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I", <br />• Colowyo Coal Mine <br />Attn: Mr. Tay Tonozzi <br />April 15, 2010 <br />Page 2 <br />which our stability analyses were based. Section E was not analyzed because it is not a <br />critical section by inspection. <br />4 Figure 13 - Total and Effective Stress Mohr Envelopes Boring C -104 has been revised <br />to remove the reference to Appendix B. <br />5 Figure 14 — Summary of Overburden Strength Test Results has been revised to change <br />Curve Nos. 4 and 6 from a friction angle of 24 degrees to 23 degrees. This corrects an <br />error in the 2006 report. We then re -ran the affected slope stability cases, the results did <br />not change to the one significant digit to which they are reported in Appendix D <br />(specifically we re -ran Cases 3 and 4, Section B -B', Table D -1 and Cases 9 and 10, <br />Section D -D', Table D -4). <br />6 Added a revised discussion of Highwall Mining to the Conclusions and <br />Recommendation section. <br />• In summary, the revisions do not materially change the results of our stability analyses or our <br />conclusions and recommendations regarding the temporary or permanent spoil stability. <br />• <br />STABILITY ANALYSIS MODEL AND PARAMETERS <br />Stability Models <br />The location and configuration of the proposed temporary fills are shown on Figure 10 along <br />with the four sections chosen for stability analysis. Section E from Map 45 was not analyzed <br />because it is not a critical section by inspection. <br />The analysis sections upon which our models were based are shown on Figure 11, Sheets 1 to <br />4. The sections in Figure 11 illustrate the proposed permanent spoil slopes and the temporary <br />slopes. In our stability models we used a 24- foot -thick clay layer at the base of the model <br />except where indicated otherwise on Figure 11. Additional model details are shown on the <br />selected analysis output sheets in Appendix D. <br />Exhibit 21, Item 1 Revision Date: 12/06/10 <br />Addendum No. 3- 2 Revision No.: TR -Al <br />