Record-breaking TBM Completes Monumental Voyage
In August 2019, a 6.8 m (22.3 ft) diameter Robbins Single Shield TBM accomplished an epic feat of tunneling. The unique machine, designed to statically hold up to 20 bar pressure, had bored below the Hudson River for 3,794 m (12,448 ft) over 582 days with penetration rates of 6 m (20 ft) per hour.
The tunnel depth—ranging from nearly 270 m (900 ft) deep where the TBM was launched in Newburgh, New York to over 180 m (600 ft) deep at the exit shaft—the water volume and pressure were significant challenges. “Things went really well. The machine did what it was supposed to do, the rock behaved, the learning curve for running the machine was reached before we hit any problems. We hit a curve, and the crew got used to it—we hit good limestone, and then harder rock. We also hit water, and we learned how to deal with that,” said Ted Dowey, Portfolio Manager for the New York City Department of Environmental Protection (NYCDEP), project owner.
Successfully surmounting the obstacles required unique TBM design and skillful machine operation by JV contractor Kiewit–Shea Constructors (KSC). The TBM excavated through three bedrock formations, starting with the Normanskill shale formation on the west side of the Hudson River, the Wappinger Group limestone formation, and finishing in the Mt. Merino shale formation on the east side of the river. The location and condition of these bedrock formations was well documented by New York City when it originally built the Delaware Aqueduct in the 1930s and 40s. Engineers used that historical information to design the TBM for the bypass tunnel and plan for its excavation.
Stemming the Flow
At 137 km (85 miles) long, the Delaware Aqueduct is cited in the Guinness Book of World Records as the world’s longest continuous tunnel. On average the aqueduct supplies about 50 percent of the water consumed by 8.6 million residents of New York City and an additional 1 million residents in four counties north of the City. In the 1990s, NYCDEP discovered that a section of the aqueduct below the Hudson River was leaking up to 75 million liters (20 million gallons) of water per day.
The resulting project – known as the Delaware Aqueduct Bypass Tunnel – is the largest repair in the 177-year history of New York City’s water supply. The 3.8 km (2.5-mile) bypass tunnel was excavated parallel to the existing aqueduct, and it will be connected to structurally sound portions of the existing tunnel. Notably, the new tunnel was bored while the aqueduct was still in service—only after excavation of the bypass would the flow be switched off to allow for the bypass to be connected to the existing tunnel. That shutdown, expected to begin in October 2022, is planned to last for five to eight months. The leaking section of the Delaware Aqueduct will be plugged at that time and taken out of service forever.
“We have a massive ongoing program to make sure that the rest of the water supply can suffice during a six- to eight-month shutdown,” explained Dowey. “Some towns have gotten water from this tunnel for a long time and have no alternative water source. We are working to make sure they have water while the tunnel is out of service.”
The tunneling project began with the construction of two shafts, one on either side of the Hudson River. The launch shaft was constructed 258 m (845 ft) deep in Newburgh, New York, on the west side of the river, and the retrieval shaft was mined to a depth of 197 m (645 ft) in Wappinger, New York. The shafts were constructed by drill-and-blast with concrete lining installed every 30 m (100 ft). Their construction was completed in March 2016.
The TBM was launched from a bell-out chamber with a 12 m (40 ft) high ceiling, but getting the components underground was an intricate process. The Newburgh shaft featured a complex logistical setup including an elaborate hoisting system designed to service the shaft, rather than a crane. The hoisting system provided a lifting capacity of up to 90 metric tons (100 US tons) to be used during TBM assembly. The same hoisting system was reconfigured after the launch to provide an individual muck hoist to lift out 15 cubic meter (20 cubic yard) muck boxes during the drive. Two other hoists were part of the overall hoisting system—one was used as a supply hoist to lower precast segments and linear plant down the access shaft, and the other was used as a dedicated personnel hoist to operate a 28-person man cage, all within the confines of a 9 m (30 ft) diameter shaft.
Robbins worked closely with KSC to ensure that TBM components were designed and sized so all parts were less than 90 metric tons (100 US tons) and could be lifted with the contractor’s hoist system to fit down the narrow, 258 m (845 ft) deep shaft window. “They were huge pieces of machinery. The cutterhead support came in on one massive truck with 97 wheels. KSC had to calculate the center of gravity for huge loads in the hoists. The cutterhead support was lowered with inches to spare,” said Dowey.
A factory acceptance test was held in February 2017. After being shipped to the site, the TBM was assembled on a moving cradle at the bottom of the shaft that could then be moved to the tunnel face.
A Specialized TBM
The TBM design had to account for challenges presented by the aqueduct repair, such as difficult geology and considerable water inflows. “There was the potential for high water pressures and inflows. We had a lot of interaction with the client and Robbins during TBM design. We had it set up correctly with dewatering, drilling and grouting systems in place. We had the capacity to pump 9,500 liters (2,500 gallons) of water per minute. We were well set up with the TBM to encounter anything predicted,” said KSC Tunnel Manager Niels Kofoed.
Difficult Ground Solutions (DGS), including powerful drilling, grouting and water inflow control systems, were incorporated into the machine’s design to overcome the expected challenges. But one feature was particularly important for NYCDEP: “We needed a bulkhead to seal off the machine if we encountered water inflows. We were boring 180 m (600 ft) below the Hudson River, so we also needed a tunneling solution that could work with bolted, gasketed segments,” said Dowey.
The unique Single Shield TBM was designed to be quickly sealed to protect the machine and personnel from sudden inrushes of water. The closeable bulkhead allowed the excavation chamber to be sealed off in the event that groundwater inflows (shunt flows) from the excavated portion of the tunnel caused washout of the annulus grout. Once the bulkhead was closed the groundwater flows would be stopped and secondary grouting of the precast liner could be performed, effectively cutting off the flow path.
To seal the TBM required several steps. Knife gates over the muck chute were closed, followed by retraction of the conveyor frame and the belting from the cutting chamber. The bulkhead sealing plate was retracted and finally the stabilizer doors were closed. Other aspects of the sealing system protected the main bearing, including emergency inflatable seals as well as the normal lip seals. The inflatable seals were not in running contact with moving parts of the sealing system during boring and could be activated when needed for additional pressure protection of the main bearing of the TBM. The seals were flushed and lubricated with grease to provide better protection when exposed to water with fines.
The project specification required a mandatory probe drilling program for the entire tunnel alignment, which included water inflow measurements at the probe hole locations. The TBM crew was thus required to drill four probe holes every 115 m (380 ft) to measure water inflows. When water inflows exceeded contract-allowable values, grouting would be required to reduce water inflows to acceptable levels. The TBM could then advance inside the grouted area of the alignment.
To accomplish this feat, the TBM was equipped with two types of grouting systems. The pre-excavation grouting system was a mono-component grout system used to grout ahead of the TBM. The two-component (A+B) grout system was used to backfill the annular gap between the segmental lining and the bored tunnel. The machine was equipped with two drills in the shields for drilling through the head in 16 different positions and a third drill on the erector to drill through the shields in an additional 14 positions. To add to that, water-powered, high pressure down-the-hole (DTH) hammers allowed for drilling 120 m (400 ft) ahead of the machine at pressures up to 20 bar if necessary.
“The DTH hammers are a novel drilling system for TBM tunnel application,” said Kofoed. While the drills have been used on a project in Europe, this is their first use in North America. “We needed to be able to bore two to four probe drills up to 120 m (400 ft) ahead of the machine, then mine 115 m (380 ft), then drill out 120 m (400 ft) again. The straightness of the drill holes is a huge advantage, as DTH hammers do not go off of tunnel alignment. The distance ahead of the TBM is also quite far.” Kofoed explained that more typically top hammer drills are used, meaning that the hammer action is on top of the drilling rod. In such a configuration, the hammer action only allows the drill string to reach 45 to 60 m (150 to 200 ft) ahead of the TBM and the string has a tendency to drift.
Boring the Bypass
The TBM was launched on Jan. 8, 2018, and KSC was ready with a plan to combat water inflows. “In the geotechnical baseline report, 4,900 liters per minute (1,300 gallons per minute) were predicted if we did not do any pre-excavation grouting,” said Kofoed. “Obviously we did probe drilling and pre-excavation grouting. The most we had was 1,300 liters per minute (350 gallons per minute) or so. We were also able to divert much of that water from the excavation chamber to drain ports drilled out through the liner. We then did dewatering through the liner behind the TBM.” Water pressure during mining, added Kofoed, was in the range of 200 psi—significantly less than the predicted groundwater pressure. “This was in part because of the dewatering effort. Current measurements are close to hydrostatic pressure now.”
The tunnel drive was divided into four distinct reaches, with Reach 1 consisting of shale, Reaches 2 and 3 of limestone, and Reach 4 of shale again. “We were lucky in that there turned out to be less water and volume of grouting needed. All pre-excavation grout was done in Reach 3 – the first pre-excavation grout was done in January 2019 around ring 1,500 and the last pre-excavation grouting was done at Ring 1,850 – so a stretch of approximately 530 m (1,750 ft) of the total 3,800 m (12,500 ft) of TBM tunnel had to be grouted,” said Kofoed. The grout consisted of water cement type grout, which was progressively thickened as needed from a 4:1 to 3:1 mix and finally to a 1:1 mix.
The DTH hammers were used throughout the drive. “We required that they probe ahead of the machine, and they were able to use the hammers 120 m (400 ft) ahead of the machine. It’s a huge step forward in tunneling. If you can double your probing length when it is on the critical path, that helps,” said Dowey.
“The DTH hammers were successful. Upkeep is challenging because now the drill water becomes your way to drive the hammer. Like an engine, you need to keep it clean, much like hydraulic fluid. We had to filter out the water frequently, and this maintenance and operation was a challenge with a learning curve. But overall the hammers saved us time, as we could drill 120 m (400 ft), then mine 115 m (380 ft) without having to stop again to probe drill,” said Kofoed.
According to data tracked by NYCDEP, the machine excavated 27.4 m (89.8 ft) on its most productive day, 108.1 m (354.8 ft) during its best week, and 288 m (945 ft) during its most productive month. “It is a very powerful machine with tremendous thrust capacity based on high rock strength and potential for squeezing ground. Overall the TBM design worked out well and was able to advance through shale, limestone and more challenging sections,” said Kofoed.
The depth of the tunnel and limited access meant that toward the end of tunneling, mucking out was on the critical path. “Early on when the drive was short the TBM was on the critical path. But by the end the shaft was on the critical path because of the train travel time and the muck cars, which had to be taken out of the tunnel. Crews mined and built rings faster towards the end of tunneling as well. A typical cycle was perhaps one hour, and in softer rock the machine could do 100 mm/min or 20 ft per hour. However, TBM-wise we were limited at the end because the shaft was the bottleneck,” explained Kofoed.
Throughout the drive, Robbins personnel played a key role in worker training and troubleshooting any machine issues. “Kiewit and Robbins collaboration was good. The Robbins people helped with assembly, and they stayed onsite to help keep the machine up and running,” said Dowey.
Kofoed agreed: “The local unions provided a group of very experienced and skilled craft personnel to the job; however, with no previous history of TBM tunneling in the area there was little TBM mining experience in the local unions. With the assistance of Robbins supervisors during the TBM startup and throughout the drive, the operators and laborers went through an initial steep learning curve but quickly got the hang of it and did a good job both during the TBM assembly and the tunnel drive. This included training of local TBM operators, which worked out really well.”
With tunneling complete, about 2,800 m (9,200 ft) of the tunnel will now be lined with steel interliner. The steel interliner will be much longer than what was initially used on the Delaware Aqueduct, to cover the entire section of poor ground. After segments are placed, the 4.9 m (16 ft) diameter, 12 m (40 ft) long sections of steel liner will be pulled into place before receiving a final concrete lining. This triple-pass tunnel will be among one of the most robustly reinforced tunnels in the world.
“TBM demobilization will be ongoing for the next month or so, and then we’ll move to steel liner installation,” said Kofoed. “Inside of the liner is another reinforced concrete pipe cast in place. Basically, we are going from 5.5 m (18.2 ft) ID to 4.9 m (16 ft) ID with steel liner, ending with 4.3 m (14 ft) ID after we place the concrete pipe. We’re looking to put the system online by early 2022.”
This article was contributed by The Robbins Company.