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Ms. Christine Johnston <br />Mountain Coal Company <br />Page 19 <br />April 14, 1997 <br />failed slope situations. An observed slope failure presents a situation in which the slope's <br />factor of safety passed below 1.0. At that instant, under the observed field conditions, the <br />forces driving failure exceeded the forces resisting failure, resulting in a slope failure. <br />The slope configuration was known. The degree of saturation and structure can often be <br />obtained from first hand observations of the failure. With these parameters fixed, a <br />family of material shear strength parameters can be calculated. If multiple parameters, <br />such as shear strength and the degree of saturation, cannot be precisely determined than <br />iterative analyses can be performed to determine the sensitivity of the analytical result to <br />variance in the input parameters. MCC and Golder Associates have chosen not to respond <br />to these eazlier comments by the Division. Instead, MCC has chosen to rely upon the <br />eazlier stability analyses, which the Division considers to be unrepresentative. <br />In the Division's opinion, Golder Associates' chosen methodology for stability analysis <br />of the operational and reclaimed configuration of the SGFA is not an acceptable <br />methodology for the completion of stability analyses for [he critical sections. This method <br />has been applied throughout the West Elk mine site with disappointing success. The <br />Division will not belabor this point by reiterating additional comments contained in its <br />original adequacy comment #71. The Division anticipates, as a result of ongoing slope <br />stability problems at the West Elk mine site, that establishment of an acceptably stable <br />post-mining reclaimed configuration will prove to be extremely challenging. The Division <br />does not want this problem to proliferate into the SGFA. The Division believes that <br />MCC's experience to date should allow the construction of less problematic operational <br />and reclaimed configurations within the SGFA. If MCC chooses not to avail itself of <br />other analytical approaches to demonstrating the stability of the proposed operational and <br />reclaimed site configurations, the Division believes it has no choice but to await the <br />completion and submittal of site specific stability evaluations based upon additional site <br />specific geotechnical sampling and laboratory determination of appropriate site specific <br />strength parameters. <br />§ 3.0 POTENTG4L RISK iY1ITICATION DURING CONSTRUCTION <br />Golder Associates concurs with the Division's recommendation for on-site geotechnical <br />engineering observation during construction suggestions. MCC, however, makes no <br />commitment to concur. In addition, Golder Associates list of possible mitigative measures <br />to be effected during construction appears in keeping with standard geotechnical practice. <br />§.?.2 Post-Construction Stability <br />The Division, as discussed at length above, continues to consider these stability analyses <br />to be unrepresentative ofthe actual conditions to be encountered with the operational mine <br />plan configuration of the SGFA. Golder closes this section with the comment; "Factors <br />of safety are based on assumed soil strength parameters that will be better defined <br />