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6.5- Geotechnical Stability Exhibit Methodology SB-Slope, v. 3.0 {Von Gunten Engineering Software, Inc.) is the slope stability application utilized for this application. The program uses the "Simplified Bishop Method of Slices" based on the limiting equilibrium circular failure surface method as developed by A.W. Bishop (1955). The program was used to predict the factor of safety for a given setback from the mine excavation where the factor of safety is defined as: Z(available shear strength)/T~(required shear strength) = Factor of Safety (FS) For each setback analyzed, the critical failure surface was determined. Data Input Model input data is based on test hole boring data. Soil strength data is based on published values and data provided by the Colorado Division of Minerals and Geology. Table 6.5.1 -Soil Strength Parameter data Soil Density Cohesion Number Stratigraphic Layer (pfc) (psf) Phi (~) 1 Overburden (ML) 90 3.6 34 2 Unsaturated Gravel 130 0 35 (GW-UNSAT) Saturated Gravel 3 (GW-SAT) 137 0 35 4 Weathered Shale 142 0 14 5 Shale 150 500 25 The mine slope was divided into three different sections: east, southwest and northwest mine slopes. For each section, the stratigraphic layer thickness was determined by averaging the nearest test holes. Soil thickness information is shown in Appendix I. The model input phreatic surface was determined by the following analytical solutions. The Theis method was used for approximating drawdown at distances greater than 100 ft. Dupuit-Forsheimer solution was used for approximating the drawdown geometry (phreatic surface) close to the seepage face. The mine slope was set to 0.5 to 1. Model Output The model output information is shown in Appendix I. The minimum setback and critical surface factor of safety are shown below.