As flyover bridges leading onto and off Interstate 35 continue to come down through the Lincoln Park neighborhood in Duluth, the demolition of old bridges to make room for new ones is only the most visible aspect of the ongoing $343 million Twin Ports Interchange reconstruction project.
On the ground, pivotal work to build a massive box culvert is well underway.
Standing on the earthen roadbed of the closed southbound lanes of I-35 last week, Pete Marthaler showed the News Tribune the four-celled culvert designed to carry the combined Miller and Coffee creeks underneath the future roadway and into the St. Louis Bay. MnDOT eschewed precast culverts in favor of doing concrete pours on-site.
"Everything you're looking at was formed and poured," said Marthaler, the Minnesota Department of Transportation's major projects manager in charge of reconstruction. "The benefit there is we can improve the designs to handle the higher fill heights."
Given the trend toward heavier storms, the culvert was designed to handle major rainfall events. Each of the four cells of the culvert appear large enough to drive a sedan through.
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“The flood we had in 2012 was a big factor in the design, because there’s a lot of debris that comes down,” Marthaler said. “I wouldn’t say it’s overbuilt, just that we have a lot of future-proofing in that down there.”
Crews from the joint venture between Ames Construction, of Burnsville, Minnesota, and Kraemer North America, of Plain, Wisconsin, have been pouring the concrete structure in recent weeks — both at the outlet end by the bay and farther inland, where the creeks will be joined later in the project.
Marthaler described how the twin creeks will use all four chambers of the culvert when there’s higher water flow. When the water flow is lower, the creeks will be diverted through one chamber.
“We designed in weirs (fixed barriers across the creeks),” Marthaler said. “It’s a fish thing, so they’ll always have a channel.”
The rerouting and merging of the creeks is among the most complex and challenging aspects of what are four years of project construction, ending in 2024.
It’s necessary to divert Coffee Creek, because its current underground configuration bends through a field of I-35 bridge piers, where hundreds of ground improvement columns will be installed next spring after crews demolish elevated portions of I-35 and rebuild much of it at ground level.
Both creeks run mostly underground through the neighborhood. A design aspect of the interchange project is to feature an open channel of Coffee Creek in the neighborhood for the first time since 1891. The open channel is now evident leading up to the culvert, a deep cut into the earth that looks like it runs 200 yards or so. Marthaler described it as a “constructed natural area.”
“We have a lot of other successive layers to go on top of this to make it more natural — riprap, live plantings, special water pools,” Marthaler said, describing the open channel as a future wetland home to frogs and other animals.
Once work on the inlet and outlet ends of the culvert is completed, traffic will be reconfigured to four lanes again on I-35. Southbound I-35 traffic will be diverted to the Lower Michigan Street bypass currently under construction, and northbound traffic will use the south lanes of I-35.
When that happens, crews will have access to the north lanes of I-35, and they will be able to connect the inlet and outlet ends of the four-barrel, creeks-carrying culvert.
“We will essentially open up the gap in between the inlet and outlet ends, and start that work this winter,” Marthaler said.
Before that happens, there are less than two more months of work to go on the inlet and outlet ends of the culvert.
“Roughly speaking, we’re looking at putting southbound traffic onto Lower Michigan near Oct. 1,” Marthaler said of the return to four-lane I-35 traffic. Currently, both directions of I-35 traffic are reduced to a single lane in what are the north lanes.
In the meantime, bridges continue to come down, leaving less and less access from I-35 to either U.S. Highway 53 toward the mall area or Interstate 535 over the Blatnik Bridge to Superior.
“Come winter, if you’re on I-35, 27th Avenue West (exit) and Fifth (Avenue) West will be your two options,” Marthaler said.
Two more major closures are expected Wednesday, MnDOT said in a news release last week:
Southbound Highway 53 to southbound I-535 will close. The signed detour will follow southbound Highway 53 to 21st Avenue West, to West First Street, to Lower Piedmont, to Garfield Avenue, and onto southbound I-535.
Southbound Highway 53 to northbound I-35 will close. The signed detour will follow southbound Highway 53 to eastbound Highway 194/Central Entrance to southbound Mesaba Avenue, to West First Street, to Sixth Avenue West, to West Michigan Street, to Fifth Avenue West, and onto northbound I-35.