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iColowyo Coal Mine <br />Attn: Mr. Tay Tonozzi <br />April 15, 2010 <br />Page 5 <br />Section C -C' <br />The proposed slope at 2.0H:1 V indicated marginal stability (FS less than 1.3). We re- analyzed <br />a slightly flatter slope at 2.3H:1 V. The analysis calculated along term FS for the base case of <br />2.0 (Cases 1 and 2 using mean or best estimate strength properties). We checked stability for <br />other cases using lower bound strength estimates (Cases 3 through 6) with the analysis <br />indicating a FS of 1.3 or more. <br />Section D -D' <br />Our analysis of the proposed slope at 2.2H:1 V calculated a long term FS for the base case of <br />1.8 (Cases 1 and 2 using mean or best estimate strength properties). We checked stability for <br />other cases using lower bound strength estimates (Cases 3 through 10) with the analysis <br />indicating a FS of 1.3 or more. <br />. There is a wedge of bedrock shown in Section D (Figure 11, Sheet 4 of 4). This wedge <br />represents bedrock left in place after completion of mining (mining stops short of coal outcrop) <br />and does not represent a constructed berm or buttress. As shown, a layer of CMS was included <br />in the model to represent a worst case condition where this lower strength layer is present at the <br />• <br />base of the wedge. <br />CONCLUSIONS AND RECOMMENDATIONS <br />Summary of Analysis and Recommendations <br />The results of our stability analysis, as described above, indicate that the spoil valley fills <br />constructed with the current revised slopes have calculated long term FS values of 1.5 or <br />greater. The Colorado Mined Land Reclamation Board regulations (2005) require a long term <br />FS of 1.5 or greater for permanent spoil fill slopes; they do not specifically address temporary <br />slopes. <br />Exhibit 21, Item 1 Revision Date: 12/06/10 <br />Addendum No. 3- 5 RevisionNo.. 'VP,-%A <br />