Under Second Avenue

Beneath Second Avenue on Manhattan�s Upper East Side, a 485-ton monster is chewing its way through bedrock, mining tunnels for New York City�s first new subway line in 50 years.

When completed in 2016, the Second Avenue line will run from 96th to 63rd Streets, where it will hook up with the existing 63rd Street and Broadway lines. Three new stations will be built at 96th, 86th and 72nd Streets along with new entrances to the existing Lexington Avenue/63rd Street Station.

The $4.45 billion project is the first phase of the Metropolitan Transit Authority�s multi-phase plan for the Second Avenue Subway. Subsequent phases will extend the line north to 125th Street and south to Hanover Square in lower Manhattan.

The line is expected to carry 213,000 riders each day, improving mass transit access for Manhattan�s Upper East Side and reducing overcrowding and delays on the Lexington Avenue line. Crowding on the Lexington Avenue line is projected to decrease 13 percent with travel times reduced by up to 10 minutes.

The MTA awarded the project�s first contract, valued at $350 million, to S3 Tunnel Constructors, a joint venture of Skanska USA Civil, Whitestone, N.Y., Schiavone Construction, Secaucus, N.J., and J.F. Shea Constructors, Walnut, Calif. A joint venture between AECOM, Los Angeles, and ARUP, New York designed the project.

The contract encompasses the construction of two tunnels between 92nd and 63rd Streets, excavation of the launch box for the tunnel boring machine (TBM) and access shafts at 69th and 72nd Streets. Work started in April 2007 and is expected to be completed in November 2011.

Preliminary designs for the project, done in the 1970�s, called for more cut-and-cover excavation than tunnel boring. The current project minimizes surface impacts to the public and commercial establishments along the route by going as much as possible underground, explains Geoffrey Fosbrook, senior vice president and project director, AECOM.

Using a TBM to mine the tunnels required construction of a launch box. The launch box, measuring 815-ft. long by 63-ft. wide and 56-ft. deep, extends beneath Second Avenue from 96th Street to south of 92nd Street and provides space for assembling and launching the TBM along with managing the debris excavated from the tunneling operation.

Prior to excavation of the launch box, the extensive network of underground utilities at the site were relocated. �I do not think I have ever seen a street so full of utilities,� says David Caiden, principal at ARUP.

Slurry walls and secant pile walls were built to reinforce the perimeter of the launch box. The slurry walls constructed in the northern half of the launch box are permanent walls that will be incorporated into the 96th Street Station, explains William Goodrich, program executive, MTA Capital Construction.

The cut-and-cover excavation of the launch box started in June 2009. A combination of controlled blasting and mechanical means removed 117,000 cubic yards of rock and soil.

Concrete decking installed over the excavation area allows traffic to utilize Second Avenue during construction. Two levels of lateral steel braces support the walls of the launch box. A mud matt forms the floor of the box.

During construction, preserving sidewalk and street access to businesses along Second Avenue restricted the size of work areas. �Working in such a tight area while maintaining traffic and pedestrians flow through the area was very challenging,� explains Mike Attardo, senior vice president with Skanska USA Civil.

Blasting took place in close proximity to the buildings lining Second Avenue. Sophisticated instrumentation and monitoring systems continuously monitor the building facades adjacent to the launch box and the launch box construction.

�The building stock in the area is a lot of brick masonry buildings that were built in the late 1800s to early 1900s,� Goodrich says. �Even without our construction it�s pretty fragile, and very sensitive ground induced settlement or vibrations. It�s been a challenge and we worked very hard to do this heavy civil construction in close proximity to the sensitive buildings without damaging them.�

Noise mitigation efforts are employed to reduce the disturbance to the community and include a prohibition on working in the street between 10pm and 7am in the morning.

Tunnel Boring

The TBM was delivered to the launch box in pieces starting in April after undergoing rehabilitation at Schiavone�s shop in Newark, N.J. Rehabilitation work essentially replaced all the mechanical and hydraulic components to as new conditions, explains Paul Scagnelli, Schiavone�s executive vice president and chief engineer.

Pieces of the TBM, the largest weighing over 120 tons, were lowered through an opening in the decking to the floor of the launch box for assembly. When assembled the TBM weighs 485 tons and is 450-ft long, including its trailing gear.

On the front of the TBM is a 22-ft. diameter cutterhead with 44-disc cutters. During operation the cutterhead turns while six hydraulic cylinders push the disc cutters against the rock face, making concentric pats. The force of the thrusting discs against the rock makes cracks, fracturing the rock face. �When the cracks join, slivers of rock fall out of the rock face,� explains Julio Martinez, tunneling manager for Schiavone Construction.

The TBM is held in place during the thrusting operation by curved gripper shoes that press against the side of the cylindrical tunnel walls and push the front of the system forward, Scagnelli explains. �After you push ahead six-feet or so there is an alternative support on the machine that supports it vertically so you can retract the side gripper shoes and then pull the machine ahead. �It is sort of like a caterpillar making its own hole.�

As mining proceeds, rock bolts and steel ribs are installed to provide temporary tunnel support as required. At intervals the tunneling team probes ahead to insure they don�t mine into bad ground or water conditions.

�Average penetration is expected to be 50 to 60-feet a day,� Scagnelli says. The pace takes into consideration the installation of rock supports, maintenance, changing cutter discs, grouting as required and probing.

The rotating cutterhead equipped with buckets that scoop up the debris and deposit it on a conveyor belt inside the TBM. The conveyor system transfers the material to the rear of the machine for removal.

Excavated material from the TBM is transported by another conveyor system to the launch box. Instead of using a conventional crane hoisting system, a vertical conveyor with buckets will lift the excavated materials to the surface and deposit it in waiting trucks. �It is quieter and more reliable and we believe it will be accepted by the community,� Scagnelli explains.

To meet the schedule, tunnel mining will occur 24 hours a day. Above ground operations must cease after 10pm to minimize noise.

�To work 24 hours we came up with a system that uses the full length of the launch box to store the excavation underground using a series of stacking conveyors,� Scagnelli says. The material is transported to the surface during the day, starting at 7am in the morning.

Mining of the 5,006-ft. long west tunnel started in late May and is expected to take five-months. When completed the TBM will be pulled back to the launch box and slid over to drive the 5,800-ft. east tunnel, which will take seven months.

The concrete liner for the west tunnel will be constructed during mining of the east tunnel. The length of both tunnels will be lined except where the tunnels pass through future stations. That work, along with excavation of the stations, will be bid under further contracts.

The poured-in-place liner will use concrete reinforced with steel and polypropylene fibers for fire protection. �During a fire the polypropylene fibers melt, creating voids,� Caiden explains. �The voids allow space for the water held within the concrete to expand into as it becomes vapor. This minimizes explosive spalling that would otherwise occur and reduces fire damage.�

Additional work under this contract includes construction of two 30-ft diameter shafts at 72nd Street and 69th Street and a pumping station at 78th Street. The shafts will be used by a future contract to construct the 72nd Street Station.

Follow-on Contracts

At the contract�s completion, the launch box will be turned over to an E.E. Cruz�Tully Construction joint venture to do the structural work for the 96th Street station. Cruz-Tulley will also extend the station to connect with a tunnel segment stretching from 99th to 105th Streets built in the 1970�s. The segment will be used for train storage and maintenance.

Additional contracts will be awarded for fit-out of the 96th Street Station; excavation and construction of the 86th and 72nd Street Stations; rehabilitation work on the existing 63rd Street-Lexington Avenue Station; and installation of track, power, signal, and communications systems.