It’d be an understatement to say the Second Avenue Subway is long-awaited by New York’s residents. There have been plans and proposals dating back to 1929, and in the early 1940s the elevated trains along Second and Third Avenues were even demolished to make way for future development.

Fast forward to 2011—certainly some would dispute the use of the word ‘fast’— when the city saw the first breakthrough of the first phase to construct the line. And some may scoff at celebrating this step toward completion. The city has seen several sections of tunnel built for the Second Avenue Subway since the 1960s and ‘70s that never materialised into a working line due to budgetary issues. But the New York Metropolitan Transportation Authority Capital Construction (MTACC) reports the project is on schedule to finish in December 2016.

Phase One
Phase One entails a two-track line connecting 105th Street and Second Avenue to existing services at 63rd Street and Third Avenue, with new stations at 96th, 86th and 72nd Streets.

Although the full length of the Second Avenue line won’t be completed, when Phase One finishes in five and a half years, the subway service is projected to carry more than 200,000 weekday riders. Future Phases Two through Four will complete the line from 125th Street down to Hanover Square in lower Manhattan with 16 new stations (Figure 1).

A Skanska, Schiavone and Shea joint venture (S3 Tunnel Constructors) secured the contract to build new tunnels between 92nd and 63rd Streets, excavation of the TBM launch box and excavation of access shafts at 69th and 72nd Streets. These shafts will be used for constructing the 72nd Street station, which is part of a different contract. Likewise, excavation of the 96th and 86th street stations come under separate contracts, as does the connection to existing tunnels from 99th to 105th Streets (see Current Contracts box). DMJM Harris and Arup is providing design and engineering services.

Tunnelling started at 92nd Street to make use of the Manhattan Schist, which continues to the south (Figure 2 below). The launch box measures roughly 810ft (247m) going north to south, approximately half in rock at the southern end and half in soil at the northern end (Figure 3, p20). In the rock cut, the width is around 66ft (20m), and in the soil cut, the width is 58ft (17.7m).

The box walls consist of two different types of construction, secant walls at the southern half and slurry walls at the northern half. The slurry walls at the middle portion of the box are founded within rock. Northward from there, the slurry walls are founded within the soil with approximately 40ft (12m) of toe below the final excavation limits. From the southern end to the middle portion of the box, the walls are constructed with secant piles founded within the rock.

S3 had been awarded the contract in March 2007, but it wasn’t until early 2010 that the contractor was ready to begin underground work. “We had to perform a significant amount of utility and other preparatory work in order to build the walls of the box and that took about a good year, year and a half, ” explains Julio Martinez, project engineer with Schiavone Construction Co., working in the S3 JV. “And then we had to install decking on top of the future excavation since traffic still had to be able to transverse above us while we excavated within the box.”

Monitoring movement
MTACC surveyed 225 buildings on Second Avenue that could be affected by tunneling work, and 51 buildings were deemed ‘too fragile’ in autumn 2010. These are primarily located near the TBM launch box, and are four- to five-storey brick buildings built in the 1800s with shallow foundations, supported on soil or timber piles.

Martinez describes them as basically floating on the soil, which is a mix of sand, silt and clays. Prior to the construction of these buildings, this part of Manhattan was shoreline, which was then reclaimed to amplify the city’s limits. “This fill was not controlled. It wasn’t compacted as fill is placed in modern times. So whatever soil you move, it’s actually going to react much quicker,” he says.

The buildings were affected by work to excavate the launch box and construct its walls, and there were some so sensitive, they started to move while S3 was doing utility work. Reinforcement is being undertaken by the MTA. When it came to surveillance and monitoring the owner was prudent, says Martinez, “That’s what kept the problem from getting worse.”

But these buildings and the reinforcement work have affected excavation operations. The MTA initially predicted a progress of 50ft (12m) per day, and T&TI reported in November 2010, TBM excavation was progressing at 40ft (12m) per day.

Besides buildings there are utilities overhead—between 40ft to 20ft (12.19 to 6.1m) away as the tunnel’s elevation changes through the alignment. Lengths and feed pressures on drilling and probing have to be less aggressive.

“We’ve had to do more probes than what we normally would have with a machine at a deeper depth, which increased the mining cycle a little bit,” says Chris Cosenzo, tunnel superintendent with Schiavone Construction Co.

He later explains, “We put up a lot more ribs and major rock support that we didn’t expect to have to put up, which in turn slows the mining process down. Which is when you don’t get to see the production you expect because you’re constantly trying to fight through the bad zones and put up proper rock support.”

One down, one to go
The tunnels range in depth from 60 to 100ft (18.3 to 30.5m) from the surface, and each bore has a diameter of 22ft (6.7m) with an 18ft (5.5m) rock pillar separation between them. TBM excavation for the 7,215ft (2.3km) western bore finished in February at 65th Street.

S3 is using the same TBM for both bores to save space in the launch box and to avoid the logistics of doubling the electricity, water, ventilation and other services required. The JV is using a reconditioned open-face main beam gripper machine with 44 cutters, 17in in diameter, from Shea’s fleet—a decision that saved time in a tight schedule. Working with Herrenknecht, it took roughly five to six months to rebuild the machine.

“We took advantage of it when we bid this work, and that’s why we were the low bidder,” Martinez says.

To save time, the contractor wanted to bring the machine back through the western tunnel fully assembled, with the hydraulics and electricity still hooked up. The only disassembly needed was to remove the four segments of the cutterhead around the perimeter, the roof supports and the side supports.

S3 considered jacking up the machine to insert a sliding plate underneath. This would allow the TBM to be transported back through the tunnel under its own power. Instead the decision was made to fit the machine with transport dollies and tow it back to the launch box using the project’s locomotives.

With the TBM tunnelling the eastern bore, as of late March, S3 has started work to treat select stretches of the western bore with a PVC membrane waterproofing system, prior to receiving its 13.5in (342.9mm) thick lining of concrete with steel and polyethylene fibers.

Freezing ground
The 7,793ft (2.4km) eastern bore has proven to be more complicated than its western counterpart. While tunneling started here in March, Moretrench of New Jersey has been providing ground freezing services since January for an area approximately 147ft (44.8m) long at the portal of the east bore (around 91st Street).

While doing secondary support for the starter tunnels, S3 encountered soil and water. “We notified the owner and performed exploratory borings,” Martinez says. “In the process we did find that part of the alignment, the crown, was in soil-like material. In light of these findings the owner prudently elected to freeze this stretch of the alignment.”

Approximately 110 freeze pipes of 3in (76.2mm) internal diameter have been installed around the tunnel alignment at depths of approximately 75ft (23m), at varied angles.

“We had to angle as much as 30 degrees off of the vertical in order to avoid water lines, gas lines, all the utilities,” says Kenneth Wigg, senior engineer with subcontractor Moretrench.

He explains. “We couldn’t get exactly where we wanted to be. So we had to really look at the placement of the freeze pipes, and in some places we had to add freeze pipes, to get those pockets we couldn’t reach because of the utilities.”

Freeze operations are using calcium chloride brine, and as the machine bores the east tunnel a thin interliner is being installed to seal off and support the tunnel crown prior to the thaw. Within the next three months, subcontractor Moretrench should be able to dismantle its surface level freeze plant.

It should take between six to 10 months to breakthrough at the 63rd Street Station. However at the end of the east bore, the TBM makes a tight compound curve to meet up with existing tunnels and train services at 63rd Street.

“I think it’s probably in the extreme range of what the machine can do. It’s a pretty tight radius,” says Martinez. “It starts around 830ft (252.98m) radius and ends about 620ft (188.98m) radius. The reason they had to keep it so tight is because of eminent domain and zoning issues.”

Working in such a densely-populated city has created other constraints for the project. Trucking for the project’s spoil removal and deliveries have limited hours from 7am to 7pm because the area is residential. There is also limited space in the launch box.

“We can’t stack or store large volumes of materials. We have to truck them in and out of the tunnel as we receive them. It’s a constant chess game to get materials into the tunnel and keep everything in a productive manner,” Cosenzo says.

Martinez hopes the eastern bore will complete later this year.

The TBM mined the western bore before being drawn back almost fully assembled to work on the eastern bore Figure 1, the Second Avenue Subway will be built in four phases; Phase One (inset) is schedule to be operational by 2016 Figure 2, Phase One TBM excavation started at 92nd Street to take advantage of the Manhattan Schist Figure 3, the launch box is 247m long. Secant walls are used in the southern half, which is founded in soil and slurry walls for the northern half in rock