A Minnesota interstate has been shut down after corrosion ate away at a bridge support, reportedly to the point that inspectors could see right through it.
A section of southbound Interstate 35 in Duluth, MN, was closed Nov. 18 after bridge inspectors found deteriorated pilings during an inspection. The low-lying section of the bridge sits approximately four feet off of the ground, according to a Minnesota Department of Transportation spokesperson.
"The bridge is being closed as a precautionary measure due to the amount of deterioration that was identified during a recent inspection," Duane Hill, MnDOT District 1 engineer, said in a statement.
The piling was so corroded that MnDOT crews could see right through it, the Grand Forks Herald reported.
The bridge is a 2,100-foot section of highway that is supported by 38 piers with 1,300 steel pilings.
A MnDOT engineer said that corrosion ate away a "significant amount of steel" on the bridge support.
Originally, the agency expected the closure to last for a few days, but it announced Nov. 21 that the road would remain closed for three to four weeks.
"More substantial repairs are required than we originally thought," Hill said. "The plans we are developing will be safe and efficient for the construction crews to accomplish."
On Aug. 1, 2007, a bridge carrying I-35W over the Mississippi River in Minneapolis, MN, collapsed while 111 vehicles were on it. Thirteen people died and 145 were injured, according to the National Transportation Safety Board's accident report. Minneapolis is about 154 miles south of Duluth on I-35.
'Significant Amount' Corroded
Deterioration was found on the Pier 32 pilings at the bottom of the pier caps where the piling and the cap join. The repair plan being developed includes using a concrete cap extension to transfer the bridge load. Sound steel begins about one foot below where the deterioration was found, and the deteriorated steel will be encapsulated inside the new concrete cap extension, MnDOT explained.
"It ate away most of the steel in those piling supports," Hill told MPRNews. "So a significant amount of steel was gone."
Corrosion was found in all 13 steel pilings that support Pier 32, according to MPRNews.
"The space under that pier was like a mouse hole that you had to have your face right down in the mud to look at. So it was really a difficult thing to ever see... [A worker] just happened to look down there with a flashlight and say, 'hmmm, we should dig this out a little bit more and see what's going on,'" Hill said.
While the cause is still being investigated, MnDOT said that excessive moisture may be a contributing factor. Pier 32 is located in a low-lying section of the bridge in an area with a high water table and poor drainage.
Runoff from I-35 flows through an expansion joint on top of the pier and, combined with water from deck drains on the bridge, runoffs from the bluffs above the freeway, and overflow from Buckingham Creek, the soil beneath the bridge has become saturated.
Repair work on the bridge will take up to a month to complete. The deteriorated steel will be encapsulated in a new concrete cap extension.
MnDOT said it would start the repair work with its maintenance crew before hiring a contractor to complete the work. After assessing repair options, the agency awarded an emergency contract for an accelerated schedule, including multiple shifts per day, seven days per week.
Recent 'Mega Project'
A section of I-35, including the area currently closed, recently underwent a $68 million "mega project," which involved bridge work, including replacing three fracture-critical bridges. The project was completed June 29, 2012, according to MnDOT.
Pat Hudson, MnDOT District 1 Assistant Director of Engineer Operations, said that this bridge was not one of the three replaced.
MnDOT has a bridge preservation project planned for 2015 that will improve drainage and extend the life of this bridge, as well as another bridge. The project plans include reshaping the soil to intercept and drain away the water that has been pooling beneath the bridges.
Wisconsin Pier Corrosion
A similar issue occurred on the Leo Frigo Memorial Bridge in Green Bay, WI, in late September when a 400-foot section of the structure abruptly dropped about two feet overnight.
One of the bridge's piers sank when its steel pilings buckled, "apparently due to corrosion," the Wisconsin Department of Transportation reported. WisDOT said the corrosion was believed to have been caused by a combination of soil composition at the location and the rise and fall of the water table.
However, the Green Bay bridge's piers are situated in water, while the corroded I-35 pier is only driven into the ground.
The Leo Frigo Memorial Bridge carries Interstate 43 over the Fox River south of its mouth into Lake Michigan. An average of 40,000 vehicles use the bridge each day.
Repairing the bridge was ballparked at $50 million, and the Federal Highway Administration mobilized emergency funds after determining the state could not have anticipated the failure.