Originally placed into service in 1944, the Rondout-West-Branch Tunnel (RWBT) is a section of the Delaware Aqueduct, which supplies nearly 50% of the raw water for New York City. The 13.5‑foot-diameter (4.1 m) rock tunnel is approximately 45 miles (72 km) long, ranges in depth from 300 to 2,300 feet (91 to701 m) and operates with an internal head of up to 1,200 feet (366 m). It was constructed during the late 1930s and 1940s using drill-and-blast methods.
Since the 1990s, leakage has been observed at the ground surface near where the tunnel crosses beneath the Hudson River. To address the leakage, the DEP is constructing the Rondout-West Branch Bypass Tunnel (aka Bypass Tunnel).
As prime consultant on the 12-year-long design support contract, Delve Underground (through a New York affiliate JA Underground, PC) is leading a team of multidisciplinary firms that is assisting the DEP with planning, design, and design services during construction for the 2.5‑mile-long (4 km) Bypass Tunnel.
The Bypass Tunnel tunnel boring machine (TBM) drive beneath the Hudson River was successfully completed using a shielded TBM installing a gasketed segmental liner. Challenges included excavation through folded and faulted limestone, high head conditions beneath the Hudson River, and installation of a significant length of a final “interliner” consisting of composite steel and reinforced concrete.
The remaining work includes unwatering of the existing RWBT, excavation and lining of the connections to the RWBT, construction of high-pressure plugs, then abandonment of the leaking section of the RWBT. Other areas of the RWBT will also be inspected, and contact and pressure grouting will be conducted to mitigate another leaking area of the tunnel.