CA Transbay Tube Retrofit Underway
Earlier this month, the Bay Area Rapid Transit agency began retrofitting work to the Transbay Tube, which runs under San Francisco Bay, carrying four transbay lines. Plans include the addition of a new steel liner, waterproof coating and pipe system to help support the structure in case of an earthquake.
The three-and-a-half-year, $313 million seismic retrofit of the 3.6-mile-long transit tunnel involves attaching, welding and grouting steel plates along the structure’s interior concrete to create a liner. According to BART, the structure opened for passenger service in 1974.
Transbay Tube History
According to SFGate, the Transbay Tube is made up of 57 individual sections that were built on land, and later towed out into the bay by a large catamaran barge. Once the steel shell was finished, water-tight bulkheads were added. Concrete made for 2.3-foot-thick interior walls and a track bed.
Tube construction began in the 1960s, and the first test trains ran in August of 1973. The cost of building the Transbay Tube hiked from the originally estimated $133 million to $180 million.
In March 2018, BART officials announced a feasibility study for a second transportation tunnel running under San Francisco Bay, which would both double capacity and allow for trains to run for 24 hours a day.
In planning for the seismic retrofit, BART worked with specialists using geotechnical and structural site investigations, computer simulations and materials testing. Installation of the inner liner inside the tube’s gallery began in 2017, and the next phase began earlier this year. The $313 million contract for the work was awarded to Shimmick Construction and California Engineering Contractors Inc.
“The Transbay Tube is structurally sound, but we are preparing for a rare and devasting earthquake- defined as a 1,000-year event- something that happens once every thousand years,” BART writes. “In an event this large, the tube won’t fail but it could crack and leak.”
Plans also include an upgraded pumping system that will allow for faster removal of larger quantities of water from the tube.
During the retrofit, a plate-handling machine will be used to move sections of steel plates to form archways inside the tube. “Once put in place they will be welded together and bolted into place,” BART writes. “Grout will then be used to adhere the plates to the concrete walls via grout ports in the plates.”
Tunnel fllooring will be coated with a waterproofing polymer membrane. During nights, workers are to remove a small segment of track, leveling the concrete beneath it and then coating the concrete with the membrane. Then new concrete will be poured on top, and the track will be restored. An emergency generator that will be able to power the pumping system for two days will also be installed.
According to the San Francisco Chronicle, funding for the retrofit came from a $980 million general obligation bond that was approved by voters in 2004.
Editor's Note: Corrected liner verbiage, on Feb. 20, 2019, at 9:26 a.m.