Investigations into the collapse of the I-43 Leo Frigo Memorial Bridge in Green Bay, Wisconsin, US have determined that highly unusual environmental conditions led to the severe corrosion of the steel piles in the foundation that supported Pier 22 and which caused the vertical displacement of the bridge deck.
The Leo Frigo Bridge carries four lanes of traffic over the Fox River, and consists of two 3.6m lanes, a 1.8m median shoulder, and a 3m outside shoulder. The bridge was constructed between 1978 and 1980.
On September 25, 2013 bridge pier 22 sank 0.6m, causing the bridge deck to sag and the bridge to be closed for investigation and repair until January 2014.
Laboratory tests on soil and water samples taken from test pits at 20 out of the 51 piers determined that several factors had contributed to a highly unusual environment that caused the severe corrosion of the steel in the foundation that supported Pier 22.
The presence of a moist, porous fly ash fill with high levels of chlorides and sulphides combined with a low resistivity created the corrosive environment. In addition, a dense clay layer was present below the porous fly ash, leading to differential oxygen concentration and differential chemical concentrations within the fill layers. Bacteria were found at many of the piers and, according to the newly published report by the Wisconsin Department of Transportation, it is likely that microbiologically influenced corrosion also played a role.
Once sufficient material and support had been lost from the piles, the remaining piling became unstable, with the most heavily deteriorated sections crushing and buckling.
Visual inspection and soil borings also confirmed the presence of fly ash surrounding severely corroded steel piles at Piers 21, 23 and 25, with only some corrosion at Pier 24.
As a result of the initial investigation repairs were performed at Piers 21 through to 25, including the installation of new concrete drilled shaft foundations that are capable of supporting the entire pier design load. These new foundations have been connected to the existing piers and provide corrosion protection measures designed to offer 75 years of service life.
The investigation and repairs cost US$15 million.
Download the report below:
Volume I: text, tables & exhibits
Volume II: appendices A through F
Photo credit: yooperann