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David Berry -18- June 20, 1996 <br />committed to proper design, including subgrade density testing <br />and periodic inspections. This will be noted in the Volume IV <br />text. <br />4.3 SLOPE STABILITY <br />4.3.2 Material Properties <br />WESTEC considered four earthen materials in completing its <br />slope stability analysis for the proposed waste pile. <br />Strength parameters for the coal waste, material "1" were <br />determined by performing a direct shear test in the <br />laboratory. Strength parameters for existing sandstone- <br />derived colluvial material underlying the proposed waste pile, <br />material "2", were theoretically developed from interpretation <br />of the Standard Penetration Test (SPT) blowcounts recorded in <br />the field. Strength parameters for the Mancos Shale bedrock, <br />material "3", were similarly deduced from SPT blowcounts. <br />Strength parameters for the historic coal mine waste fill <br />material located beneath the toe of the waste pile, material <br />"4", were also deduced from SPT blowcount conversions. <br />WESTEC's stability analysis of the proposed Bowie No. 2 mine's <br />coal waste pile determined the overall static slope stability <br />safety factory of the structure to be 1.54. The corresponding <br />pseudostatic factor of safety, assuming a pseudostatic <br />horizontal acceleration factor of 0.1 g was calculated at <br />1.14. This calculated static safety factor narrowly exceeds <br />the required value of 1.5. If the ultimate waste pile volume, <br />in excess of 225,000 cubic yards of combined waste, coverfill <br />and topsoil were to fail, it could compromise the function and <br />integrity of the sediment pond adjacent to the waste pile toe. <br />Further, since the waste pile toe is within 150 feet of the <br />right-of-way of old State Highway 133, it might compromise the <br />drainage facilities adjoining that highway and subject the <br />public to a health and safety risk. <br />In consideration of the potential risk, BRL should obtain <br />samples of each of the three earthen materials for which <br />WESTEC assumed strength parameters and determine those <br />properties by appropriate laboratory testing. In the case of <br />the Mancos Shale (material "3") and the old coal mine refuse <br />(material "4"), tests should be performed in both saturated <br />and unsaturated conditions to determine the effect of ground <br />water saturation. Sensitivity-style stability analyses can <br />then be completed to determine the possible effect of <br />unanticipated saturation of the subject materials. <br />DMG's concerns were taken into consideration during field <br />investigations and prior to performing site designs. Based on <br />careful consideration and our experience with similar <br />materials in this region, we believe that the material <br />