Laserfiche WebLink
' Only the static analysis was performed for the landslide for existing conditions. No seismic analysis <br />was performed as it was only necessary to show that the proposed geometry exhibited an adequate <br />' factor of safety under seismic loading. <br />Two piezometric levels were used in the modeling: (1) the threshold water levels from Norfleet <br />' (1999) for the back-analysis and (2) the water levels measured in July 2006 when the reservoir was <br />full and the piezometers had reached near steady state. <br />9.1.4.2 Proposed Conditions <br />The proposed modifications to the dam and landslide area that have an impact on the stability of the <br />landslide include: <br />• Adding a buttress on the downstream face of the dam. This will add weight at the toe of a <br />portion of the landslide. <br />• Installing trench drains through the bench that makes up the downstream portion of the left <br />abutment. The trenches will be excavated to 25 feet deep and backfilled with freely draining <br />material. The trenches will act to draw down water levels within the landslide mass and dam. <br />• Excavating the ridge to the south of the access road for borrow material to build the buttress <br />and upstream blanket. The borrow area will consist of a flat area at Elevation 7,340 which is <br />about 150 feet wide at Cross-Section A. The back slope of the borrow area will then be <br />graded to a 3H:1V slope until it meets natural ground. This will remove a portion of the <br />driving weight of the landslide mass. <br />' • Installing an interceptor trench through the borrow area and exiting in the lower part of the <br />spillway channel. The trench will be excavated to 25 feet deep and backfilled with freely <br />draining material. The trench. will act to draw down water levels within the landslide mass. <br />Only the drawn down water levels were used in the model to simulate the impact of the trench drains <br />on the left abutment and the interceptor drain in the borrow area. <br />9.1.4.3 Static <br />Most cases were analyzed using static equilibrium in which no horizontal seismic acceleration was <br />applied in the model. <br />9.1.4.4 Seismic <br />In addition to the static analysis of the proposed geometry and groundwater conditions, a seismic, or <br />pseudo-static, analysis was also performed. The seismic loading consisted of a horizontal <br />acceleration of O.15g. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 43 <br />