Experts question soil under Green Bay bridge
GREEN BAY — Crews weren't able to rest the Leo Frigo Memorial Bridge's steel structure on sturdy limestone bedrock, which some geologists and engineers say may have contributed to the now-closed Green Bay span's sinking pavement.
Wisconsin highway officials say that's not the case, blaming steel corrosion at ground level as the likely culprit that forced the indefinite closure of the bridge, which carried about 40,000 cars daily over the Fox River. Work starts Monday to reinforce the bridge, but officials say it could be months or longer before it's usable again.
Some experts say underground soil that was sturdy enough to support the bridge when it opened in 1980 might have deteriorated in subsequent years, the Green Bay Press Gazette reported Sunday.
The bridge was built on top of 51 two-legged concrete piers. Each leg is supported underground by about 20 steel beams that extend downward vertically.
Records of soil tests show layers of sand, clay and other materials before limestone bedrock appears about 120 to 130 feet down. The beams beneath the damaged pier reached layers of hardened clay or other material that were judged capable of supporting at least 150 tons — generally to depths of 90 to 115 feet.
"You could have a soft spot," said Bill Kallman, a Michigan-based engineering consultant who has worked on bridges for the New York Department of Transportation. Kallman said he considers that a more likely explanation than corrosion, which he said would have to be widespread in order to affect all the steel beams supporting the damaged pier.
Tom Buchholz, the state's project manager for the bridge investigation, said bridge designers don't require that beams go all the way into bedrock.