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The material properties for the Bowie Member shale is important to the overall stability of the <br />coal mine waste bank. Bowie Member shale may be roactive to water such that a loss of <br />• strerngth usually ocean if ttnero is not adequate drainage. Although there are several systems <br />at the site desigrnsd to transport wetar away from waste bank and keep materials involved dry <br />and at peak strength, the shale was modeled rn a lower strength than predicted for a dry <br />state. As a contingency, the shale was modeled with a 20 degree friction angle and 2,000 <br />psf cohesion based on previous WESTEC e~erierncs in the arse. Unit weight for the shale <br />was assumed to bs 130 pcf wet and 135 pcf saturated. Keeping the shale dry is essential to <br />obtaining higher strength than those modeled. The shah is rtwdeled as material '4' on Fguros <br />4.3 and 4.4. <br />4.3.3 Reaults <br />Results of the stability analyses indicate adequate stability of the coal mine waste bank as <br />planned under the conditions modeled. Two failuro modes wero analyzed, circular, and planar <br />through the unconsolidated colluvial sandstone. The critical failure mode was calculated to <br />bs circular, within the Bowie Member shale. Ths ovxaY factor safety for the planned coal <br />mine waste bank at the Sanborn Creek Mine was calculated to W 2.1 for the circular failure <br />'~ mode, and 2.3 for planar failures, meeting the minimum requirements of 1.5 per Ruk <br />• 4.10.4(2!. long term stability, ahx reriamation should not go below this a, k-ng as drainage <br />is maintained. The pseudo-static safety factor waa calculated t0 bs 1.5 for the circular case, <br />and 1.8 for the planar case, demonstrating stability during earthquake Loading conditions. <br />Results of these anayses are located in Appenduc 0, and shown graphically in Fpures 4.3, <br />4.4, 4.5 and 4.8. <br />•, <br />962a6/1424.RfT Cod Mires Wrr~b 6uik OMgn fa tM S~Man dMk MYr <br />OetoWr 1996 WE9TEC, Ino. 21 <br />