THE REMNANTS of the Dumbarton rail bridge might look sturdy, but entropy is the inevitable fate of all things material.
A recent report threw cold water on the likelihood of rehabbing the old structure's concrete piers, which support the span, although other bridge parts could be reused for a high-speed rail route across the marshy South Bay.
Many of the struts holding up the 98-year-old bridge have been disintegrating under constant pressure of the tides, leaving piles of stones where cement has worn away and exposing fir timbers sunk in the steel-encircled cylinders.
The cost to replace the piers, or caissons, for a commuter train line already is part of the project's $595 million price tag. Another hope, though, was to make less of an environmental impact and to save money by reusing parts of the bridge which until 1982 was a freight hauling avenue.
Kuan Go, project manager with HNTB architecture firm in San Jose, told transit and local government officials last week in Newark that some of the structure is in good shape but that some has deteriorated, leaving no clear cost decision on whether to renew or replace the span.
"You'll have to grapple with that policy choice," he said, noting that the original engineering only has a life of about a century.
In November and December, engineers with HNTB and divers with underwater surveyors Halcrow examined 22 caissons under the Dumbarton rail bridge and five caissons under a smaller rail bridge over the Newark Slough.
They found that all the main channel caissons suffered corrosion of their steel casings, "from minor surface deterioration to large voids with soft concrete and exposed timber piles," according to the report given Tuesday to the Dumbarton Rail policy advisory committee.
The slough caissons' concrete was in better shape, although the center pier supporting that bridge had severe corrosion, the report said.
As for the iconic swing bridges at the center of the two spans, rust has taken its toll, while some of the steel support trusses on both sides of the main bridge were exposed to fire a decade ago and need further study.
In January, concrete specialists from CTL Group construction firm took core samples from some of the bridge caissons and found serious degradation.
Go said in some spots, loose aggregate sits piled where the binding cement had dissipated.
"It's almost like rocks that you pick up from the beach," he said.
That is essentially a reversal of the process by which the structures were built, i.e., adding cement paste to stone as if the tides turned back time for the piers.
Years ago, anticipating all the decay, officials changed the project's logo from one based on the crisscrossing swing bridges and trusses to one with a vague swoop pointing to the streamlined future, rather than crumbling history, of the Dumbarton rail line.
"That may not necessarily be the new bridge," said Christine Dunn, spokeswoman for the San Mateo County Transportation Authority, one of the rail project's partners. "We're moving forward here. It may look like something completely different."
Go said more study is needed to say whether it is worth the effort to rehab parts of the bridge for the commuter service, due to begin in 2012.
The advisory committee set a tentative next meeting July 22 in Palo Alto or Redwood City.
Reach staff writer Todd R. Brown at 510-353-7004 or email@example.com.