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The material properties for the Bowie Member shale is important to the overall stability of the • coal mine waste bank. Bowie Member shale may be reactive to water such that a loss of strength usually occurs if there is not adequate drainage. Although there are several systems at the site designed to transport water away from waste bank and keep materials involved dry and at peak strength, the shale was modeled at a lower strength than predicted for a dry state. As a contingency, the shale was modeled with a 20 degree friction angle and 2,000 psf cohesion based on previous WESTEC experience in the area. Unit weight for the shale was assumed to be 130 pcf wet and 135 pcf saturated. Keeping the shale dry is essential to obtaining higher strength than those modeled. The shale is modeled as material "4' on Figures 4.3 and 4.4. 4.3.3 Results Results of the stability analyses indicate adequate stability of the coal mine waste bank as planned under the conditions modeled. Two failure modes were analyzed, circular, and planar through the unconsolidated colluvial sandstone. The critical failure mode was calculated to be circular, within the Bowie Member shale. The overall factor safety for the planned coal mine waste bank at the Sanborn Creek Mine was calculated to be 2.1 for the circular failure • mode, and 2.3 for planar failures, meeting the minimum requirements of 1.5 per Rule 4.10.4(2). Long term stability, after reclamation should not go below this as long as drainage is maintained. The pseudo-static safety factor was calculated to be 1.5 for the circular case, and 1.8 for the planar case, demonstrating stability during earthquake loading conditions. Results of these analyses are located in Appendix D, and shown graphically in Figures 4.3, 4.4, 4.5 and 4.8. • 96256/1424.RPT Coel Mine Weete Benk Design for the Sanborn Creek Mine October 1990 WESTEC, Inc. 2t