The New York City Department of Environmental Protection on Tuesday completed excavation of the Delaware Aqueduct Bypass Tunnel, a significant milestone in the $1 billion effort to repair leaks in the longest tunnel in the world. The moment, known as “holing through,” happened at 6:51 a.m. when a tunnel boring machine broke through a wall of shale bedrock nearly 700 feet beneath the Town of Wappinger in Dutchess County.
The Delaware Aqueduct Bypass Tunnel is the largest repair project in the 177-year history of New York City’s water supply system. Its centerpiece is a 2.5-mile-long bypass tunnel that DEP is building 600 feet under the Hudson River from Newburgh to Wappinger. When the project is finished in 2023, the bypass tunnel 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 85-mile-long Delaware Aqueduct, the longest tunnel in the world, typically conveys about half of New York City’s drinking water each day from reservoirs in the Catskills.
Two leaking sections
DEP has monitored two leaking sections of the Delaware Aqueduct – one in Newburgh, and the other in the Ulster County town of Wawarsing – since the early 1990s. The leaks release an estimated 20 million gallons per day, about 95 percent of that escaping the tunnel through the leak near the Hudson River in Newburgh.
In 2010, the City announced a plan to repair the aqueduct by building a bypass tunnel around the leaking section in Newburgh, and also by grouting closed the smaller leaks in Wawarsing. The project began in 2013 with the excavation of two vertical shafts in Newburgh and Wappinger to gain access to the subsurface. These were completed in 2017.
The existing Delaware Aqueduct will stay in service while the bypass tunnel is under construction. Once the bypass tunnel is nearly complete and water supply augmentation and conservation measures are in place, the existing tunnel will be taken out of service and excavation will begin to connect the bypass tunnel to structurally sound portions of the existing aqueduct. While the Delaware Aqueduct is shut down, work crews will also enter the aqueduct in Wawarsing to seal the small leaks there.
More about the tunnel
The Delaware Aqueduct Bypass Tunnel is the first tunnel built under the Hudson River since 1957, when the south tube of the Lincoln Tunnel was finished.
It was excavated by “Nora”- one of the world’s most advanced tunnel boring machines (TBM). The machine – which measures more than 470 feet long and weighs upwards of 2.7 million pounds – was named in honor of Nora Stanton Blatch Deforest Barney, a noted suffragist and the first woman in the United States to earn a college degree in civil engineering. Nora, who worked for the City as a draftsperson during the construction of Ashokan Reservoir, was also the first female member of the American Society of Civil Engineers.
The $30 million machine arrived at the worksite in Newburgh in 2017. It was delivered in 22 pieces and took four months to assemble. The 21.6-foot diameter TBM was built to withstand more than 30 bar of pressure – believed to be the most of any TBM ever manufactured. The machine needed to withstand high pressure because workers encountered huge inflows of water under immense pressure when the aqueduct was first built more than 70 years ago. The TBM was equipped with pumping equipment to remove up to 2,500 gallons of water per minute away from the tunnel as the machine pushed forward. The TBM was also outfitted with equipment to install and grout the concrete lining of the tunnel, and to convey pulverized rock to a system of railroad cars that followed the TBM as it works. The railroad cars regularly traveled back and forth between the TBM and the bottom of Shaft 5B in Newburgh, delivering workers, equipment and rock between the two locations.
The TBM mined 12,448 feet during the 582 days that it pushed eastward from its starting point nearly 900 feet below the surface in the Town of Newburgh in Orange County. The TBM excavated through three bedrock formations, starting with the Normanskill shale formation on the west side of the Hudson River, the Wappinger Group limestone formation, and finishing in the Mt. Merino shale formation on the east side of the river. The location and condition of these bedrock formations was well documented by New York City when it originally built the Delaware Aqueduct in the 1930s and 1940s. Engineers used that historical information to design the TBM for the bypass tunnel and plan for its excavation.
As the tunnel boring forged ahead, it also lined the shale and limestone bedrock with precast rings of concrete. A total of 2,488 concrete rings were installed by the machine. Now that mining is finished, DEP will begin to install 16-foot diameter steel liners inside the first layer of concrete. After the 230 steel liners are installed and welded together, they will be coated with a second layer of concrete. This “triple-pass” design will provide the bypass tunnel with structural stability and prevent leaks from occurring again in the future. During the excavation, the TBM was driven, maintained and supported by dozens of local laborers who worked 24-hours, six days a week. They operated cranes, trucks and underground trains to collect the pulverized rock and haul it to the surface. They removed and replaced cutting discs on the front of the machine, and maintained the many complex systems that kept the TBM functioning properly.
Excavation of the tunnel was completed on budget and ahead of schedule, according to the DEP.
“I want to congratulate the engineers, project managers and local laborers who steered us toward this milestone with considerable skill and precision,” DEP Commissioner Vincent Sapienza said. “Holing through is a major achievement for any tunneling project, especially one as large and complex as our repair of the Delaware Aqueduct. The moment is also a reminder that much work remains to be done as we move steadily toward completing this project in 2023 and ensuring the long-term reliability of the water supply system that sustains 9.6 million New Yorkers every day.”
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