Components will make their way down to a 200-ft-long, 37-ft-high assembly vault that, upon project completion, will act as a fore bay. Next, segments will slide over a 360-ft-long, 3-ft-deep concrete trench using a 400-ton hydraulic jacking frame. Once assembled, a dozen people will operate the 6,000-hp, non-air-conditioned TBM, including a mechanic and electrician. Tunnel temperature is about 90° F, with high humidity.

The caterpillar-type machine will run 24 hours a day, moving up to 40 feet per eight-hour shift, barring any mechanical downtime. VTC has about $2 million in replacement parts on-site.

The pressurized TBM drill face can operate in both open and closed mode, giving it versatility for tackling both solid and soggy soil conditions. The machine will reach a peak speed of 6 rpm and remove a total of 250,000 cu yd of material while placing 2,500 supporting tunnel rings.

Each 35-ton ring consists of six separate 8,000-psi precast pieces, and the contractor hopes to average five rings per shift. Ring segments are transported using 16 trailers hitched to the back of the TBM and lifted into place with a crane. The slightly tapered ring segments are bolted together and continuously injected with grout; compression does the rest.

The TBM has already found an adopted home upon project completion, says Jim McDonald, VTC project manager. "We’ve already reached an agreement with Herrenknecht to buy back the machine once were done, as well as any unused parts," he says.