2008-03-05_REVISION - M1997054 (23)

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i7 1 Design Analysis and Criteria The proposed mining and reclamation slopes were analyzed using the XSTABLE V5.208 DOS computer program. XSTABLE was designed to analyze the slope stability of an earth embankment subjected to several critical situations that may occur during the life of the embankment. Two cases dictated by the State Engineer were investigated: a steady state case and a rapid draw down case. Case 1: Assumes the embankment is in a steady state scenario; the ' required factor of safety 1.5. Case 2: Assumes the embankment is in a rapid draw down state; the required factor of safety is 1.2. For the mining conditions, the scenario of maximum highwall dewatering was the only scenario analyzed. Methodology ' The Bishop method was used in the computer analysis for determining the safety factors. The procedure utilizes a critical failure surface searching method to generate circular surfaces. The program automatically searches for the lowest factor of safety. 600-1200 separate failure surfaces were analyzed for each case. The same process was used in determining the safety factor for each case. For the given soil parameters, the factor of safety was determined. The ' cohesion and friction angle of the granite fines were then varied to determine the soil properties of the minimum allowable safety factor. ' The mining embankment configuration shown in the computer analysis represents the estimated conditions for this site. If the mining conditions differ significantly from the estimated conditions, the slope stability will need to be reevaluated on a case by case basis. The mining highwall was found to be stable and well above the factor of safety. The reclamation embankment analysis illustrates the slope failures consist of minor sloughing r failures indicative of erosion and maintenance requirements. The attached results show the failure slopes and the associated property values and safety factor. The resulting property values 1 are conservative for anticipated site conditions. Slope Stability Results ' The minimum safety factors required are 1.2 for the rapid draw down condition and 1.5 for steady state. This design criteria was used to establish the desired minimum safety factors for this project and should be considered conservative for evaluating reclaimed slopes against alluvial mining high walls. The calculated factors of safety are within the design criteria specified for this project and can be considered indicators of high wall performances under the various conditions. The results of the slope stability analysis are presented in Table 2.