Despite a $1 million dollar study, Caltrans cannot say whether or not microscopic organisms are gouging the pits found on some of the 13 giant steel piles whose performance is critical to assure that the new Bay Bridge fulfills its 150 year design lifespan – prompting another testing program to look for firm evidence of the phenomenon known as microbiologically influenced corrosion.
The testing order came after Deepwater Corrosion Services, a Texas consulting firm, discovered pits near welds on several of the 13, 10-foot thick steel reinforced piles that hold up the new eastern span’s tower foundation. The piles are sunk down through Bay mud to near bedrock.
The firm surveyed the San Francisco Bay and found it teeming with the virulent creatures that leave behind similar pits in steel, experts say that it is a far more unpredictable force than normal rust.
Besides ordering the tests, Caltrans is also sending dive teams to inspect for similar pitting near welds on piles used to construct the Skyway part of the new bridge as well as the Richmond and San Mateo bridges during their five year inspections of those structures.
“We have no proof, or any evidence,” said Caltrans’ Ken Brown, who oversees maintenance for Bay Area bridges, “that it’s microbiologically influenced corrosion.”
“Right now we are not sure whether those little pitting areas are due to natural causes -- they could have been there originally in the original pile -- so we are just going to go back through and monitor these on a normal program and try to determine whether we do have a problem or not.”
But a Berkeley-based corrosion expert, Lisa Fulton, worries that if microbes are indeed at work, there may be deeper pits that have yet to be discovered. She says she cannot understand why divers didn’t simply take samples from the pits to test for sulphur and the iron sludge the microbes leave behind.
“That is conclusive evidence that there is bacterial corrosion going on – the fact that they didn’t do that is really, really alarming…”
Deepwater’s findings ruled out that the pits were there to begin with – leaving Fulton to suspect that if microbes are to blame, the span’s 150 year lifespan could be at risk.
Caltrans set aside about a quarter of the pile thickness to account for rust – about an inch. Some of the pits Deepwater’s report identified were so deep that as much as a quarter of that one-inch steel corrosion margin is already depleted -- in about 15 years. Fulton says at that rate, parts of the entire reserve layer could be depleted in as little as 60 years.
But as of this moment, Caltrans officials figure that there is plenty of steel left in the Bay.
“We really don’t have any safety concerns and we’re not worried about the rate of corrosion right now,” Caltrans maintenance chief Brown said. “We expect it to go down, as time continues.”