Bay Area Rapid Transit Transbay Tube Seismic Retrofit

Connecting San Francisco, California, and its neighbor Oakland underneath the San Francisco Bay, the Transbay Tube (TBT) is the most critical asset in the Bay Area Rapid Transit (BART) rail system, accommodating up to 24 trains in each direction per hour.

Built in 1969, the steel and reinforced concrete TBT runs south of Alcatraz Island 80 feet underwater, buried by sediment at the bottom of the bay, and connects popular areas such as Haight Ashbury, Nob Hill, and Union Square with locations in Oakland. Its cross-section resembles a pair of binoculars, with two 70-foot-diameter tunnel bores that carry the subway trains.

Adjacent to the bores are 8-foot-wide spaces known as galleries, used as exhaust chambers and access areas. The entire tube is 48 feet wide and 24 feet high, with seismic joints at each end allowing six degrees of freedom to move in multiple directions in the event of an earthquake.

Our Client’s Challenge

As part of their risk management initiative, BART embarked on a major retrofit of their transit system, adding an inner steel liner to critical sections of the TBT and a new water pumping system. The project mitigated damage from cracks and leaks in the event of earthquakes or landslides. It is a vital part of BART’s Earthquake Safety Program, which includes $1.2 billion in seismic improvements to the BART system to resist a “maximum credible earthquake”— a 7.1-magnitude quake on the nearby Hayward Fault or an 8.0-magnitude quake on the San Andreas Fault. Landslides caused by large quakes could move the tube, straining and possibly cracking the outer steel shell and causing leaks.

Our Solution

Gannett Fleming provided construction management and inspection services to BART for the retrofit, including:

Cost management and scheduling.
Quality assurance.
Procurement services.
Claims management.
Dispute resolution.

Our firm was also responsible for construction inspection of the retrofit work and communicating directly with the contractor, ensuring compliance of BART’s standards and that the project was completed on schedule as designed.

Installing the liner required custom-made machinery, flatcars, and 10,000 tons of steel plating. Mechanical arms lifted the plates off the work train and maneuvered sections of the curved steel plates into place. Then, crews welded the plates together and bolted them into place, using grout to adhere the steel plates to the concrete walls.

The subway tracks required the installation of waterproof membranes beneath without disrupting service. Crews raised a section of track each night, replaced its concrete bed, and inserted a layer of plastic heat membrane. This technique eliminated the need to cut the steel rails.

An upgraded pumping system featuring 33 new pumps can now remove more than twice the amount of water from the tube per minute compared to the existing system. The construction team used a plate-handling machine to maneuver sections of 3-ton, curved steel plates to form archways inside the tube.

The plates were installed with grouted anchor bolts, between 2.3” and 2.5” in diameter, drilled into the concrete inner surface of the tube, and field-welded together. The area between the back of the plates and the inner tube surface was grouted, and the plates were painted with a polymer product, avoiding the need to cut tracks and shut down rail service for prolonged periods.

Accessing the tube with several tons of equipment and materials necessitated using a specially built, 800-foot-long maintenance vehicle consist (MVC) that includes 14 flatcars powered by three locomotives hauling plate-handling devices and volumetric concrete mixers. Specially trained BART technicians operated these rail systems.

Key Features

Steel lining along key sections of the tube builds resilience against cracks and leaks caused by seismic activity.
A pumping system with new pumps ranging from 0.5 to 150 horsepower and 14,261 feet of 12-inch discharge line enhances water removal capability.
An 800-foot-long work train provides easy access to the tube, enabling efficient construction between BART stations to reduce rail service disruption.