GREEN BAY - State transportation officials revealed Thursday that they think they know what caused the Leo Frigo Bridge to sag.
Answers are coming a little more than a week after a bridge pier sank roughly two feet.
The issue is below what's called the concrete footing. About eight feet under the bottom of that, the piling buckled.
After digging holes to look at the piling, investigators found that it had corroded. The WisDOT says at least three other piers showed signs of corrosion.
Crews are now working to find out what is causing the corrosion.
Workers on the bridge's east end bored holes into the ground to get soil samples. They've also dug trenches at Piers 10 through 25 to inspect pilings and footings.
“The investigation and subsequent repair of the bridge is a massive effort but we have the right team assembled and we will get the job done,” said WisDOT NE Region Operations Manager Will Dorsey.
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The DOT said in a news conference the bridge deck and Pier 22 appear to be in good shape. However, the inspection has found pilings for that pier had buckled, apparently due to corrosion.
“Our preliminary factors to the corrosion is really that fill material that's been in place prior to the construction and a combination of water, groundwater interacting with that soil, corroding on the piles,” said Tom Buchholz with the Wisconsin Department of Transportation.
The investigation has also found the vast majority of the other bridge piers being checked remain in the same condition as when they were installed more than 30 years ago. However, there are some suspect piers.
“Adjacent piers, Pier 21, Pier 23, and Pier 25… We did see corrosion on our test pits, not to the severity obviously that caused failure, but you could see corrosion. I want to reiterate that the rest of the bridge is not in danger of collapse,” said Buchholz.
DOT officials say the fill material found in samples from the 70s show foundry sand and organic material. That fill is unique to the east side of the bridge. The new soil samples are being compared to older ones.
“To confirm that the soil borings that were done in 1974 is what we're encountering today. That there weren't land use changes under those piers that would cause concerns for corrosion further on the Leo Frigo Bridge,” said Buchholz.
The DOT says it came as a relief that the problem with Pier 22 was found just eight feet below its concrete footing.
“Very early we were concerned of this could be happening 80, 90 feet below ground which gets more difficult to actually confirm that's what happened,” said Buchholz.
The DOT says bridge has not moved since it settled last week.
Officials believe there is no danger of collapse.
The DOT did not have a timetable on when repairs could begin or how long they would take.
The investigation is expected to be wrapped up next week.
Thursday, officials also touched on traffic issues related to the bridge closure.
Basically, a reminder that they suggest Highways 41 and 172 even though construction is ongoing and sometimes a traffic stopper.
The DOT's traffic engineer said they're monitoring traffic and still plan overnight closures. But he said they could adjust, if they had to, but usually overnight is not a serious problem.
For the time being, at least one special lane on the heaviest of the heavily traveled parts of 41 will stay open.
“One of the things that we're doing is the 41 northbound auxiliary lane that exists today, between Highway 172 and Lombardi Avenue is going to remain open. That was scheduled to be closed here soon, that now is going to remain open so we can continue to have traffic move through that section of highway as best as we can,” said Randy Asman, DOT traffic engineer.
There haven't been any major traffic issues with the 41 project after the Leo Frigo closure.
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