2021-07-22_REVISION - M2010071 (6)

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Design Report Goose Haven Reservoir Cell 2A May 14, 2021 Page 2 of 7 2.1 Embankment The proposed Goose Haven Reservoir Cell 2A will be constructed by excavating up to 19 feet below existing grade and constructing a zoned earth embankment back to existing grade and up to 17 feet above existing grade, depending on the existing topography. The existing ground surface slopes downward to the north, so the earth embankment will be constructed on the west, north, and east sides of the excavation to achieve the maximum design water elevation (El. 5062.0 ft) in the reservoir. The excavation below existing grade will be extended to competent bedrock and backfilled with general embankment fill with a connecting clay core zone that extends into a 5-ft deep bedrock keyway to limit flow into and out of the reservoir. The bottom elevation of the embankment varies across the site. The portion of the embankment dam to be above existing grade will consist of a clay embankment core and general embankment fill up- and downstream of the core zone, and will incorporate chimney, blanket, and toe drains. The toe drain system will outlet northeast of the proposed reservoir footprint. Riprap will be installed on the upper portion of the upstream face to protect the embankment from erosion from wave action, with maximum slopes on the upstream face varying between 4:1 and 3:1. (H:V). The downstream face will have 3:1 (H:V) and will be seeded with native grass. The crest will have a minimum width of 20 ft and will incorporate a 6-inch sloped overbuild to maintain the embankment dam freeboard elevation after settlement. 2.2 Outlet Pipe and Transfer Box Structure Cell 2A will be connected to Cell 2 to the east with a 24-inch diameter pipeline that runs through a transfer box that incorporates an overflow structure. Flood flows will route through the Cell 2A- 2 transfer box overflow structure, and no additional spillway is planned for the reservoir. A portion of the outlet pipe between Cell 2A and Cell 2 will be installed using trenchless techniques to avoid a deep trench excavation below an existing 36-inch water main. Approximately 160 ft of 36-inch minimum diameter steel casing pipe will be advanced through bedrock between the transfer box structure and Cell 2 using guided auger bore methods. Once the trenchless drive is completed, contact grouting will be performed to fill the annulus between the tunneled ground and the outside of the casing pipe. Backfill grout will be placed in the annulus between the 24-inch carrier pipe and the steel casing. To minimize the potential for soil piping to occur along the outlet pipe, a filter diaphragm will be installed on the upstream end of the trenchless casing pipe. 2.3 Inlet Pipe and Associated Structures Cell 2A will receive water from a 30-inch diameter pipe that runs from a transfer box between Cell 2 and Cell 3 to an inlet structure on the bottom surface of Cell 2A. The inlet pipe will be founded on granular bedding material, and the trench will be backfilled with granular material to a minimum of 6 inches above the pipe. Trench plugs consisting of CLSM and clayey backfill will be installed to restrict water movement through granular bedding material. Modifications to the existing Cell 2-3 transfer box are planned to accommodate the new 30-inch inlet pipe, but this component is not considered to be part of the project that is subject to SEO review. BRIERLEY ASSOCIATES Creating Space Underground