2011-08-01_REVISION - M1978210 (15)

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4. BACKFILL MIX DESIGN 4.1 General Lyman Henn conducted a laboratory testing program to develop a mix design for the slurry wall backfill material. The backfill must meet several engineering criteria to provide adequate performance of the slurry wall. The proposed backfill is a soil - bentonite (SB) mixture. During the geotechnical investigation in May and June 2011, Lyman Henn collected bulk samples of the various components of the backfill mix. Samples of overburden, coarse alluvium, and bedrock were obtained from drill cuttings and from the test borings. Samples of clay borrow were collected from the Orr property, a borrow site designated by Hall -Irwin to be used as supplemental fines. The site derived fill (overburden) placed on the west side of the site was also sampled and is suitable to use as supplemental fines. A thin layer of overburden also exists around the alignment and its contribution as a supplemental fines source is negligible. Groundwater samples were obtained from the east and west pit as well as the on -site well. The eastern pit at the Geisert gravel mine was surrounded by silty sand that contained varying amount of fines content and varying low plasticity indices. Due to the low to non - plastic nature of the silty sand, this material is not suitable for use as supplemental fines. The laboratory testing program consisted of performing laboratory analyses of the samples of the backfill components, creating mix designs based on the results of these analyses, and preparing and conducting permeability tests on the mix design. Lyman Henn conducted the backfill mix preparation and testing, and determined engineering properties of overburden, sand and gravel, clay borrow and bedrock. Site groundwater was used in the mix design. Lyman Henn also determined the engineering properties of the bentonite slurry. Lyman Henn has designed two SB mixtures allowing Hall Irwin to utilize the west -side stockpiled overburden soils and minimize the clay borrow material. 4.2 Backfill Components The following materials were the components of the prepared SB mix designs. • select backfill (combination of clay borrow, (Orr property supplemental fines)), overburden, and coarse alluvium); and • bentonite slurry made from dry bentonite powder. The source and pertinent characteristics of these components are described in the following subsections. Index tests including fines content (passing the #200 sieve), gradation, and Atterberg Limits on the select backfill components, the results were performed and are summarized on Table V. G:\PROJECTS \111025 Geisert Gravel Pit Slurry Wa1l\0 \Ol \Geisert Slurry Wall Design Report.doc 5