San Francisco, CA

Bay Tunnel

San Francisco Public Utilities Commission


The award-winning Bay Tunnel project is part of the SFPUC’s $4.6 billion capital improvement program to upgrade the Hetch Hetchy Water System. The project extends 5 miles (8 km) under the San Francisco Bay, replacing the aging pipeline infrastructure built in the 1920s and 1930s that traverses the Bay on wooden trestles. Bay Tunnel is the first tunnel excavated under San Francisco Bay and is designed to withstand severe seismic shaking because of its close proximity to two major faults and the soft ground conditions the tunnel is in.

Scope of Work

Delve Underground led the design team and provided design services during construction 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 (up to 3.5 bar) and was excavated through a short section of highly weathered Franciscan Complex bedrock that is very hard and abrasive.

Challenges & Innovations

Delve Underground addressed numerous design challenges on this project. These included difficult ground and groundwater conditions including high water pressure; the excavation of stiff clays, sands, and hard rock within the alignment; 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.

Fast Facts
  • First tunnel excavated under the San Francisco Bay by tunnel boring machine (TBM)
  • 5 miles (8 km) of TBM-excavated soft-ground tunnels
  • 108-inch internal diameter (2,745 mm) steel liner
  • 90-foot-deep (27 m) and 124-foot-deep (38 m) shafts in wet conditions
  • Mixed ground conditions