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3.3.2 Static Stability <br />Previous static slope stability analyses for the dam described in Section 3.1.2 indicate very high <br />static factors of safety of 2.7 or better, which are well above the typical design standard <br />minimum of 1.5, for both upstream and downstream slopes. For reasons also described above, <br />these conditions currently are improved from previous analyses due to a much smaller free -water <br />pool and consolidation and drainage of the tailing. No further static analyses are deemed <br />necessary. <br />3.3.3 Stability under Flood Loading <br />Under current conditions, large magnitude storm events on the order of the 100 -year event and <br />larger floods are expected to bring additional water into the impoundment, as described in <br />Section 4. These temporary impoundments of storm water have the potential to re- saturate a <br />portion of the tailing and could result in a higher phreatic line than was assumed in previous <br />static stability analyses. <br />It is not known exactly what portion of the flood would be routed out of the impoundment during <br />a large magnitude flood, so the worst possible case has been analyzed here. As discussed later <br />in Section 4.0, assuming entire capture of the PMF, water level in the impoundment would rise <br />to approximately elevation 8618. Water would then be recirculated until evaporated away, but <br />for purposes of simplifying analysis, it is assumed the water is held long term as a worst case. <br />Figure 11 illustrates the seepage front that develops during the worst case assumption of a <br />permanent PMF flood pool held in the facility. Most of the tailing become saturated with a steep <br />vertical gradient under this assumption, with an unsaturated zone forming above the liner and <br />along the upstream face of the dam where the liner drain and embankment rapidly remove <br />seepage to the collection pond. <br />Figure 12 illustrates a stability analysis computed under this assumption. Both block and <br />circular failures were computed. Block failure was the worst case, with a slip occurring along <br />the liner for which the previous design had assumed a conservative liner friction angle of 18 <br />degrees. The computed factor of safety is high at 2.3 and is very similar in shape to the worst <br />case Failure 4 from the previous design illustrated in Figure 5 that had a factor of safety of 2.9. <br />Factor of safety is likely lower due to higher phreatic conditions under a PMF than existed <br />during normal operations at the time of active tailing deposition. The computed factor of safety <br />of 2.3 greatly exceeds the typical design standard of 1.3 to 1.4 for flood loading conditions. <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 18 February 2014 <br />