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• <br />The stability analysis assumes the fill will be well drained and that no pore <br />pressures will develop in the fill. The soil strength values were varied from those listed <br />above to assess the sensitivity of the analysis to varying soil strength parameters. The <br />variations in the analysis produced relatively small and predictable changes to the <br />calculated factor of safety indicating that the stability analysis was not sensitive to slight <br />variances in the soil strength values. <br />The slope stability analysis was performed using SB-Slope by Geosystem <br />software. The slope stability analysis considered numerous possible failure surface <br />iterations to help identify the potential theoretical slope stability. The stability analysis <br />indicates a minimum factor of safety of 3.04 and 3.58 for the alluvial and colluvial strength <br />parameters respectively. A copy of the computer analysis with the assumptions made is <br />included in Appendix N. <br />The stability analysis was performed with the strength parameters utilized by SCC. <br />L~ <br /> <br />Results were similar to those listed above with a minimum factor of safety of 2.68. <br />Relatively high factors of safety are a result of the relatively flat slope, 3H:1 V, of the <br />waste pile. <br />At the worst case, the only point of instability which could occur would be at the toe <br />of the slope due to a high buildup of the phreatic surface. The stability analysis was run <br />with a phreatic surface at the base of the gob pile. The factors of safety ranged from 2.5 <br />to 3.1. Rather than building expensive underdrain systems, monitoring well GW-6 will be <br />used to monitor the phreatic surface. Ground water monitoring well GW-6 is twenty (20) <br />feet deep. The bottom of the well is at approximate elevation of 5420. The base of the <br />refuse pile will be about elevation 5434 near GW-6. If afree-standing water surface <br />elevation of 5434 feet or higher develops in Monitoring well GW-6, a toe drain will be <br />constructed as follows prior to additional placement of coal mine waste material or <br />(should the critical groundwater elevation develop following completion of waste <br />placement) prior to Phase III bond release. The toe drain will consist of 4 inch diameter <br />perforated pipe surrounded by about 4 to 6 cubic feet per linear foot of pipe of free <br />draining concrete sand. <br />Parmit Renewal <br />2-21 <br />09/03 <br />