2015-10-15_INSPECTION - M1980244

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Continued. 2.0 Current Conditions Brett Byler of Amec Foster Wheeler visited the SGVLF PSSA on September 9, 2015. Photographs taken during the site visit are presented in Appendix A. At the time of the visit, conditions at the PSSA consisted of: • Drain Cover Fill: DCF completely covers the primary geomembrane of the PSSA, with exception of some areas where the DCF has eroded down to the primary geomembrane as a result of surface water runoff of exposed upgradient geomembrane. Photos 2 and 3 (Appendix A) show areas of erosion of the DCF within the PSSA. • Spent Ore: An approximately 100 -foot high lift of spent ore has been placed along the east side of the PSSA. The spent ore lift toes out on the bottom of the PSSA which is nearly flat. Subsequent to the site visit, the geometry of the spent ore relative to the PSSA liner system was developed by comparing a Lidar survey conducted on June 3, 2015 to the design grading plan. The extent of the spent ore lift is shown on Figure 1 and Photos 1, 3 and 4. As can be seen in the photos, some sloughing of the spent ore has occurred. • Pool: Water has collected within the PSSA as a result of runoff of surface water. Based on pool elevation monitoring data conducted by Foresight West Surveying, Inc. (FWS), the pool elevation on September 10, 2015 was 9,377.9 feet. 3.0 Stability Evaluation Slope stability analyses were conducted for two cross-sections within the SGVLF PSSA based on current conditions. Stability was evaluated for both static and seismic loading conditions. Both block and circular failure surfaces were evaluated. 3.1 Methodology Stability analyses were conducted using Slide Version 6.036 (Rocscience, 2010), a commercially available computer program that uses conventional limit equilibrium methodology using a variety of methods. For the failure mechanisms considered in the analyses, slope stability was evaluated using limit equilibrium methods based on Spencer's method of analysis (Spencer, 1967). Spencer's method is a method of slices (consideration of potential failure masses as rigid bodies divided into adjacent regions or "slices," separated by vertical boundary planes) that satisfies both moment and force equilibrium. It is based on the principle of limit equilibrium (i.e., the method calculates the shear strengths that would be required to just maintain equilibrium along the selected failure plane, and then determines a safety factor by dividing the available shear strength by the driving shear stress). Consequently, safety factors calculated by Spencer's, or by any other limiting equilibrium method, indicate the percentage by which the available shear strength exceeds, or falls short of, that required to maintain equilibrium. Therefore, safety factors in excess of 1.0 indicate stability and those less than 1.0 indicate instability, while the greater the mathematical difference between a safety factor and 1.0, the larger the margin of safety (for safety factors in excess of 1.0), or the more extreme the likelihood of failure (for safety factors less than Page 2 of 7