2007-10-31_REVISION - M2006080 (2)

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IV. BACKFILL MIX DESIGN 4.01 General Lyman Henn conducted a laboratory testing progam to characterize the soils and bedrock and to assist in developing a mix design for the slurry wall backfill material. The backfill must meet the State Engineer's maximum seepage allowance of 0.03 CF/SF of wall per day to ensure effective performance of the slurry wall. The proposed backfill is a soil-bentonite (SB) mixture. During the geotechnical investigation in Apri12007, Lyman Henn collected bulk samples of the various components of the backfill mix from test pit excavations. Samples of overburden, sand and gravel, and bedrock were obtained from drill cuttings and from the test borings. Samples of overburden and coarse alluvium material were sampled from stockpiles next to the test pit by hand digging. A goundwater sample was also obtained from the area of the test pit. The laboratory testing progam consisted of performing laboratory analyses of the samples of the backfill components, creating a mix design based on the results of these analyses, and preparing and conducting a permeability test on the mix design. Ardaman and Associates (formerly Soil Testing Engineers) was retained by Compass Environmental to perform the laboratory analyses. The laboratory testing conducted included preparation and testing of the backfill mix, and deternunation of the engineering properties of overburden and sand and gavel. 4.02 Backfill Components The following materials were the components of the prepared SB mix design. ^ select baci~ll (combination of overburden and sand and gavel); and • bentonite slurry made from dry bentonite powder. The source and pertinent chazacteristics of these components are described in the following subsections. Index tests including -200 content, gadation, and Atterberg Limits were performed on the select backfill components. The laboratory test results are summarized on Table IV and are included in Appendix B. A. Select Backfill The select backfill consists of a blend of overburden and coazse alluvium expected to be encountered upon excavation of the slurry trench. Because of the variability of the overburden components, the most conservative sample (least amount of fines and lowest plasticity index) was selected for testing. The laboratory backfill mix design consisted of a blend of approximately 61 % sand and gavel and 39% overburden to achieve an overall backfill mix containing approximately 30% fines (material passing the #200 sieve). Due to the difficulty of obtaining bulls samples and deternuning the amount of break down, bedrock was not included in the G^PROIECTS~10'/021-0°0 Loveland Ready Mic Sluny WaII~R. Raords and Conepondrnce~R.l l Reports~Fiva151uny WaR Dcaign Repon.doc