Holding Up to the Pressure

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Our cover story this issue presents one of the most fascinating jobs completed recently in the United States – the Delaware Aqueduct Bypass Tunnel. The project is central to repairing the Delaware Aqueduct, which supplies approximately 50 percent of the drinking water for 9.6 million New Yorkers.

Completed in 1945 and spanning a record length of 85 miles, the tunnel was found to be leaking in the 1990s – losing an estimated 20 million gallons a day. To address the issue, the project owner – New York City Department of Environmental Protection (DEP) – devised a $1 billion repair plan, the largest repair in the city’s 177-year water supply history. Central to the plan was a new 2.5-mile long tunnel to bypass the leaking portion.

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The depth of the tunnel and its route under the Hudson River brought a unique set of challenges for contractor Kiewit/Shea Constructors, and TBM supplier The Robbins Company. Most notable was the water volume and pressure that would be encountered more than 600 ft below the surface of the Hudson.

The Robbins 22-ft diameter Single Shield TBM was designed to hold up to a record 20 bar pressure and was equipped with drilling, grouting and water inflow control systems. Additionally, the TBM incorporated a closeable bulkhead that seals the machine from potential sudden ingress of water, something that miners had experienced during the original construction of the aqueduct in the 1930s and ’40s.

The TBM was launched in January 2018 and completed its drive on Aug. 13, 2019, paving the way for DEP to begin preparations to divert water into the new tunnel, thus ensuring a reliable water supply for the nation’s largest metropolis.

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