MnDOT revises St. Croix bridge design after summer testing

Stillwater bridge
The Stillwater lift bridge and downtown Stillwater, Minn., are seen in this file photo from Sept. 28, 2011. The new St. Croix crossing is designed to alleviate traffic congestion on the old bridge.
MPR Photo/Jeffrey Thompson

Crews are working this fall to wrap up prep work on a new bridge over the St. Croix River between Oak Park Heights in Minnesota and St. Joseph in Wisconsin. The bridge is still several years away from becoming a reality and officials continue to refine plans for one of the most expensive public works projects in the state's history.

The new bridge will divert much of the traffic that now must go through downtown Stillwater to cross the current 80-year-old lift bridge. Oak Park Heights City Council member Mary McComber said she hopes that will mean less traffic speeding through Oak Park Heights, as well.

"The traffic is getting heavier and heavier all the time on Highway 36 and through the different neighborhoods," she said. "You find a lot of cut-through traffic going through the neighborhoods and that doesn't make it safer for residents either."

When the new bridge is in place, the lift bridge will be converted to a bike and pedestrian trail. Residents are getting a taste of what's to come right now, because the Stillwater bridge is closed for repairs from now until December.

The new span will be an "extradosed" bridge -- a hybrid of a concrete box girder style, similar to the I-35W bridge in downtown Minneapolis, and a cable-stayed style, similar to the Sabo Bridge in Minneapolis.

MnDOT video visualizing the new bridge:

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The mile-long freeway span will be the second of its kind in the U.S. The first extradosed bridge is now under construction in Connecticut.

Extradosed bridges are less vulnerable to wind or vibrations than traditional cable-stayed bridges, according to Jon Chiglo, the project director from the Minnesota Department of Transportation.

MnDOT plans to do wind testing on the new St. Croix design to avoid the kind of vibration problems that affected the Sabo Bridge, Chiglo said. In February, two plates on that bridge broke, causing a pair of cables to fall off the span. A study of the damage found the plates were stressed by vibrating cables.

The extradosed design has other advantages as well, Chiglo said.

Smooth traffic
Midmorning traffic moves smoothly across the historic Stillwater lift bridge, in this file photo. Congestion during weekday rush hours snarls traffic in downtown Stillwater, causing long waits and traffic backups.
MPR Photo/Dan Olson

"You can get longer spans than what you could normally get with a box girder structure, but the tower heights are not as tall as what you would normally see in a normal cable stay. They're shorter," he said.

The shorter towers will keep the bridge's height below the top of the Wisconsin bluffs. That was a priority for environmental groups and other stakeholders who worked with MnDOT on the project proposal.

There are other environmental concerns planners will have to consider, too. For one thing, the St. Croix river bottom is soft, and that makes designing the new bridge's foundation complicated.

At its deepest point, there's as much as 85 feet of muck at the bottom of the river. MnDOT testing this summer showed just how far crews will need to drill down to reach the river's limestone foundation -- 130 feet. That's one-third farther than crews drilled for the 35W bridge foundation in the Mississippi River.

Chiglo, who also led construction of the 35W replacement bridge, says this summer's testing also showed the number of piers needed to support the bridge could be reduced from six to five.

Yet MnDOT's plans don't persuade Oak Park Heights Mayor David Beaudet that the bridge will benefit his community. Beaudet was the only member of the City Council to oppose the project. Beaudet said he's concerned about how construction could affect the environment and local businesses.

St. Croix Crossing project director
St. Croix Crossing project director Jon Chiglo, left, and Minnesota Department of Transportation Commissioner Tom Sorel answer questions regarding the bridge project at the Water Street Inn in Stillwater, Minn. Tuesday, March 13, 2012.
MPR Photo/Jeffrey Thompson

The mayor sent a letter to the U.S. Army Corps of Engineers, which must approve MnDOT's permit to begin building the new bridge, asking the agency to extend its timeline for public input.

"When we are done and we come to the scenic overlook in 2016, you'll then be able to understand the physical size of the 5,000- foot long bridge in in the beautiful St. Croix River valley," said Beaudet. "We'll then understand the visual image of how much it hurt the river. We'll now have a recalculation of whether that cost-benefit analysis actually was worth it."

Several bald eagles and at least four kinds of endangered mussels make their homes near the bridge site. MnDOT's plans include steps to protect the land, water, historic structures and species in and around the river from damage.

Earlier this year, Congress exempted the St. Croix River crossing project from the Wild and Scenic Rivers Act, allowing it to move ahead after years of delays, angering environmentalists and opponents of the new bridge.

If the U.S. Army Corps of Engineers approves MnDOT's permit, work on the new bridge project could begin as early as next spring.

Costs for the project, which include road and other work on the Minnesota and Wisconsin sides of the river, range from $580 million to $676 million.