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Table 9 shows the results of the seismic modeling for proposed conditions. As described previously, <br />the seismic analyses were performed as part of the USSA only. These results are for the downstream <br />face. <br />Table 9 Summary of Factors of Safety for Stability of Dam, Seismic <br />Cross- <br />Section <br />Geomet <br />Case <br />Factor of Safet <br />B Pro osed USSA 1.21 <br />C Pro osed USSR 1.00 <br />These factors of safety are considered adequate for the case in which seismic loading is applied. <br />Seismic stability was the controlling case at Cross-Section C. The buttress and other modifications at <br />Cross-Section B increase the factor of safety to 1.21 for the seismic case, but they are also needed to <br />increase the seismic factor of safety of the landslide. The landslide stability is discussed in detailed <br />further in Section 9.1.4. <br />Table 10 shows the results of the modeling for a rapid drawdown under proposed conditions. These <br />results are for the upstream slope. <br />Table 10 Summary of Factors of Safety for Stability of Dam, Rapid Drawdown <br />Cross- <br />Section <br />Geomet <br />Case <br />Factor of Safet <br />B Pro osed USSA 1.96 <br />C Pro osed USSA 1.90 <br />It can be seen in this table that factors of safety against rapid drawdown are adequate. <br />8.3 Trench Drain Analysis and Design <br />Parallel trench drains will be installed to reduce porewater pressures within the left dam abutment <br />and the landslide mass. The depth of the drains is constrained by the depth to which a backhoe can <br />reach, which has been established at about 25 feet. The spacing of the drains, however, was <br />calculated based on the method in Forrester (2001). <br />The method is based on two-dimensional flow to parallel drains. The equation for computation of <br />drain spacing is <br />BZ < k h2 + h H' <br />qo 0.16 0.1 <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 32 <br />