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3. SOIL - BENTONITE BACKFILLED SLURRY WALL CONSTRUCTION <br />3.1 Bentonite Slurry Mixing and Delivering <br />The Contractor constructed a slurry pond using on -site material for the purpose of mixing and hydrating bentonite <br />slurry. The slurry pond was located inside the slurry wall on the land that divided the east and west cell where <br />native material has not been mined. The bentonite was mixed with water in the constructed slurry pond using a <br />shear mixer. The water for mixing was obtained from the east cell of the mine. The slurry was delivered from <br />the slurry mixing pit to the open trench using a pump and a 4 -inch HDPE surface pipeline. <br />3.2 Bentonite Slurry Quality Control Results <br />3.2.1 As -Mixed Slurry <br />To ensure proper mixing and hydration of the powdered bentonite, the as -mixed bentonite slurry was <br />tested approximately once per day. Lyman Henn performed density and viscosity measurements <br />according to API RP 13B -2. Results are presented in Table I. <br />3.2.2 In- Trench Slurry <br />Slurry within the trench was tested on average twice daily, with samples taken approximately 5 ft from <br />the trench bottom once backfilling had commenced. Lyman Henn performed density, viscosity and sand <br />content measurements according to API RP 13B -2. Results are presented in Table II. <br />3.3 Slurry Trench Excavation <br />The slurry wall trench was excavated using the continuous trench method. The slurry wall trench (not including <br />overlaps due to tie -ins) was 7,015 ft. long, a minimum of 42 in. wide, with total depths ranging from 38.5 to 75 ft. <br />below ground surface (bgs) and an average depth of 61.2 ft. bgs. The slurry wall trench was excavated through <br />the alluvium and weathered bedrock and "keyed" a minimum of 4 ft into the unweathered bedrock, except as <br />noted in Section 3.5. The total area excavated for the slurry wall installation was 439,593 square ft. The <br />direction of excavation was counter - clockwise starting at Sta. 0 +00. <br />The slurry trench alignment was excavated using track - mounted hydraulic excavator equipped with 42 in. wide <br />buckets equipped with rippers. A Komatsu PC 1250 was used for excavation of the slurry trench. Excavation was <br />typically performed in 20, 25 and 30 ft. intervals or "sets" along the alignment. <br />Excavated soil and bedrock were spoiled inside the perimeter of the working platform and inside the Geisert <br />gravel pit ponds. The Contractor excavated to the top of the unweathered bedrock and then provided a sample of <br />' }ai;; rock to Lyman Henn field personnel for verification. Once Lyman Henn verified that unweathered bedrock <br />had been reached, the depth to the bottom of the slurry trench was measured by the Hall -Irwin at three locations <br />(start, middle and end) along the excavated interval and the deepest measurement was used to determine the top <br />of the unweathered bedrock for the entire interval. Excavation continued until the trench penetrated the <br />unweathered bedrock a minimum of 4 ft as measured by the Hall -Irwin and verified by Lyman Henn at the same <br />locations. The depths to the top of unweathered bedrock and the bottom of trench are presented in Table III. <br />G \PROJECTS\201I Projects \111025 Gersen Gravel Pa Slurry Wall \O \Ol \Construction report\LH Construcuun Report- BS.doc <br />3 <br />