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Table 2. Effective Strength Parameters <br />From CU Triaxial Testing <br />Boring <br />ID <br />Boring <br />Location <br />Sample <br />ID <br />Sample <br />Depth <br />ft ) <br />Effective Stress <br />Parameters <br />r <br />(deg. <br />c' <br />BG05 -05 <br />BG05 -06 <br />N/A' <br />CWDA No. 2 <br />CWDA No. 2 <br />N/A <br />SH -1 <br />20 -21.5 <br />34.3 <br />0 <br />SH -2 <br />27.5 -29.5 <br />33.6 <br />0 <br />BS -3 <br />N/A' <br />34.0 <br />0 <br />Average: 34.0 0 <br />Notes: <br />1. Sample BS -3 remolded from bulk sample <br />5.0 STABILITY EVALUATIONS <br />Stability evaluations of the Bowie CWDA No. 4 have been updated for a 150 -foot ultimate <br />refuse bank height. Initial stability evaluations for CWDA No. 4 were for a 118 -foot ultimate <br />refuse bank height (BGI, 2007). Stability evaluations were conducted using limit equilibrium <br />methods and effective stress analyses based on pore pressure monitoring at the adjacent <br />CWDA No. 2. The following sections present the revised stability evaluation. <br />5.1 Model Geometry <br />• Stability of the Refuse Pile No. 4 was evaluated for the maximum section, located through the <br />approximate central portion of the waste bank. The approximate cross - section location is <br />indicated on Drawing 2 in Appendix A. The maximum section is considered to represent the <br />likely worst case scenario with respect to slope geometry due to the maximum height of waste. <br />Geometry of the refuse for the analyses was as follows: <br />➢ A slope of 2.5H: IV for the first 50 vertical feet of the stockpile (beginning at the toe). <br />➢ A 15 -foot wide bench at a vertical height of 50 feet. <br />➢ An overall slope of 3H: IV above the first 50 vertical feet. <br />➢ An ultimate waste height of 150 feet was modeled, as measured from the downgradient <br />toe to the top of the bank. This represents an increased ultimate height from the initial <br />stability evaluation (BGI, 2007). <br />Geometry of the native ground surface upslope and downslope of the refuse was developed <br />from a topographic survey of the pre- mining surface. Refuse placement operations at Bowie <br />have typically included blending of coal refuse with native soils. For Refuse Pile No. 4, native <br />soils are being excavated from the existing slope at the refuse pile prior to refuse placement. <br />This excavation typically occurs in a benched manner as refuse placement progresses. In <br />absence of survey data of the actual native cut slope topography below /behind the coal refuse, <br />an overall slope of approximately 1.3H: IV was assumed for the stability evaluation. Geometry <br />of the waste bank used in the stability model is presented in Figure 1. <br />• <br />Bowie No 2 Refuse Pile No 4 2009 Revised stability.doc <br />Project #06 -481 -003 <br />Page 7 of 12 <br />