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~ III IIIIIIIIIIIIIIII <br />~ ^ 999 <br />CTLfTHOMPSON, INC. II yu,,,fr II <br />CONSULTING GEOTECHNICAL AND MATERIALS ENGINEERS <br />February 14, 1992 <br />Colowyo Coal Company. <br />5731 State Highway 13 <br />Meeker, CO 81641 <br />Attention: Mr. Jim Kiger <br />Subject: Supplemental Slope Stability Analysis <br />West Pit Excess Spoil Pile <br />Colowyo Mine <br />Moffat County, Colorado <br />Job No. 18,349 <br />Gentlemen: <br />We have performed additional slope stability analysis to confirm stability of the <br />West Pit Spoil Pile. These analyses supplement those in our previous report (Job <br />No. 16,516, dated March 1, 1991). This letter contains laboratory test results, the results <br />of our analyses, and conclusions regarding the stability of the West Pit Pile, <br />We were provided a sample of clay Colowyo employees obtained from the general <br />area of TH-1 from the previous investigation, This sample represents the clay fill which <br />is to be compacted to at least 90% of maximum modified Proctor dry density below the <br />toe of the spoil pile. A Proctor compaction test (ASTM 1557-78A) indicated a maximum <br />dry density of 119 pcf and an optimum moisture content of 12.5% for the clay (Fig. 1). <br />We performed a consolidated, undrained triaxial test with pore pressure measurements <br />on a sample of clay which was remolded to 90% of maximum dry density. Figure 2 <br />shows the triaxial test results. The friction angle (~) and the cohesion intercept (c) were <br />calculated from this data as 28° and 650 psf; respectively. <br />A stability analysis was executed using the Geoslope "Stabl" computer program <br />from Geocomp, Inc. This analysis was run with the soil properties determined for the <br />various soils in this investigation and the previous investigation. The soil profile <br />considered in this analysis was representative of the design. The analysis assumed <br />natural clays at the toe of the pile would be removed and replaced with compacted clay <br />fill. The spoil was then placed above the clay fill layer. Groundwater was assumed at the <br />top of the clay till. This profile exhibited a computed factor of safety of 2.0. Figure 3 <br />shows the profile and the ten most critical failure surfaces with the most critical denoted <br />by the number 1. <br />1971 WEST 12TH AVENUE DENVER. COLORAD08020a (303)8250777 <br />