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• A plunge pool to minimize the potential for channel erosion at the point of outlet pipe <br />dischazge. <br />4.2 Landslide <br />The proposed modifications to the dam and landslide area that have an impact on the stability of the <br />landslide include: <br />• A buttress on the downstream face of the dam. This will add weight at the toe of a portion of <br />the landslide and increase the factor of safety of the landslide. <br />• Ahigh-permeability drainage blanket that will be placed underneath the buttress material to <br />prevent high porewater pressure buildup in the buttress material. <br />• Trench drains through the bench that makes up the downstream portion of the left abutment. <br />The trenches will act to draw down water levels within the landslide mass and the dam. Any <br />decrease in porewater pressures in the landslide will improve stability. <br />' Excavating the ridge to the south of the access road for borrow material to build the buttress <br />and upstream blanket. The borrow azea will consist of a flat azea at Elevation 7,340 which <br />will be about 150 feet wide at its widest. The back slope of the borrow azea.will then be <br />' graded to a 3H:1V slope until it meets natural ground. This will remove driving weight from <br />the landslide mass and improve its stability. <br />• An interceptor trench through the borrow area that will exit in the lower part of the spillway <br />channel. The trench will be excavated to 25 feet deep (the maximum reach of a backhoe) and <br />backfilled with freely draining, granular material. The trench will act to draw down water <br />levels within the landslide mass, thus improving stability. <br />• A ditch to drain the yeaz-round ponded water at the upper landslide scarp about 650 feet <br />southeast of the access road. The water will be drained into the natural drainage Swale east <br />of the borrow area and west of the camping area located south of the access road. This <br />ponded water is viewed as a significant source of water infiltration into the landslide mass. <br />Eliminating this permanent source of water will inhibit the infiltration of water into the <br />landslide mass and increase the factor of safety. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 6 <br />