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• The thickness of the colluvia] clays along the centerline of the proposed refuse pile was based on the geophysical study. The existing contours of the natural ground were based on topographic maps supplied by Oxbow. The strength of the coal refuse, friction angle of 33 degrees, a cohesion of 0 psf, and a unit weight of 100 pcf, was the minimum value obtained from the Mining and Reclamation Plan, Sanborn Creek and Elk Creek Mines, Volume 11, August 2000, permit submittal to the CDMG for the original refuse pile. For this analysis, it was assumed that Oxbow will compact the refuse to 90 percent of maximum dry density according to AASHTO Specification T99-74 as is presently done in the existing refuse pile, in lifts not to exceed 24 inches in thickness. These assumptions are based on Rule 4.10.4 (3) requirements. Variance from these construction methods may be approved later by the Mine Safety and Health Administration per Rule 4.10.4 (3)(c). The strength of the colluvial soil was also obtained from the laboratory shear tests of the colluvium in the vicinity of the existing refuse pile. This value was shown as a friction angle of 30 degrees, a cohesion of 0 psf and a unit weight of l 18 pcf. This shear test result can be found in Appendix B. The phreatic surface was assumed to be at the bedrock contact. No water was observed during either the geologic mapping or the borings at the existing refuse pile. Per Rule 4.09.2, an up- gradient clean-water diversion will intercept any ninon to the refuse pile and route it azound the • refuse pile. Designed perimeter ditches will intercept any sideslope drainage and collect any runoff from the refuse pile and route flows to Sedimentation Pond E. Designs for all surface drainage structures are provided in Exhibit 2.05-E3, Drainage and Sediment Control Plan, prepared by Montgomery Watson Harza. A designed under-drain will collect and transmit any seepage or infiltration through the refuse pile to Sedimentation Pond E. Design information for the underdrain is provided in Exhibit 2.05-E4, Mine Development Rock/Coal Refuse Facility Design, prepared by Montgomery Watson Harza. Where the slope in the disposal azea is steeper than 2.8h:1v (36 percent), the need for keyway cuts (excavations to stable bedrock) or rock toe buttresses was evaluated. Where the toe of the fill rests on a downslope, stability analyses was performed in accordance with 2.05.3(6)(6), (c) and (d). The steepest outslope was analyzed as presented in Appendix C. Refuse Pile Slope Stability Analysis Results Circulaz and wedge slope stability analysis results for the critical outslope aze presented in Appendix C for static conditions and Appendix D for earthquake analysis. The results show that a minimum allowable factor of safety of 1.5, required by the CDMG for static conditions, was achieved for multiple landslide conditions assuming both wedge and circulaz failures. These analyses assumed both shallow and deep-seated failures as presented in Appendix C. The earthquake analysis also showed a factor of safety Beater than 1.1 was achieved as presented in Appendix D. • The results indicate toe buttresses or keyways are not required. 3