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Colowyo Coal Company, L.P. <br />Attn.: Mr. Juan Garcia <br />January 4, 2013 <br />Page 4 of 7 <br />Stability Models <br />SHANNON 8WILSON, INC. <br />We analyzed stability along a typical maximum slope profile at Section A -A'; the section is also <br />located where the rock slope is closest to the spoil fill (Figure 1). As discussed previously, <br />analysis of a section through the northern slope of the spoil fill is not necessary. <br />We performed our analysis using Spencer's limit - equilibrium method, which satisfies force and <br />moment equilibrium, using SLOPE /W software (GEO- SLOPE, 2007). We analyzed circular slip <br />surfaces and enabled SLOPE /W's optimization procedure. The optimization procedure varies <br />the location of individual points on the critical circular slip surface until an optimized (non - <br />circular) slip surface is found upon which the minimum factor of safety (FS) is calculated. <br />Three material layers were included in the model; spoil fill, a 5 -foot thick layer of clay below the <br />spoil, and then rock below the clay. For each model, we considered combination of mean and <br />low -bound strength estimates. Using this model three general stability cases were evaluated: <br />Case 1 - Failure paths though spoil only (Figures 2 and 3). Failure path entry points <br />were limited to the crest of the slope, and exit points were located near the toe of the <br />slope. This results in analysis of a more global failure path. The interbench slope angles <br />are at the angle of repose for the spoil material (1.3H:1 V based on several surveys of <br />spoil in active pits) and by definition are at a FS of 1.0. Theses slopes will be flattened to <br />post mining topography. Smaller scale failure paths were considered in Case 2. <br />• Case 2 — Failure paths through the spoil and underlying clay (Figures 4 to 7). Failure <br />path entry points were limited to the second bench and above, and exit points were <br />located near the toe of the slope above the base of the clay. <br />• Case 3 — Failure paths through the spoil, clay and rock (Figures 8 and 9). Failure path <br />entry points were limited to the second bench and above, and exit points were located <br />from the crest of the rock slope down. <br />Results of Stability Analyses <br />The results of our analyses are summarized on Table 1. The lowest calculated FS was 1.3 for <br />Case 2c that considered failure paths through the spoil and underlying clay, with low -bound <br />strength estimates for both materials. A FS of 1.4 was calculated for Cases 1 a and 2a, again <br />using a conservative low -bound shear strength of 30 degrees, no cohesion for the spoil. For <br />these cases the failure surface was developed to evaluate the overall slope from the upper crest to <br />the toe. FS of 1.7 was calculated for Case 3a with low -bound strength estimates for both the <br />23 -1- 01105 -600 <br />