Excavation complete in Delaware Aqueduct Bypass 22 Aug 2019

Excavation is now complete for the Delaware Aqueduct Bypass under New York City. The breakthrough for joint venture, Kiewitt-Shea and CM Parsons, and the New York Department of Environmental Protection, DEP, was achieved on budget and ahead of schedule and marks a milestone in the USD$1 billion effort to repair water leaks in the aqueduct.

Breakthrough for bypass

The Delaware Aqueduct Bypass is the largest repair project in the 177-year history of New York City’s water supply system. It includes a 2.5mile (4km) bypass tunnel that DEP is building 600ft (183m) under the Hudson River. When the project is finished in 2023, the bypass will be connected to structurally sound portions of the existing Delaware Aqueduct on either side of the Hudson River to convey water around a leaking section of the tunnel. The 85mile (136.8km) Delaware Aqueduct conveys about half of the daily drinking water needs of New York City from reservoirs in the Catskills.

The excavation was undertaken by a Robbins single shield 6.8m TBM, which bored 2.4miles (3.8km) in 582 days from its starting point nearly 900ft (274m) below the surface of Newburgh in Orange County. It achieved a best day of 89.8ft (27.4m), a best week of 354.8ft (108.1m), and a best month of 945ft (290.1m). The machine was built to withstand more than 30 bar of pressure, believed to be the most of any TBM ever manufactured, as workers had encountered huge high-pressure inflows of water when the aqueduct was built. It was also equipped with pumping equipment to remove up to 2,500 gallons of water per minute during excavation.

The TBM cut through three bedrock formations, the Normanskill shale formation on the west side of the Hudson River, the Wappinger Group limestone formation, and the Mt Merino shale formation on the east side of the river, lining the route with 2,488 precast rings of concrete. DEP will now install and weld 230 16ft (4.9m) diameter steel liners inside the first layer of concrete, before coating with a second layer of concrete, to provide the bypass with structural stability and prevent leaks from occurring again.

Steel liners will help leak-proof bypass

The Delaware Aqueduct Bypass is the first tunnel built under the Hudson River since 1957, when the south tube of the Lincoln Tunnel was finished. The project is expected to be complete in 2023.

DEP has monitored two leaking sections of the aqueduct, at Newburgh and Wawarsing, since the early 1990s. The leaks release about 20 million gallons per day with 95% of that escaping near the Hudson River in Newburgh. In 2010, the City announced a plan to repair the aqueduct by building a bypass around the leaking section in Newburgh and grouting the smaller leaks in Wawarsing. The project began in 2013 with the excavation of two vertical shafts. These shafts, at 845ft (257.6m) and 675ft (205.7m), were completed in 2017.

The existing aqueduct will stay in service while the bypass is under construction. When work is nearly complete and water supply augmentation and conservation are in place, the existing tunnel will be taken out of service and work will begin to connect the bypass. While the Delaware Aqueduct is shut down, work crews will also repair the aqueduct in Wawarsing. The project will mark the first time that the aqueduct will be drained since 1958. In 2013, DEP installed new pumps ahead of draining the tunnel in 2022. The nine pumps can remove a maximum of 80 million gallons a day. The largest of the pumps are 23ft (7m) tall vertical turbine pumps that each weigh 9 tonne.