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• to be 130 pcf wet and 135 pcf saturated. Keeping the shale dry is essential to obtaining <br />higher strength than those modeled. The shale is modeled as material '3' on Figures 4.3 and <br />4.4. <br />4.3.3 Sediment Pond Slope Stability <br />Slope stability of the sediment pond embankment was evaluated because the combined scopes <br />were steeper than 5:1. The situation considered for the analysis was a maximum pool <br />inducing steady-state seepage conditions. The critical slope stability section was determined <br />to be the southern embankment adjacent to the highway. A[ this location, the 2:1 <br />downstream slope transitions directly into the steeper road cut. Figure 4.5 and Figure 4.6 <br />shows this section and the material properties used. Both pseudostatic and static conditions <br />were analyzed for this model. <br />Soil representing the foundation material was modeled with the same material properties as <br />the 'fill' material in the waste bank analyses. The embankment material properties were <br />modeled as the 'fill' material in a compacted state, in the event that the cut 'fill' material from <br />the interior of the sediment pond is used in the embankment. Properties for this material <br />• include an internal angle-of-friction of 28 degrees, with 200 psf cohesion. Unit weights used <br />include a 120 pcf wet unit weight, and a 125 pcf saturated unit weight. <br />For the purposes of this analysis, steady-state seepage was assumed as a conservative <br />approach. This situation is not expected to occur, since a pool will only develop in the event <br />of a rain storm, and is not likely to be permanent. <br />4.3.4 Results <br />Results of the stability analyses indicate adequate stability of the coal mine waste bank as <br />planned under the steady state conditions modeled. The failure mode was calculated to be <br />within the Mancos shale. Terraces created by the benches and the haul roads were not the <br />critical failure mode. The overall stability of the planned coal mine waste bank at the Bowie <br />No. 2 Mine was calculated to be 1.54, meeting the minimum requirements of 1.5 per Rule <br />4.10.4(2). Long term stability, after reclamation should not go below this as long as drainage <br />is maintained. The pseudo-static safety factor was calculated to be 1.14, demonstrating <br />stability during earthquake loading conditions. Results of these analyses are located in <br />• Appendix D, and shown graphically in Figures 4.3 and 4.4. <br />96266/1424.RPT Coel Mine Weste Bank Final Design (or the Bowie No. 2 Mine <br />June 1996 W E S T E C 22 <br />ADLQUACY-Ol <br />