Deworming the can of worms is getting more expensive.

The latest cost estimate for the five-year project to improve the much discussed and cussed interchange of interstates 35 and 535 and U.S. Highway 53 is $342 million, people who attended an update by the Minnesota Department of Transportation learned on Monday.

Even before the first traffic cones and detour signs have been placed for what's officially known as the Twin Ports Interchange Reconstruction Project, the price estimate has been spiraling upward. In October, it was estimated at as much as $275 million. Last June, it was between $216 million and $252 million.

Much of the reason for the sticker shock, project manager Roberta Dwyer explained, is that the land the interchange sits on is even worse for road construction than MnDOT realized.

"It's a very poor soil," Dwyer told the 40 or so people who attended the second of two sessions MnDOT hosted at Clyde Iron Works. "It's very interesting to note that much of the area for the interchange was at one time (under water). And starting in about 1870 as Duluth developed, they just kept dumping fill in there. And so it's full of everything you can possibly imagine.

"There was also a sawmill in there. So at one point when we were doing some exploration, we found 15 feet of sawdust."

The problem is that MnDOT doesn't have $342 million for the project, Dwyer said. It has $300 million, including the $20 million federal grant that was awarded in December.

MnDOT is required to have a balanced budget for the project, Dwyer said, so "on paper," it's taking out a rebuild of the I-535 and Garfield interchange just in front of the Blatnik Bridge.

To a question, Dwyer agreed that this part of the project is essential for getting heavy loads from the port off of city streets and on to the interstate system. MnDOT "fully intends" to manage costs in such a way to get that interchange back in the project, she said.

Another $2 million was added with a decision to keep Highway 53 through Lincoln Park as a bridge rather than a filled-in structure, Dwyer said. That was in response to concerns about maintaining access in a neighborhood in which 25 percent of residents don't have motor vehicles.

Dick Haney, who lives north of Duluth and attended the session, said the change was good for another reason. The fill would have disrupted movement by wild animals, perhaps forcing them into more heavily trafficked areas, he said.

Haney and Mike Casey, who lives in the Smithville neighborhood, both expressed concerns that MnDOT wasn't paying enough attention to pedestrian and bicycle travel in its planning.

Casey noted that construction would disrupt the Cross City Trail, used primarily by bicyclists, and he said MnDOT was leaving it up to the city to come up with an alternative route.

"We would really hope the city works hard with MnDOT in funding a robust ... Cross City Trail detour," said Casey, who represents Duluth Bikes.

Dwyer said more than 100 people attended the noon meeting, and people continued to visit MnDOT's display throughout the day.

To learn more

Staff of MnDOT's Twin Ports Interchange Reconstruction Project will hold "office hours" to answer questions and receive input about the project at the Ecolibrium 3 Office, 2014 W. Superior St., on the following times and dates:

• 1-3 p.m. today

• 4:45-7 p.m. Feb. 7

• 1-3 p.m. Feb. 12

• 4:45-7 p.m. Feb. 21

• 1-3 p.m. Feb. 26

• 4:45-7 p.m. March 4