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06-04. The calibration of the model made possible the close agreement between computed and <br />measured values. Also at Cross-Section C, seepage is shown emerging on the downstream face at two <br />locations: (1) at about Elevation 7,320 at the slope break between the left abutment bench and the <br />upper dam and (2) at the toe of the left abutment bench at about Elevation 7,275. The drain pipe that <br />is installed at about Elevation 7,300 in the field is likely responsible for the fact that no seepage was <br />observed by Barr personnel above the left abutment bench. This pipe was not included in the analysis <br />because information about the details of the pipe is not readily available. <br />The seepage for the proposed geometry at Cross-Section C also shows a similar pattern. The main <br />difference is the significant drop in porewater pressures at the location of the trench drains, though <br />some head loss will occur across the upstream clay blanket. In the piezometers installed at SB-06-03, <br />the heads are predicted to drop by 5 to 8 feet, while the heads in all three piezometers in the SB-06- <br />04 boring are expected to drop by about 10 feet. <br />The transient seepage analysis indicates that the water stored in the embanlanent during steady state <br />conditions drains out of the embanlanent on the upstream face of the dam, as expected. The plots <br />showing the total heads and phreatic surfaces at a time corresponding to August 28, 2006, are <br />included in Appendix C. This date is the first measurement after the reservoir had drained completely <br />and all rapid drawdown stability analyses were performed based on this worst-case condition. See <br />Section 8.2.3.2 for a description of the rapid drawdown stability analysis. <br />8.2 Slope Stability Analysis <br />The main objective of the slope stability analysis was to evaluate the stability of the slope under <br />existing, proposed, and seismic loading conditions. A special emphasis was placed on evaluating the <br />impact of groundwater flow on stability. The slope stability analyses were performed on Cross- <br />Sections Band C. <br />Two types of stability analyses are typically performed for dams and slopes: the Undrained Strength <br />Stability Analysis (LTSSA) and the Effective Stress Stability Analysis (ESSA). The USSA is <br />performed to analyze the case in which loading or unloading is applied rapidly and excess porewater <br />pressures do not have time to dissipate during shearing. This scenario typically applies to loading <br />from, for example, embankment construction where the loading takes place quickly relative to the <br />permeability of the soils. Seismic loading is also assumed to mobilize the undrained shear strength of <br />materials. It is often referred to as the "end-of-construction" case. <br />t The ESSA is performed to account for much slower loading or unloading, or no external loading, in <br />which the drained shear strength of the materials is mobilized and no excess porewater pressures are <br />' allowed to develop. For example, a slowly moving landslide is best analyzed using the ESSA. For <br />this reason, the ESSA is often referred to as the "long term" case. <br />' P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 24 <br />