Second Avenue Subway TBM Tunnels, New York

MTA Capital Construction President Dr. Michael Horodniceanu led a tour of the Second Avenue Subway Phase I project when it was 82% complete in May 2015.

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 included.

Scheduling was important when deciding to use a cast-in-place liner in lieu of precast segments. Using steel fibers in the concrete mix saved the contractor time needed to erect the reinforcing steel, and provided the flexural strength and crack control required to meet the liner's strength and serviceability requirements.

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.

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. The contractor placed more than 18,000 cubic yards of concrete to form the tunnel liners.

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.

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 tube.

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