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Seepage Analyses <br /> Seepage into the reservoir through the slope liner was modeled using the Seep/W 8.0.2 computer <br /> program. The embankment modeled was based on the assumed maximum typical section located on the <br /> northwest side of the Phase II mine. <br /> The typical highwall scenarios were modeled using the design slope liner geometry(i.e.4-foot cutoff in <br /> low permeable bedrock),conservative groundwater levels outside of the slope liner and typical hydraulic <br /> conductivities. Results of the seepage analyses and the hydraulic conductivities used are summarized on <br /> Table 3. A computer generated cross-section summarizing the analysis is shown in Attachment D. <br /> Parameters and calculated results,including hydraulic conductivities,groundwater levels(phreatic <br /> surface), and resulting flux through the slope liner are shown on the cross-section. <br /> The State Engineer allows a design standard seepage rate of 0.03 ft3/day/ft2 of vertical slope liner. <br /> Multiplying the allowable rate by the height of the section used in the analysis results in an allowable <br /> seepage rate per linear foot for that particular section. The allowable seepage rates and modeled seepage <br /> rates are summarized in Table 3. The modeled seepage rates are all below those allowed under State <br /> Engineer regulations. <br /> Stability Analyses <br /> Limit equilibrium stability analyses were performed using the Slope/W 8.0.2 computer program on the <br /> same typical slope liner embankment as the seepage analysis. Conservative strength parameters were <br /> utilized based on our experience in the area. The slope liner was assumed to be constructed with <br /> compacted sandy clay or clayey sand obtained from the overburden on-site. <br /> The strength parameters and properties utilized in the analyses are summarized in Table 4. Analyses were <br /> performed for the following scenarios: <br /> • Empty reservoir <br /> • Full reservoir <br /> • Rapid drawdown <br /> • Full reservoir with a seismic component(0.1g horizontal acceleration) <br /> Resulting factors of safety are summarized and compared to State Engineer required factors of safety for <br /> jurisdictional dams on Table 4. Annotated cross-sections of the analyses are provided in Attachment E. <br /> The analyses show that with conservative soil parameters and groundwater levels, the slopes meet the <br /> State Engineer's required factors of safety for a jurisdictional dam. For each case, Slope/W created 2,100 <br /> potential failure planes(slip circles)and calculated the factor of safety. The factor of safety for the most <br /> critical surface is shown on Table 4 and in the cross-section in Attachment E. The factor of safety for <br /> each slope surface was calculated using the Spencer limit equilibrium method. The Spencer method is the <br /> most rigorous, taking into account both force and moment equilibrium. The Slope/W program <br /> incorporates a search routine to locate those failure surfaces with the least factor of safety within user <br /> defined search limits. Trial failure surfaces were defined with "entry and exit" parameters, resulting in a <br /> range of possible locations to search for the critical (lowest factor of safety) potential failure surface. <br /> Analyses were performed using Mohr-Coulomb failure criteria for the materials. <br /> Page 4 of 5 <br />