The city of San Francisco dedicated in October the first seismically sound tunnel built under San Francisco Bay, the $288 million Bay Tunnel.

The Bay Tunnel will connect Hetch Hetchy and East Bay water supplies with San Franciscans and nearby communities.

The project was completed under budget and ahead of schedule and was built almost directly on top of three major earthquake fault lines, including the San Andreas and the Hayward faults.

The tunnel is part of the Water System Improvement Program (WSIP), a multi-billion-dollar program created in response to the 1989 Loma Prieta earthquake. That earthquake served as a catalyst for the San Francisco Public Utilities Commission to launch WSIP ro prepare for the next big quake, which the U.S. Geological Survey says the Bay Area can expect sometime during the next 30 years.

“The new Bay Tunnel will ensure that we have reliable access to Hetch Hetchy water at all times, particularly within 24 hours of a major earthquake,” says SFPUC General Manager Harlan Kelly, Jr.

Creating a Lifeline

The Bay Tunnel replaces two aging pipelines that sit on the Bay floor and is one of the last WSIP projects to be completed.

The Bay Tunnel acts as a "seismically-reliable lifeline" connecting the Hetch Hetchy and East Bay water supplies with customers on the Peninsula and in San Francisco,” the SFPCU says in a news release.

Construction on the 5-mile-long tunnel began in April 2010 and was delivered at a cost of $288 million, some $25 million below the original estimate.
Early in the project, Johanna Wong, project manager, realized that some of the biggest challenges would include seismic performance and reliability, the environment the tunnel was being built in and keeping the project within budget and on schedule.

Wong managed an RFP process designed to find a design consultant with tunnel experience, especially with the geotechnical factors found in the Bay Area. The SFPUC chose Jacobs Associates as the lead design consultant and then integrated its staff with the firm to perform a pipeline seismic analysis.

A decade ago, Jacobs Associates completed a seismic upgrade to the 80-year-old Claremont Tunnel, which carries 175,000 gallons of water a day to 800,000 residents in the East Bay area. The upgraded Claremont tunnel won two awards from the American Society of Civil Engineers for its seismic safety and innovative design features.

As the prime consultant, Jacobs Associates led the tunnel design team—which also included URS, Fugro West, and Telamon Engineering.

Building a Quake-proof Tunnel

One of the first decisions the SFPUC had to make with the WSIP project was to decide whether to build a pipeline or tunnel. After consideration, the decision was that a tunnel would be more seismically sound than a pipeline.

“When you’re a surface pipeline, you have more shaking than when you’re deeper,” Wong says. “The tunnel is enclosed, which makes it more stable than the pipeline, and why we recognized it as the better option.”

The tunnel was a more challenging option to undertake and they’re harder to maintain because of environmental restrictions and limited access underground.

The tunnel structure is made up of several layers; a pre-cast concrete segment, a layer of cellular concrete and then steel pipe lined with cement mortar. Design engineers also specified a combination of fiber steel and concrete to create more stability in the event of an earthquake.

The cement rings that make up the tunnel were built at the Traylor Shea Precast facility, which produced 20 rings a day on average at its peak for a total of 5,230 rings.

The Hitachi Zosen earth-pressure-balance tunnel boring machine. Source: Jacobs Associates

The tunnel lies at depths ranging from 75 to 110 feet 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 hard and abrasive. The tunnel was lined with precast concrete segments followed by steel pipe as final lining, resulting in an internal finished diameter of 108 inches.

“You have to think far enough ahead for what you will need, instead of realizing the needs during the construction process,” Wong says.

That’s where the extensive geotechnical research comes in—to ensure the project will not run into surprises halfway through.

The environmental design phase alone took five years with an extensive geotechnical investigation that cost $4 million. The engineering design team tried to cover the entire shoreline but ended up 7,000 feet short because some property owners resisted having marine drilling and investigation done on their property. Engineers completed 17 land borings up to 300 feet deep and 17 marine borings 200 feet deep. The engineers also conducted cone penetration testing, pressure meter testing, vane shears, piezometers and suspension logging.

Building Shafts

One of the most difficult steps in completing the tunnel was building the shafts, which had to be excavated from the inside, Wong says.

The shaft's concrete bottom is 136 feet below the surface, and the team had to bring in divers to do work on the concrete base to ensure it was level. However, because of the depth of the water, divers could only work for 15 minutes at a time before returning to the surface.

The walls of the shaft were built using diaphragm slurry wall construction methods. One of the biggest challenges with the shafts was the inability to build and use intermediate construction shafts because of environmentally sensitive habits, according to Jacobs Associates.

The tunnel's completion and dedication in October 2014 means WSIP is one step closer to completing its goal of 83 different projects to upgrade infrastructure in preparation for an earthquake.

“This tunnel will survive an earthquake, and enable everyone to continue to get water,” Wong says.

And Dr. Kit Miyamoto, CEO of Miyamoto International, Inc., a global earthquake and structural engineering firm, applauds San Francisco's effort to prepare for future seismic activity and having a sense of urgency about that preparation.

“Memories of earthquake disasters fade, and when memory fades that’s often when danger strikes,” Miyamoto says. “Across the country we need to go faster. The next big earthquake could happen tomorrow.”