2007-08-10_PERMIT FILE - C1980007 (3)

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' Spencer's method was used as the search technique to determine the factor of safety of the embankment in this stability analysis. This method is considered the most adequate because it ' satisfies all conditions of static equilibrium and provides a factor of safety based on both force and moment equilibrium. ' 8.2.3.1 Existing Conditions Cross-Sections B and C were modeled using the existing geometry. Both USSA and ESSA modeling scenarios were performed. Only the static analysis was performed for the dam for existing conditions. No seismic analysis was performed as it was only necessary to show that the proposed geometry exhibited an adequate factor of safety under seismic loading. The porewater pressures used in modeling were those from the seepage model calibration based on piezometer measurements from July 21, 2006. 8.2.3.2 Proposed Conditions The proposed modifications to the dam include: • Adding a buttress on the downstream face of the dam. This will add weight at the toe of the dam. Also, a pervious drainage blanket will be installed underneath the buttress material to prevent high porewater pressures from accumulating. • Placing an upstream clay blanket that will act to limit seepage into the higher permeability "soft sediment" layer and lower porewater pressures throughout the dam. • Installing trench drains through the bench that makes up the downstream portion of the left abutment. The trenches will be excavated to 25 feet deep and backfilled with freely draining material. The trenches will act to draw down water levels within the dam. Because the trenches will only be installed on the left abutment, they were only included in the model at Cross-Section C. In addition to the static analysis of the proposed geometry and groundwater conditions, a seismic analysis was performed using the pseudo-static approach. Furthermore, a rapid drawdown stability analysis was also performed on the upstream face of the ' dam. The seepage portion of the rapid drawdown analysis, also referred to as a transient analysis, is described in Section 8.1. The limit equilibrium calculations were performed for the August 28, 2006, t reading when the reservoir had just emptied completely. This is assumed to be the most critical moment at which the factor of safety reaches a minimum value. It should be noted that the USSA strength parameters were used for all materials except for the "soft sediment". The drained strength P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 30