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(ESSA) was used because of its higher permeability relative to the other materials in the cross- <br />sections. <br />8.2.3.3 Static <br />Most cases were analyzed using steady-state seepage equilibrium in which no horizontal seismic <br />acceleration was applied in the model. <br />8.2.3.4 Seismic <br />Horizontal seismic accelerations were applied to the model to simulate earthquake loading for the <br />proposed geometry. This was done using the pseudo-static approach, in which a fraction of the <br />acceleration due to gravity g is applied to the slices in the model. The acceleration increases the loads <br />on the slices and acts to reduce the factor of safety accordingly. <br />The seismic loading consisted of a horizontal acceleration of O.15g. For these model runs, only the <br />USSA was performed, thus undrained shear strengths were used for each material type. An <br />earthquake load is assumed to arrive quickly, not allowing significant drainage to occur as the load is <br />applied. <br />8.2.4 Results of Stability Analyses <br />Table 8 shows the results of the dam stability modeling for existing and proposed conditions for both <br />the USSA and ESSA cases. These results are for the downstream face and static conditions only. <br />Table 8 Summary of Factors of Safety for Stability of Dam, Static <br />Cross-Section Case Existin Pro osed <br /> USSA 1.07 1.91 <br />B ESSA 1.28 2.42 <br /> USSA 1.24 1.87 <br />C ESSA 1.00 1.69 <br />It can be seen in Table 8 that the factors of safety for the existing USSA at Cross-Section B and the <br />existing ESSA at Cross-Section C are quite low, indicating inadequate stability. They are 1.07 and <br />1.00, respectively. It should be noted that the factor of safety for the existing ESSA at Cross-Section <br />C was the result of aback-calculation and the actual factor of safety could be higher. However, the <br />dam exhibits factors of safety under existing conditions which are not adequate. <br />It can also be seen that the proposed modifications increase the dam factor of safety considerably. <br />The lowest factor of safety reported for static conditions is 1.69 for the ESSA at Cross-Section C. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 31 <br />