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-20- <br />• method of mining results in removal of coal down to a Bard, massive <br />sandstone on the west wall. Strength tests on intact pieces of the <br />sandstone, as well as observations of the existing pit wall which is <br />currently standing as much as 200 feet high with no benches, indicate <br />that the west wall is not the critical stability consideration. The <br />planned configuration includes benches approximately 120 feet in height <br />to provide safety benches and protection in the event of rockfall. The <br />high shear strengths available from the sandstone formation and the <br />geometry proposed do not indicate that the west wall will be critical <br />within the depths proposed. The east wall will be the critical wall <br />from a point of view of stability for the proposed mining sequence. <br />Minor failures have occurred along the east wall of the existing pit. <br />These failures generally have occurred relatively high in slope. <br />As discussed above, failures within the pit were analyzed to <br />• back calculate the shear strengths. The failures which have occurred <br />appear to have occurred approximately on a plane that is oriented about <br />90 degrees away from the angle of dip. The sedimentary rocks on the <br />east wall dip into the slope at an angle ranging from 40 to 50 degrees. <br />This results in a plane of weakness occurring at about 45 to 50 degrees <br />from the horizontal, which is roughly 90 degrees from the bedding plane. <br />The anticipated plane of weakness has been observed in some minor <br />failures which have occurred in ,the upper 60 feet of the existing east <br />pit wall. <br />As a part of our boring program, we have observed that the ma- <br />terials occurring within the upper 100 feet are substantially softer <br />than the materials below 100 feet. Our air photo interpretation and <br />• <br />