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Prep work underway for 'can of worms' interchange project in Duluth

Dylan Buhl, a specialist with the Minnesota Department of Transportation, reseals the probe atop a length of steel bore pipe before using the cone penetration rig to send it deep into the ground below the"can of worms" interchange last week. Tom Lund (left), Twin Ports Interchange project supervisor, looks on. Bob King / rking@duluthnews.com1 / 5
Soil testing is underway beneath the Interstate 35 "can of worms" interchange to determine what kinds of loads it can support; a planned reconstruction project would bring the elevated roadways down to ground level. Bob King / rking@duluthnews.com2 / 5
Dylan Buhl works from a box truck that houses the cone penetration testing rig he operates beneath the "can of worms" interchange in Duluth. Bob King / rking@duluthnews.com3 / 5
Dylan Buhl clears snow from beneath the truck so the cone penetration device has a clean start into the ground. Bob King / rking@duluthnews.com4 / 5
Dylan Buhl, a transportation specialist with the Minnesota Department of Transportation, records readings from his laptop during a cone penetration testing run below the Interstate 35 "can of worms" interchange last week. Bob King / rking@duluthnews.com5 / 5

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. The rig was parked near the railyard off Michigan Street in the Lincoln Park neighborhood of Duluth.

A specialist with the Minnesota Department of Transportation, Buhl works out of the Office of Materials and Road Research based in Maplewood. 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, the series of ramps and elevated sections of freeway connect I-35 with U.S. Highway 53 and I-535. It's being targeted for replacement due to the traffic safety and maintenance concerns that 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."

Dylan Buhl works a pneumatic press to burrow a probe into the soil beneath the "can of worms" interchange. Bob King / DNT

Located along the St. Louis River estuary, the water table underneath the can of worms is almost right below the surface, 2-3 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 which 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 and 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.

Four years on the job, Buhl enjoys the work. He works from a predetermined grid, centering the behemoth rig over the next hole site.

"A bad day is when I break something or get stuck," he said. "Then it's time to grab the shovel."

When the News Tribune met Buhl last week, he'd reached the area near 20th Avenue West on his way to 27th. He figures to be in Duluth deep into the warm weather. He said he likes learning what's underfoot and that it makes him feel like he's connecting with history.

"There used to be a woodyard here if I'm not mistaken," he said.

This probe relays data about the soil density and core pressure when it's bored into the ground. Bob King / DNT

Buhl is working alongside a commercial outfit contracted by MnDOT to drill and gather soil samples in the areas Buhl is also working. The commercial company declined to speak with the News Tribune.

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 — what with the series of steel pilings, some 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 figures to 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."

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