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1 <br />The calculated friction and cohesion values for the various <br />materials corresponded very closely to data used in previous <br />reports for the same project site. <br />1 The shear strength for the proposed bench fill material was based <br />on previous testing on typical refuse material as stated in the <br />August 25, 1981 Rocky Mountain Geotechnical report concerning the <br />' refuse disposal area slope stability analysis. This study <br />' indicated that a friction angle of 28 degrees and cohesion of 200 <br />psf can be expected for this type of material. <br />The assumed position of the water table is extremely important to <br />a stability analysis. Information supplied to us by Western <br />' Associated Coal Company indicated that the current water table at <br />the site near the existing sedimentation pond in the immediate <br />t vicinity of the proposed portal cut is currently approximately <br />Elevation 6306. To be conservative, the water table was set at <br />t Elevation 6320 near the portal face. Although the water table <br />' for the proposed benches should remain relatively low or <br />completely dry, a Case II groundwater profile as described in the <br />' book Rock Slooe Enaineerina was assumed. This case assumes that <br />the phreatic surface slopes upward from the toe of the berm to a <br />point 8 times the height of the berm behind the toe of the slope. <br />5.2 FAILURE ~~ <br />In soil slopes, although planar-type failures are possible in <br />stratified deposits, the most common mechanism of failure <br />' 8 <br />