SACRAMENTO, Calif. (CN) – Despite the discovery of rust pits in steel pile casings and corrosive bacteria in soil around the new eastern span of the San Francisco-Oakland Bay Bridge, engineers remain confident the structure can survive a major earthquake for the next 135 years.
“Nothing gives me any cause for concern,” said Brian Gibbs, a structural integrity engineer with the consulting firm Deepwater Corrosion Services, which performed a detailed corrosion survey on the bridge.
The results of the $853,000 corrosion survey were delivered to the three-member board that oversees the bridge during a meeting in Sacramento Monday.
As part of the study, a special team of divers swam underwater and used ultrasonic waves to test parts of the bridge. The survey found six holes, or rust pits, in two of the 160 casings for steel piles that stretch from the seafloor to the bridge’s sea-level support structures.
Gibbs and other engineers say the rust pits don’t pose a threat to the bridge’s integrity because they are small and limited to a few random spots. The six holes dig an average 3 millimeters into the 94-milimeter-thick steel pile sleeves.
The survey also found steel pile sleeves lost about two millimeters of thickness in welded spots over the last 15 years. Engineers say that level of corrosion puts the bridge on target to survive its 150-year lifespan, which started when the bridge’s foundation structure was placed underwater in 2003.
“These results verify that the structure performs the way it was designed to,” said Marwan Nader, bridge designer and project manager with the civil engineering firm T.Y. Lin.
The discovery of corrosion-causing bacteria in underwater soil also failed to raise alarms for engineers. That’s because the bacteria haven’t yet led to higher than expected rust levels on tested parts of the bridge.
Nevertheless, Gibbs recommended California transportation officials consider installing steel coupons, or samples, into underwater soil near other Bay Area bridges. The steel samples could then be surveyed periodically for potential corrosion, he said.
The survey also found no unexpected corrosion in the pile cap, or steel-in-concrete frame that holds up the bridge’s only support tower.
Board Chairman Steve Heminger had asked the engineering team to look into possibly installing a cathodic protection system to protect the bridge from corrosion. An active cathodic protection system would divert the corrosive effects of seawater from critical bridge structures to an insignificant, “sacrificial” piece of metal.
Given the results of the recent corrosion survey, Bay Bridge Chief Engineer Brian Maroney insisted that a cathodic protection system is not necessary at this time.
“If we ever needed to, we could always add cathodic protection,” Maroney said.
Engineers continue to monitor corrosion sensors in 18 of 424 anchor rods designed to provide an extra layer of protection during an earthquake. Two anchor rods failed critical strength tests in 2015 after a botched sealing and grouting job allowed corrosive saltwater to seep into shafts and weaken rods.
Backfill that stabilizes the rods was replaced in June 2017 for $8.5 million, and two contractors were fined for shoddy construction work. No corrosion has been detected in the anchor rod shafts so far.
Fears about corrosion and the bridge’s overall integrity have persisted for years. The bridge’s $6.4 billion eastern span, completed in 2013, was designed to withstand an 8.5-magnitude earthquake, but several defects – including cracks in the foundation, brittle support rods, bolt holes that leak water through the deck and weak spots in welded steel plates – have raised alarms about safety.
Also on Tuesday, the bridge panel congratulated itself on the results of a recent state auditor’s report that found its oversight of the seismic retrofit program has helped avoid more than $455 million in potential cost overruns and seven years of potential delays. The oversight panel was established by the state Legislature in 2005 amid frustrations about swelling costs and long delays.
Not straying from its reputation for cost-cutting, the board on Tuesday refused to approve an extra $25.5 million for a $49 million project that aims to convert leftover structures from the now-demolished, former eastern span into publicly accessible docks. The board asked the project team to provide a more detailed list of costs and other cost-cutting options in two weeks.
Additionally, the panel discussed dismantling the oversight panel now that the $9 billion seismic retrofit program is winding down.
Board Chairman Steve Heminger said this could be one of the panel’s last meetings.
Separately, Heminger announced he will retire at the end of February 2019.
Aside from being chairman of the Toll Bridge Program Oversight Committee, Heminger has served as executive director of the Bay Area’s Metropolitan Transportation Commission for the last 17 years.