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' San Luis Mine Phase II, Raise 2 Design Report ' Stability analyses of the upstream face of the embankment raise considered potential failure surfaces which involved the full height of the raise. As the analyses considered the critical condition for this ' portion of the raise whereby no tailings are impounded by the raise, a failure corresponding [o the conditions analyzed would not compromise containment of the tailings. If a failure of the upstream face occurs, there would be an opportunity to re-construct the failed portiod prior to it being necessary to contain tailings. As the tailings level in the impoundment rises, stability of the upstream face will increase. ' The following sections present the results of the stability analyses conducted under static and dynamic loading conditions. 4.5 Static Loading Conditions ' Through repetitive analyses, the lowest factor of safety under static loading for any failure surface considered was found to be 2.76 indicating that stability of the structure is within normally ' acceptable limits. This factor of safety was calculated for a rotational failure of the downstream face which involves only the embankment raises. A factor of safety of 2.87 was indicated for both ' a sliding wedge type of failure with the VLDPE liner as the basal sliding plane and a circular failure extending from the crest to the toe of the structure. These potential failure surfaces are depicted on Figure 11. The lowest factor of safety computed under static loading for the upstream face of the embankment in its raised configuration was 2.94. This failure surface is depicted on Figure 10 as well as the ' lowest factor of safety sliding wedge type failure surface for the upstream face. Also included are potential failure surfaces which extend deeper, into the saturated tailings. As the level of tailings within the facility increases the factor of safety for these potential failure modes Of the upstream face will increase. ' 4.6 Dynamic Loading Conditions ' 4.6.1 Pseudostatic Stability As per the approved Permit and Amendment, the pseudostatic seismic coefficient adopted for the ' design of facilities with long-term exposure is O.15g. The lowest factor of safety computed for any failure surface utilizing this seismic coefficient is 1.1. This factor of safety was calculated for a ' sliding block type failure of the downstream face of the embankment involving basal sliding along the VLDPE liner as shown on Figure 11 and reflects the conservative shear strength parameters employed and the two dimensional nature of the analysis. Studies have shown that the "end effects" associated with the actual three dimensional nature of a failure surface typically result in a 10 u y 4-6 ro~ect o.