The Second Avenue Subway project will be the first new subway line in New York City in more than 70 years. The project manager, Metropolitan Transit Authority Capital Construction (MTACC) needed an innovative solution: A dust-dry tunnel, built on a tight schedule, and cast-in-place concrete provided the answer.
The first contract of the $4.4 billion 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 built using a tunnel boring machine (TBM) and two 30-foot diameter construction shafts 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 Avenue Station.
The 19-foot, 9-inch interior diameter running tunnels were first 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 and was selected.
The constructed final liner included 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, rather than reinforcing steel, in its final liner.
The high-strength, low-permeability concrete mix included 650 pounds of cement per cubic yard, 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 rapid chloride permeability test result of 1450 coulombs at 56 days, and 2% air entrainment that achieved over 8000 psi compressive strength in 28 days and over 9000 psi after 56 days.
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. 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 with a vertical drop of almost 60 feet. Overall, the contractor placed more than 18,000 cubic yards of concrete to form the tunnel liners.