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Mountain Coal Company, L.L.C. Exhibit 51 <br />West Elk Mine ~ ~ Lower Refuse Pile <br />conditions. Three sections were then selected for analysis. These sections represented the followin_e <br />• conditions. <br />1 • The potential for failure affecting Highway 133. <br />• The potential for failure into Sylvester Gulch. <br />• The potential far failure into the existing sedimentation basin. <br />The selected cross-section locations do not pass through the maximum height of the pile in all cases. <br />Therefore, in all cases where stability analyses were to be performed, cross-sections were drawn to depict <br />the maximum pile height and maximum steepness of slopes in order to model [he most critical stability <br />conditions. Analyses were performed on SectionsA-.4'.D-D',andE-E'. <br />A slope stability analysis was performed for the phase 1 through phase IV pile configuration. A critical <br />cross section with an overall average slope of 2.SH:IV was analyzed for both shallow and deep slope <br />failures. Earthquake slope stability was also analyzed. The critical cross section analyzed bisected the pile <br />in a north-south direction. This was the same cross section as utilized in the 198 design. The subsurface <br />geology, estimated water surface and shear strength data were the same as presented in the 198 permit <br />submittal. All input parameterdata are visually presented in Figure 3.0 in Appendix K. <br />The stability analysis was performed for the four-phase refuse pile utilizing the computer program STABL <br />developed by Purdue University. This program is a two dimensional, limit equilibrium slope stability <br />program. The Spencer s method of slices was utilized in this analysis. Both shallow and deep failure modes <br />were analyzed. The factor of safety for deep and shallow failures were 1.82 and 2.01 respectively. In both <br />cases, the factor of safety exceeded the minimum factor of safety of I .~ as required by the CDMG for refuse <br />~. piles. The results of the analyses are visually presented in Figure 3.0. The data for the slope stability <br />computations can be found in Appendix K. The four-phase refuse pile was also analyzed for earthquake <br />stability. An acceleration factor of 0.10 was used for this analysis, which is conservative for this area. The <br />factor of safety for this case was 1.25 which also meets the CDMG criteria <br />5.5.2 Parameter Selection <br />Selection of the strength parameters for the foundation soils were based upon testing results of thin-wali <br />tube samples obtained from the on-site investigation, grain size analysis, and standard penetration testing <br />from the on-site investigation, past experience of Rocky Mountain Geotechnical with similar materials in <br />the North Fork Valley, and data provided by previous investiea[ors. Conservativeparame[erstyere selected. <br />Available data pertinent to selection of strength parameters are summarized on Table 7. Strength <br />parameters selected for the various materials are as follows. <br /> Effective Stress Effective Stress <br />Material Descriotion Friction Anele Cohesion (pfs) <br />Alluvium 38 degrees 0 <br />Debris Flow 34 degrees 0 <br />Interfin^_ered Debris <br />flow & Alluvial 36 decrees 0 <br />Col{uvium 19 de~ees 200 <br />Selection of strength parameters for the waste material were based upon results of tests performed by MCC <br />in 1984 at the Tucson Research Center (direct shear testing) and the results of investigations by previous <br />19 <br />