The work and the technique: San Francisco Metro Line, Central Subway Project, USA

Works to build the Central Subway included building two parallel tunnels for trains, measuring approximately 2.6 km each and also 4 stations along a 2.7-km-route. The Webuild Group built the two tunnels, the preliminary works for the future stations and consolidation ones, also with the entrance ramp and the extraction shaft for the TBM that measures 12.2 m x 12.2 m and is 15 m deep.

Tunnel excavations were built with 2 twin Robins TBMs (Tunnel Boring Machines), both of the EPB type, with a diameter of 6.3 m, capable of excavating with a stabilizing pressure up to 4 bars at the front.  The station bulkheads have been built by using benthic sludge excavations to install concrete diaphragms (for the Mscone/Yerba Buena) stations and also by installing drilled poles (for Union Square Station). The material consolidation of one of the cross passages under the aquifer level, was built by freezing the soil (a special technique), to minimize impacting the local community.

The main challenge was facing the technical difficulties when building the two tunnels that cross a city centre like that of San Francisco; in fact, to meet this complex situation, premium engineering skills had to be used, with constant geo-technical controls, frogmen and specialized medical personnel for support during maintenance interventions to be carried out with a pressure greater than the atmospheric one. Particular technical innovations have been applied to save materials and workmanship during the construction of the retaining walls and rooms, to ensure managing transport flows to and from an efficient and not-very invasive site in a densely populated area.