Second Avenue Subway TBM Tunnels
The Second Avenue Subway project is a $4.4-billion entity that will be first new subway line in New York City in over 70 years. This latest addition to the storied New York City subway system will run under Second Avenue from 96th Street south to 63rd Street and will connect to the existing Lexington Avenue/63rd Street Station. Simply stated, the project manager, Metropolitan Transit Authority Capital Construction (MTACC) needed an innovative solution: A dry, dusty tunnel, built on a tight schedule—and concrete—provided the answer.
The first construction contract of the project, awarded to S3 Tunnel Constructors (S3), a tri-venture of Skanska, Schiavone and Shea, consisted of an 800-foot-long Launch Box near 92nd Street: 15,000 linear feet of circular running tunnels constructed by Tunnel Boring Machine (TBM) and two 30-foot diameter construction shafts located at 69th and 72nd Streets. The TBM tunnels run from 92nd Street and connect to the existing stub tunnel that is part of the 63rd St/Lexington Ave. Station.
High-strength, durable concrete that could last for the required 100-year design life of the new subway line was the material of choice, a decision made early on in the design phase. The goal of that “dry and dusty” tunnel was achieved with the use of cast-in-place concrete for the tunnel liner.
The 19-foot, 9-inch interior diameter running tunnels were initially proposed to be lined with precast concrete segments, but during the bidding phase the option of using a cast-in-place tunnel liner was extended to the bidding teams. The constructed final liner consisted of a 12-inch thick concrete liner reinforced with both steel and polypropylene fibers. It is believed that this is the first cast-in-place tunnel in New York City that used steel fibers and not reinforcing steel in its final liner.
The concrete was a high-strength, low-permeable mix that included 650 pounds of cement, 150 pounds of fly ash, 3/4-inch aggregate, a high-range water reducer, 30 pounds of steel fiber, and 1.75 pounds of polypropylene fiber. The result was a concrete liner with a water cement ratio of 0.37, a coulomb test result of 1450 at 56 days, and 2% air entrainment that achieved over 8000 psi in 28 days and over 9000 psi after 56 days. To place the concrete tunnel liner, the contractor used a 240-foot long invert form and three 40-foot long arch forms to place the arch liner in 120-foot segments. The invert was placed first, followed by the arch. All construction joints had key joints and PVC water stops. The tunnels were wrapped with a PVC waterproofing membrane and had both contact and remedial grout tubes.
To pump the concrete, the joint tri-venture had an access shaft near 78th Street and pumped the concrete distances greater than 1000 feet horizontally, and a vertical drop of nearly 60 feet. Overall, the contractor placed over half million cubic feet, or 18,000 cubic yards of concrete, to form the tunnel liners.
Scheduling was an important issue when deciding to use a cast-in-place liner in lieu of precast segments for the TBM tunnel liner. Using steel fibers in the concrete mix saved the contractor time needed for erection of the reinforcing steel, and provided the flexural strength and crack control required to meet both the strength and serviceability requirements for the liner.