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REFUSE PILE EXPANSION j. ..' of the existing topography and for each construction phase. The regional stability of the mountainside was investigated using a profile extending down the entire hillside, through Highway 133, to the North Fork of the Gunnison River. The computational results are presented in Appendix K. The Purdue University PCSTABLS and Geo-Slope International Ltd. SLOPE/W (Version 3, 1995) software programs were used for the slope stability analysis to determine factors of safety against failure. The factor of safety computed for each run is the ratio of the forces resisting the slope movement to the forces which cause the slope to fail. The resisting forces are determined by the shear strength of the imported soils to resist failure along a specific surface. Factors which influence slope failure include, but are not limited to: (a) angle of the slope; (b) location of existing roads and drainage ditches in relation to the slope; (c) pore pressure or water table acting on the failure plane; and (d) geotechnical characteristics of the coal refuse, native soil and bedrock found in [he slope. A factor of safety of ].0 implies that the slope is marginally stable for the given stratigraphic profile, native clay/bedrock material strengths, groundwater levels, and slope conditions. A factor of safety less than one indicates impending failure. HLA has designed the RPE to have a minimum factor of safety of I.5 under static loading, and 1.0 under psuedostatic loading. A factor of safety of 1.5 is the minimum level of safety required by the Regulations of the CMLRB for Coal Mining regarding slopes on which coal refuse is placed. 5.5.1 Critical Sections Four profiles were used to model the internal stability of the RPE and the underlying native soil; Figure B-1 shows the location of the cross-sections. A fifth profile, shown on Figure K-I, was used [o model the Highway 133 stability. A sixth profile, Figure K-3, was used to model [he regional stability. The physical conditions of each profile is as follows: • Profile One encompasses the western half of the proposed RPE. A haul road will be developed perpendicular to [his profile. Sylvester Gulch lies parallel ro this profile. • Profile Two bisects the thickest and widest wedge of the RPE during Phase 3 and Phase 4. • Profile Three is located in the central portion of the proposed RPE. This profile also bisects the sediment control pond at the base of the proposed RPE. • Profile Four encompasses the eastern edge, and steepest portion of the natural terrain found in the proposed RPE. • Profile Five bisects the Highway 133 road fill above the culvert that drains the wetland. This slope has experienced shallow soil erosion in the past. The proposed RPE sediment control pond will utilize this slope as part of the pond • Profile Six, used for regional stability investigation. spans from upslope of the RPE, northward to across Highway 133 and down to the North Fork of the Gunnison River. 5.5.2 Parameter Selection Parameters used in the slope stability software programs to determine factors of safety for the proposed RPE are as follows: Proposed RPE Geometry: The RPE stability analysis incorporates the proposed slope geometry of Phase I, 2, 3 and 4 as it occurs during the RPE development. Hiehwav 133 Geometry: The stability analysis for the road fill above the culvert draining the wetland incorporates present slope geometry. Groundwater: Groundwater levels are assumed to be located at the bedrock interface. Groundwater has been noted at this interface during previous drilling programs at the WEM. The slope stability analysis assumes a moderate to low groundwater level in all models. Soil Streneth Properties: Five soil types were used in the model. The soil strength parameters are based on recent RPE Harding Lawson Associates 9