Advances in Design of Landfil

Over CCB Ponds and Landfill

Mike Houlihan, P.E., BCEE, F.ASCE

Geosyntec Consultants

Columbia, Maryland

16 January 2013
Ov

 Current State of Practice Properties of CCBs Key Design Issues Case Histories Closing Comments
Current State of the P

• There is a broad range of operating and reg

standards in the USA

• New CCB disposal facilities (i.e., those cons

between 1994-2004) generally have liner syst

• Hundreds of existing CCB disposal facilities h

liner systems

• Many of these sites are candidate

vertical/lateral expansions if they can be deto comply with coming standards and good p

Reference: U.S. Environmental Protection Agency and U.S. Department of the Energy “CoaWaste Management at Landfills and Surface Impoundments”, 1994-2004, DOE/PI-0004, [2006]
...Current State of P

• Disposal methods have varied historically from w

the South and in parts of the Midwest) to dry, froengineered disposal facilities to structural fills.

• USEPA will soon (?) decide on whether Subtitle 

liner requirements will apply 

• Placement of dry CCBs in engineered monofill

containment facilities will likely become the requdisposal method in the future

• Relevant containment technologies are well unde

but the unique nature of CCB materials and past practices requires a new look at engineering pra

CCB containment system design
Typical Coal-Fired Electric Pow
Overview of 

 Fly ash  Bottom ash  Flue gas desulfurization (FGD) byproducts (i. Boiler feedwater treatment wastewater solids Wastewater impoundment solids Metal cleaning wastewater solids Coal washing operation solids Also, water from these operations:

 Coal pile runoff and wash water Boiler cleaning wastewaters Boiler blowdown water Cooling tower blowdown water
Change in Pa

for Power Comp
Focus on Fly 

SilicateSilt sized (can vary)Round particles - apparently uniformLow unit weight/specific gravityHigh capillary tensionPhysically and chemically – remnants of parent cIt is difficult to obtain and transport undisturbed 

for lab testing- it easily liquefies in transit

BUT, excellent drained strength
What is Fly 

Particle Shape & Mineralogy

• Aluminum silicates 

• Dominantly spherical • Significant internal voids• Void ratio and Gs difficult to measure

Particle size

• 75% finer than 45μm

• Significant silt size fraction• Typically well graded• Varies by coal, facility, combustion process
Insitu Properties of Deposi

N
Perhaps ana

to a House of C

1
Strength of F

0

50

100150200

0 100 200 300 400

Effective mean stress, p=(

σ1'+

σ3')/2 [kPa]

Deviatoric stress, q=( σ1-σ3)/2 [kPa] e=0.87 e=0.80 e=0.54 e=0.48 e=0.42

φ=30°

φ=35°

0

50

100150200250

0 100 200 30

Effective mean stress, p=(

σ

Deviatoric stress, q=( σ1-σ3)/2 [kPa]

σc=50σc=100σc=300σc=400Series1Series2

Triaxial Shear Strength in Lab

• Tests performed on lab remolded specimens 

• Undrained strength similar to granular soils• Nominal beneficial effect of “density”• Generally dilatative response, even at high stress• Minimal tendency to contract• Strength and response similar to frictional 

response of sands

(Geosyntec/Georgia Tec
Agin

1

Shear wave velocity measurements of remolded and fresh (i.e., un-hy

Arithmetic Time Log Time

Unanticipated results, as KIF ash is Class F ash with little to no fre

Similar indications observed in previously hydrated ash and block sa

(Georgia 
Fly Ash Characterization Ch

 Limited accurate information available regarding fl

material properties

 Low CPT tip resistance can be misleading and cause 

concerns regarding strength

 Even Class F ash has been shown to “age” with time 

confined

 Key element is control of water (infiltration and intern

drainage)

 Each facility will have unique conditions based on 

properties, construction methodology, operational and facility layout
Fly Ash Characterization: Implic

Closure and Overfil

 Fly ash and CCB materials are not typical 

geotechnical construction materials and need well understood to characterize a site correctldevelop a good design

 Fly ash properties vary from site to site, and c

within a site, as a function of many factors (cosource, combustion process, air pollution contdisposal method, disposal site operations, etc

 There is a growing body of knowledge of CCB 

ash properties, although not all of it is publicaavailable... (EPRI Study should help in this re

1
State of Practice is A

field data

 Large Body of Knowledge from the 

Kingston failure investigations 

 EPRI Studies

 Geotechnical properties of coal ash Expansion over coal ash ponds Groundwater monitoring at CCB 

disposal sites

 Coal Combustion Ash and Research 

Institute, UNC Charlotte

 Center for Applied Energy Research, 

University of Kentucky

 Practicing Engineers (although they are 

not always at liberty to share their data)

 Technology Transfer through 

Conferences and Seminar
Key Design 

Overfill Foundation Stability 

 Settlement Slope Stability

 Seismic Liquefaction Static Liquefaction

Leachate Collections SystemsPerformance Assessment of Existing CCR l
Potent

Settlement Im

Industrial WasteCCB- Dry StackHazardous Waste Land Clearing WasteCCB – Wet StackConstruction/Demo DebrisMunicipal Solid Waste

Greater Less

Potential Potential

“Area” Settlement “Local” Settle

Industrial WasteCCB’s Hazardous WastMunicipal Solid WLand Clearing WConstruction/De

Greater Less

Potential Potential
Potent

Settlement Im

General• Settlement of CCBs in wet ponds is much greater than in dry s• Settlement in wet ponds can be in the range of 10% of total th

ash (Class F) pond (based on Geosyntec experience at severa

• Estimates are sensitive to material properties and can vary widPossible Localized Effects• Collapse of a void or poorly compacted CCB • Distortion/damage to liner• LCS grade reversal• Typically evaluated/quantified using analytical methodsPossible Area-Wide Effects• Excessive total settlement and grade reversal• Excessive differential settlement and grade reversal• Evaluated/quantified using conventional analytical methods or n

methods (FEM)
How Can The Accu

Settlement Estimates Be Imp

1. Specialized Undisturbed Sampling2. In Situ and Lab Testing3. Field Tests: Monitored Preload Fill-Pilot T
Monitored Prelo

• A MPF provides a method to verify engineering calcu

measuring settlements on a pilot scale.

• Basically: a large pilot scale test, somewhat expensive s

the first choice of settlement estimate methods

• Consists of application of load (soil layers) in incre

measurement of:

• Settlement/deflection of the surface and subsurface laye• Pore pressures in the subsurface layers

• It’s possible that, after a period of MPF performance and 

the practice and regulators will depend on preload fill demless and less
MPF Instrument 

D

SP-1 SP-2 SP-3 SP-4 SP-5 SP-6

4” HDPE Profiler 

Pipe

PW-6 (40’)PW-7 (70’)

PW-1 (40’)PW-2 (70’)

PW-3 (30’)PW-4 (60’)PW-5 (90’)

SONDEX Pipe

MPF Boundaries
Profiler “Settlement Measur
Profiler Settlement R

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 100 200 300 400 500 600 700 

Settlement Profiler Line (southwest to northeast), (ft)

Total Settlement (ft) 
Pore Pressure Measure

0

100020003000400050006000

3/19/07 4/8/07 4/28/07 5/18/07 6/7/07 6/27/07 7/17/07 8/6/07 

Date

Pore Water Pressure (psf) PW-6 (40-ft)

PW-5 (90-ft)

PW-4 (60-ft)

PW-1 (40-ft)

PW-3 (30-ft)

PW-2 (70-ft) PW-7 (70-ft)
Key Design 

Foundation Stability 

 Settlement Slope Stability

 Seismic Liquefaction Static Liquefaction

Leachate Collections SystemsPerformance Assessment of Existing CCR l
Causes of Slope Ins

• Increased Stress

• External Loads – stockpile, equipment

*

• Increase unit weight

*

• Remove material by cut/excavation• Undermine by seepage erosion

*

• Shock – earthquake• Tension crack• Water pressure in crack

*

* Influenced by water (after Sowers, 1979)
Causes of Slope Ins

• Decreased Strength

• Swelling and adsorption of water

*

• Pore pressure increase

*

• Structural changes in soil

*

• Cracking caused by shrink-swell

*

• Strain and progressive failure• Thawing

*

• Weathering of soils

*

* Influenced by water (after Sowers, 1979)
Key Design 

Foundation Stability 

 Settlement Slope Stability

 Seismic Liquefaction Static Liquefaction

Leachate Collections SystemsPerformance Assessment of Existing CCR landfi
Seismic Lique

Liquefaction of saturated, non-cohesive, uniform

soils under seismic loading:
Seismic Lique

 The prevention of seismic liquefaction requires expensive 

methods:

 Construction of a toe berm (soil embankment) or buttress beyond the e

landfill to maintain confinement of the subgrade; 

 Installation of a shear key at the edge of the landfill to resist the lateral 

below the edge of the landfill; 

 Performing a preload fill program to promote consolidation and reduce 

liquefaction potential or reduce the effects of liquefaction, should it occu

 Foundation improvements, such as deep soil mixing and solidification

densification, etc. to eliminate the conditions conducive to liquefaction. 

 Additional study is necessary using laboratory cyclic triaxial an

simple shear testing correlated to the project setting

 Preliminary laboratory data suggest that the risk of seismica

liquefaction using the traditional analytical methods is over estimat
Static Lique

 Caused by relatively rapid loading of the subgrade that 

strain-induced excess pore pressures that canadequately dissipated, resulting in loss of material sand, typically, a bearing-capacity type failure

 For proposed developments, can be analyzed during 

and avoided by construction methods

 Recent lab testing is finding it difficult to induce 

liquefaction in CCB materials
Key Design 

Foundation Stability 

 Settlement Slope Stability

 Seismic Liquefaction Static Liquefaction

Leachate Collections SystemsPerformance Assessment of Existing CCR landfi
Leachate Collection Sy

 Decreases in functionality caused by:

 Settlement-induced grade changes in LCS components Clogging by fine particles and calcification Overloading of existing leachate collection systems un

landfills

 Addressed during design through testing and design 

filtration capacity

 This is a key issue in CCB landfills compared to MSW 

the very large working areas that are common at CCB land

 Ponding of surfacewater on working “deck” is a design co

that is relatively unique to CCB landfills
Oper

Surfacewater C
Leachate Collection Sy

 To prevent clogging, landfill LCS layers are typically overlain by a g

layer, which must have a relatively small apparent opening size (AOS) CCR material from passing through the geotextile.

 Large overburden pressures caused by the overlying CCR can 

effectiveness of a geotextile filter.

 To address these design challenges:

(i) perform a particle-size analysis of the CCR material to confirm the 

the material that the geotextile must retain;

(ii) test the geotextile filter under the expected overburden pressures 

filtration properties;

(iii) provide excess filtration layer surface area over/around the colle

gravel as a contingency in case the geotextile filter experiences sand

(iv) provide a method of cleaning out the leachate collection and trans

should precipitates form or sediments accumulate that could conveyance capacity
Leachate Collection Sy

 To prevent clogging, landfill LCS layers are typically overlain by a g

layer, which must have a relatively small apparent opening size (AOS) CCR material from passing through the geotextile.

 Large overburden pressures caused by the overlying CCR can 

effectiveness of a geotextile filter.

 To address these design challenges:

(i) perform a particle-size analysis of the CCR material to confirm the 

the material that the geotextile must retain;

(ii) test the geotextile filter under the expected overburden pressures 

filtration properties;

(iii) provide excess filtration layer surface area over/around the colle

gravel as a contingency in case the geotextile filter experiences sand

(iv) provide a method of cleaning out the leachate collection and trans

should precipitates form or sediments accumulate that could conveyance capacity
LCS Clogging Potential for CC

10oz/yd2 NW GEOTEXTILE FILTER

8” DIA. SDR-9 

PERFORATED HDPE 

NO. 2 STONE

SUBGRADE

COMPACTED CLAY LINER

SOIL COVER
Graded Filter Drainag
Key Design 

Foundation Stability 

 Settlement Slope Stability

 Seismic Liquefaction Static Liquefaction

Leachate Collections SystemsPerformance Assessment of Existing CCR landfi
Performance Assessm

Existing F

Evaluate During Expansion Design: Liner System of Underlying Landfill

 Data collection – is it available? Consider lack of siting criteria if a very old fill Stability of underlying materials

 Older design standard May not allow vertical expansionLeachate Collection System Pipe strength - may not allow vertical expansion Access to cleanouts, leachate removal, etc. Resulting slopes may not result in positive drainage
CCB Expansion Project E

TVA Kingston, TN – Peninsula Landfill, 

2010-2012

Cardinal Landfill, OH: Improvement of 

Poor Foundation, 2008

R. Paul Smith, MD/WV – MSE B

Capacity Enhancement, 2008
Case Hi

CASE HISTORY ONE:

Vertical Expansion Over Closed CCB Pond

CASE HISTORY TWO:

Lateral Expansion and Overfill on CCB Landfill

CASE HISTORY THREE:

Overfill and Lateral Expansion on CCB Landfill and P
Case History #1 – 

Expansion Over Closed CC

 Overview:

 110-acre Ash Pond Subgrade 35 m of Mostly Saturated Fly Ash Subgra Class I High Hazard Dam Downstream Lateral Groundwater Inflow

 Design Challenges:

 Significant site geometry constraints

 Desire to construct expansion in small inc Utilize the existing footprint as much as p
Case History #1: Site 
Case History #1: Cross S
Design Ap

1. Design Investigation

 Undisturbed and disturbed sampling Borings and cone penetrometer with dissipation testing

2. Liquefaction Evaluation

 Liquefaction potential was considered in the design Facility designed with a horizontal confining layer to restrain mo Consolidation water drained using an underdrain layer of bot

dissipate pore pressures
Design Ap

3. Hydrostatic Uplift Potential- Lateral Groundwater Inflow

 Mitigated using Bottom Ash Underdrain below Liner to Dis

Pressures

 Mitigated using Sand Underdrain on Side Slopes

4. Settlement Evaluation

 Mitigated using a Moving Preload to Remove Significant 

before Construction
Hydrostat
Hydrostati
Cap/Liner Design Cross-S
Geotechnical Design Consid

Hydrostatic Uplift

Challenges: high GWT, increased pore pressures, groundwater seepage.

Solutions: Side slope drainage blanket of 60-cm thick granular material below 

Groundwater underdrain of 60-cm thick granular material below isolation 

Slope Stability

Challenges: Static and seismic slope stability for side slopes over fly ash, side the mine spoil berm, and side slopes adjacent to the high wall.

Solutions: Several failure modes, including filling scenarios analyzed. Strengt

foundation through preloading. Liner, cover, and interim berm slopes limit

Settlement

Challenges: Limited data on flyash behavior, large settlements, liner strains, eff

Solutions: Insitu (CPT/SPT), lab (consolidation and shear strength), and moni

fill programs. Steeper slopes for underdrain and LCS pipes to account Preload sequence before liner construction.

Liquefaction 

Challenges: Material deposited in an ash pond is known to be susceptible to se

Limited data on liquefaction behavior of flyash was available.

Solutions: Identified potentially liquefiable zones using grain size and CPT. Us

strengths using Stark and Olson approach. Increased soil layer thicliquefied zone to achieve a calculated FS > 1.
Geotechnical Mo

Monitored Preload:• 800 ft X 500 ft, 3H:1V, 35 ft of flyash and mine spoils, 5 ft lifts• Settlement plates• Vertical extensometer (SONDEX pipe)• Horizontal settlement profiler• Vibrating wire pore pressure transducers• Topographic settlement plate surveys

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 100 200 300 400 500 600 700 800 900

Settlement Profiler Line (southwest to northeast), (ft)

Total Settlement (ft) 5/3/2007 5/14/2007 5/25/2007 6/9/2007

7/10/20078/7/2007

856.3876.3896.3916.3936.3956.3976.3996.3

-0.5 0 0.5 1 1.5 2 2.5 3 3.

Total Settlement (ft)

Elevation (ft)

4/12/07 4/22/07 5/3/07 5/14/07 5/25/2007 6/9/2007 7/10/20078/7/2007
Monitoring, Lessons L

• Fly ash materials, while very loose and saturated, were significantly 

compressible and more free-draining than previously understood.

• Use redundant instrument systems.• Perceived fatal flaws were addressed through collection of relevant 

information.

• Vertical drains were not needed.

Lessons learned 

from the monitored 

preload program: 
Case History #2 – 

Expansion and 

 Overview:

 90-acre Dry Ash Fill Operated periodically as wet settling pond

excavated

 10-15 years of disposal capacity needed Adjacent to downhill, undeveloped area

 Design Situation:

 Provide expansion capacity but anticipate 

retirement of plant

 Construct expansion in small increments
CASE HISTORY 

Expansion and O

EXISTING 

LANDFILL

PHASE 1 

LATERAL 

EXPANSION

PHASE 2 RSSVERTICAL 

EXPANSION
CASE HISTORY 

Expansion and O
CASE HISTORY 

Expansion P

Three Expansion Phases:1. Lateral, lined expansion2. Vertical, lined expansion using reinforced soil slope (RSS3. Vertical, unlined expansion over new facility footprint
CASE HISTORY TWO- P

Lateral Exp
CASE HISTORY 

Phase 2 – Vertical/RSS Exp
Case History #3 – 

Expansion Over Closed CC

 Overview:

 Existing 250-acre CCB landfill A 50,000,000 yd

3 expansion needed

 Adjacent to a 300-acre inactive (not closed

 Design Challenges:

 Utilize the existing footprint as much as pra Expand laterally over ash pond Significant site geometry constraints Significant mine voids below edge of footpr
CH #3: Design Ap

 Site characterization of ash in the pond using b

and CPT

u probes

 Specialized evaluation of existing LCS pipes plu

inspection

 Geometry constraints overcome through CCB 

excavation/relocation and increasing vertical ex

 Mine voids mitigated primarily by excavation/ 

replacement

 Foundation water pressures managed using un

system:
Underdrain Consider

Initial Co

Ash

Native 

Subgrade

Existing 

Pond Level

Phreatic 

Surface
CCB Pond Dewaterin

Ash

Native 

Subgrade

Lowered 

Pond Level

Phreatic 

Surface
Cover/Liner System Const

Ash

Native 

Subgrade

Landfill Line

or Pond 

Cover Syste

Lowered 

Pond Level

Inf

Grou
The Situation to A

Ash

Native 

Subgrade

Landfill 

Liner 

System

Infiltra

Ground

Rising Water 

Level in the 

Pond
Underdrain S

Ash

Native 

Subgrade

Landfill 

Liner 

System

I

Gr

Breakou

along th

Perimete

Continued 

Rising Water 

Levels

Phreatic 

Surface

Underdrain

System
Case Study 3: Observ

1. Need to accurately identify geology/subgrade2. Assess structural integrity of as-built existing LC3. Understand the long-term groundwater pore 

conditions in ash/subgrade and be prepared underdrain system for groundwater as “leachat
One More Ex

Ash Facility C

• Alternative closure techniques and 

alternative (and potentially innovative) beneficial uses of ash ponds that are to be closed under the proposed EPA regulations. 

• Forecast how the most recently 

proposed regulations would affect innovation and future technology.

• Prepared a Report on Ash 

Impoundment that client can use in their near-term assessment of options.
Case History (Facility 

Saltville Soda Ash Dispos

• Two ponds at an industrial facility owned by Olin with Ammonia 

waste required closure. Pond dikes were located next to Holston Virginia. 

• Pond 5: 1925 to 1971, 76 acres, 100 ft high dikes, 35 to 70 feet 

ash, also received mercury contaminated wastewater, liquids cotreated on site prior to discharge. 

• Pond 6: 1964 to 1972, 45 acres, 20 – 30 ft high dikes, less merc

contaminated water, liquids were decanted and discharged to th
Case History (Facility C

Saltville Soda Ash Dispos

Geosyntec provid

• Geotechnical characterizat

and test fill program

• Evaluation of capping alter

likely long-term performanc

• Detailed design of cover sy

Pond 5 and 6

• On-call support during con

Dikes constructed using 

process waste mixed with 

slaked lime
Saltville: Geotechnical Mo

• 10 ft high test fill• Total Settlements

• Areal settlement sin

8 feet in some area

• Test fills - 0.5 to 1.0 

construction

• 2 to 3 feet in first 9 

• Differential Settlements

• Differential settleme

to be generally propsettlements, but tesindicate that there irelationship
Saltville: R

Measurewithin bathan 6 inyears aftand are iagreemepredicted based on grading d

“Landform 

Grading”
Analogous D

Avtex Fibers NPL Superfund Site, Virginia (ET cover, 

lightweight fill, reinforced cap, consolidation)

Wood River Impoundment, Ill

Final Closure Design and C

Onondaga Lake Containment Facility over Waste 

Impoundments and Subaqueous Lake Capping Other Overfills over Coal A

1. Allen Steam Generating Sta

NC, Duke Energy (operatin

2. Rush Island and Meramec 

Louis MO, Ameren (propos

3. Also: Others in Illinois, India

Up

solid

Liquefaction 

evaluation of soft 

lake sediments and 

proposed cap 

Impoundments 

located within 

100-yr flood plain
Closing Com

• Landfills can be, and have been, developed over CCB ponds and fills.• At the current state of the practice, site-specific field and laboratory 

properties is strongly recommended.

• Pore pressure buildup is a key design consideration, and can likely be m

the typical timeframe for development of a CCB overfill.

• CCB disposal facilities can be designed to mitigate the effects of seismi

liquefaction.

• Develop a plan to manage groundwater and saturated CCRs below 

avoid buildup of pore pressures that could lead to foundation instability.

• For overfills, conduct a thorough evaluation of the existing landfill leach

system and liner stability to verify that the LCS will can function under pressures resulting from the overfill CCR materials.
Acknowledge

1. Contributing Clients:

 American Electric Power Allegheny Energy Supply/First Energy Tennessee Valley Authority

2. Co-workers/contributors:

 John Seymour, Paul Sabatini, William Steier, Rod

Sancio, John Esser, Sarah Fick, Scott Sheridan
THANK