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Table 5. Factors of Safety For Critical Failure Surfaces <br /> Failure Coal Refuse Stren th Factor of <br />Condition Surface ~~ (deg) c' Safety <br /> Circular 1.93 <br /> 34.0 0 <br /> Block 1.94 <br />Phreatic Surface Based on Pore Circular 1.78 <br />Pressure Measurements Block 32.0 0 1.83 <br /> Circular 1.65 <br /> 30.0 0 <br /> Block 1.68 <br /> Circular 1.75 <br /> 34.0 0 <br /> Block 1.81 <br />Assumed Increased Phreatic Circular 1.63 <br />Surface Block 32.0 0 1.69 <br /> Circular 1.50 <br /> 30.0 0 <br /> Block 1.56 <br />All critical factors of safety exceeded minimum requirements as specified by the Regu/ations of <br />the Co%rado Mined Land Reclamation Board for Coal Mining (DRMS, 2005). Factors of safety <br />using the actual measured shear strength for the coal waste and the actual measured pore <br />pressures were slightly greater than 1.90. Factors of safety for the more conservative analyses <br />that consider reduced shear strength for the coal waste and increased pore pressures still result <br />in critical factors of safety of 1.50 or greater. <br />6.0 CONCLUSIONS AND RECOMMENDATIONS <br />BGI has conducted a revised stability evaluation for the Bowie CWDA No. 2. This evaluation <br />was revised based on on-going instrumentation monitoring conducted at the waste bank over <br />the past three years. Vibrating wire piezometers installed within the coal mine waste <br />demonstrate pore pressure dissipation as well as pore pressures well below the failure <br />conditions measured during triaxial shear tests for similar confining pressures. Based on this <br />monitoring, effective stress analyses were conducted using effective strengths for the coal mine <br />waste. Pore pressures were conservatively accounted for by approximation with a phreatic <br />surface within the coal waste. The critical factors of safety for waste bank slopes of 2.5H:iV <br />from 0 to 50 feet and 3H:iV above 50 feet were demonstrated to meet minimum requirements <br />as specified by Rules 4.09 and 4.10 of the Regu/ations of the Co%rado Mined Land Rec/amation <br />Board for Coal Mining (DRMS, 2005). <br />The current conditions at the CWDA No. 2 consist of 2.5H:iV slopes from 0 to 50 feet and <br />5H:1V slopes above 50 feet. Cover soil has been placed on the slopes from 0 to 50 feet and a <br />portion of the slope above 50 feet. In order to increase the coal waste slope to 3H:1V, all <br />topsoil must be first removed and stockpiled for later use. Leaving the topsoil in place may <br />present a weak layer within the waste. <br />• <br />Bowie No 2 2008 Reevalution of Coa] Mine Waste Stability.doc <br />Project #OS-126-GEO <br />Page 13 of 15 <br />