Laserfiche WebLink
• Colowyo Coal Mine <br />Attn: Mr. Tay Tonozzi <br />April 15, 2010 <br />Page 4 <br />of 17 degrees and cohesion of 4.9 pounds per square inch (psi) (or 706 psf) was <br />used in the base case. <br />Bedrock. The bedrock was modeled as impenetrable for all sections except <br />section D -D'; in this case failure along a potential CMS seam was allowed to <br />simulate worse case conditions. <br />RESULTS OF STABILITY ANALYSIS <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 <br />slip surfaces and enabled SLOPE /W's optimization procedure. The optimization procedure <br />varies the location of individual points on the critical circular slip surface until an optimized <br />(non- circular) slip surface is found upon which the minimum factor of safety (FS) is calculated. <br />• We analyzed a base case for each model /section with the mean or best estimate of material <br />strength for each layer. We then varied strength values to include a lower bound strength <br />estimate for the spoil, clay, or carbonaceous mudstone (CMS) below the spoil. We also <br />analyzed cases with a 20- foot -thick water layer at the base of the spoil. <br />n <br />�J <br />The general configurations of the slopes we analyzed are presented on Figure 11, Sheets 1 <br />through 4. The results of the stability analysis are presented in Appendix D. <br />Section A -A' <br />We initially analyzed Section A -A' at the proposed 2.3H:1 V out slope. The calculated Factors <br />of Safety (FS) were marginal. We re- analyzed the slope at a slightly flatter slope of 2.5H:1 V. <br />The analysis calculated a long term FS for the base case of 1.5 (Cases 1 and 2 using mean or <br />best estimate strength properties). We checked stability for other cases using lower bound <br />strength estimates (Cases 3 through 8) with the analysis indicating a FS of 1.3 or more. <br />Section B -B' <br />Analysis of the proposed slope at 2.9H:1 V indicates FS greater than 1.5 for all cases <br />considered. <br />Exhibit 21, Item 1 Revision Date: 12/06/10 <br />Addendum No. 3- 4 Revision No.: TR -M <br />