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Not-so-good vibrations: Mpls. condo residents fear SWLRT construction will damage their homes

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Southwest LRT Project Director Jim Alexander speaks to a group
Southwest LRT project director Jim Alexander speaks to a group of residents and elected officials at the Calhoun Isles building in Minneapolis Tuesday.
Matt Sepic | MPR News

Residents of the Calhoun Isles complex near the Midtown Greenway fear the digging of a proposed train tunnel just inches from the converted concrete grain silos they call home could do irreparable damage due to the unusual nature of their building. 

When hulking, utilitarian grain terminals reach the end of their useful life, they're often torn down to make way for something new. But 35 years ago, developers in Minneapolis turned a cluster of 14 concrete grain bins into a condominium complex.  

The work took a year and a half. A 1983 write-up in the trade magazine Concrete Construction says crews used diamond-tipped saws with blades 3 feet in diameter to slice openings for doors and windows. They bolted new concrete floors — up to 10 inches thick — to the silo walls.  

It's a solid building to be sure. But Nina Katzung says that a few years ago, she noticed a problem. 

"The antique furniture that I own was humming. I could feel the vibrations on my dining room table," she said.

Those vibrations were the result of pile driving at the construction site of an apartment building about 160 feet from Katzung's condo. She says cracks formed in her drywall and the noise became unbearable. 

"It was so bad that I just took my purse and left because I couldn't stand to sit there in that terrible noise and vibration," she said.

The Calhoun Isles Condominium Association says the work next door did about $30,000 in damage to their building. But they fear that project — now complete — was just a preview of headaches to come from construction of the $2 billion Southwest light rail line from downtown Minneapolis to Eden Prairie, where plans call for a train tunnel to be dug just 24 inches from the concrete footings that support some of the former grain silos.

Planned route for the Southwest light rail line
The planned route for the Southwest light rail line, as of May 2018.
Courtesy of Metropolitan Council

And residents say they're not just worried about damage during construction. They fear noise from the frequent light rail trains will cause problems once the line is built. 

Engineers the condo association hired confirmed that vibrations are more pronounced higher up in the building. That's the complete opposite of the way concrete structures usually react, said Lee Petersen of the Itasca Consulting Group, which analyzed data gathered by engineers working for the Metropolitan Council. 

"I would speculate that it's because the building is so rigid that the vibrations that are coming up the structure in the walls don't really attenuate at all," Peterson said. "And then when the vibrations get to a floor, the floors themselves are susceptible to resonances." 

After building residents raised these concerns with the Met Council, light rail project managers promised to mitigate vibrations, both during construction and when the trains are running.

Met Council Chair Alene Tchourumoff promises construction crews will turn down the volume, mainly by using a pile driver that uses a relatively quiet hydraulic press rather than a giant hammer.

"We've committed to a different type of construction that's really different than what you might associate with pile driving and loud noise," Tchourumoff said. "We're committed to trying to innovate there to minimize vibration, and also working closely with residents of the condominium association to make sure that we try to mitigate those impacts to their homes." 

Katzung says she's glad the officials are listening to their concerns, but at a meeting with project managers Tuesday night, she and many of her neighbors expressed skepticism about the mitigation plans. 

The Met Council has also promised to take before and after photos of the entire building to make it easier for residents to file claims for any damage. 

Tchourumoff says work on the Southwest light rail line could begin as soon as this year if the Federal Transit Administration gives the go-ahead. The Met Council expects the FTA to sign off on a critical $929 million grant for the project in 2019.