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Herdlnp Lawson Aasodates <br />Ms. Christine Johnston <br />Mountain Coal Company " ' <br />August 8, 1996 <br />• Profile Two bisects the northern slope of the LRP. The critical parameter in this profile is the location of <br />Highway 133, a public thoroughfare, located at the base of the coal refuse slope. <br />• Profile Three is located in the central portion of the eaztem slope of the LRP. The critical parameter in this <br />profile is the location of the WEM access road, a private thoroughfare, located at the base of the coal refuse <br />slope. <br />PARAMETER SELECTION: <br />A brief description of the criteria used in the GEO-SLOPEn" program to evaluate the factors of safety fdr the <br />perched water table at the LRP is az follows: <br />LRP Geometry: The LRP stability analysis incorporates the slope geometry of the three profiles <br />identified in the Critical Section paragraph. <br />Groundwater: Groundwater az defined az water extending from the bedrock up into the LRP has not <br />been identified in the piezometers located along the flanks of the LRP. However, a perched water table, <br />confined between two fine grained-low permeability layers has been assumed to exist at a general <br />elevation of 6204 feet above mean sea level at the base of the LRP eazt flank (Profile 3). This perched <br />water table has been incorporated into the sensitivity slope stability models (see attachments). <br />oil Ctr np,L ffonerties: Two coal refuse types and a native soil were used in the model. The coal refuse <br />strength parameters are hazed on recent laboratory soil testing, and previous triaxial shear tests taken at <br />the initial stages of LRP development az identified in Exhibit 51. The native soil strength values were <br />taken Crom the Exhibit 51. They are az follows: <br />Original Coal Refuse: Coal refuse placed on the LRP during the initial stages of development, <br />cohesion 288.0 psf, internal fiction angle 30.0 degrees, moist unit weight 80.0 pcf, saturated unit <br />weight 90.0 pcf. <br />Current Coal Refuse (sensitivity analysis): <br />1. Coal refuse having 7.6 % fines, cohesion 1100 psf, internal friction angle 35.0°, moist unit <br />weight 58.0 pcf, saturated unit weight 68.0 pcf. <br />2. Coal refuse having 10.0 % fines, cohesion 0.0 psf, internal friction angle 39.0°, moist unit <br />weight 58.0 pcf, saturated unit weight 68.0 pcf. <br />3. Coal refuse having 20.0 % fines, cohesion 300 psf, internal friction angle 39.0°, moist unit <br />weight 58.0 pcf, saturated unit weight 68.0 pcf. <br />4. Coal refuse having 30.0 % fines, cohesion 1100 psf, internal friction angle 36.5°, moist unit <br />weight 58.0 pcf, saturated unit weight 68.0 pcf. <br />Alluvial soils: Soils deposited by the North Fork of the Gunnison River, soils are clays and <br />gravel's, cohesion 447.0 psf, internal friction angle 19.0 degrees, moist unit weight 100.0 pcf, <br />saturated unit weight 110.0 pcf. <br />The input information for the analyses includes slope geometry, groundwater conditions, soil strengths, and <br />loading conditions. <br />The perched water table slope stability analysis was evaluated using GEO-SLOPE. This computer program <br />uses the ordinary, Janbu, Bishop, and Spencer method of slices. It compares the forces resisting slope failure to <br />forces which cause slope failure along a specified failure surface. The analysis utilizes a limit equilibrium <br />