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• degrees for the internal angle of friction, 120 pcf for the unit weight and a cohesion value of 5,000 psf for <br /> the underlying bedrock materials. <br /> Using the information above, we have determined the critical section would be the fill slope on the north <br /> side of the proposed CRDA. The stability analysis was performed using the Slope/W 2007 computer <br /> program developed by GEO -SLOPE International, Ltd. A circular slip surface was assumed to be the most <br /> probable mode of failure; therefore, the Bishop's Method of Slices was used to calculate the factors of <br /> safety against slope failure. The slope stability model analyzes the driving forces (i.e., forces causing the <br /> slope to move) versus the resisting forces (i.e., the shear strength of the soils tending to resist movement). <br /> The ratio of these forces yields the factor of safety of the slope against movement (failure). A factor of <br /> safety of 1.0 indicates failure of the slope and subsequent movement. A minimum factor of safety of 1.5 <br /> under static conditions is required for the proposed slopes for the coal refuse disposal area. <br /> Based on our analysis, the variability of the unit weight for the coal refuse and reclaim spoils did not have a <br /> significant effect on the factor of safety. It was also determined that using an internal angle of friction of 25 <br /> degrees, with a cohesion value of 0 psf, for the coal refuse yielded a factor of safety of 1.2 for the proposed <br /> fill slope. This indicated a shallow failure on the face of the fill slope. We believe this low value for the <br /> angle of friction is very conservative and would likely only occur where insufficient compaction is attained <br /> (particularly near the edge of the slope). Factors of safety of 1.5 to 1.8 were obtained when the friction <br /> angle for the coal refuse was increased to 30 and 35 degrees. These failure surfaces were also relatively <br /> shallow and in the face of the fill slope. <br /> • It should also be noted that if finer- grained soils are encountered in the coal refuse materials, resulting in a <br /> lower friction angle, the value for the cohesion will also be increased resulting in a higher factor of safety. <br /> Factors of safety increased to greater than 2.0 when the cohesion of the waste coal materials was increased <br /> to 100 psf, with an internal angle of friction of 25 degrees. <br /> The overall global stability of the proposed CRDA failing through the foundation reclaim spoil materials <br /> yielded a factor of safety in excess of 2.0. <br /> Internal Draina¢e <br /> As noted previously, the proposed CRDA does not classify as a valley fill in the regulations. Therefore, the <br /> CRDA is not required to have a full -sized valley fill drain. However, considering possible water sources <br /> entering the fill slope (storm water, snowmelt and free water from the coal refuse fill materials), we <br /> recommend that an underdrain system be constructed at the base of the fill materials. The location of the <br /> underdrain is shown in Figure C -101 and typical cross - sections of the underdrain are presented in Figure C- <br /> 301. The design of the underdrain is presented in the Surface Water Control Plan, which is being presented <br /> in a separate report. <br /> • <br /> Job Number: 08 -7915 NWCC, Inc. Page 7 <br /> Revised 10/12/09 <br />