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• 4.3 SLOPE STABILITY <br />A slope stability analysts was performed for the coal mine waste bank to ensure static <br />stability with along-term factor of safety of 1.5. Stability analyses were performed using <br />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 performed 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, 19941, and was <br />determined to be 0.1 g. Both the static and pseudo-static analyses were conducted modelling <br />the coal waste bank at maximum height. Output from the computer analyses Is presented in <br />Appendix D. <br />4.3.1 Groundwater Conditions <br />Groundwater conditions at the site were modeled according to the piezometer located in <br />WBH-84 at the toe of the planned waste bank. The groundwater was read to be 27 feet bgs. <br />From this information, the phreatic surface as modeled was assumed to run along the coal- <br />• colluvium interface to the ground surface at the toe as a conservative measure, as shown in <br />Figures 4.3 and 4.4. A phreatic surface within the coal waste is not expected to develop, <br />considering the placement of dry coal waste, the planned underdrain system, and the surface <br />runoff diversion systems designed to route rainwater directly off the coal waste bank before <br />it can infiltrate. <br />4.3.2 Material Properties <br />Material properties were determined using various methods. Borehole investigations <br />conducted by WESTEC, and laboratory analysis on potential coal waste material that would <br />be placed in the waste bank, obtained from Bowie Resources, were used to determine <br />properties. Figures 4.3 and 4.4 show a summary of material properties used for the stability <br />analyses. Material properties required for analysis are Coulomb strength parameters: internal <br />angle-of-friction, cohesion, and unit weight. <br />Coal waste samples obtained by WESTEC from Bowie Resources were tested in a laboratory <br />contracted by WESTEC. A Standard Proctor compaction test (ASTM D-698) was performed <br />• on a sample of the coal waste. The results were used to remold another sample to 90 <br />85268l1424.RPT Coal Mine Weste Benk Finel Design for the Bowie No. 2 Mine <br />June 1996 WESTEC 20 <br />ADOpUACY-Ol <br />