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SEC'iIONSIX Soil-Bentonfte Cutoff Wail <br />6.1 CUTOFF WALL DESIGN AND PROFILE <br />A soil-bentonite cutoff wall was evaluated for construction around the entire perimeter of <br />Sharkey's Lake Gravel Pit from the existing ground surface to a minimum of 4 feet below the <br />top of bedrock. The average depth to the top of the bedrock is approximately 34 ft. 'The design <br />profile and design cross-sections for the soil-bentonite cutoff wall are presented in Drawings 2, 3 <br />1 and 4. <br />6.2 MIX DESIGN <br />The purpose of the laboratory bench scale test program was to design asoil-bentonite backfill <br />mix appropriate for use in the construction of a vertical groundwater seepage bamer around the <br />Shazkey's Lake Gravel Pit. The proposed mix consists of onsite soil, site mix water and a clay <br />mineral additive (bentonite). This mix design provides the basis for onsite backfill mixing to <br />achieve the required hydraulic conductivity after construction. <br />6.2.1 Laboratory Testing <br />Samples Tested <br />Materials to be tested were delivered to ATT Laboratory in Lakewood, CO during September <br />and October, 2005. These samples consisted of buckets of onsite material, one bag of Wyo-Ben <br />Hydroge190, meeting API 13A Section 4 specifications for a treated bentonite, one 5-gallon <br />bucket of site groundwater, and one 5-gallon bucket of site surface water for mixing. <br />Composite No. 1 is a mixture of material from TP-1 through TP-6. Composite No. 2 is a mixture <br />of sand and gravel obtained from the SPTs performed in the test holes. Composite No. 3 is <br />comprised of 70% cf Composite No. 1 and 30% of Composite No. 2. Composite No. 4 is <br />comprised of 50% of Composite No. 1 and 50% of Composite No. 2. Composites No.l, No. 3, <br />and No. 4 were each mixed with bentonite slurry for backfill mix testing as described below. <br />Test Procedures <br />Below is an explanation of the test procedures performed on the bentonite water slurry and <br />proposed backfill mixes for the cutoff wall. <br />^ Slurry Density <br />Slurry densities were determined using the mud balance method, as described in Section <br />1 of API RP 13B-1. <br />^ Slurry Viscosity <br />Slurry viscosities were determined using the Marsh Funnel method, as described in <br />Section 2 of API RP 13B-1. <br />^ Relative Filtrate Loss of Slurry <br />Filter Press Testing was conducted on the hydrated slurry in accordance with API 1994 <br />standards. After the creation of a filter cake using the standazd filtration method, the <br />1 N\PROJECTS\22238240_PLATTE_SAND_GRAVEL\SUB OOU 2.0_WORD_PROC\PLATTE SANDB GRAVEL GEOTECHNICAL DESIGN REPORT4ODOCV S-NOV-0S\\ V-1 <br />