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Mr. Steve Magnuson <br />May 5, 1987 <br />Page 3 <br />MATERIAL PROPERTIES USED IN STABILITY ANALYSES <br />The material properties used in the stability analyses are given below: <br />Unit Weight (pcf) Shear Strength <br />----------------- ------------------------------------------ <br />Material Moist Saturated Cohesion (psf) Friction Angle (degrees) <br />1. Refuse 104 109 0. 40. <br />2. Sedimentary 120 135 4000. 50. <br />Rock <br />The rock properties are typical values for sandstones and shales, and were used <br />in previous stability analyses for Wyoming Fuel Company (WWL, 1984). The <br />refuse material properties were hosed upon the geotechnical test results <br />summarized in this letter. The refuse material shear strength used in the <br />analyses was based upon the residual strength meas~ ~d in the direct shear <br />test. <br />The stability analyses were conducted assuming steady seepage conditions <br />(outlined in U.S. Army Corps of Engineers, 1982; and Wilson and Marsal, 1979). <br />Under these conditions, the shear strengths listed above were assumed to be <br />• effective shear strengths. <br />Pore pressures in the materials were represented by steady seepage conditions <br />governed by Laplace's equation (Cedergren, 1970). The phreat'c surface in the <br />waste pile and foundation was estimated from a procedure developed by <br />Casagrande (1937) for flow through homogeneous embankments (from typical flaw <br />nets illustrated in Cedergren, 1970; and Lambe and Whitman, 1969). <br />STABILITY ANALYSIS RESULTS <br />The stability analysis results are summarized below and shown in Figures 2 <br />through 5. <br />Minimum Calculated Factor of Safety <br />Description of Case Static Pseudostatic <br />Low phreatic surface (at toe of pile) <br />High phreatic surface (at top of bedrock) <br />2.53 2.18 <br />2.41 2.12 <br />The minimum calculated factors of safety are well above Colorado MLRD criteria <br />of 1.5 and 1.1 for static and seismic conditions, respectively. As mentioned <br />• above, analyses using non-circular failure surfaces yielded minimum factors of <br />safety higher than those listed above. <br />