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CESARE,INC. <br /> Rapid drawdown of the reservoir was analyzed with a drawdown rate of 1 foot per day. Pore <br /> pressures were computed using transient seepage analysis. Slope stability analyses were performed <br /> during drawdown which generated a minimum factor of safety versus time plot. <br /> SECTIONS ANALYZED <br /> Two sections were chosen for analysis to capture stratigraphic differences across the site. Boring <br /> IT-B9 at Station 31+52 was chosen to represent areas within the site covered by relatively deep <br /> sand and gravel deposits. This section contains a relatively deep deposit of permeable soil and <br /> represents areas where seepage potential is expected to be the highest. This section was analyzed <br /> for both steady state seepage and slope stability during rapid drawdown. <br /> Boring IT-B12 at Station 44+60.39 was chosen to represent areas within the site covered by <br /> relatively deep clay deposits. Because the clay soils were assigned the lowest shear strength <br /> values, this section will represent areas where the risk of slope instability is the highest during rapid <br /> drawdown. This section was analyzed for slope stability during rapid drawdown. <br /> Bedrock and groundwater elevations were judged to be relatively stable across the site and were <br /> not significant factors in the choice of sections for analysis. For conservatism, a groundwater <br /> elevation located at the top of the gravel/sand layer was used in our analyses. <br /> MATERIAL PROPERTIES <br /> Hydraulic conductivity values for analysis were developed. For the weathered shale bedrock, <br /> hydraulic conductivity values were based on the Packer test results performed in Borings IT-B1, JT- <br /> B3, JT-135, JT-B7, IT-B9, JT-B11, and IT-613. The results are presented in Table 2. <br /> TABLE 2. Packer Test Results <br /> LAVerowilwow <br /> ground level) <br /> JT-B1 - 0 72- 89 <br /> JT-B3 8.46 x 10-5 72.5-82.5 <br /> JT B5 1.59 x 10*s 70—83 <br /> _ JT-137 1.44 x 10`5 76.5—85.5 <br /> JT-B9 9.53 x 10'S 69—76 <br /> JT-B11 9.74 x 10-' 72—84 <br /> JT B13 0 72 - 90 <br /> For sand, clay, and gravel, hydraulic conductivity values were based on Seepage, Drainage, and <br /> Flow Nets, 3rd Edition by Cedergren. For the slurry cutoff wall, the hydraulic conductivities of 1x10- <br /> 6 and 1x10"' were evaluated. Presumptive ratios of horizontal to vertical hydraulic conductivity for <br /> each material were assigned as found in the USACE/CA DWR guidance document. <br /> The unit weight, cohesion, and friction angles of the materials encountered are estimated values <br /> based on published literature and our experience in the area. <br /> 11.124 GP Ranches Memo 03.21.1303.21.13 2 <br />