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• Table 4. Summary of Stability Evaluation Results <br />Notes: <br />1. Buckhorn Geotech, Inc. (2008). <br />2. Spencer's procedure <br />0 6.0 CONCLUSIONS AND RECOMMENDATIONS <br />BGI has conducted a revised stability evaluation for the Bowie CWDA No. 4. This evaluation <br />was revised for an increased ultimate refuse height of 150 feet. On -going instrumentation <br />monitoring conducted at the adjacent CWDA No. 2 over the past three years demonstrates pore <br />pressure dissipation as well as pore pressures well below the failure conditions measured during <br />triaxial shear tests for similar confining pressures. Based on this monitoring, effective stress <br />analyses were conducted using effective strengths for the coal refuse. Pore pressures were <br />conservatively accounted for by approximation with a phreatic surface within the coal waste. <br />The critical factors of safety for refuse bank slopes of 2.5H: IV from 0 to 50 feet and 3H: IV <br />above 50 feet for an ultimate height of 150 feet were demonstrated to meet minimum <br />requirements as specified by Rules 4.09 and 4.10 of the Regulations of the Colorado Mined <br />Land Reclamation Board for Coal Mining (DRMS, 2005). <br />Proper management of coal refuse moisture content during placement will reduce the pore <br />pressures within the fill and enhance stability. As practical, waste materials in excess of the <br />optimum moisture content should be allowed to dry prior to placement. Blending wet material <br />with drier material may also prove to be an effective moisture content management strategy. <br />Bowie No 2 Refuse Pile No 4 2009 Revised stability.doc <br />Project #06 -481 -003 <br />Page 11 of 12 <br />Failure <br />Coal Refuse Stren th <br />Factor of <br />Condition <br />Surface <br />� (deg) <br />c' (psf) <br />Safety <br />Circular <br />34.0 <br />0 <br />1.77 <br />Block <br />1.80 <br />Assumed Phreatic Surface Based <br />on Pore Pressure Measurements <br />Circular <br />32.0 <br />0 <br />1.64 <br />at Bowie Refuse Pile No. 2' <br />Block <br />1.67 <br />Circular <br />Block <br />30.0 <br />0 <br />1.52 <br />1.63 <br />Circular <br />34.0 <br />0 <br />1.77 <br />Block <br />1.83 <br />Circular <br />Block <br />32.0 <br />0 <br />1.64 <br />1.67 <br />Assumed Increased Phreatic <br />Surface <br />Circular <br />30.0 <br />0 <br />1.52 <br />Block <br />1.65 <br />Notes: <br />1. Buckhorn Geotech, Inc. (2008). <br />2. Spencer's procedure <br />0 6.0 CONCLUSIONS AND RECOMMENDATIONS <br />BGI has conducted a revised stability evaluation for the Bowie CWDA No. 4. This evaluation <br />was revised for an increased ultimate refuse height of 150 feet. On -going instrumentation <br />monitoring conducted at the adjacent CWDA No. 2 over the past three years demonstrates pore <br />pressure dissipation as well as pore pressures well below the failure conditions measured during <br />triaxial shear tests for similar confining pressures. Based on this monitoring, effective stress <br />analyses were conducted using effective strengths for the coal refuse. Pore pressures were <br />conservatively accounted for by approximation with a phreatic surface within the coal waste. <br />The critical factors of safety for refuse bank slopes of 2.5H: IV from 0 to 50 feet and 3H: IV <br />above 50 feet for an ultimate height of 150 feet were demonstrated to meet minimum <br />requirements as specified by Rules 4.09 and 4.10 of the Regulations of the Colorado Mined <br />Land Reclamation Board for Coal Mining (DRMS, 2005). <br />Proper management of coal refuse moisture content during placement will reduce the pore <br />pressures within the fill and enhance stability. As practical, waste materials in excess of the <br />optimum moisture content should be allowed to dry prior to placement. Blending wet material <br />with drier material may also prove to be an effective moisture content management strategy. <br />Bowie No 2 Refuse Pile No 4 2009 Revised stability.doc <br />Project #06 -481 -003 <br />Page 11 of 12 <br />