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4.3 SLOPE STABILITY <br />• A slope stability analysis was performed for the coal mine waste bank to ensure static stability <br />with along-term factor of safety of 1.5 (Rule 4.10.41211. Stability analyses were performed <br />using computer software called PCSTABL Version 6H, produced by Purdue University (Purdue <br />University, 1988). Moment equilibrium was determined using the Modified Bishop Method. <br />A seismic stability analysis also was pertormed to ensure stability during seismic loading <br />conditions. Apseudo-static factor was added in the horizontal direction to the slide mass to <br />model an earthquake occurring in the area. The pseudo-static factor is based on seismic data <br />obtained through the National Oceanic and Atmospheric Association (NOAA, 1994), and was <br />determined to be 0.1 g. Both the static and pseudo-static analyses were conducted modeling <br />the coal mine waste bank at maximum height. Output from the computer analyses is <br />presented in Appendix D. <br />4.3.1 Groundwater Conditions <br />Groundwater conditions at the site were modeled according to the piezometer located in WBH- <br />S01 in the West Storage Yard, east of the coal mine waste bank. The groundwater was read <br />to be 50 feet bgs and is suspected to be remnants of drilling fluid. The phreatic surface as <br />• modeled was assumed to run along the coal mine waste bank-colluvium interface to the <br />ground surface at the toe as a conservative measure, as shown in Figures 4.3 and 4.4 instead <br />of at the observed piezometer level. A phreatic surface within the coal mine waste bank is not <br />expected to develop, considering the placement of dry coal mine waste, the planned <br />underdrain system, and the surface runoff diversion systems designed to route rainwater <br />directly off the coal mine waste before it can infiltrate. <br />4.3.2 Material Properties <br />Material properties were determined using various methods. Borehole investigations conducted <br />by WESTEC, and laboratory analysis on potential coal mine waste material that would be <br />placed in the waste bank, obtained from OXBOW, were used to determine properties. Figures <br />4.3 and 4.4 show a summary of material properties used for the stability analyses. Material <br />properties required for analysis are Coulomb strength parameters: internal angle-of-friction, <br />cohesion, and unit weight. <br />C~ <br />96266/1424.RPT Coel Mine Weate Bank Design for the Sanborn Creek Mine <br />October 1998 WESTEC, Ina. 19 <br />