A construction approach called “deep tunneling” will be used to construct the Central Subway. Deep tunneling allows most of the work to be done below ground, reducing disruption on the surface. The tunneling will be accomplished with a Tunnel Boring Machine (TBM), detailed in the next section, that has been used extensively throughout the world. Deep tunneling has great potential for controlling project costs by minimizing surface construction staging, reducing utility relocations and shortening construction time.
The only visible tunneling activity will occur at the portal construction location (4th Street between Bryant and Harrison streets) and at the excavation site (Columbus Avenue at Union Street).
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Tunnel Boring Machine (TBM)
A TBM consists of a rotating cutterhead within a cylindrical steel shell that is pushed forward along the axis of the tunnel while excavating the ground through the cutterhead. The steel shield supports the excavated ground as required until the final tunnel lining is built in the rear of the shield. The shield is propelled using hydraulic jacks that thrust against the erected tunnel lining system. The TBM is used in conjunction with a prefabricated ground support system, which consists of pre-cast concrete segments that are bolted and gasketed to form a watertight lining.
Pressure-face TBMs that are capable of exerting a balancing pressure against the tunnel face are used to control excavation rates and groundwater inflow, as well as to maintain stability of the tunnel face.
After completion of TBM excavation and installation of the lining, the temporary rail and conveyor system are removed, the invert is cleaned, and a flat invert for the permanent rail fixation and a raised walkway are constructed as reinforced, cast-in-place concrete. The invert contains embedded pipes and inlets for track drainage.
TBM’s will be used to construct the entire length of both tunnels. The tunnels will be approximately 8300’ long and range in depth from approximately 40’ to 120’.