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Extreme challenge for Rondout bypass TBM 28 Apr 2016

Peter Kenyon, TunnelTalk

Robbins is selected to supply the 6.6m diameter single shield TBM that will operate under extremes of pressure to complete the largest single repair project in the 170-year history of New York City’s water supply.

Fig 1. Delaware Aqueduct Bypass Tunnel cross section alignment
Fig 1. Delaware Aqueduct Bypass Tunnel cross section alignment

The order is placed by the Kiewit/Shea Joint Venture contractor, which is now mobilised on site following award in 2015 of the US$706.6 million build-only main underground construction contract of the Delaware Aqueduct Bypass Tunnel Project. The new deep bypass will prevent leakage of up to 20 million gal/day through the fractured limestone geology of the 72-year-old existing tunnel as it runs under the Hudson River. The tunnel conveys more than half of New York City’s drinking water. The contracting authority for the US$1 billion project is the New York Department of Environmental Protection (DEP). Design has been completed in-house.

Full scope of works includes TBM excavation of the segmentally-lined 4,000m-long bypass of the Rondout Tunnel; installation of 2,800m of steel interliner along the limestone section of the bypass to prevent recurrence of the leaks; excavation by drill+blast of 46m-long tunnels to connect both the TBM launch and retrieval shafts of the new bypass to the existing tunnel; and grouting work during tunnel drain down in another lesser effected section of the main tunnel some 29km north of the TBM drive, in Ulster County, New York.

Kiewit/Shea is reported to be on target to begin drill+blast excavation of the TBM bell-out chamber in summer this year (2016). This follows completion last month (March 2016), under a separate US$101.6 million contract awarded to Schiavone, of the two deep shafts – 5B and 6B – at either end of the bypass. The chamber is scheduled for completion in 2017 ahead of arrival later in the year of the TBM.

Schiavone completed excavation of Shafts 5B and 6B in March 2016
Schiavone completed excavation of Shafts 5B and 6B in March 2016

The single shield machine will launch from Shaft 5B, which measures 845ft x 36ft o.d. (30ft i.d.), and is situated on the Newburgh side of the Hudson River. From here it will pass 600ft below sea level, some 100ft below the river under a maximum anticipated hydrostatic pressure of up to 20 bar, through to the 675ft x 38ft o.d. (33ft i.d.) reception shaft on the Wappinger side (Fig 1). The shafts will remain in place following the completion of the TBM drives as permanent access structures.

Desiree Willis, for Robbins, told TunnelTalk: “The TBM will be designed to passively hold potentially high water pressure of up to 30 bar using a cutterhead drive sealing system and backfill grouting through the tailskin.

“The machine is equipped with high thrust to get through challenging ground and sophisticated drilling and pre-grouting equipment for detection. Water-powered, high-pressure down-the-hole hammers are capable of accurate drilling 60–100m ahead of the TBM, while blow-out preventers enable drilling at high pressures up to 20 bar.”

The TBM will be built in Robbins’ Solon manufacturing facility in Ohio, with a factory acceptance test scheduled for the last quarter of this year (2016). The machine will then be shipped to the jobsite for an estimated start-up in Spring 2017.

Installing structural steel support prior to applying the secondary concrete lining in the Rondout West Branch Tunnel, Nov 5, 1942
Installing structural steel support prior to applying the secondary concrete lining in the Rondout West Branch Tunnel, Nov 5, 1942
Applying concrete in the Rondout West Branch Tunnel, June 12, 1941
Applying concrete in the Rondout West Branch Tunnel, June 12, 1941

Anticipated geology features competent rock of densely compacted schists and shales at either end of the alignment, but with a middle section under the river itself that includes a folded and faulted limestone formation which proved problematic during drill+blast construction of the original tunnel in 1939–44. It is through this highly porous short limestone section, and through another similar one in Ulster County further north, that the majority of leakage has been found to occur.

Adam Bosch, for NYDEP, told Tunneltalk: “During original tunnel construction, the sections running through limestone had a steel inter-liner installed to provide additional support. The main leak that showed up following detailed camera and sonar surveys using Automated Underwater Vehicles (AUVs), was coming from the end of that liner.”

Just like the original tunnel, which will be decommissioned and taken out of service following watering up of the new bypass, the new section will include a steel inter-liner. Critically, however, at 2,800m long (70% of the total bypass length) it will be considerably longer than the previous steel liner so as to prevent a recurrence of the leakage problem. “During original construction in the 1940s, tunnel workers dealt with huge inflows of water coming in at them while they were drilling,” said Bosch. “Total inflows were recorded at approximately 2–4 million gal/day, with the largest single section of inflow coming in at roughly 1,500 gal/minute. This was inflow from both groundwater and river influence, and managing water will also be a major challenge for this project.”

To handle these expected inflows a grout curtain will be installed ahead of the TBM, and the contractor will also be prepared to pump out water where necessary. Despite the highly pressurized conditions at the face, hyperbaric interventions are not expected. “We will be relying on drill probes of the ground ahead, installing a grout curtain where necessary ahead of the face, and there are expected to be adequate sections of competent rock in which the contractor can perform maintenance to the cutterhead and the cutting tools in free air,” said Bosch.

Following TBM breakthrough, scheduled for 2020-21, the flow into the Delaware Aqueduct will be stopped by closing shutoff valves at the Rondout Reservoir, where the 85-mile long tunnel starts. In 2022 the tunnel will then be drained using a pumping system that has been installed at Shaft 6 on the east side of the Hudson River close to the new Shaft 6B. Final connections of approximately 46m at either end of the bypass will then be made using the drill+blast method. These short sections will then be lined with sprayed concrete and the portion of the existing tunnel to be abandoned will be plugged and permanently taken out of service ahead of watering up of the system.

During a shutdown period of approximately six months New York City will continue to be supplied by the Croton and Catskill systems. The bypass project has been in the planning since the leaks were first detected in 1991, but until fairly recently it would have impossible to shut off one of New York’s three main sources of fresh water. Although the city has grown in population by 1.4 million people since 1991, new technology and metering have brought about a 30% reduction in water consumption. This, together with activation of the Croton Water Filtation Plant that will supply an extra 290 million gal/day during the shutdown period, will enable a six month window during which one of the city’s three supply tunnels can be shut down.

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