WILMINGTON, Del. --Highway engineers say a crucial bridge on the Eastern Seaboard's interstate highway system could imperil drivers if traffic is allowed back on it.
The bridge, near Wilmington, Delaware, was closed Monday when its support pillars were found to be tilting. The Interstate 495 bridge won't reopen anytime soon, highway officials said Tuesday, and the 90,000 vehicles that cross it every day are being diverted onto the main north highway, I-95, further overloading one of the most crowded arteries in America.
Engineers say ground under the columns moved and caused the supports to tilt. Officials said they believe the mile-long bridge over the Christina River is not in any danger of collapsing under its own weight. But out of concern for public safety, they do not want to allow traffic back on it until they find out more about what caused the pillars to shift.
"We never said that it was ready to fail. We were concerned about the tilt because that was abnormal behavior for that structure," said Rob McCleary, chief engineer for DelDOT.
In a worst-case scenario, such as a crash that forced traffic to back up and stall in both directions on the six-lane bridge, certain parts of it might not be able to handle the weight load within acceptable safety standards, officials said.
"If all the traffic was stopped and you were directing maximum stress and load on that bridge, you could get overload of certain members, ... that could potentially lead to failure," state Transportation Secretary Shailen Bhatt said in an interview Tuesday with The Associated Press. "But you never want to overload."
Four pairs of 50-foot-tall columns that are 5 feet in diameter were leaning, with the top of one roughly two feet out of line with the bottom. They are tilting by as much as 2.4 degrees, or 4 percent, from vertical. The whole bridge, built in 1974, needs to be inspected, officials say.
Paul Moffitt a geotechnical engineer for a private firm working with DelDOT, noted that the bridge was built in a flood plain area that includes soft organic clay. Ground movement could put stress on steel piles, measuring 140 feet to 160 feet long, driven in deep to try to bypass that softer soil into firmer ground to support the bridge columns.
"We're in an industrialized area here, ... so there could be a corrosion potential involved," he added.
The closing forces more traffic onto I-95, which runs through downtown Wilmington, and already is heavily clogged during the morning and afternoon rush hours. McCleary said a system to shore up and brace the bridge will have to be designed, a process that will take weeks, not days, and there is no exact timetable for the important roadway to reopen.
"It's not going to be open anytime soon," Bhatt said.
The problem was discovered by crews working on an unrelated project.
Two employees of private geoscience consulting company Duffield Associates saw some cracking in the soils around a dirt pile dumped on the east side of the bridge - the direction in which the columns are tilting. They then looked at the leaning columns and contacted transportation officials, said the firm's chairman, Jeff Bross.
"They were significant cracks," Bross said.
Bhatt could not say whether the dirt pile was there legally but noted that part of it appears to be in DelDOT's right of way.
It's the latest crisis involving the half-century-old interstate system.
An AP analysis of more than 600,000 bridges last year showed that more than 65,000 were classified as "structurally deficient" and more than 20,000 as "fracture critical." Of those, nearly 8,000 were both - a combination of red flags that experts say indicate significant disrepair and similar risk of collapse.
The now-closed Delaware bridge was classified as "fracture critical." A bridge is deemed that when it doesn't have redundant protections and is at risk of collapse if a single, vital component fails. Bridge collapses are not common, but some of the more prominent ones in recent history took place on fracture critical spans.
As for the detours and traffic, the transportation department has sent out notices from Maine to Florida, telling people the best routes to take to avoid the 11-mile closure.
Joe Erthal heard about the problem Tuesday morning while heading to Maryland from Philadelphia, where he lives. Driving home that afternoon, he got mixed up while trying to bypass the closure and mistakenly ended up on I-495.
"I guess we're just going to have to find another way around 'til everything's fixed again," Erthal said while consulting a map app on his cellphone for directions.