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' 9.1.5 Summary of Landslide Stability Analysis <br />Table 15 shows the results of the stability modeling of the landslide when water levels were varied <br />' within the sliding mass. As described above, no seismic loading was applied to the existing <br />geometry. <br />Table 15 Summary of Factors of Safety for Landslide Stability, Existing <br /> <br /> Case Water Levels Factor of Safet <br />' Static Threshold 1.00 <br /> Static Jul 2006 1.07 <br />It can be seen in Table 14 that, as described above, the factor of safety is one when the threshold <br />' water levels are used. This is expected because the constraints of the back-analysis dictate that the <br />friction angle of the failure surface material is adjusted until a factor of safety of one is achieved. <br />' When the July 2006 water levels were used, the factor of safety increased to 1.07, which represents <br />the stability of the landslide under normal conditions (i.e. not during spring thaw when the porewater <br />pressures increase in the landslide). <br />Table 16 shows the modeling results when all of the proposed modifications described in Section <br />9.1.4.2 have been integrated. <br />Table 16 Summary of Factors of Safety for Landslide Stability, Proposed <br />Case Factor of Safet <br />Static 2.18 <br />Seismic* 1.00 <br />'-Pseudo-static with horizontal seismic <br />acceleration of 0.158 <br />It can be seen in this table that the controlling case was the condition in which seismic loading was <br />applied. The geometry of the borrow area was increased until enough weight had been removed from <br />the landslide and the interceptor trench had drawn down water levels sufficiently that a factor of <br />safety of one was obtained. When the static case was modeled using the same geometry, the factor of <br />safety increased dramatically to 2.18. This shows that under typical conditions, the stability of the <br />landslide (through Cross-Section A) will be significantly improved. <br />9.2 Landslide Drain Design <br />In addition to the trench drains that will be installed in the bench comprising the downstream, left <br />dam abutment, two drains will act to reduce porewater pressures in the landslide mass. They are the <br />interceptor drain and the upper pond ditch. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 44 <br />