Caltrans Probes Tunnel Corrosion Risk
Caltrans officials have a new corrosion investigation on their hands: The Bay Bridge’s Yerba Buena Island tunnel, which links the western span and the new eastern span of the San Francisco-Oakland Bay Bridge.
The issue came to light after a chunk of concrete wall fell into the roadway, just missing a vehicle in the travel lane, SFGate reported Saturday (Feb. 6).
Caltrans is now looking into whether the damage is a result of water intrusion, as well as whether it is an indication of a larger problem.
“The possibility of water damage is one of the many factors that they are going to be investigating,” Caltrans spokesman Bob Haus told the news site. “They are looking at the whole gamut of potential issues.”
Water Stains, Cracking
While the threat of corrosion on the new span Eastern span of the Bay Bridge itself has been an issue scrutinized by Caltrans for some time, the Yerba Buena Island tunnel is “some distance” from the structure, the news site indicated.
The 1,800-foot-long, five-lane tunnel holds the distinction as the longest, largest single-bore tunnel in the world, according to the Guinness book of World Records, Caltrans says. The 76-foot wide, 58-foot high structure was lined with concrete casing during its construction and was then split into its double-decker roadway, each half carrying the east- or westbound traffic decks.
According to reports, the chunk of concrete—a 3-inch thick semicircular piece about 2 feet in length—fell into the eastbound, lower deck of the 80-year-old tunnel on Jan. 30. The point of failure was where the ceiling of the lower deck (which also acts as the deck for the westbound deck on top) connects to the tunnel wall about one-third of the way into the south end of the structure.
Caltrans is investigating the failure, but believes it to be confined to an isolated area. The structure was last inspected in July 2015, officials said, with no problems identified.
Water stains are evident at the site of the failure, SFGate reported. Additionally, a crack is visible in the concrete that remains near the break, and more concrete cracking appears to be hidden by the tunnel ceiling.
Experts Weigh In
Haus indicated that Caltrans would investigate all possible causes, telling the NBC Bay Area affiliate, including whether an external origin might be at fault: “Was this caused by an internal structure problem? Wait [sic] it hit by an oversized vehicle? Was it some combination of both?" Haus suggested.
Metallurgical engineer Lisa Fulton disagreed. "This is not damage from any vehicle, or any other thing," she told the news station. "This is definitely deterioration."
Fulton believes the problem was caused by water seepage invading the concrete over time and ultimately rusting the tunnel’s steel reinforcements.
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| By Jacob Davies / CC BY-SA 2.0 via Wikimedia Commons |
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Caltrans is investigating the failure, but believes it to be confined to an isolated area. The structure was last inspected in July 2015, officials said, with no problems identified. |
"The things about this type of damage is you can't see how deep it is because you only see superficially," Fulton told the news station, suggesting that the corrosion could continue deep into the tunnel. "If you have a steel beam there, the water can cause corrosion."
Georgia Tech professor Larry Kahn agrees that the concrete shows signs that water is infiltrating the concrete and rusting the steel framework, which would eventually cause it to push against the concrete, resulting in failure.
Kahn, who specializes in corrosion and concrete structures, told the SFGate, “We call it a ‘pop-out,’ where you have some type of metal embedment that is corroding, causing it to expand. Then all of a sudden, it falls apart.”
Having examined a photo of the tunnel, he believes he sees one other area, at least, that may run the risk of just such a sudden failure.
“This is probably due to long-term deterioration,” he said. “Corrosion is very, very slow, but it can be extremely dangerous—I would go through that whole tunnel and find out if there is more corrosion.”
Kahn added, “Some mitigation work should be performed very quickly—structural engineers should be out there very soon and be investigating the entire wall. You want to mitigate the problem and not have it happen in another place.”
Caltrans will not speculate on a time frame for any necessary repairs until its investigation is complete.
