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Supplemental Geotechnicai Engineering Report Proposed Slurry Cut-off Walls @ St. Vrain Development Weld County, Colorado Terracon Project No. 25045300 Table No. 2 Terracon Approximate Approximate Top of Approximate Bottom Approximate Boring thickness of No. goring Elevation Bodrock Elevation of Slurry Wall bedrock excavation (feet) (feet) Elevation (feet) (feet) 44 4828 4807 4800 7 45 4828 4811 4804 7 46 4827 4818 4805 13 47 4825 4815 4805 10 46 4825 4816 4806 10 A general profile of the borings and bedrock elevation are shown of Figures 2 and 2A, Bedrock Elevation Maps. For design purposes, the slurry cut-off walls should be at least 30 inches wide and keyed in a minimum depth of about 5 feet into competent bedrock to be delineated by a representative of the geotechnical engineer during construction. Based on the test borings an average excavation depth of 7 feet is recommended to penetrate through the upper two feet of weathered/fractured bedrock and provide an adequate key of 5 feet into competent bedrock. However, as outlined in Tabie 2, some areas will require deeper depths to penetrate through hard, fractured bedrock, which exhibited excessive seepage characteristics detected during the packer permeability testing. Excavation depths into potentially hard to very hard bedrock in this area could approach or exceed about 10 to 15 feet. To keep the slurry trench open, the slurry used should be composed of potable water with about 4 to 8 percent sodium bentonite having a density less than the soil-bentonite backfill mixture (at least 15 pounds per cubic feet (pcf) lighter} to be used. The soil- bentonite backfill mixture should be composed of a minimum of 2 percent bentonite and well graded soils with about 20 to 50 percent fines. The mixture should have a permeability on the order of about 1 x 10~' cm/sec (approximately 4 x 10$ in/sec) or lower. Based on the overburden soils encountered in the test borings at the packer testing locations, sources from outside the slurry wall boring locations may be needed to provide the recommended percent fines type of materials. As an alternative, higher percentages of bentonite could be considered (up fo about 4 to 6 percent). Formal mix designs and permeability testing will need to be performed prior to construction to assess that the minimum permeability value has been attained. This testing is usually provided by the slurry wall contractor, though we are available to assist as needed. Other design and construction recommendations outlined by the Colorado Department of Natural Resources (CDNR) and bentonite specifications as outlined by the American Petroleum Institute (API) should be followed. 9