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Case 4 -Existing overhead electric transmission line and WCR 25 right-of-way to <br />the east. The mining operation is adjacent to an overhead electric transmission line and <br />the WCR 25 right-of way in this area. The proposed setback for mining is 60 feet from <br />the transmission line and right-of way. The mining depth was assumed to be 37 feet in <br />this area based on bore log information in the Terracon report and liner investigation. <br />The mining side slope in this location will be near vertical for the bottom half of the slope <br />and 0.5h:1v for the top half of the slope. <br />Case 5 -Existing overhead electric transmission line to the west. The mining <br />operation is adjacent to an overhead electric transmission line in this area. The proposed <br />setback for mining is 55 feet from the transmission line. The mining depth was assumed <br />to be 27 feet in this area based on bore log information in the Terracon report and liner <br />investigation. The mining side slope in this location will be near vertical for the bottom <br />half of the slope and 0.5h:1v for the top half of the slope. <br />The cross-sections located in Appendix B show the proposed mining slope geometry, estimated <br />phreatic surface, location of the man made structures relative to the mining slope, and location <br />of the most critical slope failure surface for each case. <br />V-METHODOLOGY <br />The mining embankment configuration shown in the computer analysis represents the estimated <br />conditions for this site. If mining conditions differ from the estimated conditions, the slope <br />stability will need to be re-evaluated on a case by case basis. The Bishop Method was used in <br />the computer analysis for determining safety factors. The procedure searches for circular shear <br />failures and automatically searches for the lowest safety factor. 1,000 separate failure surfaces <br />were analyzed for each case. The required minimum safety factors are based on the current <br />standards used by the Colorado State Engineer's Office (SEO) in evaluating embankment <br />dams, and industry accepted standards for the evaluation of temporary structures during <br />construction. <br />VI -SLOPE STABILITY RESULTS <br />The SEO requires minimum factor of safety of 1.25 for static condition analyses and 1.0 for <br />pseudo-static (earthquake loading) condition analyses for Class I (high hazard) embankment <br />dams. This design criteria was used to establish the desired minimum factors of safety for this <br />project and should be considered as highly conservative for evaluating alluvial mining high <br />walls. The calculated factors of safety are within the design criteria specified for this project and <br />can be considered indicators of the high wall pertormance under the various conditions. The <br />results of the static condition and pseudo-static condition slope stability analyses are presented <br />in Table 2 and Table 3. <br />NCCI Pii #1 <br />Slope Stability Analysis <br />Page 3 <br />