During the past four years, as millions of New York City subway commuters daily made their way through the transit system, tunnelers below were making history. In 2009, for the first time ever, tunnel-boring machines burrowed through ground that was artificially frozen, with crews placing precast segments in the TBMs’ wake. The twin tunnels form a 1.5-mile extension of the No. 7 subway line, from Manhattan’s Times Square station to the far West Side.
For the most part, the TBMs dug through bedrock up to about 100 ft deep. But in one 300-ft-long segment, the top of the rock layer dips, which meant tunnelers had to dig through a mix of hard rock and soft soils in that stretch.