Bay Tunnel

The Bay Tunnel project is part of the San Francisco Public Utilities Commission’s (SFPUC) $4.6 billion capital improvement program to upgrade the Hetch Hetchy Water System, which brings water through gravity-fed pipes from Yosemite National Park to the San Francisco Bay Area. This tunnel will extend 5 miles (8 km) under San Francisco Bay, replacing the aging pipeline infrastructure built in the 1920s and 1930s that traverses the Bay on wooden trestles, and will provide seismic and delivery reliability after a major earthquake for about 2.5 million customers in the San Francisco Bay area. This is the first tunnel to be excavated under San Francisco Bay.

As the prime consultant, Jacobs Associates led the tunnel design team—which also included URS, Fugro West, and Telamon Engineering—and is currently providing construction support services for this challenging project. The tunnel lies at depths ranging from 75 to 110 feet (23 to 34 m) in sandy and silty clays under high groundwater pressures of up to 3.5 bar and through a short section of highly weathered Franciscan Complex bedrock that can be very hard and abrasive. Because of these ground conditions, a 15-ft-diameter (4.56 m) earth pressure balance tunnel boring machine (TBM) is being utilized for excavation between two deep slurry diaphragm wall shafts. The tunnel will be lined with precast concrete segments followed by steel pipe as final lining, resulting in an internal finished diameter of 108 inches (2,743 mm).

Jacobs Associates faced numerous design challenges on this project. These included difficult ground conditions, the proximity of California’s largest active seismic faults (the San Andreas and the Hayward faults), and the presence of environmentally sensitive habitats which required that no intermediate construction shafts be built.

The project is currently under construction and scheduled for completion in spring 2015. Jacobs Associates is providing design support services during construction. The 58-ft inside diameter (17.7 m), 124-ft-deep (37.8 m) Ravenswood launch shaft is located in Menlo Park and was completed in May 2011. It was built using diaphragm slurry wall construction methods. The TBM, manufactured by Hitachi Zosen (Hitz) in Osaka, Japan, has arrived on site.