This video from Herrenknecht AG shows the operation of the enormous tunnel boring machine that will conduct the deep tunnelling for San Francisco's new subway lines. The machine obviates the necessity of tearing up city streets for subway construction, and somehow manages to be gentle enough to avoid shaking the buildings above it. There's a much older version of this monster on display at the fabulous London Transport Museum in Covent Garden that is truly awesome to behold.
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.
(Thanks, Fipi Lele!)