New Bay Bridge corrosion probe: Concrete chunk falls in tunnel

The Yerba Buena island tunnel. The Yerba Buena island tunnel. Photo: Paul Chinn, The Chronicle Photo: Paul Chinn, The Chronicle Image 1 of / 7 Caption Close New Bay Bridge corrosion probe: Concrete chunk falls in tunnel 1 / 7 Back to Gallery

Caltrans is investigating possible corrosion in the Bay Bridge’s Yerba Buena Island tunnel after a chunk of the concrete wall tumbled into the roadway and narrowly missed hitting a motorist, officials said.

The semicircular piece of the wall, about 2½ feet square and as much as 3-inches thick, fell into the path of a car in the slow lane of the eastbound, lower-deck tunnel the afternoon of Jan. 30. The car, a Ford Fusion, suffered scrapes and a blown tire when it ran over the concrete, but the driver escaped injury.

Of concern to Caltrans, and to an expert who examined a photo of the damage, is whether the incident is a sign that water is causing more widespread problems in the 80-year-old tunnel.

Corrosion fears have been among the construction issues Caltrans has been working to alleviate on the new eastern span of the bridge. Anchor rods in the tower and on seismic-stability structures, as well as the span’s cable, have been the focus of such concerns.

The spot in the tunnel where the concrete fell away, however, is some distance from the new span. The failure occurred at a point where the ceiling of the lower deck — which serves as the deck for the westbound lanes above — is connected to the tunnel wall.

‘Safety is first’

Although Caltrans is still investigating, an agency official said, first indications are that it was an isolated case of corrosion that caused the concrete to fail.

“Safety is first for us, so this was not a good situation at all,” said Caltrans Deputy District Director Dan McElhinney. “We are lucky that it wasn’t more severe.

“These are opportunities to figure out what is going on with our aging infrastructure,” he said. “This wall has seen many, many years of service.”

To further investigate, Caltrans is planning to shut down at least one lane of the eastbound tunnel in the next few days to allow crews to check the entire area. The immediate focus of the agency’s investigation has been where the concrete fell away — on the south side of the tunnel, about a third of the way in.

“They haven’t determined the exact cause of it,” Caltrans spokesman Bob Haus said Friday. “They last inspected it in July, and they found no obvious problems.”

The nearly 1,800-foot-long tunnel opened in 1936 and remains the longest, largest single-bore tunnel in the world. The five-lane structure knifes through the solid rock of Yerba Buena and was lined with concrete casing during construction, then was split into the upper and lower halves.

There are apparent water stains where the piece broke away, along with a crack where fractured concrete remains in place. There is evidence of more concrete cracking that is obscured by the tunnel ceiling.

“The possibility of water damage is one of the many factors that they are going to be investigating,” Haus said. “They are looking at the whole gamut of potential issues.”

He added, “One of the reasons we have these investigations is to determine the cause, so this kind of thing doesn’t happen again.”

Corrosion signs

Larry Kahn, a Georgia Tech professor who specializes in corrosion and concrete structures, examined a photograph of the tunnel and said the concrete showed signs of corrosion damage.

One likely explanation would be water invading the concrete, causing rust in the steel framework, he said. Eventually, Kahn said, the steel would expand and push against the concrete, ultimately causing it to fail.

“We call it a ‘pop-out,’ where you have some type of metal embedment that is corroding, causing it to expand,” Kahn said. “Then all of a sudden, it falls apart.”

He added that the photo showed at least one other spot nearby where the concrete appears to be at risk of failing abruptly and hitting the road.

“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.”

McElhinney said he is optimistic that Caltrans can avoid more such failures.

“At least at this point in time, there’s nowhere else in the tunnel where there is an issue,” McElhinney said.

But, he added, “the more we keep our eye on this, the better.”