The Transbay Tube turns 50: Inside the groundbreaking history and future of the Bay's underwater crossing

A diagram of the Transbay Tube that shows the outer steel shell, concrete liner, and steel plates that line the Tube's interior.

Loma Prieta had a magnitude of 6.9. What would happen to the Tube in the event of a once-in-a-thousand-years earthquake?

This was the thought rumbling around the mind of Jim Dunn, BART's then-Chief Engineer, when he instigated what became known as BART's Earthquake Safety Program (ESP) in the early 2000s. The huge project was officially completed this year.

"Dunn knew Loma Prieta was not the worst-case scenario. He wanted to know if a real big one would give us a problem," said Tom Horton, who was BART Group Manager of ESP before retiring in 2017.

With ESP, BART was not just preparing for a major earthquake, but one that might be centered closer to the core system, which runs directly adjacent to the San Andreas and Hayward faults.

On November 2, 2004, voters in Contra Costa, San Francisco, and Alameda counties approved Measure AA, which allowed BART to issue general obligation bonds to fund up to $980 million of the $1.4 billion total cost of earthquake safety improvements. The team hit the ground running. The Tube retrofit began years later, in 2017.

"We started with a major evaluation of the system and how vulnerable it was to earthquakes," Horton said. When Horton started, he had three employees. At the height of the project, more than 100 people were working on the program, 30 of whom were devoted to ESP full-time.

"I have immense respect and appreciation for the personnel who worked on this," said Zach Amare, BART Assistant Chief, Infrastructure Delivery. Nearly every department at BART had a role to play, including but not limited to the Operations Control Center, Traction Power, Track Maintenance, System Safety, Power/Mechanical Engineering and Maintenance, Operations Planning, Locomotive Operators, Operations Liaisons, Government and Community Relations, Communications, and many, many more.

He credits the project's success to four things: dedicated BART personnel; executive management who offered full support from the get-go; skilled contract personnel "who were up for the challenge to solve problems"; and a cohesive project management team.

"Without these elements, this complicated, challenging project would not have been successful," Amare said.

Chuck Bernardo, BART Group Manager of Capital Projects, said he "didn't realize how one-of-a-kind the Tube was until I started working on it."

"The challenge of the retrofit appealed to me," he said. "You're working at night and running the system the rest of the time. It was like doing open heart surgery every night then bringing your patient back to life, over and over and over again. You had to just dive right in. No one could tell you how to do it."

A key piece of the Earthquake Safety Project was the seismic retrofit of the Transbay Tube, deemed BART's most valuable asset.

"It was the most difficult engineering project I have worked on with many unexpected challenges along the way, both in design and construction," said Mark Salmon, who helped coordinate the retrofit design work for BART. "It really showed me the power of teamwork and perseverance. In many respects the Tube is unique in the world; there is no other similar example we could use to draw experience from and guide us. We had to solve the design and construction problems ourselves."

We want to be very clear: The Transbay Tube is structurally sound, but we are preparing for a rare and devasting earthquake, defined as something that happens once every thousand years. In an event this large, the Tube won't fail, but it could crack and leak.

"Underground structures in general are less of an issue because they don't see the same level of ground motion as the surface," Horton said. "The wave is confined by the soil if it's underground, but when it hits the surface, you get big movements." As evidenced by the Tube travelers during Loma Prieta, the Tube may be one of the safer places to be during a big quake.

BART worked with earthquake retrofit specialists using a combination of geotechnical and structural site investigations, computer simulations, and testing of materials and models to develop the retrofit design.

Part of the modeling process including traveling to UC Davis, which has an old NASA centrifuge once used to train astronauts. Now, researchers use the centrifuge to simulate the pressures earthquakes could apply to buildings, and in our case, the pressures the Tube might face in a major shaker.

Back in the 1960s, engineers designed the Tube to flex with earthquake ground motions to dissipate earthquake forces and absorb shock. A stiff Tube would become brittle and break in an earthquake, so the "Tube wiggle" is crucial.

Public link

Please use the above public link if you want to share this noodl on another website.