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proposed mining setback was set approximately 60 feet from the existing <br />overhead poles. <br />Case West — Oil /Gas Structure (WEST). The mining operation is adjacent to <br />private property with an existing oil and gas structure. The mining depth was <br />assumed to be 26 feet in this area based on bore log information in the <br />geotechnical data. The proposed mining setback was set approximately 60 feet <br />from the existing structure. <br />The cross - sections located in Appendix B show the estimated phreatic surface <br />associated with each case as well as the geometry used in the mining. <br />METHODOLOGY <br />The mining embankment configuration shown in the computer analysis represents the <br />estimated conditions for this site. If mining conditions differ from the estimated <br />conditions, the slope stability will need to be re- evaluated on a case by case basis. The <br />Bishop Method was used in the computer analysis for determining safety factors. The <br />procedure searches for circular shear failures and automatically searches for the lowest <br />safety factor. 1000 separate failure surfaces were analyzed for each case. The required <br />minimum safety factors are based on the current standards used by the Colorado State <br />Engineer's Office (SEO) in evaluating embankment dams, and industry accepted <br />standards for the evaluation of temporary structures during construction. <br />SLOPE STABILITY RESULTS <br />The SEO requires minimum safety factors of 1.3 for static condition analyses and 1.0 for <br />pseudo- static (earthquake loading) condition analyses for Class I (high hazard) <br />embankment dams. This design criteria was used to establish the desired minimum <br />safety factors for this project and should be considered as highly conservative for <br />evaluating alluvial mining slopes. The calculated factors of safety are within the design <br />criteria specified for this project and can be considered indicators of the mining slope <br />performance under the various conditions. The slopes were analyzed using full and <br />empty reservoir conditions. The results of the static condition and pseudo- static <br />condition slope stability analyses in Table 2 and Table 3. <br />Northern Colorado Constructors, Inc. — Perry Pit <br />Slope Stability Analysis <br />JdtT Consulting, Inc. Pa e 3 <br />9 <br />