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SHANNON i;WILSON, INC. <br /> <br />6.0 STABILITY ANALYSIS <br />The following sections outline the spoil fill geometry; selection of strength and density values for <br />spoil, soil, and bedrock; groundwater conditions modeled; and the results of the stability <br />analyses. Drain requirements and construction monitoring recommendations are also presented. <br />6.1 Fill Geometry <br />Figure 2 shows the Post Mining Topography (PMT) proposed at the time of this report, along <br />with the location of the pit crest. These two factors in combination define the areas of excess <br />spoil. The excess spoil is located north and west of the South Taylor pit. Several sections <br />through the fills were developed for initial analysis to determine the most critical section for <br />further parametric analysis. <br />Sections A-B and A-C are located along the forks of the western valley. Section D-E cuts across <br />an area of spoil with a steeper slope of about 3H:1 V. This area is located over the pit and is <br />technically not excess spoil; however, because it was the steepest portion of the fill, the section <br />was analyzed. Section F-G cuts the eastern valley fill along the existing valley centerline. These <br />. sections and (coordinates defining the analysis sections) are shown on Figure 8. <br />The out-slopes of the permanent valley fills are planned at a maximum of 3.8H:1 V (Figure 8). <br />The maximum slope angle allowed by the CDMG Regulations (Reference 1) is 2H:1V. <br />Generally, flatter slopes have a higher FS against slope instability, allow for easier topsoil <br />placement, and have less erosion potential. <br />Our stability analysis consisted of analyzing a three-layer system of spoil, soil, and bedrock. <br />Mohr-Coulomb strength parameters were used to model the three-layer system. Based on <br />borings ST-1 and ST-2, we used a 24-foot-thick overburden soil layer along the valley bottom. <br />The geometry of the analyzed cross-sections, including data points, is shown on Figure 8. <br />6.2 Soil/Rock Engineering Properties <br />The strength and density properties of the spoil, overburden soils, and bedrock were evaluated as <br />discussed in the following subsections. We also included shear test results of similar soils found <br />in the Lower Wilson (LW) and Collom (CO) Mine Areas. These results are not referenced <br />because the have not yet been reported. <br /> <br />zs-i-o~ ws-zoo-R~.aoc 23-1-01105-200 <br />