Aging tunnels beneath Twin Cities could collapse, potentially cause floods

Minneapolis officials are beginning to focus on the neglected tunnels.

;MINNEAPOLIS (AP) – Miles of aging storm water tunnels underneath the Twin Cities are splitting apart under the pressure of heavy rains, leading to a risk of collapse that could flood streets and buildings above.

Fifteen miles worth of tunnels under Minneapolis, some more than 100 years old, need $75 million in repairs to prevent such failures, engineers studying the problem told the Star Tribune. The city of St. Paul and the state Department of Transportation, which own 28 miles of storm water tunnels, face similar problems.

The tunnels, mostly built with unreinforced concrete or masonry, collect storm water from streets and rooftops and carry it to the Mississippi River. Increased urban runoff and heavy rains frequently fill some tunnels to capacity, creating pressure they weren’t designed to bear.

“We force more and more water into these tunnels, and the pressure starts to blow them apart,” said Leonard Krumm, an engineer with CNA Consulting Engineers, a Minneapolis firm wrapping up a six-year analysis of tunnel conditions in the city.

Excessive water pressure has cracked or burst tunnel walls in some spots, Krumm and other engineers said. If such a tunnel collapsed, falling debris could block it, causing storm water to back up and flood the neighborhood that the tunnel was built to protect.

Pressure is so intense in one tunnel underneath Interstate 35W in south Minneapolis that a geyser erupts from a manhole in the median during some storms.

St. Paul is spending $10 million to fix cracks and breaches in concrete tunnels underneath Frogtown, Interstate 94 and downtown. At one fracture, water blasted through the wall into the adjoining soft sandstone, gouging a cavern 90 feet long and up to 20 feet high. Workers filled the cavern with concrete so the tunnel wouldn’t collapse.

But tunnel repairs also carry risk. Two workers filling cracks in a St. Paul tunnel drowned in July after they were stranded underground during a deluge. Minnesota’s Occupational Health and Safety Division is investigating the accident.

Engineers said it will take 10 years to make all the needed repairs, a pace dictated more by weather and safety than money. Crews prefer to do most of the work in January and February, when risk of washouts is lowest.

Minneapolis officials concede that tunnel maintenance has been a low priority, but say that’s changing.

“I think the way folks managed in the past was in the reactive mode,” said Rhonda Rae, director of the city’s storm water program. “The checkbook’s getting smaller, and as things get older they need more maintenance. So where do we put our priorities?”

Still, finding the answer to that could be a struggle. The city’s current repair budget is $2 million a year, but just replacing three south Minneapolis tunnels would cost an estimated $19 million.

But if a tunnel collapses in an urban area, it could mean immediate and expensive flooding. In 2006, a deluge collapsed a 100-year-old storm water tunnel in Pittston, Pa., undermining the foundations of two commercial buildings and flooding basements in the commercial district, including the city’s library.

Rae said she has pondered the risk of a tunnel collapse. “Do I lose sleep over it? No,” she said. “But am I concerned? Yes.”