MnDOT studying how to bring elevated freeway to the ground in Duluth
DULUTH—The estimated $205 million reconstruction of the "can of worms" section of Interstate 35 through Duluth isn't scheduled to begin until 2019. But that doesn't mean work isn't already happening.
"We're the first part of any project right here," said Dylan Buhl as he approached a rugged box truck reminiscent of a woolly mammoth. A specialist with the Minnesota Department of Transportation, Buhl works out of the Office of Materials and Road Research based in Maplewood, Minn. He's been in Duluth most weekdays since November, using the cone penetration testing rig to probe the earth underneath the interchange. The reconstruction plan is to bring much of the elevated roadway to the ground — something that wasn't possible in the 1960s when the interchange was constructed.
"We need to know what type of foundation — what type of material — is under there," said Duluth-based MnDOT Project Manager Roberta Dwyer. "We want to be sure it can support (a roadway). If it can't, we can strengthen it so it can. But the most important thing is first finding out what's there."
Touring the worksite with Buhl, a constant whooshing and humming of traffic came from the curved ramps overhead. Formally, the Twin Ports Interchange is the series of ramps and elevated sections of freeway connecting I-35 with U.S. Highway 53 and I-535. It's being targeted for replacement due to traffic safety and maintenance concerns, which dog the interchange.
Buhl is working his way through a predetermined grid, tapping one hole after another to learn what the earth brings. So far, the data jives with what engineers encountered 50-plus years ago when the roadway was built.
"It's pretty dicey," Buhl said. "It gets swampy in a hurry."
Located along the St. Louis River estuary, the water table underneath the can of worms is almost right below the surface, two to three feet down under a layer of sand and silt. Tapping deeper tends to reveal dirt, clay and even harder stuff, Buhl said.
Buhl works from inside the box of the truck, where he operates a pneumatic press that drives a pointed steel probe as far as it can go — up to 100 feet or more in some cases.
The apparatus itself works like a boring machine with Buhl manually applying new sections of pipe one at a time as the press grabs and pushes the probe deeper. The probe is like a warhead that Buhl screws onto the first pipe. It collects no samples. Instead, it relays an abundance of data, on things such as soil density and core pressure, onto a computer screen as it drives down into the unknown.
For Dwyer and the project planners, the samples and data will help them determine what they'll need to do to reduce the number of bridges involved in the can of worms. Engineers back in the day didn't have the technology to build the can of worms roadway on the soft earth. Instead, they drove pilings deep into the ground and elevated the whole thing.
The result has been a lot of areas difficult to maintain, including the steel pilings, some of which are concentrated in crawl spaces, corroding in the high water table. In 2013, the roadway had to be closed to address some pilings that were rapidly deteriorating, Dwyer said. Bringing the roadway to the ground will reduce maintenance and annual bridge inspection costs and make the roadway safer for drivers by giving it a more level profile, Dwyer said.
"They just didn't have the methods back then that we do nowadays," she said. "We have so many different options we can put in there. We can put different types of fill, everything from lightweight concrete to styrofoam blocks capable of holding a roadway."
Once started, work to reconstruct the can of worms will be done in phases across multiple construction seasons. Until then, the prep work carries on — 17,000 pounds of pressure per square inch at a time.
"You feel that?" Buhl said, starting a new probe. "That's the layer of frost. It's tough to get through that."