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• Model Development <br />The models analyzed were based upon the existing and proposed geometry <br />along Section A-A through the RDA, perpendicular to the face. The location of the <br />section is shown on Sheets 1 and 2. Conceptual sections of the model are shown <br />on Fig. 6. We first analyzed the existing RDA. We then performed an analysis of the <br />initial RDA configuration with a top elevation of 7870 and either 2.0:1 <br />(horizontal:vertical) of 2.5:1 slopes. The final RDA configuration was then developed <br />and analyzed. The final configuration has 2.2:1 slopes between benches with a <br />maximum elevation of 8035. The existing RDA geometry was based upon survey <br />information provided by Basin Resources. The location of the refuse/soil/bedrock <br />interface at the bottom of the RDA was based upon the results of our borings and <br />the topographical information from Reference 12. <br />• For each of these models a three layer soil configuration was used, with the <br />layers consisting of the coal refuse, a 10-foot thick soil (or weathered bedrock) layer, <br />and then bedrock. We do not believe a continuous, weak soil layer is present below <br />the pile. The soil layer was included in the model to evaluate the effect of a disturbed <br />zone of soillrefuse at the base of the pile. <br />Based upon the geometry and sail properties selected, the model was <br />evaluated to determine the critical method of analysis. This involved evaluating <br />circular and sliding block type failures with various locations, depths and failure <br />surface entry and exit points to produce the minimum calculated factors of safety. <br />For this analysis both sliding block and circular models were critical depending on <br />the strengths and/or configuration analyzed. We therefore used both methods <br />• throughout the analysis and reported the lower of the two. <br />17 <br />