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"D" Seam Slope Stability Page 5 October 19, 1998 <br />The Section 7450 dry slope factors of safety for the 1976 <br />ground surface slide mass segments are all greater than one. with <br />• the exception of Segment Ib at the crest and Segment Vb near the <br />toe, saturated segment factors of safety are less than one. The <br />overall Section 7450 slope factors of safety for the 1976 ground <br />surface, shown on the last page of Table 2, indicate stability for <br />a dry slide slope (FS = 1.38) and predicts failure if the slope <br />approaches saturation (FS = 0.92). This suggests that the Section <br />7450 slope failed when the uplift pore pressure reduced the <br />frictional resistance to sliding. <br />The impact of the planned excavation of part of the slide mass <br />above the Section 7450 sliding surface, indicated by the "cut <br />surface" curve on Figure 2, is considerable. The only segment <br />factors of safety to drop below one when the slide slope is assumed <br />to be fully saturated is Segment IVa (FS = 0.87). The overall <br />saturated slope factor of safety increases to 1.38 for the cut <br />surface slide from 0.92 for the 1976 ground surface when saturated. <br />The preliminary slope stability analysis demonstrates that the <br />planned cut slope should stabilize the slide area lying to the west <br />of the "D" Seam portal area. The preliminary slope stability <br />analysis also indicates the importance of pore water to slope <br />stability. It is recommended that efforts be made to divert <br />surface water away from the top of the slide area and to prevent <br />water from collecting at the base of the slide area. <br />• POST MINING RECLAMATION SLOPE STABILITY <br />The final regrading of the fill platform in the "D" Seam <br />portal area will not reduce the overall slope stability. Under dry <br />conditions, the cut slope sliding resistance along Section 7300 <br />will increase 6.7 T/ft (1.48) and the cut slope sliding resistance <br />along Section 7450 will decrease 5.4 T/ft (0.78). Under saturated <br />conditions, the cut slope sliding resistance along Section 7300 <br />will increase 3.6 T/ft (1.08) and the cut slope sliding resistance <br />along Section 7950 will decrease 4.6 T/ft (0.98). These changes <br />are slightly beneficial, but insignificant. <br />I hope this will be of some assistance in your planning for <br />the "D" Seam mining. <br />Sincerely, <br />~. ~,~. <br />ohn F. Abel, Jr. <br />Colorado P.E. 5642 <br />• <br />