A TBM designed specifically to repair the Hudson River crossing of the Delaware Aqueduct in New York City has completed factory testing to meet the demanding tunnel drive. The 137km (85 mile) long tunnel delivers about 50% of the drinking water for the city each day.
Representatives of the project owner, the New York Department of Environmental Protection (NYDEP), the consultant McMillen Jacobs, and the JV contractor Kiewit-Shea Constructors (KSC) witnessed the factory acceptance testing of the custom TBM at the Robbins fabrication facility in Solon, Ohio.
The Robbins single shield TBM will bore a bypass tunnel to replace a 3.8km (2.4 mile) stretch of the Delaware Aqueduct which runs below the Hudson River and currently leaks about 75 million litres (20 million gallons) of water/day. Investigations of the aqueduct over the last decade revealed cracks in the lining of the existing aqueduct tunnel, and while several inspections with an automated underwater vehicle showed the cracks were stable, it was determined they could not be fixed from within the operating aqueduct. The new bypass tunnel will be built below the existing tunnel to resolve the leakage.
The existing Delaware Aqueduct tunnel was completed in 1944 and construction crews working on the under river section documented groundwater inflows of 7.5 to 15 million litres/day (2 to 4 million gallons/day) into the tunnel heading at 183m (600ft) below the Hudson River. Considerable water inflows combined with difficult geology requires that the new Robbins 6.8m (21.6ft) diameter single shield TBM for the replacement bypass tunnel be designed to safely seal against pressures of up to 30 bar, and operate in variable hard rock conditions.
Key design features of the new TBM include difficult ground solutions (DGS) technology with powerful drilling, grouting, and water inflow control systems.
“One particular feature of this TBM is the closeable bulkhead, which allows the excavation chamber to be sealed off,” said Niels Kofoed, KSC Tunnel Manager. “We expect this to be a key feature in the event that groundwater flows that will constitute shunt flows from the excavated portion of the tunnel, will cause washout of the annulus grout behind the tunnel’s segmental lining. Once the bulkhead is closed, the groundwater flows are stopped and secondary grouting of the precast liner can be performed, effectively cutting off the flow path of the shunt flows.”
Robbins project manager Martino Scialpi further noted that, “the TBM was designed with a 9,500 litre/min (2,500 gallon/min) dewatering capacity. The machine is equipped with two drills in the shields for drilling through the cutterhead in 16 different positions and a third drill on the erector to drill through the shield in an additional 14 positions. Drilling and pre-excavation grouting will be a routine job to control and minimize water inflows.” In addition, water-powered, high pressure down-the-hole-hammers will allow for drilling 60m to 100m (200-330ft) probe holes ahead of the machine at pressures up to 20 bar if necessary.
In order to provide access to launch and retrieve the TBM and its trailing equipment at the start and end of the tunnel drive, two deep shafts were constructed either side of the Hudson River in the towns of Newburgh and Wappinger. Limited space available at these launch and reception working shafts pose challenges in themselves to the assembly, launch and retrieval of the TBM.
Robbins worked closely with KSC to ensure that TBM components were designed and sized so that all could be lifted and lowered into the narrow 270m (885ft) deep shaft. Once assembled, the machine is expected to begin boring in Autumn 2017.