By Tim Jimenez and Jan Carabeo

DOVER, Del. (CBS) — The Firefly Music Festival in Dover opened its campgrounds to festival goers on Wednesday, a change this year meant to ease opening day traffic, which will likely be worse because of the I-495 bridge closure.

This comes as authorities finally name an official cause of the tilting bridge.

READ: Guide To Firefly Music Festival

The Federal Highway Administration now says the huge pile of dirt located next to the bridge displaced soil underneath the support columns, causing them to tilt.

The owner of that pile of dirt, Jim Thomas of Keogh Contracting, declined comment on Wednesday.

The Federal Highway Administration also says the bridge was properly designed, constructed, inspected and maintained, adding that the state could not have done anything more to prevent the failure.

Meanwhile, work at the bridge along Christiana Avenue in Wilmington continues around the clock. Crews are operating on 12-hour shifts. The focus right now is building the bridge’s new foundation by drilling 150 feet down to bedrock.

Officials go on to say repairs will be covered by federal emergency relief funds.

For commuters, it’s shaping up to be a summer of detours and delays, with the tilting I-495 bridge forcing 90,000 drivers to find another route.

“Last Friday it took me an hour and a half to do a 20 minute commute,” Florence Kiger said.

That will only get worse as 70,000 more people head to the Firefly Music Festival. the event runs for four days, starting Thursday and going through Sunday.

“It’s going to be crazy. Luckily, I’ll be out of the area starting Friday on vacation,” Kyle Buzzard said.

For those attending the Firefly Music Festival, DelDOT suggests taking I-295 or the New Jersey Turnpike to the Delaware Memorial Bridge.

All the detours can be found here.

Festival gates open Thursday at 5 p.m.

The I-495 bridge is expected to stay closed through the summer.

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