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Colowyo Coal Company, L.P. <br />Attn.: Mr. Juan Garcia <br />January 4, 2013 <br />Page 2 of 7 <br />STABILITY ANALYSIS <br />Spoil Fill Configuration <br />SHANNON &WILSON, INC. <br />A plan and section view of the planned spoil fill is shown on Figure 1. Section A -A' through the <br />southern slope of the fill and located above the endwall of the South Taylor Pit is considered the <br />critical case and was selected for stability analysis. <br />The typical slope profile of the temporary spoil fill will consist of 65 -foot wide benches, with a <br />50 -foot vertical height between benches, and a slope of 1.3:1 (horizontal to vertical) between <br />benches (at spoil angle of repose). This configuration is shown in more detail on the stability <br />analyses output on Figures 2 through 9. The average overall slope of the out - slopes are about <br />2.3H:1 V. The lower portion of the fill is permanent excess spoil; after mining is completed, the <br />upper portion of the fill will be removed and the slopes will be re- graded to the planned post - <br />mining topography (PMT). The planned underdrains for the permanent excess spoil fill will <br />extend below this fill as shown on Figure 1. <br />The northern slope of the fill is about 200 feet in height at a 2.3H:1 V slope and is located above <br />the permanent fill located in the valley to the north. Analysis of a similar slope was performed in <br />Addendum 3 (Shannon & Wilson, 2010). As shown in Analysis Section C -C' (Figure 11, Sheet <br />3 of 4) in this report, a taller, 2.3H:1 V slope, 265 feet in height, was analyzed considering the <br />presence of a weak carbonaceous mudstone layer (CMS) at the bases. The CMS layer has lower <br />estimated shear strength than the clay layer that would be expected at below the fill considered in <br />this report. The analysis indicated Factors of Safety of 2.0 for mean shear strength estimates, <br />and FS of 1.3 or more for low bound shear strength estimates (Table D -3). Therefore, adequate <br />FS was indicated for a taller spoil fill, with similar slope angle, and lower shear strength material <br />at the base. Based upon this, it is our opinion that analysis of a section through the northern <br />slope of the fill is not required. <br />Material Properties <br />Laboratory test results, which form the basis for our selection of strength parameters for spoil <br />and clay materials, are presented on Figures 10 through 12. The basis for utilizing these shear <br />strength parameters was discussed in our Revised Addendum 3 (Shannon & Wilson, 2010). The <br />strength values used in our analysis are shown on Table 1. <br />23 -1- 01105 -600 <br />