Bessac: a first step towards the SVCW project in the United States

As reminder, at the end of 2017 Bessac won a design & build project for a more than 5 km long tunnel, suppling the waste water to the treatment plant of municipalities of San Carlos, San Mateo, Belmont and Redwood City, located on the shore of San Francisco Bay.


On March 25, 2020, Bessac teams celebrated the breakout of the first drive, 1,562 m long, in the Bair Island outlet well.

This breakout occurred in a 17.70 m long flooded rectangular shaft, and with an exit plug poured in low strength concrete inside the shaft.

The excavation was carried out in very good conditions thanks to many factors which made it possible to reach record production rates with an average of almost 30 m per day in final configuration and a maximum production of 64 m in 24 hours:

  • Geology made up of consolidated clays with some sand lenses,
  • Muck-out by continuous conveyor,
  • Use of two-components mortar,
  • Possibility to store 3 segment rings of in the tunnel boring machine.

As a reminder, the overall project – done in consortium with an US partner – consists in the construction of a waste water transport tunnel of a total length of 5,313 m, excavated with a 4.94 m diameter earth pressure balanced tunnel boring machine (EPB TBM) and 3 associated shafts.

The next step is the disassembly, transport and reassembly of the machine in the launching shaft for the 3751 m long second drive.

TBM/ tunnelier pression de terre / tuneladora a presión de tierra / tunnel boring machine