R.S. Wallace Power Station

Completed in 1925, the former R.S. Wallace Power Station was a coal-fired facility situated on the Illinois River across from downtown Peoria. Fly ash generated from coal combustion was originally sold as a concrete additive, but when this practice ceased in the 1980s, the ash was deposited in an on-site impoundment close to the river bank. The dike surrounding the pond had been constructed from back ll containing zones of construction debris salvaged from demolition in the local area. When the facility owner, Central Illinois Light Company (CILCO), determined that there was groundwater communication between the ash pond and the river, Moretrench was retained to fill voids in the debris zones and construct a soil-bentonite slurry trench to permanently cut off the flow path.

Scope of Work

The pond was trapezoidal in shape with a perimeter of approximately 3,600 ft and situated just 30 ft from the Illinois River at its closest point. The purpose of the work was to form a 1,800-ft long, impermeable “core” through the dike to prevent seepage flow. Grouting of voids in the debris zones was required in order to prevent excessive loss of the trench stabilizing fluids during the subsequent slurry trench construction.

Grouting program

Grouting was focused on an approximately 800-ft length of the dam alignment identified by the owner as containing construction debris. Moretrench installed an initial 33 tube à manchette (TAM) grout pipes on 25-ft centers to a depth of approximately 1-2 ft below the bottom of the target zone. At the same time, piezometers were installed to monitor groundwater levels during slurry trench installation.

Bentonite cement grout was pumped under controlled flow rates and pressures until relative refusal was reached. Where low pressures and high grout takes indicated signi cant voids, additional grout pipes were installed between primary locations. Split-spoon samples were taken during installation and grouting of the secondary TAM pipes and checked for grout content using pheno- thylene as an indicator. A total of approximately 145,000 gallons of grout was injected through 56 grout pipes to complete the grouting program.

Slurry Trench Construction

Typically, excavated trench material is utilized in the backfill. Mixing is accomplished alongside the trench and the backfill dozed into place as excavation progresses. However, for this project trench excavation had to be accomplished through the dike itself, which had a maximum height and width of 12 ft and 16 ft respectively. Space restrictions and the construction debris content therefore precluded normal procedures. Instead, backfill was mixed at a central location and pumped to the work area through a 5-inch diameter aluminum pipe over a distance of as much as 800 ft.

Several backfill mixes were prepared by Moretrench and laboratory tested by the project geotechnical consultant to determine design parameters that would meet the specified 1 X 10-6 cm/sec permeability requirement. The final mix consisted of 40% fly ash, 40% sand, and 20% clay, with bentonite slurry added to achieve the desired 4-5-inch slump.

The 30-inch trench was excavated to design depth at 30 ft below the top of the dike through bentonite slurry to maintain sidewall stability, and keyed into underlying impermeable soil. Backfill was piped directly into the trench, at the top of the angle of repose of previously placed fill. Excavated trench material and displaced slurry were transported by truck to a designated dump area within the facility. In places where trench excavation was too far from the mixing station for practical pumping, a front end loader was utilized to transport the back ll material to the trench.

Verification

During the work, a comprehensive quality control program was conducted by Moretrench to verify that the backfill material would ensure a completed wall that met the design permeability requirement. Measures taken included twice-daily density testing of trench grab samples; testing to ensure that sand content in the trench was maintained at less than 8%; slump testing at the trench back ll delivery point; and filter testing to verify permeability.

Following completion of the trench, Moretrench installed a cap consisting of filter fabric covered by compacted loamy material. Subsequently, the site was reclaimed for commercial development.

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