THE GEOLOGICAL FINDINGS OF THE NYCDEP BYPASS TUNNEL ALIGNMENT AND IMPLICATIONS ON THE STRUCTURAL GEOLOGY OF THE MID-HUDSON VALLEY OF NEW YORK

ABSTRACT: New York City’s Department of Environmental Protection (NYCDEP) is presently constructing a new 20-foot diameter deep bedrock, drinking water tunnel in the Mid-Hudson Valley of lower New York. The new tunnel is part of the Delaware Aqueduct system, which conveys approximately 820 million gallons per day or 50 percent of the City’s drinking water on a typical day. Put into service in 1944, the Aqueduct stretches over 85 miles north of the City, making it the longest tunnel in the world.

A section of the Aqueduct, known as the Rondout West Branch Tunnel (RWBT) is leaking at a rate between 15 million and 35 million gallons per day. The leaking sections coincide with areas of challenging geology and high groundwater pressure. The City researched a variety of options to mitigate the leaks and arrived at a plan to construct a new parallel tunnel to the RWBT, called the Bypass Tunnel. Construction of a new tunnel was a reliable, relatively low risk option to mitigate the leaks.

The new tunnel alignment, located almost 1,000 feet below the surface, was investigated during the pre-design, exploratory phase. Results were used to procure a new, state-of-the-art Robbins manufactured, single shield, Tunnel Boring Machine (TBM), using a concrete segmental liner to mine through variable ground conditions with hydrostatic pressure approaching 30 bar.

Construction encountered variable and difficult ground, water bearing fault zones, and high hydrostatic pressure, that required innovative solutions to relieve the pressure.

The contract for the new Bypass Tunnel employed geologists to monitor, sample, map and develop 3-D projections of the new tunnel bore as it was advanced upwards of 2.5 miles. The TBM segmental lining prevented the geologists from employing standard tunnel mapping techniques. Instead, the geologists used innovative approaches to log, monitor, and map the geology of the tunnel, in a systematic approach using the TBM’s unique probe drill systems. The systems allowed the geologist to survey, track drilling performance data, perform downhole geophysics, and collect rock samples to assess conditions at and in front of the TBM. This paper will present the geotechnical characteristics and geological findings of the new Bypass Tunnel, to help enhance the structural geology maps of lower New York.