Schoharie Reservoir – Gilboa

Marine Construction & Tunnel Support

Project: Schoharie Reservoir Low Level Outlet

Owner: New York City Department of Environmental Protection

Prime Contractor: Southland Construction / Contract Amount: $142,636,000.00

Project Highlights: The Schoharie Reservoir Low Level Outlet Tunnel Project (Gilboa Tunnel), located in the vicinity of the Gilboa Dam in Schoharie County, New York, consists of the construction of a total of approximately 2,250 linear feet of 9 foot finished diameter steel water pipe to be installed in order to facilitate drawdown of the Schoharie Reservoir for the New York City Department of Environmental Protection (NYCDEP).  Ballard Marine Construction is proud to be supporting Southland Construction in both marine and tunnel support services.

The tunneled portion of this project has been divided into two sections or “legs”, the Land Leg and the Water Leg. The Land Leg consists of approximately 1,188 linear feet of pipe and the Water Leg consists of approximately 930 linear feet of pipe. Construction of the Gilboa Tunnel project will be accomplished through micro tunneling, cut-and cover, and dredging methods. The project specifications require that the Land Leg and Water Leg tunneled sections be excavated using a micro tunnel boring machine (MTBM). The Land Leg tunnel will daylight into a portal area where it will be tied into an approximately 92-foot-long cut and- cover section. The Water Leg tunnel will end underwater in a cofferdam to be constructed in the Schoharie Reservoir using dredging methods and tremie concrete in the reservoir bottom. A wet retrieval of the MTBM will be required for the Water Leg tunnel. Additionally, a 40 to 43-foot diameter main work shaft, the Gate Shaft, will be constructed using conventional excavation and drill-and-blast excavation methods to a total depth of approximately 184 feet below ground surface. Both the Land Leg and Water Leg tunnels will be constructed from the Gate Shaft. The Land Leg tunnel is specified to be constructed first, followed by the Water Leg tunnel. Other items include construction of a Gate Shaft containing two roller gates adjacent to the east side of Gilboa Dam, construction of Valve Chamber containing two fixed cone valves and two knife gates at the east bank of Schoharie Creek, downstream of Gilboa Dam, installation of tunnel piping to connect the Intake Structure, Gate Shaft and valve Chamber, construction of Roller Compacted Concrete (RCC) berm along the north training wall, decommissioning and removal of existing siphons, installation of electrical and instrumentation components, reconstruction of the east overlook area and Schoharie Stream restoration.