- LocationSan Francisco, California
MethodsEarth pressure balance (EPB)
Custom-built tunnel boring machine (TBM)
Slurry shaft construction
Tremie concrete pouring
Ground freezing shaft construction
Precast concrete segments
Deep shaft construction
Michels Trenchless, Inc. was the managing partner of a joint venture that constructed the San Francisco Bay Tunnel. As part of a $4.5 billion water system upgrade to provide a reliable clean water source for 2.5 million people, the project included the construction a 5-mile tunnel under San Francisco Bay (the first tunnel under the bay), and launch and retrieval shafts with no intermediate shafts.
The tunnel was excavated using a 14'10" Earth Pressure Balance Tunnel Boring Machine (EPB TBM), and supported by precast concrete segments with an interior diameter of 12'10". A 108" interior diameter steel water carrier pipe was installed in the concrete lined tunnel. The steel pipe segments were joined by a full depth butt weld, and then lined with cement mortar. The annulus was backfilled with cellular grout. Four curves exist in the tunnel's alignment, each with a radius of 1,000'.
The project also included installing two shafts, up to 141' in depth. The 58' diameter launch shaft was supported by a circular concrete diaphragm wall. The shaft invert was 141' deep. The retrieval shaft excavation was stabilized using ground freezing methods with the addition of steel rings and timber lagging with an interior diameter of 22' and a depth of 98'.
The project was located in an environmentally sensitive area between the San Andreas and Hayward faults, both active earthquake zones. The tunnel alignment crosses beneath railroads, large diameter water and gas pipelines, and is up to 110' below the water. The geology of the tunnel excavation included stiff clay, lenses of sand and gravel, and basalt rock. This required the EPB TBM to be designed for interchangeable soft and hard ground cutting tools.
The collaborative approach led to many innovations that saved the project owner nearly $13 million, with no construction claims. One of those innovations was using ground-freezing technology for the exit shaft which saved money, reduced risk of water intrusion and prevented damage to the surrounding marshland ecosystem.
The project was also completed seven months ahead of schedule with 0.63 lost work days per 1,000 hours worked by Michels, which is nearly half the industry average. The project received the Tunnel Achievement Award for Project Excellence in 2015 and the National APWA Project of the Year in 2015.