C150365 Feasibility Study - Laserfiche WebLink

Home Browse Search

SECTIONFOUR Formulation of Alternatives 4-2 4.3 SPILLWAY REHABILITATION AND EMBANKMENT INITERNAL SEEPAGE SYSTEM ALTERNATIVE The proposed modifications to Continental Dam include placement of a two-stage filter blanket on the downstream embankment slope and toe drain for seepage collection purposes and a stability buttress. The existing riprap on the downstream slope will be removed and stockpiled and the slope re-graded to 2H:1V above elevation 10215.00 feet. Based on the 1990 Hydro- Triad, Ltd. construction drawings, it appears that a rock fill toe of unknown gradation is located below elevation 10215 feet to approximately the invert of the outlet works, which is assumed at elevation 10193 feet. The slope will be regarded to 1.5H:1V between elevation 10215 and 10193 feet to expose the outlet works and partially remove the rock fill. Some rock fill might be left in place after excavation of the 1.5H:1V slope is complete. A filter blanket will be placed to the invert elevation of the outlet works conduit to prevent piping through the rock fill and along the conduit. A trench will be excavated in the existing downstream rock fill toe to protect the existing head wall. The reservoir is assumed to be empty during the construction. The filter blanket will be a two-stage filter consisting of ASTM 33 sand (filter sand) and No. 89 gravel (drain gravel). The filter sand and drain gravel placed against the re-graded 2H:1V slope will be 3 feet (horizontal distance) thick. The buttress fill will consist of common fill. It will be placed to widen the existing crest by 15 feet at elevation 10290 feet with a 33-foot wide bench at elevation 10265 feet. Drain gravel will be placed closest to the rock fill in the excavated trench followed by a 5-foot wide (horizontal distance) filter sand layer. A toe drain pipe will be placed within the drain gravel above the outlet works conduit for seepage collection. Eighteen- inch diameter HDPE slotted pipe will be installed within the drain gravel to convey the collected seepage water to a metering manhole located left of the existing outlet works headwall. The collected seepage water will outfall in the existing North Clear creek. The existing left groin drain will be demolished approximately below elevation 10215 feet. If no cloudy seepage is observed from the existing left groin drain, it is proposed to leave the drain in place to outfall within the new two-stage filter blanket. If cloudy seepage is observed, the drain will be grouted. The proposed modifications also include rehabilitation of the spillway chute and installation of a USBR Type II stilling basin along the right abutment. The existing concrete spillway chute will be demolished near the toe of the embankment, and a new rectangular