Technical Manual:

Plastic Pipe Used in Embankment Dams

Best Practices for Design, Construction, Problem Identification and 

Evaluation, Inspection, Maintenance, Renovation, and Repair

 November 2007

Plastic Pipe Used in Embankment Dams

Best Practices for Design, Construction, Problem Identification and 

Evaluation, Inspection, Maintenance, Renovation, and Repair 

Federal Emergency Management Agency 

 November 2007 

Preface

Plastic pipe has been used for many decades in water and sewer applications. More 

recently, plastic pipe has been used in new embankment dam construction and in the 

renovation of existing conduits. However, most of the available design information 

is targeted toward water distribution and sewer pipes and does not address the 

unique factors involved in using plastic pipe in embankment dams. In general, 

information on plastic pipe is too dispersed for the best use of lessons learned from 

past performance, and compilation of information into a more readily available 

source was needed. Due to the absence of any single recognized standard for plastic 

pipe used in embankment dams, there is significant inconsistency in the design and 

construction rationale. In an effort to deal with this problem, this document has 

been prepared to collect and disseminate information and experience that is current 

and has a technical consensus. The goal of this document is to provide a single, 

nationally recognized standard to promote greater consistency between similar 

project designs, facilitate more effective and consistent review of proposed designs, 

and result in increased potential for safer, more reliable facilities. 

This document is intended to supplement the plastic pipe information in the Federal 

Emergency Management Agency’s (FEMA) Technical Manual: Conduits through 

Embankment Dams (2005). This document provides in-depth analyses of loading 

conditions, structural design, and hydraulic design of plastic pipe. 

This document attempts to condense and summarize the body of existing 

information, provide a clear and concise synopsis of this information, and present a 

recommended design approach. The authors reviewed most of the available 

information on plastic pipe as it relates to use within embankment dams in preparing 

this document. Where detailed documentation exists, they cited it to avoid 

duplicating available materials. The authors have strived not to reproduce 

information that is readily accessible in the public domain. Where applicable, the 

reader is directed to selected portions of FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005) and other consensus-accepted references for additional 

guidance. This document is intended for use by personnel familiar with 

embankment dams and conduits, such as designers, inspectors, construction 

oversight personnel, and dam safety engineers. 

In preparing this document, the authors frequently found conflicting procedures and 

standards in the many documents they reviewed. Where conflicts were apparent, the 

authors focused on what they judged to be the “best practice” and included that 

judgment in this document. Therefore, this document may differ from some of the 

participating agencies’ own policies. 

iii 
Plastic Pipe Used in Embankment Dams 

Since this is a supplemental document, the authors adopted the same approach 

toward hazard potential classification as used in FEMA’s Technical Manual: Conduits 

through Embankment Dams (2005). The reader is directed to that document for a 

complete discussion of hazard potential classification. The hazard potential 

classification does not reflect in any way on the current condition of the dam (i.e., 

safety, structural integrity, or flood routing capacity). The three hazard potential 

classification levels used in this document are low, significant, and high as defined in 

FEMA 333, Federal Guidelines for Dam Safety: Hazard Potential Classification Systems for 

Dams (1998): 

• Low hazard potential.—Embankment dams assigned the low hazard potential 

classification are those where failure or misoperation results in no probable loss 

of human life and low economic and/or environmental losses. Losses are 

principally limited to the owners’ property. 

• Significant hazard potential.—Embankment dams assigned the significant hazard 

potential classification are those dams where failure or misoperation results in 

no probable loss of human life, but can cause economic loss, environmental 

damage, or disruption of lifeline facilities, or can impact other concerns. 

Significant hazard potential classification dams are often located in 

predominantly rural or agricultural areas, but could be located in areas with 

population and significant infrastructure. 

• High hazard potential.—Embankment dams assigned the high hazard potential 

classification are those where failure or misoperation will probably cause loss of 

human life. 

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 expected 

Yes (but not necessary for this 

classification) 

The authors consider the guidance in this document to be technically valid without 

regard to the hazard potential classification of a particular dam. However, some 

design measures that are commonly used for design of high and significant hazard 

potential dams may be considered overly conservative for use in low hazard potential 

dams. As an example, the authors recommend chimney filters that extend across the 

entire width of the embankment fill section for most high hazard potential 

embankments. Many smaller, low hazard potential embankments are constructed 

iv
Preface 

without this feature. This document recommends that even low hazard potential 

dams should contain other currently accepted design measures that address seepage 

and internal erosion along the conduit. Specifically, this document recommends a 

filter diaphragm or filter collar around the conduit for all embankment dams 

penetrated by a conduit. 

FEMA, as the lead agency for the National Dam Safety Program, sponsored 

development of this document in conjunction with the Association of State Dam 

Safety Officials, Bureau of Reclamation, Mine Safety and Health Administration, 

Natural Resources Conservation Service, and U.S. Army Corps of Engineers. 

The primary authors of this document are Wade Anderson, P.E. (Natural Resources 

Conservation Service), Chuck Cooper, P.E. (Bureau of Reclamation), John Fredland, 

P.E. (Mine Safety and Health Administration), Michele Lemieux, P.E. (Montana 

Department of Natural Resources and Conservation), Mark Pabst, P.E. (Bureau of 

Reclamation), David Pezza, P.E. (U.S. Army Corps of Engineers), and Hal Van 

Aller, P.E. (Maryland Department of the Environment). The technical editor for this 

document was Lelon A. Lewis (Bureau of Reclamation). Illustrators for this 

document were Bonnie Gehringer (Bureau of Reclamation), John Markley (Bureau 

of Reclamation), and Wendy Pierce (Natural Resources Conservation Service). 

Additional technical assistance was provided by Cynthia Fields (Bureau of 

Reclamation), Cindy Gray (Bureau of Reclamation), and Gia Price (Bureau of 

Reclamation). 

Peer review of this document was provided by Darren Blank P.E. (Mine Safety and 

Health Administration), Kurt Hafferman P.E. (Montana Department of Natural 

Resources and Conservation), Bruce Harrington P.E. (Maryland Department of the 

Environment), Greg Hughes P.E. (U.S. Army Corps of Engineers), John LaBoon 

P.E. (Bureau of Reclamation), Danny McCook P.E. (Natural Resources 

Conservation Service), Chuck Redlinger, P.E. (Bureau of Reclamation), Greg 

Reichert P.E. (URS Corporation), Sal Todaro P.E. (URS Corporation), and Ken 

Worster P.E. (Natural Resources Conservation Service). 

The National Dam Safety Review Board (NDSRB) reviewed this document prior to 

issuance. The NDSRB plays an important role in guiding the direction of the 

National Dam Safety Program. The NDSRB has responsibility for monitoring the 

safety and security of dams in the United States, advising the Director of FEMA on 

national dam safety policy, consulting with the Director of FEMA for the purpose of 

establishing and maintaining a coordinated National Dam Safety Program, and 

monitoring State implementation of the assistance program. The NDSRB consists 

of five representatives appointed from federal agencies, five State dam safety 

officials, and one representative from the U.S. Society on Dams. 

A number of additional engineers and technicians provided input in preparation of 

this document, and the authors greatly appreciate their efforts and contributions. 

The authors also extend their appreciation to the following agencies and individuals 

v 
Plastic Pipe Used in Embankment Dams 

for graciously providing additional reviews, information, and permission to use their 

materials in this publication: 

Advanced Drainage Systems, Inc., Jim Goddard and Robert Slicker 

American Concrete Institute 

American Society of Civil Engineers 

American Water Works Association 

Association of State Dam Safety Officials 

 ASTM International 

Bureau of Reclamation, Mark Baker, Richard D. Benik, Richard Fuerst, Mark 

Gemperline, Ernest Hall, Walter Heyder, Steven Robertson, and Jay 

Swihart 

Bureau of Indian Affairs 

Federal Emergency Management Agency 

Florida Department of Environmental Protection, Steve Partney 

Geo/Environmental Associates, Inc., Barry Thacker 

 Amster Howard 

Inuktun Services, Ltd. 

ISCO Industries, Dudley Burwell 

Knight Piesold, Allen H. Gipson Jr. 

Maryland Department of the Environment 

Mine Safety and Health Administration, Carol L. Tasillo 

Montana Department of Natural Resources and Conservation 

Montana Tech. of the University of Montana, Rich McNearny 

Natural Resources Conservation Service, Bill Irwin 

 Jim Norfleet 

Robert Peccia and Associates, Robert Peccia 

Performance Pipe, Larry Petroff 

Plastic Pipe Institute 

Simpson Gumpertz & Heger Inc., Timothy J. McGrath 

Tetra Tech. Inc., Larry Cawlfield and Mike Hatten 

Uni-Bell PVC Pipe Association, Michael Luckenbill 

URS Corporation, Scott Jones 

U.S. Army Corps of Engineers, Ed Ketchum, Terry J. Matuska, and Kevin L 

Pavlik 

Utah State University, Steve Folkman, A.P. Moser, Blake Tullis, and Reynold K. 

Watkins 

Virginia Tech University, Michael J. Duncan 

Designers must continue to explore the advantages and limitations of plastic pipe. 

No single publication can cover all of the requirements and conditions that can be 

encountered during design and construction. Therefore, it is critically important that 

when plastic pipe is used within an embankment dam, the designer must be 

experienced with all aspects of the design and construction of these structures. 

vi 
Preface 

The authors caution the users of this document that sound engineering judgment 

should always be applied when using references. The authors have strived to avoid 

referencing any material that is considered outdated for use in modern designs. 

However, the user should be aware that certain portions of references cited in this 

document may have become outdated in regards to design and construction aspects 

and/or philosophies. While these references still may contain valuable information, 

users should not automatically assume that the entire reference is suitable for design 

and construction purposes. 

The authors utilized many sources of information in the development of this 

document, including: 

• Published design standards and technical publications of the various federal and 

State agencies and organizations involved with the preparation of this 

document. 

• Published professional papers and articles from selected authors, technical 

journals and publications, and organizations. 

• Experience of the individuals, federal and State agencies, and organizations 

involved in the preparation of this document. 

This document is available in two formats: print copy and digital versatile disc 

(DVD). The DVD format includes built-in Adobe Acrobat Reader software, 

hyperlinks, and search capabilities. A hyperlink is a highlighted word or image within 

the document which, when clicked, takes the user to another place within the 

document or to another location altogether. Hyperlinks are especially useful when 

the user wants to see the full reprint of a cited reference or the exact location in a 

reference from which the material was cited. The available document formats and a 

description of their contents are as follows: 

• Print copy.—Some users of this document may not have direct access to a 

computer and may find print copies more valuable. A print copy would be 

especially useful to those users working in the field at construction sites, where 

direct access to a computer may not be available. Users of the print copies will 

lack the hyperlinking and search capabilities available in the DVD formats. 

• DVD.—The DVD contains this document, portable document format (PDF) 

copies of the cited references that were available in the public domain or where 

permission for reprint was granted, plus “additional reading” references in PDF 

format. The “additional reading” references are references that have not been 

specifically cited in this document, but may be of additional interest to the user. 

As DVD drives become more common, the DVD format will eventually 

become the preferred format for all users, since it allows the user to utilize all 

the available features. 

vii 
Plastic Pipe Used in Embankment Dams 

This document is intended solely for noncommercial and educational purposes. 

PDF copies of references available in the public domain have been included on the 

DVD whenever possible. For references not readily available in the public domain, 

the authors tried to obtain copyright permission. Users should be aware that PDF 

copies for a number of cited references were unavailable due to size constraints, lack 

of availability in the public domain, or permission for reprint not being granted. 

These references have been hyperlinked to a PDF file titled “Document 

unavailable.” For these references, users may want to contact the author or 

publisher directly for reprint information. 

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

to: 

Bureau of Reclamation 

Denver Federal Center, Bldg. 67 

6th Avenue and Kipling 

Denver CO 80225-0007 

 Attention: Chuck Cooper (86-68130) 

Please reference specific pages, paragraphs, or figures within the document, together 

with proposed new material in any convenient format. Sources of proposed new 

material should be completely cited. Submission of material signifies permission for 

use in a future revised edition of this document, but credit for such new material will 

be given where appropriate. 

The material presented in this document has been prepared in accordance with 

recognized engineering practices. The guidance in this document should not be used 

without first securing competent advice with respect to its suitability for any given 

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

representation or warranty on the part of individuals or agencies involved, or any 

other person named herein, that this information is suitable for any general or 

particular use, or promises freedom from infringement of any patent or patents. 

Anyone making use of this information assumes all liability from such use. 

Any use of trade names and trademarks in this document is for descriptive purposes 

only and does not constitute endorsement. The information contained herein 

regarding commercial products or firms may not be used for advertising or 

promotional purposes and is not to be construed as an endorsement of any product 

or firm. 

viii 
Contents 

page 

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

Common Abbreviations......................................................................................................xx

Conversion Factors............................................................................................................xxii

Symbols...............................................................................................................................xxiii

AASHTO Standards .........................................................................................................xxvi

ASTM Standards ..............................................................................................................xxvii

AWWA Standards.............................................................................................................xxxi

Websites.............................................................................................................................xxxii

Introduction ............................................................................................................................ 1

Chapter 1—General............................................................................................................... 5

1.1 Historical Perspective ................................................................................................. 5

1.2 Common Types of Plastic Pipe Used in Embankment Dams ............................. 6

1.2.1 Thermoplastic....................................................................................................... 7

1.2.1.1 HDPE ............................................................................................................ 9

1.2.1.2 PVC ..............................................................................................................17

1.2.1.3 Other types of thermoplastic pipe ...........................................................20

1.2.2 Thermoset plastic...............................................................................................21

1.3 Common Uses for Plastic Pipe................................................................................23

1.3.1 HDPE..................................................................................................................24

1.3.1.1 Solid wall and dual-wall containment pipe..............................................24

1.3.1.2 Corrugated pipe ..........................................................................................26

1.3.2 PVC......................................................................................................................27

1.3.3 CIPP.....................................................................................................................29

1.4 Design Life .................................................................................................................30

Chapter 2—Loading Conditions........................................................................................33

2.1 Soil Loading................................................................................................................33

2.1.1 Soil prism load for trench conduits, positive projecting conduits in the 

trench condition, and negative projecting conduits ...............................................39

2.1.2 Marston load for positive projecting conduits...............................................41

2.1.3 Increase in soil loading due to a dam raise.....................................................44

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Plastic Pipe Used in Embankment Dams 

2.2 Hydraulic Loading .....................................................................................................45

2.2.1 Internal hydrostatic pressure ............................................................................46

2.2.2 Internal vacuum pressure..................................................................................49

2.2.3 External hydrostatic pressure...........................................................................50

2.3 Construction Loading ...............................................................................................50

Chapter 3—Structural and Hydraulic Design ..................................................................53

3.1 Flexible Pipe...............................................................................................................54

3.1.1 Wall crushing ......................................................................................................58

3.1.2 Wall buckling ......................................................................................................60

3.1.3 Deflection............................................................................................................62

3.1.4 Internal hydrostatic pressure ............................................................................69

3.1.5 Strain ....................................................................................................................71

3.2 Rigid Pipe....................................................................................................................73

3.3 Encased Plastic Pipe..................................................................................................74

3.3.1 Wall crushing ......................................................................................................74

3.3.2 Wall buckling ......................................................................................................74

3.3.3 Internal hydrostatic or vacuum pressure ........................................................76

3.4 Summary of Design Considerations for Flexible and Encased Plastic Pipe

Design................................................................................................................................76

3.5 Embedment and Encasement Material Considerations ......................................78

3.5.1 Soil........................................................................................................................79

3.5.2 Concrete ..............................................................................................................80

3.5.2.1 Reinforced concrete cradle (encasement to springline) ........................81

3.5.2.2 Reinforced cast-in-place concrete encasement (completely 

encased) ....................................................................................................................81

3.5.2.3 Unreinforced cast-in-place concrete encasement (completely 

encased) ....................................................................................................................83

3.5.3 Controlled low strength material (flowable fill).............................................83

3.5.3.1 Design considerations for using CLSM ..................................................86

3.5.3.2 Problems with using CLSM ......................................................................86

3.5.4 Grout ...................................................................................................................87

3.5.5 Comparison of embedment and encasement materials................................88

3.5.6 When to use flexible or encased plastic pipe design.....................................88

3.6 Expansion and Contraction .....................................................................................91

3.7 End Restraint Design................................................................................................92

3.8 Other Design and Construction Considerations ..................................................92

3.8.1 Foundation problems ........................................................................................93

3.8.2 Leak testing .........................................................................................................93

3.8.3 Thrust blocks......................................................................................................94

3.8.4 Anchors and spacers..........................................................................................95

3.8.5 Placement temperature......................................................................................95

3.8.5.1 Soil as embedment material ......................................................................96

3.8.5.2 Concrete, CLSM, and grout as encasement material.............................96

3.8.6 Collapse of pipes due to grout pumping pressure ........................................96

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Contents 

3.8.7 Air venting...........................................................................................................97

3.8.8 Seepage ................................................................................................................97

3.9 Hydraulic Design of Embankment Conduits........................................................97

3.10 Renovation, Replacement, and Repair of Embankment Conduits..................98

Chapter 4—Drainpipes and Filters..................................................................................101

4.1 Drainpipes ................................................................................................................101

4.1.1 Structural design...............................................................................................102

4.1.2 Hydraulic design...............................................................................................102

4.1.3 Inspection wells and cleanouts.......................................................................109

4.1.4 Renovation, replacement, and repair of drainpipes ....................................112

4.2 Filters.........................................................................................................................114

4.2.1 Zoning ...............................................................................................................114

4.2.2 Determination of filter gradation limits........................................................116

4.2.3 Flow capacity ....................................................................................................118

4.3 Backfill.......................................................................................................................118

4.3.1 Backfill for nonperforated drainpipe.............................................................119

4.3.2 Backfill for perforated drainpipe ...................................................................120

4.3.3 Zoning design...................................................................................................121

4.3.4 Improving access..............................................................................................122

4.3.5 Abandonment/grouting of the drain system...............................................124

Chapter 5—Construction Guidance ...............................................................................127

5.1 Embankment Conduits...........................................................................................127

5.2 Drainpipes and Filters.............................................................................................128

5.2.1 Foundation preparation ..................................................................................129

5.2.2 Placement around drainpipes .........................................................................130

5.2.3 Segregation........................................................................................................137

5.2.4 Compaction methods for backfill and filter and drain materials 

around drainpipes......................................................................................................137

5.2.5 Borrow sources.................................................................................................139

5.2.6 Contamination..................................................................................................140

5.2.7 Quality control and assurance........................................................................143

Chapter 6—Inspection......................................................................................................147

6.1 Embankment Conduits...........................................................................................147

6.2 Drainpipes ................................................................................................................149

Chapter 7—Plastic Pipe Used in Tailings Disposal Facilities and Slurry 

Impoundments ...................................................................................................................161

Chapter 8—Research Needs ............................................................................................171

8.1 Research Items.........................................................................................................171

8.1.1 Pipe material (PM) ...........................................................................................171

8.1.2 Embedment/encasement material (EM)......................................................172

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Plastic Pipe Used in Embankment Dams 

References ...........................................................................................................................176

Additional Reading.............................................................................................................182

Index ....................................................................................................................................188

Glossary ...............................................................................................................................199

References for Glossary................................................................................................215

Appendix A—Example Calculations

A-1 Flexible pipe design (for a drainpipe)..................................................................A-3

A-2 Encased pipe design (for an embankment conduit) .........................................A-7

A-3 Siphon design....................................................................................................... A-11

A-4 Toe drain design (filter and drain) .................................................................... A-14

Appendix B—Case Histories 

Davis Creek Dam...........................................................................................................B-3

Ganado Dam ..................................................................................................................B-7

Sediment Control Pond SP-4 Dam...........................................................................B-11

Sugar Mill Dam ............................................................................................................B-18

Upper Wheeler Reservoir Dam .................................................................................B-22

Virginia Dam ................................................................................................................B-25

Wheatfields Dam .........................................................................................................B-29

Worster Dam................................................................................................................B-34

Tables 

No. page 

1 Classification of buried conduits (required to compute soil loads using the 

Marston Load Theory) ...............................................................................................40 

2 Design values for the settlement ratio, rsd ................................................................43 

3 Typical modulus of elasticity values for HDPE and PVC pipe ...........................54 

4 Average values of the modulus of soil reaction, E, for the Modified Iowa 

Equation .......................................................................................................................67 

5 Hartley-Duncan’s (1987) values of E', modulus of soil reaction .........................68 

6 Temperature reduction factors..................................................................................71 

7 Summary of design considerations and required analyses for plastic pipe in 

embankment dams......................................................................................................78 

8 Comparison of embedment and encasement materials.........................................89 

9 Flexible pipe design versus encased pipe design for plastic pipe, as a function 

of encasement material...............................................................................................90 

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Contents 

10 Coefficient of thermal expansion..............................................................................91 

11 Drainpipe diameter based on dam size and foundation type .............................103 

12 Gradation of ATSM C 33 fine aggregate with additional requirement.............117 

13 Gradation for ASTM C 33 drain materials (percent passing by weight)...........117 

14 Maximum perforation dimension for ASTM C 33 Drain Materials..................118 

15 Example gradation for drainpipe embedment......................................................119 

Figures 

No. page 

1 Plastic pipe is lightweight, which facilitates installation........................................... 3 

2 Resin. Photo courtesy of Uni-Bell PVC Pipe Association Members and 

Associate Members ....................................................................................................... 7 

3 Conventional extrusion line. Photo courtesy of the Plastic Pipe Institute.......... 9 

4 Types of HDPE pipe walls........................................................................................10 

5 Solid wall HDPE pipe to be used for sliplining of an existing outlet works 

conduit ..........................................................................................................................10 

6 Dual-wall containment HDPE pipe. A 14-inch diameter carrier pipe is 

being inserted into a 20-inch diameter containment pipe. Intermediate 

spacers are attached to the carrier pipe. Grout lines for grouting of the 

annulus between the existing conduit and containment pipe can be seen .........11 

7 Corrugated single wall HDPE pipe has corrugations on both interior and 

exterior surfaces. Photo courtesy of Advanced Drainage Systems, Inc.............11 

8 Corrugated profile wall HDPE pipe has smooth interior and corrugated 

exterior surfaces...........................................................................................................12 

9 HDPE pipe joint being butt fusion welded ............................................................13 

10 A butt fused HDPE pipe joint being checked for gaps and voids ......................14 

11 Hand held extrusion gun............................................................................................15 

12 HDPE flange adapter connection ............................................................................16 

13 Male end of snap joint................................................................................................17 

14 External split coupler..................................................................................................17 

15 Solid wall PVC pipe has occasionally been used in embankment conduit 

applications within low hazard potential embankment dams. However, the 

bell and spigot joint connection used for this type of pipe can experience 

separation and seepage when improperly installed ................................................18 

16 PVC pipe joint (bell and spigot)................................................................................18 

17 The splined joint has a machined groove in the PVC pipe and in the 

coupling to allow insertion of a flexible thermoplastic spline that provides a 

360-degree restrained joint. Photo courtesy of Uni-Bell PVC Pipe 

Association Members and Associate Members ......................................................19 

xiii 
Plastic Pipe Used in Embankment Dams 

18 The heat fusion process is used to join PVC pipe, resulting in a continuous 

length of pipe. Photo courtesy of Uni-Bell PVC Pipe Association Members 

and Associate Members..............................................................................................19 

19 This type of mechanically restrained joint is used to prevent overinsertion of 

bell and spigot gasket PVC pipe. Photo courtesy of Uni-Bell PVC Pipe 

Association Members and Associate Members ......................................................20 

20 PVC pressure pipe with external mechanically restrained joints. Photo 

courtesy of Uni-Bell PVC Pipe Association Members and Associate 

Members.......................................................................................................................20 

21 CIPP liner exiting from an existing outlet works conduit, via the hydrostatic 

inversion method.........................................................................................................22 

22 Contractor installing a resin-soaked CIPP liner into an existing outlet works. 

Installation begins by hauling the liner up to the top of the platform. On 

the platform, water is run into the liner causing it to pressurize and expand 

downward. As the liner reaches the outlet works pipe opening, laborers on 

the ground maneuver the water filled liner into the outlet works. Water 

pressure continues to cause the liner to advance upstream in the outlet 

works pipe and un-invert itself. Photo courtesy of Tetra Tech Inc ...................23 

23 HDPE dual-wall containment pipe arriving at a job site ......................................25 

24 A toe drain being constructed using corrugated profile wall HDPE pipe..........27 

25 A temporary siphon constructed using PVC pipe. This siphon is being used 

for short term operation.............................................................................................28 

26 Classification of buried conduits for the Marston theory (Spangler and 

Handy’s Soil Engineering, 1982). Note: Use of incomplete trench and special 

case should not be used for embankment dam applications ................................35 

27 Trench conduit. Backfill soil moves downward relative to the soil at the side 

of the trench. A drainpipe buried beneath natural ground is an example of a 

trench conduit. (Note: A trench conduit should not be used for an 

embankment conduit.)................................................................................................36 

28 Positive projecting conduit in a projection condition. The pipe is installed 

above the ground surface or compacted fill, with fill placed around and 

above the conduit. The exterior prisms settle more than the interior prism, 

causing load to be transferred to the interior prism. An embankment 

conduit is an example of a positive projecting conduit in projection 

condition. (Note: This figure is not intended to show all the design details 

required.).......................................................................................................................37 

29 Positive projecting conduit in a trench condition. If the foundation is 

yielding or the conduit deflects, the interior prism settles more than the 

exterior prisms. The soil load on the conduit is less than the weight of the 

soil above it. A drainpipe in an embankment dam is an example of a 

positive projecting conduit in a trench condition. (Note: This figure is not 

intended to show all the design details required.)...................................................37 

30 Negative projecting conduit. The pipe is installed in a shallow trench such 

that the top of the pipe is below natural ground or compacted fill, and then 

covered with fill material. Negative projecting conduits should not be used 

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Contents 

for embankment conduits or drainpipes (Note: This figure is not intended 

to show all the design details required.)...................................................................38 

31 Arching action of a negative projecting conduit. Negative projecting 

conduits should not be used for embankment conduits. For an 

embankment conduit, an excavation with 2:1 side slopes or flatter should be 

used. This causes the conduit to behave as a positive projecting conduit. 

(Note: This figure is not intended to show all the design details required.)......38 

32 Complete condition. The complete condition exists when the fill height (H) 

is less than or equal to the height to the plane of equal settlement (He

). 

(Note: This figure is not intended to show all the design details required.)......41 

33 Incomplete condition. The incomplete condition exists when the fill height 

(H) is greater than the height to the plane of equal settlement (He

). (Note: 

This figure is not intended to show all the design details required.) ...................41 

34 The soil prism load is the weight of the soil directly above the conduit. 

(Note: This figure is not intended to show all the design details required.)......42 

35 Projection ratio, p = depth of the foundation material below the top of the 

conduit divided by the outside diameter of the pipe (DO). (Note: This figure 

is not intended to show all the design details required).........................................44 

36 Values for the positive projection load coefficient (Cc

).........................................45 

37 Internal hydrostatic pressure .....................................................................................46 

38 Internal vacuum pressure...........................................................................................49 

39 External hydrostatic pressure ....................................................................................50 

40 The crown of this single wall corrugated HDPE pipe has been damaged due 

to construction traffic crossing over it. Insufficient cover over the pipe was 

the likely cause .............................................................................................................51

41 Load is transferred differently for rigid and flexible pipe .....................................55 

42 Examples of two- and three-dimensional meshes..................................................56 

43 Typical failure modes for flexible pipes...................................................................59 

44 Single wall corrugated HDPE pipe experiencing wall crushing...........................59 

45 Single wall corrugated HDPE drainpipe experiencing failure due to 

buckling.........................................................................................................................61 

46 Single wall corrugated HDPE drainpipe experiencing excessive deflection 

leading to buckling ......................................................................................................63 

47 Unconstrained collapse pressure vs. minimum pipe stiffness for single wall 

corrugated HDPE pipe ..............................................................................................77 

48 Unconstrained collapse pressure vs. standard dimension ratio (SDR) for 

HDPE and PVC solid wall pipe. Plot is based on a minimum factor of 

safety of 1.0 ..................................................................................................................77 

49 Compacting earthfill under the haunches of plastic pipe is very difficult and 

quality compaction can not be achieved..................................................................80 

50 Until further research is completed, concrete cradles beneath plastic pipe 

should not be used. ....................................................................................................82 

xv 
Plastic Pipe Used in Embankment Dams 

51 The CLSM is typically transported to the construction site in ready mix 

concrete trucks. In this figure, CLSM is being used as a pipe encasement. 

CLSM should not be used for embankment conduits in significant and high 

hazard potential dams.................................................................................................84 

52 Leak test being performed on an HDPE slipliner for an outlet works 

renovation ....................................................................................................................94 

53 Installation of profile wall corrugated pipe for a drainpipe replacement 

during a modification of an embankment dam ....................................................103 

54 Profile wall corrugated HDPE pipe with slotted perforations...........................104 

55 Profile wall corrugated HDPE pipe with circular perforations..........................104 

56 Slotted PVC pipe.......................................................................................................105 

57 Placement of a cast-in-place base slab for an inspection well ............................110 

58 The first ring of a drainpipe inspection well .........................................................110 

59 Invert of an inspection well. The sediment trap is painted white, so any 

sediment can be easily observed. The dark material in the trap is algae ..........111 

60 Cleanout designed to accommodate CCTV inspection in pipes with 

diameters of 8 inches or larger ................................................................................112 

61 A drainpipe cleanout with a steel encasement and lockable protective 

cover............................................................................................................................112 

62 Idealized cross sections of single (left) and double (right) stage drainpipes. 

The placement of the drain material around the pipe can result in a variety 

of geometries based on placement method. Minimum cover requirements 

should always be met independently of geometry................................................115 

63 Old drain.....................................................................................................................119 

64 Barrier condition introduced by a replacement drain resulting in poor 

seepage collection and high pore pressure ............................................................119 

65 Construction equipment can travel safely over plastic pipe when adequate 

cover above the pipe is provided............................................................................121 

66 Prefabricated 22.5-degree bend for profile wall corrugated HDPE pipe .........129 

67 Trapezoidal side slopes used in drainpipe construction......................................131 

68 Trench excavation.....................................................................................................133 

69 Initial filter placement...............................................................................................133 

70 Drain material placement.........................................................................................133 

71 Excavate for pipe ......................................................................................................133 

72 Set pipe, place ballast ................................................................................................133 

73 Backfill haunch by hand...........................................................................................134 

74 Place remaining drain material ................................................................................134 

75 Place remaining filter ................................................................................................134 

76 Place the final miscellaneous fill or protective cap to the specified level .........134 

77 Initial filter placement in a trapezoidal trench ......................................................135 

78 Drain material being placed over a drainpipe .......................................................135 

79 Filter material being placed over drain material....................................................136 

80 A trench box or “doghouse” has been used to place material around 

drainpipes in vertically sided trenches in low hazard potential dams. 

xvi 
Contents 

Vertically sided trenches should not be used in significant and high hazard 

potential dam construction......................................................................................136 

81 Naturally occurring segregated soil. During deposition, the gravel sizes 

were segregated from the sand sizes. Poor construction practices can lead 

to similar segregation. The deposit is a broadly graded mixture of silt, sand, 

and gravel and is internally unstable. Note that silt has eroded into the 

gravel sizes and coated the particles .......................................................................138 

82 Water being added to filter immediately preceding compaction. Note that

this placement is occurring during a heavy rain, and wetting of the filter 

must continue ............................................................................................................139 

83 Typical borrow area including processing plant. Produced material is in 

foreground and the plant is in the background ....................................................140 

84 Roadway crossing over a filter. Photo courtesy of ASDSO..............................141 

85 Contaminated materials being excavated beneath a roadway crossing over a 

filter. Photo courtesy of ASDSO...........................................................................141 

86 A butt fused, solid walled HDPE pipe joint in an outlet works slipliner as 

viewed using CCTV inspection equipment...........................................................148 

87 Looking upstream toward the control gate in an HDPE sliplined outlet 

works conduit using CCTV inspection equipment..............................................149 

88 CCTV inspection of a newly installed slipliner for an outlet works 

renovation ..................................................................................................................149

89 Inspection well provides access to the drainpipe for CCTV inspection...........150 

90 Cleanout provides access to the drainpipe for CCTV inspection......................150 

91 Profile wall corrugated HDPE pipe has corrugated exterior and smooth 

interior surfaces .........................................................................................................151 

92 Single wall corrugated HDPE drainpipe experiencing failure due 

to buckling..................................................................................................................153 

93 This HDPE drainpipe has experienced joint separation allowing backfill 

materials to enter the drainpipe...............................................................................154 

94 Slotted PVC pipe used for toe drain has experienced longitudinal cracking. 

The cracking occurred during construction ..........................................................155 

95 PVC pipe used for toe drain has experienced transverse cracking. The 

cracking occurred during construction ..................................................................155 

96 Calcite deposits have blocked many of the slots in this HDPE drainpipe.......156 

97 Calcite deposits have formed at joints and perforations in this HDPE 

drainpipe.....................................................................................................................156 

98 Iron bacteria have partially blocked the perforations in this HDPE 

drainpipe. Note that the only open perforation passing seepage is in the 

lower left corner of figure........................................................................................156 

99 Sediment deposit on the invert of a HDPE drainpipe........................................157 

100 Operator water jetting a drainpipe..........................................................................160 

101 Decant pipe with multilevel inlets ..........................................................................165 

102 Deflection monitoring of a decant pipe.................................................................165 

xvii 
Plastic Pipe Used in Embankment Dams 

Figures in the Appendices 

Appendix A 

No. page 

A-1 Cross section of an embankment dam and drainpipe (filter not shown)..........A-3 

A-2 An 18-inch diameter solid wall HDPE slipliner installed in an existing 

24-inch diameter CMP outlet works conduit........................................................A-7 

A-3 This figure illustrates a siphon extending over the crest of an embankment 

dam. Alternative siphon designs may want to consider the addition of 

earthen ramps over the siphon or embedment into the crest of the dam to 

facilitate vehicular traffic on the dam crest......................................................... A-11 

A-4 Proportion of fines, sand, and gravel for foundation soils .............................. A-20 

A-5 Individual gradations for foundation soil ........................................................... A-21 

A-6 Design of filter material......................................................................................... A-22 

A-7 Design of drain material........................................................................................ A-23 

A-8 Individual gradations of borrow material ........................................................... A-24 

A-9 Average of filter, drain, and borrow materials ................................................... A-25 

Appendix B 

No. page 

B-1 Locations of observation wells and cleaned reaches............................................B-3 

B-2 The typical amount of sediment deposition observed in toe drain during the 

November 2000 inspection......................................................................................B-4 

B-3 A buckled left toe drain pipe at approximately Sta. 23+25 stopped the 

camera-crawler. A cleaning removed fine materials from the pipe invert.......B-5 

B-4 Erosional features typically associated with dispersive soils as seen at 

Ganado Dam. The common name for such features is “jughole.”..................B-8 

B-5 Subsurface fissure located at the downstream toe of the dam east of the 

outlet works. The subsurface soil had a very high moisture content ...............B-8 

B-6 Modification cross showing embankment zones and toe drain system............B-9 

B-7 Cross section of toe drain ........................................................................................B-9 

B-8 Looking upstream through the breach. Embankment was 18 feet high at 

breach........................................................................................................................B-13 

B-9 Breach showing 36-inch diameter HDPE pipe and riser..................................B-13 

B-10 View looking downstream along the pipe. The breach was 26 feet wide ...B-14 

B-11 The gradation used for the filter diaphragm.....................................................B-16 

xviii 
Contents 

B-12 In the early 1990’s the spillway capacity of the dam was increased by 

construction of a system of 5 PVC siphons embedded in concrete in a 

shallow trench through the dam.........................................................................B-18 

B-13 After about 10 years of operation, seepage was observed on the 

downstream slope of the dam in the vicinity of the siphons, and the 

overlying embankment material was excavated to expose the pipes to 

determine the source of the seepage..................................................................B-19 

B-14 A small hole drilled through the concrete bedding between the siphons 

did not encounter voids under the concrete, even though the designer 

suspected that seepage was occurring directly under the pipes .....................B-20 

B-15 After removal of portions of the PVC siphons, it was determined that the 

original concrete bedding had been improperly placed, resulting in voids 

under the centers of the pipes. Portions of the siphons were replaced, 

and the bedding was replaced with a high slump concrete ............................B-20 

B-16 Looking upstream towards toe of dam: View of old box conduit and 

new downstream concrete encasement, during grout operation. Grout 

pipe is shown in top of photo ............................................................................B-22 

B-17 Looking downstream from toe of dam after excavation of collapsed 

portion of HDPE pipe (located in original box conduit, upstream of new 

concrete encasement)...........................................................................................B-23 

B-18 Position of 48-inch diameter, SDR 32.5 HDPE pipe in unreinforced 

concrete encasement ............................................................................................B-25 

B-19 View of interior of deformed HDPE pipe from CCTV camera...................B-26 

B-20 Approximate upward distortion of bottom of HDPE from outside 

hydrostatic pressure between the pipe and its concrete encasement............B-27 

B-21 The profile of the existing outlet works prior to modifications ....................B-30 

B-22 The profile of the modified outlet works..........................................................B-31 

B-23 HDPE pipe being unloaded at the site using a John Deere 230LC 

excavator................................................................................................................B-33 

B-24 Guiding the 20-inch pipe into the upstream end of the CMP.......................B-33 

xix 
Plastic Pipe Used in Embankment Dams 

Common Abbreviations

AASHTO, American Association of State Highway and Transportation Officials 

ABS, acrylonitrile-butadiene-styrene 

ACI, American Concrete Institute 

ADS, Advanced Drainage Systems, Inc. 

AGI, American Geological Institute 

ASCE, American Society of Civil Engineers 

ASDSO, Association of State Dam Safety Officials 

ASTM, ASTM International 

AWWA, American Water Works Association 

CANDE, Culvert Analysis and Design 

CCFRPM, centrifugally cast fiber reinforced polymer mortar 

CCTV, closed circuit television 

CIPP, cured in place pipe 

CLSM, controlled low strength material 

CLSM-CDF, controlled low strength material—controlled density fill 

CMP, corrugated metal pipe 

DOS, disk operating system 

DR, dimension ratio 

DVD, digital versatile disc 

EM, embedment/encasement material 

ESC, environmental stress cracking 

F, Fahrenheit 

FEMA, Federal Emergency Management Agency 

FFP, fold and formed pipe 

FHWA, Federal Highway Administration 

FS, factor of safety 

HDB, hydrostatic design basis 

HDPE, high density polyethylene 

HDS, hydrostatic design stress 

LL, liquid limit 

MSA, maximum size aggregate 

NAWIC, National Association of Women in Construction 

NCHRP, National Cooperative Highway Research Program 

NCLS, notched constant ligament stress 

NDSP, National Dam Safety Program 

NDSRB, National Dam Safety Review Board 

NRCS, Natural Resources Conservation Service 

xx 
Plastic Pipe Used in Embankment Dams 

PB, polybutylene 

PC, pressure class 

PDF, portable document format 

PE, polyethylene 

P.E., Professional Engineer 

PI, plasticity index 

PM, pipe material 

PP, polypropylene 

PPI, Plastic Pipe Institute 

PR, pressure rating 

PS, pipe stiffness 

PUR, polyurethane 

PVC, polyvinyl chloride 

ROV, remotely operated vehicle 

SCC, self consolidating concrete 

SCR, stress crack resistance 

SCS, Soil Conservation Service 

SDR, standard dimension ratio 

SI, International System of Units 

SSHB, Standard Specifications for Highway Bridges 

SIDR, standard inside dimension ratio 

UP, unsaturated polyester 

USACE, U.S. Army Corps of Engineers 

USCS, Unified Soil Classification System 

USSD, United States Society on Dams 

USU, Utah State University 

UV, ultraviolet 

xxi 
Plastic Pipe Used in Embankment Dams 

Conversion Factors 

To the International System of Units (SI) (Metric) 

Pound-foot measurements in this document can be converted to SI measurements 

by multiplying by the following factors: 

Multiply By To obtain

acre-feet 1233.489 cubic meters

cubic feet 0.028317 cubic meters 

cubic feet per second 0.028317 cubic meters per second 

cubic inches 16.38706 cubic centimeters 

degrees Fahrenheit (°F-32)/1.8 degrees Celsius

feet 0.304800 meters

feet per second 0.304800 meters per second 

gallons 0.003785 cubic meters

gallons 3.785412 liters

gallons per minute 0.000063 cubic meters per second 

gallons per minute 0.063090 liters per second

inches 2.540000 centimeters

miles 2.589988 kilometers

mils 0.000025 meters

mils 0.025400 millimeters

pounds 0.453592 kilograms

pounds per cubic foot 16.01846 kilograms per cubic meter 

pounds per square foot 4.882428 kilograms per square meter 

pounds per square inch 6.894757 kilopascals

pounds per square inch 6894.757 pascals

square feet 0.092903 square meters 

square inches 6.451600 square centimeters 

xxii 
Plastic Pipe Used in Embankment Dams 

Symbols

%ΔY/D, percent deflection 

α, coefficient of thermal expansion, in/in/°F 

γ, total unit weight of soil, lb/ft3 

γb, buoyant unit weight of soil, lb/ft3 

γm, moist unit weight of soil, lb/ft3 

γs, saturated unit weight of soil, lb/ft3 

γw, unit weight of water, 62.4 lb/ft3 

ΔH, increase in dam height, ft 

ΔH, surge pressure, feet of water 

ΔPs

, increase in soil loading due to dam raise, lb/ft2 

ΔP, surge pressure, lb/in2 

ΔT, change in temperature, °F 

ΔV, change in velocity of water, ft/s 

ΔY/DM = %ΔX/D, percent deflection expressed as a decimal 

ε, maximum combined strain in pipe wall, in/in of pipe wall circumference 

εall, allowable strain for the pipe material, in/in 

εf, maximum strain in the pipe wall due to ring deflection, in/in 

εh, maximum strain in the pipe wall due to hoop stress, in/in 

η, porosity, percent of void volume, % 

μ, coefficient of friction, tan φ 

φ, effective friction angle of backfill 

ρ, density of water, slugs/ft3 

, allowable long-term compressive stress, lb/in2 

a, percentage of soil passing the No. 200 sieve, fines content 

a, velocity of the pressure wave, ft/s 

A, filter or foundation area through which flow passes, ft2 

Apw, area of the pipe wall, in2/in of pipe length 

AR, required area of the end restraint, ft2 

B, empirical coefficient of elastic support 

c, distance from the inside surface to the neutral axis, in 

C, constant ranging from 0.2 to 0.6, averaging 0.35 

C, reduction factor for buckling pressure 

Cc

, positive projection load coefficient 

cu, coefficient of uniformity, D60 / D10 

D10, particle size diameter in millimeters of the 10th percentile passing grain size 

xxiii 
Plastic Pipe Used in Embankment Dams 

D15, particle size diameter in millimeters of the 15th percentile passing grain size 

D50, particle size diameter in millimeters of the 50th percentile passing grain size 

D60, particle size diameter in millimeters of the 60th percentile passing grain size 

D85, particle size diameter in millimeters of the 85th percentile passing grain size 

D15B, particle size diameter in millimeters of the 15th percentile passing grain size of 

the base soil 

D85B, particle size diameter in millimeters of the 85th percentile passing grain size of 

the base soil 

D15E , particle size diameter in millimeters of the 15th percentile passing grain size of 

the envelope 

D85E, particle size diameter in millimeters of the 85th percentile passing grain size of 

the envelope 

D10F, particle size diameter in millimeters of the 10th percentile passing grain size of 

the filter 

D15F, particle size diameter in millimeters of the 15th percentile passing grain size of 

the filter 

D85F, particle size diameter in millimeters of the 85th percentile passing grain size of 

the filter 

Di

, inside diameter of the pipe, in 

DL, deflection lag factor 

DM, mean pipe diameter, in 

DO, outside diameter of the pipe, ft 

e, base of natural logarithms, 2.7183 

E, modulus of elasticity of pipe material, lb/in2 

E, short-term modulus of elasticity of pipe material, lb/in2 

E, modulus of soil reaction, lb/in2 

F, force due to expansion/contraction of the pipe, lb 

FS, factor of safety 

g, acceleration due to gravity, 32.2 ft/s2 

h, height of fill above the top of pipe, in 

hw, height of water above the top of the pipe, ft 

H, height of soil above the top of the pipe, ft 

He

, height of plane of equal settlement above the top of the pipe, ft 

Hi, initial height of existing dam, ft 

HDB, hydrostatic design basis of the pipe, lb/in2 

HDS, hydrostatic design stress, lb/in2 

i, hydraulic gradient, head loss outside the pipe divided by the distance over which 

that head loss occurs, ft/ft 

Ipw, pipe wall moment of inertia, in4/in of pipe length 

K, bedding constant (typically 0.1 for soil embedment) 

K L, bulk modulus of water, lb/in2 

K, Rankine’s active lateral earth pressure coefficient, tan2(45-φ/2) 

k, coefficient of permeability of the surrounding filter or foundation, whichever is 

greater, ft/yr 

xxiv
Plastic Pipe Used in Embankment Dams 

L, distance within the pipe that a pressure wave moves before it is reflected back by 

a boundary condition, ft 

LL, liquid limit, % 

P, design pressure, lb/in2 

p, projection ratio 

PC, pressure class, lb/in2 

PCR, unconstrained collapse pressure, lb/in2 

PG, external hydrostatic pressure, lb/ft2 

PI, plasticity index, % 

PR, pressure rating, lb/in2 

Ps, pressure due to weight of soil on top of pipe, lb/ft2 

PS, pipe stiffness, lb/in2 

PV, internal vacuum pressure, lb/in2 

PW, pressure on the pipe from a wheel load, lb/ft2 

qa, allowable buckling pressure, lb/in2 

qAll, allowable soil bearing capacity, lb/ft2 

qar, reduced allowable buckling pressure, lb/ft2 or lb/in2 

Q, rate of flow of water into a drainpipe, ft3/yr 

r, mean pipe radius, in 

rsd, settlement ratio 

Rw, water buoyancy factor 

SDR, standard dimension ratio of pipe, D0/t 

SEC, stress due to temperature change, lb/in2 

SIDR, standard inside dimension ratio 

t, wall thickness of the pipe, in 

tw, top width of existing dam crest, ft 

TCR, critical time, s 

Tpw, thrust in pipe wall, lb/in 

v, Poisson’s ratio 

w/c, water-cement ratio by volume 

Wc

, soil load, lb/linear foot of pipe 

WL, wheel load, lb 

Wv, vacuum load per linear foot of pipe, lb/ft 

xxv 
Plastic Pipe Used in Embankment Dams 

AASHTO Standards 

AASHTO Standard Title 

M252 Corrugated Polyethylene Drainage Pipe. 

M294 Corrugated Polyethylene Pipe, 300- to 1200-mm Diameter 

SSHB Standard Specifications for Highway Bridges 

T99 Moisture-Density Relations of Soils Using a 2.5-kg (5.5-lb) 

Rammer and a 305-mm (12-in.) Drop 

xxvi 
Plastic Pipe Used in Embankment Dams 

ASTM Standards 

ASTM Standard Title 

C 33 Standard Specification for Concrete Aggregates 

C 117 Standard Test Method for Materials Finer than 75-m (No. 200) 

Sieve in Mineral Aggregates by Washing 

C 136 Standard Test Method for Sieve Analysis of Fine and Coarse 

Aggregates 

C 150 Standard Specification for Portland Cement 

C 618 Standard Specification for Coal Fly Ash and Raw or Calcined 

Natural Pozzolan for Use in Concrete 

D 653 Standard Terminology Relating to Soil, Rock, and Contained 

Fluids 

D 698 Standard Test Methods for Laboratory Compaction 

Characteristics of Soil Using Standard Effort (12,400 ft-lbf/ft3 

[600 kNm/m3]) 

D 1504 Standard Specification for Folded Poly(Vinyl Chloride) (PVC) 

Pipe for Existing Sewer and Conduit Rehabilitation 

D 1556 Standard Test Method for Density and Unit Weight of Soil in 

Place by the Sand-Cone Method 

D 1785 Standard Specification for Poly(Vinyl Chloride) (PVC) Plastic 

Pipe, Schedules 40, 80, and 120 

D 2241 Standard Specification for Poly (Vinyl Chloride) (PVC) PressureRated Pipe (SDR Series) 

D 2321 Standard Practice for Underground Installation of Thermoplastic 

Pipe for Sewers and Other Gravity-Flow Applications 

xxvii 
Plastic Pipe Used in Embankment Dams 

D 2412 Standard Test Method for Determination of External Loading 

Characteristics of Plastic Pipe by Parallel-Plate Loading 

D 2434 Standard Test Method for Permeability of Granular Soils 

(Constant Head) 

D 2487 Standard Classification of Soils for Engineering Purposes 

(Unified Soil Classification System) 

D 2657 Standard Practice for Heat Fusion Joining of Polyolefin Pipe and 

Fittings 

D 2837 Standard Test Method for Obtaining Hydrostatic Design Basis 

for Thermoplastic Pipe Materials or Pressure Design Basis for 

Thermoplastic Pipe Products 

D 2922 Standard Test Methods for Density of Soil and Soil-Aggregate in 

Place by Nuclear Methods (Shallow Depth) 

D 3034 Standard Specification for Type PSM Poly(Vinyl Chloride) (PVC) 

Sewer Pipe and Fittings 

D 3035 Standard Specification for Polyethylene (PE) Plastic Pipe (DRPR) Based on Controlled Outside Diameter 

D 3261 Standard Specification for Butt Heat Fusion Polyethylene (PE) 

Plastic Fittings for Polyethylene (PE) Plastic Pipe and Tubing 

D 3350 Standard Specification for Polyethylene Plastics Pipe and Fittings 

Materials 

D 4221 Standard Test Method for Dispersive Characteristics of Clay Soil 

by Double Hydrometer 

D 4253 Standard Test Methods for Maximum Index Density and Unit 

Weight of Soils Using a Vibratory Table 

D 4254 Standard Test Methods for Minimum Index Density and Unit 

Weight of Soils and Calculation of Relative Density 

D 4318 Standard Test Methods for Liquid Limit, Plastic Limit, and 

Plasticity Index of Soils 

D 4439 Standard Terminology for Geosynthetics 

xxviii 
Plastic Pipe Used in Embankment Dams 

D 4647 Standard Test Method for Identification and Classification of 

Dispersive Clay Soils by the Pinhole Test 

D 5813 Standard Specification for Cured-In-Place Thermosetting Resin 

Sewer Piping Systems 

D 6572 Standard Test Methods for Determining Dispersive 

Characteristics of Clayey Soils by the Crumb Test 

F 412 Standard Terminology Relating to Plastic Piping Systems 

F 477 Standard Specification for Elastomeric Seals (Gaskets) for Joining 

Plastic Pipe 

F 679 Standard Specification for Poly(Vinyl Chloride) (PVC) LargeDiameter Plastic Gravity Sewer Pipe and Fittings 

F 714 Standard Specification for Polyethylene (PE) Plastic Pipe (SDRPR) Based on Outside Diameter 

F 794 Standard Specification for Poly(Vinyl Chloride) (PVC) Profile 

Gravity Sewer Pipe and Fittings Based on Controlled Inside 

Diameter 

F 894 Standard Specification for Polyethylene (PE) Large Diameter 

Profile Wall Sewer and Drainpipe 

F 949 Standard Specification for Polyvinyl Chloride (PVC) Corrugated 

Sewer Pipe With a Smooth Interior and Fittings 

F 1216 Standard Practice for Rehabilitation of Existing Pipelines and 

Conduits by the Inversion and Curing of a Resin-Impregnated 

Tube 

F 1668 Standard Guide for Construction Procedures for Buried Plastic 

Pipe 

F 1743 Standard Practice for Rehabilitation of Existing Pipelines and 

Conduits by Pulled-in-Place Installation of Cured-in-Place 

Thermosetting Resin Pipe (CIPP) 

F 1803 Standard Specification for Poly (Vinyl Chloride)(PVC) Closed 

Profile Gravity Pipe and Fittings Based on Controlled Inside 

Diameter 

xxix 
Plastic Pipe Used in Embankment Dams 

F 2164 Standard Practice for Field Leak Testing of Polyethylene (PE) 

Pressure Piping Systems Using Hydrostatic Pressure 

F 2306 Standard Specification for 12 to 60 in. [300 to 1500 mm] Annular 

Corrugated Profile-Wall Polyethylene (PE) Pipe and Fittings for 

Gravity-Flow Storm Sewer and Subsurface Drainage Applications 

xxx 
Plastic Pipe Used in Embankment Dams 

AWWA Standards 

AWWA Standard Title 

C900 Polyvinyl Chloride (PVC) Pressure Pipe, and Fabricated 

Fittings, 4 - 12 in. (100-300 mm), for Water Dist. 

C901 Polyethylene (PE) Pressure Pipe and Tubing, 1⁄2 in. (13 mm) 

Through 3 in. (76 mm), for Water Service 

C905 Polyvinyl Chloride (PVC) Pressure Pipe and Fabricated 

Fittings, 14 - 48 in. (350-1,200 mm) 

C906 Polyethylene (PE) Pressure Pipe and Fittings, 4 in. (100 mm) 

Through 63 in. (1,575 mm), for Water Dist. and Trans. 

xxxi 
Plastic Pipe Used in Embankment Dams 

Websites 

The following websites can provide additional information and publications related 

to plastic pipe, embankment conduits, drainpipes, and embankment dams: 

American Society of Civil Engineers: http://www.asce.org 

American Society of Civil Engineers Publications: http://www.pubs.asce.org 

Association of State Dam Safety Officials: http://www.damsafety.org 

Bureau of Reclamation: http://www.usbr.gov 

Bureau of Reclamation Publications: 

http://www.usbr.gov/pmts/hydraulics_lab/pubs/index.cfm 

Canadian Dam Association: http://www.cda.ca 

Federal Emergency Management Agency: 

http://www.fema.gov/plan/prevent/damfailure 

Federal Emergency Management Agency Publications: 

http://www.fema.gov/plan/prevent/damfailure/publications.shtm 

Federal Energy Regulatory Commission: 

http://www.ferc.gov/industries/hydropower.asp 

International Commission on Large Dams: http://www.icold-cigb.org 

Mine Safety and Health Administration: http://www.msha.gov 

National Performance of Dams Program: http://npdp.stanford.edu 

Natural Resources Conservation Service: http://www.nrcs.usda.gov/technical/eng 

Natural Resources Conservation Service Publications: 

http://www.info.usda.gov/ced 

Plastic Pipe Institute: http://www.plasticpipe.org 

U.S. Army Corps of Engineers: http://www.usace.army.mil 

xxxii 
Plastic Pipe Used in Embankment Dams 

U.S. Army Corps of Engineers Publications: 

http://www.usace.army.mil/publications 

United States Society on Dams: http://www.ussdams.org 

Uni-Bell PVC Pipe Association: http://www.uni-bell.org 

xxxiii 
Plastic Pipe Used in Embankment Dams 

xxxiv
Introduction

Plastic pipe used in embankment dams serves different purposes than pipe used in 

water and sewer applications. Failure of plastic pipe in water and sewer applications 

rarely results in loss of life. However, failure of plastic pipe in dams can have 

catastrophic consequences. Removal and replacement can be difficult, time 

consuming, and costly. Plastic pipe used in dams must be conservatively designed to 

provide for a long service life, strength to accommodate all loading conditions and 

foundation movements, and have adequate access for cleaning and inspection. For a 

discussion of the importance of good design and construction and the ramifications 

that can result if these are lacking, see the Introduction of FEMA’s Technical Manual: 

Conduits through Embankment Dams (2005). 

Recommendations in this document are based on well founded engineering 

principles fundamental to the safety dams. However, in some cases, a distinction is 

made between plastic pipe used in significant/high hazard potential dams and low 

hazard potential dams. Significant and high hazard potential dams require stringent 

and conservative design measures, because failure or misoperation could result in 

loss of human life or economic damages. Generally, this is not the case for low 

hazard potential dams. While low hazard potential dams could certainly benefit from 

the design measures discussed in this document, some measures may be considered 

overly conservative for this type of structure. The designer of low hazard potential 

dams needs to carefully consider the requirements of their particular application. 

Plastic pipe has been used in the construction and renovation of conduits and 

drainpipes within embankment dams (i.e., earthfill and rockfill) since about the 

1980’s. The term “conduit” as used in this document refers to conduits used for 

outlet works, spillways, and siphons in embankment dams. These types of conduits 

regulate or release water impounded by the dam and are grouped together as 

“embankment conduits.” The term “drainpipe” is used to refer to toe drains that act 

as a downstream extension of the dam’s internal drainage system to collect and 

transport seepage passing through the dam or foundation to a desired outfall 

location. Plastic pipe has also been used for decants and drainpipes in tailings 

disposal and slurry impoundment facilities since about 1980. 

Plastic pipe is lightweight, abrasion resistant, and inert to most forms of chemical 

attack. This facilitates installation and benefits durability and service life. Plastic 

pipe is often used in toe drain systems for collecting and measuring seepage and 

safely discharging it into a channel located downstream from the dam. Plastic pipe is 

1 
Plastic Pipe Used in Embankment Dams 

commonly used for toe drain construction, since it is relatively inexpensive, readily 

available in many diameters, can be manufactured with slots or perforations, and can 

be rapidly installed (figure 1). Another frequent use of plastic pipe is for the 

sliplining of deteriorating outlet works conduits. Plastic pipe is preferred for 

sliplining due to its ease of installation, ability to re-establish the watertightness of 

the conduit, and improved hydraulic performance. 

Dam designers and dam safety officials often rely upon precedent and recognized 

guidelines to design critical features of dams; therefore, many dam designers and 

dam safety officials have been reluctant to use plastic pipe. Currently, the primary 

source of design information for plastic pipe is from manufacturers. However, most 

of this information is targeted to sewer and water pipe installations and does not 

address the unique factors involved in using plastic pipe within embankment dams. 

Most dam designers have never had training on the behavior of plastics and must 

weigh decisions on the use of plastic pipe by considering the initial costs, operating 

requirements, maintenance costs, dependability, and long-term performance. Some 

State dam safety officials have attempted to address the use of plastic pipe in their 

policies and regulations since the early 1990’s. Their efforts have resulted in 

imposing various design requirements, including reinforced concrete encasement, 

restrictions on the use of plastic pipe, and use restrictions based upon dam hazard 

classification. However, because of the many potential benefits, more projects are 

being designed and constructed using plastic pipe. The manufacture of plastic pipe 

will continue to evolve, based on the requirements of the engineering community. 

Continued improvements in manufacturing processes will provide products with 

enhanced strength, durability, and efficiency. This document is intended to serve as 

a guide for dam designers and dam safety officials to address the unique design 

requirements of plastic pipe used in dams for embankment conduits and drainpipes. 

This document provides the reader with detailed procedures for design, inspection, 

maintenance, renovation, and repair for plastic pipe applications used in 

embankment dams. 

This document specifically addresses plastic pipe applications involving embankment 

conduits and drainpipes in traditional water-retention embankment dams. The 

information in this document also applies to the design and use of plastic pipe for 

conduits and drainpipes in tailings or mine waste-disposal impoundments. However, 

chapter 7 discusses how the unique characteristics of these impoundments can affect 

the design of plastic pipe when used for this application. 

This document does not address other uses of plastic pipe often associated with 

embankment dams, such as instrumentation (e.g., piezometer riser pipes), relief wells 

(relief wells are considered part of the foundation drainage system), and structure 

underdrains (i.e., drains located under spillway floor slabs). Also, this document does 

not address plastic pipe used to deliver tailings or slurry to a mine-waste-disposal 

impoundment. However, some portions of this document may have limited 

applicability to these uses of plastic pipe. 

2 
Introduction 

3 

Figure 1 .—Plastic pipe is lightweight, which facilitates installation. 

Figure 1.—Plastic pipe is lightweight, which facilitates installation. 

Flat drains (edge drains) may have very limited application for drainpipes within 

embankment dams where overburden depths are small and future access is not a 

problem. However, concerns exist that the geotextile fabric wrapped around the 

drain has the potential for clogging, rendering the drain ineffective. Due to concerns 

with the potential for clogging, numerous inspection difficulties associated with flat 

drains, unknown performance under large fill loads, and the lack of precedent for 

use, they will not be addressed further in this document. Another recent innovation 

involving plastic pipe that will not be discussed in this document includes 

prefabricated riser intake structures. These prefabricated units are typically used to 

replace deteriorated corrugated metal pipe (CMP) risers. 

New and improved plastic pipe products are continuously being developed. Some 

may have potential applicability for use in embankment dams and others may not. 

For any new plastic pipe product without a proven record of successful use in 

embankment dams, the designer must exercise a cautious approach and closely 

evaluate all the characteristics and properties of the particular pipe. A number of 

research needs are presented in chapter 8 to better understand the performance of 

plastic pipe and embedment/encasement materials used in embankment dam 

applications. 
Plastic Pipe Used in Embankment Dams 

4 
Chapter 1 

General

Many types of plastic pipe are available from manufacturers and suppliers. However, 

certain types of plastic pipe are preferred for use within embankment dams due to 

their ability to accommodate a variety of internal and external loading conditions that 

may be experienced during the service life of the project. This document is intended 

to address parameters unique to plastic pipe and its applications within dams. The 

designer should understand that design criteria for plastic pipe used in dams differ 

from criteria used in design of plastic pipe in other types of applications, such as 

municipal water distribution and sewers. The most significant differences are the 

limited accessibility should something go wrong and the resulting potential impacts 

to downstream populations. Plastic pipe used in dams is often buried deeply where 

access is nearly impossible due to the amount of overburden existing above it and 

the existence of a reservoir pool. For these reasons, dam designers considering the 

use of plastic pipe must be cautious and select pipe that meets or exceeds 

conservative design criteria affecting watertightness, durability, structural 

performance, and design life. 

This chapter discusses the history, common types of plastic pipe, and their 

advantages and disadvantages for use in the construction of embankment conduits 

and drainpipes within embankment dams. Chapters 2 and 3 provide guidance on 

loading conditions and structural/hydraulic design. 

1.1 Historical Perspective 

Plastic pipe has been commonly used for embankment dam drainage systems (e.g., 

drainpipes) since the early 1970’s. Drainage applications in dams are typically 

nonpressurized. The use of plastic pipe for embankment conduit applications (e.g., 

outlet works and spillways) within traditional earthen dams is less common. Plastic 

pipe has been used in the construction and modification of embankment conduits 

since the early 1990’s. These types of applications can either be pressurized or 

nonpressurized. Typically, the designs for embankment conduits have been 

prepared without the use of a nationally recognized guideline and by default, have 

largely been based upon manufacturers’ information developed for differing and less 

critical applications. 

5 
Plastic Pipe Used in Embankment Dams 

The mining industry has used plastic pipe in dams since the mid-1980’s for decant 

pipes, internal-drain collector pipes, and delivery pipes for slurry or tailings disposal. 

As with embankment conduits, no nationally recognized design guideline is available 

for this type of application. 

While no standardized guidelines exist for the design of plastic pipe used for 

embankment conduits and drainpipes, numerous codes, standards, and 

recommended practices do exist that regulate and influence the plastic pipe industry. 

These publications cover a wide range of product performance requirements, 

materials, manufacture, and test methods related to plastic pipe. ASTM International 

(ASTM) publishes standard specifications, practices, and test methods. Standard 

specifications define specific performance and product requirements, standard 

practices define how a particular activity is to be performed, and standard test 

methods define how a particular test is to be performed. The American Water 

Works Association (AWWA) also publishes standards. ASTM and AWWA are 

consensus standards and are voluntary. They only become mandatory when 

specified by some user or entity such as a government agency. For example, if an 

agency specifies that pipe must meet AWWA C900, the finished product 

specifications found in AWWA C900 must be met. At the same time, the ASTM 

requirements called out in AWWA C900 also become mandatory. As changes in 

plastic pipe are made and newer products, applications, or test methods are 

developed, the standards are revised accordingly. The use of up-to-date publications 

is strongly advised. 

Additional information concerning plastic pipe is available in a number of 

publications, such as AWWA’s PE Pipe—Design and Installation (2006) and PVC 

Pipe—Design and Installation (2002), the Plastic Pipe Institute’s (PPI) Handbook of 

Polyethylene Pipe (2006), and Uni-Bell PVC Pipe Association’s Handbook of PVC Pipe— 

Design and Construction (2001). 

1.2 Common Types of Plastic Pipe Used in Embankment Dams 

Many types of plastic pipes are available, but not all types should be used in dams. 

The formulations used for the production of plastic pipe can vary slightly from 

manufacturer to manufacturer. The designer must specify the type, grade, and class 

required for each plastic pipe application. Due to the numerous options available, 

selection of the proper plastic pipe can become a bewildering experience for the 

designer. Fortunately, many standards, such as those from ASTM and AWWA, have 

been developed to ensure plastic pipe products have uniform characteristics, 

regardless of the manufacturer. This section will discuss some of the types of plastic 

pipe that have been “commonly” used in dams. Section 1.3 discusses how these 

types of plastic pipes are used in embankment conduit and drainpipe applications. 

The types of plastic pipes discussed in these sections have been successfully used in 

the past for applications in dams. If the designer wants to consider other types of 

6 
Chapter 1—General 

Figure 2.—Resin. Photo courtesy of

Uni-Bell PVC Pipe Association Members

and Associate Members.

plastic pipe not discussed in this document, all design implications must be carefully 

evaluated. Also, as the industry introduces newer plastic pipe products, the designer 

will need to carefully determine their applicability for the intended project. The 

information contained in this document should be used to assist in this 

determination. 

Plastic pipe used in dams primarily consists of two types: thermoplastic and 

thermoset plastic. The differences between thermoplastic and thermoset plastic 

pipes are discussed in the following sections. Some of the information in these 

sections has been adapted from FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005). Selected information has been updated where applicable. 

1.2.1 Thermoplastic 

Thermoplastics are plastics that can be 

repeatedly softened by heating and hardened 

by cooling without deterioration of their 

properties. In thermoplastics, the polymer 

molecules are not crosslinked (not chemically 

bonded to other polymer molecules). The 

molecules not being connected by crosslinks 

allows the molecules to spread farther apart 

when the plastic is heated. With the 

application of heat, thermoplastics may be 

shaped, formed, molded, or extruded. This is 

the basic characteristic of a thermoplastic. 

Plastics used for the manufacture of 

thermoplastic pipe are compounds consisting 

of resins (figure 2) mixed with additives. 

Each additive serves a specific purpose, such 

as (Willoughby, 2002, p. 2.3): 

• Antioxidants.—Extends the temperature range and service life. 

• Colorants.—Provides color to the plastic material. 

• Coupling agents.—Improves the properties of the plastic material. 

• Fibrous reinforcements.—Improves the strength to weight ratio. 

• Fillers and extenders.—Improves the properties of the resin. 

7 
Plastic Pipe Used in Embankment Dams 

• Heat and ultraviolet stabilizers.—Helps prevent degradation from heat and 

sunlight. 

• Preservatives.—Helps prevent bacterial attack on the plastic material. 

The formulations, proportions, and actual ingredients used provide the specific 

properties dictated by the particular application. 

The thermoplastics class of materials commonly includes polyethylene (PE), 

polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene (ABS), polybutylene (PB), 

and polypropylene (PP). However, the thermoplastics most commonly used in the 

construction of embankment dams are PE and PVC: 

• Polyethylene.—Polyethylene pipe is classified into several different categories 

based mostly on its density and branching. These categories include, among 

others, low, medium, and high density PE. The mechanical properties of PE 

depend significantly on variables such as the extent and type of branching, the 

crystal structure, and the molecular weight. ASTM D 3350 is used to classify 

polyethylene materials used for piping. High density polyethylene (HDPE) is 

the most common type of PE used in dam construction. 

• Polyvinyl chloride.—Polyvinyl chloride pipe is classified into several categories: 

pressure class (AWWA C900), pressure rating (ASTM D 2241 and AWWA 

C905), schedule 40, 80, and 120 (ASTM D 1785), and nonpressure (ASTM 

D 3034). The pressure class and rating products offer a pressure capacity 

independent of pipe size, whereas the schedule product pressure ratings vary 

between different pipe diameters. 

The general properties, advantages, and disadvantages of HDPE and PVC pipe in 

dam construction are discussed in section 1.3. 

Thermoplastic pipe is produced by the extrusion process, as illustrated in figure 3. 

The extrusion process produces an inherently strong finished product. The 

extrusion process continuously forces molten polymer material through an angular 

die by a turning screw. The die shapes the molten material into a cylinder. The 

speed at which the molten material is drawn away from the extruder determines the 

wall thickness. After a number of additional processes, such as cooling of the 

extruded pipe, the final product can be handled without distortion and can be cut 

into the specified pipe lengths. The process described here is typically used for solid 

wall pipe. Additional steps are required in the manufacturing process for adding 

corrugations or belling the ends of the pipe. For example, to add the bell end to a 

PVC pipe, one end of the PVC pipe is reheated and placed into a belling machine to 

enlarge the pipe diameter. The bell is formed by means of a belling mandrel which is 

slipped through the heated end of the pipe to enlarge it and shape it into the bell. In 

8 
Chapter 1—General 

Figure 3.—Conventional extrusion line. Photo courtesy of the Plastic Pipe Institute.

this machine, the bell is formed along with a groove for installation of a rubber 

gasket. 

Thermoplastic pipe fittings are required for changes in alignment, size, or 

connections (e.g., bends, wyes, tees, and reducers). Pipe fittings can be manually 

fabricated or made by the injection mold process (for nominal diameters of 

12 inches or less). Manually fabricated fittings are normally constructed by joining 

sections of pipe or machined from blocks. Pressure rated fittings are joined by heat 

fusion. To ensure that manually fabricated fittings have the same exact dimensions 

and properties as the pipe to which they will be connected, straight lengths of the 

same type of pipe are used to fabricate the fitting. The straight lengths of pipe are 

precision cut and joined together using heat fusion to form the fitting. 

1.2.1.1 HDPE 

Two general classes of HDPE materials are commonly used to make pipe for dam 

applications. One material (ASTM F 714) is classified by ASTM D 3350 as having a 

hydrostatic design basis and is suitable for pressure applications. The other material 

(ASTM D 3035) is classified by ASTM D 3350, but is not pressure rated. This 

material is used to make corrugated pipe manufactured to American Association of 

State Highway and Transportation Officials (AASHTO) standards M252 and M294 

respectively. In some special cases, corrugated pipe can be made from “pressure 

rated” material. The four HDPE pipe types, described in the following paragraphs, 

have been used in dam construction (figure 4 shows cross-sectional illustrations of 

each type). Other plastic pipe wall configurations exist, but have had very infrequent 

9 
Plastic Pipe Used in Embankment Dams 

10 

Figure 4.—Types of HDPE pipe walls.

use or have not been used in embankment dam applications. Those pipes will not be 

discussed in this document. 

HDPE plastic pipe used in dam construction includes: 

• Solid wall.—Solid wall pipe is made of a continuous wall of HDPE with uniform 

thickness. Solid wall pipe has smooth interior and exterior surfaces. Although 

solid wall pipe is pressure rated to meet the requirements as specified in ASTM 

F 714, dual-wall containment pipe should be used for pressurized embankment 

conduit applications in dams due to its added factor of safety. Solid wall pipe is 

available in diameters up to about 63 inches in typical lengths of 40 to 50 feet. 

Figure 5 shows an example of solid wall HDPE pipe. 

Figure 5.—Solid wall HDPE pipe to be used for sliplining of an existing 

outlet works conduit. 
Chapter 1—General 

11 

• Dual-wall containment.—Dual-wall containment 

pipe is made from two solid wall pipes. Dualwall containment pipe consists of an inside pipe 

(carrier pipe) which is centered within an outer 

pipe (containment pipe). Dual-wall 

containment pipe should be used use in 

pressurized embankment conduits, since it 

affords the added protection of a second pipe. 

The annular space between the carrier and 

containment pipes allows for quick detection of 

leaks in the carrier pipe. The manufacturer can 

preassemble this type of pipe at the factory, or 

the pipe can be assembled at the job site using 

two solid wall pipes. End spacers (centralizers) 

located at each end of a section of pipe center 

the carrier pipe within the containment pipe. 

The end spacers are made to form a tight fit and 

are extrusion welded in place. Intermediate 

spacers (known as spiders) are placed at 

intermediate points between the end spacers to 

provide additional support. Figure 6 shows an 

example of a dual-wall containment pipe. The 

containment pipe and carrier pipe should be 

pressure rated to meet the requirements as 

specified in ASTM F 714. Dual-wall 

containment pipe is available in diameters up to 

about 54 inches for the carrier pipe and 

63 inches for the containment pipe. The 

Wheatfields Dam Case History in appendix B 

discusses the use of dual-wall containment pipe 

for an outlet works conduit renovation. 

• Corrugated (single wall).—Single wall corrugated 

pipe has corrugated interior and exterior 

surfaces. This pipe is manufactured using a 

corrugated cross section for increased strength 

to allow the pipe wall to support soil loads. 

Single wall corrugated pipe is distributed in coils 

(figure 7). Single wall corrugated pipe is 

available in both perforated and nonperforated 

products. Perforations can be slots or circular 

holes. Single wall corrugated pipe is available in 

diameters up to about 24 inches. 

Figure 6.—Dual-wall 

containment HDPE pipe. A 

14-inch diameter carrier 

pipe is being inserted into a 

20-inch diameter 

containment pipe. 

Intermediate spacers are 

attached to the carrier 

pipe. Grout lines for 

grouting of the annulus 

between the existing 

conduit and containment 

pipe can be seen. 

Figure 7.—Single wall 

corrugated HDPE pipe has 

corrugations on both 

interior and exterior 

surfaces. Photo courtesy of 

Advanced Drainage 

Systems, Inc.
Plastic Pipe Used in Embankment Dams 

Figure 8.—Profile wall 

corrugated HDPE pipe has 

smooth interior and 

corrugated exterior 

surfaces.

• Corrugated (profile wall).—Profile wall corrugated 

pipe has a smooth interior surface and a 

corrugated exterior surface (figure 8). The 

corrugations add ring stiffness to the pipe to 

assist in maintaining cross-sectional shape. The 

smooth interior surface reduces friction and 

resistance to flow. Profile wall corrugated pipe 

economizes on the amount of material needed 

for fabrication; by altering the wall the same 

stiffness may be achieved with less material. 

However, not all types of wall corrugation are 

equal. Parametric studies were conducted by 

Burgon, Folkman, and Moser (2006) to examine 

the influence of profile height, shape, and 

thickness. Results of this research show that the 

corrugation shape had a dramatic effect on 

profile stability. Profile wall corrugated pipe is 

available in both perforated and nonperforated 

products. Perforations can be slots or holes. 

This pipe is supplied in standard 20-foot lengths. Profile wall corrugated pipe is 

available in diameters up to about 60 inches. 

HDPE pipe is typically black due to the addition of carbon black during the 

manufacturing process. The addition of carbon black prevents degradation of the 

pipe when exposed to ultraviolet (UV) radiation. HDPE pipe is also available in 

shades of gray to reduce glare and improve conduit inspection using closed circuit 

television (CCTV) equipment. 

The most common method used to join solid wall pipe and dual wall containment 

pipe is by heat fusion (ASTM D 2657; and PPI, 2005). Although a number of 

different fusion techniques exist, the butt fusion technique is the most widely used 

and industry-accepted method for joining sections of HDPE pipe. Butt fusion is 

typically used to join pipes that have the same nominal outside diameter and wall 

thickness. Butt fusion is accomplished by heating two surfaces to a designated 

temperature, and fusing them together by application of sufficient force. The 

application of force causes the melted materials to flow and mix together. As the 

joint cools, the molecules return to their crystalline form, the original joint interfaces 

are gone, and the two pipes have become one homogenous pipe. If performed 

according to recommended procedures, the fused joint is watertight and as strong or 

stronger than the HDPE pipe in both tensile and compressive properties. Butt 

fusion is performed at the site, by an operator who has been trained by an 

experienced pipe distributor, fusion equipment manufacturer, or pipe manufacturer 

using a portable fusion machine. Improper operation of the equipment can produce 

a poor fusion. Six steps are involved in making a properly performed butt fusion 

12 
Chapter 1—General 

Figure 9.—HDPE pipe joint being butt fusion welded. 

joint using a fusion machine (figure 9) (PPI, 2005, p. 14; Performance Pipe, 2006, 

p. 9): 

1. Securely fasten the pipe components into the clamping jaws of the fusion 

machine, so that they will not move. 

2. Face (trim and square off) the ends of the pipe components to establish clean, 

parallel mating surfaces. Most fusion machines have a rotating planer to 

perform this task. Poor preparation and any contaminants remaining on the 

pipe surfaces will produce a poor joint. 

3. Align the pipe ends to minimize mismatch of pipe walls. 

4. Heat both ends of the pipes (usually to about 400 to 450 °F). The heating tools 

are integrated into the fusion machine. A melt pattern that penetrates into the 

pipe must be formed around both pipe ends. 

5. Join the ends of the pipe by bringing them together with sufficient pressure to 

properly mix the molten pipe materials on the ends of the pipe components. A 

small melt bead will form at the joint on the interior and exterior surfaces of the 

pipe as the ends are joined. A properly performed fusion will form a double 

melt bead that is rolled over to the surface on both ends of the pipe. The pipe 

manufacturer will specify proper pressure required for the thickness and 

diameter of the pipe. 

13 
Plastic Pipe Used in Embankment Dams 

6. Hold the molten joint together under pressure until it has cooled adequately to 

develop proper strength. The amount of time required for cooling depends 

upon the material, pipe diameter, and wall thickness. The manufacturer will 

specify proper cooling times for their product. 

Fusion machines are available for pipe sizes up to 63 inches in diameter. Modern 

butt fusion machines are hydraulically assisted and semiautomatic requiring only one 

operator. Hydraulic power is used to operate all fusion functions including the 

clamping jaws, heater, and facer. Some machines have the capability to record 

important data, such as heater surface temperature, and heating, fusion, and cooling 

times. A printed record for each joint can be created to ensure consistency. Trial 

fusions should be considered at the beginning of the day, so the fusion procedure 

and equipment settings can be verified for the actual job site conditions. During 

cold weather, additional time is required to warm up the fusion machine and to heat 

the ends of the HDPE pipe. A temporary shelter may need to be constructed for 

joining the sections of HDPE pipe in case of inclement weather to avoid 

precipitation, wind, and heat loss. For additional cold weather procedures, see 

ASTM D 2657. Dual-wall containment pipe is typically butt fused together 

simultaneously or by staggering the welds of the carrier and containment pipes. 

Manufacturers’ recommended procedures should always be observed for butt fusion. 

HDPE pipe cannot be joined by field threading or solvent bonding. 

The need for melt bead removal is uncommon, and has negligible impact on the 

hydraulic performance of the pipe. If melt bead removal is required, it can be 

accomplished using special tools after the joint has thoroughly cooled to ambient 

temperature. Personnel using the debeading tool should be properly trained, so the 

pipe is not needlessly gouged. 

Figure 10.—A butt fused HDPE

pipe joint being checked for gaps 

and voids.

The beads should be thoroughly inspected for 

uniformity and proper size around the entire 

joint. Visual inspection criteria should be 

obtained from the pipe manufacturer. 

Nondestructive evaluation methods have been 

performed using ultrasonic equipment to detect 

voids or other discontinuities. Radiographic 

methods are considered unreliable because x-rays 

are a poor indicator of fusion quality. For 

destructive testing, a bent strap test (ASTM 

D 2657) can be performed in the field to confirm 

joint integrity, operator procedure, and fusion 

machine setup (PPI, 2006, p. 8). Figure 10 

shows a joint being tested. The test is easy to 

perform on thin wall pipes, but can be difficult 

on thick wall pipes (greater than about 

11⁄2 inches). For thicker walled pipes, 

14 
Chapter 1—General 

nondestructive evaluation methods should be considered. Field fusion should not 

proceed until joint quality on a test sample has been properly evaluated. Use of 

fusion machine operators who are skilled, knowledgeable, and certified will produce 

a good joint. Improperly butt fused joints cannot be repaired and must be cut out, 

and the ends must be properly joined (ASTM D 2657). Upon completion of the 

repair, the HDPE pipe should be retested for leaks. For guidance on leak testing, see 

section 3.8.2 

Unlike plastic pipe joined by couplers—as in corrugated HDPE, bells and spigots in 

PVC, or flanged joints—butt fusion creates a continuous joint-free pipe of nearly 

constant outside diameter. In sliplining applications for embankment conduits, the 

butt fusion joint does not take up any additional space, so a larger inside diameter 

slipliner can be used. This is an advantage over bell and spigot pipe or pipe with 

flanged joints. 

Other joining methods for solid wall pipe include: 

• Joints made by extrusion welding.—Many prefabricated fittings (i.e., elbows, bends, 

and tees) can be joined to HDPE pipe with heat fusion (ASTM D 3261) in the 

field using an extrusion gun. Extrusion welding is a manual process utilizing a 

hand held extruder (figure 11). The process involves continuously extruding 

molten HDPE onto the plastic components to be joined. The welding gun has 

the appearance of an electric drill with a small extrusion barrel attached to the 

front. The extrusion barrel is heated either by cartridge heaters or hot air. 

Figure 11.—Hand held extrusion gun. 

15 
Plastic Pipe Used in Embankment Dams 

HDPE rod or granule feedstock is fed into the rear of the extrusion barrel and 

the material is heated as it is drawn through the barrel. The molten HDPE is 

continuously ejected through a specially designed shoe attached to the front of 

the extrusion barrel. At the leading edge of the shoe, hot gas is used to preheat 

the surfaces where the molten HDPE is to be applied, so a proper weld can be 

formed. Generally, no further work is required to complete the joint. Typical 

welding speeds are 1 to 3 feet per minute. Extrusion-welded joints are 

significantly weaker than butt fusion joints. Weld quality depends upon the skill 

of the operator. Proper training and certification are required to maintain high 

standards of fabrication. Extrusion welding has also been successfully used for 

connecting HDPE grout and air vent pipes to plastic pipe slipliners. Extrusion 

welding cannot be used to repair damaged HDPE pipe. 

• Mechanical joints.—Mechanical joints are used to 

join HDPE pipe and fittings to themselves or to 

other types of pipe materials. The most common 

mechanical joint is the flange adapter (figure 12). 

Flanged connections are often used to connect 

HDPE pipe to steel pipe. The flange adaptor 

consists of a stub end, which is typically butt fused 

to the HDPE pipe, and a flanged end, which is 

joined with bolts and nuts to the flanged end of 

another pipe. A backup ring should be used with 

flanged connections. The backup ring is placed 

behind the HDPE flange. When the flange bolts 

are tightened, the backup ring compresses against 

the HDPE flange to the steel pipe flange, 

providing a seal. Flanged connections allow for 

easy assembly and disassembly of the joint. Flange 

joints tend to require more annular space than butt fusion joints. Depending 

on the application, the use of a gasket may be required with the flange adaptor 

connection. Other mechanical joining methods, such as couplings, are available 

from various manufacturers, but have not had much applicability for use in 

embankment dam construction. Although couplings are meant to allow HDPE 

connections to other pipe materials, there are special concerns. These include 

the low coefficient of friction of HDPE making gripping of the outside of the 

pipe more difficult than for other materials and the need for internal stiffeners. 

• Snap joints.—This type of patented joint is used in ISCO’s Snap-Tite pipe 

joining system and consists of solid wall HDPE pipe specially machined to 

form two grooves around the circumference on both ends of the pipe section. 

The grooves on the male end are on the exterior surface, and the grooves on 

the female end are on the interior surface. Each new piece of pipe is snapped 

onto the proceeding pipe. A lubricant and gasket is normally used with this 

type of joint. Snap joints allow sections to be easily joined using chains 

Figure 12.—HDPE flange 

adapter connection. 

16 
Chapter 1—General 

wrapped around the pipe, come-alongs, and a 

backhoe. This type of pipe joint has been used in 

sliplining of nonpressurized embankment conduits 

in low hazard potential dams, but should not be 

used in significant or high hazard potential 

applications. Figure 13 shows an example of the 

male end of a snap joint. 

Corrugated pipe is most often used in embankment 

dams for drainpipe applications, requiring nonrated and 

nonpressure joints. Manufacturers typically offer a 

variety of joints to meet specific project 

requirements (i.e., prevent the infiltration 

of soil, exfiltration of water, etc.). 

Corrugated pipe products are joined 

using the following methods: (1) single 

wall pipe using an external split or snap 

coupler and (2) profile wall pipe using an 

external split coupler, snap coupler, 

bell/bell gasketed coupler, or integral 

bell and spigot gasketed joint. Figure 14 

shows an example of an external split 

coupler. 

Figure 13.—Male end of 

snap joint. 

1.2.1.2 PVC Figure 14.—External split coupler. 

Pressure- and nonpressure PVC pipe is available in solid wall, which has smooth 

interior and exterior surfaces (figure 15). Solid wall PVC pipe is commonly available 

in 4- to 48-inch diameters in standard 20-foot lengths for pressure pipe. ASTM 

D 3034 nonpressure pipe is available in 14- or 20-foot lengths and ASTM F 679 

nonpressure pipe is available in 14-foot lengths. Note that AWWA C900 and C905 

are the only standards that specify a length. All others may vary from manufacturer 

to manufacturer. Open profile (single and double wall) (4- to 48-inch) ASTM F 794 

and F 949, and closed profile (double wall) (18- to 60-inch diameter) ASTM F 1803 

are also available, but have not been used in dam applications. 

The common joining system for PVC pipe is a bell and spigot flexible gasketed joint 

(figure 16). The gasketed joint is designed so that when it is assembled, the 

elastomeric gasket(s) is compressed radially between the pipe spigot and bell to form 

a positive seal (Uni-Bell, 1995, p.1). Gasket materials should comply with the 

physical requirements as specified in ASTM F 477. Assembly of gasketed joints is 

facilitated by use of a lubricant as recommended and applied in accordance with the 

pipe manufacturer’s instructions. Best practice for bell and spigot connections 

17 
Plastic Pipe Used in Embankment Dams 

Figure 15.—Solid wall PVC pipe has occasionally been used in 

embankment conduit applications within low hazard potential 

embankment dams. However, the bell and spigot joint connection used 

for this type of pipe can experience separation and seepage when 

improperly installed. 

requires the bell ends pointing in the direction of the 

work progress, since it is easier to insert the spigot into 

the bell rather than push the bell over the spigot. Care 

must be taken to avoid over- or underinsertion of the 

spigot end into the bell end. Since gasketed joints 

permit some flexibility, they are preferred for drainpipe 

installations, especially where settlement is expected. 

However, since embankment conduits in significant 

and high hazard potential dams must be designed with 

a high degree of conservatism, bell and spigot joints 

should not be used. Bell and spigot joints are 

susceptible to separation as the embankment dam 

settles. 

Other joining systems are available for PVC pipe. 

These proprietary joining systems include spline (figure 

17), heat fusion (figure 18), and mechanical (figures 19 and 20) joints. These types of 

joints are being used on water distribution and sewer installations, but have not been 

used in applications for dams. The designer needs to carefully evaluate the 

watertightness and long-term suitability of these joints before they are considered for 

use in dam applications; see research need PM-6 in chapter 8. 

Figure 16.—PVC pipe 

joint (bell and spigot). 

18 
Chapter 1—General 

Figure 17.—The splined joint has a machined 

groove in the PVC pipe and in the coupling to allow 

insertion of a flexible thermoplastic spline that 

provides a 360-degree restrained joint. Photo 

courtesy of Uni-Bell PVC Pipe Association Members 

and Associate Members. 

Figure 18.—The heat fusion process is used to join 

PVC pipe, resulting in a continuous length of pipe. 

Photo courtesy of Uni-Bell PVC Pipe Association 

Members and Associate Members. 

19 
Plastic Pipe Used in Embankment Dams 

Figure 19.—This type of 

mechanically restrained joint 

is used to prevent 

overinsertion of bell and 

spigot gasket PVC pipe. 

Photo courtesy of Uni-Bell 

PVC Pipe Association 

Members and Associate 

Members.

Figure 20.—PVC pressure pipe with 

external mechanically restrained joints. 

Photo courtesy of Uni-Bell PVC Pipe 

Association Members and Associate 

Members.

1.2.1.3 Other types of thermoplastic pipe 

Another type of thermoplastic is called fold-and-formed plastic (FFP). This system 

has not been used for renovation of embankment conduits, but may have 

applicability at some low hazard potential dams. The FFP system utilizes 

thermoplastic materials that have been folded from a circular shape to produce a 

smaller net cross-section and can be inserted into an existing pipe (USACE, 1995, 

pp. 2-8). These pipe products are usually extruded PVC or HDPE pipe that is 

flattened and folded longitudinally. The plastic pipe is fed from a spool into an 

existing pipe, and hot water or steam is applied until the liner reaches a uniform 

temperature throughout the material elevated enough for rounding. For one system, 

a special rounding device is inserted in the upstream end of the FFP and propelled 

by steam pressure to the downstream termination point. As the rounding device 

progresses, it expands the FFP tightly against the walls of the host pipe. Other 

systems use only heat and pressure to round the FFP. Any liquids in the host pipe 

are pushed out ahead of the expanding liner. The flexible FFP molds to the shape of 

the host pipe and normally forms distinct dimples at service connections. Pressure is 

maintained in the rounded FFP until it cools to a rigid state. The completed FFP 

liner has no joints and a very small annular space. No bonding occurs between the 

FFP and host pipe. The diameter range is limited to the manufacturing limits of this 

system (4 to 18 inches). Lengths up to 700 feet are possible. Due to its limited 

potential for use in embankment dams, FFP will not be discussed further in this 

document. For additional guidance on FFP, see ASTM D 1504. 

20 
Chapter 1—General 

1.2.2 Thermoset plastic 

Thermosetting plastics (thermosets) refer to a variety of polymer materials that cure, 

through the addition of energy, to a stronger form. The energy may be in the form 

of heat or through a chemical reaction (e.g., two-part epoxy). The curing process 

transforms the resin into a plastic by cross-linking. Thermoset plastic polymer 

molecules are cross-linked (chemically bonded) with another set of molecules to 

form a “net like” or “ladder-like” structure. Once cross-linking has occurred, a 

thermoset plastic does not soften, melt, or flow and will disintegrate when sufficent 

heat is added. However, if the crosslinking occurs within a mold, the shape of the 

mold will be formed. A thermoset material cannot be melted and remolded after it is 

cured. The thermoset class of materials includes unsaturated polyester (UP), epoxy, 

and polyurethane (PUR). Thermoset materials are generally better suited to hightemperature applications than thermoplastic materials. However, they do not lend 

themselves to recycling like thermoplastics, which can be melted and remolded. 

The most commonly used thermoset plastic in dam applications has been cured-inplace pipe (CIPP) (figure 21). CIPP is also referred to as an “elastic sock.” CIPP 

liners have been used mainly for sliplining of embankment conduits, as an alternative 

renovation method. CIPP liners are constructed to be slightly smaller than the inner 

diameter of the existing pipe that is being renovated. CIPP consists of a flexible 

polyester needle-felt or glass fiber/felt tube preimpregnated with resin. The 

preimpregnation process is usually done at the factory for quality control purposes. 

Unsaturated polyester, vinyl ester, and epoxy resins are available, with unsaturated 

polyester being the most widely used. These resins have a wide range of capability 

allowing CIPP to be designed for specific applications, unlike other types of plastic 

pipe, which have fixed properties. The fabric tube carries and supports the resin 

until it is in the final position and cured. The fabric tube must withstand stresses 

from installation and stretch to expand against irregularities within the existing pipe. 

On the inner surface of the CIPP liner is generally a coating or membrane of 

polyester, polyethylene, surlyn, or polyurethane, depending on the type of 

application. The membrane provides a low friction and hydraulically efficient inner 

surface to the CIPP liner. 

A variety of installation methods are available, including using water or air pressure 

to invert the tube through the existing pipe or a winch to pull the tube through the 

existing pipe (figure 22). When pressure is applied for rounding out the tube, the 

saturated fabric stretches to conform to the inner surface of the existing pipe. 

Although inversion is the preferred method of installation, winching may be pursued 

in situations where sufficient water pressure is unavailable or scaffold towers 

required for inversion are not practicable (USACE, 2001, p. 11). Combinations or 

variations of these methods are sometimes used. Hot water or steam is used to heat 

the resin and allow it to harden and cure after the liner has been formed within the 

21 
Plastic Pipe Used in Embankment Dams 

Figure 21.—CIPP liner exiting from an existing 

outlet works conduit, via the hydrostatic 

inversion method. 

existing pipe. Other curing methods are possible (i.e., UV and ambient), but typically 

have not been used with embankment conduits. When completed, the CIPP process 

forms at continuous tight-fitting, pipe-within-a-pipe containing no joints. 

Many CIPP systems are available today. The primary differences between these 

systems are in the composition and structure of the tube, method of resin 

impregnation, installation procedure, and curing process (USACE, 1995, pp. 2-6). 

Commonly used standards for specification and installation of CIPP are ASTM D 

5813 and F 1216. CIPP is applicable for lining existing conduits with diameters 

ranging from 4 to 132 inches. Maximum lengths of CIPP liners can exceed 1,000 

feet. At the larger diameters, the weight and cost of the materials become significant 

and the economics of the process may be adversely affected. Some mechanical 

bonding of the resin to the inner pipe surface can occur in practice. Whether it is 

effective in enhancing the structural performance of the CIPP liner depends to a 

great extent upon the condition of the existing pipe (USACE, 1994, pp. 14-15). 

Grouting of the annulus is typically not possible due to the small size of the gap 

between the existing pipe and a properly installed CIPP liner. 

Fiberglass pipe is another type of thermoset plastic, but has had very infrequent use 

in dam applications. Fiberglass pipe generally consist of two types, filament wound 

and centrifugally cast. In the filament-wound process, glass fiber is drawn, and a 

gelatinous or glutin like substance is applied. This substance helps protect the fiber 

as it is wound onto a bobbin. The particular substance applied relates to the end use 

22 
Chapter 1—General 

Figure 22.—Contractor installing a resin-soaked CIPP liner into an 

existing outlet works. Installation begins by hauling the liner up to the 

top of the platform. On the platform, water is run into the liner 

causing it to pressurize and expand downward. As the liner reaches the 

outlet works pipe opening, laborers on the ground maneuver the water 

filled liner into the outlet works. Water pressure continues to cause 

the liner to advance upstream in the outlet works pipe and un-invert 

itself. Photo courtesy of Tetra Tech Inc. 

of the pipe. The winding process takes place at a very high speed. In the centrifugal 

casting process, materials are placed in multiple layers, building from the outside to 

the inside using mold rotation. Centrifugally cast fiber reinforced polymer mortar 

(CCFRPM) pipe is manufactured in this fashion. 

The main advantage fiberglass pipe has over other types of plastic pipe is the 

availability in larger diameters. Fiberglass pipe typically has standard designs up to 

110 inches and nonstandard designs for larger sizes. Fiberglass pipe uses bell and 

spigot joints and should only be used on low hazard potential dam applications. For 

further guidance on the design of fiberglass pipe, see AWWA’s, Fiberglass Pipe Design 

Manual (2005). 

1.3 Common Uses for Plastic Pipe 

Not all plastic pipe can be used in the same way within dams. This section discusses 

some of the common applications of plastic pipe used in dam construction. 

23 
Plastic Pipe Used in Embankment Dams 

1.3.1 HDPE 

1.3.1.1 Solid wall and dual-wall containment pipe 

Solid wall pipe is mainly used in nonpressurized sliplining applications for renovation 

of existing outlet works conduits, construction of siphons, and the construction of 

decants in tailings and slurry impoundments. Sliplining is a renovation method 

where a new plastic pipe is pulled or pushed through the interior of an existing 

embankment conduit (i.e., outlet works), forming a watertight barrier. HDPE pipe 

has been used in sliplining of existing conduits since the early 1990’s. The annulus 

between the new and existing pipes is typically filled with grout. HDPE pipe is an 

inert material and as such is not subject to corrosion or deterioration, has a long 

service life, and requires little maintenance. This is especially important in small 

embankment conduits that are not easily renovated and cannot be easily inspected. 

The Worster Dam case history in appendix B illustrates how a HDPE slipliner can 

be used to renovate an outlet works. Dual-wall containment pipe is mainly used in 

pressurized sliplining applications. Use of dual-wall containment pipe in dam 

applications began after 2000. Figure 23 shows an example of HDPE dual-wall 

containment pipe arriving at a job site. 

The advantages of using solid wall or dual-wall containment HDPE pipe for new 

construction and for renovation include: 

• High strength and stiffness resists internal pressures and external loads, when 

properly designed. 

• Lightweight material facilitates installation requiring less equipment and fewer 

personnel. However, dual-wall containment pipe is roughly twice as heavy as 

solid wall pipe. 

• Resists corrosion and is not affected by naturally occurring soil and water 

conditions. May be preferable in certain embankment conduit applications 

where aggressive water or soil chemistry would limit the life of concrete or 

metal pipe. 

• Smooth interior surface reduces friction and resistance to flow. 

• Smooth interior surface minimizes adherence of soluble encrustants (e.g., 

calcium carbonate). 

• Minimizes biological growth and attack by microorganisms, such as bacteria and 

fungi. 

• Pipe joints can be butt fused, which provides a strong, watertight joint. 

24 
Chapter 1—General 

Figure 23.—HDPE dual-wall 

containment pipe arriving at a job 

site.

• Good resistance to abrasion. 

• Remains flexible at subfreezing temperatures. 

The disadvantages of using solid wall or dual-wall containment HDPE pipe for new 

construction and for renovation include: 

• Has a higher coefficient of thermal expansion relative to other types of plastic 

pipe, which can cause movement of the pipe, requiring the use of end restraints. 

• Pipe can be damaged or deformed by construction and compaction equipment. 

• Pipe can be displaced during compaction of earthfill against the pipe due to its 

light weight. 

• Heat fusion of pipe joints requires special equipment and a trained operator. 

• Compaction of earthfill under the haunches of the pipe is difficult and labor 

intensive. 

• Due to concerns with internal erosion, a properly shaped, reinforced cast-inplace concrete encasement is required for significant and high hazard potential 

embankment dams to accommodate compaction of earthfill against the 

embankment conduit. 

• Combustible and can melt in fire situations. 

For guidance on the design and construction of embankment conduits for new 

installations and renovations, see chapters 2 and 3 in this document and FEMA’s 

25 
Plastic Pipe Used in Embankment Dams 

Technical Manual: Conduits through Embankment Dams (2005). For guidance on solid 

wall pipe used in drainpipe applications, see chapters 4 and 5 of this document. 

While solid wall HDPE pipe has occasionally been used in drainpipe applications, 

dual-wall containment pipe has not been used for drainpipes. 

1.3.1.2 Corrugated pipe 

Corrugated HDPE pipe (single wall and profile wall) is most often used in 

embankment dams for drainpipe applications, such as toe drains (figure 24). Single 

wall corrugated pipe was first used for drainpipes in the early 1980’s. The use of 

profile wall corrugated pipe began in the 1990’s. Plastic pipe for drainpipes has 

largely replaced other pipe materials including clay tile, corrugated metal, and cast 

iron. Most designers prefer profile wall corrugated pipe over single wall pipe due to 

its higher wall strength and smoother interior. Also, CCTV inspection has shown 

the existence of structural integrity issues with single wall pipe (see section 6.2). 

Underground installation of corrugated pipe should follow the guidance in ASTM 

D 2321 and manufacturers’ instructions. 

The advantages of using HDPE corrugated pipe for a drainpipe include: 

• Lightweight material facilitates installation requiring less equipment and fewer 

personnel. 

• Resists corrosion and is not affected by naturally occurring soil and water 

conditions. 

• Smooth interior surface of profile wall pipe reduces friction and resistance to 

flow. 

• Minimizes biological growth and attack by microorganisms, such as bacteria and 

fungi. 

• Remains flexible at subfreezing temperatures. 

The disadvantages of using HDPE corrugated pipe for a drainpipe include: 

• Corrugated interior surface of single wall pipe will result in lower discharge 

capacity (use of profile wall pipe avoids this problem). 

• Interior surface corrugations can trap sediments and allow biofouling to 

develop (use of profile wall pipe avoids this problem). 

• Interior surface corrugations are more difficult to clean (use of profile wall pipe 

avoids this problem). 

26 
Chapter 1—General 

Figure 24.—A toe drain being constructed using profile wall corrugated 

HDPE pipe. 

• Pipe can be damaged or deformed by construction and compaction equipment. 

• Pipe can be displaced during compaction of earthfill against the pipe, due to its 

light weight. 

• Compaction of earthfill under the haunches of the pipe is difficult and labor 

intensive. 

• Combustible and can melt in fire situations. 

For guidance on the design and construction of drainpipes, see chapters 4 and 5. 

1.3.2 PVC 

PVC pipe was first introduced to North America in the early 1950’s. However, use 

of PVC pipe did not appear in dam applications until about the early 1970’s. 

Nonpressure PVC pipe is often used in dam applications for drainpipes. Some 

pressure rated pipe has been used for embankment conduits in low hazard potential 

dams and for siphons (figure 25). However, PVC pipe should not be used in 

significant and high hazard potential dams for embankment conduits. Primary 

27 
Plastic Pipe Used in Embankment Dams 

Figure 25.—A temporary siphon constructed using PVC pipe. This 

siphon is being used for short term operation. 

concerns involve the potential for leakage of the bell and spigot joints due to 

foundation movement. 

The advantages of using PVC pipe for embankment conduits in low hazard potential 

dams and drainpipes include: 

• High strength and stiffness resists internal pressures and external loads when 

properly designed. 

• Lightweight material facilitates installation, requiring less equipment and fewer 

personnel. 

• Resists corrosion and is not affected by naturally occurring soil and water 

conditions. May be preferable in certain applications where aggressive water or 

soil chemistry would limit the life of concrete or metal pipe. 

• Smooth interior reduces friction and resistance to flow. 

• Smooth interior surface minimizes adherence of soluble encrustants (e.g., 

calcium carbonate). 

28 
Chapter 1—General 

• Minimizes biological growth and attack by microorganisms, such as bacteria and 

fungi. 

• Higher beam strength than HDPE pipe helps alignment and grade control 

during installation. 

• Greater modulus of elasticity than for HDPE pipe. This allows for thinner 

sections of pipe to be used for the same conditions when properly designed. 

The disadvantages of using PVC pipe for embankment conduits in low hazard 

potential dams and drainpipes include: 

• More potential leak points at bell and spigot joints since joints are located every 

10 to 20 feet. 

• Susceptible to impact during cold weather and requires reasonable care. 

• Susceptible to extended UV exposure resulting in reduced resistance to impact 

and gradual decline in pipe strength. However, providing an opaque surface 

between the sun and pipe prevents UV degradation. Burial provides complete 

protection. 

• Pipe can be damaged or deformed by construction and compaction equipment. 

• Pipe can be displaced during compaction of earthfill against the pipe, due to its 

light weight. 

• Compaction of earthfill under the haunches of the pipe is difficult and labor 

intensive. 

• Limited resistance to cyclic loading under very high stress amplitudes. 

For guidance using PVC pipe in the design and construction of drainpipes, see 

chapters 4 and 5. For guidance on design and constructions of conduits, see 

chapters 2 and 3 in this document and FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005). 

1.3.3 CIPP 

CIPP has been successfully used in renovating of deteriorated embankment conduits 

since about the mid-1990’s. However, the use of CIPP has been relatively small 

compared to other applications using thermoplastic pipe. 

29 
Plastic Pipe Used in Embankment Dams 

The advantages of using CIPP lining for embankment conduits include: 

• Resists corrosion and is not affected by naturally occurring soil and water 

conditions. May be preferable in certain conduit applications where aggressive 

water or soil chemistry would limit the life of concrete or metal pipe. 

• Smooth interior surface reduces friction and resistance to flow. 

• Smooth surface minimizes, adherence of soluble encrustants (e.g., calcium 

carbonate). 

• Minimizes biological growth and attack by microorganisms, such as bacteria and 

fungi. 

The disadvantages of using CIPP lining for embankment conduits include: 

• High material and installation costs require a trained crew with special 

equipment. 

• Not suited for conduits with significant bends or changes in diameter. 

For guidance on the use of CIPP in embankment conduit renovation applications, 

see chapter 12 in FEMA’s Technical Manual: Conduits through Embankment Dams 

(2005). CIPP has not typically been used for drainpipe applications. 

1.4 Design Life 

Plastic pipe has many desirable characteristics. Unlike metal and concrete pipe, 

which can deteriorate over time from galvanic or chemical corrosion, plastic pipe 

does not rust, rot, or corrode. Aggressive soils do not affect plastic pipe, and it 

tolerates subzero temperatures well. Plastic pipe was introduced to the United States 

in about the 1950’s, but its use in embankment dam applications did not begin until 

about the mid 1970’s. The long-term performance of plastic pipe, like any pipe 

product, depends primarily on the quality of the installation. Excessive deflection of 

flexible pipe caused by inadequate compaction of the backfill material in the haunch 

area and at the sides of the pipe affects the long-term performance of the pipe. The 

plastic pipe industry has addressed other factors affecting design life by updating and 

improving materials. Current ASTM standards for plastic pressure pipe require use 

of high quality plastic materials that are designed for long-term performance under 

field conditions. Pipe manufacturers are continuously testing and evaluating their 

products in accordance with ASTM procedures to ensure the long-term strength and 

performance. 

30 
Chapter 1—General 

The design life for HDPE pipe in pressure service is based on the hydrostatic design 

basis testing for thermoplastic pipe (ASTM D 2837) and provides for a factor of 

safety of 2.0. Solid wall HDPE pressure pipe and corrugated HDPE pipe have 

significantly different properties and are not generally used in the same applications 

nor designed in the same way. The base resins used to manufacture these pipes are 

normally different. Polyethylene pipe resin is identified by an ASTM Material 

Designation Code or grade. Pressure pipe base resin material has a Cell Class of 

345464C or higher as designated in ASTM D 3350. ASTM D 3350 resin cell 

classification provides the means for identification, close characterization, and 

specification of material properties for polyethylene. This is a modern improved 

material and provides the longest life available for pipe in pressure flow applications. 

Current ASTM standards allow manufacturing of corrugated pipes with base resin 

materials having a cell classification of 323410C or 333410C. AASHTO 

requirements for corrugated pipes generally require better resistance to long-term 

stress than specified by ASTM. In some cases, corrugated pipe can be manufactured 

with materials similar to those used for pressure pipes. 

Manufacturers have used accelerated testing and statistical prediction methods to 

determine the expected life expectancy of plastic pipe. The basis recommendation 

for the design life of plastic pipe is 50 years. The Plastic Pipe Institute cites a recent 

report (PPI, 2003) that there is justification for assuming a greater design service life 

for corrugated polyethylene pipe when properly installed and used for gravity flow 

end-use applications. The PPI report pertains only to corrugated HDPE pipe that is 

gravity flow and operates primarily in compression. However, there is no uniformly 

accepted agreement concerning design life exceeding 50 years. The Florida 

Department of Transportation has initiated a program to verify the design life of 

corrugated polyethylene pipe (Hsuan and McGrath, 2005, and Hsuan, Zhang, and 

Wong, 2006). The study pertains only to corrugated HDPE pipe used in gravity flow 

applications that operate primarily in compression stress (i.e., low demand because 

slow crack growth is a tension failure mode). Pressure pipe operates primarily under 

tension and therefore requires polyethylene resins with a hydrostatic design basis 

(HDB) rating. 

HDPE pipe resins have differing amounts of stress crack resistance (SCR). A 

number of early drainpipe failures have occurred in single wall corrugated pipes. 

These failures are often attributed to the effects of environmental stress cracking 

(ESC) (also called slow crack growth). This phenomenon can occur during the 

handling and installation of HDPE pipe or under long-term service loads. The 

HDPE pipe could be gouged, scratched, kinked, or stressed resulting in a weak spot 

on the pipe wall and subsequent cracking. Failures from ESC tend to be due to the 

development of cracks in areas of tensile stress that slowly grow and propagate over 

time. Specifying HDPE pipe made with ASTM D 3350 cell classification 345464C 

grade resin provides the highest level of resistance to slow growth cracking and can 

virtually negate the possibility of this type of failure. This ensures a virgin, high 

grade resin that has been found highly resistant to environmental stress cracking. 

31 
Plastic Pipe Used in Embankment Dams 

The cell classification designated in the applicable product specification identifies the 

stress crack requirement. The product design and end-use applications determine 

the required stress crack requirement. The stress crack or notched constant ligament 

stress (NCLS) requirement designated in the applicable product specification have 

been established to insure that the HDPE resin provides the highest level of stress 

crack resistance for the intended end-use application for the pipe. Other grades of 

resin often contain some percentage of low grade recycled resins. The designer 

should be aware that ASTM cell classifications have changed over time and ASTM 

D 3350 should always be consulted for current classification designations. For 

additional information concerning resistance to slow growth cracking, see AWWA’s 

PE Pipe – Design and Installation (2006). 

The Bureau of Reclamation recommends that corrugated polyethylene pipe used in 

embankment dams comply with the requirements specified in AASHTO M252 (3- to 

10-inch diameter), AASHTO M294 (12- to 60-inch diameter), or ASTM F 2306 (12- 

to 60-inch diameter). AASHTO M252 specifies an ESC resistance requirement and 

AASHTO M294 and ASTM F 2306 specify a notched constant ligament stress 

requirement for the resins used to manufacture the larger diameter products. In 

addition, an ESC resistance requirement is specified for the finished product in both 

the AASHTO and ASTM product specifications. Additional research may be 

necessary to determine if a higher stress-crack-resistant resin should also be required 

in smaller diameter pipe (see chapter 8, research need PM-1). 

Research has shown that the actual performance of plastic pipe has exceeded the 

performance predicted by the long-term pressure tests more than 60 years ago. 

(Hulsmann and Nowack, 2004, p.8) reported that the extrapolation of 10,000-hour 

pressure testing is conservative and the actual service life of PVC pipe is likely to be 

greater than 50-years. Utah State University conducted an extensive survey of 

utilities in 1994 to evaluate performance of PVC in both gravity and pressure 

applications (Moser, 2001). The study showed that 50 percent of all problems 

occurred within the first year. Material-related long-term problems are few and are 

decreasing with time, which indicates that the problems are not a result of aging. 

Although much has been written regarding the projected design life for plastic pipe, 

there is general agreement that 50 years is a conservative estimate. As discussed in 

section 1.1, the performance history of plastic pipe used in embankment dam 

applications has been limited. Therefore, a number of research items are proposed 

in section 8.1.1 (PM-1 through PM-6) to further evaluate the use of plastic pipe in 

dams. The designer should consider all aspects of the project, installation 

conditions, end-use application, product specifications compliance, and established 

codes of practice when designing for a design life of more than 50 years. If high 

quality materials are used in the manufacture of plastic pipe and installation is 

performed in compliance with established codes of practice, a design life exceeding 

of 50 years may be possible. 

32 
Chapter 2 

Loading Conditions 

Embankment conduits and drainpipes are subjected to stresses and strains from 

external and internal loadings. External loads can include the soil above the pipe, 

vehicular loads, external hydrostatic pressure, and vacuum pressure. Internal loads 

can include fluid pressure and water hammer. This chapter discusses the 

determination of the various loadings on plastic pipe. Chapter 3 discusses the 

structural design principles necessary to accommodate these loadings. 

2.1 Soil Loading 

Many classic references have used the terms ‘buried conduit’ or ‘conduit’ when 

discussing loading conditions. In embankment dam applications, the term ‘buried 

conduit’ or ‘conduit’ often is interpreted to mean either embankment conduits (i.e., 

outlet works, siphon) or drainpipes. Generally, for significant and high hazard 

potential dams, embankment conduits constructed of plastic pipe are encased in a 

properly shaped reinforced cast-in-place concrete section to facilitate compaction of 

earthfill against the conduit. However, in some low hazard potential dams, the pipe 

may or may not be encased in concrete. Discussions in chapter 2 primarily focus on 

the application of load on plastic pipe assuming no concrete encasement. Figures 27 

through 34 are intended only to illustrate the principles involved with soil loads on 

buried pipe. However, these figures do not present all the required details for the 

proper design of conduits within embankment dams. The discussions presented in 

this chapter are best suited for applications involving conduits within low hazard 

potential dams or renovation (i.e., sliplining) where no support from the existing pipe 

or conduit is typically assumed. See section 3.5.2.2, for discussion of the reinforced 

concrete encasement as it relates to plastic pipe. For further design and construction 

guidance for conduits within significant or high hazard dams, see FEMA’s Technical 

Manual: Conduits through Embankment Dams (2005). To conform with commonly used 

terminology whenever possible in this chapter, the terms ‘buried conduit’ and 

‘conduit’ will be used interchangeably with ‘buried pipe’ and ‘pipe.’ 

Loads applied to buried conduits consist of dead and live loads. Dead loads are 

generally permanent, consisting of the soil above the conduit. Live loads, such as 

construction loadings (section 2.3), may or may not be permanent. Estimated soil 

loads on buried conduits have historically been computed using the Marston load 

33 
Plastic Pipe Used in Embankment Dams 

theory. Soil loads may also be computed by the soil prism theory. The differences 

in these loading theories are as follows: 

• Marston load theory.—This theory considers the transfer of load to or from the 

soil directly above the buried conduit due to the relative settlement between the 

soil directly above the conduit and the adjacent soil. The vertical load is made 

up of two parts: (1) the weight of the soil element directly over the buried 

conduit and (2) frictional forces acting either upward or downward on the sides 

of the soil column. If the soil on the sides of the column settles due to 

compressible soils or foundation, poor compaction, or other causes, downward 

friction forces will develop on the soil column. When this occurs the pressures 

on the buried conduit is greater than just the weight of the soil column above it. 

If the soil in the column above the buried conduit settles more than the 

surrounding soil or if a compressible foundation allows the conduit to move 

downward or if the conduit deflects vertically, upward friction forces will 

reduce pressures on the conduit. Illustrations of the relative load transfer are 

shown in this section (figures 28, 29, and 30). 

• Soil prism theory.—The soil prism theory is considered the simplest method for 

determining vertical earth soil loading above a buried conduit. This method 

assumes no load is transferred to or from the prismatic soil column directly 

above the buried conduit and includes only the load from the entire soil column 

directly above the conduit (figure 34). 

The designer should be aware that full load transfer onto the buried conduit may 

require months or even years and might not be realized until after construction is 

completed. However, both theories of soil loading presented in this document are 

only estimates of the soil loadings on the buried conduit. The designer should 

always consider the range of possible soil loadings based on the potential range of 

each of the parameters included in the soil load computations. Additional discussion 

of the Marston load and soil prism theories is included in this and the following 

sections. Recommendations for the soil load method to use are provided in section 

2.1.1 and 2.1.2 with further recommendations provided in table 9 in section 3.5.6. 

Classifying buried conduits is required to compute soil loads using the Marston 

theory. Figure 26 shows the classification of buried pipe with additional details 

provided in figures 27 through 33. Additional guidance on buried conduit 

classification is available in Spangler and Handy’s Soil Engineering (1982). 

Figures 27 through 35 show circular pipes in a discussion of loading conditions for 

plastic pipes, which are ordinarily circular in cross section. Other sections of this 

document caution against using circular pipes in significant and high hazard potential 

embankment projects because attaining intimate contact with the surrounding 

embankment soils is difficult with circular pipes. Circular pipes used in embankment 

dams require special considerations. A filter diaphragm as discussed in chapter 6 of 

34 
Chapter 2—Loading Conditions 

Figure 26.—Classification of buried conduits for the Marston theory (Spangler and Handy’s 

Soil Engineering, 1982). Note: Use of incomplete trench and special case should not be 

used for embankment dam applications. 

FEMA’s Technical Manual: Conduits through Embankment Dams (2005) should always be 

used. Encasing plastic conduits in a cross section of concrete with a battered shape 

avoids the problems of compacting soil under the haunches of a circular conduit. 

An example of a good cross section for encasements is shown in figure 46, chapter 4 

of the 2005 FEMA Technical Manual. Precautions discussed in section 3.8.8 are 

important for plastic conduit encasements. 

Trench conduits are installed in a relatively narrow trench in passive or undisturbed 

soil and backfilled to the ground surface, as shown in figure 27. The consolidation 

and settlement of the backfill along with the settlement of the conduit cause the 

backfill soil to move downward relative to the soil at the side of the trench. Some 

load is transferred from the backfill soil to the trench sidewalls due to friction. A 

drainpipe buried beneath the natural ground/foundation surface is often considered 

a trench conduit depending on the width and side slopes of the trench excavation. 

Embankment conduits should not be installed as trench conduits because of the 

potential for seepage in the zone where reduced stresses and hydraulic fracture can 

occur. 

Projecting conduits consist of those covered by fill material, such as embankment 

conduits. According to the Marston load theory, projecting conduits may be positive 

projecting or negative projecting. Positive projecting conduits are installed with the 

pipe projecting above the ground surface or compacted fill, with fill placed around 

and above the pipe. Positive projecting conduits are most typical for conduits 

through embankment dams and include conduits placed in wide or sloped-back 

35 
Plastic Pipe Used in Embankment Dams 

Figure 27.—Trench conduit. Backfill soil 

moves downward relative to the soil at 

the side of the trench. A drainpipe buried 

beneath natural ground is an example of a 

trench conduit. (Note: A trench conduit 

should not be used for an embankment 

conduit.) 

trenches. An outlet works conduit, spillway conduit, penstock, buried siphon, or 

drainpipe installed within an embankment drain or filter are considered projecting 

conduits. 

A positive projecting conduit may be in a projection condition or trench condition. 

If the exterior prisms settle more than the interior prism, as shown in figure 28, load 

is transferred from the exterior prisms to the interior prism, and a projection 

condition exists. The soil load on the conduit in a projection condition is greater 

than the weight of the fill above the conduit (soil prism load). This is caused in part 

by the loads from the exterior prisms as they deform being transmitted by soil 

shearing stresses to the interior soil prism. This increases the downward force 

applied to the pipe. This is sometimes referred to as negative or reverse arching. 

This often happens with rigid pipe because it undergoes only small deformations 

when loaded. If the interior prism settles more than the exterior prisms, as shown in 

figure 29, due to yielding foundation conditions or deflection of the pipe, a trench 

condition exists. The soil load on the conduit in the trench condition is typically less 

than the weight of the fill above the conduit (soil prism load). A flexible conduit that 

is installed as projecting conduit is typically considered a projecting conduit in the 

trench condition since the deflection of the conduit causes the interior prism to settle 

more than the exterior prisms. 

36 
Chapter 2—Loading Conditions 

Figure 28.—Positive projecting conduit in a 

projection condition. The pipe is installed 

above the ground surface or compacted fill, 

with fill placed around and above the 

conduit. The exterior prisms settle more 

than the interior prism, causing load to be 

transferred to the interior prism. An 

embankment conduit is an example of a 

positive projecting conduit in projection 

condition. (Note: This figure is not intended 

to show all the design details required.) 

Figure 29.—Positive projecting conduit in a 

trench condition. If the foundation is yielding 

or the conduit deflects, the interior prism 

settles more than the exterior prisms. The 

soil load on the conduit is less than the 

weight of the soil above it. A drainpipe in an 

embankment dam is an example of a positive 

projecting conduit in a trench condition. 

(Note: This figure is not intended to show all 

the design details required.) 

Negative projecting conduits are installed in shallow trenches, such that the top of 

the pipe is below the natural ground or compacted fill and backfilled and covered 

with fill material, as shown in figure 30. The soil load on a negative projecting 

conduit is less than that on a positive projecting conduit and typically less than the 

weight of the fill above the conduit (soil prism load). A negative projecting conduit 

may apply to a conduit through an embankment dam that is set in a narrow valley or 

foundation excavation, or to a drainpipe buried in the foundation beneath the 

embankment dam. However, good design practice requires embankment conduits 

and drainpipes not to be constructed as negative projecting conduits because soil 

arching above the conduit (figure 31) can cause potential seepage paths through the 

embankment. Arching can occur in all soils that have an internal angle of friction 

greater than zero. This includes all granular soils and most fine grained soils in the 

drained state. Arching is the result of grain-to-grain contact of the soil particles and 

37 
Plastic Pipe Used in Embankment Dams 

Figure 30.—Negative projecting conduit. 

The pipe is installed in a shallow trench such 

that the top of the pipe is below natural 

ground or compacted fill, and then covered 

with fill material. Negative projecting 

conduits should not be used for embankment 

conduits or drainpipes (Note: This figure is 

not intended to show all the design details 

required.) 

Figure 31.—Arching action of a negative 

projecting conduit. Negative projecting 

conduits should not be used for embankment 

conduits. For an embankment conduit, an 

excavation with 2:1 side slopes or flatter 

should be used. This causes the conduit to 

behave as a positive projecting conduit. 

(Note: This figure is not intended to show all 

the design details required.) 

is a form of shear resistance. Arching is as stable and permanent as other forms of 

shear resistance (Petroff, 1990, p. 286). 

Arching of the soil above the conduit can result in reduced lateral effective stress. If 

water pressure exceeds this stress, hydraulic fracture can occur, allowing internal 

erosion to develop. See chapter 5 in FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005) for further discussion on hydraulic fracture. To avoid 

installing a negative projecting embankment conduit, use a trench with at least 2 to 1 

(horizontal to vertical) or flatter side slopes (figure 31). The conduit will behave as a 

positive projecting conduit. 

Positive projecting and negative projecting conduits are further divided into 

complete and incomplete conditions (figure 26). The transfer of load from exterior 

prisms to the interior prism and vice versa causes different strains in the interior and 

38 
Chapter 2—Loading Conditions 

exterior prisms. At some point above the conduit the accumulated strain and 

settlement in the exterior prisms will equal that of the interior prism. This is defined 

as the plane of equal settlement. Above the plane, the interior and exterior prisms 

settle equally, and no shear or friction forces are transferred between the prisms. A 

complete condition exists when the embankment height is less than or equal to the 

height of the plane of equal settlement, as shown in figure 32. An incomplete 

condition exists when the embankment height is greater than the height of the plane 

of equal settlement, as shown in figure 33. Most concrete-encased embankment 

conduits are in the incomplete condition. At some height of fill above the pipe, but 

before the top of the embankment, the interior and exterior prisms are settling the 

same. 

A summary of the classifications of buried conduits is shown in table 1. A thorough 

understanding of table 1 is crucial for any buried conduit design. The difference in 

“projecting conduits” and “trench conduits” and “projection condition” and “trench 

condition” must be understood. The terms “projecting conduits” or “trench 

conduits” refer to a classification based on construction methods while “projection 

condition” or “trench condition” refers to a subclassification based on relative 

settlements of a positive projecting conduit. 

The range of potential soil loading on the buried conduit should be determined using 

the potential range of total unit weight of the soil. The soil prism load is not 

recommended for a projecting conduit in the projection condition, since settlement 

of the exterior prisms cause additional load on the interior prism that would be 

ignored using the prism theory. Thus, projecting conduits in a projection condition 

are typically designed using the Marston load theory. Conduits through embankment 

dams should be designed as shown in chapter 3, table 9. The effects of arching are 

typically ignored in computing the loading using the prism theory as a conservative 

measure. The soil prism theory typically estimates a greater (more conservative) soil 

load than that estimated from the Marston load theory for trench conduits, 

projecting conduits in a trench condition, or negative projecting conduits and should 

be used for these pipe classifications. Trench conduits should not be used for 

embankment conduits. The soil prism load theory is discussed in section 2.1.1. The 

Marston load theory is discussed in section 2.1.2 and by Spangler and Handy (1982). 

2.1.1 Soil prism load for trench conduits, positive projecting conduits in the 

trench condition, and negative projecting conduits 

The soil load on trench conduits, positive projecting conduits in the trench 

condition, and negative projecting conduits is determined by the soil prism theory as 

shown in figure 34 and the following equation: 

Ps = γ H (2-1) 

39 
Table 1.—Classification of buried conduits (required to compute soil loads using the Marston Load Theory) 

Classificationbyconstructionmethod

Trench conduit(figure 27)—Installed in a narrow trench in a passive orundisturbed soil, then backfilled to ground surface. Consolidationof backfill and settlement of conduitcause backfill to move downward relative to soil at side. Someload transferred frombackfill to trench due to friction. 

Projecting conduits—Covered by a fill material Induced trench

conduit—Aninduced trenchconduit is originallyconstructed as a positiveprojectingconduit. Uponfilling at least one conduit diameter abovethe top of the conduit, a trench is excavated to the top of the conduit and backfilled withcompressiblematerial.Induced trenchconduits shouldnot be used forembankmentconduits due tothe potential forpreferentialseepage throughthecompressiblematerial placed above theconduit.

Positive projecting conduit—Installed with conduit projecting above the grade or compacted fill with fill placed around and above the conduit.

Negative projecting conduit (figure 30)— Installed in a shallowtrench such that top of conduit is below the natural ground or compacted fill and then backfilled and covered with fill.Negative projecting conduits should not be used for embankmentconduits.

Subclassification by relativesettlement

Projection condition (figure28)—Exists when exteriorprisms settle more than interior, load is transferredfrom exterior prisms to interiorprism. Soil load on conduit is greater than weight of fill above the conduit. Typical for conduits through embankmentdams; includes conduits placed in wide trenches.

Trench condition (figure 29)—Exists when interior prism settles more than exteriorprism due to yieldingfoundation condition or deflection of conduit. Soil load on conduit is less than weightof the fill above the conduit.

Completecondition

Incompletecondition

Completecondition(figure 32)— Exists when theembankmentis higher than the plane of equalsettlement.*

Incompletecondition(figure 33)— Exists when theembankmentis lower than the plane of equalsettlement.*

Completecondition

Incompletecondition

* Plane of equal settlement is some point above the conduit where the accumulated strain and settlement in the exterior prisms equals interior

prism. Above the plane, exterior and interior prisms settle equally, and no shear or friction forces are transferred between the prisms.
Chapter 2—Loading Conditions 

Figure 32.—Complete condition. The 

complete condition exists when the fill 

height (H) is less than or equal to the height 

to the plane of equal settlement (He).

(Note: This figure is not intended to show all 

the design details required.) 

Figure 33.—Incomplete condition. The 

incomplete condition exists when the fill 

height (H) is greater than the height to the 

plane of equal settlement (He). (Note: This 

figure is not intended to show all the design 

details required.) 

where: 

Ps = pressure due to weight of soil on top of pipe, lb/ft2

γ = total unit weight of soil, lb/ft3

H = height of soil above the top of the pipe, ft 

2.1.2 Marston load for positive projecting conduits 

The soil load on a positive projecting conduit may be computed by: 

Wc = Ccγ DO 

2 (2-2) 

where: 

Wc = soil load, lb/linear foot of pipe 

Cc = positive projection load coefficient 

 = total unit weight of soil, lb/ft3

DO = outside diameter of the pipe, ft 

Cc

 depends on whether the buried conduit is in the projection or trench condition 

and in the complete or incomplete condition. 

41 
Figure 34.—The soil prism load is the weight 

of the soil directly above the conduit. (Note: 

This figure is not intended to show all the 

design details required.) 

The positive projection load coefficient, Cc, is defined as: 

H  2K μ  

Cc = 

e  D0  −1 when H ≤ He (complete condition) 2K μ 

(2-3)

or 

2K μ 

He 

 D 2K μ





He 



Cc = 

e 0  −1 

+ 

H − He 



e  D0  when H > He (incomplete condition) 2K μ  D0 D0 

(2-4)

Plastic Pipe Used in Embankment Dams 

where: 

Cc = positive projection load coefficient 

e = base of natural logarithms, 2.7183 

K = Rankine’s active lateral earth pressure coefficient, (tan2

(45o-φ/2)) 

μ = coefficient of friction (between backfill and sides of trench), tan φ 

φ = effective friction angle of backfill 

H = height of soil above the top of the pipe, ft 

He = height of plane of equal settlement above the top of the pipe, ft 

DO = outside diameter of the pipe, ft 

42 
The height to the plane of equal settlement, He

, may be determined by the following 

equation developed by Spangler (Spangler and Handy, 1982) (The solution for He 

requires an iterative procedure): 

2K μ 

He 

 1 H He  rsd p 	 e  D0  −1 1 He 

2 


 +  −  +  +    2K μ  D0 D0  3 
 2K μ 2  D0 

He 

rp H H  2K μ 

D  1 H H H H

+ sd  − e e  0  − e − e = rsd p 3  D0 D0  2K μ D0 D0 D0 D0 

(2-5) 

Chapter 2—Loading Conditions 

where: 

K = Rankine’s active lateral earth pressure coefficient, (tan2

(45-φ/2)) 

μ = coefficient of friction of fill material, tan φ 

φ = effective friction angle of backfill 

H = height of soil above the top of the pipe, ft 

DO = outside diameter of the pipe, ft 

He = height of plane of equal settlement above the top of the pipe, ft 

rsd = settlement ratio (table 2) 

p = projection ratio, as defined by figure 35, p is computed based on the 

dimensions of the installation 

e = base of natural logarithms, 2.7183 

Note: The value of Kμ is limited to 0.19 for a projection condition. 

Recommended design values for the settlement ratio (rsd,) are provided in table 2. 

These values are used to determine the Marston load on positive projecting conduits. 

The settlement ratio is a function of the type of installation and foundation 

condition. 

43 

Table 2.—Design values for the settlement ratio, rsd 

Settlement ratio, rsd 

Installation and foundation condition Range Design value

Positive projecting Rock or unyielding 1.0 1.0

conduit in projection soil

condition

Dense/well 0.5–0.8 0.7

compacted soil*

Loose/poorly 0.0–0.5 0.3

compacted soil

* The value of the settlement ratio is a function of the degree of compaction of the fill 

material adjacent to the pipe.
Plastic Pipe Used in Embankment Dams 

Figure 35.—Projection ratio, p = depth of the foundation material below 

the top of the conduit divided by the outside diameter of the pipe (DO).

(Note: This figure is not intended to show all the design details required.) 

Figure 36 provides values for the positive projection load coefficient, Cc , for various 

values of the product of the settlement ratio, rsd, and the projection ratio, p. Since the 

effect of μ is minimal, Kμ is assumed to be 0.19 for the complete projection 

condition and 0.13 for the complete trench condition in figure 36. 

The pressure on the top of the pipe may be determined by: 

Wc Ps = DO 

(2-6)

where: 

Ps = pressure due to the weight of soil on top of the pipe, lb/ft2

Wc = soil load, lb/linear foot of pipe (see equation 2-2) 

DO = outside diameter of the pipe, ft 

The range of potential soil loading on the conduit should be determined using the 

potential range of parameters, such as the total unit weight of the soil, settlement 

ratio, and projection ratio. Example A-2 in appendix A compares soil loading using 

both the Marston and prism theories. 

2.1.3 Increase in soil loading due to a dam raise 

The height of an embankment dam may be increased to provide additional flood 

protection or to enlarge the reservoir. The soil loading resulting from a dam raise 

may not be the same soil load as a dam originally constructed to the new height. The 

existing embankment and foundation will have experienced some, if not all, of the 

consolidation from the original embankment construction. The increase in soil load 

may be determined by finite element programs, (see section 3.1), or estimated using 

the following equation based on the stress distribution of an infinite footing: 

44 
Figure 36.—Values for the positive projection load coefficient (Cc).

ΔH t γ w Δ = Ps t w i + H

Chapter 2—Loading Conditions 

(2-7) 

where: 

Ps = increase in soil loading due to a dam raise, lb/ft2 

H = increase in dam height, ft 

 = total unit weight of the soil, lb/ft3

tw = top width of existing dam crest, ft 

Hi = initial height of existing dam, ft 

2.2 Hydraulic Loading 

Embankment conduits may experience hydraulic loading from internal hydrostatic 

pressure, surge pressure, internal vacuum pressure, or external hydrostatic pressure. 

Drainpipes are assumed to operate in a nonpressurized condition and typically do 

not experience this type of loading. 

45 
Plastic Pipe Used in Embankment Dams 

Figure 37.—Internal

hydrostatic pressure. 

2.2.1 Internal hydrostatic pressure 

Internal hydrostatic pressure must be resisted by tensile 

stress (hoop stress) in the pipe walls, as shown in 

figure 37. The internal hydrostatic pressure is typically 

no more than the pressure due to the full reservoir head. 

As with internal hydrostatic pressure, surge pressure 

must also be resisted by tensile stress (hoop stress) in the 

pipe walls. Surge pressure (water hammer) occurs when 

the flow velocity in the pipe is suddenly stopped or 

changed. When flow is suddenly stopped, the mass 

inertia of the flowing water is converted into a pressure 

wave or high static head on the pressure side of the pipe. Some of the most 

common causes of surge pressure in an embankment conduit occurs during the 

opening and closing of gates or valves, starting and stopping pumps, or entrapped 

air. 

Surges may generally be divided into two categories: transient surges and cyclic 

surges. Transients are described as the intermediate conditions that exist in a system 

as it moves from one steady state condition to another. Cyclic surging is a condition 

that recurs regularly with time. Plastic pipe may eventually fatigue if exposed to 

continuous cyclic surging at sufficiently high frequency and stress. 

Recurring surge pressures occur frequently and are inherent to the design and 

operation of the system (such as normal pump startup or shutdown and normal gate 

or valve opening and closure). Occasional surge pressures are caused by emergency 

operations. Occasional surge pressures are usually the result of a malfunction, such 

as power failure or system component failure, which includes pump seize-up, gate or 

valve-stem failure, and pressure-relief-valve failure. 

The pressure wave caused by the surge travels back and forth in the pipe, getting 

progressively lower with each transition from end to end. The magnitude of the 

pressure change caused by the surge pressure wave depends on the elastic properties 

of the pipe and water as well as the magnitude and speed of the velocity change. The 

maximum surge pressure is equal to: 

a VΔ Δ = H

g (2-8)

or 

a VΔ γ w Δ = P

g 144 (2-9) 

46 
where: 

ΔH = surge pressure, feet of water 

a = velocity of the pressure wave, ft/s 

ΔV = change in velocity of water, ft/s 

g = acceleration due to gravity 

= 32.2 ft/s2 

ΔP = surge pressure, lb/in2 

γw = unit weight of water, lb/ft3 

= 62.4 lb/ft3 

The maximum surge pressure results when the time required to stop or change the 

flow velocity is equal to or less than (2L/a) such that: 

(2-10)

where: 

TCR = critical time, s 

L = distance within the pipe that the pressure wave moves before it is 

reflected back by a boundary condition, ft 

a = velocity of the pressure wave, ft/s 

The velocity of the pressure wave, a, may be estimated by: 

TCR ≤ 



2L 



 a 

KL 12 

ρ 

a = 

KL Di  1+     E  t 

Chapter 2—Loading Conditions 

(2-11)

or 

12 

a = 

γ w 

 1 

+ 

Di 

 g KL Et  

(2-12) 

where: 

KL = bulk modulus of water, lb/in2 

= 300,000 lb/in2 

ρ = density of water, slugs/ft3 

= 1.93 slugs/ft3 

47 
E = modulus of elasticity of pipe material, lb/in2

 (140,000 lb/in2 for HDPE, 

and 400,000 lb/in2 for PVC. Note: The modulus of elasticity for 

surge/water hammer analysis is conservatively assumed to be 

higher than the value used for buried pipe analysis.) 

Di = inside diameter of the pipe, in 

t = wall thickness of the pipe, in 

γw = unit weight of water, lb/ft3

= 62.4 lb/ft3

 g = acceleration due to gravity, ft/s2 

= 32.2 ft/s2

For solid wall plastic pipe, the velocity of the pressure wave, a, may be expressed as: 

K 12 L 

ρ 

a = 

K SDR − L ( 1+ 

E 

2) 

or 

Plastic Pipe Used in Embankment Dams 

(2-13)

12 

a = 

γ 1 SDR − 2  w  +  gK E  L 

(2-14)

where: 

KL = bulk modulus of water, lb/in2 

= 300,000 lb/in2 

ρ = density of fluid, slugs/ft3 

= 1.93 slugs/ft3 

SDR = Standard Dimension Ratio 

= DO/t 

DO = outside diameter of the pipe, in 

t = minimum wall thickness of the pipe, in 

E = modulus of elasticity of pipe material, lb/in2 (140,000 lb/in2 for HDPE, 

and 400,000 lb/in2 for PVC. Note: The modulus of elasticity for 

surge/water hammer analysis is conservatively assumed to be 

higher than the value used for buried pipe analysis.) 

γw = unit weight of water, lb/ft3

= 62.4 lb/ft3

g = acceleration due to gravity, ft/s2 

= 32.2 ft/s2

48 
The term “standard dimension ratio (SDR)” is widely used in the plastic pipe 

industry. SDR is sometimes used interchangeably with the term “dimension ratio 

(DR).” Both terms refer to the same ratio, which is a dimensionless term that is 

obtained by dividing the average outside diameter of the pipe by the minimum pipe 

wall thickness. These ratios were developed out of convenience rather than out of 

necessity. They have been established to simplify standardization in the specification 

of plastic pipe internationally. Since these define a constant ratio between outer 

diameter and wall thickness, they provide a simple means of specifying product 

dimensions to maintain constant mechanical properties regardless of pipe size. In 

other words, for a given SDR or DR, pressure capacity and pipe stiffness remain 

constant regardless of pipe size. 

2.2.2 Internal vacuum pressure 

Embankment conduits may be subject to an effective 

external pressure because of an internal vacuum pressure, 

PV. Sudden valve closures, shutoff of a pump, or 

drainage from high points within the system often creates 

a vacuum. Embankment conduits (e.g., outlet works and 

siphons) are subject to internal vacuum pressures 

(figure 38) if they are not adequately vented. Internal 

vacuum pressure can lead to buckling (collapse) of the 

conduit. Internal vacuum pressure may be intermittent 

(short term), for long durations, or continuous (long 

term). The internal vacuum pressure determined by: Figure 38.—Internal

vacuum pressure. 

PV = 

12×WV 

Di 

Chapter 2—Loading Conditions 

(2-15)

where: 

PV = internal vacuum pressure, lb/ft2 

WV = vacuum load per linear foot of pipe, lb/ft 

Di = inside diameter of the pipe, in 

Example A-3 in appendix A demonstrates the principles involved with 

accommodating internal vacuum pressure in a siphon design. 

49 
Plastic Pipe Used in Embankment Dams 

2.2.3 External hydrostatic pressure 

Embankment conduits and drainpipes beneath 

the water table or phreatic line within the 

embankment are subject to external 

hydrostatic pressure. Even pipes encased in 

concrete or grout are subject to external 

hydrostatic pressure as a result of water 

reaching the outside surface of the pipe by 

entering through cracks in the encasement 

material or simply seeping through the porous 

encasement material. External hydrostatic 

pressure, as shown in figure 39, may lead to 

buckling or collapse of the pipe. The external 

Figure 39 hydrostatic pressure may be determined by: .—External hydrostatic 

pressure. 

PG = γw hw (2-16) 

where: 

PG = external hydrostatic pressure, lb/ft2 

γw = unit weight of water, lb/ft3 

= 64 lb/ft3

hw = height of water above the top of the pipe, ft 

External hydrostatic pressure is often the controlling loading condition for plastic 

pipe used for conduits in embankment dams. This is due to the critical buckling 

pressure being directly proportional to the modulus of elasticity of the pipe. The 

long term modulus of plastic pipe can be as low as 1/100th as that for concrete pipe 

and 1/1000th of steel pipe. See sections 3.1.2 and 3.3.2 for guidance on 

accommodating external hydrostatic pressures. 

2.3 Construction Loading 

Buried conduits may be subjected to wheel loads during construction or throughout 

the life of the project. Pressures on the pipe depend on many factors, such as the 

vehicle’s weight, speed, tires, surface smoothness, and depth of the pipe. Wheel 

loadings diminish as the depth of fill over the pipe increases. Loads from light duty 

vehicles tend to have little impact on buried pipe, but heavy construction equipment 

can seriously damage pipe with inadequate cover. For example, during construction, 

rough surfaces over pipes can cause scrapers to accelerate and decelerate vertically 

(i.e., bounce). Research has measured stresses representing impact factors with large 

magnitudes caused by bouncing scrapers (Bureau of Reclamation, 1984, p. 1). The 

higher the speed and greater the roughness, the higher the impact factor. Controls 

should be placed on construction practices for buried pipe. Limits on the speed of 

50 
construction equipment over the pipe should be implemented. Although this is 

surface-roughness dependent, as a general rule, the speed of equipment crossing over 

pipe should be limited to 5 mi/hr until there is 2 to 4 feet of cover. Figure 40 shows 

an example of an HDPE pipe that has experienced damage due to insufficient cover. 

The effect of wheel loads lessens with the depth of fill. When the depth of fill is 2 

feet or more, wheel loads may be considered as uniformly distributed over a wider 

area above the pipe (trapezoid shape with sides equal to 13⁄4 times the depth of fill) 

(NRCS, 2005, p. 52-8). Wheel loads may also be computed by AWWA’s PE Pipe— 

Design and Installation (2006). 

The pressure may be estimated by: 

(2-17) WL Pw =

(1.75 H )

2 

Chapter 2—Loading Conditions 

Figure 40.—The crown of this single wall corrugated HDPE 

pipe has been damaged due to construction traffic crossing 

over it. Insufficient cover over the pipe was the likely 

cause.

where: 

Pw = pressure on the pipe from a wheel load, lb/ft2 

WL = wheel load, lb 

H = height of soil above the top of the pipe, ft 

Soil and encasement 

materials requiring 

compaction within 2 feet 

of the pipe should be 

compacted with manually 

operated compaction 

equipment. Heavier 

compaction equipment 

may be used once the 

depth of soil over the pipe 

has reached 2 to 4 feet. A 

more detailed analysis 

procedure for wheel 

loading may be found in 

NRCS’s Structural Design of 

Flexible Conduits (2005, p. 

52-7) or in chapter 2 of 

Buried Pipe Design (Moser, 

2001). An example of the impact construction loads may have on a buried plastic 

pipe is included in NRCS’s Structural Design of Flexible Conduits (2005, p. 52B-29). 

51 
Plastic Pipe Used in Embankment Dams 

52 
Chapter 3 

Structural and Hydraulic Design 

The design of embankment conduits and drainpipes generally is divided into two 

categories: rigid and flexible. Rigid design assumes the pipe maintains its shape 

under loading by transferring the load to the foundation through the pipe wall. Rigid 

pipe is considered stiffer than the surrounding soil and does not require support 

from the surrounding fill. Rigid pipe will only allow minimal deflection without 

structural distress. Reinforced cast-in-place and precast concrete, clay, and cast iron 

pipe are examples of rigid pipe. 

Flexible design assumes that the pipe is less stiff or only slightly less stiff than the 

surrounding soil and deforms without experiencing structural damage. Steel, ductile 

iron, CMP, aluminum, fiberglass, HDPE, and PVC are examples of flexible pipe. A 

flexible pipe derives its load-carrying capacity from its ability to transfer load to the 

surrounding soil. Under external load, the pipe deflects, developing soil support 

along the sides of the pipe. The deflection of the cylindrical pipe relieves the pipe of 

some of the load by transferring load to the soil surrounding the pipe. A flexible 

pipe is defined as one that deflects at least 2 percent out-of-round in cross-section 

without structural distress. The load that ultimately reaches the buried pipe from the 

dead weight of the soil and any surcharge depends upon the shear strength of the 

soil, its stiffness, and the buried pipe classification (see chapter 2). The transfer of 

load from the pipe to the surrounding soil results in lower bending and compressive 

stresses than would be experienced by rigid pipes. However, even small earth 

loadings can result in pipe deflection, if the surrounding soil provides insufficient 

support. For guidance on the loading conditions applied to buried pipe see 

chapter 2. Example A-1 in appendix A demonstrates the principles used in flexible 

pipe design. 

As discussed in chapter 1, thermoplastic pipe such as HDPE and PVC has 

commonly been used in embankment dams. However, thermoset pipe (i.e., CIPP) 

has been used only in limited application. Therefore, this chapter will not address 

the structural and hydraulic design of thermoset plastic pipe. The reader is directed 

to ASTM F 1216 for guidance on the design considerations for CIPP and to 

AWWA’s Fiberglass Design Manual (2005). Although structural and hydraulic design 

of thermoset pipe will not be discussed, the reader may find some of the guidance 

presented in chapter 3 beneficial in understanding the basic principals of plastic pipe 

which are of critical importance to applications for dams. 

53 
Table 3.—Typical modulus of elasticity values for HDPE and PVC pipe 

Modulus of elasticity, lb/in2

Material Short-term Long-term

HDPE 110,000 - 140,000 22,000 - 30,000 

PVC 360,000 - 400,000 100,000 - 140,000 

Plastic Pipe Used in Embankment Dams 

3.1 Flexible Pipe 

Flexible pipe design requires the load on pipe to be transferred to the soil 

surrounding the pipe. As the loading increases, the vertical diameter decreases and 

the horizontal diameter increases. Figure 41 illustrates the differences in load 

transfer for rigid and plastic pipe. 

Since plastic pipe deflects under load, the modulus of elasticity of the plastic is an 

important material parameter used in the structural design of plastic pipe. The 

modulus of elasticity of plastic is a material property that describes the stress/strain 

behavior of a material in the linearly elastic region. However, for viscoelastic 

materials like HDPE and PVC, generally, the stress/strain curve is not linear, and the 

modulus of elasticity is often called “apparent modulus of elasticity,” meaning it 

changes depending on the load amplitude and duration. As stress relaxation occurs 

under constant load, the modulus of elasticity decreases from a short-term modulus 

to a long term modulus. The ratio of the short-term to the long-term modulus of 

elasticity is approximately 3 for PVC and 5 for HDPE. Typical modulus of elasticity 

values are given in table 3: 

The short-term modulus of elasticity is recommended for conditions that change 

through time, such as deflection or strain. Research shows that the short-term 

modulus of elasticity does not decrease after long-term loading. The calculated 

short-term modulus of elasticity actually increased when an incremental load was 

applied and increased the deflection (Moser, 2001, p. 415). The concept for this 

recommendation allows that soil settlement around a buried pipe occurs in dynamic, 

discrete, multiple events as the soil consolidates or soil grains are reoriented. Once 

movement occurs, soil arching redistributes the load, and no further deflection 

occurs for that particular event (AWWA, 2006, p. 58). However, as the next event 

occurs, these load increments are felt like impulse loads and the pipe resists them 

with its short-term elastic properties. In analysis for buckling, the modulus of 

elasticity and Poisson’s ratio (for HDPE) should represent the expected duration of 

the expected load (i.e., the short-term properties should be used for live loads and 

the long-term properties for static loads, such as soil loading). The long-term 

modulus of elasticity should be used for all analysis on solid wall pipe with SDR 

values less than 13.5 because these pipes typically carry a substantial portion of the 

54 
Chapter 3—Structural and Hydraulic Design 

(a) Load transfer in (b) Load transfer in flexible pipe 

rigid pipe 

Figure 41.—Load is transferred differently for rigid and flexible pipe 

(Howard, 1996). 

load and the long-term modulus is more conservative. For additional discussion of 

short- and long-tem modulus, see PPI’s Handbook of Polyethylene Pipe (2006). 

The designer can use finite element programs for structural design and analysis and 

evaluation of buried pipes to solve complex pipe-soil interaction problems. The soil 

surrounding the pipe is set up as a mesh of soil elements, and each element can be 

assigned different properties. Finite element programs not only allow for more 

realistic soil models than design equations, but also can model the effects of 

construction and special loadings. 

Often, buried pipes in low hazard potential dams are installed with little or no field 

monitoring or other quality controls. In these types of situations, pipes should be 

designed with simplified procedures that are known to be conservative and provide 

ample protections against poor construction procedures. Finite element analyses are 

best used to investigate behavior and design buried pipes for projects with unusual 

installation conditions, such as deep fills or proximity to structures, on large projects 

where a significant investment warrants extra effort in design, or for projects where 

the consequences of failure are significant. Pipes buried in dams can meet all of 

these criteria. 

Finite element analysis allows for modeling of pipe and soil using discrete elements 

that can each be assigned separate properties, accurate modeling of backfill soils, 

natural soil strata and inclusions, and pipe material behaviors. Computers now have 

the power to complete complex analyses in both two and three dimensions 

(figure 42). 

55 
Plastic Pipe Used in Embankment Dams 

56 

2-D analysis 3-D analysis 

Figure 42.—Examples of two- and three-dimensional meshes.

Any general purpose finite element program can be used to investigate buried pipe 

behavior, but only two have soil models specifically developed for modeling soil 

behavior and are widely used, Culvert ANalysis and DEsign (CANDE) (Musser, 

1989) and Plaxis (Brinkgreve and Broere, 2004). CANDE is an older program 

developed by the U.S. Federal Highway Administration specifically to investigate the 

performance of pipes and culverts. This program exists only as a disk operating 

system (DOS) program that is relatively laborious in conducting analyses, although 

Soil Structure Interaction Specialists (Webb, 2005) have developed one interface 

using AutoCAD. The National Cooperative Highway Research Program instituted a 

project in 2005 to update CANDE and develop a Windows interface (NCHRP 

Project 15-28). Plaxis is a much newer program with an excellent user interface. 

Plaxis was developed to investigate soil-structure interaction for a wide variety of 

buried structure problems, including buried pipes. Both programs have soil models 

that incorporate nonlinear, stress-dependent soil stiffness and strength, which has 

been shown to be critical for accurate modeling of buried pipe problems. Both of 

these soil models require numerous input parameters, but CANDE includes standard 

properties for a variety of soils that allow users to select only a soil type and 

compaction level. CANDE and Plaxis have been used for analysis of a range of 

buried pipe applications both rigid and flexible, including round and arch shapes 

with large spans. CANDE has been and continues to be the most common program 

for long-span, flexible pipes. Application to long-span, flexible pipes was one of the 

key types of applications it was developed for. Plaxis has not been as widely used, 

but it is also quite suitable for analysis. 

CANDE allows the user to select from several soil models. The model that has been 

most widely accepted incorporates the hyperbolic Young’s modulus developed by 

Duncan, et al. (1980) and the hyperbolic bulk modulus developed by Selig (1988, 

pp. 99-116). The properties for this model developed by Selig have been used to 
Chapter 3—Structural and Hydraulic Design 

develop the design procedures currently adopted by AASHTO’s Bridge Design 

Specifications (2005) for concrete and thermoplastic pipe. For backfill soils, 

parameters are available for three types of soils: (1) coarse-grained soils with little or 

no fines, (2) sandy or gravelly silts or silty or clayey coarse grained soils, with low 

plasticity, and (3) clay soils. These three sets of parameters have been shown to be 

suitable for most design problems. Selig (1988, pp. 99-116) describes the procedures 

necessary to develop parameters for specific soils and provides parameters for a 

range of in situ soils. A significant drawback in the use of Plaxis is that it does not 

include suggested parameters for typical soils. This is in part because Plaxis was 

developed primarily to model in situ soils, which are much more difficult to 

characterize than compacted soils. 

Key elements to consider in finite element analysis of buried pipe problems include: 

• The power of finite element analysis lies in accurate modeling of the pipe-soil 

system. This power can only be truly realized with accurate input and careful 

interpretation of results. 

• Models should include incremental construction where the backfill soils are 

placed incrementally as in actual construction. This has been found to be 

important to accurately model displacements and stresses. 

• Live load analysis, though often completed with two-dimensional analysis, is 

actually a three-dimensional problem. This complicates interpretation 

somewhat, but experience has shown that by reducing the applied load at the 

surface of a finite element mesh to account for live load attenuation in the third 

dimension between the ground surface and the top of the pipe, reasonable 

accuracy can be achieved. 

• Selecting the backfill soil model can present the same problems as selecting the 

appropriate modulus of soil reaction (E) value in the Iowa formula (Spangler, 

1941). If the fill height is high enough to raise concerns, monitoring of actual 

deflections might be considered, so that deflection predictions (and soil model) 

can be checked based on back-calculated calibrations of the soil model under 

the design fill. Until enough case studies have been performed to demonstrate 

and document actual field performance versus CANDE’s predicted 

performance, the designer may want to compare the results of CANDE to 

those obtained using more traditionally accepted methods. 

• The design method selected by the designer should be no more sophisticated 

than the construction procedure. For pipe that is buried without much control, 

simplified, conservative design procedures are appropriate. For special 

conditions (large pipe, deep fills, long pipes), significant economies can be 

achieved with more sophisticated design, and the cost of increased quality 

control in the field is justified. 

57 
Typical failure modes of flexible pipes are shown in figure 43. Flexible pipe design 

of buried plastic pipe includes analyses of the wall crushing, buckling resistance, 

allowable long-term deflection, and allowable strain. Deflection and buckling most 

often control the design of flexible pipe. Table 9 in section 3.5.6 provides the 

appropriate method of determining the soil load based on soil type and type of 

conduit. 

3.1.1 Wall crushing 

Wall crushing in plastic pipe is characterized by localized yielding when the in-wall 

stress reaches the yield stress of the pipe material (Moser, 2001, p. 499). Wall 

crushing typically occurs at the 3 and 9 o’clock positions as illustrated in figure 43a. 

Figure 44 shows an example of wall crushing. This localized yielding can occur in 

improperly designed stiff flexible pipes installed in deep, highly compacted fill. Less 

stiff flexible pipe more frequently fails from wall buckling, as discussed in 

section 3.1.2. 

Resistance to wall crushing of plastic pipe is evaluated by: 

PD Tpw = 

2 

O 

(3-1) 

where: 

Tpw = thrust in pipe wall, lb/in 

DO = outside diameter of the pipe, in 

P = design pressure (PS+PV+PW), lb/in2

 (see equations 2-6, 2-15, and 2-17) 

The required wall cross-sectional area is determined by: 

T 

Apw = pw 



(3-2)

where: 

Apw = area of the pipe wall, in2

/in of pipe length 

Tpw = thrust in pipe wall, lb/in 

 = allowable long-term compressive stress, lb/in2 

= HDB/2 

HDB = hydrostatic design basis of the pipe, lb/in2 

The actual area for a solid wall pipe wall may be computed as: 

(DO − Di ) Apw = or t 

2 

Plastic Pipe Used in Embankment Dams 

(3-3)

58 
Chapter 3—Structural and Hydraulic Design 

59 

(a) Wall crushing (b) Wall buckling (c) Excessive 

deflection 

(d) Excessive 

internal 

hydrostatic 

pressure 

Figure 43.—Typical failure modes for flexible pipes.

Figure 44.—Single wall corrugated HDPE pipe experiencing wall crushing.

where: 

Apw = area of the pipe wall, in2/in of pipe length 

DO = outside diameter of the pipe, in 

Di = inside diameter of the pipe, in 

t = wall thickness of the pipe, in 

The actual area of the pipe wall for corrugated (single and profile wall) may be 

obtained from the manufacturer or ASTM standard. 
3.1.2 Wall buckling 

External loadings from soil pressures, external hydrostatic pressure, or internal 

vacuum can cause in inward deformation known as wall buckling (collapse). Wall 

buckling is characterized by localized yielding, as illustrated in figure 43b. Figure 45 

shows an example of single wall corrugated HDPE drainpipe which has failed due to 

buckling. Wall buckling can occur due to insufficient pipe stiffness. The more 

flexible the plastic pipe, the more unstable the wall structure will be in resisting wall 

buckling (Moser, 2001, p. 110). Plastic pipe encased in soil may buckle due to 

excessive loads and deformations. The total load must be less than the allowable 

buckling pressure. If good backfill is used with sufficient stiffness, wall buckling is 

often not a concern and deflection will normally govern the design. This is true in 

most cases, with the exception for dam applications with shallow cover and internal 

vacuum pressures or fine grained backfill around embankment conduits used in low 

hazard potential dams. 

The allowable buckling pressure may be computed with various equations (Moser, 

2001; Chevron Phillips, 2002, p. 105; AWWA, 2006, p. 61-63; or Uni-Bell, 2001, 

p. 252). Equation 3-4 is from Moser (2001, p. 112). 

q = ′′ 1 

32R BE

EI pw 

12 

a FS 

 w DO 

3 



Plastic Pipe Used in Embankment Dams 

 (3-4)

where: 

qa = allowable buckling pressure, lb/in2 

FS = factor of safety 

= 2.5 for (h/DO) ≥ 2 

= 3.0 for (h/DO) < 2 

where h = height of fill above the top of pipe, in 

DO = outside diameter of the pipe, in 

Rw = water buoyancy factor 

= 1 – 0.33(hw/h), 0 < hw < h (3-5) 

where hw = height of water above top of the pipe, in 

B= empirical coefficient of elastic support 

4(h 2 + Dh ) = O (3-6) 1.5(2 h D+ O )

2

E= modulus of soil reaction, lb/in2

E = modulus of elasticity1 of pipe material, lb/in2

1 A long-term modulus of elasticity is recommended if the pipe is subject to external soil or internal 

vacuum pressure in normal operations. If the pipe is subject to the pressure for short time periods 

and infrequently, use of the short-term modulus of elasticity is recommended. 

60 
Chapter 3—Structural and Hydraulic Design 

Figure 45.—Single wall corrugated HDPE drainpipe experiencing 

failure due to buckling. 

Ipw = pipe wall moment of inertia, in4/in of pipe length 

t 3 

= (for solid wall pipe) (3-7) 12 

where t = wall thickness of the pipe, in 

(Note: To determine Ipw for corrugated single and profile wall pipe, 

contact the pipe manufacturer) 

The allowable buckling pressure depends on the surrounding soil pressure. The 

allowable buckling pressure increases/decreases as the effective soil pressure 

surrounding the pipe increases/decreases. The effective soil pressure decreases as 

the height of water above the pipe increases. The water buoyancy factor, Rw, 

accounts for the reduction in effective soil pressure for water levels in the soil above 

the top of the pipe. 

For a siphon extending over the crest of a dam that does not have the support of 

surrounding soil or controlled low strength material, the pipe should be designed to 

withstand unconstrained wall buckling as described in section 3.3.2 and illustrated in 

Example A-3 in appendix A. 

If plastic pipe is encased in a rigid material, such as grout, the potential for the pipe 

to buckle as a result of external hydrostatic pressure needs to be considered in 

accordance with the guidance provided in section 3.3.2. 

61 
Plastic Pipe Used in Embankment Dams 

3.1.3 Deflection 

Deflection of plastic pipe in cross section is the decrease in vertical diameter and the 

simultaneous increase in the horizontal diameter resulting from the loadings 

encountered. The amount of deflection along the length of pipe can vary 

significantly due to the inherent differences in soil compaction, type, and loading. 

Deflection of a flexible pipe is a performance limit to prevent cracking of the pipe, 

avoid reversal of curvature, limit bending stress and strain, avoid pipe flattening, and 

reduce the potential for leaking joints. Deflection is illustrated in figure 43c. 

Figure 46 shows an example of a single wall corrugated HDPE pipe experiencing 

excessive deflection leading to buckling. 

Excessive deflection can eventually lead to the collapse of the pipe. The normal 

sequence involved in pipe collapse is summarized as follows (Spangler, 1941): 

1. The embankment is built high enough to cause enough loading so the pipe 

deflects. The vertical diameter becomes smaller and the horizontal diameter 

becomes greater. 

2. The outward movement of the sides of the pipe against the enveloping earth 

brings into play the passive pressure of the earth, which acts horizontally 

against the pipe and reduces the rate at which the deflection occurs. 

3. As the embankment is constructed higher, the deflection continues until the 

top of the pipe becomes approximately flat. 

4. Additional load causes the curvature at the crown to reverse direction, 

becoming concave upward. 

5. The sides of the pipe pull inward, which eliminates most of the side support 

of the pipe since it is a passive force and cannot follow the inward moment. 

6. The pipe rapidly collapses. 

The Iowa Deflection Formula was developed by Spangler (Spangler, 1941) based on 

research of corrugated metal pipe (CMP) under earthen embankments. Spangler 

realized that deflection of CMP was not a function of pipe strength alone, but rather 

the soil-pipe system. The formula was later modified by Watkins (Watkins and 

Spangler, 1958) as the modified Iowa Equation to predict deflection of a buried 

flexible pipe. The deflection of buried, nonpressurized, flexible pipe increases with 

time as the supporting soil around the conduit consolidates and the soil-pipe system 

approaches equilibrium. The rate of deflection and ultimate deflection vary with the 

surrounding soil properties, particularly material type and density. Deflection 

continues to increase as long as the soil around the pipe continues to consolidate. 

To account for this, the Modified Iowa Equation includes a deflection lag factor, DL. 

62 
Chapter 3—Structural and Hydraulic Design 

Figure 46.—Single wall corrugated HDPE drainpipe 

experiencing excessive deflection leading to buckling. 

A DL value of 1.0 to 1.5 is often recommended. A DL of 1.0 has been used when the 

soil load is determined by the soil prism theory (Uni-Bell, 2001, p. 230). Plastic pipes 

designed with a DL value of 1.5 have historically performed well in embankment 

dams. 

At a depth of about 50 feet, these equations for deflection become conservative 

since they neglect the load reduction due to arching and increases in Edue to lateral 

earth pressure (stiffening of the soil surrounding the pipe due to over burden 

pressure). Other than in mine tailing impoundments, flexible plastic pipe for 

embankment conduits and drainpipes are rarely used in fill heights greater than 

50 feet in depth, so this should not be a concern. Deeply buried pipes are outside 

the scope of this document and the reader should consult other methods of analysis, 

such as those discussed in PPI’s Handbook of Polyethylene Pipe (2006) or by finite 

element analysis. 

Wheel loads should be considered when estimating deflection and vehicles may cross 

the pipe alignment. See section 2.3 for guidance on computing wheel loadings on 

top of pipes. 

Internal vacuum loads should be considered for siphons or when the internal 

hydraulic behavior of the system may allow an internal vacuum to develop. See 

section 2.2.2 for a discussion of internal vacuum pressure. 

Deflection of nonencased plastic pipe could potentially allow pathways to open 

within the soil and result in the development of internal erosion along the pipe. 

Deflection can also reduce flow capacity and cause joint leakage. For these reasons, 

nonencased plastic pipe should not be allowed within significant and high hazard 

potential dams (see section 3.5 for guidance on encasement). Although there is not a 

63 
uniformly accepted deflection limit for nonencased plastic pipe within low hazard 

potential embankment dams, it is often limited to 5 percent (NRCS, 2005, 

pp. 52-11). In mine tailings dams that are periodically monitored, a 7.5-percent 

deflection limit is often used (see chapter 7). Deflection limits are set to avoid the 

development of “reversal of curvature,” limit bending stress and strain, and avoid 

pipe flattening. The NRCS has installed plastic pipe in hundreds to thousands of 

small (less than 25 feet in height), low hazard potential dams. The design deflection 

limit of 5 percent has resulted in satisfactory performance. 

The use of a filter zone surrounding the pipe is a valuable defensive design measure, 

even for low hazard potential dams with favorable conditions. Some designs for low 

hazard potential dams may not employ a filter zone around the pipe, but eliminating 

this valuable feature should be carefully considered and justified. Filter diaphragms 

should only be eliminated when extremely favorable soil conditions, good conduit 

construction materials and methods, reliable construction practices, and favorable 

foundation conditions exist. See chapter 6 of FEMA’s Technical Manual: Conduits 

through Embankment Dams (2005) for guidance on the design and construction of 

filters. In addition, designers of mine waste-disposal dams should consider the 

discussion in chapter 7 when determining the installation requirements for decant 

conduits in these types of dams. 

For drainpipes, it is recommended that the allowable deflection be limited to 

7.5 percent, as is often recommended by manufacturers for other plastic pipe 

applications. However, the designer will need to carefully consider where the 

drainpipe is being installed within the dam and may need to require more stringent 

deflection limitations, if deflection of the drainpipe could potentially result in internal 

erosion concerns. Deflection of the pipe may be decreased by the use of higher 

quality or more compact backfill or a thicker or stiffer pipe wall. Higher quality or 

more compact backfill has a greater impact on the deflection of the pipe than the 

stiffness of the pipe. 

The Modified Iowa Equation may be modified as follows to compute the percent 

deflection of each type of pipe. 

Solid wall pipe: 

%ΔY (DP + P + P ) (100) K 

= Ls W V 

D 2E  	 
  + 0.061 E ' 


 3(SDR −1)

3     


(3-8) 

Corrugated single and profile wall pipe: 

% Δ Y (DP LS + PW + P V )K (100) 

= 

D [0.149 PS + 0.061 E '] 

Plastic Pipe Used in Embankment Dams 

(3-9) 

64 
Chapter 3—Structural and Hydraulic Design 

where: 

%ΔY/D = percent deflection 

D = DO = outside diameter of the pipe, in 

DL = deflection lag factor 

Ps = pressure due to the weight of soil on top of the pipe, lb/in2 

(see equation 2-6) 

PW = pressure on the pipe from a wheel load, lb/in2

 (see equation 2-17) 

PV = internal vacuum pressure, lb/in2 

K = bedding constant (typically 0.1 for soil embedment) 

E = short-term modulus of elasticity of pipe material, lb/in2 

(see section 3.1) 

SDR = standard dimension ratio of pipe, DO/t 

t = wall thickness of the pipe, in 

E= modulus of soil reaction, lb/in2

 PS = pipe stiffness, lb/in2 

SDR is the ratio of the outside diameter of the pipe (DO) to its wall thickness (t). A 

low SDR means a very strong pipe and a high SDR means a thinner wall, more 

flexibility, and less strength. Pipes with different outside diameters with the same 

SDR will tend to have similar flexibility. An SDR equal to or lower than 26 should 

be used for solid wall pipe since higher SDR values are extremely dependent upon 

the support provided by the surrounding soil. 

The Modified Iowa Equation is only a guide and can be an imprecise prediction of 

deflection in certain situations. The accuracy of the predicted deflections is normally 

adequate for designs within the range of pipe and soil stiffness relationships covered 

by research. However, for very stiff and very flexible pipes, the Modified Iowa 

Equation excessively overstates the deflections on one end (very stiff) of the scale 

and understates them on the other end. The equation demonstrates the importance 

of the soil and the relatively small contribution of ring stiffness to ring deflection. 

The equation should never be used alone to design the wall thickness of a flexible 

pipe and should only be used to determine pipe deflection. The required pipe wall 

thickness for flexible pipe should also be determined as discussed in sections 3.1.1, 

3.1.2, and 3.1.4. Limitations on the use and potential misuse of the Modified Iowa 

Equation are discussed in Schluter and Capossela (1998), Smith and Watkins (2004), 

Jeyapalan and Watkins (2004), and Howard (2006). The Modified Iowa Equation 

was originally developed to predict horizontal deflection, but has traditionally been 

used as an estimate for vertical deflection. Plastic pipe tends to deflect into a nearly 

elliptical shape, and the horizontal and vertical deflections may be considered to be 

equal for small deflections (Uni-Bell, 2001, p. 230). 

The modulus of soil reaction, E, is an empirical soil stiffness used for many years to 

model the soil contribution to control deflection and in-ground buckling. The soil 

modulus cannot be measured in the laboratory or by an in-situ test and has usually 

65 
Plastic Pipe Used in Embankment Dams 

been determined by measuring pipe deflection under a known load and calculating 

E. Amster Howard (1977) developed Evalues based on the soil prism load theory 

as shown in table 4 (Howard, 1996). The Howard parameters are the most 

commonly used Evalues in general design practice. These values were backcalculated from measured vertical deflections at a number of flexible pipe 

installations. They provide a constant value for soil stiffness regardless of depth of 

fill and subsequent confinement. These values can be used to a cover depth up to 

50 feet. The values for Evary with soil type and compacted density. A conservative 

value for Eis recommended. The Evalues presented in table 4 are average values 

for the type of material and percent compaction or relative densities shown. Many 

designers often reduce the values provided in table 4 by 25 percent to account for 

values below the average and variability along the length of pipe (PPI, 2006, p. 210). 

While Howard’s Evalues are most widely used in the Modified Iowa Equation in 

standards, in guidelines, and by designers, it should be noted that these values were 

derived using the prism load, a time factor to estimate long-term deflection, and 

vertical deflections. Howard recommends that soil loads be calculated using prism 

load theory, a deflection lag factor of 1.0, and a time factor that varies with backfill 

material and compaction. Further discussion on the use Howard’s Evalues and 

time factors can be found in Howard (2006). 

Note that the Modified Iowa Equation was originally developed for a Marston load 

with a deflection lag factor of 1.5. However, the Marston load for a flexible 

embankment conduit would be similar to the soil prism load (it is sometimes less 

depending on the parameters assumed for the Marston load). In addition, for many 

years, the NRCS has predicted vertical deflection using the Modified Iowa Equation 

with the soil prism load and a deflection lag factor of 1.5, In summary, it is 

recommended that deflection be determined for a cover depth up to 50 feet by using 

the Modified Iowa Equation, soil prism loads, and a deflection factor of 1.5. 

Whether or not the modulus of soil reaction varies with depth has been the subject 

of much research and conflicting opinion. Howard reported no correlation between 

Eand depth of fill. However, others (Hartley and Duncan, 1987) have 

demonstrated empirically and analytically that the value of Eincreases with 

increasing depth of cover over the pipe. This is because of the increased 

confinement of the soil embedment by the surrounding soil. The increased 

confinement stiffens the soil embedment and raises its E(AWWA, 2006, p. 59). 

Table 5 gives Evalues for cover depths up to 20 feet as determined by Hartley and 

Duncan. The designer should compare Evalues in both tables 4 and 5 and use the 

most conservative. The designer should base their selection of Eon project 

conditions, project requirements, judgment, and experience. If experience is lacking, 

an expert should be consulted on how to establish these values. For depths of cover 

exceeding 50 feet, the designer should consider alternate deflection calculations, as 

discussed in PPI (2006). 

66 
Table 4.—Average values of the modulus of soil reaction, E, for the Modified Iowa Equation

Soil type—Pipe bedding

material (Unified Soil 

Classification System—ASTM

D 2487) 

Efor degree of compaction of bedding, lb/in2

Dumped

Slight, Moderate, High,

<85% Standard 85%-95% Standard >95% Standard 

Proctor, Proctor, Proctor,

<40% relative 40-70% relative >70% relative 

density density density

Fine-grained soil (LL ≥ 50) Soils No data available; consult a competent soils engineer;

with medium to high plasticity otherwise, use E= 0 

CH, MH, CH-MH. No data 

available

Fine-grained soil (LL < 50) Soils 50 200 400 1,500

with medium to no plasticity 

CL, ML, ML-CL, with less than 

30% coarse-grained particles 

Fine-grained soil (LL < 50) Soils 150 400 1,000 2,500

with medium to no plasticity 

CL, ML, ML-CL, with more than 

30% coarse-grained particles. 

Coarse-grained soils with fines 

GM, GC, SM, SC contains more 

than 12% fines 

Coarse-grained soils with little 200 700 2,000 3,000

or no fines GW, GP, SW, SP

contains less than 12% fines 

Crushed rock. Not more that 1,000 1000 3,000 3,000

25% passing -in sieve and not 

more that 12% fines; maximum

size not to exceed 3 in. 

Notes:

• See table 6 for a description of soil classifications

• LL = liquid limit, %

• For use in predicting initial deflections only; appropriate deflection lag factor must be applied for

long-term deflections

• If bedding falls on the borderline between two compaction categories, select lower Evalue or 

average the two values

• Fines are soil particles that pass a No. 200 (75-μm) sieve 

• Percent Proctor based on ASTM D 698 or AASHTO T99

• Percent Relative Density based on ASTM D 4253 and D 4254 

• Values applicable only for cover of about 50 feet or less

• Evalues are in lb/in2

• Dumped − No compactive effort

• Slight − Some compactive effort,

In-place density <85% standard Proctor, or <40% Relative Density 

• Moderate − Intermediate level of compactive effort,

In-place density ≥85% and <95% standard Proctor, or ≥40% and <70% Relative Density 

• High − Considerable compactive effort,

In-place density ≥95% standard Proctor, or ≥70% Relative Density

Chapter 3—Structural and Hydraulic Design 

67 
Table 5.—Hartley-Duncan’s (1987) values of E', modulus of soil reaction 

Efor standard AASHTO relative 

Type of soil 

Depth of 

cover, ft 

compaction, lb/in2 

85% 90% 95% 100% 

Fine-grained soils with less than 0-5 500 700 1,000 1,500

25% sand content 

5-10 600 1,000 1,400 2,000 

10-15 700 1,200 1,600 2,300 

15-20 800 1,300 1,800 2,600 

Coarse-grained soils with fines 0-5 600 1,000 1,200 1,900 

(SM, SC) 5-10 900 1,400 1,800 2,700 

10-15 1,000 1,500 2,100 3,200 

15-20 1,100 1,600 2,400 3,700 

Coarse-grained soils with little or 0-5 700 1,000 1,600 2,500 

no fines (SP, SW, GP, GW) 5-10 1,000 1,500 2,200 3,300 

10-15 1,050 1,600 2,400 3,600 

15-20 1,100 1,700 2,500 3,800 

Plastic Pipe Used in Embankment Dams 

Issues concerning the use and misuse of the Modified Iowa Equation and the 

conflicting opinions of Evarying with depth only apply to flexible pipes. If a 

flexible plastic pipe is to be used as an embankment conduit in significant and high 

hazard potential dams, it should be encased in properly shaped reinforced cast-inplace concrete to facilitate compaction of earthfill against the conduit or grouted in 

place when used as a slipliner. Calculation of deflection is not necessary for encased 

plastic pipes. The principles of encased plastic pipe design are discussed in 

section 3.3. 

Standard Proctor density or AASHTO relative compaction and relative density are 

not the same. Standard Proctor density is typically used for fine grained soils while 

relative density is typically used for coarse grained soils with few or no fines such as 

SW, SP, GP, and GW. Table 5 does not include the values for relative density. In 

order to use Table 5 for coarse grained soils compacted to a relative density, some 

experience in determining which AASHTO relative compaction column is 

appropriate for the percent relative density of the coarse grained soil is required. 

For pipes installed in trenches, the support stiffness developed depends on the 

combined stiffness of the embedment material immediately adjacent to the pipe, plus 

68 
the native soil in the trench. For this situation, the designer will need to determine 

the composite modulus of soil reaction. For guidance on determining this value, see 

AWWA’s PE Pipe-Design and Installation (2006) and PVC Pipe-Design and Installation 

(2002). 

Internal vacuum pressure (Pv

) and pressure on the pipe from a wheel load (Pw) 

seldom occur at the same time. However, this could occur for siphon applications 

and should be considered in the computation of percent deflection. 

3.1.4 Internal hydrostatic pressure 

The internal hydrostatic pressure capacity of plastic pipe is given as a pressure rating 

for pipe manufactured in accordance with ASTM standards and as a pressure class 

for pipe meeting AWWA standards. Figure 43d illustrates internal hydrostatic 

pressure. The pressure class of PVC pipe manufactured in accordance with AWWA 

C900 is reduced by the surge pressure from an instantaneous velocity change of 

2 ft/s. (AWWA C905 does not include an allowance for surge pressure). If surge 

pressure is not anticipated, the allowable internal pressure of AWWA C900 pipe may 

be increased accordingly. The pressure class or rating of PE pipe manufactured in 

accordance with AWWA C906 or ASTM standards has not been reduced for surge 

pressure. Surge pressure is typically not a concern for drainpipes or embankment 

conduits. See section 2.2.1 for a discussion of surge pressure. The design of PE and 

PVC pipe for surge pressure is described in AWWA’s PE Pipe—Design and Installation 

(2006) and Uni-Bell’s Handbook of PVC Pipe (2001), respectively. The designer 

should not rely on the surrounding fill to resist internal hydrostatic or surge 

pressures. 

The manufacturing process of solid wall plastic pipe controls either the outside or 

inside diameter of the pipe. Either SDR or standard inside dimension ratio (SIDR) is 

provided in the applicable ASTM or AWWA standard and by the manufacturer 

depending upon the manufacturing process. The outside diameter of a pipe is the 

same for the available range of SDR values in outside-diameter-controlled pipe while 

the inside diameter of the pipe is the same for the available range of SIDR values of 

inside-diameter-controlled pipe. The pressure rating for solid wall plastic pipe may 

be determined by one of the following formulas. 

For outside-diameter-controlled pipe: 

PR = PC = 2(HDS ) 

SDR −1 

Chapter 3—Structural and Hydraulic Design 

(3-10)

69 
For inside-diameter-controlled pipe: 

2(HDS ) PR = PC = 

SIDR + 1 

Plastic Pipe Used in Embankment Dams 

(3-11) 

where: 

PR = pressure rating, lb/in2 

PC = pressure class, lb/in2 

HDS = hydrostatic design stress, lb/in2 

HDS = HDB/FS 

where HDB = hydrostatic design basis, lb/in2 

FS = factor of safety (2.5 for AWWA C900 pipe, 2.0 for all others 

[ASTM; AWWA C901, C905, and C906]) 

SDR = standard dimension ratio 

SDR = Do/t 

where DO = outside diameter of the pipe, in 

t = wall thickness of the pipe, in 

SIDR = standard inside dimension ratio 

SIDR = Di

/t 

where Di

 = inside diameter of the pipe, in 

t = wall thickness of the pipe, in 

The hydrostatic design basis (HDB) is an approximate measure of the amount of 

stress a plastic material can resist over a long time period. The hydrostatic design 

stress (HDS) is the maximum stress the plastic can resist over a long period of time 

with a high degree of certainty that it will not fail. The HDS is based on the HDB, 

which is reduced by a design factor or factor of safety. A complete description of 

HDB and HDS is included in ASTM D 2837. 

Corrugated plastic pipe (single and profile wall) typically is not pressure rated and 

should not be used in sustained pressure applications. Due to the limited allowable 

pressure for watertight joints in corrugated plastic pipe (single and profile wall) and 

the variability in the types of joints, the manufacturer’s recommendations should 

always be consulted. 

All pressure ratings are determined in an environment of approximately 73.4 o

F. As 

the temperature of the water or surrounding soil environment increases, the pipe has 

a reduction in strength and stiffness. The pressure rating should be decreased by the 

factors shown in table 6 or by the manufacturer’s recommended service factors. The 

temperature reduction factor is applied directly (by multiplication) to the calculated 

pressure rating. 

70 
Table 6.—Temperature reduction factors

Temperature, °F PVC factor HDPE factor

73.4 1.00 1.00

80 0.88 0.92

90 0.75 0.81

100 0.62 0.70

110 0.50 0.65

120 0.40 0.60

130 0.30 0.55

140 0.22 0.50

Source: AWWA (2002) and PPI (2006). 

For embankment dam applications, the pipe temperature rarely exceeds 73.4 °F and 

a temperature reduction factor of 1.0 is used. As the pipe temperature falls below 

73.4 °F, the pressure capacity of the pipe increases. The pressure rating (or pressure 

class) are considered to be the same when the pipe temperature is 73.4 °F. 

3.1.5 Strain 

Total strain in a pipe wall can be caused by two actions: (1) hoop stress due to 

internal or external pressure in the pipe wall and (2) flexure of the pipe as it deforms. 

Longitudinal strain is typically not a concern in buried pipe applications with mild 

and constant slopes since the load and surrounding support is relatively constant 

along the length of pipe. If a homogeneous wall is assumed and pressure 

concentrations are neglected, the following formulas can be used to estimate strain. 

Strain due to hoop stress in the pipe walls: 

PDM ε h = 

2A Epw 

(for single and profile wall corrugated pipe) 

ε h = 

PDM 

2tE (for solid wall pipe) 

Chapter 3—Structural and Hydraulic Design 

(3-12) 

(3-13) 

where: 

εh = maximum strain in the pipe wall due to hoop stress, in/ in 

P = design pressure, lb/in2 

71 
DM = mean pipe diameter, in 

= Di

 + 2c (for corrugated pipe) 

Di = inside diameter of the pipe, in 

c = distance from the inside surface to the neutral axis, in, (as supplied by 

the manufacturer) 

Apw = area of the pipe wall, in2

/in of pipe length 

E = short-term modulus of elasticity of pipe material, lb/in2

 (see section 3.1) 

DM = DO - t (for solid wall pipe) 

where DO = outside diameter of the pipe, in 

t = wall thickness of the pipe, in 

Strain from ring deflection: 

Maximum strains due to ring deflection or flexure may be determined by 

assuming the pipe remains an ellipse during deflections. The resulting 

equations are: 

ε f = 6 

D

t 

M 

Δ

D 

Y 

M 

 (for single and profile wall corrugated pipe) (3-14) 

or 

 M 1  

  3 Δ Y D/ 

 1 2 − Δ

t 3Δ Y D/ M ε = = (for solid wall pipe) f D Y / DM SDR  M M  1 − Δ2 Y / D 

Plastic Pipe Used in Embankment Dams 

(3-15) 

where: 

εf = maximum strain in the pipe wall due to ring deflection, in/in of pipe 

wall circumference 

t = wall thickness of the pipe, in 

DM = mean pipe diameter, in 

ΔY/DM = %Δ Y /D = percent deflection expressed as a decimal 

SDR = standard dimension ratio 

DM = Di

 + 2c (for single and profile wall corrugated pipe) 

where Di

 = inside diameter of the pipe, in 

c = distance from the inside surface to the neutral axis, in 

DM = DO - t (for solid wall pipe) 

where DO = outside diameter of the pipe, in 

t = wall thickness of the pipe, in 

In a buried pipe these strain components act simultaneously. The maximum 

combined strain in the pipe wall can be determined by summing both components. 

ε = εf ± εh (3-16) 

72 
Chapter 3—Structural and Hydraulic Design 

where: 

ε = maximum combined strain in pipe wall, in/in of pipe wall circumference 

εf = maximum strain in the pipe wall due to ring deflection, in/in of pipe 

wall circumference 

εh = maximum strain in the pipe wall due to hoop stress 

In calculating the maximum combined strain, the strain due to hoop stress in the 

pipe wall, εh, resulting from applied internal pressure, if any, should be added to the 

maximum strain due to deflection, εf

 . If the strain due to hoop stress in the pipe 

wall is due to external load or internal vacuum pressure, the strain due to hoop stress 

in the pipe wall from the applied internal pressure should be subtracted to obtain the 

maximum combined strain, ε. 

Utah State University (USU) has conducted research on PVC pipes deflected in cross 

section up to 20 percent. These pipe have not experienced failure after years of 

deflection. Additional research by USU indicates PVC can withstand strains up to 

100 percent. Thus, the allowable deflection for PVC pipe limits strain in standard 

PVC pipes to an acceptable value. Research by Janson (1991) showed that pressure 

rated HDPE pipe (solid wall) would not fail due to strain. Therefore, computation 

of strain and comparison to an allowable strain is not recommended for PVC pipe 

and HDPE made of the resins recommended in this document. A strain limit of 

5 percent is recommended for single and profile wall corrugated HDPE pipe. 

The maximum strain in the pipe should be limited to: 

 (3-17) 

where: 

ε = maximum combined strain in pipe wall, in/in of pipe wall circumference 

εall = allowable strain for the pipe material, in/in 

An example of a flexible pipe design is provided in appendix A, example A-1. 

ε  εall

3.2 Rigid Pipe 

Rigid pipe, typically reinforced cast-in-place concrete pipe, is designed to transfer the 

load from the pipe wall to the foundation; the pipe wall is strong enough to take the 

load without deflecting in the cross section when the load transfer occurs. 

Plastic pipe encased in reinforced cast-in-place concrete serves as only an interior 

form and water tight barrier. Plastic pipe surrounded by concrete does not become a 

rigid pipe. For guidance on the design of reinforced cast-in-place concrete conduits 

73 
or precast concrete conduits, see chapter 4 in FEMA’s Technical Manual: Conduits 

through Embankment Dams (2005). 

3.3 Encased Plastic Pipe 

Encased plastic pipe design applies to plastic pipe encased in concrete, flowable fill, 

grout in the annular space of a slipliner, and plastic pipe on a concrete cradle. The 

encasement provides uniform circumferential support to the pipe. Plastic pipe in 

this configuration should be designed as an encased pipe, rather than a flexible pipe, 

and cross-sectional deflection should be considered negligible. If the groundwater 

table is above the encasement, the potential exists to develop hydrostatic pressure 

between the encasement and the pipe through cracks, joints, imperfections in the 

encasement. Complete grouting of the annulus around a slipliner pipe is difficult and 

inspection is impractical. With these concerns, the structural design of encased 

plastic pipe should consider the wall crushing due to the soil load, internal 

hydrostatic pressure and wall buckling caused by external hydrostatic pressure. 

Example A-2 in appendix A demonstrates the principles used in encased plastic pipe 

design. 

3.3.1 Wall crushing 

Encased plastic pipe is analyzed for wall crushing due to the soil load using the 

equations for wall crushing described in section 3.1.1. Any support from the 

encasement or an existing pipe is ignored. If an encased conduit extends through an 

embankment dam, soil loads should be calculated for an embankment conduit in the 

positive projecting condition as discussed in section 2.1.2. 

3.3.2 Wall buckling 

The potential exists to develop an opening within the grouted annulus of a slipliner 

or between the concrete encasement and the plastic pipe. Therefore, plastic pipe 

should be designed to withstand external hydrostatic pressure on the pipe due to 

loadings from the reservoir or internal vacuum pressure. The pipe should be 

conservatively designed to withstand unconstrained buckling pressure by: 

3EI pw PCR = (1− vr 

for all pipe 2 ) 3 

Plastic Pipe Used in Embankment Dams 

(3-18)

74 
0.447PS PCR = (1−v ) for short-term loading1

 of corrugated plastic pipe 2 

3 

PCR = 

2E

(1− v 2 ) 







1 

SDR −1 







 for solid-wall pipe 

Chapter 3—Structural and Hydraulic Design 

(3-19)

(3-20) 

where: 

PCR = unconstrained collapse pressure, lb/in2 

E = modulus of elasticity of the pipe material,* lb/in2 (see section 3.1) 

Ipw = pipe wall moment of inertia, in4

/in of pipe length 

v =Poisson’s ratio (0.38 for PVC, 0.35 for short-term loading of HDPE, and 

0.45 for long-term loading of HDPE) 

r = mean pipe radius, in 

PS = pipe stiffness, lb/in2

 (as determined in accordance with ASTM F 894 

and D 2412) 

SDR = DO/t 

where DO = outside diameter of the pipe, in 

t = wall thickness of the pipe, in 

* A long-term modulus of elasticity and Poisson’s ratio are recommended if the pipe is subject to 

the pressure in the normal operations. If the pipe is subject to the pressure for short time 

periods and infrequently, the short-term modulus of elasticity is recommended. The hydrostatic 

pressure from the maximum reservoir pool would be considered long term. 

Research conducted by Ian Moore (El-Sawy and Moore, 1997) has shown that for 

plastic pipes fully encased in concrete, the unconstrained collapse pressure can be 

increased by an enhancement factor of 4 to 5 depending upon the pipe SDR and 

ovality. This assumes that the grouting process completely encases the pipe. 

However, for plastic pipe used in sliplining applications, a more conservative design 

is required for withstanding unconstrained buckling pressure, since complete 

grouting of the annulus (see section 3.5.4) can not be reasonably assured. 

Pipes that are significantly out-of-round or deflected have less collapse (buckling) 

resistance than round pipes. Pipes that are out-of-round due to manufacturing or 

deflected due to external pressure from soil and wheel loads or internal vacuum 

pressure have a lower allowable buckling pressure due to an increase in the bending 

moment. The allowable buckling pressure for these out-of-round or deflected pipes 

should be reduced by the following factor: 

1 Equation 3-19 is to be used only for wall buckling due to short-term loads since the pipe stiffness 

(PS) is representative of short-term material properties. 

75 
 %ΔY 1  	

3 


 1 −  
  D 100 



C =  
%ΔY 1 

2  
1 +    D 100  


Plastic Pipe Used in Embankment Dams 

(3-21)

qar = qaC (3-22) 

where: 

C = reduction factor for buckling pressure 

%ΔY/D = percent deflection 

qar = reduced allowable buckling pressure, lb/ft2

 or lb/in2 

qa = allowable buckling pressure, lb/ft2

 or lb/in2 

Figure 47 illustrates how pipe stiffness for single wall corrugated HDPE pipe relates 

to its ability to withstand unconstrained collapse pressure. A minimum factor of 

safety of 2 applied to the unconstrained collapse pressure is often recommended for 

external hydrostatic pressure or internal vacuum pressure (Chevron Phillips, 2002, 

p. 102). Figure 48 illustrates how standard dimension ratio relates to solid wall pipe. 

A more detailed wall buckling analysis may be completed as described by Watkins 

(2004). 

An example of a encased plastic pipe design is provided in appendix A, example A-2. 

The Virginia Dam case history in appendix B illustrates how concrete encased plastic 

pipe can be damaged due to improper design and construction. 

3.3.3 Internal hydrostatic or vacuum pressure 

If the encased pipe is subject to internal pressure, the pipe should have a pressure 

rating as described in section 3.1.4. The pipe should also be designed to withstand 

unconstrained buckling pressure as described in section 3.3.2. Design for internal 

vacuum pressure is discussed in section 3.3.2. 

3.4 Summary of Design Considerations for Flexible and Encased Plastic Pipe 

Design 

A number of considerations must be taken into account when using a plastic pipe in 

an embankment dam, as discussed in previous sections of this manual. Table 7 

summarizes these different design considerations and provides a reference to 

sections in the manual where additional information can be found. 

76 
Chapter 3—Structural and Hydraulic Design 

Figure 47.—Unconstrained collapse pressure vs. minimum pipe stiffness for single wall 

corrugated HDPE pipe. 

Figure 48.—Unconstrained collapse pressure vs. standard dimension ratio (SDR) for HDPE 

and PVC solid wall pipe. Plot is based on a minimum factor of safety of 1.0. 

77 
Table 7.—Summary of design considerations and required analyses for plastic pipe in 

embankment dams

Design

consideration Required analysis

Pipe diameter Hydraulic design (section 3.9 for embankment conduits and section 

4.1.2 for drainpipes)

Access requirements (section 6.1 for embankment conduits and 6.2 for 

drainpipes)

Pipe material Stress crack resistance (section 1.4) 

Soil loading Soil prism (section 2.1.1, and examples A-1 and A-2) 

Marston load (section 2.1.2, and example A-2)

Hydraulic loading Internal hydrostatic pressure (section 2.2.1)

Surge pressure (section 2.2.1) 

Internal vacuum pressure (section 2.2.2, and example A-3) 

External hydrostatic pressure (section 2.2.3, and example A-2)

Other loading Construction (section 2.3) 

Structural design Flexible pipe (section 3.1, and examples A-1 and A-3) 

• Wall crushing (section 3.1.1) 

• Wall buckling—constrained (section 3.1.2) 

• Wall buckling—some siphons—unconstrained (section 3.3.2) 

• Deflection (section 3.1.3)

• Internal pressure (section 3.1.4) 

• Strain (section 3.1.5)

Encased pipe (section 3.3, example A-2) 

• Wall crushing (sections 3.3.1 and 3.1.1) 

• Wall buckling—unconstrained (section 3.3.2) 

• Internal pressure (sections 3.3.3, 3.1.4, and 3.3.2) 

Plastic Pipe Used in Embankment Dams 

3.5 Embedment and Encasement Material Considerations 

The embedment or encasement is the material immediately surrounding the pipe. 

The nature and placement of this material are critical to the structural performance 

of the plastic pipe installation. For instance, a properly shaped reinforced cast-inplace concrete encasement is required in significant and high hazard potential dams 

to facilitate the compaction of earthfill against the conduit to minimize differential 

settlement and the potential development of internal erosion. As discussed in 

sections 3.1 and 3.3, the type of embedment or encasement material dictates whether 

the plastic pipe is designed according to flexible or encased plastic pipe design 

procedures. In a flexible plastic pipe design, the pipe deflects into the embedment 

material. As the pipe deflects, load is transferred to the material surrounding the 

pipe, which results in a shifting of load away from the pipe. The embedment 

78 
Chapter 3—Structural and Hydraulic Design 

material should provide adequate strength, stiffness, uniformity of contact, and 

stability to minimize deformation of the pipe due to earth pressures. An encased 

plastic pipe design is necessary if stress redistribution is limited by concrete or grout, 

which limits deflection. An encased plastic pipe design is also necessary if the 

encasement material has an E= 0 (fine grained soils with a high liquid limit), 

offering little or no stiffness compared to the pipe. The four most common 

embedment and encasement materials are soil, concrete, controlled low strength 

materials (flowable fill), and grout (contained within the annular space of a sliplined 

pipe). 

3.5.1 Soil 

Soil has been widely used as bedding and backfill for flexible pipe. However, soil can 

be problematic in obtaining adequate compaction under pipe haunches. The quality 

of the backfill and its placement, particularly in the haunch area (figure 49) and at the 

sides of the pipe, are the most important factors in limiting pipe deflection. 

Although flexible pipe is designed to deflect in cross section, excessive deflection can 

lead to unsatisfactory performance or structural failure. For this reason, the use of 

soil as bedding material for plastic pipe embankment conduits is only acceptable in 

low hazard potential embankment dams. Due to concerns with the potential for the 

development of internal erosion along embankment conduits in significant and high 

hazard potential dams, a reinforced cast-in-place concrete encasement should be 

used. 

Soil stiffness is defined as the soil’s ability to resist deflection. As discussed in 

section 3.1.3, soil stiffness is represented as E, the modulus of soil reaction. Loose 

soils have a relatively low E, while dense, well compacted soils have a high E'. 

Tables 4 and 5 in section 3.1.3 show average values of modulus of soil reaction. 

Highly plastic soils, provide minimal resistance to pipe deflection. If the embedment 

material is primarily comprised of highly plastic soils, or soft organic material, the 

modulus of soil reaction should be assumed to be zero. 

Research has shown that plastic pipe is generally resistant to structural failure. 

However, three conditions are known to cause pipe collapse: high internal vacuum 

pressure, excessive external hydrostatic pressure, and burial in loose or poorly 

compacted fine grained soils. Thus, the importance of having adequate compaction 

cannot be overemphasized. Section 5.2.4 in this document and section 5.3 in 

FEMA’s Technical Manual: Conduits through Embankment Dams (2005) discuss 

construction techniques that will ensure adequate compaction and pipe support. 

If the pipe is not externally supported by embedment or if embedment provides little 

or no support, unconstrained pipe wall buckling may be a concern. External 

79 
Plastic Pipe Used in Embankment Dams 

Figure 49.—Compacting earthfill under the haunches of plastic pipe is 

very difficult and quality compaction can not be achieved. 

pressures such as hydrostatic load from groundwater will also need to be analyzed 

using methods described in sections 2.2.3 and 3.3.3. 

As with any type of pipe, the maximum size aggregate (MSA) of the backfill for 

plastic pipe should be considered. For drainpipe applications, the MSA is controlled 

by the processed drain material described in other sections of this document. For 

general backfill of embankment conduits, the MSA is a function of pipe diameter. 

For pipes less than 1 foot in diameter, the MSA should not exceed 3⁄4 inch. For pipe 

greater than 1 foot in diameter, the MSA should not exceed 1.5 inches. 

Backfill material should not be angular or subangular since sharp protrusions on the 

particles could puncture or gouge the pipe. Rounded and subrounded particle 

shapes are acceptable. 

For dams, the type of backfill and bedding material for embedment used depends on 

where the pipe is located within the cross section, and whether the pipe is acting as 

an embankment conduit or a drainpipe. Section 4.3 and FEMA’s Technical Manual: 

Conduits through Embankment Dams (2005) provide guidance on selecting appropriate 

backfill. 

3.5.2 Concrete 

Concrete is often used as an encasement material for plastic pipe embankment 

conduits in significant and high hazard potential dams. There are three 

configurations: (1) reinforced concrete cradle up to springline (horizontal diameter) 

of the pipe, (2) reinforced cast-in-place concrete encasement, and (3) nonreinforced 

concrete encasement. In all cases, the use of concrete as encasement material limits 

80 
Chapter 3—Structural and Hydraulic Design 

deflection (longitudinal and cross-sectional) of the plastic pipe and changes the 

design approach. As discussed in section 3.3.2, if outward deflection is restrained, 

the plastic pipe has to buckle inward. The inward buckling takes more energy; thus 

the critical buckling pressure is higher when a pipe is encased in concrete. 

3.5.2.1 Reinforced concrete cradle (encasement to springline) 

There are arguments both for and against the use of concrete cradles. Continuous 

concrete cradles have been used to eliminate concerns of inadequate compaction 

beneath pipe haunches. The cradle also can provide anchor points for placement of 

pipe restraints, necessary to prevent pipe movement. Since a rigid cradle restrains 

cross-sectional deflection of the pipe, one concern has been the potential to create 

stress concentrations at the points where the pipe contacts the top of the cradle. 

Since the pipe does not deflect, it must be designed according to encased plastic pipe 

design principles (see to section 3.3). Additional research is necessary to better 

define the effect of stress concentrations on plastic pipe in concrete cradles (see 

research need EM-7 in chapter 8). Figure 50 shows an example of a concrete cradle 

used to support an HDPE pipe. 

If contraction and expansion of in HDPE pipe after placement is an issue, flanges 

can be fusion welded onto the plastic pipe and encased by the cradle. When plastic 

pipe is used with bell and spigot joints, continuous concrete cradles are sometimes 

used to prevent excessive joint displacement and associated leakage potential. 

Another concern when using a concrete cradle is that bond between the plastic pipe 

and the concrete cradle cannot be achieved due to material differences between the 

pipe and concrete and shrinkage of the concrete during curing. Thus, it is important 

that a diaphragm filter and drainage system be incorporated to prevent migration of 

fines along the conduit. For a discussion of filters, see chapter 6 of FEMA’s Technical 

Manual: Conduits through Embankment Dams (2005). 

Until the additional research is completed as discussed in research need EM-7, 

concrete cradles beneath plastic pipe should not be used. 

3.5.2.2 Reinforced cast-in-place concrete encasement (completely encased) 

Many States and federal agencies require that on significant and high hazard potential 

dams, plastic pipe conduits be completely encased in reinforced cast-in-place 

concrete. The plastic pipe acts primarily as an interior form, as well as a watertight 

liner. When properly designed, the concrete encasement provides a good exterior 

shape to compact earthfill against the conduit. The reinforced encasement should be 

designed according to reinforced concrete design principals. Design of reinforced 

cast-in-place concrete is beyond the scope of this document. For guidance on 

81 
Plastic Pipe Used in Embankment Dams 

Figure 50.— Until further research is completed, concrete cradles beneath

plastic pipe should not be used. 

reinforced cast-in-place concrete design, see chapter 4 in FEMA’s Technical Manual: 

Conduits through Embankment Dams (2005). 

Although it is recommended that the strength of the plastic pipe be ignored when 

determining the structural strength of the reinforced concrete, the plastic pipe must 

still be designed to prevent collapse from both excessive hydraulic pressures and 

concrete pressures. Full reservoir head can be transferred through cracks to small 

annulus spaces in between the plastic pipe and reinforced concrete encasement. 

With a reinforced cast-in-place concrete encasement, it is tempting to place the pipe 

and concrete in a vertical wall trench (causing the pipe to be negatively projecting). 

As discussed in section 2.1, negative projecting conduits should not be used due to 

the potential for soil arching above the conduit creating potential seepage paths 

through the embankment. To avoid installing a negative projecting conduit, the side 

slopes of the trench excavation must be sloped 2H:1V or flatter. The conduit then 

behaves as a positive projecting conduit. In addition, sloping trench sides facilitate 

compaction and bonding of the backfill with the sides of the excavation. 

Special precautions are necessary to prevent floating the pipe during concrete 

placement. These precautions may include: (1) strapping the pipe to anchors and (2) 

welding or bolting end caps on to the pipe and filling it with water. Since each 

installation is unique, it is recommended that the pipe manufacturer or supplier 

should be contacted for recommendations. Often, they have installation manuals 

with specific instructions on how to prevent pipe movement during construction. 

82 
Chapter 3—Structural and Hydraulic Design 

The reinforced cast-in-place concrete encasement should be completed within one 

monolithic placement around the pipe to prevent the need for construction joints. 

An alternative to a monolithic placement is the use of properly treated horizontal 

construction joints. Regardless of how the encasement is constructed, a diaphragm 

filter system is recommended. For guidance on filters, see chapter 6 in FEMA’s 

Technical Manual: Conduits through Embankment Dams (2005). 

3.5.2.3 Unreinforced cast-in-place concrete encasement (completely encased) 

Complete encasement of conduits for plastic pipe in unreinforced cast-in-place 

concrete is sometimes used with low hazard potential embankment dams, but would 

not be acceptable in significant or high hazard potential dams. Concrete is strong in 

compression, but weak in tension. The reinforcement in concrete carries the 

tension. Also, reinforcement prevents encasement joints from opening. As with a 

continuous concrete cradle, complete concrete encasement eliminates compaction 

concerns under pipe haunches. However, when the concrete encasement is not 

reinforced, it is not structurally adequate to withstand large fill heights, without 

relying on the strength of the interior plastic pipe. Since both cross-sectional and 

longitudinal deflection of the plastic pipe is restrained, encased plastic pipe design 

principles should be applied. Additional research is proposed in section 8.1.2 

(EM-9) to further investigate the use of unreinforced concrete encasement. 

As discussed in section 3.5.2.2, the pipe should be designed for external hydraulic 

pressures, equal to the full reservoir head. Experience has shown that concrete 

encasements do not bond with the outer surface of the plastic pipe, allowing a 

potential seepage path for reservoir water between the pipe and concrete. When the 

pipe is unwatered, full reservoir head is transferred to the pipe exterior, and collapse 

can occur. 

3.5.3 Controlled low strength material (flowable fill) 

Controlled low strength material (CLSM), also known as flowable fill, is a selfcompacted, cementitious material used primarily as an encasement in place of a 

compacted backfill. Use of low strength materials began in the early 1970’s for 

trench backfill, pavement base, and other construction applications. More recently, 

CLSM has been used as an encasement material for flexible pipe embankment 

conduits in low hazard potential applications (figure 51). CLSM should not be used 

in significant and high hazard potential dams until further research is performed to 

evaluate the potential concerns discussed in section 3.5.3.2. 

CLSM is defined as a cementitious material that is in a flowable state at the time of 

placement and has a specified compressive strength of 1,200 lb/in2

 or less at the age 

of 28 days. Most applications require unconfined compressive strength of 300 lb/in2 

or less, which is equivalent to a compacted fill. CLSM is made of Portland cement, 

83 
Plastic Pipe Used in Embankment Dams 

Figure 51.—The CLSM is typically transported to the construction site in 

ready mix concrete trucks. In this figure, CLSM is being used as a pipe 

encasement. CLSM should not be used for embankment conduits in 

significant and high hazard potential dams. 

fly ash, water, fine aggregate and sometimes entrained air. The Portland cement type 

should be appropriate for most project requirements and site conditions. Typically, 

CLSM uses Type I or Type II Portland cement. The cement provides cohesion and 

strength. The fly ash improves flowability while reducing shrinkage and 

permeability. Water is necessary for flowability and hydration. Aggregate is the 

major constituent of CLSM. Usually ASTM C 33 fine concrete aggregate is used. If 

the fine aggregate does not meet ASTM C 33 (i.e., reactive, slaking), care must be 

taken to assess how the aggregate affects the CLSM performance. For example, 

aggregate with a higher fines content or a finer gradation will adversely affect the 

flowability. 

The most critical parameter for use of CLSM as an embankment conduit encasement 

material is the strength. The strength must be kept low so that the CLSM can 

accommodate deformation of the pipe without creating large cracks and maintains a 

seal around the pipe. If the CLSM is too strong, the CLSM will be more brittle and 

may not accommodate deformation of the pipe. This could result in large cracks 

that may create seepage paths. The water content also affects the strength of the 

CLSM and the amount of bleed water that forms. Excessive bleed water may 

accumulate under the pipe, resulting in a void. Excessive water can cause shrinkage 

and cracking. Quality control is important for any successful application of CLSM. 

Proper attention must be given to uniformity of materials used in the mix design, 

equipment, and transportation of the mix to the project site. Specifications using 

ASTM standards C 33, C 150, and C 618 for concrete aggregate, cement, and fly ash 

84 
Chapter 3—Structural and Hydraulic Design 

respectively will help to ensure uniformity and control flowability and strength in 

CSLM mixtures (Brewer and Hurd, 1993, p. 29). 

Air entrainment can be used to improve flowability, as well as to limit the maximum 

strength. Air entrainment is also used to reduce bleed and segregation. The 

additional cost of this or other admixtures must be weighed against the benefits. 

Mixture proportioning of the CLSM is critical to achieving the required performance. 

The mixture must be appropriate for the project requirements and site conditions. 

The Department of Transportation or Department of Roads in some States has 

CLSM mix designs for backfilling culverts which might be applicable for use. Trial 

batching of the CLSM should be performed in a laboratory and at the batch plant. 

Due to differences in cement, fly ash, and aggregate sources, it is important to trial 

batch using the materials that will be used at the project. Field testing should verify 

that the mix and placement methods are within specification requirements for dams. 

Some considerations for material selection are sulfate resistance, improved 

flowability, thermal reduction, and bleed and segregation control. For instance, 

CLSM made with natural sand will be more flowable than the same proportioned 

CLSM made with manufactured sand. Trial batching needs to consider the time 

effects on the CLSM during placement and then with regard to strength gain. The 

trial batch should be evaluated with respect to how long the CLSM maintains 

flowability over time. Flowability should be retained long enough to complete the 

entire placement or lift. Another important consideration is that CLSM continues to 

gain strength with time. Compressive strength should be evaluated with trial 

batching over time. Strength tests should be made at least at 7, 28, and 90 days, but 

3, 14, and 56 days would also be beneficial to evaluating performance. Although the 

CLSM is specified by a 28-day strength, additional strength gain beyond 28 days may 

result in the CLSM becoming stronger and more brittle than what is appropriate for 

the application. Testing durations longer than 90 days should also be considered 

when the project schedule allows for advance testing of the CLSM. Other 

considerations include evaluation of the bleed and shrinkage characteristics during 

trial batching. 

CLSM should not be confused with lean mix concrete. Lean mix concrete is a term 

used for reducing the cement in a Portland cement concrete mixture and is usually 

designed using Portland cement concrete principles. CLSM is not designed by 

Portland cement concrete principles (Brewer, 1990, p. 109). CLSM is not designed 

to resist freezing or abrasion. This should not be a problem in dam construction, 

since most applications are buried. The designer should be aware that the quantity 

of CLSM used in low hazard potential applications is small and there may not be any 

significant cost savings compared to lean concrete. CLSM should also not be 

confused with soil cement, which is a much drier mix and requires compaction. 

Although internal concrete vibrators may be used to facilitate the flow of CLSM to 

ensure that no air is trapped under the pipe, compaction is not required. For 

85 
Plastic Pipe Used in Embankment Dams 

additional basic information on CLSM (see ACI’s Controlled Low-Strength Materials, 

1999). 

3.5.3.1 Design considerations for using CLSM 

A concern with the use of CLSM as an encasement material is the hydrostatic 

pressure it exerts (ACI, 1999). CLSM is not self-supporting and places a load on the 

pipe. For large, flexible wall pipes, CLSM should be placed in lifts, so that lateral 

support can develop along the sides of the pipe before fresh CLSM is placed over 

the pipe. Since a plastic pipe will deflect into cured CLSM, loads are determined 

using the prism theory (section 2.1.1). Although it is tempting to place CLSM in a 

vertical walled trench, care should also be taken to adequately slope the sides of the 

pipe trench to make sure the conduit behaves as a positively projecting conduit. 

As with other encasement materials, plastic pipe encased in CLSM should be 

checked for external water pressure and vacuum pressure as well as internal loading 

conditions. 

As discussed in section 3.1.3, deflection is a function of the wall thickness and the 

soil structure interaction, and is calculated using the Modified Iowa Equation. 

Although CLSM and soil behave similarly, soil variables used in the Modified Iowa 

Equation are different for CLSM. Some research has shown that CLSM results in 

less horizontal deflection than for conventional backfill (Brewer and Hurd, 1993 p. 

28). As shown in table 4 in section 3.1.3, Efor soils range from 50 for fine grained 

soils to 3,000 lb/in2

 for crushed rock. Laboratory tests have been performed to 

determine correlations between CLSM compressive strength, Young’s modulus, and 

E' (Brewer, 1990, p. 118). For 100 lb/in2

 compressive strength CLSM, E' ranged 

from 1,000 to 1,800 lb/in2

, depending on the Poisson’s ratio of the CLSM (which is 

a function of the aggregate filler). Also, Efor CLSM does not increase with depth 

of cover since CLSM does not consolidate. Efor CLSM also does not depend on 

compaction. Additional research is necessary to better define the modulus of soil 

reaction for CLSM (see chapter 8, research need, EM-3). 

3.5.3.2 Problems with using CLSM 

CLSM shows promise as an encasement material since it provides adequate support 

under pipe haunches, is easy to place, and does not require compaction. However, 

there are several uncertainties with the use of CLSM. Additional research is needed 

to evaluate the various performance considerations of CLSM before it can be 

recommended for use in significant and high hazard potential embankment dams. A 

number of research needs related to CLSM are proposed in section 8.1.2 (EM-3 

through EM-8). Due to the number of uncertainties currently existing with the use 

of CLSM and until additional research is completed, it is recommended that CLSM 

only be used in low hazard potential embankment dam applications. 

86 
Chapter 3—Structural and Hydraulic Design 

The main concern with CLSM is that shrinkage and cracking would create seepage 

paths. CLSM cracking can create localized stresses in the pipe wall. Shrinkage and 

cracking tendencies depend on the mixture proportioning, and need to be evaluated 

during the trial batching of the CLSM. There is also concern that the heat of 

hydration that is created when the cement and fly ash react could cause pipe 

deformation and affect the contact between the pipe and CLSM. In addition, bleed 

and segregation tendencies need to be evaluated since this may affect the contact of 

the CLSM with the pipe. In-place performance should be investigated with regard to 

the effects of bleed, shrinkage, and cracking. Also, the placement of CLSM in lifts 

should be evaluated to ensure that the cold joint between lifts does not create a 

seepage path. 

CLSM is assumed to behave similarly to soil, allowing cross-sectional pipe deflection. 

The compressive strength of the CLSM influences the amount of pipe deflection. 

Laboratory testing is needed to determine recommended compressive strength for 

the use of CLSM as encasement in dam applications. Additional research is needed 

to better quantify the modulus of soil reaction for CLSM. Full-scale laboratory tests 

would also be useful in evaluating the response of plastic pipe encased in CLSM 

exposed to large vertical loads. 

When using CLSM with plastic pipes, the absence of soil overburden will cause the 

pipe to float because the weight of the pipe does not offset the uplift forces of the 

CLSM. The designer will need to properly anchor the pipe to prevent floatation. 

The use of multiple lifts can reduce the uplift forces acting on the pipe by the CLSM. 

3.5.4 Grout 

From a structural design perspective, grout can be considered an encasement 

material. For example, when sliplining a deteriorating conduit, the presence of grout 

in the annulus between the slipliner and the existing conduit prevents the plastic pipe 

from deflecting. As the existing conduit continues to deteriorate, external soil loads 

can be transferred to the interior pipe/grout system. With deflection limited, the 

interior plastic pipe may be designed using encased plastic pipe design principles 

(section 3.3). 

A common conservative approach is to not consider the existing conduit in 

calculations for the design of the plastic pipe slipliner. Also, when checking the 

buckling resistance of the slipliner, conditions during grouting and after grouting 

must be considered. During the grouting operation, the slipliner is not confined and 

its unconfined buckling resistance versus the grouting pressure must be checked. 

Once the pipe is grouted in place, the long term buckling resistance should be 

checked assuming the slipliner is exposed to external hydrostatic pressure, as 

discussed in section 2.2.3. External hydrostatic pressures can be transmitted to the 

87 
Plastic Pipe Used in Embankment Dams 

plastic pipe through voids in the exterior pipe and shrinkage cracks in the grout. The 

plastic pipe must be designed to withstand full reservoir head and maximum 

embankment soil load. For additional guidance on grouting of the annulus, see 

section 12.1 in FEMA’s Technical Manual: Conduits through Embankment Dams (2005). 

The most common grout used for filling the annular space is cement grout. Cement 

grout is a mixture of cement, fly ash, and water. A superplasticizer is often utilized 

to facilitate pumping of the grout. The grout used as an encasement material 

typically has 28-day compressive strength of approximately 4,000 lb/in2

. For large 

diameter conduits, grouting is often done in stages to prevent floating the slipliner. 

Cement grouts can be ordered from the local ready mix company, making it readily 

available and inexpensive. For neat concrete grout (cement and water) and 

prepackaged dry grout mixed with water, a standard grout mixer/pump allows onsite 

mixing at the point of placement. Since specialized equipment is not required, the 

overall cost of using cement grout is low. One concern with cement grout is that the 

pressure used for pumping must be closely monitored to avoid danger associated 

with collapsing the plastic pipe. Also, verification of the complete filling of the 

annulus space is not achievable. Close attention to actual grout quality used must be 

made during construction. 

Cellular grout should not be used in embankment conduit applications in significant 

and high hazard potential dams due to its porous nature and lack of strength as an 

encasement material. Cellular grout is a mixture of cement, water, and foam. And 

generally has a 28-day compressive strength of approximately 300 lb/in2

. Specialty, 

licensed contractors generally install cellular grout. The contractor has specialized 

equipment for generating the foam. The foam is usually introduced into the grout as 

it is being pumped, and this develops the most stable mixture. 

3.5.5 Comparison of embedment and encasement materials 

Many factors should be evaluated when choosing an embedment or encasement 

material. Table 8 compares the different types of embedment and encasement 

materials, and some of the issues that should be evaluated when making a selection. 

A thorough understanding of table 8 is crucial in the selection of the proper 

embedment and encasement material. 

3.5.6 When to use flexible or encased plastic pipe design 

The preceding sections have demonstrated that the behavior of plastic pipe depends 

on its surrounding medium, whether it is soil, concrete, CLSM, or grout. Table 9 

summarizes the various situations where flexible and encased plastic pipe designs 

apply. When soil is the embedment material, the pipe can deflect, allowing the 

interior prism to settle more than the exterior prisms. Flexible plastic pipe design 

theory should be used (section 3.1). A flexible conduit surrounded by soil installed 

88 
Table 8.—Comparison of embedment and encasement materials

Soil Concrete CLSM Grout

Mechanical Yes, placed in Vibration required No, consolidates No

compaction compacted lifts under its own weight 

required

Weather issues Yes, cannot be Special protection Must be protected Must be protected

placed in rain; required after from freezing until it from freezing until it 

special placement in cold cures cures

protection weather; surface

required in cold integrity deteriorates

weather in rain

Placement time Slow, placed in In most cases, can be Generally rapid Generally rapid

lifts done with a single placement; cures 

placement; use of rapidly

reinforcement

requires additional

time; slow curing

Permeability Easily Low when not Similar to compacted Low, with proper mix

controlled— cracked granular fills; must proportioning and 

Depends on soil use higher fines injection methods

type content to decrease

permeability;

shrinkage and 

cracking may be a

concern

Pipe support Difficult to Excellent Excellent Excellent

compact

adequately under

haunches

Homogeneous No Yes; with proper mix Yes; with proper mix Yes; with proper mix

proportioning proportioning proportioning

Strength of pipe Derives most Reinforced: strength Derives strength from Encased pipe design

strength by a function of concrete deflecting into necessary (deflection

deflecting into and reinforcement; flowable fill; mix is limited)

soil Unreinforced: design critical for 

encased plastic pipe proper performance;

design may require deflection less than

lower SDR (stiffer with soil backfill

pipe) than a pipe

enclosed in well 

compacted soil

Construction Pipe can move Pipe restraints Must prevent pipe Spacers required to

concerns during necessary to prevent from floating during keep pipe from 

compaction, pipe from floating; placement; pipe must floating and to

difficult to pipe must be be designed to promote even 

restrain designed to withstand withstand load CLSM distribution of grout

the load concrete places on pipe during in annular space;

places on pipe during placement bridging can be a 

placement concern

Primary Compaction Construction joints Potential for high Full encapsulation

concerns under haunches can be source of permeability; with grout is rarely

is difficult to seepage and stress potential for cracking achievable and

achieve concentrations and seepage paths. E' cannot be confirmed

value is not well in field 

known.

Chapter 3—Structural and Hydraulic Design 

89 
Table 9.—Flexible pipe design versus encased pipe design for plastic pipe, as a function of 

encasement material

Positive projecting Trench

conduits conduits Flexible Encased

(Embankment (Drainpipes plastic plastic

conduits and not under pipe pipe

Embedment/encasement drainpipes under embankment design? design?

material embankment fill) fill) (sec 3.1) (sec 3.3) 

Soil embedment Prism (trench Prism Yes

condition)

No

Reinforced concrete cradle to Marston (projection Not applicable No Yes

springline condition)

Reinforced cast-in-place Reinforced concrete design principals apply 

concrete encasement

Unreinforced cast-in-place Marston (projection Not applicable No Yes

concrete encasement condition)

CLSM placed in lifts Prism (trench Not applicable Yes

condition)

No

Grout Marston (projection Not applicable No

condition)

Yes

Plastic Pipe Used in Embankment Dams 

as a positive projecting conduit (i.e., located under embankment fill) is considered a 

positive projecting conduit in the trench condition, since the deflection of the conduit causes 

the interior prism to settle more than the exterior prisms (refer to figure 29). The 

soil load on a conduit in the trench condition is typically less than the weight of the 

fill above the conduit (soil prism load). Thus, soil prism load theory is conservative 

and should be used (section 2.1.1). A flexible conduit surrounded by soil that is 

installed as a trench conduit (i.e., located beneath natural ground), where the interior 

prism settles more than the exterior prisms (refer to figure 27), behaves similarly. 

The soil prism load should be also used. 

When the plastic pipe is encased in a rigid material such as concrete or grout, 

deflection is limited. Encased plastic pipe theory should be used (section 3.3). An 

encased conduit installed as a projecting conduit (located under embankment fill) is 

considered a projecting conduit in the projection condition, since deflection is limited and the 

exterior prisms settle more than the interior prism (refer to figure 28). The soil load 

on the conduit is greater than the weight of the fill above the conduit. Thus, the soil 

prism theory underestimates the load. Marston load theory should be used in this 

situation (section 2.1.2). 

If the plastic pipe is encased in CLSM placed in lifts, the CLSM behaves similarly to 

soil. Deflection occurs, and the interior prism settles more than the exterior prisms. 

Flexible pipe design theory and soil prism load theory should be used. 

90 
Chapter 3—Structural and Hydraulic Design 

If the plastic pipe is encased in a single placement of CLSM, one of the concerns is 

that lateral support does not develop and a load is placed on the pipe until the CLSM 

cures. If there is no lateral support, flexible pipe design theory cannot be used; loads 

must be determined using encased plastic pipe design theory. Eventually, after the 

CLSM sets, flexible theory can be used, but during curing, which is the worst case 

situation, the conservative encased plastic pipe theory should be used. 

3.6 Expansion and Contraction 

All pipes expand and contract with changes in temperature. The designer needs to 

consider these changes and the effects on the selected length for installed pipe. 

Table 10 presents approximate coefficients of thermal expansion. In buried 

applications, the pipe will not typically experience significant changes in temperature, 

and thermal stress or dimension change will be minimal. However, changes in the 

ambient temperature prior to backfilling around the pipe may lead to excessive 

expansion or contraction. Plastic pipe may also experience a change in temperature 

once it is buried if the ambient temperature or temperature of pipe exposed to the 

sunlight is different than the buried condition. Contraction of the pipe can also 

occur if the water released is colder than the pipe’s installation temperature during 

construction, or if the pipe is drained in the winter and open at the downstream end. 

If the conduit is empty during the winter and an air vent is provided at the upstream 

end, a cold draft can develop causing freezing conditions. 

Table 10.—Coefficient of thermal expansion 

Pipe material Coefficient, in/in/°

F

PVC 3.0x10-5

HDPE 1.2x10-4

Source: AWWA, 2002 

Any change in pipe length due to thermal expansion or contraction depends on the 

pipe material’s coefficient of thermal expansion and variation in the temperature. A 

pipe restrained or anchored at both ends will experience a change in stress with 

changing temperature due to expansion and contraction. The stress due to 

temperature change should be less than the allowable stress represented by the 

hydrostatic design stress for the plastic material. The longitudinal stress in the pipe 

wall due to temperature changes may be estimated by: 

SEC = EαΔT (3-23) 

91 
where: 

SEC = stress due to temperature change, lb/in2 

E = short-term modulus of elasticity, lb/in2 

α = coefficient of thermal expansion, in/in/°F 

ΔT = change in temperature, °

F 

The modulus of elasticity of plastic pipe is a function of the temperature. Heat 

transfer occurs at relatively slow rates through the wall of the pipe and temperature 

change does not occur rapidly. The average temperature is often recommended for 

use in determining the appropriate modulus of elasticity. The modulus of elasticity 

should be adjusted for temperature by the factors shown in table 7 in section 3.1.4. 

3.7 End Restraint Design 

Often, the friction between the soil and the plastic pipe or the slipliner and the grout 

surface provides enough restraint against the forces of expansion and contraction. 

However, if a structure such as an intake tower, principal spillway riser, impact basin, 

or manhole is intended to resist the expansion and contraction forces, the structural 

design and stability of the structure must consider these forces. If restraints are 

buried in the soil, the bearing capacity of the soil must resist the forces from 

expansion and contraction. The end thrust (force) due to expansion or contraction 

may be estimated by: 

F S = EC A

pw (3-24)

The required area of an end restraint in soil may be determined by: 

F AR = qAll

Plastic Pipe Used in Embankment Dams 

 (3-25)

where: 

F = force due to expansion or contraction of the pipe, lb 

SEC = stress due to temperature change, lb/in2 

APW = area of the pipe wall, in2

/in of the pipe length 

AR = required area of the end restraint, ft2 

qAll = allowable soil bearing capacity, lb/ft2 

3.8 Other Design and Construction Considerations 

Other design and construction considerations are briefly discussed in the following 

sections. The pipe manufacture or supplier should be consulted for additional 

92 
Chapter 3—Structural and Hydraulic Design 

guidance. Often they have installation manuals with specific instructions on how to 

avoid problems with their particular brand of pipe. 

3.8.1 Foundation problems 

A nonuniform bedding can result from unstable or variable foundation materials, 

nonuniform compaction, collapsible soils, soft clays, and undermining or erosion 

from water flowing on the outside of the conduit. Fortunately, flexible pipe can 

deform away from many pressure concentrations. Axial bending is rarely a cause of 

failure in a flexible pipe. However, the designer should consider the possibility of 

joints opening due to soft foundations. For reinforced concrete encased plastic pipe, 

joint movement is generally not a concern because the concrete encasement and 

longitudinal reinforcement through the joint limits deformation. 

For nonencased plastic pipe, foundation conditions must be carefully considered. 

Flexible pipe generally has enough flexibility to allow the pipe to conform to minor 

foundation movements without structural distress. However, bell and spigot joints 

can be susceptible to separation and leakage with foundation movement. Openings 

of pipe joints are often not uniform, and large concentrated openings can occur at 

isolated joints. See section 4.3.1 in FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005) for guidance on joints. If soft, loose, expansive, or 

liquefiable soils are present where significant ground movement can be anticipated, 

butt fusion joints should be considered. 

Differential settlement of a valve or other structures to which a pipe is rigidly 

connected can induce high bending moments and shearing forces. A support pad 

should be provided below the pipe and for at least two pipe diameters length under 

the connecting pipe. The support pad should be compacted soil or placed concrete. 

The designer must look at potential for settlement and determine whether the pipe 

and joints have adequate flexibility to withstand anticipated vertical movement. Pipe 

manufacturers can provide information on maximum allowable pipe joint deflection. 

A mud slab can be used to protect the conduit foundation. A mud slab is a 2- to 

6-inch layer of concrete typically placed over soft, wet soil or used to prevent 

degradation that can occur between the time the foundation is excavated and the 

concrete encasement is constructed. The mud slab is commonly placed within 24 

hours of exposure of the foundation to protect the foundation from construction, 

erosion, and environmental causes. 

3.8.2 Leak testing 

Plastic pipe used for embankment conduits should be leak tested before being put 

into service. The purpose of a leak test is to find any defects before they result in 

leakage or rupture. Hydrostatic testing using water is the preferred method, although 

93 
Plastic Pipe Used in Embankment Dams 

other methods are acceptable. Serious safety concerns exist if compressed air is used 

because failure of a portion of pipe or joint could be extremely hazardous to 

personnel. The pipe should be restrained against movement in the event of rupture 

before any pressure is applied. The manufacturers’ recommendations should always 

be reviewed for guidance on leak testing of plastic pipe. 

The most common method of leak testing for HDPE pipe is to butt fuse or 

mechanically join flanges on each end of the plastic pipe (these flanges can also be 

later used to attach a gate or valve to the pipe). End caps can then be bolted onto 

the flanges and the conduit filled with water to 

test for leakage (figure 52). A correctly made 

butt fused joint should not leak. Any joints 

showing leakage must be repaired. No repairs 

should be done while the pipe is under 

pressure. For sliplining applications where a 

plastic pipe has been inserted into an existing 

embankment conduit, repairs are not possible, 

and the entire slipliner will most likely need to 

be pulled back out and the repair made. For 

further guidance on leak testing of HDPE 

pipe, see ASTM F 2164. 

PVC pipe has been used for embankment 

conduit applications in low hazard potential 

dams. Hydrostatic testing is commonly 

performed to prove the integrity of the pipe. 

For further guidance on leak testing of PVC 

pipe, see AWWA’s PVC Pipe—Design and 

Installation (2002). 

All important data from the leak test should be recorded, including pressure, 

duration, ambient temperatures, leaks, and repairs. Drainpipes are assumed to 

operate in a nonpressurized condition and do not require leak testing. 

3.8.3 Thrust blocks 

Typically, plastic pipe used in embankment conduits or drainpipes does not require 

thrust blocks. When needed, a concrete thrust block is often used to transfer the 

tensile loading in the pipe into the surrounding soil. The tensile load in the pipe 

must first be transferred into the concrete. Since the concrete will not grip the pipe 

smooth surface, a branch saddle or flange must be fused to the pipe and embedded 

in the concrete. The concrete thrust block must be sized based on the bearing 

capacity of the soil. Additional information on thrust block design and construction 

is contained in ASTM F 1668 and NRCS’s Structural Design of Flexible Conduit (2005). 

Figure 52.—Leak test being performed

on an HDPE slipliner for an outlet 

works renovation.

94 
Chapter 3—Structural and Hydraulic Design 

3.8.4 Anchors and spacers 

Plastic pipe can float during the installation process and must be anchored prior to 

placement of embankment or encasement material. This is particularly a concern 

with the use of CLSM. Because the difference in unit weights between CLSM and 

water is substantial, the uplift force of CLSM can be greater than two times the 

hydrostatic uplift. The problem with floating pipe is not new to the plastic pipe 

industry. Many manufacturers provide guidelines and recommendations for anchor 

spacing depending on the type of pipe and diameter. There are several options for 

anchoring plastic pipe. The pipe can be anchored with metal straps or rebar placed 

in an X pattern above the pipe and tied into wooden planks placed underneath the 

pipe at regular intervals. Another technique of filling the pipe with water will weight 

the pipe sufficiently to allow placement of concrete or CLSM to facilitate 

compaction of soil around the pipe and provide uniform support along the length 

and therefore minimizing deformation. However, in some applications, the 

additional weight of the water is not great enough to overcome the buoyancy of the 

pipe. For instance, grout has a density greater than water and the pipe would tend to 

float, even if it is filled with water. Designers should also be aware that in shallow 

burial applications, the cover over the top of the pipe must be sufficient to resist 

hydrostatic uplift pressure. Placing concrete and CLSM in lifts can minimize the 

effects of uplift. 

The problem with floating pipe also applies to pipe used in a sliplining application. 

Pipe manufacturers often provide guidance on the proper location of spacers to 

maintain an even annular space around the pipe and facilitate a complete grout seal. 

A thorough discussion on the use of spacers in sliplining applications is provided in 

section 12.1.1 of FEMA’s Technical Manual: Conduits through Embankment Dams (2005) 

3.8.5 Placement temperature 

As discussed in section 3.6, plastic pipe expands and contracts radially and 

longitudinally with changes in temperature. Some of the problems associated with 

expansion and contraction can be overcome with special attention to placement 

temperature. Section 12.1.1 of FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005) provides guidance on techniques that can be used to avoid 

excessive expansion or contraction of the plastic pipe during construction. Although 

this discussion is primarily targeted toward sliplining applications, the information 

can apply to other plastic pipe installations as well. 

Temperatures near or below freezing affect thermoplastic pipe by increasing 

stiffness, vulnerability to impact damage, and sensitivity to suddenly applied stress. 

Significant impact or shock loads against a PVC pipe that is at freezing or lower 

95 
Plastic Pipe Used in Embankment Dams 

temperatures can fracture the pipe. Extra care must be used when placing a plastic 

pipe in freezing temperatures. 

3.8.5.1 Soil as embedment material 

In direct burial installations, embedment material friction normally restrains 

longitudinal pipe movement caused by seasonal temperature changes. If the pipe is 

not anchored at the ends to resist movement, a few inches at each end may expand 

or contract as the temperature changes. This zone will extend into the burial trench 

to a point at which the friction resistance of the embedment material is equal to the 

thermal force. When installing HDPE pipe that is warmer than the soil, a slightly 

longer length may be required to compensate for contraction of the pipe as it cools. 

The change in length as a function of temperature change can be estimated using 

methods described in section 3.6. 

3.8.5.2 Concrete, CLSM, and grout as encasement material 

The effects of heat of hydration on plastic pipe should be evaluated. A significant 

rise in temperature could allow the pipe to expand during curing of the grout, 

concrete, or CLSM. As the pipe cooled, it would shrink back to its original size, 

possibly leaving a gap between the encasement material and the plastic pipe. For 

sliplining applications involving embankment conduits, the annulus between the 

existing pipe and the new slipliner is typically very small (i.e., a few inches). The 

restricted size of the annulus results in a limited amount of grout surrounding the 

plastic pipe. Although no research has been done on the effects of heat of hydration 

caused by the grout as it cures within the annulus, it is suspected that an inadequate 

amount of heat is generated that would affect the thermal expansion properties of 

the slipliner. Additional research has been proposed in section 8.1.2 (EM-2) to 

evaluate the effects of heat of hydration. If the volume of grout is increased 

dramatically relative to the thickness of the plastic pipe, consider the use of additives 

such as flyash, or embedding small diameter pipes to circulate cold water in the grout 

mass to lower the heat of hydration. The volume of grout mass can be minimized by 

selecting the circulating cold water within the plastic pipe or using the largest 

possible plastic pipe diameter for sliplining. This reduces the grout mass in the 

annulus and the possible effects of the heat of hydration. Proper mix portions of the 

grout is usually the most effective method in controlling the heat of hydration. 

3.8.6 Collapse of pipes due to grout pumping pressure 

As discussed in section 2.2.3, external hydrostatic pressure can cause plastic pipe to 

collapse. The external hydrostatic pressure can be exerted by reservoir head or 

grout. The collapse pressure of plastic pipe should not be exceeded while grouting 

the annulus during sliplining. Most manufacturers provide information on the safe 

96 
Chapter 3—Structural and Hydraulic Design 

maximum differential pressures that can be applied to unsupported pipe without 

buckling or collapsing the pipe. The collapse pressure of the pipe may also be 

determined as described in section 3.3.2. Section 12.1.1.2 of FEMA’s Technical 

Manual: Conduits through Embankment Dams (2005) provides several options for 

grouting the annular space that minimize the potential of pipe collapse. The Upper 

Wheeler Dam case history in appendix B illustrates the potential for pipe collapse 

during the grouting process. 

3.8.7 Air venting 

As with any conduit system, adequate air venting is critical. Extrusion welding is 

commonly used to join an air vent to HDPE conduits. For guidance on the location, 

airflow rates, and structural considerations of air vents, refer to Air-Water Flow in 

Hydraulic Structures (Bureau of Reclamation, 1980). 

3.8.8 Seepage 

Seepage along the contact between the embankment conduit and encasement 

material is a special design concern. Because there is no bond between the pipe and 

the embedment/encasement, a seepage path can develop. This problem can be 

worsened by radial expansion and contraction of the pipe with temperature change. 

Contraction can cause a void to develop between the pipe and embedment/ 

encasement and result in a seepage path along the outside of the pipe. Additional 

research has been proposed in section 8.1.2 (EM-1, EM-10 and EM-11) to further 

evaluate the concern. 

This problem is of greater concern with plastic pipe because of the high coefficient 

of thermal expansion. The contraction could result from: (1) cooling of the 

concrete during curing, (2) venting of a drained outlet pipe in cold weather, and (3) 

change in temperature of the pipe after the pipe fills. The use of a diaphragm filter 

around the pipe mitigates some of this concern. Diaphragm filters are described in 

detail in chapter 6 of FEMA’s Technical Manual: Conduits through Embankment Dams 

(2005). 

The Sediment Control Pond SP-4 Dam and the Sugar Mill Dam case histories in 

appendix B illustrate the concerns with seepage along the conduit. 

3.9 Hydraulic Design of Embankment Conduits 

The hydraulics design principles for HDPE and PVC pipe are well established. 

Many empirical formulas and equations are available to the designer to solve 

problems involved with flow through plastic pipe. Therefore, this section will not 

provide detailed hydraulic analysis, but will direct the reader to appropriate resources 

97 
Plastic Pipe Used in Embankment Dams 

where this information can be found. The following recommended references 

provide guidance and sound engineering principles for the hydraulic design of outlet 

works, spillway conduits, power conduits, and siphons. 

Recommended references include: 

• AWWA’s PE Pipe-Design and Installation (2006) 

• AWWA’s PVC Pipe-Design and Installation (2002) 

• The Bureau of Reclamation’s Design of Small Dams (1987a) 

• FEMA’s Conduits through Embankment Dams (2005) 

• Natural Resources Conservation Service Hydraulics (1956) 

• USACE’s Hydraulic Design of Reservoir Outlet Works (1980) 

For guidance on drainpipe hydraulics, see section 4.1.2. The reader should also 

consult the “additional reading” section of this document. Additional references are 

provided to further the reader’s understanding of topics related to plastic pipe and 

hydraulics. 

3.10 Renovation, Replacement, and Repair of Embankment Conduits 

As a result of the advancing age of the nation’s inventory of embankment dams, the 

deterioration of conduits through embankment dams is becoming a common 

deficiency that must be addressed. Designers must consider a wide range of factors 

before selecting the method best suited for a particular application. 

For a detailed discussion of methods of renovation, replacement, and repair 

involving plastic pipe, see FEMA’s Technical Manual: Conduits through Embankment 

Dams (2005). Discussions within this reference include: 

• Renovation (chapter 12).—Many embankment conduits are too small to enter for 

renovation. Traditionally, removal and replacement of the entire conduit has 

been one of the most frequently pursued alternatives, but one which can be 

very costly and time consuming. Removing and replacing the entire conduit 

requires excavation of a large portion of an existing embankment dam. 

Removal and replacement typically requires draining of the existing reservoir 

resulting in significant economic impacts. Recently, sliplining small, inaccessible 

conduits using plastic pipe has become the renovation method of choice. 

Sliplining typically results in minimized excavation, shorter construction 

periods, and less construction cost. Sliplining usually requires access to both 

98 
Chapter 3—Structural and Hydraulic Design 

the upstream and downstream ends of the conduit, requiring draining of the 

reservoir or construction of a cofferdam. 

• Replacement (chapter 13).—Removal and replacement of an existing conduit 

generally consists of draining the reservoir or constructing a cofferdam, 

excavating the dam down to the existing conduit, stockpiling the material, 

removing the existing conduit, constructing a new conduit and possibly new 

entrance and terminal structures, installing a filter diaphragm or collar around 

the downstream portion of the conduit, and replacing the embankment 

material. Plastic pipe used in this method for significant and high hazard 

potential embankment dams should be encased in properly shaped reinforced 

cast-in-place concrete to assure quality compaction of earthfill against the 

conduit. Plastic pipe used in low hazard potential embankment dams is 

sometimes not encased in reinforced cast-in-place concrete. However, use of a 

filter zone surrounding the conduit is a valuable defensive design measure, even 

for low hazard potential classification sites with favorable conditions. Some 

low hazard potential embankment dam designs may not employ a filter zone 

around the conduit, but eliminating this valuable feature should be carefully 

considered and justified. Filter diaphragms should only be eliminated when 

extremely favorable soil conditions, good conduit construction materials and 

methods, reliable construction practices, and favorable foundation conditions 

exist. 

• Repair (chapter 14).—Damage to plastic pipe may occur from improper 

shipping, handling, or improper construction technique. Damage can be in the 

form of kinks, punctures, breaks, or abrasion. Plastic pipe that undergoes this 

type of damage cannot be repaired, and the damaged section of pipe should be 

removed and replaced. 

See section 4.1.4 for guidance on the renovation, replacement, and repair of 

drainpipes. 

99 
Plastic Pipe Used in Embankment Dams 

100 
Chapter 4 

Drainpipes and Filters 

Most modern embankment dams designed since about the mid 1970’s include 

drainage and filter zones that are sized to protect against seepage-related failure 

modes without relying solely on a system of drainpipes. Drainpipes provide extra 

capacity in drain systems and provide added conservatism and redundancy to the 

design of these important features. Collecting and measuring seepage flows through 

and under embankment dams is an integral part of safe and reliable monitoring of 

embankment dam performance. This flow is typically collected and conveyed 

through filters and drainpipes as part of a embankment drain collection system. 

Collected seepage can be measured to detect changes in seepage flows that may 

indicate changes in the condition of the dam or foundation, or possible clogging of 

drains. Collected seepage can also be inspected for the presence of sediments that 

may indicate a possible loss of soil materials. 

This chapter will present methodology for the structural design of the drainpipe and 

hydraulic design for the collection of water into the drainpipe. This chapter will also 

discuss the relationship between soil backfill and the drainpipe. Types of backfill are 

separated into several groups, including backfill for perforated and nonperforated 

pipe as well as impervious caps to prevent surface water infiltration. In the 

discussion for perforated pipe backfill, the issue of single versus two-stage filters is 

addressed including the recommended minimum thickness for those materials. 

Placing drainpipes beneath embankment dams in inaccessible locations should be 

avoided. Drainpipes system designs should include inspection wells or cleanouts to 

allow easy access for inspection, cleaning, and maintenance. These features should 

be accessible without disruption of the embankment. Each drainpipe segment 

should be accessible from both ends. 

Example A-4 in appendix A demonstrates the principles involved for drainpipe and 

filter design. 

4.1 Drainpipes 

Drainpipes as described in this document are structural pipes used to convey seepage 

water collected in a drain system to a discharge at some point downstream of the 

101 
Plastic Pipe Used in Embankment Dams 

dam. The materials used for these pipes have changed over time. Early dam 

construction typically used rigid pipe (i.e., clay tile) with flexible plastic pipe 

becoming more popular since the 1980’s. This section will address structural and 

hydraulic design for these flexible pipes. Figure 53 shows an example drainpipe 

construction using flexible plastic pipe. 

A variety of materials and pipe cross sections are available for use as drainpipes. The 

most common materials are HDPE and PVC as described in other sections of this 

document. Commonly available cross sections include solid wall, corrugated single 

wall, and corrugated profile wall as described in section 1.2.1. Single wall corrugated 

plastic pipe is easily crushed during typical construction installations and should not 

be used. Solid wall pipe is available in PVC and HDPE materials. Solid wall PVC 

pipe should be a minimum schedule 80 gauge (schedule refers to the thickness of the 

pipe wall). While solid wall HDPE pipe offers sufficient strength, it is the most 

costly. Since quality of construction can vary, CCTV inspection should be 

performed to verify possible deficiencies within these pipes. The manufacturer’s 

recommendations for installation should always be consulted. Improper installation 

can result in a number of deformations, punctures, etc. Good installation practice 

should always be used. 

Corrugated metal pipes were commonly used in drainpipe systems at one time, but 

deterioration and subsequent piping of surrounding filters into the pipes has caused 

these materials to be regarded as a poor choice. Asbestos cement pipe was also used 

in many drainpipe systems, but the hazard from asbestos in manufacturing has 

caused this product to no longer be available. 

4.1.1 Structural design 

Drainpipes should be structurally designed by the design procedures described in 

chapter 3. The soil and hydraulic loadings on the pipe should be determined by the 

methods described in chapter 2. Drainpipes beyond the footprint of the 

embankment are typically trench conduits while those beneath the embankment are 

typically positive projecting conduits. For guidance on evaluating drainpipe 

configurations to accommodate CCTV inspection equipment, see section 6.2. 

4.1.2 Hydraulic design 

Determining the anticipated seepage that will be collected by a drainpipe and backcalculating the size of pipe required to carry that flow can be complicated. The 

Bureau of Reclamation has developed simple rules-of-thumb for sizing drainpipes 

based on the size of embankment and type of foundation soils in which the drain is 

embedded. Table 11 summarizes those recommendations. Smaller pipes can be 

justified by more detailed flow compilations. Drainpipes should be sized to maintain 

a piezometric surface below the top of ground in most situations. 

102 
Table 11.—Drainpipe diameter based on dam size and 

foundation type 

Dam height (ft) 

Foundation type 

Semipervious or 

Pervious impervious 

< 15% fines >15% fines (SM, GM, 

(SP, GP) ML, CL, SC, GC) 

< 30 min. 12 in min. 8 in 

30–100 12-18 in min. 12 in 

> 100 18-24 in min. 12 in 

Chapter 4—Drainpipes and Filters 

Figure 53.—Installation of profile wall corrugated pipe for a drainpipe 

replacement during a modification of an embankment dam. 

While the load-carrying capacity of nonperforated pipe is well documented, the 

strength of perforated pipe is less commonly addressed. Since the corrugations carry 

the majority of the load for both single-wall and profile-wall HDPE pipe, 

perforations through the corrugation valley have negligible effect on pipe strength 

(less than 1 percent). However, for all types of solid-wall plastic pipe (PVC, HDPE, 

etc), perforations will reduce the load-carrying capacity (loss in strength proportional 

to perforation percent open area). Additional research (PM-3) is needed as proposed 

in chapter 8. 

Solid and profile wall corrugated pipe have the additional benefits of a smooth 

interior, which increases flow capacity, and no interior corrugations to collect and 

trap soil particles (which should be trapped at the measurement point sediment trap). 

Joints for corrugated pipe are typically bell and spigot with a gasket. Solid wall 

103 
Plastic Pipe Used in Embankment Dams 

HDPE pipe has become popular recently for drainpipe applications due to its 

strength and leakproof butt fused joints. The two major limitations in using this type 

of pipe in drainage applications are its cost and lack of factory produced perforations 

(perforations have to be drilled or cut in the field). 

Typically, perforations in drainpipes are available in three geometries: 

• Circles, which are drilled 

• Slots, which are made with a saw blade or furnished from a factory 

• Well-screen configuration 

A fairly wide range of slot widths can be chosen to meet filter criteria related to 

either surrounding drain material or constructed filter zones. For slot-shaped 

perforations, the controlling dimension is the slot width. Typically, the slot length is 

a function of the slot width since the size of the tool used to make the slot is a 

function of the desired width. The slot length is also a function of how far the 

manufacturer advances the tool into the pipe to make the perforation. The 

manufacturer will select a slot length that satisfies desired strength and inflow 

requirements for a particular pipe product. Slotted pipe has considerable higher 

capacity than pipe with circular perforations (see figures 54 and 55). Corrugated 

slotted HDPE pipe is readily available in a variety of pipe diameters (see AASHTO’s 

M252 and M294). If desired, perforations can be drilled into solid wall HDPE pipe 

to provide a perforated pipe. PVC pipe is available with circular perforations as well 

as slots (figure 56). The use of geotextile socks surrounding perforated drainpipes 

should not be used (see section 4.2.1 for additional discussion). 

Figure 54.—Profile wall corrugated 

HDPE pipe with slotted 

perforations. 

Figure 55.—Profile wall corrugated 

HDPE pipe with circular perforations. 

104 
Chapter 4—Drainpipes and Filters 

Figure 56.—Slotted PVC pipe. 

Drainpipes can also be constructed using plastic well screen products. These pipes 

have the largest unit open area and highest capacity of the available products, and 

consequently have the lowest strength. They are usually more expensive than other 

pipe types. 

Consideration should also be given to the amount of inlet open area of perforation 

for a unit length of pipe. As a rule, perforations should be used that incorporate the 

entire pipe circumference (AASHTO Class II Perforation, M252). These patterns 

typically consist of perforations on a 45- or 60-degree pattern (8 or 6 equally spaced 

perforations around the drainpipe, respectively). Perforation patterns that only 

utilize half of the pipe circumference (AASHTO Class I Perforations, M252) should 

not be used due to reduced collection capacity. 

Another consideration is the percent of the openings that will be blocked by the 

surrounding filter material. NRCS Soil Mechanics Note No. 3 (1971, p. A-4) 

recommends that the effective open area for circular perforations be considered as 

30 percent of the total perforation area in computing inflow capacity. For 

rectangular slots, the recommendation is to use 60 percent of the total area of slots 

as the available flow area. The rate of flow into any given pipe per foot can be 

obtained from the manufacturer literature. 

Flow capacity and pipe size of drainpipes can be calculated using Manning’s equation 

or Hazen-Williams equation, or obtained from table B-3 in the Bureau of 

Reclamation’s Design of Small Dams (1987a). The depth of flow in the pipe is typically 

no more than a maximum 75 percent full so that flow does not become pressurized. 

Pressurization limits the effectiveness of the drain. The amount of flow into a plastic 

pipe is a function of the opening size and the number of openings per foot of pipe. 

Perforations or slots in drainpipes are sized to prevent surrounding drain materials 

from passing through them, which would result in a piping condition. If the size and 

number of perforations and slots limits capacity, a well screen type product should 

be considered. Since the requirement for soil retention is to prevent particles from 

105 
Plastic Pipe Used in Embankment Dams 

passing through a perforation, care should be taken to not use slot length values 

as the maximum dimension. An inability to install the pipe uniformly (i.e., no sags 

within segments or from segment to segment) will reduce the flow capacity of the 

pipe (the pipe may flow full through sags). Even correctly installed pipes can 

develop sags after construction due to differential settlement. 

Calculating the amount of water that can be collected from a foundation or 

embankment is not as simple as calculating flow in a pipe. Seepage analysis and 

collection prediction is complicated by lack of data and understanding of geologic 

conditions. Depending on site conditions and the complexity of the foundation, 

seepage analysis can be quite complicated, although lack of data in a small, simple 

foundation can be just as problematic (see USACE’s Seepage Analysis and Control for 

Dams, 1993). 

Depending on the site conditions and the complexity of the foundation, seepage 

analysis can be subject to significant errors. For instance, if high permeability lenses 

are ignored or not detected in an investigation, errors can be dramatic. In simple 

foundations with few strata, computations of flow quantities are more accurate. 

The simplest way of calculating foundation flow contribution into a drainpipe is by 

using Darcy's Law: 

Q = kiA (4-1) 

where: 

Q = rate of flow into a drainpipe, ft3

/yr 

k = coefficient of permeability of the surrounding filter or foundation, 

whichever is greater, ft/yr 

i = hydraulic gradient, head loss outside the pipe divided by the distance 

over which that head loss occurs, ft/ft 

A = filter or foundation area through which flow passes, ft2 

The coefficient permeability may be estimated from empirical relationships, 

presumptive values, laboratory tests, or field tests. Units for the coefficient of 

permeability should be consistent with other terms in the equation. 

Empirical methods for estimating the permeability for filter materials and coarsegrained foundation soils with no fines are available. These methods are usually based 

on the grain-size distribution curve of the materials. Most empirical estimates use 

the effective grain size, or D10 size from a soil or filter’s gradation curve. Some 

estimates use the D15 size, which is obtained similarly. The D10 and D15 sizes 

represent the particle size diameter (in millimeters) of the 10th and 15th percentile 

respectively, passing grain size of a material. 

McCook (2002, p. 5) obtained an empirical relationship for coefficient of estimating 

the permeability of a soil based on its D10 size and porosity. The equation is: 

106 
η 9.3071 × 

100 2 k(cm s) = 0.01047e D10 

Chapter 4—Drainpipes and Filters 

(4-2) 

where: 

k = coefficient of permeability, cm/sec 

e = base of the natural logarithms, 2.7183 

 = porosity, percent of void volume, % 

D10 = particle size diameter in millimeters of the 10th percentile passing grain 

size 

Empirical methods for estimates for the coefficient of permeability for granular 

filters are also available from extensive testing performed by the Soil Conservation 

Service (now NRCS) Soil Mechanics Laboratories, reported in Sherard, et al. (1984). 

The study concluded that for clean sands and gravels in the tests, the D15 parameter 

from the grain size curves provided the best empirical estimate of permeability. The 

empirical relationship for that study is: 

k(cm/s) = CD2 

15 (4-3)

where: 

k = coefficient of permeability, cm/sec 

C = constant ranging from 0.2 to 0.6, averaging 0.35 

D15= particle size diameter in millimeters of the 15th percentile passing 

grain size 

The range for the C value is between 0.2 and 0.6, with an average value of 0.35. The 

range in values is based on the scatter in the data and the differences in materials 

tested. Generally, sands with rounded particles were slightly more permeable than 

those with angular shapes and have higher C values. See NRCS Soil Mechanics Note 

No. 9 (1984) for more details on this relationship. 

A number of sources present presumptive values for soil permeability, such as table 

A1 in the Bureau of Reclamation’s Design Standard for Seepage Analysis and Control 

(1987b), NRCS’s Soil Mechanics Note No. 9 (1984), figure 7.6 in Holtz (1981), table 

2.1 in Peck et al. (1974), and Sherard et al. (1984). 

Common laboratory permeability tests can be found in ASTM D 2434, the Bureau of 

Reclamation’s Earth Manual (1989) and USACE’s Laboratory Soils Testing (1986). 

Foundation field testing can be done in single or multiple drillhole arrangements. 

While field tests give the most accurate prediction, they are also the most expensive. 

Typical testing methods are described in geotechnical engineering textbooks as well 

as the references previously mentioned. 

107 
Caution should be applied when using empirical, presumptive, and laboratory testing 

estimates of permeability. These methods tend to predict higher than actual 

permeability values, so actual seepage flows will be less because stratification and 

heterogeneity of foundation material are not considered. Naturally occurring soil 

deposits also almost always have greater horizontal permeability than vertical 

permeability. Estimates for k, i, and A should be on the high side in order to 

calculate a larger Q in the interest of not undersizing the pipe. Requiring drainage 

capacities 10 times greater than the calculated values is common. 

Seepage analysis using computer software permits more detailed calculations, 

although the limitations of selecting permeability values described above also pertain 

to this method. Computer modeling allows the designer to utilize anisotropy in the 

calculations, which can have a significant effect on seepage calculations. If 

instrumentation data exists for an existing dam, model calibration should be done. 

This calibration consists of modeling known conditions for the existing structure. 

Once the model is constructed, analysis is made for a given reservoir elevation. The 

measured pressures in the instrument are compared to those produced by the model. 

If there is disagreement, permeabilities are adjusted until there is agreement. This 

trial and error method can be time consuming, and it should be noted since the 

number of unknowns exceeds the number of knowns, there are multiple valid 

solutions for any given model. Engineering judgment is required to discern whether 

a valid solution has been found. This calibration scheme aids in reducing uncertainty 

about the permeability and thus the flow. 

The next design issue for drainpipes is determining perforation size. The U.S. Army 

Corps of Engineers (2004, p. B-4) recommends the following criterion: 

Minimum 50 percent size ( D50 ) of filter material >1 

maximum opening of pipe drain (4-4)

The Bureau of Reclamation has adopted two criteria for grain size of filter materials 

in relation to perforation openings in drainpipes (Bureau of Reclamation, 2007, 

p. 14). The first is for use with uniformly graded materials: 

D85 of the filter nearest the pipe > 2 (uniformly graded) maximum opening of pipe drain 

Plastic Pipe Used in Embankment Dams 

(4-5)

This criterion applies to multistage filter/drain combinations surrounding the 

drainpipe. 

The second criterion is based on recent studies by the Bureau of Reclamation (1997, 

p. 24) that indicate that single stage broadly graded sand and gravel filter 

combinations should have a smaller slot size to prevent plugging. The following 

criterion may be used: 

108 
D85 of the filter nearest the pipe > 4 (broadly graded) maximum opening of pipe drain 

Chapter 4—Drainpipes and Filters 

(4-6)

Note: The maximum opening of drainpipe in equations 4-4, 4-5, and 4-6 is the 

diameter for hole perforations and the width for slot perforations. 

The NRCS recommends the following criterion which is about two times larger than 

the previous two other criteria (NRCS, 1994, p. 26-5). For critical structure drains 

where rapid gradient reversal (surging) is probable, it is recommended that the D15 

size of the material surrounding the pipe be no smaller than the perforation size. 

D85 of filter material >1 

perforation size (4-7)

4.1.3 Inspection wells and cleanouts 

Drainpipes should be installed with inspection wells or provided with cleanouts, so 

there is easy access for inspection and maintenance, and reasonable access without 

compromising the dam for repair, if necessary. 

Inspection wells are commonly used along or at the end of drainpipes. They 

typically serve three functions; access to the drainpipe, inclusion of a flow 

measurement device such as a weir or flume, and inclusion of a sediment or stilling 

basin that collects any sediment that may be included in the drainpipe flow. 

Inspection wells are typically manufactured from precast reinforced concrete and 

range in size from 8 to 12 feet in diameter. The base can be a cast-in-place 

(figure 57), or precast and set in place. Combined slab/ring units should not be used 

due to poor strength and difficulty in installation. Experience has shown that 

handling of the combined units has resulted in separation issues. Precast rings come 

in standard lengths and several may be required to reach the desired surface 

elevation. Figure 58 shows the bottom ring of an inspection well. The inlet and 

outlet opening sizes should be supplied to the manufacturer prior to fabrication, so 

proper reinforcement and opening size can be included at the factory. Opening sizes 

are larger than the maximum outside diameter of the drainpipe, so that the pipe can 

penetrate the well and the annulus can be dry packed with a lean concrete. The 

invert of an inspection well is shown in figure 59. See section 4.3.3 for a discussion 

of backfill around inspection wells. 

109 
Plastic Pipe Used in Embankment Dams 

Figure 57.—Placement of a cast-in-place base slab for an inspection well. 

Figure 58.—The first ring of a drainpipe inspection well. 

110 
Chapter 4—Drainpipes and Filters 

Figure 59.—Invert of an inspection well. The sediment trap is painted 

white, so any sediment can be easily observed. The dark material in 

the trap is algae. 

When flow measurement and sediment traps are not required, a more economical 

cleanout type drainpipe access can be used. Cleanouts provide access for CCTV 

inspection equipment and cleaning tools and are used at the upstream end of 

drainpipes. These cleanouts consist of a “sweeping” end from its invert to the 

ground surface by two 22.5-degree bends. This results in the pipe daylighting at the 

ground surface at a 45-degree angle (figure 60). A protective encasement (typically 

CMP pipe) is placed around the plastic drainpipe to protect against vandalism and 

the elements. The encasement pipe should provide a minimum 6-inch free space 

between itself and the plastic pipe. The annular space between the two pipes is 

backfilled with gravel to provide support to the plastic pipe. The encasement pipe is 

embedded a minimum of 5 feet in the ground and a lockable protective metal cover 

is used to secure the end of the cleanout. Figure 61 shows an example of a cleanout. 

Lateral cleanouts can also be used on long drainpipes. The layout of a lateral 

cleanout is the same as described above except the “sweep” consists of 22.5-degree 

bends that transition in both the horizontal (away from the drain alignment) and 

vertical (toward the ground surface) planes. An alternative to the sweep concept can 

be used for drainpipes of great length requiring intermediate cleanouts. A vertical 

riser consisting of nonperforated pipe of the same material and diameter is 

111 
Plastic Pipe Used in Embankment Dams 

Figure 60.—Cleanout designed to accommodate CCTV inspection in pipes with diameters of 

8 inches or larger. 

Figure 61.—A drainpipe cleanout with a steel 

encasement and lockable protective cover. 

connected to the drainpipe. The top of the riser is protected by a CMP pipe, lid, and 

lockable latch similar to end cleanouts. This alternative can be used for drainpipes 

with diameters greater than 12 inches. See section 6.2 for additional guidance on the 

design of drainpipes to accommodate CCTV inspection equipment. 

4.1.4 Renovation, replacement, and repair of drainpipes 

Experience has show that CCTV inspection often reveals damaged or collapsed 

drainpipes. In many cases, the drainpipes appear to have failed during original 

112 
Chapter 4—Drainpipes and Filters 

construction due to equipment travel over the drain alignment, inadequate pipe 

support, pipe material defects, or other factors. In other cases, the drains are in a 

state of failure due to the deterioration of the drainpipe, differential settlement along 

the alignment of the drain. Replacement of the drainpipe may be an appropriate 

response in these situations. Considerations for repairing or replacing the drainpipe 

include: 

• Failure mode.—If considering replacement of a drain, the designer should 

consider relocating the drain alignment, elevations, outfalls, etc. to better 

address seepage conditions at the dam or to reduce or eliminate potential failure 

modes associated with the drainage feature. 

• Address why the drain failed.—In repairing or replacing the drainpipe, the designer 

should consider reasons why it failed, such as poor construction practice, 

reasons related to the pipe material, or drainpipe plugging due to problems with 

the drain envelope material. The repair or replacement should be designed to 

address these issues. 

• Design considerations.—Drain repairs or replacements offer excellent 

opportunities to provide additional access to a drain system. If practicable and 

reasonable, access points should be added at least every 500 to 1,000 feet to 

facilitate inspection, cleaning, maintenance, and monitoring activities. Access 

points should include features for monitoring flow and material movement 

within the drain system, personnel safety features, and access for CCTV 

inspection and cleaning equipment. 

• Quality assurance.—Many drainpipe failures are the result of construction 

activities. Drainpipe repair or replacement projects should include provisions 

for thorough inspection during construction and following completion of 

construction. A CCTV inspection of the drain alignment at the completion of 

construction is required. As-built drawings with accurate surveys must also be 

completed as part of the modifications. 

When damaged or collapsed existing drainpipes are encountered, complete removal 

and replacement may not be possible due to a large amount of fill over the pipe or 

cost constraints. When met with such a situation it may be possible to slipline the 

existing pipe. The simplest way to perform this sliplining is to insert a new pipe into 

the damaged pipe. Since joint offset, deformation, and cracking can lead to a 

significant reduction in interior cross section of the existing pipe, the new pipe may 

have to be significantly smaller than the existing pipe. 

Generally, it is not practical to design the replacement pipe to meet filter criteria. 

Usually the intent of sliplining repairs is to provide structural support to the existing 

damaged pipe. Flow measurement and a sediment trap should be installed at the 

113 
Plastic Pipe Used in Embankment Dams 

downstream end of sliplined drainpipe, so changes in flow and material movement 

can be monitored. 

Introducing the new pipe into the existing pipe can be problematic. If both ends of 

the existing pipe are accessible this will make installation of the liner easier. Having 

to install the liner from one end will be much more difficult and if the amount of 

liner to be installed is large, it could be impossible. 

Successful installation techniques when both ends of the drain are accessible include 

sending a fish line through the segment to be sliplined, attaching a torpedo to the 

fish, with the slipliner attached to the torpedo. The force required to pull this type 

of an arrangement may be large. Mechanical means may be required to make the 

pull, but care should be taken to not exceed the fish line or connection strengths of 

the apparatus. Breaking a fish line, or getting the torpedo stuck in the pipe can lead 

to a bigger problem than what was originally being corrected. 

4.2 Filters 

Properly designed filters adjacent to drainpipes serve two functions—allow 

foundation flow into the pipe, and prevent foundation and embankment soil from 

migrating into the pipe. 

4.2.1 Zoning 

Drainpipes have been designed in single and double stage configurations. A single 

stage system consists of one zone of filter material, usually sand, surrounding a 

drainpipe. The double stage system consists of a coarse drainage zone (gravel) 

surrounding the pipe and a filter (sand) zone surrounding the coarse element. 

Figure 62 illustrates these two types of design. 

Single stage designs have been used on smaller jobs, such as low hazard potential 

dams in the interest of reducing costs and simplifying construction. Because the 

perforations in commonly available drainpipe are too large to meet infiltration 

criteria for typical sand filters, one of the following conditions must be met: 

• The perforated pipe must be wrapped in a geotextile. 

• Screen type pipe must be used. 

• The trench must be lined with geotextile. 

• A broadly graded filter must be used. 

None of these approaches is entirely satisfactory. A geotextile used to prevent sand 

from infiltrating into perforations in the drainpipe can become clogged from ochre 

biofilm. Ochre formation results from microbial colonization by bacterial 

114 
Chapter 4—Drainpipes and Filters 

Figure 62.—Idealized cross sections of single (left) and double (right) stage drainpipes. 

The placement of the drain material around the pipe can result in a variety of geometries 

based on placement method. Minimum cover requirements should always be met 

independently of geometry.

consortia (biofilm) that may include various iron bacteria and its affinity to iron 

compounds (Mendonca, Ehrlich, and Cammarota, 2006, p. 34). Factors that 

influence the formation of ochre biofilm on geotextile include space between fibers, 

roughness of the fibers, and thickness of the geotextile. Geotextile wraps have also 

been known to clog when a filter seal forms caused by concentrated flow through 

the geotextile at perforations in the pipe. The concentrated flow transports (erodes) 

soil particles that concentrate on the face of the fabric. Clogging can more easily 

occur if the surrounding drainage medium contains some fines or the perforations 

are circular holes rather than slots. Most major design agencies, including the Bureau 

of Reclamation, Natural Resources Conservation Service, and the U.S. Army Corps 

of Engineers, do not permit use of geotextiles in critical drain applications due to the 

potential for clogging and particle migration around the edges of thin geotextile 

sections. 

Poor performance of broadly graded filters have been noted in a number of case 

histories and laboratory tests. These filters may have small particle sizes that pass 

through the slots while larger particles may become lodged in the slots. Meeting the 

slot size requirements described in section 4.1.2 is difficult with broadly graded 

materials. Two stage filter/drain combinations have higher permeability and will be 

more efficient in collecting seepage than single stage filters. For these reasons, single 

stage filters should be avoided and high hazard potential dams and two stage filters 

are preferred by designers. However, for economy and simplicity, sometimes single 

stage drainage elements are used in low hazard potential dams. When considering 

this type of filter, consideration should be given to internal stability and plugging of 

perforations within the drainpipe. A number of methods are available to check for 

internal instability and are presented in the literature (Kenney and Lau, 1986; Laflaur, 

Mlynarek, and Rollin, 1989; Milligan 1986; Ripley, 1986). The designer should also 

115 
Plastic Pipe Used in Embankment Dams 

be aware that a broadly graded sand and gravel filter has a lower permeability than a 

uniformly graded sand filter. 

If single stage filters are used, slots should be no larger than the D50 of the filter. 

This can lead to small slot size, which requires the use of screen type pipe or custom 

made perforation. Caution should be exercised in the use of screen pipe due to its 

low strength. 

4.2.2 Determination of filter gradation limits 

Determination of the required gradation limits for filter and drain material is a 

function of the “base” material it is protecting. The current state of practice for 

these limits is that the material performs two functions. The first is that it prevents 

the movement of the base material into the filter and the second is that the filter be 

sufficiently permeable that pore pressures do not build up as a result of the filter 

itself. For a single stage drain, the base material is the foundation soil. For a two 

stage drain, the base material of the outer filter is the foundation soil and the base for 

the inner filter (gravel) is the outer filter. The details for this design are covered in 

chapter 6 in FEMA’s Technical Manual: Conduits through Embankment Dams (2005). See 

example A-3 in appendix A for an illustration of required calculations. Guidance can 

also be found in the NRCS (1994) design standard. 

In lieu of complete filter design, experience has shown that fine concrete aggregate 

designated in ASTM C 33 meets the design requirements for many foundation 

materials with between 40 and 85 percent passing the No. 200 sieve. The No. 200 

sieve must be restricted to meet the permeability requirement of the filter design. 

Table 12 gives the gradation for this material, which is commonly referred to as 

“C 33 concrete sand.” Because foundation conditions differ from site to site, this 

filter should always be checked against the gradation of the base soil (foundation 

soil). Because foundation conditions differ from site to site, this filter should always 

be checked against the gradation of the base materials (foundation soil) before use. 

In a similar manner, when modified ASTM C 33 concrete sand is used as a filter, 

there are standard materials that can be used as the gravel drain that surrounds the 

pipe. Several coarse aggregates in ASTM C 33 have been checked against modified 

C 33 concrete sand and are included in table 13. When using modified C 33 

concrete sand, the coarse aggregates does not have to be checked since the filter size 

is fixed. Six materials have been included since not all materials will be available at 

all locations. 

Based on the D85 size of these materials, the maximum slot size can be calculated as 

described in section 4.1.2 using the Bureau of Reclamation criteria (equation 4-4, 4-5, 

and 4-6, respectively). Table 14 summarizes the resulting perforation sizes. 

116 
Table 12.—Gradation of ATSM C 33 fine 

aggregate with additional requirement1

Percent passing,

Sieve size by weight 

-inch 100

No. 4 95-100

No. 8 80-100

No. 16 50-85

No. 30 25-60

No. 50 5-30

No. 100 0-10

No. 2001

 0-22

1 Note qualifications of No. 200 sieve. 

2 2% stockpile, 5% in-place. For discussion 

of material breakdown, see section 4.3.2. 

Table 13.—Gradation for ASTM C 33 drain materials (percent passing by weight) 

Blend

 No. 467 No. 57 No. 67 579* No. 8 No. 89 

Sieve size D15F  9 × D85B D15F  4 × D85B

2-in 100 - - - - -

11⁄2-in 95-100 100 - 100 - -

1-in - 95-100 100 90-100 - -

3⁄4-in 35-70 - 90-100 75-85 - -

1⁄2-in - 25-60 - - 100 100

-in 10-30 - 20-55 45-60 85-100 90-100

No. 4 0-5 0-10 0-10 20-35 10-30 20-55

No. 8 - 0-5 0-5 5-15 0-10 5-30

No. 16 - - - 0-5 0-5 0-10

No. 50 - - - - - 0-5

* This gradation is a blend, in equal parts, of gradations No. 5, 7, and 9 and is not an 

ASTM standard aggregate. 

Chapter 4—Drainpipes and Filters 

117 
Table 14.—Maximum perforation dimension for ASTM C 33 Drain Materials*

No. 467 No. 57 No. 67 Blend 579 No. 8 No. 89 

USACE

(mm)

0.53 in. 

(13.5)

0.41 in. 

(10.3)

0.35 in. 

(8.8)

0.28 in. 

(7.2)

0.23 in. 

(5.8)

0.16 in. 

(4.1)

Bureau of 

Reclamation

(mm)

0.53 in. 

(13.4)

0.38 in. 

(9.6)

0.35 in. 

(9.0)

0.37 in. 

(9.5)

0.19 in. 

(4.8)

0.18 in. 

(4.5)

* The minimum dimension should be used. For circular perforation, that is the 

diameter; for slots, the width measurement should be used. 

Plastic Pipe Used in Embankment Dams 

While some design standards allow for D15F  9 x D85B, in this instance (Bureau of 

Reclamation, 2007), other standards only allow D15F  4 x D85B (NRCS, 1994). 

Table 13 illustrates which materials meet each standard. The Blend 579 material is a 

blend of the No. 5, No. 7, and No. 9 gradations from the C 33 specification. 

Although it is not a standard ASTM gradation, it is included since it allows a greater 

pipe perforation size as shown in table 14. 

4.2.3 Flow capacity 

When designing drainpipes or other drainage collection systems for pervious 

foundations where seepage is expected to be significant, consideration should be 

given to the permeability of the filter in relation to the permeability of the 

foundation. In situations where the foundation consists of interbedded silts, sands, 

and gravels, design criteria require sizing the filter for the silt sizes. This can result in 

a filter composed primarily of sand sizes being placed over the gravel layers that 

carry the majority of seepage. This filter then acts as a barrier to the flow in the 

gravel, resulting in poor seepage collection and high pore pressures. If this issue 

cannot be resolved by adjusting the filter design, additional water barrier elements 

upstream of the centerline of the dam (i.e., cutoff wall, upstream blanket, or reservoir 

liner) may be required. Figure 63 illustrates an existing (old) drain that produces a 

large flow, although it does not meet modern filter criteria. Figure 64 illustrates the 

barrier situation that can arise for a replacement drain when filter criteria are 

followed. 

4.3 Backfill 

The following sections address two types of backfill around drainpipes. The first 

describes backfill around nonperforated drainpipe, and the second describes backfill 

around perforated drainpipe. ASTM D 2321 also provides guidance on backfill for 

drainpipes installed in trenches with vertical sides. 

118 
Table 15.—Example gradation for 

drainpipe embedment 

Sieve size Percent passing, by weight 

3⁄4-inch 100

No. 4 50-75

No. 50 10-25

No. 200 0-5

Chapter 4—Drainpipes and Filters 

Figure 63.—Old drain. 

Figure 64.—Barrier condition introduced by a replacement drain resulting in poor seepage 

collection and high pore pressure. 

4.3.1 Backfill for nonperforated drainpipe 

For ease of construction and placement, backfill should be a material composed of 

natural gravel and sand, and free of silt, clay, loam, friable or soluble materials, and 

organic matter. Table 15 gives the gradation requirements for an acceptable material, 

although other granular gradation may also be satisfactory. 

No backfill materials should be placed in the drain when either the materials or the 

foundation on which it would be placed is frozen or flooded. No brush, roots, sod, 

or other organic or unsuitable materials should be placed in the backfill. 

Backfill should be carefully placed and spread in uniform layers. Backfill should be 

placed to approximately the same elevation on both sides of the pipe to prevent 

119 
Plastic Pipe Used in Embankment Dams 

unequal loading and displacement of the pipe. The difference in elevation of the 

backfill on both sides of the pipe should not exceed 6 inches at any time. Adequate 

earth cover (minimum 2 to 4 feet) should be provided over the pipe to prevent 

damage to it from construction equipment loads. Figure 65 shows equipment 

passing over a pipe with a 4-foot cover of material, with no damage to the pipe. 

4.3.2 Backfill for perforated drainpipe 

Durability and material quality go hand in hand. Concerns with these characteristics 

are associated with material breakdown during construction. Once they leave the 

processing plant, the aggregate particles can break down during handling and placing 

procedures. Typically, loaders and possibly dozers place these materials in stockpiles 

in order to build larger piles. Then the materials are loaded into trucks, dumped 

onto the fill, bladed to a uniform lift thickness, and compacted. Each of these 

operations can cause individual aggregate particles to break down. This breakdown 

will lead to a change in gradation between the material produced at the sieving plant 

and what is in place in the dam. Typically, filters are required to have no more than 

5 percent fines measured in the fill. Breakdown between the stockpile and fill is 1 to 

2 percent, thus requiring 3 percent limit on the fines when measured in the stockpile. 

While it is beneficial to specify measurement in the stockpile for construction 

operations, testing of the fill should also be done in accordance with ASTM C 117 

and ASTM C 136 to measure the amount of breakdown caused by placement 

operations. The amount of breakdown is a function of the durability of the raw 

material and the amount of handling between the plant and the fill. Breakdown is 

usually a greater concern for smaller grain sizes used for filters than it is for larger 

grain sizes, which are used for drain material. 

As a minimum, the filter material should meet the durability requirements of 

concrete aggregate as defined in ASTM C 33 class designation 1N. In addition to the 

quality requirements of ASTM C 33, the material should be nonplastic. Since it is 

desirable that filter materials “flow” or self heal, adhesion such as plasticity or 

cementing is undesirable. Plasticity can be determined in accordance with ASTM D 

4318 on material passing the No. 40 sieve. Nonplastic material is defined as having a 

plasticity index (PI) of zero as per the previous procedure. Additionally, the material 

should be free of cementing agents, such as, but not limited to, carbonate minerals, 

gypsum, sulfide minerals, and sand-sized volcanic (pyroclastic) ash. Cementing is 

indicated by cohesive behavior of granular material. Cementing agents can be 

detected by checking for reaction of the material to hydrochloric acid. 

McCook (2005, p. 3) suggests performing compressive strength tests on samples of 

fine filter materials to determine if undesirable cementitious properties may be 

present in a given sample. The “sand castle” test proposed by Vaughan and Soares 

(1982, p. 29) may also be helpful for evaluating self-healing properties of sand filters. 

For small projects, it may not be feasible to determine aggregate quality by laboratory 

120 
Chapter 4—Drainpipes and Filters 

Figure 65.—Construction equipment can travel safely over plastic pipe 

when adequate cover above the drainpipe is provided. 

testing. In this instance, the designer should consider the mineralogy of the parent 

material. Aggregates that are derived from metamorphic and igneous based rocks 

will usually have higher quality than aggregates that come from sedimentary rocks. 

For materials obtained from commercial sources, stockpiles should be examined for 

slope uniformity. Piles with irregular slopes or portions of near vertical surfaces 

indicate high fines content or possibly binders or cementing agents in the material. 

4.3.3 Zoning design 

Filter and drainage materials surrounding the drainpipe should have a minimum 

thickness equal to 12 inches. The ease of placement and inspection of the filter and 

drainage material around a drainpipe should serve as a guide to the designer on 

setting the thickness of these materials. The thickness may need to be increased for 

more difficult placements and inspection conditions. For two stage filters, care must 

be exercised to ensure the gravel stage is completely surrounded by the sand stage to 

ensure the foundation does not erode into the gravel stage. 

A capping layer of relatively impervious material (>12% fines) should be used to 

differentiate groundwater and surface water flows in order to more completely 

understand the performance of an embankment dam. For this reason, only 

groundwater flow, or seepage through the dam, should be collected and measured. 

Drainpipes should be designed to isolate the surface flow (by use of drainage ditches) 

and infiltration by the use of a surface cap. Therefore, the drainpipe filter envelope 

121 
Plastic Pipe Used in Embankment Dams 

should be capped with a relatively impervious layer to prevent precipitation from 

entering the drainpipe. 

Relatively impervious material should also be used as backfill around inspection 

wells. This relatively impervious material, or “underground dam” acts as a barrier to 

flow in surrounding drainage materials. This barrier directs this flow into the 

drainpipe and through the measurement device in the inspection well. 

4.3.4 Improving access 

CCTV inspection equipment is sometimes limited by the length of cable tether and 

by sharp bends in the drainpipe. The same is true of drain cleaning equipment. In 

general, CCTV inspection equipment and cleaning equipment can travel up to about 

1,000 feet from the access point (under ideal conditions), depending on the grade of 

the pipe and its smoothness. Some selected equipment may have extended 

capabilities, but 1,000 feet is a good rule of thumb. Some existing embankment 

dams have very long sections of drainpipe with no intermediate access points. 

Access is further complicated by sharp bends in the drainpipe alignment. Sediment 

accumulation, roots, organic debris, or damaged pipes can further limit access to the 

drainpipes. These problems may limit the possibility for inspection, monitoring, 

cleaning, or maintenance of significant portions of the drainage feature. For 

additional guidance on inspection and cleaning of drainpipes, see section 6.2. 

The cost and feasibility of improving access to drainpipes warrant careful 

consideration of the need for such access. When evaluating the need to improve 

access to the drain system, the following factors should be considered, in addition to 

those considerations discussed in the previous section: 

• Constructability.—Constructability has a major influence on the decision to 

provide additional access to an existing drain system. The location and 

configuration of drainpipes vary from dam to dam. Drain alignments near the 

downstream toe of embankments are generally much easier to access than 

alignments deeper under the dam. Some drain systems have multiple 

alignments parallel to the crest of the dam, possibly necessitating multiple 

access points. Others are constructed as a grid under the embankment. Access 

constructability considerations include: 

1. Potential to cause harm 

2. Depth of excavation 

3. Disruption to the embankment and foundation 

4. Need for unwatering and dewatering 

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Chapter 4—Drainpipes and Filters 

5. Need for reservoir restrictions or need to schedule the work during the 

normal reservoir filling and drawdown schedule to facilitate construction 

6. Number of access points needed 

• Alternatives for providing access to drainpipes.—The selection of an alternative should 

be based upon evaluation factors listed under Constructability above. Possible 

alternatives include: 

1. Construct or expose embankment drain outfall.—Many structures have 

drainpipes with buried outfalls. If the embankment drain is located as a 

result of exploration, constructing an outfall would be considered the 

minimum access necessary to provide monitoring capability. This may be 

appropriate in cases where little or no history of flow is apparent in the 

pipe or surrounding area. 

2. Construct access at junction of drain and outfall.—Construction of an access 

point at the juncture of the outfall and drain can be accomplished either 

by casing the excavation, which may be appropriate if the junction is 

located well within the embankment, or by normal excavation if the 

junction is located near the downstream toe of the embankment. Once 

this access is established, additional inspection can be conducted, and 

intermediate access points can be located, if necessary. 

3. Locate access at upstream terminal points of drains.—The upstream end of the 

drainpipe can be utilized as an access point to the drainpipe. The 

advantages of constructing access at the upstream end can include: 

a. Shallower excavation 

b. Less reservoir loading at the point of excavation 

c. Once established, can be used to locate intermediate points of access 

Disadvantages include difficulty in locating the upstream end of the 

drainpipes. Generally, as-built drawings that accurately locate the elevation 

or alignment of the drainpipe do not exist. Locating the drainpipe often 

requires extensive exploratory excavation. 

• Other considerations.—When implementing recommendations to provide 

improved access to drainpipes, the following items should be considered: 

1. Flow and sediment monitoring.—Access points should include provisions for 

measuring flow and monitoring sediment movement through the system. 

123 
Plastic Pipe Used in Embankment Dams 

The access points should include sediment traps and flow measurement 

devices. 

2. Material sampling.—Collect and analyze samples of surrounding 

embankment, foundation, and drain envelope material when installing new 

access points to assess their erodibility and determine if filter criteria are 

met. 

3. Personnel safety.—The access points should include provisions for safe 

entrance and egress for personnel to measure flow and sediment 

accumulation. 

4. Configuration.—The design of improved access must take into 

consideration the size requirements needed to accommodate use of CCTV 

inspection equipment. 

4.3.5 Abandonment/grouting of the drain system 

Abandonment may be an appropriate alternative in cases where the drainage is not 

considered a critical feature in the performance of the dam, where historic flows 

have been small or nonexistent, and where the results of the examination reveal 

damage or failure of the drain system that could lead to a future “incident,” and 

abandonment cannot cause harm. 

Abandonment would likely be most appropriate in those cases where there is not a 

likely failure mode that would lead to failure of the embankment. Rather, this 

alternative could be selected to prevent development of an “incident,” such as 

development of a depression over the alignment of the drain, and may also be an 

appropriate alternative when replacing a drain system. 

When making the decision to abandon or grout the drain, the designer should 

consider temporary measures to evaluate the impact of plugging the drainpipe. The 

designer should assess all sections of the drain to make sure that plugging would not 

cause detrimental pressures to rise. One alternative would be installation of a packer 

to temporarily plug one or more sections of the drainpipe. This would allow 

evaluation of changes in seepage conditions prior to implementing permanent 

measures to plug the drains. An adequate length of time should be allowed for any 

changes in seepage to be monitored. 

Options for plugging the drain system include filling the drain and outfall pipes with 

sand or grouting the drains. The sand alternative would have the advantage of being 

a less permanent measure, in that the sand could be jetted from the drain if changing 

conditions warrant such an action. However, grout may be easier to place and assure 

124 
Chapter 4—Drainpipes and Filters 

complete filling of the drain. The existing conditions within the drainpipe, as 

observed with CCTV inspection, may govern the alternative selected. 

125 
Plastic Pipe Used in Embankment Dams 

126 
Chapter 5 

Construction Guidance 

This chapter describes appropriate construction practices for the placement of 

plastic pipe that functions as an outlet works, spillway, siphon, or drainpipe in an 

embankment dam. Good construction practice is critical in accommodating the 

flexible nature of plastic pipe to avoid creating inherent deficiencies that would result 

in deformation or failure. Key construction issues are foundation preparation, the 

placement and compaction of earthfill, the selection of filter and drainage media, and 

the placement of filter and drainage systems. 

For guidance on transportation, handling, and storage of plastic pipe, the reader 

should consult the manufacturer’s recommendations and guidance provided in 

publications such as AWWA’s PE Pipe—Design and Installation (2006) and PVC 

Pipe—Design and Installation (2002), and the PPI’s Handbook of Polyethylene PIPE (2006), 

AWWA’s, and Uni-Bell PVC Pipe Association’s Handbook of PVC Pipe Design and 

Construction (2001). 

5.1 Embankment Conduits 

If conduits are located on foundations that are not uniform or homogenous, 

differential settlement can lead to problems within the conduit. If foundations 

consist of low strength or highly compressible materials, unacceptable deformations 

and lateral movements can damage the conduit. Zones of designed filter material 

have become the accepted method of preventing failures caused by uncontrolled 

flow of water through the embankment materials and foundation soils surrounding a 

conduit through an embankment dam. Plastic pipe that is used in the construction 

of new significant and high hazard potential embankment dams should be encased in 

properly shaped reinforced cast-in-place concrete to ensure quality compaction of 

earthfill against the conduit. Plastic pipe used in low hazard potential embankment 

dams is often not encased in reinforced cast-in-place concrete. However, use of a 

filter zone surrounding the conduit is a valuable defensive design measure, even for 

low hazard potential classification sites with favorable conditions. Some designs for 

low hazard potential embankment dams may not employ a filter zone around the 

conduit, but eliminating this valuable feature should be carefully considered and 

justified. Filter diaphragms should only be eliminated when extremely favorable soil 

conditions, good conduit construction materials and methods, reliable construction 

127 
Plastic Pipe Used in Embankment Dams 

practices, and favorable foundation conditions exist. For detailed construction 

guidance involving conduits, see FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005). Discussions within that reference include: 

• Understanding the importance of excavation and foundation preparation for 

the installation of conduits. Special attention is needed for any excavations 

made transverse to the centerline or axis of the dam where the excavation 

backfill may be different in compressibility than the adjacent foundation 

materials. 

• How the settlement of the dam near the conduit can create a hydraulic fracture 

mechanism. 

• Recommendations for proper backfilling of embankment materials against the 

conduit. Problematic soils such as broadly graded soils and dispersive clays are 

defined, and the potential problems associated with them are included. 

• The theory behind the concept for using filter zones to prevent erosion of 

earthen embankments near conduits caused by the uncontrolled flow of water 

through soils surrounding conduits that penetrate the dam. 

• The type and configuration of the filter zone depends on site conditions and 

soils used in the embankment dam. Three basic designations for filter zones 

associated with conduits are discussed: filter diaphragms, filter collars, and 

chimney filters. Examples of typical designs used by the major design agencies 

are included. 

5.2 Drainpipes and Filters 

Construction of drainage systems consisting of drainpipes and filters is critical to the 

successful performance of embankment dams. Incorrect drainpipe and filter 

construction techniques can lead to contractor claims and unexpected performance 

during first filling, which can lead to embankment dam failure. 

Corrugated plastic pipe is supplied in coils and straight tubes, depending on the 

pipe’s wall thickness and stiffness. Manufacturers’ recommendations should be 

followed for installation of plastic pipe. While axial bending or “snaking” may be 

permissible for less stiff products to accommodate directional changes in the 

alignment, it should not be allowed for products distributed as tubes. Some small 

changes in direction may be allowable for specially designed gasketed joints in PVC 

pipe, but this should be minimized to avoid the potential for joint leakage. Changes 

in directional alignment for tube products should always be accomplished using 

prefabricated fittings (figure 66), such as elbows or sweeping bends. See NRCS’s 

128 
Chapter 5—Construction Guidance 

Figure 66.—Prefabricated 22.5-degree

bend for profile wall corrugated HDPE 

pipe.

Structural Design of Flexible Conduits, chapter 

52 (2005) or Uni-Bell’s Handbook of PVC 

Pipe (2001), for additional guidance on 

longitudinal bending of plastic pipe. 

At some sites, rodents, snakes, amphibians, 

and other animals may take up residence in 

drainpipes and other small outlets. This can 

be problematic if nests or blockages are built 

by the animals. To prevent animal entry 

into drainpipes, screens, bars, or flap gates 

can be installed at the downstream end. 

Caution should be used when installing 

screens, since they can become clogged with 

sediment or algae growth. Typically, 

clogging by algae growth is a function of the 

screen opening size where smaller openings 

lead to a greater chance of clogging. 

Additionally, the design of the end protection should allow easy access by CCTV 

inspection equipment. 

The following sections will address critical construction techniques for both 

drainpipe placement and earthwork associated with the filter zones. Discussion is 

also presented that addresses issues associated with filter processing and handling 

prior to placement. 

The Ganado Dam case history in appendix B illustrates the importance of proper 

drainpipe installation. 

5.2.1 Foundation preparation 

While foundation preparation is important for drainpipe installations, it is not as 

critical as it is for embankment conduits. The major issue related to foundation 

preparation for drainpipes is the same as for conduits (e.g., settlement). The 

likelihood for differential settlement is greater than for uniform settlement due to the 

heterogeneous nature of most foundations. This differential settlement can lead to 

sags and joint separation, even in well constructed pipe laid to the correct grade 

during installation. For a detailed description of foundation preparation, see 

chapter 5 in FEMA’s Technical Manual: Conduits through Embankment Dams (2005). 

Discussions in that reference, and variance, include: 

• Proof rolling is recommended on soft foundations to help limit differential 

settlement and provide a uniform grade to lay the pipe on. Any offset in grade 

129 
Plastic Pipe Used in Embankment Dams 

produced by the proof rolling can be corrected during the placement of the 

filter material. 

• Rock foundations can be excavated by ripping or blasting. If blasting is used, 

care should be taken not to damage the foundation by incorrect blasting 

techniques. Line drilling is the preferred excavation method in this instance. 

For drainpipe installations, cleaning and backfilling of joints and fractures is not 

required. The amount of cleanup required in rock foundations for drainpipe is 

limited to sufficient effort to produce a foundation that is readily mapped for 

foundation acceptance requirements. Removal of sharp edges and other rough 

surfaces is not required since these irregularities will be covered with filter 

material. 

• Rock foundations in material that is subject to slaking should be cleaned of 

slaked material no more than 24 hours prior to filter placement. Protective 

slabs (mud slabs) are not required for drainpipe installations. 

• Soil foundations should be free of organic material, such as roots and stumps, 

sod, topsoil, wood trash, or other foreign material. Other objectionable 

materials that may require removal include very low shear strength, highly 

compressible, and collapsible soils. 

• Water control and removal are critical for both soil and rock foundations. As a 

minimum, the foundation should be free of water to enable the foundation 

mapping and acceptance to be performed. Placement of filter materials should 

not be made through standing water. Mud or other saturated soil should be 

removed prior to filter placement. 

5.2.2 Placement around drainpipes 

The following steps describe a common method for installation of drainpipe in a 

trapezoidal trench. Figure 67 shows a drainpipe being properly installed. Other 

methods have also been successfully used, but are not discussed in this manual. This 

installation method is for a two stage filter/drain system: 

1. Excavate the trench as shown in figure 68. 

2. Place the filter material on the trench invert and side slopes to the specified

thickness and compact as shown in figure 69. 

3. Place the drain material to a thickness of at least the crown of the pipe. In 

some instances, the entire thickness of drain material is placed as shown in 

figure 70. 

130 
Chapter 5—Construction Guidance 

Figure 67.—Trapezoidal side slopes used in drainpipe construction. 

4. Excavate a small trench in the drain material to the invert elevation of the pipe 

as shown in figure 71. 

5. Install the plastic pipe into the small trench, being careful to prevent debris or 

zone material from entering the pipe as shown in figure 72 (the use of 

temporary pipe caps is recommended). In inclement weather or other 

unsuitable situations, positive measures must be placed at the edges of the drain 

system to ensure that the materials are not contaminated. These may include 

for example, earthen berms, straw bales, and silt fences. 

6. Place drain material around the pipe at select locations (approximately on 5-foot 

centers) as shown in figure 73. This material acts as an anchor so the pipe stays 

in place during the main backfilling. As an alternative, steel rods can be driven 

on either side of the pipe to anchor it. The drainpipe must be constantly 

monitored to ensure that its alignment remains as specified. 

7. Place the remaining drain material over the pipe to the specified elevation as 

shown in figure 74, as this placement is made, ensure that drain material is 

placed in the haunch by hand labor using hand tools leaving no large voids or 

loose material. The drainpipe must be constantly monitored to ensure that its 

alignment remains as specified. 

8. Place the remaining filter material to the specified grade with compaction as 

shown in figure 75. Note: Minimal compaction is needed for the preceding 

drain material placement. For guidance in compaction, see section 5.2.4. 

131 
Plastic Pipe Used in Embankment Dams 

9. Place the final miscellaneous fill or protective cap to the specified grade as 

shown on figure 76. 

Figures 77, 78, and 79 show examples of filter and drain material placement around a 

drainpipe. 

A number of poor practices are commonly encountered in pipe installation and 

should be avoided. These practices typically result in crown collapse or, in the worst 

case scenario, complete crushing of the pipe. They include, but are not limited to: 

• Compaction of backfill using the backhoe bucket by “thumping” or setting the 

bucket on the backfill and lifting the end of the backhoe using the bucket. 

• Wheel rolling either parallel or transverse to the pipe by any kind of 

construction equipment or vehicle. 

• Haul roads or equipment crossing the pipe without sufficient cover. A 

minimum depth of 2 to 4 feet should be provided over the top of the pipe for 

H-20 highway truck loading (front axle load of 8,000 pounds and rear axle load 

of 52,000 pounds) in accordance with AASHTO standards (more depth may be 

required if recommended by the manufacturer). Construction equipment that 

exerts a loading on the top of the pipe larger than H-20 requires special 

consideration, and the contractor and dam owner should closely evaluate the 

proposed crossing method. See section 2.3 for additional guidance concerning 

loading from construction equipment. 

• Not placing or fully compacting material under haunches of the pipe. 

Placing material around plastic drainpipes can lead to damage and poor drainage 

when installed incorrectly. Manufacturers’ literature typically describes how to install 

pipe in vertically sided trenches (often using a trench box such as the “doghouse” 

shown in figure 80). However, vertically sided trenches should not be used for 

drainpipe construction in significant and high hazard potential dams; a trapezoidal 

section is preferred. Low hazard potential dams often utilize vertically sided 

trenches. Pipe installed in vertical trenches encounters arching that occurs in the fill 

above the pipe. This arching reduces load not only on the pipe, but also in the 

adjacent and overlying fill. Since arching is less likely to develop in a trapezoidal 

trench and load on the pipe is greater, pipe installation and backfill are critical in 

order to offer haunch support to the pipe so its maximum strength can be 

developed. 

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Chapter 5—Construction Guidance 

Figure 68.—Trench excavation. 

Figure 69.—Initial filter placement. 

Figure 70.—Drain material placement. 

Figure 71.—Excavate for pipe. 

Figure 72.—Set pipe, place ballast. 

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Plastic Pipe Used in Embankment Dams 

Figure 73.—Backfill haunch by hand. 

Figure 74.—Place remaining drain material. 

Figure 75.—Place remaining filter. 

Figure 76.—Place the final miscellaneous fill or protective cap to 

the specified level. 

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Chapter 5—Construction Guidance 

Figure 77.—Initial filter placement in a trapezoidal trench. 

Figure 78.—Drain material being placed over a drainpipe. 

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Plastic Pipe Used in Embankment Dams 

Figure 79.—Filter material being placed over drain material. 

Figure 80.—A trench box or “doghouse” has been 

used to place material around drainpipes in 

vertically sided trenches in low hazard potential 

dams. Vertically sided trenches should not be 

used in significant and high hazard potential dam 

construction. 

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Chapter 5—Construction Guidance 

5.2.3 Segregation 

Segregation during processing and placement is a common problem. Segregation 

may result in overly coarse filter/drain materials in contact with adjacent finer 

materials, which negates the effect of the filter. Incompatibility at the interface 

materials is the result. Many designers consider that segregation control during 

construction is the most important aspect of constructing a filter/drain. Segregation 

can have a significant bearing on the ultimate performance of the embankment dam. 

Figure 81 shows naturally occurring segregation. 

A common cause of segregation is the manner in which material is handled. Material 

placed in a pile off a conveyor, or loaded from a chute, or from a hopper segregates 

because the larger particles roll to the sides of stockpiles or piles within the hauling 

unit. Material dumped from a truck, front loader, or other placing equipment almost 

always segregates, with the severity of the segregation corresponding to the height of 

the drop. When material is dumped on the fill, segregation occurs. 

Segregation can be satisfactorily controlled in several ways. First, the designer should 

specify a uniformly graded filter or drain. Secondly, construction techniques to 

control segregation should be specified and enforced. Use of rock ladders, spreader 

boxes, and “elephant trunks” for loading hauling units, and hand working the placed 

materials help prevent segregation. If material is dumped, limiting the height of drop 

helps. Placing filter/drain material with belly dumps sometimes adequately limits 

segregation during placement. Limiting the width of the belly dump opening by 

chaining or other means can limit segregation. Using baffles in spreader boxes and 

other placing equipment can help reduce segregation. The personnel inspecting the 

filter/drain production, placement, and compaction should be apprised of the 

importance of limiting segregation. 

5.2.4 Compaction methods for backfill and filter and drain materials around 

drainpipes 

Compaction of filter and drain materials should be adequate to produce sufficient 

density to preclude liquefaction, limit consolidation, and provide adequate strength. 

However, excess compactive effort can cause particle breakdown and reduce 

permeability. Therefore, the amount of compactive effort should be limited to that 

required to produce the required strength and consolidation parameters, yet not 

cause excess particle breakage and unnecessarily high densities which both reduce 

permeability. Thought should be given to the number of passes specified instead of 

just using what has been used previously. If two passes will get the required density, 

then four passes are not justified because they will reduce permeability by causing 

more particle breakdown and increased density. Also, the roller operator should be 

made aware that it is undesirable to continue to roll after the required passes have 

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Plastic Pipe Used in Embankment Dams 

Figure 81.—Naturally occurring segregated soil. During deposition, the 

gravel sizes were segregated from the sand sizes. Poor construction 

practices can lead to similar segregation. The deposit is a broadly graded 

mixture of silt, sand, and gravel and is internally unstable. Note that silt 

has eroded into the gravel sizes and coated the particles. 

been made. The idea often exists that, if two passes are necessary, three are better. 

This may not be the case, and the contractor and his operators should be aware so 

that additional passes are not made to ensure no failing densities or to fill in operator 

slack periods. When the specified density is not achieved during construction, 

typically the cause is insufficient water content in the fill (dry of optimum). A 

contractor may be resistant to applying water to the fill and instead will prefer to 

make many passes in an effort to achieve density. This will lead to material 

breakdown and the required density will still not be achieved. Vigilance should be 

exercised in assuring the contractor has the fill at sufficient water content prior to 

compaction (figure 82). For a pervious filter this may require that the application of 

water immediately precede the roller and in many cases the roller literally follows the 

water truck. 

Most current equipment used for compaction of granular material used today 

possess vibratory capabilities where dynamic loading is used to achieve density. In 

addition to equipment compaction, granular materials can also be densified by 

flooding (i.e., applying sufficient water in order to achieve 100 percent saturation). 

The recommended minimum density ranges from 50 to 75 percent relative density 

(the lower value can be used for small structures in areas of low seismic activity and 

the higher value used for large structures or structures in higher seismic active areas). 

Relative density can be determined in accordance with ASTM D 4254. Whenever 

grain-size limits for filters/drains are specified, the grain-size tests should be made 

on materials compacted to simulate as closely as possible the grain sizes and soil 

structure after particle breakdown caused by construction. Ring permeability tests 

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Chapter 5—Construction Guidance 

Figure 82.—Water being added to filter immediately preceding 

compaction. Note that this placement is occurring during a heavy rain, 

and wetting of the filter must continue. 

made at various levels in test fills are one way to obtain realistic permeabilities 

representing vertical permeabilities of compacted filters and drains. Laboratory 

procedures that closely duplicate field placement and compaction methods can also 

provide reasonable values for levels of permeability to be expected in filters and 

drains. If proposed materials do not have sufficient permeability after compaction, 

changes in grain sizes should be made that will provide the required permeability. 

Also, designers should consider changes in layer thickness or geometries of drains 

that will increase discharge capacity to the required levels, while providing the 

needed filter protection. 

In-place density tests should be taken to verify the required density is being met. 

The sand cone density test, such as ASTM D 1556, will meet this need. Nuclear 

testing (ASTM D 2922) can also be used, although it may underestimate density in 

sand filters. This underestimation can lead to lift rejection and direction to the 

contractor to perform additional compaction. This additional compaction can lead 

to additional breakdown of the material, increasing its fines content. Opinions vary 

on the efficacy of these two test methods. The reader is directed to chapter 6 of 

FEMA’s Technical Manual: Conduits through Embankment Dams (2005) for a more 

detailed explanation of density testing methods and their shortcomings. 

5.2.5 Borrow sources 

In general there are two potential sources for filter/drain material: undeveloped 

sources and existing commercial sources. For small dams it may be cost effective to 

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Plastic Pipe Used in Embankment Dams 

use commercial sources and for larger projects, more economical to develop a new 

source specifically for the job, if suitable undeveloped material exists near the job 

site. The availability and suitability of material must be factored into the design. For 

example, if suitable material is limited in quantity or expensive to obtain, it may be 

more economical to use thin or narrow zones (less than placement equipment width) 

and more intensive placement and inspection techniques to ensure construction of 

adequate filter/drain zones. On the other hand, if ample material is near the job site 

and can be economically developed, equipment width dimensions of filter/drain 

zones with less intensive placement and inspection techniques may be more cost 

effective. The designer must ensure that there is sufficient volume available to 

construct the work. Generally, it is prudent to have at least four times the volume of 

material available in borrow than is necessary to produce the final in-place volume of 

the filter/drain zones. For large jobs a sieve by sieve analysis should be made in 

order to find out which grain sizes are critical for a specific pit. If thinner zones are 

used, the dimensions must be checked for adequate hydraulic capacity, as discussed 

herein. Logical sources must be investigated and, for approved sources, appropriate 

information such as location, availability, ownership, drill logs, test pit logs, 

appropriate lab tests, and geotechnical considerations provided in the specifications. 

Figure 83 shows an example of a typical borrow area and processing plant. 

5.2.6 Contamination 

To avoid contamination of filter/drain zones with excess fines during construction, 

several techniques should be used. The zone should be maintained higher than the 

surrounding fill surface, and the fill should be placed to maintain drainage of surface 

water (and sediments) away from the filter/drain zones. This will prevent the flow 

of muddy water into the filter or drain. Traffic should be well controlled, with 

crossings limited to prepared roadways which will be removed entirely prior to 

placing of additional filter/drain materials (figures 84 and 85). Crossings should be 

Figure 83.—Typical borrow area including processing plant. Produced material is in 

foreground and the plant is in the background. 

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Chapter 5—Construction Guidance 

Figure 84.—Roadway crossing over a filter. Photo courtesy of ASDSO 

(Hammer, 2003). 

Figure 85.—Contaminated materials being excavated beneath a 

roadway crossing over a filter. Photo courtesy of ASDSO (Hammer, 

2003).

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Plastic Pipe Used in Embankment Dams 

staggered to remove any possibility of vertical transmissibility of the filter/drain zone 

being reduced. Durable materials should be specified, and compactive effort held to 

the minimum needed to obtain desired in-place density, to minimize particle 

breakdown during placement and compaction. Equipment for placement and 

compaction of filter/drain zones should be restricted to operation only on the 

filter/drain zones, or cleaned before moving onto the filter, to avoid unnecessary 

internal contamination. Commonly, equipment operators (spreading and 

compacting) will want to move off of the filter/drain zone when their operation is 

done for a particular lift. This will lead to cross contamination between zones and 

should be avoided. Operators should be instructed “once on the filter, stay on the 

filter.” In cold climates, construction seasons are often short. When the 

construction season is terminated, the surface of the filter/drain zones should be 

covered (in addition to surface drainage requirements) and the covering material 

removed completely before the resumption of placement in the subsequent season. 

Contamination can also occur during loading, hauling, placing, and compaction 

because these processes tend to cause breakdown of the materials, sometimes to the 

extent of causing the gradation to be out of specification requirements. Specifically, 

trucks used to haul high fines content material may end up with that material stuck in 

corners of the truck box. When these trucks are then used to haul filter or drain 

material, this material can dislodge and end up in the filter. Truck boxes should be 

clean of such material before hauling filter or drain materials. Contamination can 

occur in the stockpile. Dust abatement control procedures and use of equipment 

around the stockpile that is maintained in a clean condition will reduce this problem. 

Reprocessing or not using the bottom foot or so of the stockpile may be necessary, 

since this is where the greatest contamination of the stockpile generally occurs. 

Generally, the concern is for an increase in the fines content, that is, material finer 

than a U.S. Standard, No. 200 sieve, because these fine materials can drastically 

reduce the filter permeability. However, breakdown of any particle size can be 

detrimental because this may alter the ability to filter or be filtered. 

Generally, the percent fines after compaction should not exceed 5 percent to ensure 

that permeability is not decreased to an unacceptable degree when tested in 

accordance with ASTM C 117 and C 136. To achieve this, the material has to 

contain less than about 2 or 3 percent fines in the stockpile, depending on the 

durability of the particles. Durability requirements equal to those used for concrete 

aggregate as described in ASTM C 33 Class Designation 1N, are preferred, and will 

usually ensure that the material can withstand necessary processes to get them in 

place and compacted without excessive breakdown, and will also help ensure longterm durability during operation. Making the specifications requirement for filter 

material gradation in place after compaction is necessary. In some cases, such as in 

the case when material is preprocessed in a prior contract, after-compaction 

requirements are not desirable. In these cases, specifying clean material (less than 

2 or 3 percent fines in the stockpile) and adequate durability becomes even more 

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Chapter 5—Construction Guidance 

important as well as thorough inspection to ensure the materials are not mishandled 

or over rolled. 

5.2.7 Quality control and assurance 

Individuals responsible for quality control and assurance should be experienced 

professional engineers with at least 2 years experience with design and construction 

of the type of drainpipe system being employed. Quality control and assurance 

should include the following: 

For the subgrade: 

• Equipment.—Visually inspect and verify soil processing, placement, and 

compaction equipment meet the requirements described in the specifications. 

• Weather conditions.—Verify that soil placement, grading, or compaction does not 

occur during periods of freezing temperatures, if it is raining excessively, or if 

other detrimental weather conditions exist. 

• Subgrade preparation execution. — 

1. Subgrade preparation.—(1) Ensure the elevation of the top surface of the 

subgrade is correct. (2) Verify the subgrade is smooth, free of voids, and 

composed of satisfactory materials. Also, verify the subgrade is 

compacted as specified. (3) Standard moisture and density tests are taken 

at the same location as the rapid tests so that results can be easily 

compared. Ensure that large equipment is turned off in the vicinity where 

sand cone tests are being performed. 

2. Subgrade protection.—(1) Ensure the contractor removes puddles and excess 

moisture from the soil surface prior to placement of additional soil, 

bedding, filter, or drain rock. (2) Look for areas of erosion after each 

rainfall. (3) Inspect for damage due to freezing and/or desiccation. 

(4) Ensure the contractor repairs damaged areas and reestablishes grades. 

3. Subgrade repairs.—If the subgrade does not conform to the specifications, 

the designer should assist in defining the extent of the area requiring 

repair. This should be done through the use of additional testing and 

visual inspection. Material from areas that are to be repaired should be 

removed and replaced. After repairs have been made, ensure retests are 

performed to check the repaired areas. In general, tests should be 

performed at the same frequency as the rest of the project. Additional 

testing should be performed in suspect areas. 

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Plastic Pipe Used in Embankment Dams 

For the bedding, filter, and drain materials: 

• Equipment.—Verify equipment used to place and compact the materials are in 

accordance with the specifications and the pipe manufacturer’s 

recommendations. 

• Delivery, storage, and handling.—The inspector shall be present during delivery, 

unloading, and stockpiling and should verify the following: 

1. Materials have not been segregated or mixed with deleterious materials 

during shipping, storage, and handling. 

2. Deliveries are properly recorded. 

3. The correct material type and gradation have been delivered. 

4. The materials are stockpiled with proper protection and handling. 

5. Materials that have been contaminated are rejected before placement. 

• Weather conditions.—Verify weather conditions are acceptable for material 

placement. 

• Material properties.—Verify that material is sampled and tested in accordance with 

the specifications and test results not meeting the requirements specified result 

in the rejection of applicable materials. 

• Installation execution.— 

1. Oversize and deleterious material which could damage the performance of 

the system has been removed prior to placement. 

2. Drainpipes are not being damaged or moved out of alignment by 

placement equipment. Placement equipment should be observed from the 

front side as material is being spread over the plastic pipes. 

3. Excessive fines have not been generated as a result of handling and 

placement of the drainage materials. 

4. Wind-borne and water-borne fines do not contaminate the drainage 

system after placement. 

5. Erosion controls are placed such that drainage systems are not 

contaminated by fines. 

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Chapter 5—Construction Guidance 

6. Watch for ponds of water on top of the drainage system which may be an 

indication that an excessive amount of fines have contaminated the 

drainage materials. 

For the drainpipe: 

• Equipment.—Verify equipment used to place and cover pipe is in accordance 

with the specifications and the manufacturer’s recommendations. 

• Delivery, storage, and handling.—The inspector should be present during delivery 

and unloading and should verify the following: 

1. Pipe and appurtenances are not damaged during shipping, storage, and 

handling. 

2. Deliveries are properly recorded. 

3. The correct material type, strength, and pipe sizes have been delivered. 

4. The size, number and location of pipe perforations are as specified. 

5. Pipes with gouges deeper than 10 percent of the wall thickness are rejected 

or repaired before use. 

6. Out-of-round pipe which cannot be properly joined together is rejected. 

• Weather conditions.—Verify weather conditions are acceptable for pipe placement. 

• Material properties.—Verify that pipe is sampled and tested in accordance with 

the approved manufacturer’s quality control manual and test results not meeting 

the requirements specified results in the rejection of applicable pipe. 

• Installation execution.— 

1. Pipe.—Verify the following during pipe placement: 

a. Pipe is carefully carried or pulled to the place of installation. 

b. Defective or damaged pipe is not used. 

c. Pipe is not laid when trench conditions or weather is unsuitable. 

d. Pipe is not installed if standing or flowing water is present. 

e. Pipe and accessories are carefully lowered into the trench. 

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Plastic Pipe Used in Embankment Dams 

f. Pipe is placed at the lines and grades indicated in the plans and 

specifications. Verify the contractor does not lay pipe on blocks to 

produce the specified grade. 

g. Specified bedding is used and the bedding is graded to provide 

proper support of the pipe. 

h. The full length of each section of pipe rests solidly upon the pipe 

bedding layer with recesses excavated to accommodate couplings and 

joints. 

i. Compaction requirements are being met for bedding layers and 

haunch areas located around the pipe. 

j. Continually monitor the pipe for alignment and shape deformation 

during placement of “fill materials.” Correct immediately any 

problems identified. 

k. Partially perforated pipe is installed with the perforations facing 

down unless otherwise specified. 

l. Pipe and fittings are free of dirt, oil, or other contaminants. 

m. The interior of pipe and accessories are thoroughly cleaned of foreign 

matter before being lowered into the trench. 

n. Pinch bars and tongs for aligning or turning pipe are used only on the 

bare ends of pipe. 

o. Bell and spigot connections are seated properly with no foreign 

material introduced into the connection. 

p. If piping is butt fused, the fusion is allowed to set for the required 

cure time and within the recommended temperature range. 

q. All required leak tests are performed successfully prior to backfilling. 

r. When work is not in progress, open ends of pipes, fittings, and valves 

are securely plugged or capped so that no trench water, earth or other 

substance enters the pipe and fittings. 

s. The entire length of the drainage system is CCTV inspected initially 

when about 3 to 5 feet of fill is placed over the pipe and again prior 

to cleaning and completion. 

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Chapter 6 

Inspection

Periodic inspection of the condition of plastic pipe is essential in detecting problems 

and evaluating its long-term safety and reliability. Periodic inspection may reveal 

trends that indicate more serious problems are developing. However, plastic pipe 

used in embankment conduits and drainpipes is often not inspected as part of an 

overall inspection of the embankment dam and appurtenant features. Generally, 

structural defects and deterioration develop progressively over time. A trained and 

experienced inspector can identify defects and potential problems before existing 

conditions in the dam and conduit become serious. On occasion, situations can arise 

suddenly that cause serious damage in a short period of time. Chapter 7 in FEMA’s 

Technical Manual: Conduits through Embankment Dams (2005) provides guidance 

concerning modes of failure involving embankment conduits. This chapter will 

address the inspection of plastic pipe used in embankment conduits and drainpipes. 

If changes are made in the field during construction and not accurately recorded, 

confusion may result during the inspection. Once plastic pipe is buried, it is difficult 

to find, making it difficult to service the pipe and more likely that unintentional 

damage will result from nearby digging. Accurate as-built construction drawings will 

facilitate the inspection process. Plastic pipe cannot be located using common metal 

pipe detection systems. Acoustical methods have been problematic when used on 

plastic pipes. 

6.1 Embankment Conduits 

For detailed guidance involving the inspection of embankment conduits, the reader 

is directed to chapter 9 in FEMA’s Technical Manual: Conduits through Embankment 

Dams (2005). Discussions within this reference include: 

• Preparing for and performing an inspection.—Good planning and preparation will 

ensure the successful outcome of an inspection. The inspection team should 

know what to look for and evaluate as the inspection progresses. The 

inspection team must keep proper documentation using written records and 

photographs. This documentation provides valuable information on changing 

conditions that could indicate a serious problem is developing. Confined space 

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Plastic Pipe Used in Embankment Dams 

precautions and proper ventilation must be considered for any man-entry in an 

embankment conduit. 

• Specialized inspections.—Specialized inspection includes the use of a dive team, 

climbing team, remotely operated vehicle (ROV), or closed circuit television. 

Specialized inspection is required for embankment conduits that are inaccessible 

for man-entry. 

Figures 86 and 87 show examples of CCTV 

inspection of an HDPE sliplined outlet works 

conduit. Figure 88 shows CCTV inspection of 

a white HDPE pipe. Manufacturers are moving 

away from white HDPE pipe and are using gray 

or black pipe to provide better viewing of the 

interior of the pipe. Inspections have found 

that white reflected too much light, and gray or 

black provides a better picture. 

Figure 86.—A butt fused, solid 

walled HDPE pipe joint in an 

outlet works slipliner as viewed 

using CCTV inspection equipment.

Figure 87.—Looking upstream toward the control gate in an HDPE 

sliplined outlet works conduit using CCTV inspection equipment. 

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Chapter 6—Inspection 

Figure 88.—CCTV inspection of a newly installed slipliner for an outlet 

works renovation. 

6.2 Drainpipes 

Before the 1990’s, most drainpipes were too small in diameter to allow for adequate 

inspection and typically were not inspected. However, the use of CCTV equipment 

has allowed for inspection of many previously inaccessible drainpipes. 

Unfortunately, in many existing dams, unless new access is provided, the drainpipes 

will likely remain uninspected, since older designs often have excessively long 

reaches of drainpipe or sharp bends. New drainpipe installations should always be 

designed to accommodate CCTV inspection equipment. The designer needs to 

consider the proper locations for inspection wells and cleanouts (figures 89 and 90). 

The reader is directed to chapter 9 in FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005) for a complete discussion of CCTV inspection. 

The Bureau of Reclamation conducted a CCTV equipment performance study 

(Bureau of Reclamation, 2004) in order to assist their designers with the proper 

design of drainpipes to better accommodate CCTV equipment. This study evaluated 

the influence of drainpipe diameters, bends, invert slopes, and invert conditions on 

CCTV inspection equipment. The study was performed using varying configurations 

of profile wall corrugated HDPE drainpipe (figure 91). 

The study was based on the assumption that a camera-crawler would travel up the 

pipe from a downstream location. Drainpipe designs that provide an upstream 

(upslope) access location from which the camera-crawler can enter allow for 

149 
Plastic Pipe Used in Embankment Dams 

Figure 89.—Inspection well provides access to the drainpipe for 

CCTV inspection. 

Figure 90.—Cleanout provides access to the drainpipe for 

CCTV inspection. 

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Chapter 6—Inspection 

Figure 91.—Profile wall corrugated HDPE pipe has corrugated exterior 

and smooth interior surfaces. 

improved cable tether pulling capacity, since the camera-crawler can move more 

easily downward (downslope) on a sloping decline. Sloping declines generally do not 

result in camera-crawler traction issues. For the camera-crawler backout process, the 

transport vehicle had a free-wheeling clutch mechanism on the track unit that 

allowed for high speed retrieval either manually or by a cable take up reel. Although 

not tested in this research program, an upstream access location would also benefit 

camera-crawler navigation around pipe bends and allow for the use of steeper invert 

slopes because the effect of cable drag would be lessened. Providing upstream 

access locations would be especially important where steeper invert slopes may be 

required, such as on abutments. The following summarizes the conclusions from the 

Bureau of Reclamation’s study and provides recommendations concerning the layout 

of drainpipe systems to accommodate inspection using CCTV equipment: 

• Pipe diameters.—The minimum recommended pipe diameter to successfully 

accommodate CCTV equipment is 8 inches. Although camera-crawlers are 

available for pipes smaller than 8 inches, they are very limited in cable tether 

pulling capacity and generally do not have sufficient traction for use in 

drainpipe inspection. In addition, the cameras typically only have a fixed lens, 

and the transport vehicle is not steerable. Camera-crawlers used in pipes with 

diameters between 8 and 12 inches generally have cameras with some pan, tilt, 

and zoom capabilities but generally are not steerable. Camera-crawlers used in 

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Plastic Pipe Used in Embankment Dams 

pipes with diameters of 15 inches or larger are steerable, have a greater cable 

tether pulling capacity, and have cameras that can provide a wider array of 

optical capabilities, including pan, tilt, and zoom. Where practical, the use of 

pipes with diameters 15 inches or larger is strongly encouraged. This allows for 

the use of more powerful and versatile camera-crawlers. The selection of larger 

pipe diameters allows for some accommodation of sediment accumulation on 

the pipe invert. Larger diameters also increase the likelihood of cameracrawlers getting past many types of obstructions that may exist in the pipe. 

• Pipe bends.—The maximum recommended horizontal bend angle to successfully 

accommodate CCTV equipment is 22.5 degrees. In pipes with diameters of 

8 and 10 inches, some camera-crawlers encounter difficulties navigating bends 

of 45 degrees or greater because the camera cannot clear the pipe crown as it 

travels through the bend, and drag friction on the tether cable reduces pulling 

capacity. Sweeping bends should always be used to facilitate camera-crawler 

navigation. For best practice in pipes of all diameters, a series of 22.5-degree 

bends is recommended. Each 22.5-degree bend should be connected to a 

minimum 5-foot length of pipe to allow the camera-crawler to navigate around 

the sweeping bend and provide adequate crown clearance. 

• Invert slope inclination.—The maximum recommended invert slope inclination to 

successfully accommodate CCTV equipment is 5 degrees. The difference in 

invert slope inclination between flat and 10 degrees can reduce cable tether 

pulling capacity by as much as 70 percent depending upon the pipe diameter, 

degree of pipe bend, and the invert condition. Flat to 5-degree invert slopes 

would appear to be the most reasonable inclination. Slopes with inclinations 

greater than 10 degrees are not recommended, due to the significant loss of 

traction that occurs when camera-crawlers are pulling long cable tethers. If 

slopes greater than 5 degrees are required, upstream access locations should be 

provided within the pipe. 

• Distance between manholes or access entry locations (cleanouts).—The maximum distance 

between manholes or access entry locations should be between 500 and 

2,000 feet, but depends highly upon the pipe diameter, bends, invert slopes, and 

invert conditions. The designer needs to take these limitations into account 

when selecting the appropriate distance between manholes or access entry 

locations. In pipes with diameters of 8, 10, and 12 inches, the maximum 

distance should not exceed about 1,000 feet. This assumes that access is 

available on both ends of the pipe. If access will only be available on the 

downstream end of the pipe, then the maximum distance should be limited to 

about 500 feet. In pipes with diameters of 15 and 18 inches, the maximum 

distance should not exceed about 2,000 feet. This assumes that access is 

available on both ends of the pipe. If access will only be available on the 

downstream end of the pipe, then the maximum distance should be limited to 

about 1,000 feet. 

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Chapter 6—Inspection 

The results of this study are also considered applicable for pipes constructed with 

PVC. 

The primary cause of pipe failure is not always known. Reasons for failure could be 

singular or a combination of events. Failures observed are not indicative of all 

plastic pipe installations, but do assist in the understanding of drain reliability issues. 

The Bureau of Reclamation has been performing CCTV inspection of drainpipes as 

part of their dam safety program since about 1999. In performing these inspections 

a database has been developed to track the problems found within various types of 

drainpipe materials (Cooper, 2005). The results of these inspections show that early 

installations of single wall corrugated HDPE drainpipe experienced dimpling shape 

deformation, and/or failure in about half of all drainpipes inspected. Shape 

deformation ranged from minor to extensive. Figure 92 shows a single wall 

corrugated HDPE drainpipe experiencing buckling. Joint offsets and separations 

were observed in about 10 percent of all HDPE drainpipes inspected. Joint offsets 

and separations ranged from minor to extensive. Figure 93 shows a single wall 

corrugated HDPE drainpipe joint that has experienced an extensive separation and 

has allowed materials surrounding the drainpipe to enter through the separated joint. 

HDPE is not the only type of plastic pipe that has experienced problems. Figures 94 

and 95 show examples of PVC drainpipe that have failed. These pipes were 

damaged during construction. 

The Davis Creek Dam case history in appendix B illustrates how CCTV can be 

utilized to inspect drainpipes. 

Figure 92.—Single wall corrugated HDPE drainpipe experiencing failure 

due to buckling. 

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Plastic Pipe Used in Embankment Dams 

Figure 93.—This HDPE drainpipe has experienced joint separation 

allowing backfill materials to enter the drainpipe. 

While the nature of plastic pipe minimizes the likelihood of plugging mechanisms 

(i.e., soluble encrustants, biofouling, etc.) developing within the pipe, they can still 

occur. For instance, calcite precipitates out of solution and forms deposits where ion 

concentrations in the seepage increase to the point where it exceeds the solutioning 

capacity of the water (Bureau of Reclamation, 2004, p. 173). This can occur at slots, 

perforations, and joints in the pipe. Figures 96 and 97 show examples of calcium 

carbonate that has precipitated out of solution as the mineral, calcite. Biofouling is 

the result of certain life process activities of bacteria. Bacterial growth can occur 

anaerobically (without oxygen) or aerobically (with oxygen). Plastic pipe lacks any 

nutrients in its composition, but fungi may still grow upon pipe surfaces, feeding 

upon nutrients that may settle or be deposited on the surface by seepage and serve as 

a physical support for the life cycle. Such surfaces are generally not attacked and 

may suffer only slight surface etching (PPI, 2000, p. 2). Bacterial growths can be soft 

and easily removed or can become hard and mineralized. Iron bacteria form the 

most common bacterial deposit (figure 98). Iron bacteria are often characterized by 

orange, red, brown, or black slime, unpleasant odor in water, and an oil-like film on 

water. Other microflora can exist in drainpipes, such as sulfate-reducing bacteria, 

sulfur-oxidizing bacteria, heterophic bacteria, and algae. Sampling and testing may 

be required to assist in planning the best course of action in dealing with plugging 

mechanisms. 

Sediments are often encountered during a CCTV inspection (figure 99). Sediments 

may be transported by seepage flowing within the drainpipe and could be an 

indication of internal erosion occurring within the dam or foundation. Sampling and 

petrographic examination of the sediments may be required to assist in evaluating 

evidence of internal erosion. 

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Chapter 6—Inspection 

Figure 94.—Slotted PVC pipe used for toe drain has experienced 

longitudinal cracking. The cracking occurred during construction. 

Figure 95.—PVC pipe used for toe drain has experienced transverse 

cracking. The cracking occurred during construction. 

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Plastic Pipe Used in Embankment Dams 

Figure 96.—Calcite deposits have blocked many of 

the slots in this HDPE drainpipe. 

Figure 97.—Calcite deposits have formed at joints and 

perforations in this HDPE drainpipe. 

Figure 98.—Iron bacteria have partially blocked the 

perforations in this HDPE drainpipe. Note that the 

only open perforation passing seepage is in the lower 

left corner of figure. 

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Chapter 6—Inspection 

Figure 99.—Sediment deposit on the invert of a HDPE drainpipe. 

HDPE drainpipe has been used in many drainpipes constructed or modified after 

about 1980. HDPE drainpipe, while lightweight and easily handled and installed, has 

experienced a significant number of shape deformation and failure instances. Many 

of the HDPE drainpipe failures may be related to stress cracking or improper 

installation of the pipe. Stress cracking is a failure mechanism which develops over 

time at stresses less than the yield strength. In the past, HDPE drainpipe resins have 

differed in the amount of SCR. Proper installation of HDPE drainpipe requires 

good compaction and quality control of the backfill to insure good support under 

the haunches. If the drainpipe is not well supported by the backfill, the drainpipe 

will deflect excessively and stresses will be concentrated at the crown, invert or 

springline. These stress concentrations can lead to premature failure. Other failures 

could be the result of isolated point loads from construction loading, such as 

equipment crossings. 

The following guidelines are recommended for inspection of plastic drainpipe: 

1. A preliminary CCTV inspection should be performed when 3 to 5 feet of 

backfill has been placed over the drainpipe. The purpose for this inspection 

would be to identify and repair any abnormalities, cracks, bulges, etc. early 

before construction is completed. 

2. Another CCTV inspection should be performed when the final backfill loading 

over the drainpipe is completed. CCTV inspection should be performed prior 

to the contractor pulling the torpedo-shaped plug or pig through the drainpipe 

and prior to any cleaning. The purpose for this inspection would be to identify 

any abnormalities, cracks, bulges, etc. that may have developed since the 

preliminary inspection. CCTV inspection could replace the need for pulling the 

plug or pig through the drainpipe. 

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Plastic Pipe Used in Embankment Dams 

3. Subsequent periodic inspection should be performed based on the performance 

of the drainpipe, changes in the characteristics or quantity of the flow, or other 

events. 

Cleaning can remove biofouling, mineral encrustation, roots, and soil deposits. 

Caution must be exercised in each step of the cleaning process to prevent damage. 

Before attempting to remove soil deposits, the engineer should consider if they are 

benign, like those that enter a pipe during the development of the filter (filter set), or 

if their removal could initiate more soil movement and make the condition worse. 

The fundamental guiding principle for any type of drainpipe cleaning should be “do 

no harm.” 

After a newly installed drain system has been through first filling of the reservoir, 

sometimes soil material is seen in the pipe invert or in a downstream sediment trap 

during CCTV inspection. Concern may arise that this material could be from the 

foundation, but it is possible it could be from the filter and drain material itself. In 

order to address this concern, the method of filter and drain processing and 

construction quality will have to be determined. During material processing three 

methods can be used to produce a material of the desired gradation; crushing, dry 

screening, and washing. Details of the process used to produce the material in 

question will be needed to determine the likelihood that fine grain material exists 

within the filter and/or drain materials. Note that it is possible to produce filter and 

drain materials with small amounts of fines and still meet the specification 

requirements. 

As an example, consider a borrow area material consisting of silt, sand, and gravel 

that will used to produce a filter composed of sand and drain material composed of 

gravel. Upon entering the processing plant the material is dry screened separating 

the gravel from the silt and sand. The silt and sand go onto to further processing to 

remove the silt, typically by using washers. This is the 'wet' side of the plant. 

Meanwhile, the gravel continues for additional dry screening until the final drain 

material gradation is achieved. This is the 'dry' side of the plant. Complications can 

arise on this side of the plant in that fine material, perhaps with some plasticity, can 

adhere to the gravel particles. For this reason drain material should be visually 

examined during construction for fines adhesion to the gravel particles. Experience 

indicates that sometimes a large amount of effort is needed to remove the fines 

contamination. This material can then show up in the drain system as described 

above. While this example describes one way fine grain material can show up in a 

drain, others can occur also. For this reason it should be expected that some finer 

material will flush out of newly constructed drain systems and plans should be made 

from the beginning of the job to clean the system after initial reservoir filling. This 

cleaning should occur after the reservoir has reached its maximum normal level so 

the drain system is wetted to the maximum extent. 

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Chapter 6—Inspection 

No clear guidance exists on how to tell the difference between fines that flush out of 

a newly constructed drain system and material that may be coming from the 

foundation. However, material derived from the filter and drain materials should 

have a uniform appearance and should occur uniformly along pipe segments which 

have flow. The amount of material should also be modest, at the most, no more 

than one cup over a 100-foot drain length. The best indication though of the source 

of the material is re-examination in the second year. If no material is found in year 

two, then it most likely was from the filter/drain materials themselves. 

Cleaning of drain systems is not yet routine (figure 100). Most drains have never 

been cleaned, and based on their performance, cleaning may not always be justified. 

CCTV inspection should precede any cleaning attempt, to ensure that cleaning will 

not degrade existing conditions. The proper method of cleaning a drainpipe varies 

according to the conditions within the pipe and the structural integrity of the pipe. 

Commercially available water-jet cleaning is most often used that utilizes highpressure water spray from a nozzle. The orientation of individual jets on the nozzle 

of the cleaner can also be varied, depending on site conditions. In some cases, low 

pressure/high volume flow is best suited for sediment removal and high 

pressure/low volume flow is best suited for root or mineral encrustation removal. 

Sometimes both methods may be required at a particular site. The condition of the 

pipe is paramount for any cleaning attempted, and this may actually govern the 

cleaning method selected at a given site 

Typically, it is difficult to ascertain how effective the cleaning has been, due to 

limited instrumentation and variations in drain flows caused by factors other than the 

reservoir, such as infiltration from precipitation. However, follow-up CCTV 

inspection after cleaning that used high pressure jet washing has shown that the 

biofouling and mineral encrustation was generally removed from the interior surface 

and some improvement of discharge from the drainpipe was often observed. No 

determination can usually be made as to the extent of the plugging mechanism 

remaining in the backfill materials surrounding the exterior of the pipe. 

Any cleaning system used should always be proven effective in a similar situation and 

on similar pipe materials. If a new cleaning system is used, it should be tested on a 

piece of pipe similar to the drain to be cleaned to ensure the process will not damage 

the pipe. 

The recommended process for drain cleaning generally includes the following six 

steps: 

1. Record all pertinent information, including measuring drain outflows, reading 

all piezometers and observation wells, and walking the alignment of the drain to 

observe the preexisting conditions. 

2. Perform an initial CCTV inspection to document existing conditions. 

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Plastic Pipe Used in Embankment Dams 

Figure 100.—Operator water jetting a drainpipe. 

3. Test the cleaning system on the first short segment of pipe. 

4. In cleaning the remainder of the pipe, use care to observe the entire process, 

including advancement rates, effluent, etc. Steps 2 through 4 may require an 

iterative process to ensure that cleaning procedures are not damaging previously 

uninspected portions of the drainpipe. 

5. Reinspect the pipe using CCTV and record all other pertinent information 

again. This could be completed by the contractor doing the cleaning, if they 

have appropriate CCTV inspection equipment. 

6. Document all information for use in future cleanings, if needed, and 

information beneficial to an evaluation of the cleaning by others. Disseminate 

copies of the cleaning report to the engineer and other appropriate parties. 

For guidance on improving drainpipe access for inspection and cleaning activities, 

see section 4.3.4. For additional guidance on cleaning, see section 9.6 in FEMA’s 

Technical Manual: Conduits through Embankment Dams (2005). 

160 
Chapter 7 

Plastic Pipe Used in Tailings Disposal Facilities 

and Slurry Impoundments 

The mining industry constructs dams for waste disposal, water supply, water 

treatment, and sediment control. Tailings dams are used for the disposal of “metal 

and nonmetal” mine waste or tailings. Slurry impoundments or coal waste 

impoundments are used for the disposal of fine waste from the processing (i.e. 

removing impurities) of coal. Tailings dams and slurry impoundments differ in many 

ways from traditional water storage embankment dams. For a discussion of the 

differences, see the Introduction of FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005). This chapter will focus on the use of plastic pipes in 

tailings and slurry impoundments. Plastic pipe has been used since about 1980 in 

mining-industry dams for decant pipes, for internal-drain collector pipes, and for 

delivery pipes for slurry or tailings disposal. A decant pipe typically serves the 

functions of removing clarified water from the impoundment, controlling the normal 

water level, and drawing down the pool level following rainfall events. 

The main reasons why plastic pipe has come into use, versus other types of pipe, in 

mine-waste disposal applications include its resistance to chemical attack, its 

capability to be constructed with watertight joints, its ease of construction, and its 

ability to tolerate deformation. More specifically, consider that: 

• The drainage and the seepage from mine waste impoundments can cause 

chemical deterioration of pipe materials due to its acidity or alkalinity. Waste 

from the processing of materials such as coal and phosphate, for example, can 

be highly acidic. 

• Some tailings dams are required to have impervious liners due to the acidity of 

the leachate. Plastic pipe has been used in these applications with a watertight 

seal provided by a boot at the point where the pipe penetrates the liner. In 

these cases, the potential for seepage along the pipe, and the development of a 

problem associated with such seepage is limited, provided careful attention is 

paid to design, construction, and monitoring. Some designers of tailings 

disposal facilities consider it best practice to avoid penetrations through 

embankments, whether the impoundment is lined or unlined, and use bargemounted pumps to control the water level. 

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Plastic Pipe Used in Embankment Dams 

• When pressure-testing became more prevalent for decant pipes – such as in 

coal slurry impoundments – the fused joints of plastic pipe were able to meet 

the testing requirements. 

• Designers have considered HDPE pipe to be beneficial for the type of 

foundation conditions and construction practices found at mining 

impoundments. The locations of these impoundments are limited to areas near 

the processing plants, meaning that designers need to deal with varied, and 

often times less than ideal, foundation conditions. Furthermore, decant pipes 

are extended as the impoundment is expanded and the pipes can become 

relatively long, sometimes exceeding 1,000 feet. Over such lengths, a flexible 

pipe can tolerate some differential movement due to varying foundation and 

installation conditions. Additionally, in the coal fields, many slurry 

impoundments have underground mining in their vicinity and the possibility of 

subsidence, or mining-induced ground movement, needs to be considered. 

HDPE pipe has been the type of plastic pipe most commonly used for decant and 

internal drain conduits in mining applications. Decants are typically solid wall pipes 

in the range of 18 to 36 inches in diameter. SDR values are commonly in the range 

of 11 to 21. 

Internal drains are used within tailings disposal facilities for various purposes such as 

to improve stability by lowering the phreatic surface; reduce the potential for the 

tailings to flow by promoting consolidation; lower the hydraulic head on an underliner to minimize seepage through the liner for environmental protection; and/or 

limit settlement after the surface of the tailings is capped and reclaimed. The 

collected seepage may be acidic, or for example, with gold tailings, contain cyanide 

from the processing of the gold ore. Profile wall corrugated HDPE pipe has been 

used for drainpipes in this type of application. The pipe has a corrugated wall on the 

exterior and smooth interior with slots in the recesses of the corrugations. This pipe 

is typically joined with snap couplings and surrounded by drainage aggregate. Pipe 

diameters in the 4- to 6-inch range are typically used for lateral drains that connect to 

main drains that may be 12 to 18 inches in diameter. The mains may be solid wall 

HDPE pipe. Numerous installations have been in operation for tens of years and 

are approaching 200 feet in height. 

Another mining-industry application for corrugated plastic pipe is in the heap 

leaching process, where large piles of ore are leached with various chemical solutions 

to extract valuable minerals such as gold and copper. Perforated pipes installed 

under the heap collect the solution for processing of the metals. Heap leach pads 

can be over 300 feet high. 

The design of tailings or slurry impoundments differs in many respects from the 

design for traditional embankment water dams. Basically, embankment dams are 

designed to store water, while tailings facilities are designed to form a basin for the 

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Chapter 7— Plastic Pipe Used in Tailings 

Disposal Facilities and Slurry Impoundments 

deposition of fine material. The tailings/slurry disposal facility designer can take 

advantage of this difference by, for example, minimizing the amount of free water 

that is stored and by using drainage to develop as “dry” a tailing/slurry deposit as is 

practical. Dryer soils are inherently more stable than saturated soils. 

The solids that settle out of suspension in a tailings/slurry impoundment can have a 

significant effect on seepage. Tailings or slurry is typically discharged from the 

upstream slope of the dam and the larger, sand-size particles tend to drop out near 

the discharge point. The finer particles settle out farther back in the impoundment, 

with the finest particles settling at the back end. A delta, which slopes back away 

from the dam, is formed by the settled material and a pool of free water accumulates 

at the back end of the reservoir. The effect of this typical configuration is that the 

free water must seep through the settled fines before seeping through the dam. 

Additionally, an objective in many tailings/slurry impoundment designs is to 

minimize the amount of free water. This is done both by site selection (i.e., 

minimizing the contributing watershed), diversion ditches, and the use of decants or 

pumping. If the impoundment is operated in a manner where ponded water can rise 

above the settled fines and contact the upstream slope of the dam for a period 

sufficient to develop steady-state seepage pressures, then the facility should be 

designed like a traditional water dam. 

The characteristics of tailings/slurry impoundments sometimes allow designers to 

employ some differences in the design of tailings/slurry impoundments versus 

traditional embankment dams. In slurry impoundments, for example, plastic pipes 

that are not encased in concrete have been commonly used. Problems with seepage 

along the pipes has not been a significant occurrence in these impoundments, likely 

because of the effect of the settled fines in limiting seepage along the pipe combined 

with the long lengths of the decant pipes. 

Another area where differences may be found, related to plastic pipe usage, is in the 

design of underdrains. For example, perforated or slotted plastic pipe is commonly 

used within underdrains, but the drainage aggregate used to surround the pipe may 

contain somewhat more than the recommended maximum of 5 percent minus 

No. 200 sieve material. This may be done, in cases where the seepage amounts and 

gradients are limited, to control the costs of installing drains to cover extensive areas. 

Where a less stringent requirement such as this is considered, however, 

tailings/slurry impoundment designers need to ensure that appropriate testing and 

analysis is performed—and construction and performance is carefully monitored— 

by engineers knowledgeable of the potential problems. Design alternatives must still 

ensure that filter criteria are met or critical exit gradients are not exceeded. 

An important consideration in the design of underdrains for tailings/slurry 

impoundments is that the chemical characteristics of the seepage may affect the 

performance of the drainage system. Potential chemical reactions of the seepage 

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Plastic Pipe Used in Embankment Dams 

solution and the drain materials must be fully evaluated and thoroughly understood. 

The potential may exist for clogging, cementation, crystal growth, or biological 

growth to render the drains inoperable. There are many examples of drains that 

have failed by these mechanisms. 

A unique situation with mine-waste disposal dams is that they are raised as needed to 

provide additional capacity. In some mine waste disposal impoundments, a decant 

pipe is initially constructed with inlets positioned at various intervals to handle the 

entire life of the facility (see figure 101). 

The inlets are blocked off as the level of waste accumulates in the impoundment. 

More commonly, the decant pipe is extended as the dam is raised with new inlets 

being installed and the old inlets being blocked off. In some cases, the amount of fill 

over the pipe is limited by installing a completely new decant pipe at a higher 

elevation when the embankment is raised, and the lower decant pipe is abandoned by 

filling it with grout. 

This method of construction, where mine waste dams are raised on a nearly 

continuous basis over the 20 or 30 year life of the facility, presents an opportunity to 

monitor the performance of conduits that is not available in traditional dams. That 

is, monitoring can be performed, and design assumptions can be verified, as the 

height of fill over the pipe is raised and well before the fill reaches a critical height. 

Additional research is proposed in chapter 8 (PM-4) to study this method of 

construction. 

Designers have proposed fill heights in the range of 200 feet over plastic decant 

pipes in mine waste dams. Due to the limited experience with plastic pipe under 

high fills, and because of uncertainties in deflection analysis, such as appropriate E

values, the Mine Safety and Health Administration has generally only accepted 

designs for high fill heights contingent upon the mining company monitoring the 

pipe deflection at various amounts of cover and over time (figure 102). Based on the 

monitoring data, the value of Eis back-figured and an estimate is then made of the 

performance of the pipe under additional fill. Initial measurements are taken when 

the pipe is installed to establish baseline values. Subsequent deflection 

measurements start at a fill height where there is confidence that pipe performance 

will be adequate. Thereafter, the monitoring depends on how fast the fill level rises. 

Monitoring is done at predetermined increases in fill height and at predetermined 

time intervals, such as annually. Deflection monitoring has been accomplished by 

pulling devices, such as mechanical deflectometers (measures the vertical diameter of 

the pipe), sonar devices, and video cameras, through the pipe. Results of monitoring 

have shown, in some cases, that deflection was at or approaching the amount 

considered allowable (typically 7.5 percent). Once the allowable deflection is 

approached, or other signs of distress are evident, the decant pipe is replaced by 

another pipe. The new pipe is installed higher in the dam cross section, and the 

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Chapter 7— Plastic Pipe Used in Tailings 

Disposal Facilities and Slurry Impoundments 

Figure 101.—Decant pipe with multilevel inlets. 

Figure 102.—Deflection monitoring of a decant pipe. 

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Plastic Pipe Used in Embankment Dams 

original pipe is abandoned by being filled with grout. Designers have come to realize 

that it is costly to measure deflection every year and at certain fill levels, so they have 

moved toward installing new pipes at a higher elevations more frequently, so that the 

amount of fill on the active pipe, before it is grouted full, is limited. 

Problems with the use of plastic pipe in mining dams are indicated by two of the 

case histories in appendix B. In the “Sediment Control Pond SP-4” case history, a 

30-foot high dam failed during first filling due to seepage along a spillway pipe. This 

failure was attributed to inadequate compaction of the backfill and/or inadequate 

contact between the backfill and the pipe. In contrast to this failure, HDPE pipes 

have been used extensively as decants for coal slurry impoundments with no known 

failures from seepage along the pipe. In these applications the pipes have not been 

encased in concrete, but have typically been backfilled using hand-held equipment, 

such as pogo-sticks or rammers. The lack of problems with this type of construction 

may be due to a combination of factors which include: the impact of the lowerpermeability settled slurry that acts to restrict seepage; the long length of the pipes - 

normally several hundred feet long; and the potential for the slurry fines to choke-off 

seepage paths along the pipe. The other problem, highlighted in the “Virginia Dam” 

case history in appendix B, involved a plastic pipe encased in concrete. In this case, 

the plastic pipe did not have adequate resistance to buckling and collapsed due to 

external hydrostatic pressure between the pipe and the concrete encasement. 

Aside from the cases indicated above, the biggest performance concern with plastic 

pipe at slurry impoundments has been with pipe deflection. Excessive deflection or 

deformation has occurred in some cases and been attributed to soft or inadequately 

compacted areas in the backfill, or to stress concentrations from oversized rock in 

the backfill. 

As stated elsewhere in this document, the “best practice” in installing a decant 

through a dam is to use a properly shaped reinforced cast-in-place concrete conduit 

through the impervious zone, so that the outside of the conduit can be battered to 

allow rubber-tired equipment to compact backfill directly against the conduit. This 

eliminates the problem with poor compaction in the haunch area. As indicated by 

Dr. Ernest Selig, “Apparently no amount of haunching effort can provide good soil 

support to the region about 20 degrees from the invert” (Selig, 1996, p. 6). 

This “best practice,” however, creates a dilemma in the case of mine waste 

impoundments. As previously explained, there are benefits to having a conduit that 

can tolerate some deformation in these impoundments. Furthermore, tailings and 

slurry impoundments do not typically have an “impervious core” and the added cost 

of reinforced cast-in-place conduits is not as suitable for the shorter life of these 

conduits, as compared to traditional embankment dam conduits. And while the 

absence of significant problems does not rule out future problems, the record does 

provide some indication that alternatives to concrete encasement may be reasonable 

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Chapter 7— Plastic Pipe Used in Tailings 

Disposal Facilities and Slurry Impoundments 

in mine waste impoundment applications. The following recommendations are 

provided for installing conduits in tailings and slurry impoundments: 

1. Although extensive problems have not been encountered with decant pipes 

through slurry impoundments, good conduit design and installation practices 

need to be followed. Slurry impoundment designers should recognize that the 

large body of evidence indicates adequate compaction cannot be achieved in the 

haunch area by conventional hand compaction methods. Using these methods, 

full contact between the pipe and the backfill cannot be ensured. 

2. Decant pipes should be provided with an adequately designed seepage 

diaphragm and filter. The diaphragm should be extended far enough out from 

the pipe to intercept areas where cracks may occur due to hydraulic fracture or 

differential movement of backfill/embankment materials. See chapter 6 of 

FEMA’s Technical Manual: Conduits through Embankment Dams (2005) for 

guidance on filter diaphragm design. 

3. The seepage diaphragm should not be considered as an adequate defense, by 

itself, against problems with seepage along the pipe. The permeability of the 

backfill material and its level of compaction need to be sufficient to restrict 

seepage and reduce the hydraulic gradient along the pipe. The seepage 

diaphragm is intended to collect the limited seepage that occurs through wellcompacted and suitable backfill and intercept particles that are being 

transported by water. The diaphragm could be overwhelmed and rendered 

ineffective by excessive seepage. 

4. If the pipe is not to be encased in reinforced cast-in-place concrete, with 

battered sides that allow compaction by heavier equipment, then an alternate 

construction method, which provides for adequate backfill density in the 

haunch area, and full contact between the backfill and the pipe, needs to be 

specified. 

5. Use of an alternate construction method should only be considered in slurry or 

tailings impoundments where it can be shown that the combination of hydraulic 

gradient and backfill material characteristics indicate adequate protection against 

internal erosion and piping. For example, slurry/tailings is typically discharged 

along the upstream face of the embankment resulting in the fine waste settling 

out and the free water collecting at the back end of the impoundment. 

However, if the slurry/tailings are discharged from the back end of the 

impoundment, free water would pond directly against the upstream slope of the 

dam. In this case, the seepage benefit from the settled fines would not be 

realized and the pipe should be designed as for a traditional embankment dam. 

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Plastic Pipe Used in Embankment Dams 

6. If the pipe is not encased in reinforced cast-in-place concrete, the installation 

options appear to be shaping the bedding, or the use of flowable fill. As 

indicated elsewhere in this document, many questions (see chapter 8, research 

items EM-3 through EM-8) need to be answered concerning the performance 

of flowable fill with respect to shrinkage, cracking, deformation properties, and 

stress concentrations at the pipe/backfill interface before it can be 

recommended for use. 

7. Shaping the bedding to conform to at least the bottom one-third portion of the 

pipe is one technique that slurry impoundment designers have used to address 

the problem with compaction in the haunch area. However, the practice of 

shaping the bedding has concerns associated with it. Perfect contact between 

the shaped bedding and the pipe is not achievable. Designers have used a thin 

layer of expansive material, such as bentonite powder, to compensate for small 

irregularities between the shaped bedding and the pipe. An extreme level of 

care and attention to detail, with close supervision by a professional engineer 

knowledgeable of the potential problems, would be required. Further research 

(EM-7) is needed on this method, as proposed in chapter 8, before it can be 

recommended for use. 

8. Specifications should include a detailed step-by-step procedure for installing the 

pipe and for achieving full contact between the conduit, bedding, and backfill. 

The type of equipment to be used to achieve the specified backfill densities 

should be specified. Construction related to critical piping installations should 

require full-time observation by experienced, qualified, and knowledgeable 

personnel. 

9. Whatever pipe installation method is specified, quality control during 

construction should be the responsibility of a registered professional engineer 

who is familiar with the project specifications and the potential problems. The 

specifications should indicate how it will be determined that the required 

backfill moisture/density specifications have been met and that full contact 

between the conduit and the backfill has been achieved. The inspector should 

periodically remove a portion of the compacted backfill and making use of a 

knife, or whatever device is necessary, ensure that the adjacent backfill is in 

intimate contact with the conduit and that no voids are present, especially along 

the bottom half of the conduit. The engineer should be required to inspect and 

accept the conduit bedding and backfill installation before the embankment fill 

is placed over the conduit. 

10. The designer(s) should always provide reasonable accommodations for 

inspection using CCTV in their designs. 

As indicated elsewhere in this document, nonencased plastic embankment conduits 

are not recommended for traditional water-retention dams with significant or high 

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Chapter 7— Plastic Pipe Used in Tailings 

Disposal Facilities and Slurry Impoundments 

hazard potential. Designers of slurry tailings disposal facilities and impoundments 

should only specify nonencased plastic decant pipe where it can be shown, based on 

the conditions which are unique to slurry/tailings structures, that potential problems, 

such as with internal erosion along the pipe, are precluded. 

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Plastic Pipe Used in Embankment Dams 

170 
Chapter 8 

Research Needs 

The National Dam Safety Program (NDSP), which was formally established by the 

Water Resources and Development Act of 1996, includes a program of technical and 

archival research. Research funding under the NDSP has addressed a cross section 

of issues and needs, all in support of ultimately making dams in the United States 

safer. 

This chapter identifies research needs that are related to the performance of flexible 

plastic pipe within embankment and tailings/slurry impoundments. The authors 

considered these research needs to be good candidates for NDSP research funding. 

8.1 Research Items 

Research is needed in two categories: the performance of pipe material and the 

performance of embedment/encasement material. 

8.1.1 Pipe material (PM) 

Research is needed for the performance of pipe material includes: 

• PM-1.—Determine minimum pipe resins and grades needed for dam related 

applications using laboratory tests or a review of existing research. 

A variety of formulations are used to produce the different kinds of plastic pipe. 

Even within a particular pipe category, such as HDPE, a number of resins are 

available. Some agencies require specific resins be used in the interest of 

obtaining sufficient pipe strength and resistance to aging. Currently there are 

no guidelines for chemical composition or grade for plastic pipe used in dams. 

A literature review would be done to determine standard practice and the 

associated issues with practitioners. At the completion of the literature review 

some laboratory testing may also be needed. Testing would focus on stress 

crack resistance, which in turn determines design life. 

• PM-2.—Service life as it relates to the wear surface. 

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Plastic Pipe Used in Embankment Dams 

HDPE and PVC are reported to have better abrasion resistance than many 

other pipe materials. Manufacturers have conducted tests to measure pipe 

performance when subjected to abrasive forces. A literature review of research 

conducted by manufacturers is necessary to help determine the design life of 

plastic pipe that will be subjected to abrasion. 

• PM-3.—Strain effects of perforations/slots on various types of pipe sectionssolid wall, corrugated wall (single and profile wall). 

Circular perforations and slots have the potential to weaken pipe. Laboratory 

tests would be used to assess how number, type, location and size of 

holes/slots affect strain under load in the pipe. 

• PM-4.—The performance of plastic pipe under staged loading associated with 

the type of stage construction unique to mine-waste dams. 

In tailings dams, the height of fill over a pipe may increase gradually over a 

period of 20 years or more. Testing should be performed, and field 

measurements collected and analyzed, to determine the affect on pipe 

performance of this type of staged construction. Issues include the allowable 

deflection, which is considered prudent under these conditions, and whether 

the short-term or long-term pipe modulus should be used in design. 

• PM-5.—New and promising plastic pipe products. 

Perform a market survey and technical evaluation of all new plastic pipe 

products currently available. Evaluate potential applications within 

embankment dams including the advantages and disadvantages associated with 

new each product. 

• PM-6.—Evaluate the watertightness and long term suitability of new joining 

systems for PVC pipe in dam applications. 

Newer joining systems have recently become available for PVC pipe. These 

newer joining systems include splined, heat fused, and mechanical joints. These 

types of joints are currently being used on water distribution and sewer 

installations, but have not been used in dam applications. Some manufacturers 

may have conducted tests for these new systems, and a literature search is 

needed. Additional laboratory testing may also be necessary. 

8.1.2 Embedment/encasement material (EM) 

Research is needed for the performance of embedment/encasement material 

includes: 

172 
Chapter 8—Research Needs 

• EM-1.—Investigate the interface bonding between plastic pipe and the 

encasement material. 

A bond between encasement materials and plastic pipe cannot be achieved due 

to material differences. Designers use a downstream filter to control internal 

erosion of embankment soil along this interface. The use of materials such as 

chemical grout and bentonite that could be used to form a seal between the 

plastic pipe and the encasement material would be evaluated for cost 

effectiveness and functionality using laboratory testing. 

• EM-2.—Investigate the effects of the heat of hydration on plastic pipe using 

laboratory testing. 

Determine if heat of hydration from the curing of grout, concrete, or CLSM 

causes a significant rise in temperature which could cause the plastic pipe to 

expand. 

• EM-3.—Quantify the Modulus of Soil Reaction for CLSM. 

The soil structure interaction between a flexible pipe and CLSM backfill is not 

clearly understood. Since the strength of CLSM is somewhere between soil and 

concrete, the reaction is somewhere between flexible and rigid restraint, 

respectively. Design of flexible pipe, as described earlier in this document, 

requires that the designer know the strength and modulus of soil reaction for 

the backfill. Limited laboratory tests have been completed (Brewer, 1990) 

which have provided estimates of the Modulus of Soil Reaction for CLSM. 

Additional testing is necessary to better define the Modulus of Soil Reaction for 

a wider variety of CLSM mixes and materials. 

• EM-4.—Quantify compressive strength for CLSM used as backfill in dam 

applications by literature review and laboratory testing. 

CLSM is assumed to behave similar to a soil, allowing pipe deflection. The 

compressive strength of the CLSM will dictate the amount of pipe deflection. 

Laboratory testing is needed to determine recommended compressive strength 

for the use of CLSM as backfill in dam applications and changes with time as 

the CLSM ages. Full-scale laboratory tests would also be useful in evaluating 

the response of plastic pipe encased in CLSM exposed to large vertical loads. 

• EM-5.—Evaluate the behavior of plastic pipe fully encased in CLSM. 

A laboratory testing program would confirm the assumption that CLSM 

behaves like a soil. A testing program would confirm this assumption and 

explore additives, which could make CLSM more flexible. 

173 
Plastic Pipe Used in Embankment Dams 

• EM-6.—Evaluate the shrinkage, permeability, and cracking potential for 

different CLSM mixes. 

A laboratory testing program would be used to measure the shrinkage, cracking 

potential, and erosion resistance for a variety of CLSM mix designs. The mix 

design would consist of evaluating a number of materials for improved 

performance of the mix. An example additive would be the use of nonshrink 

cement to see if shrinkage can be eliminated. 

• EM-7.—Evaluate the response of plastic pipe partially encased with reinforced 

cast-in-place concrete (cradle) or CLSM. 

Some designers are using plastic pipe partially encased by a cradle. There are 

concerns about the effect of stress concentrations in the pipe at the top of the 

cradle and the failure mechanism with deflection limited to the top half of the 

pipe. Full scale laboratory testing would be used to determine if CLSM and 

concrete could be used as cradle material. 

• EM-8.—Investigate if CLSM should be placed in lifts so lateral support can 

develop. 

Concerns exist that use of a single placement of CLSM can lead to pipe 

collapse, as lateral support has not developed prior to pipe being loaded 

vertically. There are also concerns with the placement of CLSM in lifts. In 

addition, the heat of hydration of the CLSM may heat the pipe, thus reducing 

its strength and potentially contributing to pipe collapse. Full scale laboratory 

testing would be used to evaluate CLSM placement methods. 

• EM-9.—Evaluate the response of plastic pipe fully encased in nonreinforced 

concrete. 

For the unreinforced concrete case, a laboratory testing program would 

determine if the plastic pipe does perform as a rigid pipe, as currently assumed, 

or is there some deflection. Testing would also determine if there is a minimum 

concrete strength at which the pipe behaves rigidly. 

• EM-10.—Investigate the use of self-consolidating concrete (SCC). 

SCC is a high-performance concrete that can flow easily into tight and 

constricted spaces without segregating and without requiring vibration. 

Determine if SCC can be economically used as an encasement material to 

improve consolidation under the haunches of circular pipes. 

• EM-11.—Investigate the use of the “cut-earth cradle” method for installing 

plastic pipe. 

174 
Chapter 8—Research Needs 

Obtaining adequate density in the haunch area of nonencased circular pipes is a 

problem. The cut-earth cradle method was developed in an attempt to address 

this concern. The cut-earth cradle method involves compacting the backfill to 

the level of the springline of the conduit and excavating a cradle through the 

compacted backfill to conform to the shape of the pipe. An expansive material, 

such as powdered bentonite is used to compensate for small irregularities in the 

contact between the backfill and the pipe. The effectiveness of this technique 

would be evaluated and limitations and guidelines for use in dam construction 

developed. 

175 
References 

The following references have been specifically cited in this document. 

Advanced Drainage Systems, Standard Pipe Perforations (4-60 N-12 Pipe), Product 

Note 3.106, 2003. 

American Association of State and Highway Transportation Officials, LRFD Bridge 

Design Specifications, Washington, DC, 2005. 

American Concrete Institute, Controlled Low-Strength Materials, ACI 229R-99, 1999. 

American Concrete Institute, Cement and Concrete Terminology, ACI 116R, Committee 

Report, 2000. 

American Geological Institute, Glossary of Geology, 1987. 

American Society of Civil Engineers, Guidelines for Instrumentation and Measurements for 

Monitoring Dam Performance, 2000. 

American Water Works Association, Concrete Pressure Pipe, M9, 1995. 

American Water Works Association, Fiberglass Pipe Design Manual, M45, 2000. 

American Water Works Association, PVC Pipe—Design and Installation, M23, 2002. 

American Water Works Association, PE Pipe—Design and Installation, M55, 2006. 

Brewer, W.E., “The Design and Construction of Culverts Using Controlled Low 

Strength Material—Controlled Density Fill (CLSM-CDF) Backfill,” Proceedings of the 

First National Conference on Flexible Pipes, 1990. 

Brewer, W.E. and J.O. Hurd, “Controlled Low Strength Material—Controlled 

Density Fill (CLSM-CDF) as Backfill around Flexible Structures,” Proceedings of the 

Second Conference on Structural Performance of Pipes, 1993. 

Brinkgreve, R.B.J. and W. Broere, Plaxis 3D Tunnel, Version 2, Plaxis, 2004. 

Bureau of Reclamation, Air-Water Flow in Hydraulic Structures, Engineering 

Monograph No. 41, 1980. 

Bureau of Reclamation, Results of the Currant Creek Pipeline Impact Loading Study, Central 

Utah Project, 1984. 

176 
References 

Bureau of Reclamation, Design of Small Dams, 1987a. 

Bureau of Reclamation, Embankment Dams, Design Standards No. 13, Chapter 8, 

“Seepage Analysis and Control,” 1987b. 

Bureau of Reclamation, Earth Manual, Part 2, 1989.

Bureau of Reclamation, Laboratory Pipe Box Test on Toe Drains at Lake Alice Dam and 

Enders Dam, 1997. 

Bureau of Reclamation, Design Summary—Ganado Dam, 1998. 

Bureau of Reclamation, Drainage for Dams and Associated Structures, 2004. 

Bureau of Reclamation, Wheatfields Design Summary, 2006. 

Bureau of Reclamation, Embankment Dams, Design Standards No. 13, Chapter 5, 

“Protective Filters,” 2007. 

Burgon, Rex P., Steven L. Folkman, and A.P. Moser “A Parametric Study of the 

Influence of Profile Geometry on the Structural Performance of Buried Profile-Wall 

Pipe,” Proceedings of Transportation Research Board Annual Meeting, 2006. 

Chevron Phillips Chemical Company, The Performance Pipe Engineering Manual, 2002. 

Cooper, Chuck R., Closed Circuit Television Inspection of Outlet Works and Spillway Conduits 

and Toe Drains, ASDSO Annual Conference, 2005. 

Duncan, J.M., P. Byrne, K.S. Wong, and P. Mabry, Strength, Stress-Strain and Bulk 

Modulus Parameters for Finite Element Analysis of Stress and Movements in Soil Masses, 

Report No. UCB/GT/80-01, 1980. 

El-Sawy, Khaled and Ian Moore, Parametric Study for Buckling of Liners: Effect of Liner, 

ASCE, 1997. 

Federal Emergency Management Agency, Federal Guidelines for Dam Safety: Hazard 

Potential Classification Systems for Dams, FEMA 333, 1998. 

Federal Emergency Management Agency (FEMA), Federal Guidelines for Dam Safety: 

Glossary of Terms, FEMA 148, 2004. 

Federal Emergency Management Agency, Technical Manual: Conduits through 

Embankment Dams, FEMA 484, 2005. 

177 
Plastic Pipe Used in Embankment Dams 

Hammer, David P., Construction of Vertical and Inclined Sand Drains in Embankment 

Dams, ASDSO Annual Conference, 2003. 

Hartley, James D., and James M. Duncan, “Eand Its Variation with Depth,” Journal 

of Transportation Engineering, ASCE, 1987. 

Holtz, R.D. and W. D. Kovacs, An Introduction to Geotechnical Engineering, 1981. 

Howard, Amster K., “Modulus of Soil Reaction Values for Buried Flexible Pipe,” 

Journal of Geotechnical Engineering, ASCE, Vol. 103, 1977. 

Howard, Amster K., Pipeline Installation, 1996. 

Howard, Amster, “The Reclamation ETable, 25 Years Later,” Plastic Pipes XIII 

Conference Proceedings, 2006. 

Hsuan, Y. Grace, J-Y Zhang, and W-K Wong, “Evaluate the Long-Term Stress 

Crack Resistance of Corrugated HDPE Pipes,” Plastic Pipes XIII Conference Proceedings, 

2006. 

Hsuan, Y. Grace and Timothy McGrath, Protocol for Predicting Long-term Service of 

Corrugated High Density Polyethylene Pipes, Prepared for the Florida Department of 

Transportation, Final Draft Report, 2005. 

Hulsmann, Thomas, and Reinhard E. Nowack, 70 Years of Experience with PVC Pipes, 

13th World Pipe Symposium, 2004. 

Janson, Lars-Eric, Long-Term Studies of PVC and PE Pipes Subjected to Forced Constant 

Deflection, Report No. 3, KP-Council, 1991. 

Jenkins, C.F. and A.E. Kroll, “External Hydrostatic Loading of Polyethylene Pipe,” 

Proceedings of the International Conference on Underground Plastic Pipe, ASCE, pp. 527-541, 

1981. 

Jeyapalan, Jey K. and Reynold Watkins, “Modulus of Soil Reaction (E’) Values for 

Pipeline Design,” Journal of Transportation Engineering, ASCE, Volume 130, Issue 1, 

January/February 2004, pp. 43-48. 

Kenney, T.C. and D. Lau, “Reply to Discussion of Kenney and Lau Article on 

Internal Stability of Granular Filters,” Canadian Geotechnical Journal, Vol. 23, 1986, 

pp. 420-423. 

Laflaur, Jean, Hacek Mlynarek, and Andre Rollin, “Filtration of Broadly Graded 

Cohesionless Soils,” Journal of Geotechnical Engineering, ASCE, Vol. 115, No. 12, 

December 1989, pp. 1747-1767. 

178 
References 

Lombardi, Tina, “Siphon—An Economic Lake Drain Option,” ASDSO Newsletter, 

Vol. 11, No. 2, March 1996. 

McCook, Danny, Things Your Mother Never Told You About Earth Dam Design, ASDSO 

Annual Conference, 2002. 

McCook, Danny, Supplemental Tests to Evaluate Suitability of Materials Proposed for Use in 

Critical Filter Zones, ASDSO Annual Conference, 2005. 

Mendonca, Marcoa Barreto, Mauricio Ehrlich, and Magali Christie Cammarota, 

“The Formation of Ochre Biofilm in Geotextile Filters,” Geosynthetic Magazine, 2006, 

pp. 34-42. 

Milligan, Victor, “Discussion of Kenney and Lau Article on Internal Stability of 

Granular Filters,” Canadian Geotechnical Journal, Vol. 23, 1986, pp. 255-258. 

Mine Safety and Health Administration, Investigation of Embankment Failure—Sediment 

Pond SP-4, Report No. MW04-024, 2004. 

Moser, A.P., Buried Pipe Design, 2001. 

Musser, S.C., CANDE-89 User Manual, FHWA-RD-89-169, Federal Highway 

Administration, 1989. 

National Association of Women in Construction, Construction Dictionary, 1986. 

Natural Resources Conservation Service, “Hydraulics,” National Engineering Handbook, 

Chapter 5 (NEH-5), 1956. 

Natural Resources Conservation Service, “Considerations for Embankment Dams,” 

Soil Mechanics Note No. 3, 1971. 

Natural Resources Conservation Service, Permeability of Selected Clean Sands and Gravels, 

Soil Mechanics Note No. 9, 1984. 

Natural Resources Conservation Service, “Gradation Design of Sand and Gravel 

Filters,” National Engineering Handbook, Part 633, Chapter 26, 1994. 

Natural Resources Conservation Service, “Structural Design of Flexible Conduits,” 

National Engineering Handbook, Part 636 Chapter 52, 2005. 

Natural Resources Conservations Service, “Filter Diaphragms,” National Engineering 

Handbook, Part 628, Chapter 45, 2007. 

179 
Plastic Pipe Used in Embankment Dams 

Peck, R.B., W. E. Hansen, and T. H. Thornburn, Foundation Engineering, 1974. 

Performance Pipe, Heat Fusion Joining Procedures and Qualification Guide, 2006. 

Petroff, L.J., “Stress Relaxation Characteristics of the HDPE Pipe Soil System,” 

Proceedings of Conference on Pipeline Design and Installation, ASCE, 1990. 

Plastic Pipe Institute, Resistance of Thermoplastic Piping Materials to Micro- and MacroBiological Attack, TR-11/2000, 2000. 

Plastic Pipe Institute, Design Service Life of Corrugated HDPE Pipe, TR-43, 2003. 

Plastic Pipe Institute, Generic Butt Fusion Joining Procedure for Field Joining of Polyethylene 

Pipe, TR-33/2005, 2005. 

Plastic Pipe Institute, Handbook of Polyethylene Pipe, 2006. 

Ripley, C.F., “Discussion of Kenney and Lau Article on Internal Stability of Granular 

Filters,” Canadian Geotechnical Journal, Vol. 23, 1986, pp. 414-418. 

Schluter, James C. and Theodore A. Capossela, “The Suitability of Spangler’s Iowa 

Formula for Predicting Deflection in All Pipes,” Pipelines in the Constructed Environment, 

ASCE, 1998. 

Selig, E.T., “Soil Parameters for Design of Buried Pipelines,” Pipeline Infrastructure, 

ASCE, 1988, pp. 99-116. 

Selig, E.T., Installation Effects on Pipe Design, Geotechnology in Infrastructure 

Improvement, ASCE, 1996. 

Sherard, James L., Lorn Dunnigan, and T.R. Talbot, “Basic Properties of Sand and 

Gravel Filters,” Journal of Geotechnical Engineering, ASCE Vol. 110, No. 6, June, 1984, p. 

693. 

Smith, Greg and Reynold K. Watkins, “The Iowa Formula: Its Use and Misuse 

When Designing Flexible Pipe,” Pipelines International Conference Proceedings, ASCE 

2004. 

Spangler, M.G., The Structural Design of Flexible Pipe Culverts, Iowa State College 

Bulletin No. 30, 1941. 

Spangler, Merlin G. and Richard L. Handy, Soil Engineering, 1982. 

Sugar Mill Community Association, Minutes of Board of Directors meetings: 

April 18, 2002; May 7, 2002; and January 14, 2003. 

180 
References 

U.S. Army Corps of Engineers, Hydraulic Design of Reservoir Outlet Works, EM 1110-2-

1602, 1980. 

U.S. Army Corps of Engineers, Laboratory Soils Testing, EM 1110-2-1906, 1970 

(original), 1986 (change 2). 

U.S. Army Corps of Engineers, Seepage Analysis and Control for Dams, EM 1110-2-

1901, 1986 (original), 1993 (change 1). 

U.S. Army Corps of Engineers, Long Term Structural Behavior of Pipeline Rehabilitation 

Systems, 1994. 

U.S. Army Corps of Engineers, Guidelines for Trenchless Technology: Cured-in-Place Pipe 

(CIPP), Fold-and-Formed Pipe (FFP), Mini-Horizontal Directional Drilling (Mini-HDD), and 

Microtunneling, Construction Productivity Advancement Research (CPAR) Program 

Report No. CPAR-GL-95-2, Waterways Experiment Station, 1995. 

U.S. Army Corps of Engineers, Technologies and Techniques for Drainage Pipe Maintenance 

and Repair: A Practical Guide to Technology Selection, 2001. 

U.S. Army Corps of Engineers, General Design and Construction Considerations For Earth 

and Rock-Fill Dams, EM-1110-2-2300, 2004. 

Uni-Bell PVC Pipe Association, Recommended Specification for Thermoplastic Pipe Joints, 

Pressure and Non-Pressure Applications, UNI-B-95, 1995. 

Uni-Bell PVC Pipe Association, Handbook of PVC Pipe-Design and Construction, 2001. 

Vaughan, Peter and Hermosia Soares, “Design of Filters for Clay Cores of Dams,” 

Journal of the Geotechnical Engineering Division, ASCE, Vol. 108, No. GT1, January 1982, 

pp. 17-31. 

Watkins, Reynold K., Buried Pipe Encased in Concrete, ASCE, 2004. 

Watkins, R.K. and M.G. Spangler, “Some Characteristics of the Modulus of Passive 

Resistance of Soil—A Study in Similitude,” Highway Research Board Proceedings, Vol. 37, 

pp. 576-583, 1958. 

Webb, M.C., SSIS Marketing International, CandeCAD, 2005. 

Willoughby, David A., Plastic Piping Handbook, 2002. 

Wilson, Charles and Joseph Monroe, Dam Surgery—Repairs to Sugar Mill Dam, Fulton 

County, Georgia, ASDSO Southeast Regional Conference, 2004. 

181 
Additional Reading 

The following references have not been specifically cited within this document and 

are provided as suggested “additional reading.” These references are intended to 

assist the user with furthering their understanding of topics related to plastic pipe 

and its use in embankment dams. The user will find additional references related to 

conduits and embankment dams in FEMA’s Technical Manual: Conduits through 

Embankment Dams (2005). 

Sound engineering judgment should always be applied when reviewing any of these 

references. While most of these references contain valuable information, a few may 

contain certain information that has become outdated in regards to design and 

construction aspects and/or philosophies. Users are cautioned to keep this mind 

when reviewing these references for design and construction purposes. 

The user may want to periodically visit a particular agency or organization’s website 

for updates or revisions to these references. 

Ackers, P. and A.R. Thomas, “Design and Operation of Air-Regulated Siphons for 

Reservoir and Head-Water Control,” Proceedings of the Symposium on Design and Operation 

of Siphons and Siphon Spillways, Paper No. A1, 1975, pp. A1-A20. 

American Water Works Association, Evaluation of Polyvinyl Chloride (PVC) Pipe 

Performance, 1994. 

Anderson, D. Wade, Structural Design of Flexible Conduits, ASDSO Annual Conference, 

2004. 

Babaeyan-Koopaei, E.M. Valentine, and D. Alan Ervine, “Case Study on Hydraulic 

Performance of Brent Reservoir Siphon Spillway,” ASCE Journal of Hydraulic 

Engineering, Vol. 128, No. 6, June 2002, pp. 562-567. 

Beech, S.H. and E.Q. Clutton, “Interpretation of the Results of the Full Notch 

Creep Test and a Comparison With the Notched Pipe Test,” Plastic Pipes XII 

Conference Proceedings, 2004. 

Boros, Stephen, “Development of an Accelerated SCG Cyclic Pressure Fatigue Test 

Method for Polyethylene (PE) Pipe and Correlation with Hydrostatic Stress Rupture 

Test Method ASTM D 1598,” Plastic Pipes XIII Conference Proceedings, 2006. 

Bryant and Jewell, Analysis of Siphon Lake Drain Performance for a Small Earthen Dam, 

1996 ASDSO Annual Conference, 1996. 

182 
Additional Reading

Bureau of Reclamation, Design of Small Canal Structures, Chapter 4, 1978. 

Bureau of Reclamation, Water Conveyance Systems, Design Standards No. 3, Chapter 

11, “General Hydraulic Considerations,” 1994. 

Bureau of Reclamation, Method for Prediction of Flexible Pipe Deflections, M-25, 1997. 

Bureau of Reclamation, Clogging of Perforations in Plastic Pipe, 2002. 

Burgon, Rex P., Steven L. Folkman, and A.P. Moser, “Comparing Measured and 

Computed Stiffness of HDPE Pipe,” Proceedings of Transportation Research Board Annual 

Meeting, 2006. 

Burns, J.Q. and R.M. Richard, “Attenuation of Stresses for Buried Cylinders,” 

Proceedings of the Symposium on Soil Structure Interaction, 1964, pp. 378-392. 

Corleto, Carlos and Eli Venecia, “Characterisation of Rapid Crack Propagation of 

PE Pipe Resins Using Dynamic Fracture Toughness,” Plastic Pipes XII Conference 

Proceedings, 2004. 

Corrugated Plastic Pipe Association, Hydraulic Considerations for Corrugated Polyethylene 

Pipe, 1998. 

DesLauriers, Paul J. and David C. Rohlfing, “Predicting Slow Crack Growth 

Resistance in Polyethylene Resins From Detailed Polymer Microstructure Obtained 

By SEC-FTIR,” Plastic Pipes XIII Conference Proceedings, 2006. 

Fairfield, C.A., S.A. Campbell, and D.B. Reid, “Erosion Performance of Plastic 

Pipes,” Plastic Pipes XII Conference Proceedings, 2004. 

Gumbel, John, New Approach to Design of Circular Liner Pipe to Resist External Hydrostatic 

Pressure, Symposium on Pipe Rehabilitation Design, 2001. 

Hammer, Greg, If It Ain’t Broke.....Or is It, ASDSO Annual Conference, 2002. 

Horn, R., “Cut-Earth Cradle Method for Installing Conduits through Earth Dams,” 

The Journal of Dam Safety, Vol. 3, Issue 3, ASDSO, 2005, pp. 16-26. 

Howard, Amster K., “Proposed Standard Practice for Installing Buried Pipe Using 

Flowable Fill,” The Design and Application of Controlled Low-Strength Materials (Flowable 

Fill), ASTM STP 1331, ASTM, 1998. 

183 
Plastic Pipe Used in Embankment Dams 

Hsuan, Y. Grace and Timothy McGrath, “Evaluation of Stress Crack Resistance of 

Corrugated High Density Polyethylene Pipes,” Plastic Pipes XII Conference Proceedings, 

2004. 

Janson, Lars-Eric, “Investigation of the Long-Term Creep Modulus for Buried 

Polyethylene Pipes Subjected to Constant Deflection,” Advances in Underground 

Pipeline Engineering, Proceedings of the International Conference, ASCE, 1989. 

Janson, Lars-Eric, Plastics Pipes for Water Supply and Sewage Disposal, 1995. 

Jeffrey, Jared D., A. P. Moser, Steven L. Folkman, “New Design Guidelines for 

Fatigue Failure in PVC Pipe,” Plastic Pipes XII Conference Proceedings, 2004. 

Krishnaswamy, Rajendra K., “Analysis of Ductile and Brittle Failures from Creep 

Rupture Testing of High-Density Polyethylene (HDPE) Pipes,” Plastic Pipes XIII 

Conference Proceedings, 2006. 

Krishnaswamy, R.K., P.S. Leevers, M.J. Lamborn, A.M. Sukhadia, D.F. Register, 

and P.L. Maeger, “Rapid Crack Propagation (RCP) Failures in HDPE Pipes: 

Structure-Property Investigations,” Plastic Pipes XIII Conference Proceedings, 2006. 

Lamson Vylon Pipe, Plastic Pipe Design Manual, 2000. 

Lemieux, Michele, and Laurence Siroky, Use of Plastic Pipe in Dams: Variations in Design 

and Regulatory Perspective, ASDSO Annual Conference, 2001. 

Leumas, James K., To Siphon or Not To Siphon: That is the Question (Among Others)—A 

Repair History of the Crossgate Dam, ASDSO Annual Conference, 1998. 

McCook, Danny, and James Talbot, NRCS Filter Design Criteria—A Step By Step 

Approach, ASDSO Southeast Regional Conference, 1995. 

McCook, Danny, The Filter Design Tightrope, ASDSO Annual Conference, 2003. 

McGrath, Timothy J., “Replacing Ewith the Constrained Modulus in Flexible Pipe 

Design,” Pipelines in the Constructed Environment, Pipeline Division Conference, ASCE, 

1998. 

McGrath, Timothy J., “Design of Profile Wall Thermoplastic Pipe for 

Compression,” Plastic Pipes XII Conference Proceedings, 2004. 

Meijering, Tiem, “Sustainable Municipal Pipes—The Performance of Plastics Pipes 

in Sewer Systems,” Plastic Pipes XIII Conference Proceedings, 2006. 

184 
Additional Reading

Milligan, Victor, “Some Uncertainties in Embankment Dam Engineering,” Journal of 

Geotechnical and Geoenvironmental Engineering, ASCE Vol. 129, No. 9, September, 2003, 

pp. 785-797. 

Morris, H.M., Applied Hydraulics in Engineering, pp 201-204, 1963. 

Natural Resources Conservation Service, “Structural Design of Underground 

Conduits,” National Engineering Handbook, Technical Release No. 5, 1958. 

Natural Resources Conservation Service, Hydraulics of Two-Way Covered Risers, 

Technical Release 29 (TR-29), 1965. 

Natural Resources Conservation Service, Drop Inlet Entrance Losses, Design Note 8 

(DN-08), 1969. 

Ng, Zena S. and Cindy Connolly, “Choosing PVC Resins for Formulation,” Plastic 

Pipes XIII Conference Proceedings, 2006. 

Pabst, Mark, Ochoco Dam Drain Rehabilitation, ASDSO Annual Conference, 1999. 

Pabst, Mark, Considerations for Filter Design, USSD Annual Conference, 2007. 

Phillips Drisco, Pipe Design Manual, “Table 9: Differential Pressure (Vacuum or 

External Fluid) Capability for Unsupported Pipe” and “Table 10: Multipliers for 

Temperature,” 1998. 

Plastic Pipe Institute, The Complete Corrugated Polyethylene Pipe Design Manual and 

Installation Guide, 2007. 

Rahman, Shah and Wayne Bird, “PVC Pipe Joining: The Reiber System in North 

America,” Plastic Pipes XIII Conference Proceedings, 2006. 

Roth, Bruce L. and Mathew Snyder, The Beneficial Use of Flowable Backfill Around 

Concrete Pipes, ASDSO Annual Conference, 1992. 

Sherard, J.L., Sinkholes in Dams of Coarse, Broadly Graded Soils, Thirteenth Congress on 

Large Dams, Question 49, R, 2, 1979. 

Sherard, James L. and Lorn Dunnigan, “Discussion of Kenney and Lau Article on 

Internal Stability of Granular Filters,” Canadian Geotechnical Journal, Vol. 23, 1986, 

pp. 418-420. 

Thomas, Craig and Richard Horvath, Installation and Operation of a Siphon System to 

Provide Water Supply Releases, ASDSO Northeast Regional Conference, 2003, 

pp. 183-184. 

185 
Plastic Pipe Used in Embankment Dams 

U.S. Army Corps of Engineers, Controlled Low Strength Material with Coal-Combustion 

Ash and Other Recycled Materials, ETL 110-3-496, 1996. 

U.S. Army Corps of Engineers, Conduits, Culverts, and Pipes, EM 1110-2-2902, 1997 

(original), 1998 (change 1). 

U.S. Department of Transportation, Federal Highway Administration, Hydraulic 

Charts for the Selection of Highway Culverts, HEC-5, 1965, Reprinted 1977. 

Uni-Bell PVC Pipe Association, Deflection—The Pipe/Soil Mechanism, UNI -TR-1-97, 

1997. 

Watkins, Reynold K., “Influence of Soil Characteristics on the Deformation of 

Embedded Flexible Pipe Conduits,” Highway Research Board, Bulletin 2231959, 

pp. 14-24, 1959. 

Watkins, Reynold King and Loren Runar Anderson, Structural Mechanics of Buried Pipes, 

2000. 

Watkins, Reynold King, “Pipe and Soil Mechanics for Buried Corrugated HDPE 

Pipe,” Plastic Pipes XII Conference Proceedings, 2004. 

Webb, M.C., T.J. McGrath, and Ernest T. Selig, “Field Test of Buried Pipe with 

CLSM Backfill,” The Design and Application of Controlled Low-Strength Materials (Flowable 

Fill), ASTM STP 1331, ASTM, 1998. 

Zhou, Zhiqiang Jimmy and Dane Chang, “Performance of Scratched HDPE 

Pressure Pipe,” Plastic Pipes XIII Conference Proceedings, 2006. 

186 
Index

A

AASHTO standards, xxvii, 9, 31, 32, 57, 67, 68, 104, 105, 132 

Abandonment/grouting of a drain system, 124-126 

Abbreviations, xviii 

Access, 1, 3, 5, 98, 101, 109, 111, 113, 122, 123, 124, 129, 149, 150, 151, 152, 160 

improving, 122-124 

Aggregate, 80, 84, 85, 86, 116, 117, 120, 142, 162, 163 

Anchors, 95 

Arching action of a conduit, 38 

ASTM standards, xxviii-xxxii, 6, 8, 9, 10, 11, 12, 14, 15, 17, 20, 22, 26, 30, 31, 32, 53, 

59, 67, 69, 70, 75, 84, 94, 107, 116, 117, 118, 120, 138, 139, 142 

AWWA standards, xxxiii, 6, 8, 17, 23, 32, 51, 53, 54, 60, 66, 69, 70, 71, 91, 94, 98, 

127 

B

Backfill, 118-126, 30, 35, 40, 42, 43, 55, 57, 60, 64, 66, 79, 80, 82, 83, 86, 89, 101, 109, 

118, 119, 120, 122, 128, 132, 134, 137, 157, 159, 166, 167, 168, 173, 175 

compaction methods, 137-139 

nonperforated drainpipe, 80, 118-120, 134 

perforated drainpipe, 80, 118-120, 134 

Barrier condition, 119 

Bell and spigot joints, 15, 17, 18, 20, 23, 28, 29, 81, 93, 103 

Borrow sources, 139, 140, 158 

Buckling, see Wall buckling 

Buried conduits, classification, 34, 35, 40 

Butt fused HDPE pipe joints, 12, 13, 14, 15, 16, 24, 93, 94, 104, 146 

Calcite deposits, 154, 156 

Capacity, see Flow capacity 

Carrier pipe, 11, 14 

187 

C
Plastic Pipe Used in Embankment Dams 

Cast-in-place, 25, 33, 53, 68, 73, 78, 79, 80, 81, 82, 83, 99, 109, 127, 166, 167, 168, 

174 

CCTV inspection, 12, 26, 102, 111, 112, 113, 122, 124, 125, 129, 146, 148, 149, 150, 

151, 152, 153, 154, 157, 158, 159, 160, 168 

equipment, 102, 112, 122, 124, 129, 149, 160 

CIPP, 21-23, 29, 30, 53 

Classification of buried conduits, 2, 31, 34, 35, 39, 40, 53, 67, 99, 127 

Cleanouts, 101, 109-112, 149, 150, 152 

Closed circuit television, see CCTV inspection 

CLSM, 83, 84, 85, 86, 87, 88, 89, 90, 91, 95, 96, 173, 174 

as an encasement material, 96, 83, 84, 85, 86, 87, 88, 89, 90, 91, 95, 96, 173, 174 

design considerations, 86, 83, 84, 85, 86, 87, 88, 89, 90, 91, 95, 96, 173, 174 

problems, 83, 84, 85, 86, 87, 88, 89, 90, 91, 95, 96, 173, 174 

Coefficient of thermal expansion, 25, 91, 92, 97 

Collapse of pipes, 49, 50, 60, 62, 75, 76, 79, 82, 83, 96, 132, 174 

due to grout pumping pressure, 96-97 

Compaction of earthfill, 35, 37, 38, 40, 58, 83 

methods for around drainpipes, 25, 27, 29, 89, 132, 137-139, 146 

Complete condition, 39, 40, 41 

Concrete, 2, 24, 25, 28, 30, 33, 35, 39, 50, 53, 57, 68, 73, 74, 75, 76, 78, 79, 80, 81, 82, 

83, 84, 85, 88, 89, 90, 93, 94, 95, 96, 97, 99, 109, 116, 120, 127, 142, 163, 166, 167, 

168, 173, 174 

encasement, 2, 25, 33, 74, 78-83, 89, 93, 96, 166 

reinforced cast-in-place concrete encasement, 2, 25, 33, 74, 78-83, 93, 166 

reinforced concrete cradle, 81, 74, 80, 81, 83 

unreinforced cast-in-place concrete encasement, 2, 25, 33, 74, 78-83, 93, 166 

Conduits 

arching action, 38 

buried, 33, 34, 39 

embankment, 1, 2, 5, 6, 10, 11, 15, 17, 18, 20, 21, 22, 24, 25, 27, 28, 29, 30, 33, 35, 

37, 38, 39, 40, 53, 60, 63, 69, 78, 79, 80, 83, 84, 93, 94, 97-100, 127-129, 147, 

148, 168 

embankment, construction, 97, 127, 128, 147 

embankment, inspection, 97, 127, 147-149 

Construction, 1, 2, 5, 8, 16, 24, 25, 27, 29, 33, 34, 39, 40, 44, 50, 51, 55, 57, 64, 76, 78, 

79, 82, 83, 84, 88, 89, 91, 92, 93, 94, 95, 98, 99, 102, 106, 113, 114, 119, 120, 123, 

127, 128, 129, 132, 137, 138, 140, 142, 143, 147, 153, 157, 158, 161, 162, 163, 164, 

166, 167, 168, 172 

considerations, 92-97 

drainpipe, 102, 131, 132 

equipment, 50, 120, 132 

guidance, 127-146 

loading, 33, 50, 51, 157 

188 
Index

Containment pipe, 11, 12, 14, 24-26 

dual-wall, 12, 24-26 

Contraction, 81, 91, 92, 95, 96, 97 

Controlled low strength material, see CLSM 

Corrugated HDPE pipe, 3, 9, 11, 12, 15, 17, 26, 27, 31, 32, 59, 61, 62, 64, 70, 72, 75, 

102, 103, 104, 128, 151, 153, 162, 172 

Cover soil over plastic pipe, 6, 50, 51, 60, 66, 67, 68, 86, 95, 111, 112, 115, 120, 132, 

145, 163, 164 

Cracking of PVC pipe, 31, 62, 84, 87, 89, 113, 157, 168, 174 

Cradle, see Concrete, reinforced concrete cradle 

Cured in place pipe, see CIPP 

D

Dams 

construction, 8, 9, 10, 23, 85, 102, 136, 175 

embankment, 1, 2, 3, 5-26, 30, 32, 33, 34, 35, 37, 39, 40, 44, 50, 53, 63, 64, 71, 74, 

76, 78, 79, 83, 86, 98, 99, 101, 103, 121, 122, 127, 128, 137, 147, 161, 162, 163, 

166, 167, 172 

high hazard potential, 1, 17, 18, 25, 27, 33, 34, 63, 68, 78, 79, 80, 81, 83, 84, 86, 88, 

99, 115, 127, 132, 136, 169 

low hazard potential, 1, 17, 20, 23, 27, 28, 29, 33, 55, 60, 64, 79, 83, 85, 86, 94, 99, 

114, 115, 127, 136 

raise, 44, 45 

size, 103 

Davis Creek Dam, 153 

Decant pipe, 6, 64, 161, 162, 163, 164, 165, 166, 167, 169 

Deflection of pipes, 30, 34, 36, 37, 40, 53, 54, 57, 58, 59, 60, 62, 63, 64, 65, 66, 67, 

68, 69, 72, 73, 74, 76, 78, 79, 81, 83, 86, 87, 89, 90, 93, 164, 166, 172, 173, 174 

Deposition of soil, 138, 163 

Design 

considerations, 6, 53, 76, 78, 92-97 

considerations for using CLSM, 86, 86 

encased pipe, 90 

flexible pipe, 53, 73, 91 

hydraulic design of embankment conduits, 53, 97, 98 

hydraulic design of drainpipes, 78, 102-109 

life, 30-32 

siphon, 49 

structural design of drainpipes, 33, 54, 55, 74, 87, 92, 101, 102 

values for the settlement ratio, rsd, 43 

189 
Plastic Pipe Used in Embankment Dams 

when to use flexible or encased plastic pipe design, 88-91 

zoning, 121, 122 

Drainpipe, 1, 2, 3, 5, 6, 17, 18, 26, 27, 28, 29, 30, 31, 33, 35, 36, 37, 38, 45, 50, 53, 63, 

64, 69, 78, 80, 90, 94, 98, 101-115, 118, 119, 121, 122, 123, 124, 125, 127, 128, 129, 

130, 131, 132, 135, 136, 137, 143, 144, 145, 147, 149-160, 162 

construction, 102, 131, 132 

diameter, 103 

embedment, 119 

HDPE, 153, 156, 157 

inspection well, 110 

nonperforated drainpipe, 119-120, 118, 119 

perforated drainpipe, 120-121, 104, 118, 120 

placement around, 130-137, 130 

Drains 

drain system, 101, 113, 122, 124, 125, 126, 130, 131, 158, 159 

materials, 80, 104, 105, 116, 117, 120, 130-140, 142, 144, 158, 159, 164 

replacement, 118, 119 

toe, 1, 26 

Dual-wall containment pipe, see Containment pipe, dual wall 

E

Earthfill, see Backfill 

Embankment conduits, 1, 2, 5, 6, 11, 15, 17, 18, 20, 21, 22, 24, 25, 27, 28, 29, 30, 33, 

35, 37, 38, 39, 40, 53, 60, 63, 69, 78, 79, 80, 83, 84, 93, 94, 96, 97-101, 127-129, 

147, 148, 168 

Embedment materials, 68, 78, 79, 88, 96 

comparison with encasement materials, 88, 89 

considerations, 78-91 

research needs, 172-175 

soil as embedment material, 68, 78, 79, 88, 96 

Encased pipe, 74-76, 90 

design, 76-78, 88-91 

Encasement materials, 3, 50, 51, 78-91, 95, 96, 97, 171, 172, 173, 174 

comparison of, 90 

comparison with embedment materials, 88, 89 

reinforced cast-in-place concrete, 81-83, 90 

research needs, 172-175 

unreinforced cast-in-place concrete, 83, 90 

End restraint design, 92 

190 
Index

Equipment, 12, 14, 24, 25, 26, 27, 28, 29, 30, 51, 84, 88, 111, 113, 120, 122, 132, 137, 

138, 140, 142, 143, 144, 145, 149, 151, 152, 157, 166, 167, 168 

CCTV inspection, 102, 112, 122, 124, 129, 149, 160 

construction, 50, 120, 132 

Excavation, 37, 38, 82, 98, 122, 123, 128, 130, 131, 133 

contaminated materials, 141 

trench, 35, 82, 133 

Expansion, 81, 91, 92, 95, 96, 97 

External hydrostatic pressure, 33, 45, 50, 60, 61, 74, 76, 78, 79, 87, 96, 166 

Extrusion gun, 15 

Extrusion line, 9 

F

Factor of safety, 10, 31, 60, 70, 76, 77 

Failure, 1, 31, 46, 55, 58, 59, 73, 79, 93, 94, 101, 113, 124, 127, 128, 147, 153, 157, 

166, 174 

modes, 58, 59, 101, 113 

Fill height, 41, 57, 63, 83, 164 

Fill material, 35, 37, 38, 40, 43, 146 

Filters, 34, 36, 64, 81, 83, 97, 99, 101, 104, 105, 106, 108, 113-121, 124, 127, 128, 129, 

130, 131, 132, 133, 134, 135, 137, 138, 139, 140, 141, 142, 143, 144, 158, 159, 163, 

167, 173 

gradation limits, 116-118 

material, 105, 106, 108, 114, 120, 127, 130, 131, 142 

placement, 130, 133, 135 

Flexible pipe, 30, 53-74, 79, 83, 91, 93, 102, 162, 173 

design, 53, 54, 58, 73, 76-78, 88-91 

failure modes, 59 

Flow capacity, 63, 103, 105, 118 

Flowable fill, see CLSM 

Foundations, 1, 2, 28, 34, 35, 36, 37, 40, 43, 44, 53, 64, 73, 93, 99, 101, 102, 103, 106, 

108, 114, 116, 118, 119, 121, 122, 124, 127, 128, 129, 130, 154, 158, 159, 162 

preparation, 129, 130 

problems, 93 

G

Ganado Dam, 129 

191 
Plastic Pipe Used in Embankment Dams 

Gradation, 84, 106, 116, 117, 118, 119, 120, 142, 144, 158 

filter gradation limits, 116-118 

Grout, 16, 24, 50, 61, 74, 79, 87-90, 92, 95, 96, 124, 164, 166, 173 

abandonment/grouting of a drain system, 124-126 

as encasement material, 96 

pumping pressure, 96, 97 

H

Haunches, 25, 27, 29, 35, 79, 80, 81, 83, 86, 89, 132, 157, 174 

HDPE pipe, 9-17, 20, 24, 25, 26, 29, 31, 51, 73, 81, 94, 96, 103, 104, 148, 162, 166 

calcite deposits, 156 

dual-wall containment, 24, 25 

iron bacteria blockage, 156 

joints, 13, 14 

single wall corrugated HDPE drainpipe, 60, 61, 153 

slipliner, 24, 148 

solid wall, 10, 16, 26, 31, 102, 104, 162 

typical modulus of elasticity values, 54 

Heat fusion process, 19 

Historical perspective, 5, 6 

Hydraulic design, 53, 97, 98 

Hydraulic loading conditions, 1, 5, 33, 34, 45-50, 53, 86 

Hydrostatic inversion method, 22 

Hydrostatic pressure 

external, 33, 45, 50, 60, 61, 74, 76, 78, 79, 87, 96, 166 

internal, 33, 45-49, 59, 69-71, 74, 76, 78, 86 

I-L

Idealized cross sections, 115 

Incomplete condition, 38, 40, 41 

Inspection, 1, 2, 3, 12, 14, 26, 74, 101, 102, 109, 111, 112, 113, 121, 122, 123, 125, 

140, 143, 147-160, 168 

CCTV, 12, 26, 102, 111, 112, 113, 122, 124, 125, 129, 146, 148, 149, 150, 151, 152, 

153, 154, 157, 158, 159, 160, 168 

wells, 101, 109-112, 122, 149 

Installation, 1, 6, 9, 21, 22, 23, 24, 26, 28, 29, 30, 31, 32, 43, 51, 55, 64, 69, 78, 82, 91, 

93, 94, 95, 98, 102, 103, 109, 114, 124, 127, 128, 129, 130, 132, 144, 145, 157, 162, 

167, 168 

192 
Index

Interior prism, 36, 37, 38, 39, 40, 88, 90 

Interior surfaces, 151 

Internal hydrostatic pressure, 45, 46, 49, 50, 60, 63, 65, 69-71, 73, 74, 75, 76, 78, 79 

Introduction, 1-3 

Iron bacteria, 115, 154, 156 

Joints 

bell and spigot, 15, 17, 18, 20, 23, 28, 29, 81, 93, 103 

butt fused, 14, 15, 16, 24, 94, 104, 146

restrained, 19, 20 

separation, 129 

snap, 17 

splined, 19 

Leak testing, 94, 93 

Liner, CIPP, 21, 22, 23 

Load coefficient, 41, 42, 44, 45 

M

Marston load, 33, 34, 35, 39, 40, 41, 43, 44, 66, 78, 90 

positive projecting conduits, 41-44 

Maximum perforation dimension, 118 

Meshes, two- and three-dimensional, 56 

Modes, failure, 58, 59, 101, 113 

Modified Iowa Equation, 62, 64, 65, 66, 67, 68, 86 

Modulus of elasticity, 29, 48, 50, 54, 60, 65, 72, 75, 92 

Modulus of soil reaction, 57, 60, 65, 66, 67, 68, 69, 79, 86, 87, 173 

N, 0 

Negative projecting conduit, 37, 38, 39, 82 

Nonperforated drainpipe, 118-120 

Outlet works, 1, 2, 5, 11, 22, 23, 24, 33, 36, 49, 98, 127, 148 

Outlet works slipliner, 23 

193 
Plastic Pipe Used in Embankment Dams 

P

Perforated drainpipe, 104, 118, 120, 121 

Perforations in pipe, 2, 103, 104, 105, 109, 114, 115, 145, 146, 154, 156, 172 

circular, 11, 104, 105 

slotted, 104, 163 

Pipe, see Plastic pipe 

Placement 

drain material, 115, 133 

filter, 130, 133, 135 

of a cast-in-place base slab for an inspection well, 110 

of fill, 85, 74, 130-137 

temperature, 95-97 

Plastic pipe 

carrier, 11, 14 

CIPP, 21, 22, 23, 29, 30, 53 

collapse, 49, 50, 60, 62, 75, 76, 79, 82, 83, 96, 97, 132, 174 

common types used in embankment dams, 6-24 

common uses, 23-30 

corrugated, 9, 11, 12, 17, 26, 27, 31, 72, 103 

corrugated profile wall, 102 

corrugated single wall, 102 

decant, 6, 161, 162, 163, 164, 165, 167, 169 

design, 23, 51, 68, 74, 76-78, 81, 83, 87, 88, 89, 90, 91, 127 

drainpipes, 1, 2, 3, 5, 6, 26, 27, 28, 29, 33, 37, 38, 50, 53, 63, 64, 69, 78, 90, 94, 101, 

102, 104, 105, 108, 109, 111, 112, 113, 114, 115, 118, 122, 123, 128, 129, 132, 

136, 137, 147, 149, 153, 154, 157, 162 

dual-wall containment, 10, 11, 14, 24, 25, 26 

encased, 74-76, 90 

failure modes, 31, 58, 59, 101, 113, 124 

flexible, 30, 53-74, 79, 83, 91, 93, 102, 162, 173 

HDPE, 8, 9, 10, 12, 13, 14, 15, 16, 20, 24, 25, 26, 29, 31, 48, 51, 53, 54, 71, 73, 75, 

77, 81, 91, 94, 96, 97, 102, 103, 104, 148, 153, 156, 157, 162, 166, 171, 172 

in tailings disposal facilities and slurry impoundments, 161-170 

material, research needs, 78, 91, 171, 172 

minimum pipe stiffness, 77 

outlet works, 23 

PVC, 6, 7, 8, 15, 17, 18, 19, 20, 27, 28, 29, 32, 48, 53, 54, 69, 71, 73, 75, 77, 91, 94, 

95, 97, 98, 102, 103, 104, 127, 128, 153, 172 

rigid, 36, 53, 55, 73, 74, 102, 174 

solid wall, 8, 10, 11, 12, 15, 16, 17, 24, 25, 26, 31, 48, 54, 58, 61, 64, 65, 69, 72, 73, 

76, 77, 102, 103, 104, 162, 172 

thermoplastic, 7, 20-21, 29, 31, 53, 57, 95 

194 
Index

thermoset plastic, 7, 21-23, 53 

when to use flexible or encased plastic pipe design, 88-91 

Pore pressure, 118, 119 

Positive projecting conduits, 36-44, 82, 90, 102 

Pressure 

external hydrostatic, 45, 50, 74, 78, 87 

grout pumping, 96, 97 

internal hydrostatic, 45-49, 59, 69-71, 74, 76, 78 

internal vacuum, 45, 49, 50, 60, 63, 65, 69, 73, 74, 75, 76, 78, 79

pore, 116, 118, 119 

unconstrained collapse, 75, 76, 77 

water pressure, 21, 23, 38, 86 

Prism 

exterior, 36, 37, 38, 39, 40, 88, 90 

interior, 10, 11, 12, 13, 16, 17, 24, 26, 28, 30, 36, 37, 38, 39, 40, 73, 81, 83, 87, 88, 

90, 103, 113, 146, 148, 151, 159, 162 

soil prism load, 36, 37, 39-42, 66, 90 

Problems, 32, 35, 55, 56, 57, 93, 95, 97, 113, 122, 127, 128, 146, 147, 153, 163, 166, 

167, 168, 169 

foundation, 93 

with using CLSM, 86, 87 

Processing plant, 120, 140, 158, 162 

Profile wall corrugated HDPE pipe, 73, 104, 149, 151, 162 

Projecting, see Positive projecting conduit or Negative projecting conduit 

Projection ratio, 43, 44 

Pumping, grout, 96, 97 

PVC pipe, 6, 7, 8, 15, 17, 18, 19, 20, 27, 28, 29, 32, 48, 53, 54, 69, 71, 73, 75, 77, 91, 

94, 95, 97, 98, 102, 103, 104, 127, 128, 153, 172 

modulus of elasticity, 54 

pressure pipe, 20 

slotted, 105 

solid wall, 17, 102 

R

Reinforced cast-in-place concrete encasement, 81-83, 90 

Reinforced concrete cradle, 81, 90 

Renovation, 1, 2, 11, 20, 21, 24, 25, 30, 33, 98, 99, 112 

drainpipes, 112-114 

embankment conduits, 98-100 

Repair, 2, 15, 16, 94, 98, 99, 109, 113, 143, 157 

195 
Plastic Pipe Used in Embankment Dams 

Replacement, 1, 98, 99, 103, 113, 118, 119 

drainpipes, 99, 112-114 

Research items, 171-175 

Research needs, 171-177 

Resin, 7, 21, 22, 23, 31, 32 

Rigid pipe, 73-74 

Roadway crossing, 141 

S

Sand, 68, 85, 108, 114, 116, 118, 119, 120, 121, 124, 138, 139, 143, 158, 163 

SDR, see Standard dimension ratio 

Sediment, 97, 103, 109, 111, 113, 122, 123, 124, 129, 152, 157, 158, 159, 161, 166 

Sediment Control Pond SP-4 Dam, 97 

Seepage, 1, 35, 37, 40, 82, 83, 84, 87, 89, 97, 101, 102, 106, 107, 108, 113, 115, 118, 

121, 124, 154, 156, 161, 162, 163, 166, 167 

Segregation, 85, 87, 137, 138 

Settlement ratio, rsd, 43, 44 

Silt, 118, 119, 131, 138, 158 

Single wall corrugated HDPE pipe, 51, 59, 60, 61, 62, 76, 153 

failure, 59, 61, 153 

unconstrained collapse pressure vs. minimum pipe stiffness, 77 

Siphons, 1, 24, 27, 28, 33, 36, 49, 61, 63, 69, 78, 98, 127 

Slipliners, 15, 16, 68, 74, 79, 87, 88, 92, 94, 96, 114 

Slotted pipe, 2, 11, 12, 104, 105, 115, 116, 118, 154, 156, 162, 163, 172 

Slurry impoundments, 24, 161-171 

Snap joints, 17 

Soil 

as embedment material, 79-80, 96 

as embedment material, 96 

loading, 11, 33, 34, 36, 37, 39-41, 44, 45, 54, 58, 63, 66, 74, 87, 88, 90 

modulus of soil reaction, E, 57, 60, 65, 66, 67, 68, 69, 79, 86, 87, 173 

segregation, 138 

Solid wall pipe, 8, 10, 11, 12, 15, 16, 17, 24, 25, 26, 31, 48, 54, 58, 61, 64, 65, 69, 72, 

73, 76, 77, 102, 103, 104, 162, 172 

containment pipe, 24-26 

HDPE, 10, 16, 26, 31, 102, 104, 162 

PVC, 17, 102 

Spacers, 11, 95 

Spigot, see Bell and spigot joints 

Standard dimension ratio, 48, 49, 54, 65, 69, 70, 72, 75, 76, 77, 89, 162 

196 
Index

Standards 

AASHTO, xxv, 9, 31, 32, 57, 67, 68, 104, 105, 132 

ASTM, xxvi-xxx, 6, 8, 9, 10, 11, 12, 14, 15, 17, 20, 22, 26, 30, 31, 32, 53, 59, 67, 69, 

70, 75, 84, 94, 107, 116, 117, 118, 120, 138, 139, 142 

AWWA, xxxi, 6, 8, 17, 23, 32, 51, 53, 54, 60, 66, 69, 70, 71, 91, 94, 98, 127 

Structural design, 33, 54, 55, 74, 78, 87, 92, 101, 102 

Sugar Mill Dam, 97 

Symbols, xxi-xxiv 

T

Temperature, 7, 12, 14, 20, 21, 70, 71, 91, 92, 95, 96, 97, 146, 173 

placement, 95-97 

reduction factors, 71 

Thermoplastic pipe, 7, 8, 9, 19, 20, 21, 29, 31, 53, 57, 95 

Thermoset plastic pipe, 7, 21, 22, 53 

Thrust blocks, 94, 95 

Toe drains, 1, 26 

Trapezoidal trenches, 130, 131, 132, 135 

Trenches, 35, 36, 37, 38, 39, 40, 41, 42, 44, 69, 82, 83, 86, 90, 96, 102, 114, 130, 131, 

132, 135, 136, 145, 146 

box, 132, 136 

condition, 36, 37, 39, 41, 44, 90, 145 

conduit, 35, 39-41, 90, 102 

excavation., 35, 133 

trapezoidal, 130, 132, 135 

Types of plastic pipe, 20-21 

U, V 

Unconstrained collapse pressure, 75, 76, 77 

Unreinforced cast-in-place concrete encasement, 83, 90 

Upper Wheeler Reservoir Dam, 97 

Vacuum pressure, 33, 49, 50, 63, 86 

Venting, see Air venting 

Virginia Dam, 76, 166 

197 
Plastic Pipe Used in Embankment Dams 

W-Z

Water filled liner, 23 

Water jetting, 160 

Websites, xxxii 

Wells, see Inspection, wells

Wheatfields Dam, 11 

Worster Dam, 24 

Zoning, 114-116, 121 

design, 121, 122 

198 
Glossary

The terms defined in this glossary use industry-accepted definitions whenever 

possible. The source of the definition is indicated in parentheses. 

Abrasion (ASTM, 2002): A rubbing or wearing away. 

Additive (PPI, 2006): A substance added in small amount for a special purpose 

such as to reduce friction, corrosion, etc. 

Allowable strain: A change in pipe dimension relative to the original dimension 

that provides an adequate factor of safety against unacceptable performance or 

failure. 

Angle of friction (ASTM, 2002): Angle whose tangent is the ratio between the 

maximum value of shear stress that resists slippage between two solid bodies at rest 

with respect to each other, and the normal stress across the contact surface. 

Anisotropy: Exhibiting properties with different values when measured in different 

directions. For soils, typically the horizontal permeability is greater than the vertical 

permeability due to layering introduced during deposition. 

Antioxidant: A plastic additive to extend the temperature range and service life. 

Arching: The condition in which vertical pressures within backfill in a trench is 

reduced because of the transfer of stress at the backfill/excavation surface interface. 

Backfill (FEMA, 2005): Soil or concrete used to fill excavations. 

Bead: Small ridge formed around the circumference of a polyethylene pipe joint as 

the two pipe ends are brought together during the butt fusion process. 

Bell and spigot gasket joint: See Joint, bell and spigot gasket. 

Biofouling: An accumulation and growth of deposits or contamination linked to 

microbial activity. 

Borrow (AGI, 1987): Earth material (sand, gravel, etc.) taken from one location 

(such as a borrow pit) to be used for fill at another location; e.g. embankment 

material obtained from a pit when there is insufficient excavated material nearby to 

199 
Plastic Pipe Used in Embankment Dams 

form the embankment. The implication is often present that the borrowed material 

has suitable or desirable physical properties. 

Branch saddle: A fitting which is bonded to the exterior of a pipe to assist in 

transferring tensile loads in the pipe to a concrete thrust block. 

Branching: The growth of a second chain of a polymer out of another one by 

replacement of a hydrogen atom on a monomer by a free-radical reaction, or by a 

condensation or other chemical reaction with a reactive group on a monomer. 

Breakdown: Undesired alteration of soil gradation by mechanical action such as 

loading, pushing, and compacting. 

Broadly graded: A soil consisting of a wide range of particle sizes where cu  5. 

Buckling: See Wall buckling. 

Butt fusion (PPI, 2006): A method of joining polyethylene pipe where two pipe 

ends are heated and rapidly brought together under pressure to form a homogeneous 

bond. 

Butt fusion joint: See Joint, butt fusion. 

Camera-crawler (FEMA, 2005): A video camera attached to a self-propelled 

transport vehicle (crawler). Typically, the camera-crawler is used for closed circuit 

television inspection of inaccessible conduits. 

Carbon black (PPI, 2006): A black pigment produced by the incomplete burning 

of natural gas or oil, that possesses excellent ultraviolet protective properties. 

Carrier pipe: The interior pipe of a dual containment pipe system. 

Cell classification: A method used to classify thermoplastic compounds based on 

the material's composition and select properties. 

Centralizer: Provides support to the carrier pipe within the containment pipe. 

Chimney filter: See Filter, chimney. 

Closed circuit television (CCTV) (FEMA, 2005): A method of inspection 

utilizing a closed circuit television camera system and appropriate transport and 

lighting equipment to view the interior surface of conduits. 

Coefficient of internal friction (ASTM, 2002): The tangent of the angle of 

internal friction. 

200 
Glossary

Coefficient of thermal expansion (ACI, 2000): Change in linear dimension per 

unit length or change in volume per unit volume per degree of temperature change. 

Colorant: A plastic additive used to provide color. 

Complete condition: A loading condition for an encased plastic pipe when the 

embankment height is less than or equal to the height of the plane of equal 

settlement. The frictional forces between the interior and exterior prisms extend to 

the top of the embankment. 

Compound: A mixture of ingredients before the final processing into a completed 

product. 

Conduit (FEMA, 2004): A closed channel to convey water through, around, or 

under an embankment dam. 

Containment pipe: The outer pipe of a dual containment pipe system. 

Contamination: The introduction of unwanted material into a dam, typically during 

construction, such as tracking core material onto a filter. 

Controlled low strength material (CLSM) (FEMA, 2005): A self-compacting, 

cementitious material typically used as a replacement for compacted backfill around a 

conduit. 

Corrosion (ACI, 2000): Disintegration or deterioration of a material by electrolysis 

or chemical attack. 

Corrugated metal pipe (CMP) (FEMA, 2005): A galvanized light gauge metal 

pipe that is ribbed to improve its strength. 

Coupling agent: A plastic additive to improve the properties of the plastic material. 

Coupling: A mechanical device that serves to connect the ends of pipes. 

Crosslinking: Chain-reaction polymerization which results in chemical links 

(bonds) between individual polymer chains. 

Cured-in-place pipe (CIPP) (ASTM, 2003): A hollow cylinder consisting of a 

fabric tube with cured (cross-linked) thermosetting resin. Interior or exterior plastic 

coatings, or both, may be included. The CIPP is formed within an existing conduit 

and takes the shape of and fits tightly to the conduit. 

201 
Plastic Pipe Used in Embankment Dams 

Dam (FEMA, 2005): An artificial barrier that has the ability to impound water, 

wastewater, or any liquid-borne material, for the purpose of storage or control of 

water. 

Dam failure (FEMA, 2004): A catastrophic type of failure characterized by the 

sudden, rapid, and uncontrolled release of impounded water or the likelihood of 

such an uncontrolled release. There are lesser degrees of failure, and any 

malfunction or abnormality outside the design assumptions and parameters that 

adversely affect an embankment dam’s primary function of impounding water is 

properly considered a failure. These lesser degrees of failure can progressively lead 

to or heighten the risk of a catastrophic failure. They are, however, normally 

amenable to corrective action. 

Dam safety (FEMA, 2004): Dam safety is the art and science of ensuring the 

integrity and viability of dams, such that they do not present unacceptable risks to 

the public, property, and the environment. Dam safety requires the collective 

application of engineering principles and experience, and a philosophy of risk 

management that recognizes that an embankment dam is a structure whose safe 

function is not explicitly determined by its original design and construction. Dam 

safety also includes all actions taken to identify or predict deficiencies and 

consequences related to failure, and to document and publicize any unacceptable 

risks, and reduce, eliminate, or remediate them to the extent reasonably possible. 

Decant: A structure used in mining operations to draw water off a reservoir after 

the heavier materials have settled out. 

Deflection (FEMA, 2005): The decrease in the vertical diameter of a pipe due to 

load, divided by the nominal diameter, expressed as a percent. 

Deformation (ACI, 2000): A change in dimension or shape due to stress. 

Design (FEMA, 2005): An iterative decisionmaking process that produces plans by 

which resources are converted into products or systems that meet human needs or 

solve problems. 

Designer (FEMA, 2005): A registered engineer representing a firm, association, 

partnership, corporation, agency, or any combination of these who is responsible for 

the supervision or preparation of plans and specifications associated with an 

embankments dam and its appurtenances. 

Dimension ratio: See Standard dimension ratio. 

Dimpling: Localized instability (buckling) resulting in a wavy checkerboard 

appearance on the inner surface of a pipe wall. 

202 
Glossary

Double stage filter/drain: A system consisting of a coarse drainage zone (gravel) 

surrounding the pipe and a filter (sand) zone surrounding the coarse element. 

Drainpipe: A system of pipe within a embankment dam used to collect seepage 

from the foundation and embankment and convey it to a free outlet. 

Dual wall containment pipe: A pipe systems that provides a secondary 

containment pipe around the carrier pipe. Any leakage from the carrier pipe will be 

safely contained within the containment pipe. 

Durability (ACI, 2000): The ability of a material to resist weathering, chemical 

attack, abrasion, and other conditions of service. 

Embankment dam (FEMA, 2005): Any dam constructed of excavated natural 

materials, such as both earthfill and rockfill dams, or of industrial waste materials, 

such as a tailings dams. 

End restraint: A structural member designed to resist the anticipated 

expansion/contraction forces caused by temperature change in a plastic pipe. 

Engineer (FEMA, 2005): A person trained and experienced in the profession of 

engineering; a person licensed to practice the profession by the appropriate authority. 

Environmental stress cracking (ASTM, 2001): The development of cracks in a 

material that is subjected to stress or strain in the presence of specific chemicals. 

Extender: A plastic additive used to reduce cost. 

External hydrostatic pressure: Pressure on the outside of the pipe due to water 

surrounding the pipe in the voids or in the soil surrounding the pipe. 

Exterior prism: The soil adjacent to the soil directly above the buried conduit (the 

interior prism). 

Extrusion joint: See Joint, extrusion. 

Fibrous reinforcement: A plastic additive used to improve the strength to weight 

ratio. 

Filler: A plastic additive to improve properties of the resin. 

Filter (FEMA, 2005): A zone of material designed and installed to provide 

drainage, yet prevent the movement of soil particles due to flowing water. 

203 
Plastic Pipe Used in Embankment Dams 

Chimney (FEMA, 2005): A chimney filter is a vertical or near vertical 

element in an embankment dam that is placed immediately downstream of 

the dam’s core. In the case of a homogenous embankment dam, the 

chimney filter is typically placed in the central portion of the dam. 

Collar (FEMA, 2005): A limited placement of filter material that 

completely surrounds a conduit for a specified length within the 

embankment dam. The filter collar is located near the conduit’s downstream 

end. The filter collar is usually included in embankment dam rehabilitation 

only when a filter diaphragm cannot be constructed. A filter collar is 

different from a filter diaphragm, in that a filter diaphragm is usually located 

within the interior of the embankment dam. 

Diaphragm (FEMA, 2005): A filter diaphragm is a zone of filter material 

constructed as a diaphragm surrounding a conduit through an embankment. 

The filter diaphragm protects the embankment near the conduit from 

internal erosion by intercepting potential cracks in the earthfill near and 

surrounding the conduit. A filter diaphragm is intermediate in size between a 

chimney filter and a filter collar. The filter diaphragm is placed on all sides of 

the conduit and extends a specified distance into the embankment. 

Filter collar: See Filter, collar. 

Filter diaphragm: See Filter, diaphragm. 

Filter material (NAWIC, 1986): Granular material that has been graded to allow 

water to pass through it while retaining solid matter. 

First filling: The initial filling of the reservoir behind a dam. Also, used to describe 

refilling of a reservoir after a modification has been made to a dam. 

Flanged joint: See Joint, flanged. 

Flexible pipe: A pipe that derives its load carrying capacity by deflecting at least 2 

percent into the surrounding medium upon application of load. 

Fold-and-formed pipe (FFP): A thermoset system where a plastic pipe 

manufactured in a folded shape of reduced cross-sectional area is pulled into an 

existing conduit and subsequently expanded to the internal shape by heat and 

pressure. 

Gasket: A flexible material used to form a water-tight seal between two 

components. 

204 
Glossary

Geosynthetic (ASTM, 2004): A planar product manufactured from polymeric 

material used with soil, rock, earth, or other geotechnical engineering related material 

as an integral part of a man-made project, structure or system. 

Gradation (ASTM, 2002): The distribution of particles of granular material among 

standard sizes, usually expressed in terms of cumulative percentages larger or smaller 

than each of a series of sieve openings. 

Grout (FEMA, 2005): A fluidized material that is injected into soil, rock, concrete, 

or other construction material to seal openings and to lower the permeability and/or 

provide additional structural strength. There are four major types of grouting 

materials: chemical, cement, clay, and bitumen. 

Haunch: The area beneath a pipe between the springline and the invert. 

Hazard (FEMA, 2004): A situation that creates the potential for adverse 

consequences such as loss of life, property damage, or other adverse impacts. 

Hazard potential (FEMA, 1998): The adverse incremental consequences that 

result from the release of water or stored contents due to failure of the dam or 

misoperation of the dam or appurtenances. Impacts may be for a defined area 

downstream of a dam from flood waters released through spillways and outlet works 

of the dam or waters released by partial or complete failure of the dam. There may 

also be impacts for an area upstream of the dam from effects of backwater flooding 

or landslides around the reservoir perimeter. 

• Low (FEMA, 1998): Embankment dams assigned the low hazard potential 

classification are those where failure or misoperation results in no probable loss 

of human life and low economic and/or environmental losses. Losses are 

principally limited to owners’ property. 

• Significant (FEMA, 1998): Embankment dams assigned the significant 

hazard potential classification are those dams where failure or misoperation 

results in no probable loss of human life but can cause economic loss, 

environmental damage, or disruption of lifeline facilities, or can impact other 

concerns. Significant hazard potential classification dams are often located in 

predominantly rural or agricultural areas, but could be located in areas with 

population and significant infrastructure. 

• High (FEMA, 1998): Embankment dams assigned the high hazard potential 

classification are those where failure or misoperation will probably cause loss of 

human life. 

Hazard potential classification: A system that categorizes dams according to the 

degree of adverse incremental consequences of a failure or misoperation of a dam. 

205 
Plastic Pipe Used in Embankment Dams 

The hazard potential classification does not reflect in any way on the current 

condition of the dam (i.e., safety, structural integrity, flood routing capacity). 

Heterogeneous: Consisting of parts or aspects that are unrelated or unlike each 

other. In relation to earth materials, soils that consist of any combination of clays, 

silts, sands, gravels, cobbles, and boulders. 

High density polyethylene plastic (HDPE) (ASTM, 2001): Those linear 

polyethylene plastics, having a standard density of 0.941 g/cm3

 or greater. 

High hazard potential: See Hazard potential, high. 

Homogenous: Consisting of a single material of uniform properties. In relation to 

earth dams, a design of uniform cross section. 

Hoop strain: Strain in the pipe wall due to internal or external pressure. 

Hoop stress: The tensile stress in the wall of the pipe in the circumferential 

orientation due to internal hydrostatic pressure. 

Hopper (NAWIC, 1986): A storage bin or a funnel that is loaded from the top and 

discharges through a door or chute in the bottom. 

Hydraulic loading: Loading on the pipe due to internal pressure, water hammer, 

internal vacuum pressure, or external hydrostatic pressure. 

Hydrostatic design basis (HDB) (ASTM, 2001): One of a series of established 

stress values specified in ASTM D 2837 for a plastic compound obtained by 

categorizing the long-term hydrostatic strength determined in accordance with Test 

Method D 2837. 

Hydrostatic design stress (HDS) (ASTM, 2001): The estimated maximum tensile 

stress the material is capable of withstanding continuously with a high degree of 

certainty that failure of the pipe will not occur. This stress is circumferential when 

internal hydrostatic water pressure is applied. 

Hydrostatic pressure: The force per unit area due to water within the pipe 

(internal) or surrounding the pipe (external). 

Igneous (AGI, 1987): Said of a rock or mineral that solidified from molten or 

partly molten material, i.e. from a magma; also, applied to processes leading to, 

related to, or resulting from the formation of such rocks. 

Inclination: The degree of deviation from a horizontal. 

206 
Glossary

Incomplete condition: A loading condition for an encased plastic pipe when the 

embankment height is greater than the height of the plane of equal settlement. The 

frictional forces between the interior and exterior prisms do not extend to the top of 

the embankment. 

Inspection (FEMA, 2005): The review and assessment of the operation, 

maintenance, and condition of a structure. 

Inspector (FEMA, 2005): The designated on-site representative responsible for 

inspection and acceptance, approval, or rejection of work performed as set forth in 

the contract specifications. The authorized person charged with the task of 

performing a physical examination and preparing documentation for inspection of 

the embankment dam and appurtenant structures. 

Interior prism: The prism of soil directly above the buried conduit. 

Internal erosion (FEMA, 2005): A general term used to describe all of the various 

erosional processes where water moves internally through or adjacent to the soil 

zones of embankment dams and foundation, except for the specific process referred 

to as “backward erosion piping.” The term “internal erosion” is used in this 

document in place of a variety of terms that have been used to describe various 

erosional processes, such as scour, suffosion, concentrated leak piping, and others. 

Note: For a complete discussion of internal erosion and backward erosion piping, 

see FEMA’s Technical Manual: Conduits through Embankment Dams (2005). 

Internal hydrostatic pressure: Pressure inside the pipe (typically no more than the 

pressure due to the full reservoir). 

Internal vacuum pressure: Negative internal pressure inside the pipe. 

Invert (FEMA, 2005): The bottom or lowest point of the internal surface of the 

transverse cross section of a conduit. 

Joint (ASTM, 2001): The location at which two sections of conduit or pipe and a 

fitting are connected together. 

Bell and spigot gasket (ASTM, 2001): A connection between piping 

components consisting of a bell end on one component, an elastomeric gasket 

between the components, and a spigot end on the other component. 

Butt fusion (ASTM, 2001): A joint in which the prepared ends of the joint 

components are heated and then placed in contact to form the joint. 

207 
Plastic Pipe Used in Embankment Dams 

Extrusion (ASTM, 2001): A joint formed by a process whereby heated or 

unheated plastic forced through a shaping orifice becomes one continuously 

formed piece. 

Flanged (ASTM, 2001): A mechanical joint using pipe flanges, a gasket, and 

bolts. 

Mechanical (ASTM, 2001): A connection between piping components 

employing physical force to develop a seal or produce alignment. 

Lean concrete: Low strength concrete (low cement content) used for nonstructural applications such as fill, or as a sub base for concrete pavements. 

Liquefaction (AGI, 1987): In cohesionless soil, the transformation from a solid to 

a liquid state as a result of increased pore pressure and reduced effective stress. 

Load coefficient: A coefficient used in calculating the soil load on buried conduits 

to account for the load transfer between the prism of soil directly above the pipe and 

the adjacent soil. 

Long-term modulus of elasticity: A material property describing the stress/strain 

behavior of a material in the linearly elastic region after exposed to a long period of 

time (50 to 100 years). 

Low hazard potential: See Hazard potential, low. 

Lubricant (ASTM, 2001): A material used to reduce friction between two mating 

surfaces that are being joined by sliding contact. 

Marston load theory: A theory on the magnitude of soil load on a buried conduit 

based on the construction method and relative settlements of the soil directly above 

the pipe, soil adjacent to the pipe, and soil adjacent to the soil directly above the 

pipe. 

Material quality: Physical properties of soil related to strength, absorption, density, 

etc. 

Maximum dimension: In relation to opening sizes in perforated pipe the diameter 

for circular holes and the length for slots. 

Maximum size aggregate (MSA): The smallest sieve through which 100 percent 

of the aggregate sample particles pass. 

Mechanical joint: See Joint, mechanical. 

208 
Glossary

Metamorphic (AGI, 1987): Pertaining to the process of metamorphism or to its 

results. 

Metamorphism (AGI, 1987): The mineralogical, chemical, and structural 

adjustment of solid rocks to physical and chemical conditions which have generally 

been imposed at depth below the surface zones of weathering and cementation, and 

which differ from the conditions under which the rocks in question originated. 

Miscellaneous fill: Earthfill that does not serve a specific function such as 

drainage, filtering, or water barrier. 

Modulus of elasticity: A material property describing the stress/strain behavior of 

a material in the linearly elastic region. 

Modulus of soil reaction (E): Measure of the stiffness of the material which 

surrounds the pipe. 

Multistage filter: A filter consisting of more than one zone, such as a sand filter 

zone and gravel drain zone. 

Negative projecting conduit (Spangler and Handy, 1982): A conduit installed in 

a relatively narrow and shallow trench with its top at an elevation below the natural 

ground surface and which is then covered with an embankment. 

Nuclear testing: Of or relating to the nuclear density test as described in ASTM D 

2922. 

Opening size: The minimum dimension of a perforation in a pipe. For circular 

perforations it is the hole diameter, for slots, it is the slot width. 

Outlet works (FEMA, 2004): A dam appurtenance that provides release of water 

(generally controlled) from a reservoir. 

Out-of-round: The allowed difference between the maximum measured diameter 

and the minimum measured diameter (stated as an absolute deviation). 

Particle breakdown: Undesired alteration of a soil grain by mechanical action such 

as loading, pushing, and compacting. 

Perforation: A hole or pattern made by or as if by piercing, drilling, or sawing. 

Permeability (k): The rate at which water passes through soil in accordance with 

Darcy’s law. 

Pipe stiffness: The inherent resistance of a flexible pipe to load. 

209 
Plastic Pipe Used in Embankment Dams 

Plane of equal settlement: A location above a pipe where the accumulated strain 

and settlement in the exterior prisms equal that of the interior prism. Above this 

plane, the interior and exterior prisms settle equally and no shear or friction forces 

are transferred between the prisms. 

Plastic (ASTM, 2001): A material that contains as an essential ingredient one or 

more organic polymeric substances of large molecular weight, is solid in its finished 

state, and, at some stage in its manufacture or processing into finished articles, can 

be shaped by flow. 

Plastic pipe (ASTM, 2001): A hollow cylinder of plastic material in which the wall 

thicknesses are usually small when compared to the diameter and in which the inside 

and outside walls are essentially concentric. 

Plasticity index: Numerical difference between the liquid limit and the plastic limit. 

Poisson’s ratio (v) (ASTM, 2002): Ratio between linear strain changes 

perpendicular to the direction of a given uniaxial stress change. 

Polyester (PPI, 2006): Resin formed by condensation of polybasic and monobasic 

acids with polyhydric alcohols. 

Polyethylene (FEMA, 2005): A polymer prepared by the polymerization of 

ethylene as the sole monomer. 

Polyvinyl chloride (PVC) (FEMA, 2005): A polymer prepared by the 

polymerization of vinyl acetate as the sole monomer. 

Positive projecting conduit (Spangler and Handy, 1982): A conduit installed in 

a bedding with its top projecting above the natural ground surface and which is then 

covered with an embankment. 

Preservative: A plastic additive used to prevent bacterial attack. 

Pressure pipe (ASTM, 2001): Pipe designed to resist continuous pressure exerted 

by the conveyed medium. 

Pressure rating (PR) (ASTM, 2001): The estimated maximum water pressure the 

pipe is capable of withstanding continuously with a high degree of certainty that 

failure of the pipe will not occur. 

Profile pipe: Pipe that has smooth interior and corrugated exterior surfaces. 

210 
Glossary

Projecting conduit: A conduit covered by fill material such as embankment 

material. 

Projection condition: A projecting conduit above which the exterior prisms settle 

more than the interior prism. 

Projection ratio (p): The ratio of the vertical height of the top of the conduit above 

the embankment subgrade to the outside conduit diameter. 

Proof rolling: A process accomplished by the application of heavy construction or 

compaction equipment on an excavation invert in order to locate low density areas. 

Quality assurance (FEMA, 2005): A planned system of activities that provides the 

owner and permitting agency assurance that the facility was constructed as specified 

in the design. Construction quality assurance includes inspections, verifications, 

audits, and evaluations of materials and workmanship necessary to determine and 

document the quality of the constructed facility. Quality assurance refers to 

measures taken by the construction quality assurance organization to assess if the 

installer or contractor is in compliance with the plans and specifications for a project. 

An example of quality assurance activity is verifications of quality control tests 

performed by the contractor using independent equipment and methods. 

Quality control (FEMA, 2005): A planned system of inspections that is used to 

directly monitor and control the quality of a construction project. Construction 

quality control is normally performed by the contractor and is necessary to achieve 

quality in the constructed system. Construction quality control refers to measures 

taken by the contractor to determine compliance with the requirements for materials 

and workmanship as stated in the plans and specifications for the project. An 

example of quality control activity is the testing performed on compacted earthfill to 

measure the dry density and water content. By comparing measured values to the 

specifications for these values based on the design, the quality of the earthfill is 

controlled. 

Renovation (FEMA, 2005): The repair or restoration of an existing structure, so it 

can serve its intended purpose. 

Repair (FEMA, 2005): The reconstruction or restoration of any part of an existing 

structure for the purpose of its maintenance. 

Resin (ASTM, 2001): A solid or pseudosolid organic material, often with high 

molecular weight, which exhibits a tendency to flow when subjected to stress, usually 

has a softening or melting range, and usually fractures conchoidally (shell-like 

fracture). 

211 
Plastic Pipe Used in Embankment Dams 

Rigid pipe: A pipe, typically reinforced concrete, designed to carry loads without 

support from the surrounding medium. 

Ring strain: Strain in the pipe wall due to deflection or deformation from external 

loads. 

Rock ladder: A device that lifts aggregate vertically by the use buckets attached to a 

belt. 

Sand (ASTM, 2002): Particles of rock that will pass the No. 4 (4.75–m) sieve and 

be retained on the No. 200 (0.075-mm) U.S. standard sieve. 

Sediment trap: An area, such as a pool, behind a weir or flume where the inflow 

velocity is reduced sufficiently that any soil particles included in the flow will settle 

out. 

Sedimentary rock (AGI, 1987): A rock resulting from the consolidation of loose 

sediment that has accumulated in layers; e.g. a clastic rock (such as conglomerate or 

tillite) consisting of mechanically formed fragments of older rock transported from 

its source and deposited in water or from air or ice; or a chemical rock (such as rock 

salt or gypsum) formed by precipitation from solution; or an organic rock (such as 

certain limestones) consisting of the remains or secretions of plants and animals. 

Seepage (ASTM, 2002): The infiltration or percolation of water through rock or 

soil or from the surface. 

Seepage paths (ASCE, 2000): The general path along which seepage follows. 

Segregation: The process of separating coarser soil from finer soil, typically during 

construction activities. 

Service life (FEMA, 2005): Expected useful life of a project, structure, or material. 

Settlement ratio (AWWA, 1995): The relationship between the pipe deflection and 

the relative settlement between the prism of soil directly above the pipe and the 

adjacent soil. 

Short-term modulus of elasticity: A material property describing the stress/strain 

behavior of a material in the linearly elastic region immediately upon a change in 

load. 

Significant hazard potential: See Hazard potential, significant. 

Single stage filter/drain: A system consisting of one zone of filter material, usually 

sand, surrounding a collector drainpipe. 

212 
Glossary

Siphon (FEMA, 2005): An inverted u-shaped pipe or conduit, filled until 

atmospheric pressure is sufficient to force water from a reservoir over an 

embankment dam and out of the other end. 

Sliplining (FEMA, 2005): The process of inserting a new, smaller-diameter lining 

or pipe into an existing larger-diameter conduit. 

Slot: A long, narrow aperture or slit. 

Slow crack growth (PPI, 2006): The slow extension the crack with time. 

Soil (ASTM, 2002): Sediments or other unconsolidated accumulations of solid 

particles produced by the physical and chemical disintegration of rocks, and which 

may or may not contain organic matter. 

Soil prism theory: The soil load on a buried pipe is weight of the soil directly above 

the pipe. 

Soil prism: The soil directly above the pipe. 

Soil-cement: Highly compacted mixture of soil/aggregate, portland cement, and 

water. Soil-cement differs from portland cement concrete pavement in several 

respects. One significant difference is the manner in which the aggregates or soil 

particles are held together. A portland cement concrete pavements mix contains 

sufficient paste (cement and water mixture) to coat the surface area of all aggregates 

and fill the void between aggregates. In soil-cement mixtures, the paste is 

insufficient to fill the aggregate voids and coat all particles, resulting in a cement 

matrix that binds nodules of uncemented material. 

Spillway (FEMA, 2004): A structure, over or through which water is discharged 

from a reservoir. If the rate of flow is controlled by mechanical means, such as 

gates, it is considered a controlled spillway. If the geometry of the spillway is the 

only control, it is considered an uncontrolled spillway. 

Spreader box: A device used in construction to deposit fill uniformly over the 

ground surface. 

Stabilizer: A plastic additive to prevent degradation. 

Standard dimension ratio (SDR) (FEMA, 2005): Ratio of the average specified 

outside diameter to the minimum specified wall thickness for outside diameter 

controlled plastic pipe. Also referred to as dimension ratio (DR). 

213 
Plastic Pipe Used in Embankment Dams 

Standard inside dimension ratio (SIDR): A specific ratio of the average specified 

inside diameter to the minimum specified wall thickness for inside diametercontrolled plastic pipe. 

Stockpile (NAWIC, 1986): Material dug and piled for future use. 

Strain (ASTM, 2001): The change per unit length in a linear dimension of a body, 

that accompanies a stress. Strain is a dimensionless quantity which may be measured 

in percent, in inches per inch, in millimeters per millimeter, etc. 

Stress crack resistance (SCR): Resistance to cracking from tensile stresses; a 

failure that develops over time at stresses less than the yield strength. Stress cracking 

is a macro-brittle cracking phenomenon that occurs at a constant stress significantly 

less than the yield or break stress of the material. Stress cracking is initiated at an 

internal or external “defect” in the material such as an inclusion or scratch. 

Surge pressure (water hammer): A surge in pressure caused by a sudden change 

in water velocity. Typical causes include the sudden starting or stopping of a pump, 

sudden valve movement, or air movement in a pipeline. The surge may damage or 

destroy pipelines and pumps if severe enough. 

Thermoplastic (ASTM, 2001): A plastic that can be repeatedly softened by heating 

and hardened by cooling through a temperature range characteristic of the plastic, 

and that in the softened state can be shaped by flow into articles by molding or 

extrusion. 

Thermoset (ASTM, 2001): A plastic that, when cured by application of heat or 

chemical means, changes into a substantially infusible and insoluble product. 

Tailings (FEMA, 2005): The fine-grained waste materials from an ore-processing 

operation. 

Toe drain: Typically a pipe used to collect water at the downstream toe of a dam. 

Trench condition: A projecting conduit above which the interior prism settles 

more than the exterior prism. 

Trench conduit (Spangler and Handy, 1982): A conduit installed in a relative 

narrow trench excavated in passive or undisturbed soil which is then covered with 

earth backfill. 

Undisturbed soil: In situ or in place soil unaltered by human activity. 

Uniformly graded: A soil consisting of a small range of particle sizes where cu < 5. 

214 
Glossary

Void (FEMA, 2005): A hole or cavity within the foundation or within the 

embankment materials surrounding a conduit. 

Wall buckling: Collapse of the pipe due to excessive external pressure or internal 

vacuum pressure. 

Wall crushing: Failure of the pipe wall due to excessive wall stress from loads on 

top of the pipe. 

Water content (ASTM, 2002): The ratio of the mass of water contained in the 

pore spaces of soil or rock material, to the solid mass of particles in that material, 

expressed as a percentage. 

Water hammer: See Surge pressure. 

Zoning: The cross sectional area of an embankment divided into zones that serve 

different purposes such as core, shell, chimney filter, etc. 

References for Glossary 

American Concrete Institute (ACI), Cement and Concrete Terminology ACI 116 R, 

Committee Report, 2000. 

American Geological Institute (AGI), Glossary of Geology, 1987. 

American Society of Civil Engineers (ASCE), Guidelines for Instrumentation and 

Measurements for Monitoring Dam Performance, 2000.

American Water Works Association (AWWA), Concrete Pressure Pipe, M9, 1995. 

ASTM International*, Standard Terminology Relating to Plastic Piping Systems, ASTM 

F 412, 2001. 

ASTM International*, Standard Terminology Relating to Soil, Rock, and Contained Fluids, 

ASTM D 653, 2002. 

ASTM International*, Standard Practice for Rehabilitation of Existing Pipelines and Conduits 

by Pulled-in-Place Installation of Cured-in-Place Thermosetting Resin Pipe CIPP, ASTM 

F 1743, 2003. 

ASTM International*, Standard Terminology for Geosynthetics, ASTM D 4439, 2004. 

215 
Plastic Pipe Used in Embankment Dams 

Federal Emergency Management Agency (FEMA), Federal Guidelines for Dam Safety: 

Hazard Potential Classification Systems for Dams, FEMA 333, 1998. 

Federal Emergency Management Agency (FEMA), Federal Guidelines for Dam Safety: 

Glossary of Terms, FEMA 148, 2004. 

Federal Emergency Management Agency (FEMA), Technical Manual: Conduits through 

Embankment Dams, FEMA 484, 2005. 

National Association of Women in Construction (NAWIC), Construction Dictionary, 

1986. 

Plastic Pipe Institute (PPI), Handbook of Polyethylene Pipe, 2006. 

Spangler, M.G. and R.L. Handy, Soil Engineering, 1982. 

* The ASTM definitions in this glossary were extracted, with permission, from ASTM standards, copyright 

ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. A copy of the complete standard 

may be obtained from ASTM (www.astm.org). 

216 
Appendix A 

Example Calculations 

Index

No. Topic Page 

A-1 Flexible pipe design (for a drainpipe) A-3 

A-2 Encased pipe design (for an embankment conduit) A-7 

A-3 Siphon design A-11 

A-4 Toe drain design (filter and drain) A-14 

A-1 
Plastic Pipe Used in Embankment Dams 

A-2 
Appendix A—Example Calculations 

A-1 Flexible pipe design (for a drainpipe) 

Description 

A 12-inch diameter HDPE (ASTM D 3350 cell class 345464C) solid wall pipe will be 

used as a drainpipe (figure A-1). The maximum height (H) of fill over the pipe is 15 

feet. The pipe will be embedded in a well graded sandy soil that is compacted to 

50 percent relative density. 

Assumptions 

The following assumptions are made for this example: 

• The pipe is considered a projecting conduit within the footprint of an 

embankment dam and classified as a positive projecting conduit in a trench 

condition. Since the pipe is embedded in coarse grained soil rather than 

encased in grout or concrete, flexible pipe design will be used. The soil prism 

theory is used as recommended in section 2.1.1. Table 9 in section 3.5.6 

describes the applicable soil load conditions. 

• The total unit weight of soil () is 115 lb/ft3 with an Eof 2,000 lb/in2 (table 4, 

section 3.1.3, using 75% of this value is 1,500 lb/in2

). 

• The short-term modulus of elasticity (E) of the HDPE is 140,000 lb/in2 and the 

long-term modulus is 30,000 lb/in2

 (table 3, section 3.1). The modulus of 

elasticity and allowable compressive stress depend upon the type and 

classification of plastic. 

• The pipe meets ASTM D 3035 and has an DO = 12.75 inches. 

• The allowable long-term compressive stress () (1/2 the hydrostatic design basis 

of 1,600 lb/in2

) of the HDPE (cell class 345464C) is 800 lb/in2. 

Figure A-1.—Cross section of an embankment dam and drainpipe (filter 

not shown). 

A-3 
Calculations 

Assume pressures from wheel (pw) and internal vacuum (pv

) are zero. 

Soil Load

The load due to overlying soil should be determined by the soil prism theory. 

The soil load on the pipe by the soil prism theory is: 

P = γ H = 115(15) = 1,725 lb

ft s 2 (2-1)

Wall Crushing 

The resistance to wall crushing of the plastic pipe is evaluated by: 

PDO (1725

144)(12.75) 

= 76.36 T lb 

pw = = in 2 2 (3-1)

The required wall cross-sectional area is determined by: 

Tpw 76.36 2 A = = = 0.095 in in pw σ 800 (3-2)

The area of a pipe wall may be computed as: 

(D − Di ) Apw = o or t (for solid wall pipe) 2 

Plastic Pipe Used in Embankment Dams 

(3-3)

where: 

Di = Do - 2t 

Solving for t: 

t = 0.095 in 

So the minimum wall thickness, t, is 0.095 inches. The minimum wall thickness of a 

12-inch HDPE pipe meeting ASTM D 3035 with an SDR of 26 (maximum 

recommended SDR) is 0.490 inches. 

A-4 
Appendix A—Example Calculations 

Wall Buckling 

Plastic pipe embedded in soil may buckle due to excessive loads and deformations. 

The total soil load must be less than the allowable buckling pressure. The long-term 

modulus of elasticity is recommended since the soil load is a permanent load. The 

allowable buckling pressure may be determined from: 

1/2 





(3-4) 1 ' 

EI pw 32RBE '  w



q = a 3 FS Do 



12 

(30, 000) (0.4903 

12 )  

1 

q (32)(1)(0.665)(1500) a = 

2.5 











12.753 

= 22.8 lb/in 2 = 3288 lb/ft 2 

(h 2 4 15 2 + 12.75 

12 4 + Dh ) ( (15) ) B ' = o 

2 = = 1.5 2 + o ) 1.5 (2)(15) + 12.75 )

2

( h D ( 12

(3-6) 0.665

(3-4) 

The soil pressure of 1,725 lb/ft2

 is less than the allowable buckling pressure of 

3,288 lb/ft2

 for a 12-inch diameter pipe with an SDR of 26. 

Deflection 

Since this pipe is a drainpipe for a filter, the recommended allowable deflection is 

7.5% (see section 3.1.3). The deflection may be estimated from the following 

equation for solid wall pipe: 

A-5 
Y 

= 

%Δ (D P + P + P K ) (100) Ls W V 

D 2E  	 




 − 3 

 + 0.061E' 
 3(SDR 1)   

(1.5) 0 0 (0.1)(100) 1725

144 + +

= (2)(140,000) 	 
  +(0.061)(1500) 

 3 26 1 
 


 ( − )

3 

Plastic Pipe Used in Embankment Dams 




( ( ) )

= 1.84% < 7.5% 

(3-8)

where: 

K = 0.1 as recommended in section 3.1.3 

Conclusion 

A 12-inch diameter, HDPE with ASTM D 3350 cell class 345464C resin, and SDR 

of 26 is recommended for the drainpipe. 

A-6 
Appendix A—Example Calculations 

A-2 Encased pipe design (for an embankment conduit) 

Description 

An existing 20-foot high embankment dam has a 24-inch diameter CMP outlet 

works conduit (figure A-2). The conduit does not have a gate and is not considered 

a pressurized conduit. The foundation consists of stiff clay. The existing conduit 

will be sliplined with an HDPE (ASTM D 3350 cell class 345464C) pipe with an 

outside diameter, DO, of 18-inches. The annulus between the existing conduit and 

the HDPE slipliner will be grouted. The liner pipe will be designed to withstand 

18-feet of hydrostatic pressure. 

Assumptions 

The following assumptions are made for this example: 

• Since the annulus of the sliplined pipe will be grouted, the soil load will be 

assumed to act on the encased HDPE pipe. The grouted annulus is assumed to 

prevent deflection. Therefore, the HDPE liner will be considered a projecting 

conduit in the positive projecting condition. 

• The CMP will continue to deteriorate and will not support the load. 

• A solid wall HDPE pipe will be used as the slipliner pipe. 

• The total unit weight of soil () is 110 lb/ft3. 

• The short-term modulus of elasticity of the HDPE (cell class 345464C) is 

140,000 lb/in2

 and the long-term modulus is 30,000 lb/in2 (table 3, section 3.1). 

• The allowable long-term compressive stress () (1/2 the hydrostatic design basis 

of 1,600 lb/in2

) of the HDPE (cell class 345464C) is 800 lb/in2. 

Figure A-2.—An 18-inch diameter solid wall HDPE slipliner installed in 

an existing 24-inch diameter CMP outlet works conduit. 

A-7 
• The settlement ratio (rsd) is assumed to be +0.5 since the fill around the pipe 

was a compacted earth fill (table 2, section 2.1.2). 

• The projection ratio, p, is 1.0 (figure 35). 

Calculations 

Soil Load 

By prism method: The soil load on the pipe using the soil prism load is: 

Ps = H = (110)(20) = 2,200 lb/ft2 (2-1) 

By Marston method: The Marston soil load for a positive projecting conduit in the 

projection condition may be determined from the following: 

WC = CC γ DO 

2 (2-2) 

Cc is determined from figure 30 for H/Do=(20)/(18/12)= 13.3. The load is based on 

an incomplete condition with rsd=+0.5. The value of Cc

 is approximately 20 from 

figure 36. 

Wc = (20)(110) (18

12)

2 

= 4,950 lb/ft 

The pressure on the top of the pipe may be determined by: 

c Ps = 

W

= 

4950 

= 3,300 lb 

D 18 ft 2 

O 12 

(2-6)

Plastic Pipe Used in Embankment Dams 

The Marston soil load is recommended for conduits encased in grout as shown in 

table 9 in section 3.5.6 and discussed in chapter 3. 

Wall Crushing 

The thrust in the pipe wall is 

PDo (3300

144)(18) 

= 206.25 T = = lb 

pw 2 2 in (3-1)

A-8 
The required wall cross-sectional area is: 

T

pw 206.25 A

pw

= 

σ 

= 

800 

= 0.2578 in2 

in (3-2)

The area of a pipe wall may be computed as: 

(DD − ) Apw = o i or t (for solid wall pipe) (3-3) 2 

So the minimum wall thickness, t, is 0.2578 in.. The minimum wall thickness of an 

18-inch HDPE pipe meeting ASTM D3035 is 0.554 inch with an SDR of 32.5. 

Wall Buckling 

The external hydrostatic pressure on the pipe is 

PG = γhw = (62.4)(18) = 1,123 lb/ft2

 (2-16) 

The long-term modulus of elasticity is recommended since the 18-feet of hydrostatic 

pressure act on the pipe throughout its design life. 

The unconstrained buckling pressure of the pipe with an SDR of 32.5 is: 

3 3 2E 1 (2)(30000) 

(1− 0.452 ) 

1  



 SDR −1  32.5 −1 











 = 2.41 lb 

in2 = 2.41(144) = 346 lbft PCR = = 2 (1− v 2 ) 

(3-20) 

where: 

v = 0.45 for HDPE as recommended in section 3.3.2 

The unconstrained buckling pressure of a SDR 32.5 HDPE pipe is less than the 

external hydrostatic pressure of 1,123 lb/ft2

. 

Check the unconstrained buckling pressure of an SDR 17. 

3 

PCR = (2)(30000) 

 (1 0.45  2 − )

1 





lb 2 = 18.3(144) = 2,644 lb = 18.3 2 17 −1 in ft 

Appendix A—Example Calculations 

A-9 
A factor of safety of 2.0 is applied to the unconstrained buckling pressure. 

PCR = 

2,644 

= 1,763 lb 2 > 1,123 lb 

ft 2

1.5 1.5 ft 

Plastic Pipe Used in Embankment Dams 

Conclusion 

An 18-inch diameter, HDPE with cell class 345464C resin, and SDR of 17 is 

recommended. 

A-10 
A-3 Siphon Design 

The following assumptions are made for this example: 

Description 

A 10-inch diameter, solid wall, HDPE pipe will be installed as a siphon over the crest 

of an embankment dam (figure A-3). The siphon head (H) is 14 feet. The siphon 

will provide additional drainage capacity and allow lowering of the reservoir. 

Assumptions 

• The siphon operates for short periods on an infrequent basis. The short-term 

modulus of elasticity will be used for the buckling analysis. 

• The short-term modulus of elasticity (E) of the HDPE is 140,000 lb/in2 and the 

long-term modulus is 30,000 lb/in2

 (table 3, section 3.1). The modulus of 

elasticity and allowable compressive stress depend upon the type and 

classification of plastic. 

• The allowable long-term compressive stress () (1/2 the hydrostatic design basis 

of 1,600 lb/in2

) of the HDPE (cell class 345464C) is 800 lb/in2. 

• The outside diameter of a HDPE pipe meeting ASTM D 3035 is 10.75 inches. 

• The soil load will be determined by the soil prism theory. 

• The internal vacuum pressure is 14 feet of head = 6 lb/in2. 

P = γ H = (62.4)(14) = 6 lb/in 2

144 

Appendix A—Example Calculations 

Figure A-3.—This figure illustrates a siphon extending over the crest of 

an embankment dam. Alternative siphon designs may want to consider 

the addition of earthen ramps over the siphon or embedment into the 

crest of the dam to facilitate vehicular traffic on the dam crest. 

A-11 
• The Poisson’s ratio (v) for HDPE = 0.45. 

Calculations 

Soil Load

The pipe is on top of the embankment and does not have a soil load. 

Wall Crushing

This is not an issue since there is not a soil load. 

Deflection

Deflection is not determined since there is not a soil load. 

Wall Buckling

The maximum SDR (minimum wall thickness) of a 10.75-inch HDPE pipe meeting 

ASTM D3035 is 32.5. 

The unconstrained buckling pressure of the pipe with an SDR of 32.5 is: 

PCR = 

2E 

(1− v 2 ) 







1 

SDR −1 







3 

= 2(140, 000) 

 (1− 0.45  2 ) 

1 

32.5 −1 







3 

= 11.2 lb 

in2 (3-20) 

where: 

v = 0.45 for HDPE as recommended in section 3.3.2 

A factor of safety of 1.5 is applied to the unconstrained buckling pressure. 

PCR = 

11.2 

= 7.5 lb 2 > 6 lb 

1.5 1.5 in in2 

The unconstrained buckling pressure of an SDR 32.5 HDPE pipe is greater than the 

internal vacuum pressure of 6 lb/in2

. 

Strain

Strain is not evaluated since there is not a soil load. 

Plastic Pipe Used in Embankment Dams 

A-12 
Appendix A—Example Calculations 

Conclusion 

A 10-inch diameter, HDPE with cell class 345464C resin, and SDR of 32.5 is 

recommended. 

A-13 
A-4 Toe drain design (filter and drain) 

Description 

This example will illustrate the design of a toe drain system utilizing the guidelines 

presented in this document as well as judgment required by the designer beyond 

these guidelines. The example is derived from the case history of the Keechelus 

Dam modification completed in 2002 and includes a portion of the modified dam’s 

entire drainage system. For brevity, filter materials at other toe drain locations are 

not included in this example. The filter used for the toe drain was designated Zone 

2B and the drain material Zone 3. The foundation for the toe drain is an alluvial fan 

deposit in a glacial environment during the Quaternary (Qaf). 

Zones 2B and 3 were designed in accordance with Bureau of Reclamation’s 

Embankment Dams, Design Standards No. 13, Chapter. 5, “Protective Filters,” 2007. 

The gradation data of the base material (foundation), Qaf, was determined from 

laboratory testing on 17 samples obtained from drillholes and test pits. The statistics 

for the proportions of gravel, sand, and fines is shown on figure A-4. As a rule, 

grain size distribution for soils are not uniformly distributed due to the inherent 

heterogeneity of soil. Figure A-4 illustrates the simple statistics (minimum, 

maximum, mean, 1st std deviation) to indicate the nature of the material. The actual 

gradation curves for the 17 samples are shown on figure A-5. 

Filter Design 

The first step in sizing the filter is to mathematically regrade the base material (finer 

limit only) to the minus No. 4 sieve. The regraded limits are shown on figure A-5 in 

red. An outlier was identified and eliminated from the dataset as shown on the 

figure. The percent passing the No. 200 sieve is determined as 32.3% by using the 

finer side of the regraded curve. Based on the percent passing the No. 200 sieve, the 

base material is classified as “category 3” and protection against particle movement is 

controlled by: 

(40 − a )(4 DB − 0.7 mm) DF ≤ 0.7 mm + 85 

15 25 

Plastic Pipe Used in Embankment Dams 

where: 

D85B = finer side of regraded gradation = 0.78 mm 

a = percentage of soil passing No. 200 sieve = 32.3% 

D15F  1.4 mm (particle movement limit) 

D15F  D15B (permeability limit)

 where: 

D15B: coarser side of regraded = .05 mm 

A-14 
Appendix A—Example Calculations 

D15F  (5)(0.05)  0.25 mm 

0.25 mm > 0.10 mm so 0.25 mm controls 

D15F  0.25 mm (permeability limit) 

The D15F limits for the range of acceptable filter gradations are 0.25 mm. to 1.4 mm. 

(i.e., 0.25 mm.  D15F  1.4 mm), and are shown on figure A-6. 

In order to maximize the permeability of the filter, the filter=s upper limit is set near 

the upper end of this range (the trial is shown on figure A-7). 

The next step for sizing the filter is selecting the degree of uniformity of the 

gradation. Based on the trial D10F of approximately 0.5 mm, the D85F upper limit for 

segregation is 20 mm (as shown on Table 2, Bureau of Reclamation, 2007) and is 

indicated on figure A-6. Uniformity in this example is found by matching the 

coefficient of uniformity (cu) of “concrete sand,” as shown in table A-1. Since 

“concrete sand” (ASTM C 33, fine aggregate) has shown good performance in the 

field (does not segregate), it is used as a guide for this selection (this procedure is not 

in Bureau of Reclamation’s design standard). 

Table A-1.—Selection of coefficient of uniformity for 

Zone 2B 

Degree of “Concrete Sand” Zone 2B 

Uniformity

D60 / D10 4.0 3.8

(1.63/0.425)

The third step is an estimation of the gradation limits (band width) for the filter. 

Since the D10F at this stage of the design is less than 20 mm, no requirement is given 

for the width of the prescribed gradation range (limits of gradation). Again, 

recognizing that “concrete sand” is limited to ranges no greater than 35 points, this 

limit is set for this filter (this step is not in Bureau of Reclamation’s design standard). 

The trial gradation is given in table A-2 and plotted on figure A-7. 

The permeability of this filter is checked against the foundation permeability later in 

this example. 

A-15 
Table A-2.—Gradation limits for Zone 2B

Sieve size Percent passing, by weight

3⁄4-inch 100

-inch 90 – 100 

No. 4 65 to 100 

No. 8 40 to 75 

No. 16 10 to 45 

No. 30 0 to 15 

No. 50 0 to 3 

No. 100 0 to 2 

No. 200 0 to 1 

Drain Design 

A drain material will be used to surround the perforated drainpipe in the 

downstream toe drain (the envelope). This material is bounded by two surrounding 

materials; perforation size of the pipe and the D85 size of the filter. Since this is a 

two stage filter and it is assumed the drain material will be uniformly graded, the 

following relationship is used for the perforation constraint (also see section 4.1.2). 

D85 of the filter nearest the pipe > 2 uniformly graded perforation opening of pipe drain 

Plastic Pipe Used in Embankment Dams 

Assuming a perforation width = 10 mm (HDPE 12-inch diameter, ADS N-12 pipe, 

circular perforation, ADS Product Note 3.106 (2003): 

D85E  (2)(10.0)  20.0 mm, 

where: 

D85E  20.0 mm (slot limit) 

D85E: finer side of envelope 

Next, material size is determined against the base (filter). The first step in sizing the 

envelope against the filter is to determine the category of the filter. Since Zone 2B 

contains less than 15% fines as, shown on figure A-7, it is a “category 4 soil.” The 

envelope criteria is: 

D15E  D85F (particle movement limit), 

A-16 
where: 

D85F: finer side of filter = 3.4 mm 

D15E  13.6 mm (particle movement limit) 

D15E  D15F (permeability limit), but, not less than 0.10 mm. 

D15E  (5)(0.6) mm, 

D15E  3.0 mm (permeability limit) 

D15F: finer side filter = 0.6 mm 

The D15E limits for the range of acceptable envelope gradations are 3.0 mm. to 

13.6 mm (i.e., 3.0 mm  D15E  13.6 mm), and D85E limit is minimum 20.0 mm, for 

pipe perforation size. The limits are shown on figure A-7. The uniformity of the 

envelope (Cu = 2.80) was slightly more uniform than the filter. 

A summary plot of the selected filter and drain materials is shown on figure A-7. 

The gradation specification is given for the Zone 3 drain material in table A-3. 

Table A-3.—Gradation limits for Zone 3 

Sieve size Percent passing, by weight

3⁄4-inch 100

-inch 90 – 100 

No. 4 65 to 100 

No. 8 40 to 75 

No. 16 10 to 45 

No. 30 0 to 15 

No. 50 0 to 3 

No. 100 0 to 2 

No. 200 0 to 1 

Appendix A—Example Calculations 

Permeability Check 

The permeability of the filter needs to be checked to see if its permeability is less 

than some material found in the foundation. Since this example consists of a 

relatively pervious foundation including highly pervious layers there is a concern that 

the filter could act as a barrier to these zones. 

Examination of 17 gradations for the foundation materials range from GW–GM 

material at the upper bound, as shown in figure A-5, to SW-SM material near the 

midpoint of the gradation band. GW–GM materials are estimated to have a 

A-17 
Plastic Pipe Used in Embankment Dams 

permeability in the range of 10,000 to 1,000,000 ft/yr (Bureau of Reclamation, 

1987a) and the SW-SM soils are in the range of 100 to 30,000 ft/yr. The finer side 

of the filter (figure A-7) classifies as an SP and its range of permeability is 50 to 

150,000 ft/yr. This indicates that the permeability of the filter equals the foundation 

permeability somewhere between the mid point and upper limit of the gradations. 

Assuming the four coarsest samples produce a permeability greater than 

150,000 ft/yr, then about 25% (4/17) of the foundation would be blocked by the 

filter. An increase in pore pressure would then be expected until equilibrium is 

reached. Since 25% is a relatively small portion of the foundation, the filter is 

deemed adequate. Additionally, in this instance because of site topography, the toe 

drain was 10 to 18 feet deep. Due to this deep burial there was no concern about 

excessive uplift pressures 

Borrow Area 

A local borrow site is available for use in producing filter and drain materials. 

Twelve test pits and three drillholes were used for characterization of this pit. The 

test pits ranged from 4.5 to 30 feet deep and the drillholes were about 40 feet deep. 

Laboratory analysis on forty-five samples (figure A-8) indicates that the material 

within the borrow site consists primarily of a Silty Sand (SM) to a Well Graded 

Gravel with Sand (GW)s. The percent of oversize material ranges from a trace to 30 

percent cobbles and a trace to 20 percent boulders with maximum dimension of six 

feet. Simple statistics were produced on all samples (similar to figure A-4 described 

earlier) which results in the following categorization; gravel content varies from 0 to 

76 percent, but generally ranges from 10 to 56 percent; sand content varies from 1 to 

91 percent, but generally ranges from 31 to 76 percent, and fines content varies from 

1 to 99 percent, but generally does not exceed 27 percent. The specific gradations of 

the borrow area (by grain size) are plotted on figure A-8. 

Figure A-9 is a plot of the filter and drain materials (average) designed in the 

previous sections along with the average gradation of the borrow material. This plot 

is used to compare the available grain sizes against those that are required to produce 

the filter and drain. Note: “Oversize” material (material larger than 3-inches) has 

been removed from the data set. This is done because that material is not usable for 

the products in question. Additionally, a more detailed analysis was done for 

comparison of material demands from all produced materials against available 

material within the pit (not covered in this example). The demand is calculated by 

computing the weight required, per sieve, and deducting it from the available weight 

on a per sieve basis. Performing the analysis on a per sieve basis will illustrate which 

sieve (grain sizes) are used most and which are used least. Estimation can then be 

made on the amount of waste (from washing operations) and the amount of byproducts produced (material that passes through the plant but is not used for any of 

the final products). 

A-18 
Appendix A—Example Calculations 

Figure A-9 illustrates that all grain sizes are available to produce the two required 

products. That is, no supplemental material will have to be brought in to complete 

the material. The figure also indicates that since the pit contains an average of 14% 

fines a washing operation will be required. Note that the 14% fines content is a 

product of scalping the sample to a minus 3-inch material and a number of samples 

that had very high fines content, as shown in figure A-8. At this site these samples 

were near the ground surface and were stripped prior to production. Examination of 

figure A-8 indicates that 7% fines (average) is a better indication of the actual 

amount of fines that would be expected during production. 

Figure A-9 also indicates that a large amount of oversize material would be surplused 

(by products) from the screening operation. Once this was identified this material 

was specified for use as riprap, riprap bedding and slope protection. The more 

detailed analysis (‘per sieve’ analysis) also indicated that the medium sand sizes would 

be used most extensively. In order to reduce the total yardage that would have to be 

processed through the plant, a crushing operation was recommended. 

Reference 

Bureau of Reclamation, Design of Small Dams, 1987a. 

A-19 
Plastic Pipe Used in Embankment Dams 

Figure A-4.—Proportion of fines, sand, and gravel for foundation soils. 

A-20 
Appendix A—Example Calculations 

Figure A-5.—Individual gradations for foundation soil. 

A-21 
Plastic Pipe Used in Embankment Dams 

Figure A-6.—Design of filter material. 

A-22 
Appendix A—Example Calculations 

Figure A-7.—Design of drain material. 

A-23 
Plastic Pipe Used in Embankment Dams 

Figure A-8.—Individual gradations of borrow material. 

A-24 
Appendix A—Example Calculations 

Figure A-9.—Average of filter, drain, and borrow materials. 

A-25 
Plastic Pipe Used in Embankment Dams 

A-26 
Appendix B 

Case Histories 

Index

Feature Location Topic Page 

Davis Creek Dam Nebraska CCTV inspection of a toe drain 3 

Ganado Dam Arizona Toe drain installation in an 7 

embankment dam modification 

Sediment Control Mississippi A breach occurred due to 11 

Pond SP-4 excessive seepage along an HDPE 

pipe spillway 

Sugar Mill Dam Georgia Poor construction practices lead 18 

to internal erosion along a siphon 

spillway 

Upper Wheeler Washington Collapse of a HDPE pipe during 22 

Reservoir Dam grouting operation 

Virginia Dam Virginia Collapse on a HDPE pipe encased 25 

in concrete due to external 

hydrostatic pressure 

Wheatfields Dam Arizona Sliplining a deteriorating outlet 29 

works conduit using HDPE dualwall containment pipe 

Worster Dam Colorado Renovation of an existing outlet 35 

works using an HDPE slipliner 

grouted in place 

Additional case histories involving plastic pipe used in dams are available in FEMA’s 

Technical Manual: Conduits through Embankment Dams (2005). 

B-1 
Plastic Pipe Used in Embankment Dams 

B-2 
Appendix B—Case Histories 

Project: Davis Creek Dam 

Location: Nebraska 

Summary: CCTV inspection of a toe drain 

Davis Creek Dam was completed in 1992 and is located about 6 miles southeast of 

North Loup in central Nebraska. The dam is a homogenous earthfill embankment 

with a structural height and crest length of approximately 110 feet and 3,000 feet, 

respectively. The toe drain system consists of two toe drains, one to the right of the 

outlet works centerline and another to the left of the outlet works centerline. The 

right and left toe drains consist, respectively, of approximately 1,200 feet of 8-inchdiameter and 1,440 feet of 12-inch-diameter perforated, corrugated polyethylene 

pipe. Flow from the right toe drain is measured by a V-notch weir, located about 

30 feet to the right of the outlet works centerline in inspection well No. 7. Figure 

B-1 shows the toe drain layout. Flow from the left toe drain is measured by a 

V-notch weir located at the end of a weir box. The weir box is on the ground 

surface several hundred feet to the left of the outlet works centerline. The toe drains 

meet at the location of the toe drain outfall manhole, station 98+95, where they flow 

into the Jack Canyon drainpipe. The Jack Canyon drainpipe was constructed to carry 

toe drain discharges and surface runoff. The Jack Canyon drainpipe extends for 

about 1,100 feet and consists of 18-inch-diameter perforated, corrugated 

polyethylene drainpipe. 

Figure B-1.—Locations of observation wells and cleaned reaches. 

B-3 
Plastic Pipe Used in Embankment Dams 

In the spring of 1994, a sinkhole 8 to 10 feet deep and approximately 20 feet wide 

developed above the 12-inch-diameter nonperforated, corrugated polyethylene 

outfall pipe. The sinkhole was located along the right outfall about midway between 

inspection well No. 9 and the Jack Canyon diversion drain culvert outlet transition. 

Drain rehabilitation in the fall of 1994 and the spring of 1995 consisted of replacing 

the 12-inch diameter outfall drainpipe with a 12-inch diameter perforated pipe placed 

within a gravel envelope. 

In November of 2000, the Bureau of Reclamation performed a CCTV inspection of 

the toe drains at Davis Creek Dam as part of routine drain maintenance. 

Observations from the CCTV inspection showed areas of pipe buckling, other 

potentially damaged areas of pipe, and sediment deposition. Figure B-2 shows a 

typical amount of sediment deposition that was seen in the toe drain. 

Based on CCTV inspection, selected reaches of the Davis Creek toe drains were 

cleaned using sewer cleaning equipment in January 2002. The reaches cleaned were 

located in the left toe drainpipe from stations 19+46.91 to 23+00 and from stations 

23+00 to 26+00; however, care was taken not to wash out any of the materials from 

the locations where the pipe was damaged. 

In February 2002, the Bureau of Reclamation inspected the cleaned reaches of pipe, 

including the short reach of the Davis Creek toe drain outfall replacement pipe and 

stations 98+95 to 99+12 of the Jack Canyon drain. The inspection of the left toe 

Figure B-2.—The typical amount of sediment deposition observed in toe 

drain during the November 2000 inspection. 

B-4 
Appendix B—Case Histories 

drain at Davis Creek was within the 12-inch-diameter pipe. The camera-crawler was 

inserted into the manhole at station 23+00, and proceeded downstream to station 

19+46.91. The camera-crawler was then backed out and turned around in order to 

proceed upstream. The camera-crawler proceeded upstream to approximately 

station 23+25. Originally, it was intended to inspect the entire cleaned reach to 

station 26+00, but the camera-crawler was unable to proceed when it came across a 

section of buckled pipe that was previously reported during the 2000 inspection. 

Figure B-3 shows the results of drain cleaning and the pipe damage that halted the 

camera-crawler. This photograph was taken in the Davis Creek toe drain at 

approximately station 23+25. The fine materials previously seen on the pipe invert 

have been removed. 

Lessons learned: 

• In the short term, the cleaning was effective in removing most of the deposited 

sediments within the cleaned reaches. The long-term efficiency of the cleaning 

operation is unknown. No additional damage occurred inside the drainpipe 

because of the pressure jetting. Decreases in toe drain flows were not seen 

before cleaning, nor were higher flows seen immediately following cleaning. 

However, it should be noted that only a portion of the toe drain was cleaned. If 

the entire length were cleaned, the discharge rate might have increased. Also, it 

Figure B-3.—A buckled left toe drain pipe at approximately Sta. 23+25 

stopped the camera-crawler. A cleaning removed fine materials from 

the pipe invert. 

B-5 
Plastic Pipe Used in Embankment Dams 

is possible that the sediments in the toe drain are not controlling toe drain 

flows. 

• In a 1994 field examination, it was concluded that the sinkhole developed from 

material being transported into an open, collapsed pipe. The collapse of the 

pipe could have occurred either from equipment load during construction or 

from earth pressure on the outside of the pipe. A CCTV inspection 

immediately following or during construction would have been helpful in 

pinpointing the cause of the pipe failures. Even though the cause of the 

sinkhole could not be pinpointed, the CCTV inspections in 2000 and 2002 were 

helpful in viewing the condition of the drainpipe and supporting the conclusion 

from the 1994 exam. Both inspections noted some pipe failures that could 

facilitate the development of sinkholes. 

Reference: 

Bureau of Reclamation, Drainage for Dams and Associated Structures, 2004. 

B-6 
Appendix B—Case Histories 

Project: Ganado Dam 

Location: Arizona 

Summary: Toe drain installation in an embankment dam modification 

Ganado Dam, originally constructed in the early 1900’s, was raised 5.5 feet in 1943 

for an approximate total height of 28 ft. Constructed of locally available dispersive 

soil on a dispersive foundation, numerous dam safety issues and poor performance 

led to the reservoir being drained in 1982. 

The dispersive properties of the embankment and foundation led to a number of 

dam safety deficiencies including: 

• Internal erosion initiated by seepage concentration through transverse cracks in 

the embankment. 

• Erosion of dispersive material into porous regions of the dam foundation that 

can lead to piping. 

• Erosion of dispersive material along structures that can lead to enlarged 

concentrated flow paths. 

• Soil erosion and rilling of the downstream face of the embankment. 

Most dispersive soils can be field identified by characteristic erosion features as 

shown on figures B-4 and B-5. This case history is an excellent example that even a 

low head structure was unable to store water due to the highly erosive nature of the 

soils. 

The dam was rebuilt in its entirety, including embankment, outlet works, and 

spillway. Additionally, a toe drain system was added. Prior to draining the reservoir, 

seepage through and under the dam led to standing water downstream of the 

embankment. The redesign of the embankment included a cutoff trench in the 

foundation and the inclusion of chimney and blanket filters as shown in figure B-6. 

Since the foundation consisted of alternating clay, silt, and sand layers the intent was 

to include a cutoff of sufficient depth to engage at least several of the sand layers. 

This design feature minimized seepage and pore pressures in the downstream area of 

the dam. 

The toe drain consisted of a 12-inch profile wall corrugated HDPE pipe surrounded 

by a two stage drainage system (gravel envelope surrounded by a sand filter). The 

cross section is shown in figure B-7. Profile wall corrugated pipe was selected due to 

its greater strength and smooth interior (single wall corrugated interior pipe can trap 

B-7 
Plastic Pipe Used in Embankment Dams 

Figure B-4.—Subsurface fissure located at the downstream toe of the 

dam east of the outlet works. The subsurface soil had a very high 

moisture content. 

Figure B-5.—Erosional features typically associated with dispersive soils 

as seen at Ganado Dam. The common name for such features is 

“jughole.” 

B-8 
Appendix B—Case Histories 

B-9 

Figure B-6.—Modification cross showing embankment zones and toe drain system.

Figure B-7.—Cross section of toe drain. 

Appendix B—Case Histories 
Plastic Pipe Used in Embankment Dams 

sediment which may enter the pipe and is more difficult for CCTV examinations). 

The pipe diameter was selected at 12-inches as a minimum for camera crawler access 

since the predicted flows were expected to be quite small. Perforated pipe was used 

along the toe drain trench and nonperforated pipe was used for the outfalls. Clean 

outs and inspection wells were also included in the design for access to the toe 

drainpipe along with measurement devices and sediment traps. This arrangement 

allows for complete access to the toe drain system for monitoring and inspections. 

Lessons learned: 

An effective foundation cutoff minimized the amount of seepage past the dam 

allowing a minimal toe drain installation. The available on-site sand source, although 

abundant in quantity, was composed mostly of No. 100 fine sand (locally known as 

blow sand). While acceptable as a filter, this was on the fine side of the criteria and 

was marginal in meeting the permeability requirement of the design standard. This 

was judged not an issue since the cutoff and very low permeability of the core 

rendered the cross section nearly impervious. In fact there is excellent seepage 

attenuation through the cross section and the toe drains are all dry. 

As recommended in the Bureau of Reclamation’s Protective Filters (2007) a filter 

compatibility test was performed. The design standard recommends that specific 

filter material be tested for specific sites when dispersive base soils are present. This 

check was done for the Ganado work and the prescribed filter was found to be 

adequate. 

The profile wall corrugated HDPE pipe was easily installed and capable of 

withstanding construction installation loads. 

Reference: 

Bureau of Reclamation, Design Summary—Ganado Dam, 1998. 

Bureau of Reclamation, Embankment Dams, Design Standards No. 13, Chapter. 5, 

“Protective Filters,” 2007. 

B-10 
Appendix B—Case Histories 

Project: Sediment Control Pond SP-4 Dam 

Location: Mississippi 

Summary: A breach occurred due to internal erosion along an HDPE pipe spillway 

Sediment Control Pond SP-4 Dam failed on February 13, 2004 when a 26-foot wide 

breach occurred at the location of the pond’s spillway pipe. The failure occurred 

approximately 77 days after the facility began to impound water. Approximately 

439 acre-feet of water were released as a result of the breach. No injuries or 

significant damage occurred. 

The low hazard potential pond was constructed to control runoff and collect 

sediment from upstream mining operations. The earthen embankment had a crest 

width of 20 feet, maximum height of 29.5 feet, and an overall length of 

approximately 2,700 feet. The upstream and downstream slopes of the 

homogeneous embankment were sloped at 3 horizontal to 1 vertical. The 

embankment was constructed of clay and silty-clay soils. The PI values of soil used 

to backfill the pipe were in the range of 9 to 12. The majority of the embankment 

was constructed of CL soil with a PI in the range of 10 to 18. 

The spillway pipe consisted of a drop inlet structure having a 60-inch diameter 

polymer-coated corrugated metal pipe riser and a 36-inch outside-diameter, SDR 17, 

HDPE conduit. Two slide gates on the side of the riser allowed for low level 

discharge. The length of the HDPE conduit was approximately 125 feet. Joints 

were butt fused. The pipe discharged into a plunge pool. 

The pipe was installed by compacting fill to the bottom elevation of the pipe and 

then shaping the bedding by hand excavation, for a depth of approximately 6 inches, 

to conform to the shape of the pipe. The bedding was reportedly shaped until the 

workers achieved what they considered “reasonable contact.” A transit and plywood 

template was used to maintain alignment and grade control. Workers reportedly 

rolled the pipe into and out of the cradle excavation several times to check if “full” 

contact was achieved between the pipe and the bedding. Backfill was then 

compacted in the haunch area in 6-inch lifts using powered hand-tampers. A walkbehind sheepsfoot roller was used to compact the remainder of the backfill to 

approximately 2 feet above the pipe. The fill was then raised above the pipe as the 

rest of the embankment was raised. 

A seepage diaphragm was constructed approximately 25 feet downstream of the 

centerline of the embankment. The sand diaphragm was approximately 21 feet wide, 

12 feet high, and 3 feet thick at the base (2 feet thick at the top). The diaphragm 

extended approximately 7 feet above the pipe, 10.5 feet to either side of the pipe’s 

centerline, and less than 2 feet below the pipe. The pipe was located in the fill 

B-11 
Plastic Pipe Used in Embankment Dams 

portion of the dam. Approximately 8 feet of foundation soil had been removed and 

replaced with compacted fill. The diaphragm extended less than 1.5 feet into this 

replacement fill. 

A sand filter zone extended downstream of the diaphragm as bedding for the 

spillway pipe and to act as a drainage outlet for seepage collected by the diaphragm. 

This bedding layer was approximately 6 feet wide and extended approximately 1 foot 

below the pipe. A 3-foot “gravel plug” was constructed at the downstream end of 

this sand layer. A woven geotextile separated the “gravel plug” from a layer of riprap 

that lined a plunge pool at the pipe outlet. 

The embankment had been completed on November 28, 2003. During January 

2004, the slide gates were opened on three occasions to release water from the 

reservoir. On January 20, 2004, a member of the pump crew noticed vibrations at 

the downstream end of the spillway pipe while treating the pond with a flocculent. 

Until February, the highest reported water level in the reservoir was only 4.5 feet 

above the invert of the transport section of the spillway pipe. However, on 

February 5, 2004, over 4 inches of rain fell in the area. 

A routine embankment inspection was performed on February 13, 2004 at 3:30 p.m. 

At this time, the water level had risen to the point where it was 2.5 feet below the top 

of the riser pipe, and 9.5 feet below the crest of the dam. The corresponding head 

on the invert of the transport section was approximately 8.5 feet. This was the 

highest water level that the reservoir had experienced. Nothing unusual was noted 

during this inspection. No seepage was observed around the periphery of the decant 

pipe at that time. 

The failure occurred 5 hours later (figures B-8, B-9, and B-10). Witnesses indicated 

that, just prior to the failure, they observed a stream of water, described as being 

about 10 inches in diameter, exiting at the downstream toe of the embankment 

adjacent to the spillway pipe. At the same time, a vortex was observed in the 

reservoir near the point where the pipe intersected the upstream slope. Failure of 

the embankment occurred approximately 20 minutes after the water flow was first 

observed. At the time of the failure, no water was flowing through the spillway pipe. 

The following postfailure observations were made: 

• The fused pipe joints were intact, and their workmanship appeared to be of 

high quality. 

• The seepage diaphragm that had been constructed 25 feet downstream of the 

embankment centerline was completely washed away in the failure. 

B-12 
Appendix B—Case Histories 

Figure B-8.—Looking upstream through the breach. Embankment was 

18 feet high at breach. 

Figure B-9.—Breach showing 36-inch diameter HDPE pipe and riser. 

B-13 
Plastic Pipe Used in Embankment Dams 

Figure B-10.—View looking downstream along the pipe. The breach 

was 26 feet wide. 

• The pipe bedding and backfill soil was indicated to be nondispersive based on

pinhole tests (ASTM D 4647), crumb tests (ASTM D 6572), and double

hydrometer testing (ASTM D 4221).

• The conditions observed immediately prior to the failure indicated that a direct

flow path was present along the outside of the pipe or through the pipe backfill. 

As indicated below, several explanations are possible for the presence of a 

preferential flow path: 

1. Apparently the procedures used in installing the pipe did not achieve 

adequate contact between the bedding/backfill and the pipe, and/or 

adequate compaction of the backfill immediately around the pipe. Based 

on the low permeability of the backfill when compacted to the specified 

density, voids or zones of poorly compacted material were likely present in

portions of the haunch areas, allowing the flow path to develop as quickly

as it did. 

2. Observations of the compacted fill surface that remained after the failure

show the surface prepared for placement of the pipe was highly

compacted and did not bond properly to subsequent lifts. Sheepsfoot

roller impressions were visible in the lift surfaces remaining after the

failure. This condition is favorable to hydraulic fracturing, which could 

also explain the internal erosion flow path.

B-14 
Appendix B—Case Histories 

3. The sand used to construct the filter diaphragm was much finer and more 

poorly graded than ASTM C 33 concrete sand. The gradation used for the 

filter diaphragm is shown in figure B-11 (plotted on the same graph with 

ASTM C 33 sand). The finer sand used to construct the diaphragm was 

probably more likely to crack and to sustain an open crack because it 

would likely have poor self-healing characteristics and a high potential to 

bulk during placement. 

4. The intent of filter diaphragms is to intercept any flow through 

preferential flow paths, and hydraulic fracture cracks. To effectively 

accomplish this, the diaphragm must extend well beyond the portion of 

the fill that could be affected by poor construction or hydraulic fracture. 

This filter diaphragm may not have been deep enough to encompass voids 

beneath the conduit or hydraulic fracture cracks at the contact between the 

conduit backfill and the remainder of the embankment. Current design 

criteria used by some agencies require the filter diaphragm to extend a 

distance equal to at least two times the outside diameter of the pipe below 

the pipe (NRCS, 2007). This would equal a distance of about 6 feet, but 

this diaphragm extended less than 2 feet below the conduit. 

• Either the diaphragm or its outlet drain was overwhelmed by a large quantity of 

flow through defects under the haunches of the pipe, or flow bypassed the 

diaphragm. The head likely had reached a point where the gradient was 

sufficient to cause the uncontrolled flow along the pipe to carry away the 

embedment material, causing the soil above the pipe to collapse and erode, and 

the breach to develop. 

Lessons learned: 

• In the installation of a circular pipe, full contact between the pipe and the 

backfill is difficult to achieve, and compacting backfill in the haunch area is 

particularly difficult because the energy of backfill efforts can easily lift the pipe. 

• If a pipe is not encased in concrete, then construction procedures, such as the 

shaping of the bedding for at least the lower third of the pipe diameter, must be 

used to ensure that full contact is achieved between the pipe and the 

surrounding soil and that the soil in the haunch area is adequately compacted. 

• Filter diaphragms should have dimensions both horizontal and vertical that are 

extensive enough that flow in the vicinity of the conduit cannot circumvent the 

diaphragm, particularly under the diaphragm. The filter diaphragm did not 

extend deeply enough below the conduit according to current criteria used by 

many agencies. 

B-15 
100 

90 

80 

Percent Finer 

70 

60 

50 

40 

30 

20 

10 

0 

Filter used to

construct

diaphragm

ASTM C-33 

Concrete Sand

ASTM C-33 Concrete Sand & Coarse Filter 

Sieve Size 

#20 #100 #50 #30 #16 #8 #4 3/8" 1/2" 3/4" 1" 11⁄2" 2" 3 " 

0.01 0.1 1 10 100 

Grain Size, millimeters

Figure B-11.—The gradation used for the filter diaphragm. 

Plastic Pipe Used in Embankment Dams 

• The sand material used for the diaphragm was too fine and poorly graded to

furnish properties considered desirable for filter diaphragms. Those properties 

are self-healing and a lack of bulking characteristics. ASTM C 33 concrete sand 

has been found to be an excellent filter for this purpose, but the filter used was 

significantly finer and more poorly graded than C 33 sand. The result was 

probably that the filter diaphragm could support an open crack and

consequently could not fulfill the most important diaphragm function of

collecting and filtering flow in the crack. 

• The designer should monitor pipe installations to ensure that the specifications 

and the intent of the design are complied with, and that construction difficulties

are adequately accounted for in the design requirements and construction 

specifications. 

• The downstream area where the pipe exits the structure should be monitored

closely for unusual quantities of seepage and evidence of internal erosion, 

especially during first filling of the reservoir. 

References: 

Mine Safety and Health Administration, Investigation of Embankment Failure - Sediment

Pond SP-4, Report No. MW04-024, 2004. 

B-16 
Appendix B—Case Histories 

Natural Resources Conservation Service, “Filter Diaphragms,” National Engineering 

Handbook, Part 628, Chapter 45, 2007. 

B-17 
Plastic Pipe Used in Embankment Dams

Project: Sugar Mill Dam 

Location: Georgia 

Summary: Poor construction practices lead to internal erosion along a siphon

spillway 

Sugar Mill Dam is a residential subdivision that was developed in the early 1990’s in 

north Fulton County, Georgia (Atlanta metropolitan area). A central amenity of the 

development was an existing lake impounded by an old earthen embankment with 

inadequate spillway capacity. 

In addition to widening the earthen emergency spillway, five PVC siphon pipes 

(ranging from 6 to 24-inches in diameter) were installed in a trench excavated

through the crest of the embankment and terminating in a new wall at the toe of the 

dam (figure B-12). The design called for the pipes to be bedded in concrete. 

Control valves were installed in the siphons at the top of the dam, inside of manhole 

structures. 

In 2002, about ten years after construction of the siphon spillway system, the dam 

owner noted water flowing out of a hole in the embankment adjacent to the siphons, 

approximately 15 feet downstream of the valve manhole. 

Figure B-12.—In the early 1990’s the spillway capacity of the dam was 

increased by construction of a system of 5 PVC siphons embedded in 

concrete in a shallow trench through the dam. 

B-18
Appendix B—Case Histories

The owner contacted the designer for guidance. An internal CCTV inspection of the 

siphons found no problems with the PVC pipes, and the designer recommended that 

a filter drain system be constructed to control the seepage along the pipes. However, 

this did not work and the seepage situation continued to get worse. In 2003, the 

owner attempted to operate the siphon spillways during a storm, and found that the 

manholes were full of water and that the seepage flow along the siphons had 

substantially increased. 

The designer suspected that flow was occurring under the pipes and recommended

exploratory “surgery” in an attempt to locate the source of the seepage. After

removal of the backfill over the pipes (figure B-13), a small hole drilled through the

concrete between the siphons (figure B-14) revealed no voids and additional 

excavation was required. After portions of the siphon pipes and concrete bedding

were removed it was found that the original contractor had not achieved adequate 

placement of the concrete bedding, and there were extensive voids under the center

of each siphon pipe. Constant flow through these voids had caused internal erosion

of the underlying embankment soils. 

These sections of the siphons were replaced with new PVC pipe and the bedding

was replaced with a higher slump concrete than was used originally (figure B-15). 

Figure B-13.—After about 10 years of operation, seepage was observed

on the downstream slope of the dam in the vicinity of the siphons, and 

the overlying embankment material was excavated to expose the pipes

to determine the source of the seepage.

B-19
Plastic Pipe Used in Embankment Dams

Figure B-14.—A small hole drilled through the concrete bedding 

between the siphons did not encounter voids under the concrete, even 

though the designer suspected that seepage was occurring directly 

under the pipes. 

Figure B-15.—After removal of portions of the PVC siphons, it was 

determined that the original concrete bedding had been improperly

placed, resulting in voids under the centers of the pipes. Portions of 

the siphons were replaced, and the bedding was replaced with a high

slump concrete.

B-20
Appendix B—Case Histories 

Lessons learned (adapted from Wilson and Monroe): 

• Internal erosion of soils is a real-life occurrence. 

• Neglecting minor details during construction can result in development of 

failure mechanisms. 

• Successive attempts were made to find the source of the seepage problem in a 

cost effective manner. 

• Good communication between the contractor, owner, and designer can result 

in a cost effective solution to a major problem, saving dollars for the owner and 

improving the safety of the dam. 

References: 

Sugar Mill Community Association, Minutes of Board of Directors meetings: 

April 18, 2002; May 7, 2002; and January 14, 2003. 

Wilson, Charles and Joseph Monroe, Dam Surgery—Repairs to Sugar Mill Dam, Fulton 

County, Georgia, ASDSO Southeast Regional Conference, 2004. 

B-21 
Plastic Pipe Used in Embankment Dams 

Project: Upper Wheeler Reservoir Dam 

Location: Washington 

Summary: Collapse of HDPE pipe during grouting operation 

In 1992, construction on Upper Wheeler Reservoir Dam included sliplining an old 

concrete box conduit with an HDPE pipe. The project also included extending the 

outlet downstream. The problem arose while grouting the annular space between 

the original box conduit and the new HDPE pipe. 

The grouting operation consisted of pumping grout in from the downstream end of 

the box conduit, forcing the grout upstream. The contractor was successful in only 

grouting the lower 120 feet of the existing box conduit. When they reached the 

halfway point, the sides of the old box conduit failed at the lower end, resulting in 

the loss of about 5-6 cubic yards of concrete. Because the contractor could no 

longer continue the grouting operation from the downstream end, the equipment 

was relocated to the upstream end of the conduit. At the time, no one was aware 

that the pipe had collapsed. The fact that they could not release any water was the 

ultimate “smoking gun.” Figure B-16 shows the grout tube in the original box 

conduit at the toe of the dam and the new outlet extension. Note: The State of 

Washington requires HDPE to be encased in concrete. 

Figure B-16.—Looking upstream towards toe of dam: View of old box 

conduit and new downstream concrete encasement, during grout 

operation. Grout pipe is shown in top of photo. 

B-22 
Appendix B—Case Histories 

Figure B-17.—Looking downstream from toe of 

dam after excavation of collapsed portion of HDPE 

pipe (located in original box conduit, upstream of 

new concrete encasement). 

Calculations completed after the failure (figure B-17) showed that the HDPE pipe’s

resistance to external hydraulic pressures was much lower than the grouting

pressures that were used. Due to the location of the grout pipe, high grout pressures

were necessary to push the grout upstream. 

Lessons learned: 

• Get a specialty contractor experienced in grouting.

• Reviewers should get the specialty subcontractors documentation of the

suitability of the grouting scheme and independently check their calculations of 

pipe stresses.

B-23 
Plastic Pipe Used in Embankment Dams 

• If possible, the grouting of the downstream end of the pipe should be done 

using some form of slickline grout pipe inserted from the upstream end of the 

pipe. 

• Avoid pumping grout up a pipe. 

• If practical, use a low-density grout and fill the pipe to be encased with water 

during grouting. 

Reference: 

State of Washington Department of Ecology. 

B-24 
8 inches 

16 inches 

12 

inches 

12 

inches 

Figure B-18.—Position of 48-inch diameter, SDR 32.5 HDPE pipe in 

unreinforced concrete encasement. 

Appendix B—Case Histories 

Project: Virginia Dam 

Location: Virginia 

Summary: Collapse on a HDPE pipe encased in concrete due to external 

hydrostatic pressure 

A plastic pipe that was encased in concrete collapsed during first filling of a slurry 

impoundment in 1996. The decant conduit consisted of a 48-inch diameter, SDR 

32.5 HDPE pipe encased in unreinforced concrete. The encasement had been 

formed around the pipe during conduit construction and the concrete had been 

placed in several separate sections along the pipe. Blocks had been placed under the 

pipe to hold it in position during the concrete placement. The encasement was 

square and 72 inches on each side. The concrete thickness was 16 inches above the 

pipe, 8 inches below the pipe and 12 inches at the springline (figure B-18). 

B-25 
Plastic Pipe Used in Embankment Dams 

B-26

Construction was completed in early May, 1996, and the reservoir began to fill in 

mid-May. No problems were evident up to and including an inspection of the pipe 

on May 27, 1996. On June 5, 1996, when the pool level had risen by approximately 

50 feet, a discharge of approximately 300 gallons per minute was observed from the 

decant pipe, even though the pool was still three feet below the riser inlet elevation. 

Man-entry inspection of the pipe revealed that for a distance of approximately 25

feet, the bottom of the pipe had deformed upward, with the bottom of the pipe 

contacting the top of the pipe in one area. See figures B-19 and B-20. While 

inspecting the deformed area from the upstream end, running water could be heard 

entering the pipe farther downstream. Two days later, the pipe had deformed for a 

distance of approximately 250 linear feet. Eventually, the HDPE pipe became 

deformed along most of its encased length. 

The plastic pipe had collapsed as a result of being subjected to hydrostatic pressure 

between the pipe and the concrete encasement. Possible entry points for the water 

included the joints between concrete placements, the contact area between the 

concrete and the blocks used to position the pipe, and cracks in the concrete. At the 

point where the collapse first occurred, the pipe was subjected to a potential head 

from the pool of approximately 81 feet, or a hydrostatic pressure of 35 lb/in2

. 

Figure B-19.—View of interior of deformed HDPE pipe from CCTV camera. 
Appendix B—Case Histories 

B-27

Figure B-20.—Approximate upward distortion of bottom of HDPE from 

outside hydrostatic pressure between the pipe and its concrete 

encasement. 

Station 1+50 Station 1+55

Station 1+30 Station 1+35

Station 1+40 Station 1+45
Plastic Pipe Used in Embankment Dams 

Based on information supplied by HDPE pipe manufacturers, unrestrained SDR 

32.5 pipe can collapse as a result of a short-duration external hydrostatic pressure of 

less than 7 lb/in2

, and a long-duration hydrostatic pressure of less than 3 lb/in2. In a 

study by Jenkins and Kroll (1981), samples of SDR 32 polyethylene pipe which were 

encased in grout collapsed when subjected to short-term hydrostatic pressures, at the 

interface between the pipe and the grout, in the range of 32 to 34 lb/in2

. 

As a result of the problem with the pipe, the reservoir was lowered by pumping, an 

open channel spillway was excavated, and the conduit and annulus were filled with 

grout and abandoned. 

Lesson learned: 

• Plastic pipe encased in concrete or grout must have sufficient strength to resist 

the external pressures to which it may be subjected. This includes pressures 

during the placement of the concrete or grout, as well as potential pressures 

from the reservoir. In this case, the concrete encasement had been evaluated 

for the earth loads, but the plastic pipe had not been designed to withstand 

hydrostatic pressure acting between the pipe and the encasement. 

• Designs for encased pipes need to take into account that the pipe may become 

out-of-round or a flat spot may be created during the construction process. 

The floatation forces created during concrete pouring, for example, can cause 

deflection of the pipe and/or local deformation where the pipe is restrained. If 

a pipe is deflected or otherwise out of round, its resistance to collapse from 

outside hydrostatic pressure is reduced (Watkins, 2004). 

Reference: 

Jenkins, C.F. and A.E. Kroll, “External Hydrostatic Loading Polyethylene Pipe,” 

Proceedings of the International Conference on Underground Plastic Pipe, ASCE, pp. 527-541, 

1981. 

Watkins, Reynold King, Buried Pipe Encased in Concrete, ASCE, 2004. 

B-28 
Appendix B—Case Histories 

Project: Wheatfields Dam 

Location: Arizona 

Summary: Sliplining a deteriorating outlet works conduit using HDPE dual-wall 

containment pipe 

Wheatfields Dam is an earthfill embankment located on the Navajo Indian 

Reservation in Arizona. The dam is an offstream storage facility used for irrigation 

and recreational purposes. Wheatfields Lake has a storage capacity of 3,880 acre-feet 

at the top of active conservation, elevation 7,296.6. Wheatfields Dam impounds 

flows from a small drainage basin on an unnamed tributary of Wheatfields Creek and 

diverted flows from Wheatfields Creek via a diversion canal. 

The embankment is homogenous earthfill consisting of silt, clay, gravel, and cobbles 

and was constructed to crest elevation 7,302.1 in 1963. The dam has a crest length 

of 1,600 feet and a maximum structural height of 66 feet, a 36-foot crest width, and 

upstream and downstream slopes of roughly 3H:1V. A highly traveled two-lane 

paved highway crosses over the crest of the dam. 

Appurtenant structures at the dam include a spillway and an outlet works. The 

spillway is an unlined trapezoidal cut excavated through a shallow ridge at the north 

end of Wheatfields Lake. The outlet works is located in the central portion of the 

embankment. The outlet works foundation consists of Pleistocene age alluvial red 

silty clay. The outlet works has two intake structures for low-level and irrigation 

releases. The low-level intake structure consists of a trashracked concrete box with 

an inlet sill elevation of 7,261.0 and is controlled by a 24-inch diameter slide gate. 

The low-level intake structure connects to approximately 290 feet of 24-inch 

diameter CMP that extends downstream to an exit portal. The irrigation intake 

structure consists of a trashracked concrete box with an inlet sill at elevation 7,292.0 

and is controlled by a 24-inch diameter slide gate. The irrigation intake structure 

connects to approximately 30 feet of 24-inch diameter CMP that extends vertically 

downward to a location where it merges with the low-level outlet works. The 

irrigation intake was abandoned after the sliplining of the CMP. The outlet works 

has a computed discharge capacity of 41 ft3

/s (through the low-level intake only) 

when the reservoir water surface is at the spillway crest, elevation 7,296.6. The gate 

stem to the low-level gate is broken, making it inoperable. Figure B-21 shows the 

general configuration of the existing outlet works. 

Modifications were required to the outlet works to address dam safety deficiencies 

and operational and maintenance issues. The primary deficiency involved separation 

of pipe joints and deterioration of the interior surface of the CMP. The existing 

condition of the conduit and the possibility of internal erosion of embankment 

materials either into or out of the conduit prompted concerns about increased risk 

B-29 
Plastic Pipe Used in Embankment Dams 

B-30

Figure B-21.—The profile of the existing outlet works prior to modifications.

for the development of a serious dam safety failure mode. As part of the 

modifications, new operational capabilities were added to supply pressurized flow to 

the outlet works to meet the need for future irrigation downstream from the dam. 

After evaluation of a number of alternatives, sliplining of the outlet works conduit 

was selected as the best solution for eliminating the dam safety issues associated with 

the outlet works and accommodating future needs. Since the modified outlet works 

will have pressurized flow, it will be controlled by a rate-of-flow control valve 

located in a new downstream control structure during normal operations. For 

operations that require faster drawdown of the reservoir, the flow will be controlled 

by a ball valve located in the downstream control structure. The outlet works 

conduit will remain pressurized during the irrigation season. During the winter 

months, the upstream slide gate will be closed, and the liner will be drained. The ball 

valves will be left open in the winter to prevent freezing. 

A dual-wall HDPE containment pipe was selected for sliplining. Dual-wall

containment pipe provides structural integrity to the outlet works in addition to 

addressing potential internal erosion concerns by preventing seepage either into or 

out of the conduit. The potential for internal erosion along the outside of the CMP 

is addressed by using sufficient pressures during the grouting process to encourage 

grout travel through any existing small openings in the CMP and the construction of 

a downstream filter and drainage system. 

A dual-wall HDPE containment pipe consisting of a 14-inch outside-diameter pipe 

in a 20-inch outside-diameter HDPE pipe was selected for sliplining and grouting 

into the existing 24-inch diameter CMP. In addition to sliplining, the upstream 

intake structure was removed and replaced, and a new downstream control structure 

was constructed. Figure B-22 shows the general configuration of the modified outlet 

works. 
Appendix B—Case Histories 

B-31

Figure B-22.—The profile of the modified outlet works. 

The controlling hydraulic design factor of the modified outlet works was the 

maximum diameter of dual-wall containment pipe that could be inserted into the 

existing CMP. During initial design, the outside diameter of the HDPE lining was 

made about 10 percent smaller than the pipe to be lined. However, to facilitate 

installation and annular grouting, a dual-wall containment pipe consisting of a 

14-inch outside-diameter carrier pipe (approximate inside diameter 12.9 inches) 

inside a 20-inch outside-diameter containment pipe (approximate inside diameter 

18.5 inches) was selected. The inside dimensions of the existing CMP were carefully 

measured using a CCTV crawler-camera with a template attached to it to be sure 

there were no obstructions or deformations within the CMP that would prevent a 

20-inch-diameter pipe from being installed. 

The following loading conditions and methods of analysis were used for design of 

the slipliner: 

• External loading equal to the maximum embankment load with no

consideration given for the existing CMP and no internal pressure. Maximum 

embankment depth was assumed to be 50 feet. The HDPE pipe was analyzed 

for wall crushing, wall buckling, and ring deflection. 

• Internal loading equal to the hydrostatic loading with the reservoir at the dam 

crest without side support from the surrounding embankment. Maximum 

hydraulic head rounded to 50 feet. Seventy percent of the strength of the 

internal 14-inch diameter pipe was used in the pressure calculations. This 

criterion was based on the manufacturer’s recommendation for the dual-wall

containment pipe. The strength is reduced because the fusion welds of the 

containment pipe cannot be inspected from the interior of the pipe. The welds 

will be visually inspected by CCTV before the pipe is grouted into the CMP. 

The following material properties were used in design: 

• Hydrostatic design stress = 800 lb/in2
Plastic Pipe Used in Embankment Dams 

• Standard dimension ratio = 26 

• Linear thermal expansion coefficient = 1.2 x 10-4 in/in/°F 

• Short term modulus of elasticity = 100,000 lb/in2 

• Compressive yield strength = 1,500 lb/in2 

Construction began in January 2005. The reservoir was drained prior to the start of 

construction. The existing outlet works conduit was pressure washed and the 

cleaning verified by CCTV inspection. During construction, it was decided to use 

two separate pipes to form the dual-wall containment pipe rather than using a 

fabricated prejoined system. A McElroy track-star 500 Series fusion welder was used 

to butt fusion weld the sections of HDPE pipe together. A John Deere 230LC 

excavator (figure B-23) and Case 580-super L backhoe were utilized for moving the 

HDPE pipe from the staging area and for placing the pipe onto the fusion machine. 

To facilitate butt fusion and avoid construction congestion, the 14-inch diameter 

pipe was joined together on the downstream side of the dam and the 20-inch 

diameter pipe was joined together on the upstream side of the dam. The 3⁄4-inch 

diameter HDPE grout lines were attached to the exterior surface of the containment 

pipe using a Munsch MA-40-B hand extrusion welding gun. Spacers were placed on 

8-foot centers to center the 14-inch diameter pipe within the 20-inch diameter pipe. 

Additional 3⁄4-inch diameter pipe was extrusion welded onto the bottom quadrant of 

the containment pipe to act as centering skids. 

The excavator was used for guiding the 20-inch diameter pipe into the CMP at the 

upstream end of the conduit (figure B-24), while the backhoe pulled the pipe using a 

specially designed steel pulling head attached to the pipe pulled from the 

downstream end. The installation process was reversed for pulling the 14-inch 

diameter pipe into the 20-inch diameter pipe. After installation of both pipes, water 

from the reservoir was used to separately fill each pipe for hydrostatic pressure 

testing. 

Bulkheads were constructed at the upstream and downstream ends of the outlet 

works conduit for grouting of the annular space between the 20-inch diameter pipe 

and the existing 24-inch diameter CMP. A grouting subcontractor was used for the 

grouting operations. Two identical grout plants were made available onsite, with one 

plant serving as the backup in case it was needed. Grouting was performed through 

four 3⁄4-inch diameter HDPE pipes extrusion welded to the crown of the 20-inch 

dual-wall containment pipe of different lengths. The four lengths are 25.5 feet, 

60.5 feet, 90.5 feet, and 120.5 feet. One additional 3⁄4-inch diameter HDPE pipe was 

also welded to the crown of the 20-inch dual-wall containment pipe and used as an 

air vent. An initial grout mix of 4,000 lb/in2 with 0.6:1 w/c (water-cement ratio by 

volume) and super plasticizer was used. After grouting operations began, it was 

B-32 
Appendix B—Case Histories 

B-33

Figure B-23.—HDPE pipe being unloaded at the site using a John Deere 

230LC excavator. 

Figure B-24.—Guiding the 20-inch pipe into the upstream end of the CMP. 

determined that the grout mix could not be injected into the 3⁄4-inch diameter grout 

pipe at the prescribed grout pressure of 5 lb/in2

. The grout mix was changed to 

0.8:1 w/c with the amount of superplasticizer increased and the pumping pressure 

slowly increased to 25 lb/in2

. Additional modifications to grout mix and pumping 

pressure were required to maintain a constant injection rate. The entire grouting 

process took about 8 hours for injection of 340 bags of cement. 
Plastic Pipe Used in Embankment Dams 

The modifications, including sliplining, construction of new upstream and 

downstream structures, and other toe drain modifications, were completed in 

September 2005. A CCTV inspection was performed after the completion of 

construction and showed no problems. 

Lessons learned: 

The color of the installed HDPE dual-wall containment pipe was white. This was 

selected to improve inspection using CCTV equipment. However, it was later found 

that the white causes too much contrast and gray or black is better suited for CCTV 

inspection. 

Grout pipes should be 1-inch diameter rather than 3⁄4-inch diameter to facilitate 

grouting of the annulus. 

References: 

Bureau of Reclamation, Wheatfields Design Summary, 2006. 

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Appendix B—Case Histories 

Project: Worster Dam 

Location: Colorado 

Summary: Renovation of an existing outlet works using an HDPE slipliner grouted 

in place 

Worster Dam and Reservoir, also known as Eaton Dam and Reservoir, are located in 

Larimer County northwest of Fort Collins, Colorado. The dam and reservoir is 

located on Sheep Creek, a tributary to the north fork of the Cache La Poudre River 

in a mountainous area about five miles south of the Colorado-Wyoming border. 

Worster Dam is a concrete face rockfill dam constructed in the early 1900’s. The 

exact year of construction is not known. The dam is about 72 feet in height with a 

crest width of about 12 feet and a crest length of over 700 feet. The impounded 

reservoir has a maximum storage capacity of about 3,750 acre-feet. 

The outlet works consists of reinforced cast-in-place concrete arched pipe with a 

central gate chamber. The inlet conduit consists of a 36- to 38-inch wide and 

37-inch high reinforced concrete arch. The inlet conduit then connects to a central 

gate chamber. The gate chamber is approximately 7 feet in height, approximately 

3 to 61⁄2 feet wide and about 10 feet long. Flow was controlled by two 36-inch 

diameter slide gates housed in the gate chamber. The gate stems extend vertically 

through the embankment with the operators located at the dam crest. The gate 

chamber discharges to a larger 48-inch by 48-inch reinforced concrete arch conduit 

and directly to the stream. 

An evaluation of the existing outlet works was performed by Woodward-Clyde 

Consultants (heritage firm to URS Corporation). The evaluation found that the 

central gate chamber appeared to be in relatively good condition. However, the 

evaluation also found that the outlet works conduit upstream and downstream of the 

gate chamber was composed of extremely poor quality concrete. Areas of the inlet 

structure had exposed steel and the steel reinforcement was severely corroded. 

Furthermore, much of the cement paste had been dissolved by the aggressive water. 

Renovation of the outlet works consisted of replacing the inlet structure and guard 

gate, demolishing the slide gates in the central chamber, and sliplining the outlet 

works with an HDPE pipe. The new HDPE outlet works conduit was designed to 

withstand the full reservoir for external water pressure (buckling) and internal 

reservoir pressure. The upstream portion of the outlet works conduit was lined with 

a 30-inch diameter SDR 17 HDPE pipe grouted with a 2,000 lb/in2

 cement grout. 

The downstream portion of the conduit was lined with 42-inch diameter SDR 17 

HDPE pipe grouted in place. The 30-inch and 42-inch pipes were connected using 

an eccentric reducer. The upstream end of the inlet structure was connected to a 

B-35 
Plastic Pipe Used in Embankment Dams 

steel elbow using a Dresser coupling and a steel elbow with air vent was installed at 

the inlet with a 30-inch diameter reducing thimble. 

The HDPE pipe was welded and assembled prior to installation. The conduit was 

then pushed/pulled into place using two Caterpillar D8 dozers. The integrity of the 

installed pipe was verified by pressure testing. Once a satisfactory condition was 

established, bulkheads were constructed at the upstream and downstream ends of 

the conduit. Grout discharge pipes of various lengths were installed on the 

downstream and upstream bulkheads and within the existing casing for the central 

grade chamber valve stems. In all, eight grout introduction points were established 

and utilized during the grouting process. 

Flange ends were installed at the inlet and outlet, and the outlet pipe was filled with 

water. A pressure gauge was installed to monitor external grout pressures to verify 

that external pressures did not exceed the design pipe load. 

The grout was slowly introduced at the downstream end. A combination of sand 

cement grout and neat cement grout with a superplasticizer were introduced at 

various times throughout the grouting process. The mix water was obtained from 

the reservoir. The mix water was near freezing because the source was a recent 

snowmelt. The cold grout resulted in constriction of the HDPE pipe. During the 

grouting process, the pipe water pressure began to increase. Grouting activities were 

suspended to allow the grout pressures to dissipate. Much to the surprise of the 

construction team, the water pressure continued to rise as the pipe continued to 

contract in response to the low grout temperature. Eventually the pressure became 

sufficiently high that the end couplings slipped, relieving the pressure. The internal 

water pressure returned to near zero pressure, the pressure remained stable, and 

grouting resumed. The temperature in the pipe stabilized and no further increases or 

decreases within the pipe pressure were observed. Once the grout was allowed to set 

up for 72 hours, the flanges were removed from the pipe and the outlet works was 

placed into service. 

Lessons learned: 

The following lessons were applicable to the construction described above. 

• Filling a pipe with water is an effective means to reduce risk of pipe collapse 

during grouting. 

• Continuous monitoring of the internal pipe water pressure should occur. 

• Plans should be made to consider and monitor the grout mix temperature. The 

subject project observed a contraction of the pipe due to the low water 

pressure. The opposite could occur in which the grout temperature may cause 

expansion of the HDPE pipe, resulting in a negative pressure in the pipe. 

B-36 
Appendix B—Case Histories 

• All of the eight grout introduction points were utilized during the grouting 

process. The success of the grouting project was based on the ability to 

introduce grout at these eight locations and fewer locations would have resulted 

in a failure to establish a complete grout seal around the pipe. 

Reference: 

Christopher N. Hatton, P.E., Woodward-Clyde Consultants, 2005. 

B-37 
Plastic Pipe Used in Embankment Dams 

B-38