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"C" Seam Slope Stability Page 2 October 19, 1998 <br />i.e. changing the profile of a failed clay slope. The failure of <br />the initial 30.5° clay-fill slope shown on Figure 3 demonstrated an <br />initial factor of safety of less than one. Golder used the slices <br />method of slope stability analysis to calculate the 1.06 factor of <br />safe*_y for the "Revised Profile". The initial slope angle demon- <br />strates that the residual angle of friction of the clay fill was <br />less than 30.5°. Laboratory testing of the clay materials present <br />along the sliding surface provided cohesion values along the <br />failure surface, 3.5 psi and 5.8 psi. These values for clay are <br />probably lower than the cohesion values for the mixed clay, silt <br />and sandstone rubble along the sliding surface above the "D" Seam <br />portal. Table 3 presents the rock type distributions and bed <br />thicknesses overlying the "D" Seam described for corehole EC-5, <br />from the surface to the "D" Seam approximately 622 feet below the <br />collar (Miller, June 15, 1998). <br />PHYSICAL PROPERTIES USED <br />It was necessary to use conservative estimates for the critic- <br />al shear strength properties of the slide mass for this preliminary <br />slope analysis. The required shear strength design properties are <br />residual (ultimate) angle of surface friction and cohesion. The_ <br />25° angle of residual friction used for calculating the stability <br />of the slide mass material was conservatively based on the mixed <br />rock types present in the overburden above the "D" Seam, determined <br />from the core log for corehole EC-5 (Miller, 1998) and the lower <br />• typical property ranges for these rock types, as indicated by Hough <br />(1957) and Fumagalli (1968). Hough presents 26° as the minimum <br />angle of residual friction for mixed silty, clayey, sandy soil and <br />Fumagalli recommends 25° for a "marly-clayey-sandstone mass". <br />Hough also presents a method of conservatively calculating the <br />cohesion of a mixed soil based on the height of a stable cut slope, <br />i.e. the peak shear stress for the maximum stable slope present in <br />the slide area. The long term stability of a 15-foot high slope <br />exposed at the south side of the slide mass calculates an overall <br />cohesion of approximately 7.5 psi. Fumaqalli suggests a cohesion <br />value of 14.2 psi for a mixed "marly-clayey-sandstone mass". <br />Therefore, a conservative 25° residual angle of friction and a <br />conservative cohesion value of 5 psi were used to evaluate the <br />stability of the slide mass above the planned "D" Seam portal. <br />Advanced Terra Testing, Inc. is in the process of measuring the <br />residual angle of friction and cohesion of a minus half-inch <br />fraction of a slide mass sample using the standard triaxial- <br />compression consolidated undrained test method (ASTM D-4767). <br />The density of the slide mass was conservatively assumed to be <br />150 PC F. This density was based on the mean measured density for <br />three immediate-roof samples above the "D" Seam. The measured in <br />place immediate-roof rock density was 158 PC F, as shown in Table <br />• <br />