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Excavated alluvium primarily consisted of sands, gravels, cobbles and occasionally boulders. Occasional clay <br />and sandy clay lenses were encountered in the excavation. <br />Lyman Henn and the Contractor monitored the in- trench slurry level during excavation. The excavation <br />suspended when slurry levels dropped below 2 ft from the top of the working platform. When the slurry levels <br />dropped 2 ft below the working platform, Hall -Irwin introduced fresh slurry or placed backfill as appropriate, <br />until the slurry level in the trench was acceptable. <br />3.4 Soil - Bentonite Mixing and Placement <br />3.4.1 Backfill Components <br />The slurry wall trench was backfilled with soil - bentonite (SB) backfill. The backfill consisted of a <br />mixture of excavated alluvium, imported soil, dry bentonite, and bentonite slurry. The backfill batching <br />was designed to result in a minimum of 30 percent fines (soil material passing the #200 sieve). Tables IV <br />and V present the backfill field and laboratory testing reports for the backfill testing. <br />Bentonite used was API certified and delivered on -site from Wyo -Ben and H &H Environmental in 2,750 <br />lb. bags. Bentonite slurry used in the backfill was obtained from the slurry trench using the excavators <br />and a slurry pump. <br />3.4.2 Backfill Mixing and Placement Procedures <br />Soil - Bentonite Backfill was batched in box behind the excavation of the slurry wall trench. Bentonite <br />slurry, select backfill, bentonite and alluvium were added to the mix by a trackhoe until a slump of 3 to 6 <br />inches was achieved. <br />The Contractor placed the SB backfill mixture into the trench using a lead -in slope or on top of <br />previously placed backfill. The backfill formed a slope of approximately 2.5:1 to 5:1 (horizontal to <br />vertical) in the trench, advancing through the trench by a sliding and slumping displacement action. The <br />toe of the backfill slope was typically maintained within 0 to 30 ft. of the excavation end of the open <br />trench. Batches were prepared and placed in approximately 20 -30 minutes. <br />As a daily routine, prior to placing backfill, the bottom of the trench was measured (sounded) by Hall - <br />Irwin to ensure the trench bottom was free of caved material and sand accumulation. The soundings also <br />determined the toe location and the profile of the backfill slope. The Komatsu PC 1250 was used to clean <br />out the trench bottom and determine the location of the backfill toe. <br />3.4.3 Soil - Bentonite Backfill Sampling Procedure <br />Lyman Henn collected 315 samples from the backfill prior to placing it into the trench. A sample was <br />collected twice per day to determine slump and density, and once per day for moisture content and grain <br />size distribution. Slump tests were performed according to ASTM C143-90a. Density tests were <br />performed using a 0.25 cubic foot bucket according to ASTM C 138 -92. Moisture and grain size <br />distribution were performed using ASTM D 2216 and ASTM C 136, respectively. Table V presents the <br />results for each sample. <br />G \PROJECIS\201I Proleus \111025 Ger ert Gravel Pit Slurry Wall \O \O1 \Construction report\LH Construction Report-BS doe <br />4 <br />