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Hall-Irwin Corporation ' <br /> Hokestra Slurry Walls <br /> October 30, 2014 <br /> Page 4 of 8 <br /> 2.4.3 Slurry Trench Excavation ' <br /> The slurry wall trenches were excavated using the continuous trench method. The slurry wall trench ' <br /> was excavated through the alluvium and weathered bedrock and "keyed" a minimum of 4 ft. into the <br /> less weathered bedrock. The direction of excavation was counter clockwise starting at sta. 0+00 and <br /> ending at sta. 43+58 for the northern wall and sta. 59+08 for the southern wall. ' <br /> The slurry trench alignment was excavated using track-mounted hydraulic excavator equipped with 36 <br /> in. wide buckets equipped with rippers. A Komatsu PC 800 was used for excavation of the slurry <br /> trench. Excavation was typically performed in 25 and 35 ft. intervals or "sets" along the alignment. ' <br /> Excavated soil and bedrock were spoiled inside the perimeter of the working platform. The Contractor <br /> excavated to the top of the less weathered bedrock and then provided a sample of this rock to Brierley <br /> field personnel for verification. Once Brierley verified that less weathered bedrock had been reached, <br /> the depth to the bottom of the slurry trench was measured by the Hall-Irwin at three locations (start, <br /> middle and end) along the excavated interval and the deepest measurement was used to determine the ' <br /> top of the less weathered bedrock for the entire interval. Excavation continued until the trench <br /> penetrated the less weathered bedrock a minimum of 4 ft. as measured by the Hall-Irwin and verified by <br /> Brierley at the same locations. The depths to the top of less weathered bedrock and the bottom of <br /> trench are presented in Table VI. <br /> Excavated alluvium primarily consisted of sands, gravels, and occasionally cobbles. Occasional clay <br /> and sandy clay lenses were encountered in the excavation. , <br /> Brierley and the Contractor monitored the in-trench slurry level during excavation. Excavation was <br /> suspended when slurry levels dropped below 2 ft. from the top of the working platform. When the slurry ' <br /> levels dropped 2 ft. below the working platform, Hall-Irwin introduced fresh slurry or placed backfill as <br /> appropriate, until the slurry level in the trench was acceptable. <br /> 2.5 Soil-Bentonite Mixing and Placement , <br /> 2.5.1 Backfill Components <br /> The slurry wall trench was backfilled with soil-bentonite (SB) backfill. The backfill consisted of a mixture <br /> of excavated alluvium, imported soil, dry bentonite, and bentonite slurry. The backfill batching was <br /> designed to result in a minimum of 30 percent fines (soil material passing the #200 sieve). Tables III & ' <br /> IV present a summary of backfill testing results <br /> Bentonite used was API certified and delivered on-site from Wyo-Ben and H&H Environmental in ' <br /> approximate 2,750 lb. bags. Bentonite slurry used in the backfill was obtained from the slurry trench <br /> using the excavators and a slurry pump. <br /> 2.5.2 Backfill Mixing and Placement Procedures ' <br /> Soil-Bentonite Backfill was mixed behind the excavation of the slurry wall trench. Bentonite slurry, <br /> select backfill, bentonite and alluvium were added to the mix by a DX65 bulldozer until a slump of 3 to 6 ' <br /> inches was achieved. <br /> BRIERLEY ' <br /> ASSOCIATES <br /> Creating Space Underground <br />