Manhattan’s largest undeveloped single piece of property, the Long Island Rail Road rail yard, makes its home on the far west side of Manhattan from 31st to 33rd Street. The surrounding neighbourhood is underused and underserved except for the recently expanded Jacob K. Javits Convention Center to which visitors have two choices for arrival by public transit; bus routes M34 and M42. The nearest subway line is three blocks away and parking garages in the area charge USD30-50 a day, but of course there are always taxis.

This lack of infrastructure was somewhat of a blessing when it came time to plan an extension to the No. 7 Line, which would service the convention centre. With not much existing above ground and not much below there was room to move, and to mine. But the same could not be said at the current terminus for No. 7 – Times Square Station. Here the underground space is already crowded with subway lines, Amtrak rail and tubes for the Lincoln Tunnel.

Designing an alignment to meet the current line and navigate the existing underground structures meant mixed face conditions of rock and soil near the start of the extension were unavoidable. The contractor, S3II Tunnel Constructors, a joint venture of Shea Construction, Skanska USA and Schiavone Construction, chose to freeze the ground before employing two double-shield, hard-rock TBMs to mine the twin tunnels. Their breakthrough this July sees a project that has seen smooth sailing six months ahead of schedule.

That’s good news for everyone, particularly the City of New York, which floated bonds to fund the USD2.1bn project as part of an initiative to develop that area of western Manhattan, and rezoned the rail yard from manufacturing to residential and commercial use. Currently the No. 7 Line travels from Flushing, Queens to Times Square Station. The extension will bring the line further west and south with a new station at 34th Street and 11th Avenue for the Javits Center. The extension will provide mass transit access to hundreds of thousands of people who will eventually live and work there, explains Michael Horodniceanu, president of MTA Capital Construction.

“Providing mass transit access became imperative,” he says of the development project. “The rezoning—that allows for higher density to be created there, and the possibility of a tax district will enable the city to pay back the bonds from the development.”

A running start
In December 2007 a USD1.14bn contract to construct the 6,600ft (2,012m) tunnels and the 34th Street station cavern was awarded to S3II, which started work in February 2008. The original design extended the No. 7 Line from Times Square Station, under 41st Street, and then turning south, running under 11th Avenue, with tail tracks going down to 26th Street. Initially the extension called for the new terminal station at 11th Avenue and 34th Street, plus a station at 10th Avenue and 41st Street. But the 10th Avenue Station was removed from the plan due to a lack of funding.

At 130ft (39.6m) below ground, the two new tunnels are relatively deep compared to other underground structures already there, says Peter Wahl, project manager for the No. 7 Line extension for Parsons Brinckerhoff. The deepest point of the new tunnels is at 26th Street where the TBMs were launched. Between that point and the new station at 34th Street there is a 0.5 per cent gradient, and within the station there is another 0.5 per cent gradient. Then it increases to around 3.5 per cent continuously until the alignment meets the existing tail track of Times Square Station. Not only did that mean the alignment would fall into mixed face, but also that as the tunnels reach 41st Street, and turn right, the 650ft (198m) radius curve has a gradient increase as well.

To launch the TBMs, a 38ft (11.6m) access shaft was built to the side of 11th Avenue with secant piles to support soil overburden and with shotcrete and rock bolts for the remainder. The rock section of the shaft was excavated using drill and blast. A large gallery was driven across from east to west underneath 11th Avenue, with a 200ft (61m) starter tunnel running north and a 200ft tail tunnel running south for each tunnel. These were excavated using drill and blast.

Herrenknecht supplied two identical double-shield TBMs of 6.81m (22ft 4in.) diameter and 17-in. cutter discs: 36 front and gauge discs and four twin discs in the centre. These were shipped from Germany to a port in New Jersey and trucked to the job site. They required full assembly. Each TBM cutter head came in three pieces that were welded together at the surface and lowered down, says Jim Rosteck, director of engineering for S3II. Once the TBMs were moved up into the starter tunnels the contractor had spray-concreted a round launching area where the grippers were able to react against the walls and thrust off, he explains.

The first 300ft (91.4m) of the drive was through frozen ground. Working with subcontractor Layne Christensen about 280 freeze pipes were drilled from 11th Avenue. These consisted of a 3in. diameter outer pipe capped on the bottom and second pipe running down the centre. Freeze plants using an ammonia refrigerant circulated calcium chloride solutionthrough the pipes to remove the heat from the ground.

S3II launched the first TBM in June of 2009, deciding to stagger the two as they bore through the frozen ground. The second TBM started one month later. Rosteck says the advance rates were a little bit slower through the frozen ground compared to the rest of the tunnel. But, he notes, “Whenever you’re starting up a TBM you have your learning curve. The crews are new to it and you’re sorting things out.” Once both TBMs passed through the frozen zone the freeze plants were turned off for the area to thaw on its own and the pipes were removed. Adjacent properties and streets were monitored and to date, no significant settlements have been observed.

Beyond the mixed face conditions, the ground composition is extremely hard, requiring 15 000-25 000lbf/in.2 (103-172 MPa) to drill through Manhattan schist, granite and some pegmatite intrusions. Advance rates ranged between 20ft (6.1m) and 40ft (12.2m) a day. Cutter disc wear rate varied, with most frequent changes occurring in the pegmatites – the hardest rocks – explains Wahl.

Overall on the project the TBMs each averaged 25ft (7.62m) a day, placing pre-cast concrete lining rings as permanent liner as they progressed. Manufactured by Technopref, the 5-FT (1.52-m) wide segments are in a five segment plus keystone configuration, 12in. (305mm) thick and wire reinforced.

Picking up the pace
There have been a number of factors driving the project six months ahead of schedule, including the good fortune that overall construction has gone well. A New York City Transit shaft located in the 8th Avenue and 41st Street section had originally been cleared for the contractor to use for personnel access.

“They agreed to remove some intermediate slabs and that allowed the contractor to use that shaft to bring equipment and materials down,” explains Shawn Kildare, the MTA’s program executive for the project. “Now that shaft allowed them to finish up all the work in the tail tracks, about a year and a half, two years ahead of schedule. If we didn’t have that shaft, we would have to rely on New York City Transit work trains, and there is a limited number of work trains that you can use on the line because you disrupt service.”

One of the significant contributions to the project’s progress was the TBMs finishing earlier than expected. Kildare attributes this to changing a shaft that had initially had two elbow connections. Modifying that to a straight connection allowed the contractor to use it for drill and blast operations for the 34th Street Station. “Having an additional shaft created a ripple effect for the project,” Kildare says.

Consisting of two components, an upper mezzanine level and a platform level, the station was originally slated for drill and blast in the upper portion and the TBMs excavating the lower one. But the contractor was able to make good progress with the extra shaft, and the ground behaved so well that drill and blast was used for the platform area too.

Once the excavation finished, in October 2009, the TBMs, staggered by no less than 100ft (30.5m) as required by the contract, could be walked through the cavern to the north, explains Wahl. “So the first TBM came through, they disengaged it from the trailing gear, moved it toward the centre of the cavern and then pulled it forward on rails. And then the trailing gear followed.”

This arrangement allowed the contractor to continue with concrete works within the cavern while the TBMs were being passed through, and the mucking operations to travel through the cavern during the concrete works. For muck removal the TBMs each had six buckets in the gauge area, four cone buckets and two backward buckets. Mulhauser muck cars transported the material to a vertical belt, which raised it up the launch shaft to dump trucks for disposal in New Jersey.

Another time advantage came from the Port Authority of New York and New Jersey, which allowed the contractor to work 24-hours-a-day rather than limit night hour working as initially arranged. The 200ft by 50ft by 40ft (61m x 15m x 12m) TBM reception chamber excavated by drill and blast in 2009 is adjacent to the current terminus for the No. 7 line and just below the Port Authority Bus Terminal. Drill and blast operations underneath the Port Authority Bus Terminal’s basement, where buses load passengers, were carried out around the clock, saving time and avoiding the need to stop, clean up, shut down, reopen and set-up again. “Now that particular section is nearly two years ahead of schedule,” says Kildare.

During the project, there weren’t any issues with settlement and local buildings, says Rosteck. “We did preconstruction surveys and there was an extensive instrumentation programme dictated by the contract so that we could check this and verify that we aren’t getting settlement.”

The TBMs each broke through in June and July and are being partially dismantled and backed up to a shaft excavated at 35th Street and 11th Avenue for removal.

Now the project will see three smaller core and shell contracts up for bid. These will provide ventilation, traction power substations and the primary station entrance. One was awarded in July to CCA Civil-Halmar International Civil Construction in July to mine and line a shaft/adit and construct a 2-storey ventilation building. Two other contracts are in the bidding phase, with due dates in late August and September.

Precast concrete segments built the lining as the TBMs bore the tunnels Formwork at the 34th Street Station’s mezzanine, looking south Disassembling one of the two TBMs used to bore the tunnels. Visit our digital edition to watch one of this summer’s 7 Line extension breakthroughs Looking east to the existing 7 Line tracks