WILMINGTON, Del. -- Underneath a now-silent interstate bridge over which 90,000 tractor-trailers and cars normally rumble every day, engineers and workers in hard hats are using shovels and high-tech sensors to figure out how and why the ground has shifted.
The Interstate 495 bridge in Wilmington, Delaware, was closed Monday after four pairs of support pillars were found to be tilting. Officials do not know when it will reopen.
Engineers are hoping to unearth clues that will help them determine how to shore up the bridge over the Christina River, a process officials say is going to take considerable time.
"Anything that's buried is a lot more difficult to deal with than anything that's above ground," noted Delaware Transportation Secretary Shailen Bhatt.
"This is a really serious situation that we're very lucky we caught in time," Bhatt added. "We don't know what could have happened. We're going to take every step to make sure things are safe before we reopen."
Still, the transportation department's chief engineer stressed that officials didn't believe a collapse was imminent.
"We never said that it was ready to fail. We were concerned about the tilt because that was abnormal behavior for that structure," Rob McCleary said.
Meanwhile, the six-lane, 11-mile-long I-495 bypass from south of Wilmington to the Pennsylvania line remains closed indefinitely, with motorists being shunted onto nearby Interstate 95 through downtown Wilmington, further overloading one of the most crowded arteries in America.
McCleary said a system to shore up and brace the bridge will have to be designed, a process that will take weeks, not days.
"It's not going to be open anytime soon," Bhatt said.
The problem was first discovered by crews working on an unrelated project.
Two employees of private geoscience consulting company Duffield Associates saw some cracking in the soil around a large 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 said engineers are looking at all geotechnical issues and have determined that the dirt pile, which appears to be partly in the transportation department's right of way, could be a factor.
"We don't know what the effect of that weight is" underground, he said.
Officials are trying to determine where the dirt pile came from, how long it's been there and how much it weighs. Engineers will use real-time data from tilt sensors placed on the bridge to see if they respond to removal of the dirt pile, but Bhatt said there could be other underground issues in the heavily industrialized area, such as deterioration of giant steel pilings driven deep into bedrock to support the bridge.
"There could be an issue with corrosion under there a hundred feet down that we don't know yet. ... I actually have a lot more questions now than I have answers," he said.
Four pairs of 50-foot-tall columns that are 5 feet in diameter are leaning, with the top of one roughly 2 feet out of line with the bottom. They are tilting by as much as 2.4 degrees, or 4 percent, from vertical.
Out of concern for public safety, officials do not want to allow traffic back on the bridge until they find out more about what caused the pillars to shift, and whether they are still moving.
Authorities said that in a worst-case scenario, such as a crash that forced traffic to back up and stall in both directions on the bridge, certain parts of it might not be able to handle the weight load within acceptable safety standards.
"If all the traffic was stopped and you were directing maximum stress and load on that bridge, you could get overload ... that could potentially lead to failure," Bhatt said.
The closing of the bridge is 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 indicates significant disrepair and risk of collapse.
The main span of the Delaware bridge is considered fracture critical, meaning it is at risk of collapse if a single, vital component fails. But officials say the span where the columns are tilting is not considered fracture critical. The bridge's superstructure, which supports the deck over which vehicles travel, was deemed structurally deficient in 1994 but no longer carries that rating.