For a few days, 2 bridges to nowhere

ELIZABETH FLORES , Star Tribune

Work continued on bridge construction on American Blvd. near Interstates 35W and 494 in Bloomington, Minn.

Cracks in the pavement of a busy Bloomington bridge were the first signs of a construction mistake that will cost $360,000 to fix. The overpass is structurally sound, but the western approach road has sagged because the wrong fill had been chosen to support it.

By JIM FOTI, Star Tribune

Last update: April 19, 2008 - 12:10 AM

The cracks appeared soon after the paving was finished in 2005, and now the American Boulevard bridge over Interstate 35W in Bloomington is closed because of a construction mistake that will cost $360,000 to fix.

The overpass is structurally sound, but the western approach road has sagged because the wrong fill had been chosen to support it.

Four lanes of asphalt, plus sidewalks, trees and a median, have now been torn up so that the original fill material can be replaced with something that weighs half as much and won't put too much pressure on the giant foam blocks beneath it.

The repair work is being paid for with state and federal funds, said Schane Rudlang, senior civil engineer for Bloomington. Minnesota Department of Transportation engineers familiar with the overpass, a joint project of MnDOT and the city of Bloomington, were not available for comment Friday.

The closure began April 9 and is scheduled to last until May 2, an unfortunate time for drivers in the Bloomington strip area. A half-mile to the north, the 76th Street bridge will close on April 28, meaning that two east-west alternatives to Interstate 494 will be out of service for most of that week.

The closures might inconvenience some drivers, said Beth Petrowske, a MnDOT spokeswoman, but with the scheduling demands of the nearby Crosstown Commons project and the upcoming demolition of the Lyndale Avenue bridge over I-494, "there was no way around it."

Tom Russell, a construction inspector working on the American Boulevard bridge, said that work couldn't have been done earlier in the season because asphalt isn't readily available during cold weather. Rudlang said the presence of ground frost was another factor.

The 76th Street bridge handles about 24,000 vehicles a day; the figure for American Boulevard is about 14,000.

Driving on foam

The $13 million American Boulevard bridge was built in an area with less-than-ideal soils, Rudlang said. Using heavy fill to build the entire approach would have caused the ground beneath it and nearby to sink, so a much lighter synthetic material known as geofoam was used for part of the fill.

The idea was to protect a nearby office building, Rudlang said. The precaution worked, but the building has since been sold to a new owner and knocked down. "If we could have been assured that the building was going to be demolished, we wouldn't have done that," he said.

Geofoam is similar to Styrofoam but much stronger. Such foam is fairly new in Minnesota but has been used for decades as a lightweight alternative to more conventional fill.

Because of the weight of the earthen fill placed atop it, the foam used at the American Boulevard project compressed a bit, causing the settling.

Since it's already compressed and isn't expected to squish down further, it'll be left in place. The new fill on top of it is a clay product called Gravelite, an uncommon construction material made in Louisiana.

Jim Foti • 612-673-4491

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