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• SLOPE STABILITY ANALYSIS <br />A stability analysis of the finished slope configuration was undertaken to evaluate soil and groundwater <br />conditions, and the potential for slope movement. This analysis required that a mathetnatical model be <br />established to simulate soil, water and geometric conditions at the site. Considering the subsurface <br />profile encountered during this investigation, a circulaz slip surface was assumed to be the most probable <br />mode of failure. Therefore, we used the microcomputer program "SB-SLOPE" which uses the <br />Simplified Bishop's Method of Slices to calculate factors of safety against slope failure given the slope <br />geometry, soil strength values and groundwater conditions. The method employed, utilizes principles of <br />static analysis where certain conditions must be assumed so that an idealized system can be created for <br />model simulation. The primary assumptions in the analysis are that the materials composing the slope <br />aze isotropic and homogeneous in each stratum. The cross section analyzed was taken from Detail 1 on <br />Figure 3, shown in the report prepazed by CTL/Thompson for the RDA. The subsurface information <br />obtained during this investigation and the test hole information outlined in the CTL report were used to <br />establish the subsoil profile of the RDA. <br />The properties of the waste coal materials, used in the stability analyses, were detertnined using the worst <br />• case scenario. For [his evaluation, we completed [he stability analyses assuming that all of the waste coal <br />materials consisted of the more cohesive and finer grained materials. A friction angle of 30 degrees, a <br />cohesion value of 0 and a unit weight of 100 pcf were used for the processed waste coal materials. <br />Based on our observations made during the removal of the topsoil from the RDA and the excavation of <br />[he toe buttress, prior to [he placement of the waste coal, it appears that the natural soils encountered at <br />the site generally consisted of either very stiff, natural clays or claystone bedrock. It should be noted <br />[hat we did not observe any spoils or soft clay materials in the RDA after the topsoil materials were <br />removed. Therefore, in the stability analyses we used the normal values obtained and used by <br />CTL/Thompson during the original design for the clays and claystone bedrock. A friction angle of 25 <br />degrees, a cohesion value of 1,500 psf and a unit weight of l20 pcf were used for the natural clays. A <br />friction angle of 25 degrees, a cohesion value of 5,000 psf and a unit weight of 120 pcf were used for [he <br />claystone materials. <br />The test holes advanced in the lower bench by our firm indicated that a very thin layer of clay was <br />encountered above the claystone bedrock; however, the test holes drilled by CTL/Thompson in the RDA <br />indicate dtat the depth of bedrock ranged from 7 to 27 feet beneath the ezis[ing ground surface. <br />Therefore, we assumed that a 20 foot thick layer of clay was present beneath the entire pile in the <br />• stability analysis. We believe that this is a very conservative assumption. The analyses were also run <br />with perched water table conditions, as were encountered during this investigation. <br />Job No. 57-3216 Nonhwes[ Colorado Consulunrs, inc. Page 6 <br />