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"D" Seam Slope Stabili*_y Page 3 October 19, 1999 <br />10. The density of the broken rock will be lower than the in place <br />rock. The conservative minor decrease in density used in the <br />stability analysis should result in a higher slide mass weight, a <br />higher than realistic thrust along the sliding surfaces and a <br />slightly lower factor of safety. <br />LOCATION OF SLIDING SURFACE <br />• <br />The estimated location of the sliding surface on the Section <br />7300 cross section (Figure 1) was based primarily on the depth of <br />colluvium (alluvium) above bedrock indicated on the drilling logs <br />of nearby drillholes. The colluvium depths were transferred to the <br />cross section along the adjacent contour lines of the 1976 <br />topographic map, developed by aerial photo interpretation. Section <br />7300 is at the start of the "D" Seam declines in the planned <br />location at of the "D" Seam portal. The crest and toe of the <br />sliding rock mass are two additional sliding surface locations <br />along the section. The same procedure was used to estimate the <br />sliding surface on Section 7450. The Section 7450 cross section <br />passes through the estimated location of the bedrock face along the <br />"D" Seam declines. <br />LOCATION OF SLIDING SURFACE SEGMENT SLICES ' <br />The method of slices for calculating the factor of safety for <br />a soil or rock slope requires segmenting the estimated sliding <br />surface. The sliding surface segments used in the slope stability <br />analysis are indicated by the Roman numerals and arrows below the <br />sliding surfaces, on Figure 1 for Section 7300 and on Figure 2 for <br />Section 7450. The method of slices conservatively assumes that the <br />base of each slice follows a straight downslope line. This is <br />conservative because the actual sliding surface is curved. The <br />cohesive resistance is applied td the area of assumed 1-foot wide <br />cross sections. The length of the curved sliding surface is longer <br />than the straight line connecting the ends of the individual <br />segments and, therefore, the actual estimated area along the curve <br />is greater than the straight line approximation. Figure 1 includes <br />the straight line simplification of the segment lines for Section <br />7300. <br />Tables 4 thr-0ugh 6 present the Section 7300 geometries for the <br />estimated sliding surface segments, the planned cut slope surface <br />and the ground surface in 1976 as measured by the aerial photogra- <br />phy taken at that time. Tables 7 through 9 present the correspond- <br />ing geometries for Section 7450. <br />ANALYSIS OF CALCULATED SEGMENT STABILITY <br />The first page of Table 1 presents the calculated factors of <br />safety for the seven individual segments along the estimated <br />• <br />