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November 18, 1996 Geotechnical Study Evaluation Page 3 burn, resulting in typical deflation fracturing of the overlying bedrock. The Bowie No.2 permit application contains references to a general outcrop bum zone throughout the property anticipated to be up to 300 feet in width. On August 28, 1996, we visited the site of the landslide discussed above. We also observed several smaller but appazently geologically analogous landslides m other portions of the permit area. These landslides each appear to have initiated as a bedrock failure in the steep exposed upper bedrock slopes of the site. This bedrock appears to have been weakened by relaxation along joints and bedding planes. The reddish oxidation of the sandstones and claystones suggests the effects of outcrop combustion of the coal seams within the bedrock section. Following combustion, the seams decrease in volume, subsiding the overlying bedrock, resulting in fracturing and joint sepazation. In the case of the referenced landslide, Coors Energy excavated small roads for exploratory drilling in the early 1980's which precipitated an almost immediate bedrock failure. In addition, however, a large soil failure was subsequently initiated immediately dowaslope from the rnnout debris pile of the bedrock failure. We suspect that the failure of the bedrock face loaded the adjacent ancient landslide debris, resulting in a more viscous, flow-like landslide. In fact, several lobate generations of failure, progressing downslope, appeared evident during our short and cursory examination on August 28th. We aze concerned that the anticipated storage bench and portal bench cuts of up to 80 feet in height could experience significant destabilization problems, unless the rock mass is retained as it is excavated. We suspect that this same problem might be encountered to varying degree in other cuts throughout the site. These slope situations aze currently buttressed by the soil and debris veneer of the lazge ancient landslide mass eazlier identified by Maxim Technologies. The deep excavation of the proposed benches will first remove the soil buttress, potentially exposing meta-stable bedrock faces analogous to those evidencing instability in the undisturbed situation. Unless great care is exercised, these slopes will fail, potentially catastrophically, as a result of removal of the soil and landslide debris buttress. Further, they will be exposed to the disturbance of near outcrop portal penetration, operational vibration, hydrologic alterations, and exposure to increased temperature fluctuation. All of these factors commonly contribute to mechanical destabilization of a slope. For purposes of Maxim's analyses, the above concern implies a possible shortcoming in [heir analytical assumptions. The general analyses performed in this study assume that the bedrock is a uniform homogeneous material with strength properties significantly higher than those of the reclaimed soil wedge. The excavated bedrock bench is portrayed as a stable foundation into which the soil wedge is placed. In other words, the failure scenarios are effectively limited to the backfilled soil wedge. A "conventional circulaz stability analysis" is ideally suited for such an analysis. We suspect that this assumption may be inappropriate in the situation presented by the Bowie No. 2 mine site. At this site, it is more likely that the exposed bedrock is prone to failure along specific bedding planes and joint faces. The mechanical frictional properties of the rock mass in effect along these planes of weakness are significantly weaker than the properties of the intact rock matrix determined by triaxial testing of a minuscule rock specimen in a laboratory test cell. Further, analytical techniques designed for wedge-type failure mechanisms should be evaluated for comparative projections of potential bedrock instability scenarios.