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III. KEY DESIGN ELEMENTS 3.01 Slurry Wall Alignment and Depth ' The proposed slurry wall alignment is shown on Drawings 102 and 103. The alignment length is approximately 8,450 linear ft and encloses 74.4 acres. The alignment follows along the existing dirt roads along the perimeter of the Site. Based on test boring information the total depth of the slurry wall along the alignment will range from 40 to 65 fr below the existing ground surface. ' 3.02 Slurry Wall Width The slurry wall will be a minimum of 3 ft wide. This width combined with the properties of the slurry wall backfill is designed to withstand blowout and/or hydrofracturing through the slurry wall for a maximum head differential across the wall of up [0 50 ft. 3.03 Slurry Wall Key The unweathered bedrock material exhibits an adequate permeability to be used as a "key" for the slurry wall. Investigation data indicate that the permeability and lithological properties of the unweathered bedrock are similar throughout the alignment, with a representative bedrock permeability being 5 x 10'6 cm/sec. A minimum key depth of 3 ft into unweathered bedrock has been selected to minimize seepage at the base of the slurry wall. 3.04 Slurry Wall Excavation The slurry wall will be constructed using the slurry trench method. The slurry trench method involves excavating a trench and stabilizing the excavation with viscous bentonite slurry. Trench stability is achieved by establishing a slurry elevation several feet above the groundwater elevation such that a positive hydrostatic slurry pressure exists in the trench and acts against the trench. walls to provide stability. It is anticipated that the soil and rock along the slurry wall alignment can be excavated primarily with large backhoes or hydraulic clamshells and supplemented with crane-mounted chisels, pre-augering and/or other aggressive excavation techniques when necessary. 3.05 Slurry Wall Backfill Type and Placement The slurry wall will be backfilled with soil-cement-bentonite (SCB) backfill. The backfill is required to have a permeability of less than or equal to 5 x 10' cm/sec and 28-day compressive strength of 30 to 100 pounds per square inch. SCB backfill has been selected for this project because it cures and hardens after placement due to the cement content, resulting in a slurry wall which is more resistant to blowout and hydrofracturing than asoil-bentonite slurry wall per unit thickness. Soil-bentonite (SB) was not selected for this project because the design width to prevent blowout or hydrofracturing is a minimum of 5.0 ft. This width was determined to be not feasible due costs associated with excavation.