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where <br />B =drain spacing <br />k =permeability of surrounding soil <br />qo =groundwater flow rate per area of horizontal surface <br />h =effective suppressed water table <br />Hl =depth to bottom of aquifer below bottom of trench drain. <br />The constants in the equation necessitate that all input values be in SI units. <br />The groundwater flow rate q was computed by determining the groundwater flux through the dam per <br />unit width of the dam, which was 3.04x10"' ft3/s/ft (2.38x10.8 m3/s/m) at Cross-Section C where the <br />trench drains will be installed. This quantity was then divided by the horizontal length of the dam <br />from the crest to the upstream edge of the embankment, or 100 feet (30.5 m), to obtain qo. <br />For slope stability modeling purposes, the effective suppressed water table h (height above the <br />bottom of the drains) is taken as 80% of the maximum height ho of the water table between the <br />drains. This provides an effective phreatic surface to be used for slope stability analyses. <br />Also, when H, is greater than twice the depth to the bottom of the trench drain, or H, that Hl should <br />be equal to 2xH. Because of the significant depth to bedrock near the left abutment, HI was taken as <br />2xH. The H, value was computed to be 50 feet, or 15.2 m. <br />Using the permeability of the colluvium (11.83x10"' cm/s from laboratory measured values) and a <br />computed qo of 9.28x10"10 m3/s/m, the drain spacing B was arbitrarily set and the height of the <br />effective water table above the trench bottom h was computed at the location of soil boring SB-06- <br />04. This location was selected because it is in the midst of the trench drains and a significant amount <br />of instrumentation is installed there, including three vibrating wire piezometers. The phreatic surface <br />corresponding to this h was established by setting an equivalent permeability for the trench drain in <br />the seepage model SEEP/W and iterating using different permeabilities until the particular h was <br />matched. These computed porewater pressures were then imported into the limit equilibrium model <br />SLOPE/W and a factor of safety was computed. This process was repeated until a reasonable drain <br />spacing and factor of safety were achieved. <br />The final equivalent permeability used for the trench drains in the seepage analysis was 1.22x10"5 <br />cm/s for a water table height of 0.9 feet. The final drain spacing was 30 feet. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 33 <br />