Hennepin County begins project to assess sinkhole risk

A map showing areas where bedrock is near the ground's surface
Across the Twin Cities there are areas where the bedrock is just beneath the surface. In Minneapolis this covers most of downtown and the area around MSP International Airport.
William Lager | MPR News

Hennepin County may not be standing on completely solid ground.

Geologists say densely populated portions of the county’s southeast corner could be prone to sinkholes, thanks to the soft sandstone bedrock that lies just beneath the surface.

So, this month, Hennepin County is launching a two-year study to assess sinkhole risk in its southeastern corner, an area that includes downtown Minneapolis and the Minneapolis-St. Paul International Airport.

Those involved in the study say there is no immediate risk of a major sinkhole forming, but the assessment is part of the county’s push to better understand risks from landslides and other natural disasters. The first year of the sinkhole project comes with a $30,000 price tag, to be split evenly between county funds and a federal grant.

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Sinkholes form when the ground collapses into a cavern below its surface, said Carrie Jennings, a geologist with the non-profit organization Freshwater, which until recently was known as the Freshwater Society. Sinkholes are common in southeastern Minnesota. In that part of the state, groundwater dissolves the limestone bedrock, leaving caves.

About 10,000 sinkholes pockmark Fillmore County alone. They are less prevalent near the Twin Cities, but Jennings said they’re still possible. In the Twin Cities metro area, the risk lies in a soft layer of bedrock called St. Peter Sandstone.

“You can just crumble it in your hands,” said Jennings. “You see people carving their name into it.”

A sinkhole is born

Groundwater can flush away the easily erodible sandstone. That process has formed caves along the Mississippi River, beneath parts of downtown and south Minneapolis, said Jennings.

Most of those caves haven’t collapsed into sinkholes, though there is one notable exception: a two-acre park in South Minneapolis called Seven Oaks Oval.

The park is a pit, 30 feet deep, complete with trees whose canopies poke out just above street level. This sinkhole formed thousands of years ago, so it didn’t swallow any houses or cars when the ground collapsed.

When Minneapolis was built up around it, the sinkhole’s walls were too steep to build on. So the spot remains a “pleasant little park that’s sunken into the ground,” said Jennings. “It’s a nice little refuge for kids to get out of the neighborhood.”

Jennings, along with geologists Greg Brick and BJ Bonin, will work with Hennepin County to assess whether sinkholes like Seven Oaks Oval could open up in the future. Eric Waage, who runs the Hennepin County Emergency Management department, is leading the effort.

“We’re in the surprise-prevention business,” said Waage.

Effects of climate change exacerbate threat

Jennings said assessing the risk of sinkholes is especially important now, as climate change brings heavier rains to Minnesota. More water soaking into the ground could mean more flushing of the soft sandstone.

Waage said his team’s study will eventually help engineers and developers decide how to build more safely. He wants developers to know whether they’re building above a cave.

And he said residents should learn the early warning signs of a potential sinkhole.

“There [would] be cracks in structures and things like that,” said Waage. “You’d get some kind of indication.”

The project will roll out over two years in two phases.

First, researchers will comb through newspaper archives, scientific papers and old drilling logs to compile any historical evidence of caves or old sinkholes that have been filled in.

The second stage of the assessment will involve fieldwork, with the geologists conducting new surveys in select areas. The project will focus on sinkholes that have formed naturally by the flow of groundwater, though Waage said artificial holes can also form as a result of leaky underground pipes.

The two-year assessment gets underway as Hennepin County wraps up a similar study on landslides.

For now, Waage advises residents to be aware of the possibility of sinkholes, but they shouldn’t be too concerned.

“This isn’t an occasion for people to panic,” said Waage. “No running for the hills at this time.”

Think you might have a sinkhole risk?

Hennepin County residents who spot early warning signs of sinkholes — such as newly formed depressions in the ground or cracks in a building’s foundation — can contact Carrie Jennings by email to report the findings to the sinkhole assessment team.