Here’s a riddle for you: When is corrosion on the Bay Bridge not on the bridge? The answer is “When it’s in a tunnel.”

And another–or maybe it’s more of a koan: When is bridge corrosion not Caltrans’ fault?

As usual, all credit for the reporting goes to our buddy Jaxon Van Derbeken.

A tunnel on a bridge? Well, yeah. A quick recap for those of you unfamiliar with the details of the Bay Bridge’s architecture. The Bay Bridge is actually two bridges and a tunnel. In the east, we’ve got a bridge burdened with botched bolts and creeping corrosion. On the west, another bridge burdened with an unfortunate name. In between, Yerba Buena Island, home of the largest single-bore tunnel in the world. The tunnel, just to be explicit, connects the two bridges. The western span of the bridge and the tunnel were built in the 1930s under the auspices of the WPA; the eastern span was, of course, built under the careful supervision of Caltrans.

On January 30, a chunk of concrete fell off the tunnel wall into a traffic lane, where it did severe damage to a Ford Fusion’s tires, undercarriage, and driver’s psyche. Caltrans quickly identified the cause of the problem: corrosion caused some of the reinforcing rods in the concrete walls to expand, stressing the concrete until the chunk popped loose.

Caltrans initially downplayed the incident. Jaxon quotes Deputy District Director Dan McElhinney as having said that “…at this point in time, there’s nowhere else in the tunnel where there is an issue.” However, Caltrans did do the right thing and survey the entire tunnel.

Last week, Jaxon revealed that the survey found a dozen spots where concrete is in danger of breaking loose–and that may not be the extent of the problem. The testing method used was to tap the concrete with a hammer and listen for hollow sounds*. Unfortunately, that doesn’t work where the concrete is painted, nor will it find patches of corrosion before they get large enough to endanger the concrete.

* Silly as it sounds, this is apparently the standard test procedure. Kudos to Caltrans for using it, rather than immediately jumping to expensive, high-tech methods.

So now Caltrans needs to go to those expensive, high-tech methods to test the rest of the tunnel. No word yet on how much that’s likely to cost, but Caltrans says it’ll be paid for out of an existing pool of funds earmarked for Bay Area bridge rehabilitation projects.

The immediate cause of the corrosion is rainwater flowing through drainage holes in the upper deck of the tunnel, infiltrating the Masonite pads that cushion the deck, and then migrating into the walls.

So, back to that koan. Is the tunnel corrosion Caltrans’ fault? It’s arguable. The tunnel was built before Caltrans existed. For that matter, the upper deck was installed in 1964, eight years before the California Departments of Public Works and Aeronautics merged to become Caltrans. According to Jaxon, Caltrans doesn’t even have documentation of the drainage system, which certainly limits their ability to predict danger points.

On the other hand, this sort of water-induced decay is a well-known problem in tunnels, and–as we’ve heard several times recently–preventative maintenance isn’t one of Caltrans’ strong suits. While they did inspect the tunnel last July, it was just a visual inspection. The last time the “hammer test” was done was 2004–before construction on the new eastern span was even begun.

Again, I’m not an engineer, but a twelve year gap between inspections sounds excessive, especially since I haven’t seen any indication that a hammer test was even under consideration before the incident on January 30.

Regardless of whether the corrosion is Caltrans’ fault, it’s unquestionably their problem. Here’s hoping for a speedy resolution with far fewer delays and cost overruns than we’ve seen on the bridge so far.