2002-03-29_REVISION - M1999034 (2)

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same design and construction specifications of the existing slurry wall. The new slurry wall tie- in is intended to intersect the existing perimeter slurry wall and extend a minimum of 10 feet outside of it at the bottom of the excavation. It is critical that the exact location of the perimeter slurry wall be established in the field prior to construction of the tie-in. The crest width of the interior dike where the proposed slurry wall is being constructed will be a minimum of 80 feet across. The extra width is to provide adequate construction room for the slurry wall. If a contractor would propose an alternate method of construction for the slurry wall that did not require this extra width, it would be acceptable to reduce it to the same section as the remainder of the interior dike. A detail of the design can be seen in the Arvada Reservoir Construction Plans on Sheet 7. After the proposed slurry wall tie-in, the top of the embankment will be reduced down to 25 feet across the top. The embankment will be keyed into bedrock with a trench approximately 12 feet wide at the bottom and six feet deep. The embankment will be a zoned embankment with Zone 1 a Clay-Shale Core with 2:1 side slopes and a top width of 15 feet. The Zone 2 overburden will have side slopes of 3:1 and a top width of 25 feet. The materials used in the construction of the embankment will have to meet the attached specifications. As noted on the construction plans, the interior dike section is founded on competent clay shale bedrock. This will require identifying where weathered shale ends and the competent shale begins. An engineer familiar with the plans should be on-site during this critical time to detemune how deep the excavation should go before the placement of the embankment begins. It is also critical to not expose shale bedrock foundations for any more than one work shift. All exposed foundation should be covered with a minimum of two loose lifts of fill prior to ending a shift to prevent air slaking of the bedrock. The engineer will determine if excessive time has passed on exposed foundation or if it has been protected adequately with cover. CTL Thompson completed a slope stability analysis for the interior dike. Various zone geometries and operating scenarios were evaluated to detemune the controlling case. The safety factor criteria used are the same as those used for jurisdictional dams. The rapid draw down wa; found to be the controlling case and a factor of safety was estimated to be 1.5 with conservative assumptions. The results of this analysis can be found in the attached September 18, 2001 Slope Stability Analysis Results. The designers do not have any operating criteria from the proposed owners of the reservoir to evaluate whether or not there would be any additional criteria recommended that might be necessary for safe operation. If the reservoir is operated in a manner that requires rapid filling and drawdown, there could be problems with slope failures that require additional maintenance. It is recommended that operations be determined and planned around the existing design criteria. The proposed reservoir does lie in the floodplain of the South Platte River as pointed out earlier. The entire area will be completely inundated during major flooding events. Riverbank and pitside stabilization techniques have been negotiated and agreed upon with Urban Drainage & Flood Control District. There still maybe situations where the embankment is overtopped and subsequent damage to the embankment may occur.