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The Phase II, Raise 2 Design Report states that phreatic assumptions for the analyses were <br />included on the figures in that report, but the phreatic line cannot be found on the report figures <br />and verified. From examination of the computer printouts of the stability analyses, it is believed <br />that the analyses were conducted for normal operating conditions during active tailing <br />deposition. These conditions would have evaluated slope stability with much more free water, <br />along with the saturated tailing slurry in the impoundment, compared to the relatively minor <br />amount of free ponded water and drained tailing conditions that exist now. As such, the previous <br />analyses for factors of safety for static stability are believed to be conservative when applied to <br />evaluate current conditions for static stability with a smaller free -water pool and drained tailing <br />in the impoundment area. <br />As discussed later in this report, during extreme flood events that exceed the design capacity of <br />the storm water diversion systems, storm water will enter the impoundment area. Temporary <br />flood loading is potentially greater than normal operating (static) stability conditions that were <br />analyzed at the time of design for active facility operation. This condition was not evaluated as <br />part of the original design, so additional analyses for stability under flood loading were <br />performed as part of this dam safety evaluation, as described in Section 3.3.3. <br />Rapid drawdown stability also was not included in the original design analyses, but this <br />condition would not be expected to occur, then or now, as there was /is no rapid exit for water to <br />result in a condition of rapid drawdown loading on the upstream face of the dam. <br />3.1.2 Results of Previous Studies <br />The Phase II, Raise 2 Design Report concluded that the tailing are dilative based on consolidated <br />undrained triaxial testing. This means the tailing materials are expected to have reduced strength <br />under the design earthquake event, but will not exhibit true liquefaction (zero shear strength) <br />behavior. The report concluded that saturated tailing should be assigned drastically reduced <br />shear strength for dynamic stability analyses. <br />Results of stability analyses from the original Design Reports are reproduced in Figure 4 and <br />Figure 5. As discussed in the previous section, these were performed using a pool elevation of <br />approximately 8613, which is above current normal operating conditions (free -water pool <br />elevation typically is at about 8608). The "saturated tailing" (Unit B) unit shown in the analyses <br />is likely currently unsaturated, as the current pool lies some 600 feet upstream from the dam <br />embankment. It is not likely that saturated tailing exist within the views (or failure surfaces) <br />presented in either of these analyses. Thus, Unit B should be equivalent to strengths shown for <br />Unit A in the original analyses, which would improve results of all analyses shown where <br />failures encompass Unit B. Additionally, strengths of both Unit A and Unit B have likely <br />improved since the time of the original analyses due to consolidation over time (it was <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 8 February 2014 <br />