Schoharie Reservoir Low Level Outlet

OWNER: NYC Department of Environmental Protection

ENGINEER: Gannett Fleming/Hazen &  Sawyer, A Joint Venture

SOUTHLAND HOLDINGS TEAM: Southland/Renda Joint Venture

PROJECT HIGHLIGHTS:

Located in the Catskills region of Upstate New York State, the Schoharie Reservoir and tunnel will provide a significant portion of daily water to New York City. The reservoir was created by the Gilboa Dam 90 years ago. The New York City Department of Environmental Protection saw the need to repair the dam and replace the existing output facility, part of which requires tunneling 150 ft. below the surface of the reservoir.

The tunnel is separated into a “Water Leg” and a “Land Leg.” Both will connect to the Gate Shaft, 40 ft. in diameter by 187 ft. deep, constructed in hard fractured rock, necessitating an extensive pre-grouting program.

Construction is being accomplished through microtunneling, cut-and-cover and dredging methods. Project specifications require a 9.5 ft. diameter, hyperbaric capable microtunnel boring machine (MTBM) designed for five bar of pressure for both the Land Leg and Water Leg tunneled sections. A wet retrieval of the MTBM will be necessary, along with dredging the receiving pit for its retrieval.

The intake structure was dredged and installed by certified divers utilizing an underwater liner plate coffer dam for Tremie concrete. The Land Leg tunnel is specified to be constructed first, followed by the Water Leg tunnel. The tunnels and shaft will be excavated primarily in sandstone, with some mixed-face conditions near the end of the tunnel drives. The Land Leg tunnel will daylight into a portal area, where it will be tied into an approximately 92 ft. long cut-and-cover section. The Water Leg tunnel will end underwater in a cofferdam to be constructed in the Schoharie Reservoir.

The tunnel portal is an open-cut excavation supported by soil-nail wall located at the downstream end of the project and constructed to accommodate tunneling operations and construction of the multilevel reinforced concrete valve chamber structure. O-piling and substantial drilled foundations are also part of this work.

We are also constructing the Gate Shaft’s two roller gates, two fixed cone valves in the Valve Chamber, two knife gates at the east bank of Schoharie Creek, and tunnel piping to connect the Intake Structure to the Gate Shaft and Valve Chamber. Other elements include a roller-compacted concrete berm in the creek to protect the portal excavation from a 10-year flood event, reconstruction of the East Overlook area, stream restoration, decommissioning and removal of existing siphons, and electrical and instrumentation controls.

Completion of the $142.6-million project is planned for 2020. We have met early milestones ahead of schedule. The Metro-E cutover of the NYCDEP network is already complete, as are the onsite batch and water treatment plants, the RCC berm, clearing of approximately 6.5 acres of trees, and geotechnical drilling on land and in the reservoir. Phase 1 excavation of the sites is complete, along with equipment and material mobilization, including the MTBM.