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Marding Lawson Assoeletes September 14, 1995 32640.5 iii iiiiiiiiniii iii 999 Mr. Norm Every Mountain Coal Company ~~-~ West Elk Mine ~F~'~`\ SoOmeBrset, Colorado 51434 5eP 2~ 1995 Re: ANALYSIS AND SPECIFICATIONS FOR CONSTRUCTION OF A BUTTRESS [ ~,petal5 ~Geo~o9`I WEST OF THE C-1 CONVEYOR ABOVE THE SUBSTATION O~~i5tp1,0 Dear Norm: The following letter report summarizes the analysis of the slope stability model and recommended mitigation measures for [he slope failure located below [he bin wall on [he WEM access road [o the electrical substation (refer to Figure 1). The slope failure below [he bin wail has been observed as early as 1985 (As per related to the HLA representative in discussions with [rune personnel in the field). In exvemely wet }'ears the slope continues to fail causing increasing damage to the bin wall. The purpose of the soil bu[vess is to allow repairs to the mine portal access road. The mine portal access road has started to erode at the point where the bin wall has failed. The soil buttress will provide the necessary support Io repair to the mine portal access road; but, will not intrinsically increase the overall stability of the slope. Subsequently future minor repairs to the soil buttress are possible. The proposed failure mitigation is a soil buttress design having a volume of approximately 1800 cubic yards. The soil buttress will be approximately 75 feet wide by 130 feet long and will cover the impacted area below the bin wall. For design purposes, the soil buttress will be constructed with pit-run gravel and clay mix. The following paragraphs outline the mitigation design. A slope stability analysis was conducted on four profiles encompassing the impacted failure area below the bin wall (refer to Figure 2). The slope stability analysis utilized [he computer program PCSTABLS to determine the factors of safety against failure. The factor of safety is the ratio of the forces resisting the slope movement [o the forces which cause the slope to fail. The resisting forces are deterntined by the shear svength of the imported soils to resist failure along a specific surface. The forces which tend [o cause failure include, but are not limited to: (a) the angle of the slope; (b) the location of e.~isling roads and drainage ditches in relation to the slope; (c) pore pressure or water table acting on the failure plane; and (d) the characteristics of [he native soil and bedrock found itt the slope. A factor of safety of 1.0 implies that the slope is marginally stable for the cross-section profiles, native clay/bedrock material svengths, groundwater levels, and the slope conditions. Harding Lawson Associates (iII-A) has designed [he buttress to have a minimum factor of safety of 1.3. A factor of safety of l.3 is [he mitumum level of safe[}' required by the Colorado Division of Minerals and Geology (CDMG) for slopes utilized in this mamter. Historic observations in the field and past experience in this area indicates that this is a conunon soil failure consistent [o this location. Past field observations indicate the presence of an elevated water [able level shat is at its highest during the late winter/early spring thaw. During this period (late winter/early spring thaw) the increased moisture content in the soil reduces the native soil's cohesive svength parameters substantially, which in turn resuhs in soil failure below the bin wall. The soil Cailtue below the bin wall is best described, Gom field observations as a rotational slump. The base of the slump is locate 60 (eet below the bottom of the bin troll. The top of the slump scans at the base oC the bin Engineering antl 136 East South Temple. Suite 1570, Salt Lake City. Utah 84111 Telephone. (801) 3674455 Telecopy~ (801) 363-4476 EnvROnmenla Services .1 S~M1udmn •0 H.nJlnp b,i..,un . Ot}i•rs suunn~~~.4