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locations for the water surface initiation were analyzed to encompass all possible <br />locations of the infiltration trenches. These locations were positioned beginning at the <br />back of the crest on the top slope (designated Water 1) and moving toward the crest <br />(designated Water 2 and Water 3). <br />PC STABL 5 allows for analysis of multiple computer generated trial failure surfaces <br />within specified starting and ending areas. Failures are specified to begin within 200 feet <br />of the embankment toe and terminate within a 100 -foot area on the crest. A total of 250 <br />trial failure surfaces are analyzed for the circular failure scenario. A minimum of 10 <br />failure surfaces are listed in the computer output. <br />Under the block failure scenario, two rectangles are established along the material <br />interface and the program randomly selects points within each rectangle to define a line <br />creating the base of the central block sliding surface. After the central block is created, <br />the active and passive portions of a trial failure surface are generated and a factor -of- <br />safety against sliding is computed. Fifty trial surfaces are generated and 10 minimum <br />geometries are reported. <br />Results <br />Minimum circular and block failures without water are both shown on Figure No. 4. <br />Figure Nos. 5 through 7 illustrate assumed water surfaces and resulting failure envelopes. <br />Failure envelopes for both circular and block failures are identical with and without a <br />water surface. This indicates that the presence of a water surface does not significantly <br />affect the failure mode. <br />Table I. Computed Factor -of- Safety <br />Case Analyzed <br />Minimum Factor-of-Safety <br />Circular <br />o /o of Existing <br />Block <br />o of Existing <br />/o <br />Existing-no water <br />1.148 <br />100.0 <br />1.497 <br />100.0 <br />Water surface 1 <br />1.148 <br />100.0 <br />1.497 <br />100.0 <br />Water surface 2 <br />1.144 <br />99.7 <br />1.497 <br />100.0 <br />Water surface 3 <br />1.118 <br />97.4 <br />1.497 <br />100.0 <br />Table I indicates that the factor -of- safety under a circular failure mode decreases slightly because <br />the presence of water. <br />Conclusion <br />Simulations of slope failure due to the addition of infiltrated mine discharge water on the lower <br />bench at Logan Wash Mine using the PC STABL 5 program indicates that there is not a <br />significant increase in slope instability compared to existing conditions at the mine site. Shear <br />strength values assigned to native and fill material are typical conservative values taken from <br />published literature. Lower safety factors are not expected if shear strength values are reduced. <br />