F. Johnson and W.I. Duvall, 1971, Blasting Vibrations and Their Effects on Structures, 

Bulletin 656, U.S. Department of the Interior, Bureau of Mines, Washington, DC.

*Nieto, A.S., 1979, “Evaluation of Damage Potential to Earth Dams by Subsurface Coal Mining at 

Rend Lake, Illinois,” Proceedings, Geotechnics of Mining: 10th Ohio River Valley Seminar, Lexington, 

KY, pp. 9-18.

Nilson, A.H., D. Darwin and C.W. Dolan, 2002, Design of Concrete Structures, 13th Ed., McGrawHill, New York, NY.

Nuttli, O.W., 1973, “Seismic Wave Attenuation and Magnitude Relations for Eastern North America,” Journal of Geophysical Research, Vol. 78, No. 5, pp. 876-885.

O’Brien, J.J., J.A. Havers and F.W. Stubbs, eds., 1996, Standard Handbook of Heavy Construction, 3rd 

Ed., McGraw-Hill, New York, NY.

O’Connor, K.M. and C.H. Dowding, 1999, GeoMeasurements by Pulsing TDR Cables and Probes, CRC 

Press, Boca Raton, FL.

Oglesby, C.H. and R.G. Hicks, 1982, Highway Engineering, 4th Ed., John Wiley & Sons, New York, NY.

* Ohio Department of Transportation (ODOT), 2006, Geotechnical Bulletin GB3: Rock Cut Slope and Catchment Design, Division of Production Management, Office of Geotechnical Design, Columbus, OH.

* Olin, D.A. and J.B. Atkins, 1988, Estimating Flood Hydrographs and Volumes for Alabama Streams, 

Water Resources Investigation Report 88-4041, U.S. Department of the Interior, U.S. Geological 

Survey, Water Resources Division, Tuscaloosa, AL.

Olsen, R.S. and J.K. Mitchell, 1995, “CPT Stress Normalization and Prediction of Soil Classification,” Proceedings of the International Symposium on Cone Penetration Testing, CPT ’95, Linköping, 

Sweden, Swedish Geotechnical Society, Vol. 2, pp. 257-262.

Olson, S.M. and T.D. Stark, 2003, “Yield Strength Ratio and Liquefaction Analysis of Slopes and 

Embankments,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 129, No. 8, 

pp. 727-737.

Olson, S.M. and T.D. Stark, 2002, “Liquefied Strength Ratio from Liquefaction Flow Failure Case 

Histories,” Canadian Geotechnical Journal, National Research Council of Canada, Vol. 39, pp. 629-647.

Owens, H.L., 2008, personal communication.

Owens, H.L., 1987, MSHA Report of Investigation, Impoundment Principal Spillway Pipe Failure, Montcoal Preparation Plant (ID No. 46-03430), Peabody Coal Company, Raleigh County, WV.

Owens, H.L., 1977, MESA Memorandum on Failure of the Fine Coal Refuse Impoundment (ID No. 

1211WV40009-02), Pond Fork Preparation Plant, Island Creek Coal Company, Boone County, WV. 

Packer, P.E. and E.F. Aldon, 1978, “Revegetation Techniques for Dry Regions,” Reclamation of Drastically Disturbed Land, F.W. Schaller and P. Sutton, eds., American Society of Agronomy, Madison, 

WI, pp. 425-450.
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Engineering and Design Manual Coal Refuse Disposal Facilities

Page, A.L., R.H. Miller and D.R. Keeney, eds., 1982, Methods of Soil Analysis, Part 2: Chemical and 

Microbiological Properties, Agronomy Monograph No. 9, 2nd Ed., American Society of Agronomy 

and Soil Science of America, Madison, WI.

Pariseau, W.G., 2006, Design Analysis in Rock Mechanics, CRC Press, Boca Raton, FL.

Pariseau, W.G., 1975, “Limit Design of Mine Pillars under Uncertainty,” Proceedings, 16th U.S. Symposium on Rock Mechanics, University of Minnesota, Minneapolis, MN, pp. 287-301.

Park, R. and T. Paulay, 1975, Reinforced Concrete Structures, John Wiley & Sons, New York, NY.

Patterson, R.J. and K.D. Ferguson, 1994, “The Gibraltar North Project Assessing Acid Rock Drainage,” Proceedings, 3rd International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA, 

U.S. Bureau of Mines Special Publication SP 06B-94, Vol. 2, pp. 12-21.

*Pearson, M.L., T. Preber and P.J. Conroy, 1981, Outcrop Barrier Design Guidelines for Appalachian Coal 

Mines, Open File Report 134-81, U.S. Department of the Interior, Bureau of Mines, Washington, DC.

Peck, R.B., 1972, “Observations and Instrumentation: Some Elementary Considerations,” Highway 

Focus, U.S. Department of Transportation, Federal Highway Administration, Vol. 4, No. 2, pp. 1-5. 

Reprinted in Judgment in Geotechnical Engineering: The Professional Legacy of Ralph B. Peck, 1984, J. 

Dunnicliff and D.U. Deere, eds., John Wiley & Sons, New York, NY, pp. 128-130.

Pelton, F., 2000, Guidelines for Instrumentation and Measurement for Monitoring Dam Performance, 0-

7844-0531-X, ASCE Task Committee on Instrumentation and Monitoring Dam Performance, American Society of Civil Engineers (ASCE), Reston, VA.

Peng, S.S. and H.S. Chiang, 1984, Longwall Mining, John Wiley & Sons, New York, NY.

Peng, S.S. and Y. Luo, 1993, “A New Method for Designing Support Area to Protect Surface Structures over Underground Coal Mining Areas,” Transactions, AIME-SME, Vol. 294, pp. 1927-1932.

* Pennsylvania Department of Environmental Protection (PADEP), 2000, Erosion and Sediment Pollution Control Program Manual, Document No. 363-2134-008, PADEP, Office of Water Management, 

Bureau of Watershed Management, Division of Waterways, Wetlands and Erosion Control, Harrisburg, PA.

Pennsylvania Department of Transportation (PennDOT), 2007, Specifications Publication 408/2007, 

Commonwealth of Pennsylvania, PennDOT, Harrisburg, PA.

* Pennsylvania State University (PSU), 2006, An In-Seam Seismic (ISS) Method Based Mine Void Detection 

Technique, Final Report for Phase I, prepared for MSHA by Maochen Ge, PSU, University Park, PA.

Performance Pipe, 2003, The Performance Pipe Engineering Manual, Performance Pipe Division of 

Chevron Phillips Chemical Company, LP, Plano, TX.

* Perin, R.J., 2000, “Foundation Treatment of Stress Relief Fractures ‘Hillseams’ in a Tailings 

Embankment Cutoff Trench,” Proceedings, Tailings Dams 2000, Las Vegas, ASDSO/USCOLD, pp. 

197-209.

Perlea, V.G., M.E. Rothford and D.C. Serafini, 2009, “Use of Seismic Deformation Analysis in 

Remediation Design,” Proceedings, 29th Annual USSD Conference, Nashville, United States Society 

on Dams, Denver, CO.

Phelps, L.B., W.B. Wells and L.W. Saperstein, 1981, “An Analysis of Surface Coal Mine Spoil Bulk 

Density,” Society of Mining Engineering Annual Meeting, SME Preprint No. 81-53, Chicago, IL.

Poulos, S.J., 1988, “Chapter 9 – Liquefaction and Related Phenomena,” Advanced Dam Engineering 

for Design, Construction, and Rehabilitation, R.B. Jansen, ed., Van Nostrand Reinhold, New York, NY.

Poulos, S.J., 1981, “The Steady State of Deformation,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 107, No. GT5, pp. 553-562.
R–24

References

Poulos, H.G. and E.H. Davis, 1974, Elastic Solutions for Soil and Rock Mechanics, John Wiley & Sons, 

New York, NY.

Poulos, S.J., G. Castro and J.W. France, 1985, “Liquefaction Evaluation Procedure,” Journal of the 

Geotechnical Engineering Division, ASCE, Vol. 111, No. GT6, pp. 772-792.

Pratt, H.K., R.C. McMordie and R.M. Dundas, 1972, “Foundations and Abutment - Bennett and 

Mica Dams,” Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 98, SM10, pp. 1053-1072.

Price, W.A., K.A. Morin and N.M. Hutt, 1997, “Guidelines for the Prediction of Acid Rock Drainage 

and Metal Leaching for Mines in British Columbia: Part II, Recommended Procedures for Static and 

Kinetic Tests,” Proceedings, 4th International Conference on Acid Rock Drainage, Vancouver, BC, Canada, 

American Society of Surface Mining and Reclamation, Vol. I, pp. 15-30.

Qiu, Y.J., and D.C. Sego, 2001, “Laboratory Properties of Mine Tailings,” Canadian Geotechnical Journal, Vol. 38, No. 1, pp. 183-190.

Quaranta, J.D., L.D. Banta, and J.A. Altobello, 2008, “Remote Monitoring of a High Hazard Coal 

Waste Impoundment in Mountainous Terrain Case Study,” Proceedings, Tailings and Mine Waste ‘08, 

Vail, CO, pp. 125-136.

Reddi, L.N., 2003, Seepage in Soils – Principles and Applications, John Wiley & Sons, Hoboken, NJ.

Rellensmann, O. and E. Wagner, 1957, “The Effect on Railways of the Ground Movements Due to 

Mining,” Proceedings, First European Congress on Ground Movement, University of Leeds, Leeds, UK, 

pp. 74-82.

Rich, D.H. and K.R. Hutchinson, 1990, “Neutralization and Stabilization of Combined Refuse 

Using Lime Kiln Dust at High Power Mountain,” Proceedings, 1990 Mining and Reclamation Conference and Exhibition, Charleston, WV, Vol. I, pp. 55-60.

Richter, C., 1958, Elementary Seismology, W.H. Freeman and Co., San Francisco, CA.

Rico, M., G. Benito and A. Diez-Herrero, 2008, Floods from Tailings Dam Failures, Journal of Hazardous Materials, Vol. 152, No. 2, pp. 846-852.

* Rixner, J.J., S.R. Kraemer and A.D. Smith, 1986, Prefabricated Vertical Drains, Vol. I, Engineering 

Guidelines, Report No. FHWA/RD-86/168, Office of Engineering and Highway Operations Research 

and Development, Federal Highway Administration, McLean, VA. 

* Robbins, C.H., 1986, Techniques for Simulating Flood Hydrographs and Estimating Flood Volumes for 

Ungaged Basins in Tennessee, Water Resources Investigation Report 86-4192, U.S. Department of the 

Interior, U.S. Geological Survey, Water Resources Division, Nashville, TN.

Robertson, P.K., 2008, discussion of paper, “Liquefaction Potential of Silts from CPTu,” Canadian 

Geotechnical Journal, Vol. 45, pp. 140-141.

Robertson, P.K., 2004, “Evaluating Soil Liquefaction and Post-Earthquake Deformations Using the 

CPT,” Proceedings of ISC-2 on Geotechnical and Geophysical Site Characterization, Porto, Portugal, V. 

Fonseca and P.W. Mayne, eds., Millpress, Rotterdam, Vol. 1, pp. 233-252.

Robertson, P.K., 1994, “Suggested Terminology for Liquefaction,” Proceedings of the 47th Canadian 

Geotechnical Conference, Halifax, NS, Canadian Geotechnical Society, Alliston, ON, Canada, pp. 277-286.

Robertson, P.K. and R.G. Campanella, 1983, “Interpretation of Cone Penetration Tests: Part I - 

Sands; Part II – Clays,” Canadian Geotechnical Journal, Vol. 20, No. 4, pp. 719-745.

Robertson, P.K. and C.E. Wride, 1998, “Evaluating Cyclic Liquefaction Potential Using the Cone 

Penetration Test,” Canadian Geotechnical Journal, Vol. 35, No. 3, pp. 442-459.

Rupert, G.B. and G.B. Clark, 1977, “Criteria for the Proximity of Surface Blasting to Underground 

Coal Mines,” Proceedings, 18th U.S. Symposium on Rock Mechanics, Colorado School of Mines, Keystone, CO, pp. 3C3-1 to 3C3-10.
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* Rusnak, J. and C. Mark, 2000, “Using the Point Load Test to Determine the Uniaxial Compressive Strength of Coal Measure Rock,” Proceedings, 19th International Conference on Ground Control in 

Mining, S.S. Peng and C. Mark, eds., West Virginia University, Morgantown, WV, pp. 362-371. 

Rutledge, P.C. and J.P. Gould, 1973, “Movements of Articulated Conduits under Earth Dams on 

Compressible Foundations,” Embankment-Dam Engineering: Casagrande Volume, R.C. Hirschfeld and 

S.J. Poulos, eds., John Wiley & Sons, New York, NY, pp. 209-237.

* Sabatini, P.J., R.C. Bachus, P.W. Mayne, J.A. Schneider and T.E. Zettler, 2002, Geotechnical Engineering Circular No. 5: Evaluation of Soil and Rock Properties, DTFH61-94-C-00099, U.S. Department of 

Transportation, Federal Highway Administration (FHWA), Office of Bridge Technology, Washington, DC.

Saikia, C.K. and P.G. Somerville, 1997, “Simulated Hard-Rock Motions in Saint Louis, Missouri, 

from Large New Madrid Earthquakes (Mw ≥ 6.5),” Bulletin of the Seismological Society of America, 

Vol. 87, No. 1, pp. 123-139.

* Sames, G.P. and N.N. Moebs, 1992, Roof Control of Stress-Relief Jointing near Outcrops in Central 

Appalachian Drift Mines, Information Circular 9313, U.S. Department of the Interior, Bureau of 

Mines, Pittsburgh, PA.

* Sames, G.P. and N.N. Moebs, 1989, Hillseam Geology and Roof Instability Near Outcrop in Eastern Kentucky Drift Mines,” Report of Investigations 9267, U.S. Department of the Interior, Bureau 

of Mines, Pittsburgh, PA.

Sarma, S.K., 1973, “Stability Analysis of Embankments and Slopes, Géotechnique, Vol. 23, No. 3, 

pp. 423-433.

* Savy, J.B., W. Foxall, N.A. Abrahamson and D.L. Bernreuter, 2002, Guidance for Performing Probabilistic Seismic Hazard Analysis for a Nuclear Plant Site: Example Application to the Southeastern United 

States, NUREG/CR-6607 (UCRL-ID-133494), Office of Nuclear Regulatory Research, U.S. Nuclear 

Regulatory Commission (USNRC), Washington, DC.

Saxena, S.K., D.E. Lourie and J.S. Rao, 1984, “Compaction Criteria for Eastern Coal Waste Embankments,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 110, No. GT2, pp. 262-284.

* Schaefer, M.G., 1992, Dam Safety Guidelines - Technical Note 1: Dam Break Inundation Analysis and 

Downstream Hazard Classification, Publication 92-55E, Washington State Department of Ecology, 

Olympia, WA.

* Schnabel, P.B., J. Lysmer and H.B. Seed, 1972, SHAKE: A Computer Program for Earthquake Response 

Analysis of Horizontally Layered Sites, Report No. EERC 72-12, University of California, Berkeley, CA.

* Schubert, C.E., W.J. Johnson and B.W. Hassinger, 1982, Improved Resource Characterization for Coal 

Mining(Geophysics and Drilling Technology), Contract No. USDOE AC22-80PC30114, U.S. Department of Energy, Office of Coal Mining, Washington, DC.

Schwartz, D.P. and K.J. Coppersmith, 1986, “Seismic Hazards: New Trends in Analysis Using Geologic 

Data,” Studies in Geophysics-Active Tectonics, National Academy Press, Washington, DC, pp. 215-230.

Scovazzo, V.A., 2008, personal communication regarding experience with angle of draw.

* Scovazzo, V.A., 1995, “Comparison of Empirical, Analytical, and Numerical Methods of Pillar 

Design and the Development of a Site Specific Pillar Formula,” Proceedings, 14th International Conference on Ground Control in Mining, West Virginia University, Morgantown, WV, pp. 116-123.

Seed, H.B., 1987, “Design Problems in Soil Liquefaction,” Journal of Geotechnical Engineering, ASCE, 

Vol. 113, No. 8, pp. 827-845.

Seed, H.B., 1979, “Soil Liquefaction and Cyclic Mobility Evaluation for Level Ground During 

Earthquakes,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 105, No. GT2, pp. 201-255.
R–26

References

Seed, R.B. and L.F. Harder, Jr., 1990, “SPT-Based Analysis of Cyclic Pore Pressure Generation and 

Undrained Residual Strength,” Proceedings of the H. Bolton Seed Memorial Symposium, Vol. 2, University of California, Berkeley, CA, BiTech Publishers, Richmond, BC, Canada, pp. 351-376.

Seed, H.B. and I.M. Idriss, 1982, “Ground Motions and Soil Liquefaction During Earthquakes,” 

Earthquake Engineering Research Institute, Berkeley, CA.

Seed, H.B. and I.M. Idriss, 1971, Simplified Procedure for Evaluating Soil Liquefaction Potential, 

Journal of the Geotechnical Engineering Division, ASCE, Vol. 97, No. 9, pp. 1249-1273.

* Seed, R.B., K.O. Cetin, R.E.S. Moss, A.M. Kammerer, J. Wu, J.M. Pestina, M.F. Riemer, R.B. Sancio, 

J.D. Bray, R.E. Kayen and A. Faris, 2003, Recent Advances in Soil Liquefaction Engineering: A Unified 

and Consistent Framework, Report No. EERC 2003-06, 26th Annual ASCE Los Angeles Geotechnical 

Spring Seminar, Long Beach CA.

Serafim, J.L. and J.P. Pereira, 1983, “Considerations on the Geomechanical Classification of Bieniawski,” Proceedings, International Symposium on Engineering Geology and Underground Construction, 

Lisbon, Portugal, Theme II, Vol. 1, pp. II.33-II.42.

Shackelford, R., 2000, “Underground Surveying – A Primer in Coal Mine Surveying and Mapping,” Point of Beginning Electronic Magazine, Sept. 29, 2000 issue.

Sherard, J.L., 1973, “Embankment Dam Cracking,” Embankment-Dam Engineering: Casagrande 

Volume, R.C. Hirschfeld and S.J. Poulos, eds., John Wiley & Sons, New York, NY, pp. 272-353.

Sherard, J.L. and L.P. Dunnigan, 1985, “Filters and Leakage Control in Embankment Dams,” Proceedings, Symposium on Seepage and Leakage from Dams and Impoundments, Denver, American Society 

of Civil Engineers (ASCE), Reston, VA, pp. 1-31.

Sherard, J.L., L.P. Dunnigan, R.S. Decker and E.F. Steele, 1976, “Pinhole Test for Identifying Dispersive Clays,” Journal of the Geotechnical Engineering Division, ASCE, Vol. 102, No. GT1, pp. 69-85.

Sherard, J.L., R.J. Woodward, S.F. Gizienski, and W.A. Clevenger, 1963, Earth and Earth-Rock Dams: 

Engineering Problems of Design and Construction, John Wiley & Sons New York, NY.

Shoup, D., 1992, “Protecting Geotechnical Sensors and Cable from Lightning Damage,” Sensors in 

the Real World, Slope Indicator Company.

Singh, M.M., 1992, “Mine Subsidence,” SME Mining Engineering Handbook, 2nd Ed., H. Hartman, 

ed., Society for Mining, Metallurgy, & Exploration (SME), Littleton, CO, pp. 938-971.

Singh, M.M., 1978, “Experience with Subsidence Due to Mining,” Proceedings, International Conference on Evaluation and Prediction of Subsidence, New York, American Society of Civil Engineers 

(ASCE), Reston, VA, pp. 92-112.

* Singh, M.M. and F.S. Kendorski, 1981, “Strata Disturbance Prediction for Mining Beneath Surface 

Water and Waste Impoundments,” Proceedings, 1st Conference on Ground Control in Mining, West 

Virginia University, Morgantown, WV, pp. 76-89.

* Sirles, P.C., 2006, Use of Geophysics for Transportation Projects: A Synthesis of Highway Practice, 

NCHRP Synthesis 357, Transportation Research Board, National Cooperative Highway Research 

Program, Washington, DC.

*Siskind, D.E., M.S. Stagg, J.W. Kopp and C.H. Dowding, 1980, Structure Response and Damage Produced by Ground Vibration from Surface Mine Blasting, Report of Investigations 8507, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA.

* Skelly and Loy, 1977, Guidelines for Mining near Surface Waters, Open File Report 29-77, U.S. 

Department of the Interior, Bureau of Mines, Washington, DC.

* Skousen, J.G., A.W. Rose, G. Geidel, R. Foreman, R. Evans, W.W. Hellier and members of the 

Avoidance and Remediation Working Group of the Acid Drainage Technology Initiative, 1998, 
R–27

Engineering and Design Manual Coal Refuse Disposal Facilities

Handbook of Technologies for Avoidance and Remediation of Acid Mine Drainage, National Mine Land 

Reclamation Center, West Virginia University, Morgantown, WV.

Skousen, J.G., J.C. Sencindiver and R.M. Smith, 1987, A Review of Procedures for Surface Mining and 

Reclamation in Areas with Acid-Producing Materials,” West Virginia University Energy and Water 

Research Center Publication EWRC 871, Morgantown, WV.

Slemmons, D.B. and C.M. Depolo, 1986, “Evaluation of Active Faulting and Associated Hazards,” 

Studies in Geophysics-Active Tectonics, National Academy Press, Washington, DC, pp. 45-62.

Smith, R.M., A.A. Sobek, T. Arkle, Jr., J.C. Sencindiver and J.R. Freeman, 1976, Extensive Overburden 

Potenials for Soil and Water Quality, EPA 600/1-76-184, U.S. Department of the Environment, U.S. 

Environmental Protection Agency, Washington, DC.

Smith, R.M., W.E. Grube, T.Arkle, Jr. and A. Sobek, 1974, Mine Spoil Potentials for Soil and Water 

Quality, EPA 670/2-74-070, U.S. Department of the Environment, U.S. Environmental Protection 

Agency, Washington, DC.

* Smith, S.A. and D.M. Hosler, 2006, “Current Research in Dam Drain Clogging and Its Prevention,” 

Proceedings, 2006 ASDSO Annual Conference, Boston, Association of State Dam Safety Officials, Lexington, KY.

Snow, R.E., J.L. Olson and W. Schultz, 2000, “Slurry Impoundment Internal Drain Design and Construction,” Proceedings of the 7th Conference on Tailings and Mine Waste ’00, Fort Collins, CO, pp. 65-73.

Sobel, P., 1994, Revised Livermore Seismic Hazard Estimates for Sixty-Nine Nuclear Power Plant Sites 

East of the Rocky Mountains, NUREG 1488, Division of Engineering, Office of Nuclear Reactor Regulation, U.S. Nuclear Regulatory Commission (USNRC), Washington, DC.

Somerville, P.G., N.F. Collins, N.A. Abrahamson, R.W. Graves and C.K. Saikia, 2001, Ground Motion 

Attenuation Relations for the Central and Eastern United States, Final Report to U.S. Geological Survey, 

URS Group, Inc., Pasadena, CA.

Song, S-H., Y. Song and B-D. Kwon, 2005, “Application of Hydrogeological and Geophysical Methods to Delineate Leakage Pathways in an Earth Fill Dam,” Exploration Geophysics, Vol. 36, No. 1, 

pp. 92-96.

Sowers, G.F. and G.B. Sowers, 1970, Introductory Soil Mechanics and Foundations, 4th Ed., Macmillan 

Publishing Company, New York, NY.

Spencer, E., 1967, “A Method of Analysis of the Stability of Embankments Assuming Parallel Interslice Forces,” Géotechnique, Vol. 17, No. 1, pp. 11-26.

Stewart, B.M. and G. Robinson, 1994, MSHA Accident Investigation Report, Non-Injury Inundation 

Report, Martin County Coal Corporation, 1-C Mine (ID No. 15-03752), Martin County, KY.

* Stewart, B.M. and L.A. Atkins, 1983, Physical Property Data on Coarse Anthracite Waste, Report of 

Investigations 8782, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA.

* Stewart, B.M. and L.A. Atkins, 1982, Engineering Properties of Combined Coarse and Fine Coal Wastes, 

Report of Investigations 8623, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA.

* Stewart, B.R., W.L. Daniels, L.W. Zelazny and M.L. Jackson, 2001, “Evaluation of Leachates from 

Coal Refuse Blended with Fly Ash at Different Rates,” Journal of Environmental Quality, Vol. 30, 

No. 5, pp. 1382-1391.

* Stewart, B.R., W.L. Daniels, and M.L. Jackson, 1997, “Evaluation of Leachate Quality from the 

Co-Disposal of Coal Fly Ash and Coal Refuse,” Journal of Environmental Quality, Vol. 26, No. 5, 

pp. 1412-1424.

Szymanski, M.B., 1999, Evaluation of Safety of Tailings Dams, BiTech Publishers, Richmond, BC, Canada.
R–28

References

* Talbot, J.R., S.J. Poulos, and R.C. Hirschfeld, 2000, “Remediation of Seepage Problems through 

Cutoff or Reduction of Flow and through Collection and Control of Seepage Including the Use of 

Geosynthetics,” The National Dam Safety Program, Research Needs Workshop: Seepage through Embankment Dams, White Paper No. 5, Attachment 8, Denver, U.S. Department of Homeland Security, 

Federal Emergency Management Agency (FEMA), Washington, DC.

Talwani, P. and W.T. Schaeffer, 2001, “Recurrence Rates of Large Earthquakes in the South Carolina 

Coastal Plain Based on Paleoliquefaction Data,” Journal of Geophysical Research, Vol. 106, No. B4, 

pp. 6621-6642.

Terzaghi, K. and R.B. Peck, 1967, Soil Mechanics in Engineering Practice, 2nd Ed., John Wiley & Sons, 

New York, NY.

Terzaghi, K., R.B. Peck and G. Mesri, 1996, Soil Mechanics in Engineering Practice, 3rd Ed., WileyInterscience, New York, NY.

* Thacker, B.K., 2000, “Perimeter Embankments to Increase Capacity, Mitigate Subsidence Impacts, 

and Abandon Coal Refuse Disposal Impoundments,” Proceedings, Tailing Dams 2000, Las Vegas, 

ASDSO/USCOLD, pp. 143-154.

Thacker, B.K., 1997, “Liquefaction Mitigation Procedures for Coal Refuse Dams Built by the Modified Upstream Method,” Proceedings, XXVIII Ohio River Valley Soils Seminar, Lexington, KY.

Thacker, B.K., C.R. Ulrich, J.C. Athanaskes and G. Smith, 1988, “Evaluation of Coal Refuse 

Impoundment Built by the Upstream Method,” Geotechnical Special Publicaton No. 21, Hydraulic Fill 

Structures, American Society of Civil Engineers (ASCE), New York, NY, pp. 730-749.

Thiers, G.R. and H.B. Seed, 1968, ‘’Cyclic Stress-Strain Characteristics of Clays,’’ Journal of the Soil 

Mechanics and Foundations Division, ASCE, Vol. 94, No. SM2, pp. 555-569.

Timoshenko, S. and S. Woinowsky-Krieger, 1959, Theory of Plates and Shells, 2nd Ed., McGraw-Hill, 

New York, NY.

Toro, G.R. and R.K. McGuire, 1987, “An Investigation into Earthquake Ground Motion Characteristics in Eastern North America,” Bulletin of the Seismological Society of America, Vol. 77, No. 2, 

pp. 468-489.

Toro, G.R., N.A. Abrahamson and J.F. Schneider, 1997, “Model of Strong Ground Motions from 

Earthquakes in Central and Eastern North America: Best Estimates and Uncertainties,” Seismological Research Letters, Vol. 68, No. 1, pp. 41-57.

Tuttle, M.P, E.S. Schweig, J.D. Sims, R.H. Lafferty, L.W. Wolf and M.L. Haynes, 2002, “The Earthquake Potential of the New Madrid Seismic Zone,” Bulletin of the Seismological Society of America, 

Vol. 92, No. 6, pp. 2080-2089.

Ullrich, C.R., B.R. Thacker and N.R. Roberts, 1991, “Dynamic Properties of Fine-Grained Coal 

Refuse,” Proceedings, Second International Conference on Recent Advances in Geotechnical Earthquake 

Engineering and Soil Dynamics, St. Louis, MO, pp. 393-397.

University of British Columbia-Geophysical Inversion Facility (UBC-GIF), 2005, “Three-Dimensional Modeling of Self-Potentials Caused by Seepage Flows,” Investigation of Geophysical Methods 

for Assessing Seepage and Internal Erosion in Embankment Dams, CEA Technologies, Inc. Project No. 

T992700-0205, Montreal, QC, Canada.

University of California, 2007a, Pacific Earthquake Engineering Research Center: NGA Database, 

Coyote Lake – Gilroy #6, 230 (CDMG Station 57383), August 6, 1979, M = 5.7. (http://peer.berkeley.

edu/index.html)

University of California, 2007b, Pacific Earthquake Engineering Research Center: NGA Database, 

Saguenay – W. Chicoutime Nord (Site 16 T), November 25, 1988, M = 5.9. (http://peer.berkeley.edu/

index.html)
R–29

Engineering and Design Manual Coal Refuse Disposal Facilities

University of California, 2007c, Pacific Earthquake Engineering Research Center: NGA Database, 

Whittier Narrows Station: 24399, Mt. Wilson – CIT Seismic Station October 1, 1987, M = 5.99. 

(http://peer.berkeley.edu/index.html).

* U.S. Army Corps of Engineers (USACE), 2008, Hydrologic Modeling System, HEC-HMS, Users 

Manual, Version 3.3, CDP-74A, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 2004, Cathodic Protection for Civil Works Structures, EM 

1110-2-2704, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 2003, Engineering and Design – Slope Stability, EM 1110-2-

1902, U.S. Department of the Army, USACE, Washington, DC.

U.S. Army Corps of Engineers (USACE), 2002, Coastal Engineering Manual – Part II, EM 1110-2-

1100, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 2000, Hydrologic Modeling System HEC-HMS: Technical 

Reference Manual, Report No. CPD-74B, U.S. Department of the Army, USACE, Hydrologic Engineering Center, Davis, CA.

* U.S. Army Corps of Engineers (USACE), 1999, Engineering and Design – Response Spectra and Seismic Analysis for Concrete Hydraulic Structures, Engineer Manual 1110-2-6050, U.S. Department of the 

Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1998a, Engineering and Design – Conduits, Culverts and 

Pipes, EM 1110-2-2902, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1998b, HEC-1 Flood Hydrograph Package: User’s Manual, 

U.S. Department of the Army, USACE, Hydrologic Engineering Center, Davis, CA.

* U.S. Army Corps of Engineers (USACE), 1995a, Geophysical Exploration for Engineering and Environmental Investigations, EM 1110-1-1802, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1995b, Construction Control for Earth and Rock-Fill Dams, 

EM 1110-2-1911, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1995c, Instrumentation of Embankment Dams and Levees, 

EM 1110-2-1908, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1994, Hydraulic Design of Flood Control Channels, EM 1110-

2-1601, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1993, Engineering and Design – Seepage Analysis and Control for Dams, EM 1110-2-1901, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1990a, Construction with Large Stone, EM 1110-2-2302, U.S. 

Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1990b, (Revised April 1992), Hydraulic Design of Spillways, 

EM 1110-2-1603, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1986, Laboratory Soils Testing, EM-1110-2-1906, U.S. 

Department of the Army, USACE, Washington, DC.

U.S. Army Corps of Engineers (USACE), 1985, Electrical Protection – Cathodic Protection, EM-1110-2-

1922, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1984a, Grouting Technology, EM 1110-2-3506, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Army Corps of Engineers (USACE), 1984b, User’s Manual, HMR52, Probable Maximum Storm 

(Eastern United States), U.S. Department of the Army, USACE, Hydrologic Engineering Center, 

Davis, CA.
R–30

References

* U.S. Army Corps of Engineers (USACE), 1980, Hydraulic Design of Reservoir Outlets, EM-1110-2-

1602, U.S. Department of the Army, USACE, Washington, DC.

* U.S. Bureau of Reclamation (USBR), 2007a, Design Standards No. 13 for Embankment Dams: Chapter 

5, Protective Filters, U.S. Department of the Interior, USBR, Technical Services Center, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 2007b, Uplift and Crack Flow Resulting from High Velocity 

Discharges over Open Offset Joints: Laboratory Studies, Report DSO-07-07, USBR, Technical Service 

Center, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 2001, Water Measurement Manual, 3rd Ed., Revised Reprint, 

U.S. Government Printing Office, Washington, DC.

* U.S. Bureau of Reclamation (USBR), 1998, Earth Manual, 3rd Ed., U.S. Department of the Interior, 

USBR, Earth Sciences and Research Laboratory, Technical Services Center, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 1992a, Design Standards No. 13 – Embankment Dams, Chapter 2, 

Embankment Design, U.S. Department of the Interior, USBR, Technical Services Center, Denver, CO.

U.S. Bureau of Reclamation (USBR), 1992b, Concrete Manual, Part 2: A Manual for the Control of Concrete Construction, 9th Ed., Water Resources Technical Publication, U.S. Department of the Interior, 

Technical Services Center, USBR, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 1991, Design Standards No. 13 – Embankment Dams, Chapter 10, 

Embankment Construction, U.S. Department of the Interior, USBR, Technical Services Center, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 1990, Cavitation in Chutes and Spillways, Engineering Monograph 

No. 42, U.S. Department of the Interior, USBR, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 1988, Downstream Hazard Classification Guidelines, ACER Technical Memorandum No. 11, U.S. Department of the Interior, USBR, Denver, CO.

* U.S. Bureau of Reclamation (USBR), 1987a, Design of Small Dams, 3rd Ed., U.S. Department of the 

Interior, USBR, Washington, DC.

* U.S. Bureau of Reclamation (USBR), 1989, Design Standards No. 13 - Embankment Dams, Chapter 

13, Seismic Design and Analysis, U.S. Department of the Interior, USBR, Technical Services Center, 

Denver, CO.

* U.S. Bureau of Reclamation (USBR), 1984, Design Standards No. 13 – Embankment Dams, Chapter 3, 

Foundation Treatment, U.S. Department of the Interior, USBR, Technical Services Center, Denver, CO.

U.S. Bureau of Reclamation (USBR), 1973, Design of Small Dams, 2nd Ed., U.S. Department of the 

Interior, USBR, Washington, DC.

U.S. Committee on Large Dams (USCOLD), 1999, Updated Guidelines for Selecting Seismic Parameters 

for Dam Projects, USCOLD Committee on Earthquakes, USCOLD, Denver, CO.

U.S. Department of Agriculture (USDA), 1985, Dimensioning of Filter-Drainage Diaphragms for Conduits According to TR-60, Technical Note, Series No. 709, USDA, Soil Conservation Service, South 

National Technical Center, Forth Worth, TX (see NRCS 2007b for current criteria).

* U.S. Department of Agriculture (USDA), 1978, National Engineering Handbook, Section 8, Engineering 

Geology, NEH-8 (now Part 631-Geology), USDA, Soil Conservation Service, Washington, DC.

* U.S. Department of Agriculture (USDA), 1973a, Flow Net Construction and Use, Soil Mechanics 

Note No. 5, 210-VI-SMN-5, USDA, Soil Conservation Service, Washington, DC.

* U.S. Department of Agriculture (USDA), 1973b, A Method for Estimating Volume and Rate of Runoff 

in Small Watersheds, SCS-TP-149, USDA, Soil Conservation Service, Washington, DC.

U.S. Department of Agriculture (USDA), 1972, National Engineering Handbook, Section 4, Hydrology, 

NEH-4 (now Part 630-Hydrology), USDA, Soil Conservation Service, Washington, DC.
R–31

Engineering and Design Manual Coal Refuse Disposal Facilities

* U.S. Department of Agriculture (USDA), 1969, Computation of Joint Extensibility Requirements, Technical Release No. 18, 210-VI-TR-18, USDA, Soil Conservation Service, Washington, DC.

U.S. Department of Agriculture (USDA), 1968, Hydraulics of Broad-Crested Spillways, Technical 

Release 39, USDA, Soil Conservation Service, Washington, DC.

* U.S. Department of Agriculture (USDA), 1958, Structural Design of Underground Conduits, Technical 

Release No. 5, 210-VI-TR-5, USDA, Soil Conservation Service, Washington, DC.

U.S. Department of Agriculture (USDA), 1956, National Engineering Handbook - Chapter 5: Hydraulics, 210-VI-NEH, USDA, Soil Conservation Service, Washington, DC.

U.S. Department of Energy (USDOE), 1996, Guidelines for the Use of Probabilistic Seismic Hazard 

Curves at Department of Energy Sites for Department of Energy Facilities, DOE-STD-1024-92, originally 

published December 1992, incorporating Change Notice #1, January 1996, Washington, DC.

* U.S. Environmental Protection Agency (USEPA), 2000, Coal Remining Best Management Practices 

Guidance Manual, EPA 821-R-00-007, U.S. Department of the Environment, USEPA, Office of Science and Technology, Washington, DC.

* U.S. Environmental Protection Agency (USEPA), 1993, Use of Airborne, Surface, and Borehole Geophysical Techniques at Contaminated Sites – A Reference Guide, EPA/625/R-92/007, U.S. Department of 

the Environment, USEPA, Office of Research and Development, Washington, DC.

U.S. Environmental Protection Agency (USEPA), 1983, Neutralization of Acid Mine Drainage: Design 

Manual, USEPA-600/2-83-001, U.S. Department of the Environment, USEPA, Cincinnati, OH.

* U.S. Geological Survey (USGS), 2005, Comparison of Peak Discharge and Runoff Characteristic Estimates from the Rational Method to Field Observations for Small Basins in Central Virginia, Scientific 

Investigation Report 2005-5254, U.S. Department of the Interior, Geological Survey, Denver, CO.

* U.S. Geological Survey (USGS), 1989, Guide for Selecting Manning’s Roughness Coefficients for 

Natural Channels and Flood Plains, Water Supply Paper 2339, USGS, Denver, CO.

U.S. Nuclear Regulatory Commission (USNRC), 1997, Recommendations for Probabilistic Seismic 

Hazard Analysis: Guidance on Uncertainty and Use of Experts, NUREG/CR-6372, Panel on Seismic 

Hazard Evaluation, National Research Council, Washington, DC.

U.S. Nuclear Regulatory Commission (USNRC), 1993, Revised Livermore Seismic Hazard Estimates for 

69 Nuclear Power Plant Sites East of the Rocky Mountains, NUREG-1488, USNRC, Washington, DC.

* U.S. Society on Dams (USSD), 2007, “Strength of Materials for Embankment Dams,” USSD Committee on Materials for Embankment Dams, Denver, CO.

Virginia Department of Mine Land Reclamation (VA DMLR), 2006, “Slurry Injection Permit Requirements,” presented at Underground Injection Workshop sponsored by the Office of Surface Mining 

(OSM), National Mine Land Reclamation Center, University of West Virginia, Morgantown, WV.

Van Aller, H.W., 2004, “Piping, Seepage Conduits and Dam Failure,” presented at ASDSO Southeastern Regional Technical Seminar, Jackson, MS.

* Van Aller, H.W., 1998, “Filter Diaphragm Design Considerations: A Collection of Useful References and Design Guidelines,” originally presented at 1990 ASDSO Mid-Atlantic Regional Technical Seminar, Baltimore, MD.

van der Veen, R., 2002, “Automatic Data Acquisition Systems and Databases,” Geotechnical News, 

BiTech Publishers, Richmond, BC, Canada, Vol. 20, No. 1, pp. 24-28.

van Zyl, D. and A.M. Robertson, 1980, “Subsurface Drainage of Tailings Impoundments: Some 

Design, Construction and Management Considerations,” Proceedings, Symposium on Uranium Mill 

Tailings Management, Fort Collins, CO, pp. 153-177.
R–32

References

Veneziano, D. and J. Van Dyck, 1985, “Analysis of Earthquake Catalogues for Incompleteness 

and Recurrence Rates: Seismic Hazard Methodology for Nuclear Facilities in the Eastern United 

States,” Project No. P101-29, Vol. 2, Appendix A-6, Electric Power Research Institute (EPRI), Palo 

Alto, CA.

* Vick, S.G., 2000, “Tailings Dam Safety - Implications for the Dam Safety Community,” Proceedings, 

Tailings Dams 2000, Las Vegas, ASDSO/USCOLD, pp. 1-19.

* Vogel, W.G., 1987, A Manual for Training Reclamation Inspectors in the Fundamentals of Soils and 

Revegetation, U.S. Department of the Interior, Office of Surface Mining and Enforcement, Washington, DC.

Von Thun, J.L., 1996, “Risk Assessment of Nambe Falls Dam,” Geotechnical Special Publication 

No. 58, C.D. Shackelford, P.P. Nelson and M.J.S. Roth, eds., American Society of Civil Engineers 

(ASCE), Reston, VA, pp. 604-635.

* Wahl, T.L., 1998, Prediction of Embankment Dam Breach Parameters: A Literature Review and Needs 

Assessment, Dam Safety Research Report DSO-98-004, U.S. Department of the Interior, Bureau of 

Reclamation, Dam Safety Office, Denver, CO.

Wald, D.J., V. Quitoriano, T.H. Heaton and H. Kanamori, 1999, “Relationships between Peak 

Ground Acceleration, Peak Ground Velocity and Modified Mercalli Intensity in California,” Earthquake Spectra, Vol. 15, No. 3, pp. 557-564.

* Walker, J.S., 1993, State-of-the-Art Techniques for Backfilling Abandoned Mine Voids, Information Circular No. 9359, U.S. Department of the Interior, Bureau of Mines, Pittsburgh, PA.

Wang, Z., E.W. Woolery, B. Shi and J.D. Kiefer, 2003, “Communicating with Uncertainty: A Critical 

Issue with Probabilistic Seismic Hazard Analysis,” EOS Transactions, American Geophysical Union 

(AGU), Vol. 84, No. 46, pp. 501, 506, 508.

Ward, S.H., ed., 1990, Investigations in Geophysics, No. 5: Geotechnical and Environmental Geophysics, 

Vols. I-III, Society of Exploration Geophysicists, Tulsa, OK.

Wartman, J., J.D. Bray and R.B. Seed, 2004, “Inclined Plane Studies of the Newmark Sliding Block Procedure,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 129, No. 8, pp. 673-684.

Watkins, R.K. and L.R. Anderson, 1999, Structural Mechanics of Buried Pipes, CRC Press, New 

York, NY.

Weaver, K.D. and D.A. Bruce, 2007, Dam Foundation Grouting, ASCE Press, Reston, VA.

Wells, D.L. and K.J. Coppersmith, 1994, “New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement,” Bulletin of the Seismological 

Society of America, Vol. 84, No. 4, pp. 974-1002.

West Virginia Department of Transportation (WVDOT), 1999, Materials Procedure, Nuclear Density 

Test by the Roller Pass Method, MP 700.00.24.

West Virginia Surface Mine Drainage Task Force, 1979, “Suggested Guidelines for Methods of 

Operation,” Surface Mining of Areas with Potentially Acid-Producing Materials,” Proceedings, 

West Virginia Surface Mine Drainage Task Force Symposium, West Virginia Department of Natural 

Resources, Charleston, WV.

West Virginia Water Research Institute, 2005, Guidance Document for Coal Waste Impoundment Facilities & Coal Waste Impoundment Inspection Form, West Virginia University, Morgantown, WV.

Wetzel, K.L. and S.A. Hoffman, 1989, Distribution of Water-Quality Characteristics that May Indicate 

the Presence of Acid Mine Drainage in the Eastern Coal Province of the United States, Hydrologic Investigations Atlas HA-705, U.S. Department of the Interior, Geological Survey, Denver, CO.
R–33

Engineering and Design Manual Coal Refuse Disposal Facilities

Whittaker, B.N. and D.J. Reddish, 1989, Subsidence: Occurrence, Prediction and Control, Elsevier, 

Amsterdam, Netherlands.

* Wightman, W.E., F. Jalinoos, P. Sirles and K. Hanna, 2003, Application of Geophysical Methods to 

Highway Related Problems, Contract No. DTFH68-02-P-00083, Federal Highway Administration, 

Central Federal Lands Highway Division, Lakewood, CO.

Wildeman, T.R., J.J. Gusek, and G.A. Brodie, 1993, Wetland Design for Mining Operations, Bitech 

Publishers, Richmond, BC, Canada.

Wilkins, J.K., 1956, “Flow of Water through Rockfill and Its Application to the Design of Dams,” 

Proceedings, 2nd Australia-New Zealand Conference on Soil Mechanics and Foundation Engineering, 

International Society for Soil Mechanics and Foundation Engineering, University of Canterbury, 

Christchurch, NZ.

Wilson, A.H. and D.P. Ashwin, 1972, “Research into the Determination of Pillar Size: An Hypothesis Concerning Pillar Stability,” The Mining Engineer, Vol. 131, No. 141, pp. 409-427.

Wilson, S.D. and L.R. Squier, 1969, “Earth and Rockfill Dams,” Proceedings, Seventh International 

Conference on Soil Mechanics and Foundation Engineering, Mexico, pp. 137-223.

Wizniak, L. and R.J. Mitchell, 1987, “Crown Pillar Subsidence Prediction from Centrifuge Models,” 

Proceedings, International Symposium on Prediction and Performance in Geotechnical Engineering, Calgary, Alberta, Canada, R.C. Joshi and F.J. Griffiths, eds., pp. 387-395.

Workhorse Technologies, 2006, personal communication.

Workman, J.L. and P.C. Satchwell, 1987, Blasting as an AML Reclamation Method, 2 Vols., U.S. Department of the Interior, Office of Surface Mining, Washington, DC.

Wride, C.E., E.C. McRoberts and P.K. Robertson, 1999, “Reconsideration of Case Histories for 

Estimating Undrained Shear Strength in Sandy Soils,” Canadian Geotechnical Journal, Vol. 36, No. 5, 

pp. 907-933.

Wu, K.K., H.L. Owens and J.W. Fredland, 2003, “MSHA’s Review of Impoundment Plans,” 2003 

SME Annual Meeting, Preprint 03-099, Cincinnati, Society for Mining Metallurgy and Exploration 

(SME), Littleton, CO.

Wyllie, D.C. and C.W. Mah, 2004, Rock Slope Engineering, Spon Press, New York, NY.

* Wyrick, G.G. and J.W. Borchers, 1981, Hydrologic Effects of Stress-Relief Fracturing in an Appalachian 

Valley, Water Supply Paper 2177, U.S. Department of the Interior, U.S. Geological Survey, Reston, VA.

Yearly, D.C., 2003, “Sustainable Development for the Global Mining and Metals Industry,” Mining 

Engineering, Vol. 55, No. 8, pp. 45-48.

Yoshimine, M., P.K. Robertson, and C.E. Wride, 1999, “Undrained Shear Strength of Clean Sands to 

Trigger Flow Liquefaction,” Canadian Geotechnical Journal, Vol. 36, No. 5, pp. 891-906.

Youd, T. L., I.M. Idriss, R.D. Andrus, I. Arango, G. Castro, J.T. Christian, R. Dobry, W.D.L. Finn, 

L.F. Harder, M.E. Hynes, K. Ishihara, J.P. Koester, S.S.C. Liao, W.F. Marcuson, G.R. Martin, J.K. 

Mitchell, Y. Moriwaki, M.S. Power, P.K. Robertson, R.B. Seed and K.H. Stokoe, 2001, “Liquefaction 

Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on 

Evaluation of Liquefaction Resistance of Soils,” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 127, No. 10, pp. 817-833.

Young, W.C. and R.G. Budynas, 2001, Roark’s Formulas for Stress and Strain, 7th Ed., McGraw-Hill, 

New York, NY.

Zeng, X. and J.A. Goble, 2008, “Dynamic Properties of Coal Waste Refuse in a Tailings Dam,” Geotechnical Special Publication No. 181, Geotechnical Earthquake Engineering and Soil Dynamics IV, D. Zeng, 

M.T. Manzari, and D.R. Hiltunen, eds., American Society of Civil Engineers (ASCE), Reston, VA.
R–34

References

Zeng, Y., J.G. Anderson, and G. Yu, 1994, “A Composite Source Model for Computing Realistic 

Synthetic Strong Ground Motions,” Geophysical Research Letters, Vol. 21, No. 8, pp. 725-728.

Zhang, J., W. Wagner, A. Prakash, H. Mehl and S. Voight, 2004, “Detecting Coal Fires Using 

Remote Sensing Techniques,” International Journal of Remote Sensing, Vol. 25, No. 16, pp. 3193-3220.