New York City Department of Environmental Protection (DEP) Commissioner Vincent Sapienza has announced that work has been completed on the final concrete lining for the Delaware Aqueduct Bypass Tunnel, marking another significant milestone in the project to repair the longest tunnel in the world. The Delaware Aqueduct supplies roughly half of the water consumed in New York City every day and the ongoing repair project is critical to ensuring the long-term sustainability of the City’s water supply system.
“More than 9 million New Yorkers count on us to provide them with high-quality water every single day of the year – without fail – and this complex repair of the Delaware Aqueduct will ensure we meet that essential mission for generations to come,” said DEP Commissioner Vincent Sapienza. “We are grateful to the skilled engineers, construction experts and local laborers who completed the concrete lining of the bypass tunnel and kept the largest repair in the history of New York City’s water supply system moving forward on budget and on schedule.”
Skilled workers placed the last concrete inside the new tunnel at about 1 p.m. on Feb. 22. Work on the final concrete lining began in June 2020, and work crews placed concrete inside the tunnel at a pace of about 80 linear feet per day. Workers pumped concrete from the eastern side of the tunnel in Dutchess County when they began last summer. The pumping equipment and concrete operation were moved to the western side of the tunnel, in Newburgh, once crews finished lining approximately half of the 2.5-mile-long tunnel. A total of 29,600 cubic yards of concrete was used for the finishing lining. The concrete was formed by a system of rolling formwork that was moved through the tunnel by a specially designed trolley and rail system.
The Delaware Aqueduct Bypass Tunnel is the largest repair project in the 179-year history of New York City’s water supply system. The centerpiece of this $1 billion project is a 2.5-mile-long bypass tunnel that DEP built 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.
The project began in 2013 with the excavation of two vertical shafts in Newburgh and Wappinger to gain access to the subsurface. These shafts, 845 and 675 feet deep respectively, were completed in 2016. A massive tunnel boring machine completed excavation of the tunnel on Aug. 13, 2019. The tunneling machine excavated 12,448 feet over the course of 582 days, starting deep below the Town of Newburgh and pushing east to an access shaft in the Town of Wappinger. The tunnel boring machine lined the shale and limestone bedrock with precast rings of concrete as it drove the tunnel forward. Once the tunnel was excavated, skilled workers installed 9,200 linear feet of steel inside the tunnel. The steel liner was installed in 40-foot-long segments, each one lowered carefully into the tunnel by a crane at the surface. The steel lining, which will provide structural support for the tunnel, was completed in May 2020. The final concrete lining was installed within the steel. This “triple-pass” design will provide the bypass tunnel with structural stability and prevent leaks from occurring again in the future.
With the final concrete lining finished inside the tunnel, workers will now turn their attention to placing concrete within the connection sites on either side of the Hudson River, excavation for the superstructures that will be located at each end of the tunnel, and other work necessary before the Delaware Aqueduct is shut down next year to make the final bypass-tunnel connections.
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. DEP has continuously tested and monitored the leaks since 1992. The size of the cracks in the aqueduct and the rate of leakage have remained constant over that time.
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. Upon completion of the two deep vertical shafts in Newburgh and Wappinger, workers then built a large underground chamber at the bottom of the Newburgh shaft. That chamber has served as the staging area for assembly and operation of the tunnel boring machine, and as the location from which excavated rock is brought to the surface by underground trains and a large crane.
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, roughly 15 miles northwest of the bypass tunnel.
The project will mark the first time that the Delaware Aqueduct will be drained since 1958. In 2013, DEP installed new pumps inside a shaft at the lowest point of the Delaware Aqueduct to dewater the existing tunnel before it is connected to the new bypass tunnel. Those pumps will be tested several times before the tunnel is drained in 2022. The nine pumps are capable of removing a maximum of 80 million gallons of water a day from the tunnel—more than quadruple the capacity of the pumps they replaced from the 1940s. The largest of the pumps are three vertical turbine pumps that each measure 23 feet tall and weigh 9 tons.
The Delaware Aqueduct Bypass Tunnel was excavated by one of the world’s most advanced tunnel boring machines (TBM). The machine - which measured more than 470 feet long and weighed 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 TBM 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. (That’s about 11 times the amount of pressure from a garden hose.) 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 was dismantled, brought to the surface in pieces, and removed from the worksite after it completed the excavation in August 2019.
About the Delaware Aqueduct
The Delaware Aqueduct is an 85-mile-long tunnel that delivers drinking water from Rondout Reservoir in Ulster County to Hillview Reservoir in Yonkers. The aqueduct typically conveys about 50 percent of New York City’s drinking water. The Delaware Aqueduct was put into service in 1944 when New York City Mayor Fiorello H. La Guardia opened a set of emergency gates to channel the Rondout Creek directly into the new aqueduct. It was placed into emergency service that year to support efforts related to World War II. The Delaware Aqueduct is the longest tunnel in the world, approximately 11 miles longer than a water tunnel built in the early 1980s in Finland.