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To: Chris Nyikos, Project Manager - Mountain Coal Company <br />Subject: Stability Analysis of LRP With Preparation Plant <br />Date: 8 September, 2009 <br />Project: 06/26-1004 <br />Comments: <br />1. Strength envelope represents average values from Table 1. Saturated unit weights are computed from average dry unit <br />weight values from Table 1 assuming G,=2.70. Moist unit weights assume w,=10%. <br />2. Strength envelope represents values used in Harding Lawson (1996). Saturated unit weights are computed from average <br />dry unit weight values from Table 1 assuming Gs=2.00. Moist unit weights assume w,=10%. <br />3. This unit assumed to be impenetrable in slope stability modeling <br />It should be noted that a direct shear test was performed on a coal refuse sample obtained from soil boring <br />SB-09-12. Because the strength envelope was higher than the historical values used in Harding Lawson <br />(1996), it was judged that the values used in the previous analysis represent a conservative approach and <br />they were used in the present analysis. <br />For conceptual purposes, the Preparation Plant facility load was assumed to impart a uniformly distributed <br />load of 4,000 psf across the proposed footprint. The slope stability analysis was completed using <br />SLOPE/W, included as part of the GeoStudio 2004 suite, developed by Geo-Slope International. The <br />Spencer method to evaluate force and moment equilibrium was used in the analysis. <br />A static stability analysis was performed to determine the factor of safety for long-term, drained conditions. <br />The CDRMS regulations state that a factor of safety of 1.50 is required for mining-related slopes such as <br />impoundments. This is also a typical factor of safety required for long-term stability in most geotechnical <br />applications. <br />In the absence of governing seismic design criteria, the seismic design requirements for refuse piles or <br />structures built atop refuse piles, the seismic design requirements for impoundments per CDRMS <br />regulations were followed. In accordance with these requirements, a minimum seismic factor of safety of <br />1.20 was used in this analysis. For the pseudo-static stability analysis, a peak ground acceleration (PGA) of <br />0.0698g was used to simulate seismic loading. This value was determined based on the USGS hazard maps <br />available online at the location of the LRP and for an earthquake return period of 475 years. A second, <br />more stringent PGA was also used in the pseudo-static stability analysis to account for a larger earthquake <br />as specified for buildings in ASCE 7-05. This PGA was 0.166g. <br />Results <br />The results of the analysis indicate static factors of safety of 1.85 and 2.00 for cross sections A and B, <br />respectively. The model outputs are shown in Figures 1 and 2, respectively. In both cases, the resultant <br />-4-