2011-08-01_REVISION - M1978210 (15)

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4.2.1 Backfill Mix The first backfill mix will be used in the relatively shallow reaches of the trench and will be referred to as Mix Design A (MDA). MDA consists of a blend of overburden, clay borrow and the coarse alluvium expected to be encountered upon excavation of the slurry trench. The laboratory backfill mix design consisted of a blend of approximately 20 percent overburden, 20 percent clay borrow and 60 percent sand and gravel to achieve an overall backfill mix containing approximately 30 percent fines (material passing the #200 sieve). The second backfill mix will be used in the deeper portions of the trench and will be referred to as Mix Design B (MDB). MDB consists of a blend of clay borrow and the coarse alluvium expected to be encountered upon excavation of the slurry trench. The laboratory backfill mix design consisted of a blend of approximately 40 percent overburden 60 percent sand and gravel to achieve an overall backfill mix containing approximately 30 percent fines (material passing the #200 sieve). Bedrock was not included in the laboratory mix designs because it is not expected to break down during field mixing. In addition, to develop a conservative laboratory backfill mix design, dry bentonite powder was not added to the mix. The thickness of the overburden varies slightly along the alignment and is expected to contribute negligible amount of fines to the field mix. Due to the variability of the soil properties, fines content, and mixing, the field mix design will include 0.5 percent dry bentonite powder by volume which will reduce the permeability of the field backfill mix. 4.2.2 Bentonite Slurry Bentonite slurry used in the backfill mixture was created with dry bentonite powder and with native groundwater collected from the site. Dry bentonite powder was provided H &H Environmental in Grand Junction, CO, which is the proposed bentonite to be used for the construction. A measured amount of dry powder bentonite was used to create slurry with a Marsh Funnel viscosity of 40 seconds. A summary of laboratory test results performed on the slurry is presented in Table IV. 4.3 Backfill Mix Preparation Lyman Henn prepared a calculated blend of the select backfill components to conservatively represent the conditions anticipated along the proposed alignment. Table IV shows the proportions of components in the mixes. The slurry was thoroughly blended using a hand -held electric mixer. Overburden, clay borrow and coarse alluvium components were mixed with bentonite slurry until a 4.5 inch slump was achieved for MDA. Clay borrow and coarse alluvium components were mixed with bentonite slurry until a 3 inch slump was achieved for MDB. No additional dry bentonite was added to the mixtures. A sample from each mix was placed into a cylinder container (4 inch by 8 inch) for testing. The permeability test (ASTM D 5084), performed by Advance Terra Testing (Lakewood, CO) consisted of subjecting a sample G:\PROJECTS \111025 Geiseit Gravel Pit Slurry Wall\O \Ol \Geisert Slurry Wall Design Report.doc 6