DOVER, Del. (AP) — State transportation officials said Wednesday that they believe a contractor is responsible for dumping a massive mound of dirt suspected of shifting the ground underneath an interstate highway bridge and forcing its closure.

Crews in Delaware were working with the contractor and trying to remove the mountain of dirt, nearly the length of a football field, next to the six-lane Interstate 495 bridge in Wilmington.

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Officials said that they weren’t sure were the dirt came from that they believe the contractor dumped it over a period of years.

The bridge closed Monday after four pairs of support pillars were found to be tilting. It will remain closed until engineers figure out how to brace the bridge and make it safe for the 90,000 vehicles that travel it daily.

Engineers suspect that the weight of the dirt caused soft soils underneath the bridge to shift, resulting in the tilting of the columns.

“I think anybody with an engineering background would look at that and say, ‘Here’s your problem; this is something you need to deal with immediately,'” Delaware transportation secretary Shailen Bhatt said Wednesday.

The dirt mound rises about two stories high, but experts are still trying to determine its exact measurements.

“That’s a very big pile of dirt that has a lot of mass and a lot of weight,” Bhatt said.

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Ground conditions are carefully considered when bridges are designed, but officials believe the design capabilities of the I-495 bridge may have been compromised by the dirt dumped next to the columns.

Bhatt said Delaware transportation officials did not know of the dirt mound until Monday, when engineers visited the bridge in response to a report received late last week of a possible problem. That report came from an engineer with a private company who was in the area on an unrelated project and saw cracking in the soil around the dirt pile on the east side of the bridge — the direction in which the columns are tilting. The engineer then spotted the leaning columns and contacted the state Department of Transportation.

Officials said they were examining aerial photographs in an effort to ascertain how long the dirt mound has been there.

“In 2012, there was some stuff out there but not very much, in 2013, a little more,” Bhatt said. “…. Right now obviously there’s a lot more dirt under there.”

Bhatt said the contractor has been cooperative in removing the dirt, which is being tested for contaminants.

“He’s been running to get dirt out of there since Monday night, and there’s still a lot of dirt down there,” he said.

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