Skanska was part of the S3 II Tunnel Constructors joint venture that was awarded a $1.1 billion project from New York City Transit to construct the extension of the No. 7 subway line, from Times Square to the west side of Manhattan. The project included mining approximately 12,000 feet of new 22-foot diameter tunnels, including three cross passages and one underground pump station for the system and construction of an underground pump station at 34th Street with two crossover interlocks. Additionally, a two track box structure was constructed in a decked open cut beneath the lower level of the Port Authority’s 42nd Street bus terminal, which is the world’s busiest bus terminal with 250,000 passengers per day. Excavation for the tunnels was done utilizing a combination of hard-rock tunnel boring machines (TBMs), drill-and-blast, cut-and-cover and ground freezing. Work began at 26th Street and 11th Avenue with installation of an offset access shaft and cross-adit for construction of the tail and starter tunnels. This area served as an assembly and launch area for the double-shielded TBMs and was excavated utilizing drill-and-blast techniques. Two mixed-face areas required pre-excavation soil freezing for ground improvement prior to TBM mining. The tunnels were constructed using approximately 65,500 square yards of permanent pre-cast concrete one-pass liner segments.
A rail system and vertical conveyer was used for muck removal. The 34th Street Station required approximately 118,000 cubic yards of mined rock excavation by SEM (sequential excavation method), which was removed via drill-and-blast. This underground cavern was over 1,200 ft. in length and included seven entrances and access tunnels, which were constructed in multiple drifts and headings from two 40’ diameter shafts. The joint venture was responsible for design and installation of all initial rock support elements, including rock-bolts, shotcrete and steel ribs. Construction of the reinforced concrete station required approximately 49,000 cubic yards of concrete and 4,200 tons of reinforcing steel.
Where the new tunnels connect to the existing tail tunnel at Times Square Station, the project team was required to lower the existing track bed nearly seven feet to meet the new tunnel. That work required extensive underpinning of the existing 8th Avenue subway line using a variety of methods including drilled-in low headroom mini-piles and temporary steel bents. Part of the Times Square connection was to construct a receiving chamber and double box subway structure below the lower level of the Port Authority bus terminal. This required the removal of the 6,500 cubic yards of rock using close-in blasting and installing a temporary decking system to maintain bus traffic within the terminal.
The No. 7 line extension will provide a much needed mode of mass transportation to the west side of Manhattan and is anticipated to help generate significant development there.