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Use of Bottom Ash in the Reinforced Zone of a 

Mechanically Stabilized Earth Wall for the 

Vertical Expansion of a Sluiced CCR Pond at the 

Trimble County Generating Station

Nicholas G. Schmitt, PE1, and M. Brian Cole, PE2 

1Senior Principal Engineer, AMEC Environment and Infrastructure, Louisville, KY

2Senior Engineer, AMEC Environment and Infrastructure, Louisville, KY

KEYWORDS: Bottom Ash, MSE Wall, Pond Expansion, Jersey Barrier

Background

Louisville Gas and Electric’s Trimble County Generating Station is a coal-fired power 

plant located on the Ohio River near Bedford, Kentucky. In 2010, the plant more than 

doubled its generating capacity with the addition of a second generating unit, which also 

more than doubled its production of Coal Combustion Residuals (CCR). During siting 

studies for a new CCR landfill, it was decided that a vertical expansion of the existing 

Bottom Ash Pond of its originally permitted height was necessary to provide enough

time to design and permit a new landfill. Due to site geometry constraints, it was 

decided to use over 8000 linear feet of Mechanically Stabilized Earth (MSE) walls to 

construct the vertical expansion. Wall heights of up 45 feet were necessary. Project 

economies were realized by using select bottom ash in the MSE wall reinforced zone. 

This required the contractor to “mine” over 300,000 cubic yards of bottom ash from the 

pond saving money and creating additional pond storage capacity; thereby extending its 

operational life. Clay soil used to line the inner slope of the dike vertical expansion was 

obtained from an adjacent property and the final grades of the borrow area were 

designed to function as a constructed wetland. Over the course of the project, various 

lessons were learned on selecting suitable bottom ash and other construction materials.

Other unique aspects of this project included the use of “back-to-back” MSE walls for 

new ramps, the use of Jersey barriers atop the walls to provide fall protection and serve 

as a vehicle barrier, creating a new emergency spillway, and paving of the embankment 

crest to meet particulate restrictions of the facility’s recently revised air permit.

2013 World of Coal Ash (WOCA) Conference - April 22-25, 2013 in Lexington, KY

http://www.flyash.info/
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Project Description

The Trimble County Generating Station Bottom Ash Pond (BAP), as shown in Figure 1, 

consists of a 4-sided earthen structure located in an upper terrace of the Ohio River 

which was constructed by subsurface excavation and use of the excavated soils to 

construct berms (dikes) around the excavation perimeter. The BAP is abutted to the 

north by a smaller previously unused pond formerly known as the Emergency Fly Ash 

Pond (EFAP), but now lined and serving as the Gypsum Storage Pond (GSP). The 

BAP is approximately 2,900 feet long (north/south) and 900 to 1800 feet wide 

(east/west). The footprint encompasses approximately 90 acres. Permitted as a “zero 

discharge” pond, the BAP was originally designed without a spillway and with a top of 

dike elevation of approximately 528 feet. However, only the east dike had been built to 

its permitted elevation at the time of its original construction in 1989. The remaining 

three dikes were only built to elevation 500 feet with the crest of the north and south 

dikes sloping upward to elevation 528 feet at their intersections with the east dike.

Figure 1 – BAP Plan
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The ground surface elevations within the dike footprint prior to construction ranged from 

440 feet along the western dike to 488 feet along the eastern dike. The area within the 

impoundment was excavated to elevations as low as 430 feet. A portion of Corn Creek 

previously meandered through the GSP and the northern and western dikes of the BAP. 

This stream was rerouted during initial construction in the 1980’s. The excavated 

materials and other locally available soils were used to construct the dikes. The dike fill

materials generally consisted of well-mixed clay, sand and gravel. The construction 

included a 3-foot think clay liner on the interior slopes. At the time AMEC was retained 

to provide engineering design services for the dike vertical expansion, the water surface 

elevation in the BAP was approximately 493 feet. A bottom ash recovery and re-use 

contractor operated in the northern portion of the BAP.

Operationally, as many as 14 pipes ranging from 8 to 30 inches in diameter were active 

and were aligned on the dikes. Other inactive pipes were also present. The pipes 

approached the BAP from the south on a pipe rack and through a pipe chase (culvert) 

located on the south dike near its intersection with the west dike. Bottom ash and fly ash 

were sluiced to the BAP and pipe networks were “floated” on the pond surface. The pipe 

discharges were primarily located in the northern half of the BAP to provide sufficient 

time for the solids to settle out before the water was pumped back to plant operations 

via a pump raft moored near the south dike.

The original design crest width for the BAP dikes was approximately 12 feet. The 

interior slopes were approximately 2.5:1 and the exterior slopes 3:1. Because the north, 

south and west dikes were not constructed to their original design height, their crest 

widths varied considerably. Portions of the west dike had crest width as great as 100 

feet, but the dikes were narrower in places. Plant operations were accustomed to 

having considerable horizontal space along the crest of the west and south dikes for 

pipe management. To accommodate the numerous pipes and a haul road, a final crest 

width of 45 feet was required for the vertical expansion. 

Due to the physical constraints of the site and a desire to maintain the existing pond 

capacity during construction, vertically expanding the dikes to provide the desired crest 

width by the conventional methods of upstream or downstream construction were 

deemed not feasible after being evaluated. These constraints led to AMEC’s design of 

an MSE wall for vertical expansion of the dikes to elevation 530 feet. Use of an MSE 

wall provided the advantages of greater height and greater crest width with less crosssectional area, and thus less volume of required fill material. Use of an MSE wall also 

allowed the dikes to be expanded vertically without increasing the original dike footprint, 

thereby minimizing disturbance outside of the existing pond dikes and preserving 

valuable pond capacity. 

Design and Materials

AMEC designed the vertical expansion of the BAP using a nearly-vertical MSE wall on 

the exterior slope to provide sufficient crest width to facilitate plant operations. The wall 
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design was based on the FHWA Guidelines1. Based on optimal plant operations and 

projected CCR production it appeared that the planned vertical expansion would provide 

at least an additional two years of storage capacity. Historically, CCR materials have 

been used throughout the US for roadway and structural fill construction. Selected 

bottom ash materials were already being “mined” from the BAP for beneficial re-use. 

AMEC evaluated the potential for using the BAP bottom ash as fill material for the 

vertical expansion. In the ash mining areas, ash had been retrieved using long-reach 

excavators and “dry-stacked” for further processing. AMEC developed a subsurface 

exploration plan to sample the ash via borings and test pits in the dry-stack areas. 

Based on plant historical records and AMEC’s exploration, it was determined that:

 Portions of the sluiced bottom ash materials were suitable for use in the 

reinforced zone of an MSE wall, based on strength and drainage characteristics.

 Sufficient volumes of materials (at least 300,000 cubic yards) were available. 

 The re-use of bottom ash materials from within the BAP footprint would extend 

the capacity and storage life of the BAP.

Based on these findings, AMEC developed a design for a nearly-vertical MSE wall as 

part of the vertical expansion of the existing BAP dike. The MSE wall design included 

the use of bottom ash in the reinforced zone and a 3-foot thick clay liner on the interior 

slope. Stability analyses were conducted using MSEW and STABL6H software 

programs. Direct shear testing of bottom ash samples from within the BAP 

demonstrated a shear strength characterized by a friction angle of 45 degrees. 

However, during other CCR projects that the authors and other AMEC professionals 

have been involved with, lower strength characteristics have typically been assumed. 

Therefore, based on previous experience, a friction angle of 38 degrees was used for 

bottom ash in the reinforced zone. Table 1 summarizes the soil parameters used in the 

stability analysis. 

Table 1 - Material Parameters

Summary of Embankment Material Parameters Used in Stability Analyses

Material (Type/Source) Unit Weight (pcf) Cohesion 

(psf)

Phi 

(degrees)

Silty Sand 116/120 75 34

Silty Sand 116/120 227 30

Gypsum 115/120 0 35

South Side Sand 126/130 60 36

Dickey Farm Clayey Sand 126/130 354 30

Bottom Ash 

(revised pool elevation) 92/96 350 35
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A typical slope stability model section is shown in Figure 2. Results of the stability 

analyses indicated that the target factors of safety were satisfied. 

Figure 2 – Typical Slope Stability Model Section

Part of the MSE wall design included selection of an appropriate wall facing. AMEC 

evaluated the following three options for the wall facing:

1. Articulated concrete 

2. Concrete masonry units

3. Crushed stone baskets with biaxial grid wrap

Although several options were available, the strength of the MSE wall is derived from 

the use of geogrid materials placed in the compacted fill and not from the facing 

material itself. Ultimately, the stone baskets were deemed to be the most cost-effective 

option. Although there were several cross-sectional variations on the project, the basic 

cross section for the vertical dike expansion is illustrated in Figure 3. 
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Figure 3 – Dike Detail

Geogrid Field Test Pad

Manufacturers of the geogrid materials used in MSE walls provide various material data

and characteristics used in design, one of which is an installation damage reduction 

factor which accounts for abrasion or other damage to the geogrid materials during 

installation. This data was available for geogrid placed in compacted sand or other 

typical fill materials but was not available for use of these materials with ash. In order to 

determine what the installation damage reduction factor would be for various geogrid 

products placed in bottom ash, AMEC conducted a test pad in the field. Three different 

geogrid manufacturers were invited to provide geogrid materials for the test. Two 

companies provided two products each and another company provided just one

product. The field test pad was based on similar testing conducted annually with soil 

materials by TRI in Texas.2The field test pad was conducted on-site at the Trimble 

County Generating Station in August 2008 and was observed by representatives of the

geogrid manufacturers. 

Steel plates were placed beneath each geogrid sample on a flat clean ground surface. 

These plates measured approximately 6’ wide x 7’ long , with a 1-foot gap between 

them and on either side (between the plate and the wall). Chains were attached to 

these plates at either end. An 8-inch loose lift of bottom ash was placed over the plates 

and compacted with a vibratory smooth-drum roller. The lift thickness was confirmed by 

AMEC personnel. A minimum of four passes were made with the machine, and a 

minimum compaction of 95% Standard Proctor was achieved, as confirmed with a 

nuclear-density gauge. The geogrid materials were placed over the plates, as 

confirmed by the chain locations, followed by another 8-inch lift, utilizing the same 

procedure described above. 

Once the proper compaction was achieved on the lift atop the geogrid, the plates were

tilted up to about a 45 degree angle and the geogrid samples were removed by 
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removing the bottom ash near the bottom of the plate with a shovel, and if necessary, 

banging the plate with a sledgehammer to loosen the fill. Following the completion of 

the field testing, samples of the geogrids were sent to AMEC’s San Diego, California 

laboratory for tensile strength testing per ASTM D-4595. Figures 4-7 illustrate the field 

testing procedures.

 

Figure 4 – Placement of steel plates Figure 5 - Placement of ash layer 

between steel plates and geogrid

 

Figure 6 – Placement of geogrid and Figure 7 - Removal of plates and 

top ash layer of compacted ash geogrids

The results of the tensile strength testing of the “test” and “control” coupons were used 

to determine the average installation damage reduction factor (RFID) for each geogrid 

type. When the “test” coupon strengths were higher than the “control” coupon strength, 

the control strength was assumed to be equal to the test strength and an RFID of 1.0 

was used for determining the average RFID. After determining TULT and RFID, the 

following formula was used to determine the long term design strength (LTDS), in 

accordance with the Federal Highway Administration (FHWA) design manual.1
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CR ID D

ult

RF RF RF

T

LTDS

 



LTDS = Long-term tensile strength on a load per unit width reinforcing basis.

TULT = Ultimate (or yield strength) from wide strip (ASTM D4595) for grids.

RFCR = Creep Reduction Factor1

RFD = Durability Reduction Factor1

RFID = Installation Damage Reduction Factor

Design parameters resulting from the geogrid test pad are summarized in Table 2. 

Table 2 – Grid Test Pad Results

Grid 

Manufacturer 

Average 

Ultimate 

Tensile 

Strength

(lb/ft)

Installation 

Damage 

Factor 

(RFID)

Creep 

Reduction 

Factor 

(RFCR)

1

Durability 

Reduction 

Factor 

(RFD)

1

Long 

Term 

Design 

Strength 

(LTDS)

(lb/ft)

Company X 2,993 1.04 2.0 1.2 1199

Company X 4,187 1.10 2.0 1.2 1586

Company Y 3,421 1.10 2.0 1.2 1296

Company Y 4,310 1.06 2.0 1.2 1694

Company Z 4,078 1.36 2.8 1.1 973

1 The values for RFCR and RFD LTDS were obtained from the U.S. Department of Transportation 

 Federal Highway Administration’s (FHWA) document FHWA-NHI-00-043, Section 3.5.

Ultimately, all geogrids tested were approved for use in the MSE wall.

MSE Wall Construction 

Construction of the MSE wall began in July 2009 and concluded in June 2011. AMEC 

field technicians observed and documented the contractors work and performed 

compaction testing of the soil and ash fill using a Troxler nuclear moisture/density 

gauge. Sand and ash fill was compacted with a vibratory smooth-drum roller, with 

compaction criteria based on Modified Proctor tests. Cohesive soils were compacted 

with a non-vibratory sheepsfoot roller, with compaction criteria based on Standard 

Proctor tests. The physical characteristics of the borrow materials were highly variable

so a large number of Proctor tests were performed to provide ample coverage for 
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testing of the engineered fill. Per project specifications, a minimum of 5 in-place 

compaction tests were conducted per acre-lift of dike fill. 

Along the entire length of MSE wall the bottom of the “embedment trench” was probed 

by an AMEC technician before wall construction commenced. These observations and 

other data were collected during construction to document the contractor’s conformance 

with the specifications and confirm installation of the materials with the proper wall 

lip/batter, grid length, strut spacing, etc. Bottom ash mined from the BAP was placed

and compacted in the grid zone of the MSE wall. To minimize disturbance to the stone 

filled baskets used for the wall facing a tamping sled was used to compact ash within 5 

feet of the wall face. A vibratory smooth-drum roller was used for compacting ash 

elsewhere in the grid zone. Figures 8 through12 show various locations and stages of 

construction along the MSE wall. 

 

Figure 8 – MSE wall construction Figure 9 – Measuring geogrid length

 

Figure 10 – West face of NW ramp Figure 11 – Installation of toe drain
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Figure 12 – Completed MSE wall atop the original west dike

Quality Control and Search for Ample Quantity of Usable Fill Materials

To provide quality control on the bottom ash mined from the BAP for use in the MSE

grid zone, periodic gradation analysis of field samples was conducted over the course of

the project in order to confirm that ash containing excessive fines was not placed in the 

grid zone of the MSE wall. Wash 200 samples were taken from the ash stockpiles on a 

regular basis to evaluate fines content. AMEC also developed a calibrated jar-shake 

test for use in the field. Ash samples and water were placed in clear glass jars, shaken 

and allowed to settle for up to 2 minutes. The amount of settled solids in these sample 

jars, as indicated by level marks, was compared to similar samples which had 

previously undergone Wash 200 testing as a means of field confirmation. Any sample 

that remained murky after 5 minutes was deemed unacceptable for use in the grid zone 

of the MSE wall due to fines content. Wash 200 samples of ash confirmed the relative 

accuracy of this approach. 

Once wall construction began it became apparent that there was not enough easily 

recoverable ash and quality clay material readily available to complete the entire vertical 

expansion using only the originally planned borrow areas. AMEC undertook 

investigative efforts at various locations around the facility to locate additional borrow 

sources for material that would be suitable to complete the project. Sand previously 

dredged from the river and stored in an on-site reclamation pit was identified for use to 

construct the Southwest Ramp which did not require ash for MSE walls or clay for a 
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liner. Also, the borrow area north of the GSP was expanded horizontally to obtain more 

useable soils and some bottom ash was also hauled in from the nearby Ghent 

Generating Station. Table 3 summarizes project quantities. 

Table 3 - Project Quantities

Item Quantity Units

Bottom ash recovered for use in dike extension 300,000 CY

Dredged sand used for ramp construction 77,000 CY

Borrow area converted to constructed wetland 200,000 CY

Temporary MSE Wall

A temporary MSE wall was constructed between the west and south dikes, which 

facilitated transitioning of the pipe crossing from the old Southwest Pipe Culvert 

(SWPC) to the new SWPC by allowing the old culvert to stay in use while the new 

culvert was being constructed. This wall was initially intended as a temporary means of 

grade separation, but due to weather delays, it was determined that the south dike 

would not be constructed until the following year. The geotextile wrap on the face of the 

MSE wall was not designed to withstand direct UV exposure for that duration. Tarps 

were deployed as an interim measure; but due to wind concerns, they were not deemed 

to be adequate for the entire period when the wall was to be exposed. Ultimately, the 

decision was made to apply Gunite to the wall face to protect the textile from prolonged 

UV exposure, as shown in Figure 13.

Figure 13 - Temporary MSE wall during Gunite application
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Back-to-Back MSE Walls 

Although the pond north of the BAP was converted to a Gypsum Storage Pond (GSP) 

during this project, its dikes were not raised resulting in an elevation difference from the 

top of one pond dike to the other of 55 feet. For maintenance and security purposes, a 

means for vehicle access from one pond dike elevation to the other was needed. The 

crest width of the GSP was only about 20 feet. Therefore AMEC developed a design for 

ramps that used two pairs of MSE walls constructed back-to-back to form nearvertically-walled ramps connecting the BAP and GSP dikes. The Northwest Ramp grid 

zone consisted of compacted bottom ash. Compacted crushed stone was used to 

construct the Northeast Ramp after the usable bottom ash supply had been exhausted. 

Figure 14 shows the Northwest Ramp after grading.

Figure 14 – Northwest Ramp and GSP Dike

Jersey Barriers

The original design called for a cable restraint guardrail system to minimize the risk of 

vehicles falling over the MSE wall. In June 2010, it was determined that constructability 

would be difficult for this cable restraint system due to the large concrete anchors 

required within the geogrid zone. Also, large stretches of standard W-plate guard rail 

were required to protect the anchors in the curved sections of the dikes—and this 

guardrail was not designed to resist impact (in the locations where impact seems most 
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likely to occur) from the Caterpillar 740 articulated dump trucks planned for use on the 

dikes. After evaluating multiple options, the decision was made to replace the cable 

system with a pre-cast Jersey barrier system, where the barriers were high enough (42 

inches) to be above the axle of the dump trucks. Most of these barriers were designed 

to be mounted on slide pads, and pinned together laterally to allow controlled 

deformation upon impact. Studies have shown this is an efficient way to absorb energy 

from impact by large vehicles.

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 Figure 15 shows both the pinned and free conditions of 

the barriers. On either side of the Northeast Ramp and Northwest Ramps which 

connect the BAP and GSP dikes, the design incorporated gutters into the slide pads.

Figure 15 - Jersey Barriers, pinned and free conditions

Pavement

A condition of the facility’s revised Title V air permit required the roadways on top of the 

BAP dikes be paved in order to minimize particulate emissions. Pavement design had 

to accommodate a variety of heavy loading conditions and variable subgrade 

conditions. Two distinct traffic patterns were identified at the site. The primary design 

traffic pattern is related to three specific operations at the site: 1) tractor-trailers used to 

haul bottom ash mined from the BAP, 2) Caterpillar 740 dump trucks used to haul 

bottom ash and pyrites to the proposed future landfill, and 3) Caterpillar 740 dump 

trucks used to haul bottom ash to the proposed landfill location to construct the leachate 
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collection layer. The secondary traffic pattern consisted of maintenance vehicles and 

did not affect the life cycle of the pavement. Pavement design loads are summarized in 

Table 4.

Table 4 - Pavement Design Loads

Location Truck

Truck Load 

(including vehicle 

weight), lbs

Number of Loads

North, West and 

South Dikes, 

Southwest 

Ramp

Tractor-Trailer 90,000 750 vehicles/yr

Cat 740 72,400 68 vehicles/yr

Cat 740 72,400 1969 total vehicles

East Dike and 

South Ramp

Tractor-Trailer 35,000 750 vehicles/yr

Cat 740 156,175 68 vehicles per 

year

Cat 740 156,175 1969 total vehicles

The pavement sections were analyzed using the Army Corps of Engineers Pavement 

Design and Evaluation software PCASE Version 2.09. The subgrade on the South 

Ramp and East Dike, which were part of the original BAP construction, was assumed to 

have a California Bearing Ratio (CBR) of 5. The subgrade in this area was identified as 

lean clay based on the geotechnical studies completed in 2008. The roadway surface 

in these areas consisted of a mixture of gravel and bottom ash.

The subgrade along the North, South and West dikes, where the vertical expansion had 

been constructed, was assumed to have an existing stone surface consisting of 4 

inches of KDOT #57 crushed stone on 8 inches of KDOT#2 crushed stone, and a

subgrade with a CBR of 13. This value was estimated from dynamic cone penetrometer 

(DCP) testing conducted at the site in May of 2010. The Southwest Ramp was also 

assumed to have a subgrade CBR of 13 with little to no existing stone surface 

thickness. Table 5 provides the calculated design sections that were developed based 

on the traffic and subgrade conditions identified above. 
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Table 5 – Pavement Design Sections

Design Life Location Material Thickness, in.

10 years

North, West and 

South Dikes

Asphalt 3.5

Crushed Stone 

Base 4

Stone Sub-base 12

East Dike,South and 

Southwest Ramps

Asphalt 7.1

Crushed Stone 

Base 6

20 years

North, West and 

South Dikes

Asphalt 3.5

Crushed Stone 

Base 6

Stone Sub-base 12

East Dike,South and 

Southwest Ramps

Asphalt 7.2

Crushed Stone 

Base 6

Based on these designs, the following pavement structure was recommended for use 

along the newly expanded North, West and South Dikes (areas with existing 4 inches of 

#57 stone on 8 inches of #2 crushed stone):

3 inches 0.5-in. asphalt cement PG70-22 (2 lifts of 1.5 inches)

4 inches 0.75-in asphalt cement PG70-22 (2 lifts of 2.0 inches)

4 inches Crushed Stone (DGA) Base (KYDOT Section 302)

As an alternate to the DGA layer described above, 2 layers of chip seal coat (KYTC 

Section 405) placed on the existing # 57 stone was allowed to be substituted for the 4-

inch DGA layer.

A typical pavement section along the vertically expanded BAP dike is shown in Figure 

17.
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Figure 16 – Paving Detail

References

[1] Elias, V., Christopher, B., and Berg, R., Mechanically Stabilized Earth Walls and Reinforced 

Soil Slopes Design and Construction Guidelines, FHWA-NHI-00-043, National Highway 

Institute, Washington, D.C., Section 3.5, 2001

[2] Allen, Sam R., Geosynthetic Installation Damage Testing – A Status Update, 

TRI/Environmental, Inc., Geosynthetic Services, Austin, Texas.

[3] Barnas, A. and Edl, T. Heavy Impacts on Bridge Restraint Systems, SECON International 

Conference on Bridges, Dubrovnik, Croatia, 2006, pp. 525