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Ms. Christine Johnston <br />Mountain Coal Company <br />Page 17 <br />April 14, 1997 <br />Division's opinion it is unlikely that the MR 210 sampling plan will develop sufficient <br />geotechnical data to support the demonstration of stability of the light duty access road <br />along the proposed electrical supply powerline. Further, insufficient geotechnical data <br />may result with which to support the demonstration of stability of the substation and the <br />main access road. The Division cannot render an opinion until the data have been <br />obtained and the stability analyses amended and resubmitted for review by the Division. <br />§ 2.2 Material Properties <br />Within this section of the technical memorandum, Golder Associates presents it's rationale <br />for selection of one selected pair of material strength parameters [an angle of internal <br />friction = 35°, and a cohesion of 200 pounds/ftZ] to use as representative of the entire <br />SGFA. Golder Associates bases its selection on a review of 22 historic analyses from <br />throughout the West Elk Mine site, including two triaxial analyses, and literature report <br />values for materials it considers similar (Hunt, 1983). <br />Golder Associates also observes; "...a cohesion value of 200 pounds per square foot was <br />selected because it resulted in a realistic failure surface. When higher cohesion values <br />were assumed, an unrealistically deep failure surface occurred when it was analyzed as <br />input in XSTABL." This statement, with significantly more supporting explanation <br />regarding the analytical procedure for determining "realism", may be acceptable. <br />However, on its face, it suggests that a value for cohesion is being chosen to accomplish <br />the analyst's predetermined result. In other words, "the realization of aself-fulfilling <br />prophecy" approach to stability analysis. <br />In the Division's opinion the significant vaziation in historically determined material <br />strength parameters at the West Elk mine site is informative. The William's Fork <br />formation and its parent Mesa Verde Group aze stratigraphically complex, containing <br />sandstones, mudstones, shales, siltstones and coals. Past laboratory testing has determined <br />vaziant material strength parameters because the chazacter of the Williams Fork formation <br />is that it is comprised of a broad range of materials. Further, slope stability at the West <br />Elk mine site, as at all mine sites, is influenced by intrinsic rock matrix and rock <br />substance mechanical properties, structural disturbance (joints, faults, cleats), and ground <br />water moisture content. For these reasons, the Division concludes that it is inappropriate, <br />for anything but extremely preliminary design and facility siting considerations, to rely <br />on evaluations based upon average material strength parameters hypothesized to represent <br />the performance of the total Williams Fork formation. Further, MCC's selection of <br />geotechnical consultants agree in recommending site-specific investigations and testing be <br />conducted for proposed facilities. <br />[n it's original geotechnical adequacy comment #71, the Division commented; "In support <br />of its proposed design cut and f II slopes, Golder Associates has prepared a slope stability <br />