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8 Dam Analysis and Design <br />~ This section of the report discusses the seepage and stability analyses performed at the dam. The <br />objective is to determine the stability of the dam under existing and proposed conditions and the <br />impact that groundwater has on the overall system. Further, the impact of seismic loading on the <br />dam was included in the analysis. <br />8.1 See a e Anal sis <br />P 9 Y <br />The main objective of the seepage analysis was to develop a good understanding of groundwater flow <br />and how it relates to the stability of the dam. Groundwater typically plays a major role in the <br />stability of dams and slopes. The groundwater flow simulations were initially completed <br />' independently from the stability calculations. Subsequently, the groundwater flow was incorporated <br />with the mechanical calculations to capture the effects of fluid/soil interaction and its impact on <br />t factor of safety calculations. <br />The seepage analyses were conducted at Cross-Sections B and C, which were deemed to be the most <br />' representative cross-sections through the dam. The locations of these cross-sections aze depicted in <br />Figure 2, which shows a plan view of the dam. Existing and proposed geometry was modeled at each <br />cross-section. <br />The seepage simulations presented in this report model groundwater flow under steady-state and <br />transient conditions. While steady-state conditions were used for most cases, the transient modeling <br />was performed in order to compute the factor of safety for the rapid drawdown conditions on the <br />upstream slopes at Cross-Sections B and C. <br />8.1.1 Geometry <br />' Figures 3 and 4 show the existing geometry modeled at Cross-Sections B and C, respectively. Figures <br />5 and 6 depict the proposed geometry at Cross-Sections B and C, respectively. <br />' Cross-Section B cuts through the portion of the dam where the height is at its maximum. This also <br />neazly coincides with the location of the 12-inch cast-iron pipe and the three nested piezometers <br />' installed during the geotechnical investigation on the crest in soil boring SB-06-02 (Appendix A). <br />Bedrock was encountered at a depth of about 60 feet, which corresponds to Elevation 7,260. The <br />contact between the bedrock and the overlying colluvium was assumed to be horizontal. It was <br />assumed that the cast-iron pipe was placed directly on natural ground (colluvium) at about Elevation <br />7,282.7 and a starter dam was constructed above the pipe, making up the downstream face of the <br />' present dam. It is believed that a portion of the dam was constructed ofhydraulically-placed fill <br />behind the starter dam (Hydro-Triad, 1979). This was confirmed by soil boring SB-06-02, in which <br />stratified, soft sediment was encountered between Elevations 7,317.7 and 7,292.7 (Appendix A). <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignRepori\FINAL\DesignReportFINAL.doc 1 g <br />