ASHWAUBENON - Corrosion of steel pilings below a support pier on the Leo Frigo Memorial Bridge in Green Bay caused Pier 22 to buckle last week, creating a long, deep dip in the bridge deck and forcing the bridge's indefinite closure.
The 100-foot-long pilings under the pier were degraded from a combination of water and the composition of soil surrounding the bridge support, Wisconsin Department of Transportation officials said Thursday.
Investigators have been on the job for a week but only now are starting to look at options to fix the 33-year-old bridge, which carries Interstate 43 over the Fox River.
"That fill material that was in place prior to construction, and the combination of ground water interacting with that soil, (was) corroding the piles," said Tom Buchholz, the DOT project manager for the bridge investigation.
Corrosion, to a lesser degree, was found near the surface in test pits on adjacent support Piers 21, 23 and 25 on the east side of the bridge, Buchholz said.
"This does not indicate the rest of the bridge is in danger of collapse," he said. "These foundations, these piles, have huge safety factors on them so if one pile fails there are 39 others to support the bridge."
The failure of a bridge piling is rare, DOT officials said.
Officials are working to determine how many support piers may be in a similar condition, but they say most are in good shape. There are 51 support piers on the 8,000-foot-long bridge, which is the second-longest bridge in Wisconsin.
The investigation is focused on the area from the Fox River east to North Quincy Street on the east side of the bridge, where fill materials like foundry sand and organic materials are part of the soil profile.
"We've encountered all kinds of different things," Buchholz said about soil samples in that area.
The fill is unique to that part of the bridge, he said. Other areas are marsh and wetlands. East of Quincy Street is native ground comprised largely of clay and silt.
The bridge was closed early on the morning of Sept. 25 after Pier 22 sunk 22 to 27 inches in what appears to have been a matter of hours. That caused a 400-foot-long dip in the bridge deck noticeable to drivers who called 911 to alert authorities about a possible problem with the span.
There has been no additional movement of the bridge since it was closed. The bridge has been inspected and is not in danger of collapsing, DOT Northeast Region director Will Dorsey said. No one was injured when the support pier buckled.
The closure has forced thousands of drivers to find alternate routes around the city. Officials say, for the most part, that drivers have adapted well to life without the bridge, which carried about 40,000 cars a day.
"(Drivers) should prepare for life without the Leo Frigo Bridge through Packer season, the winter, and maybe even longer," said Kim Rudat, communications manager for the DOT's Northeast Region.
The Federal Highway Administration's participation in the investigation has not been affected by the federal government's partial shutdown, Dorsey said.
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