A threat builds deep beneath the Twin Cities

Deep beneath the Twin Cities, aging storm water tunnels are splitting apart under the pressure of heavy rains, posing a risk of collapse that could flood streets and buildings above.

Minneapolis' 15 miles of tunnels -- some more than 100 years old -- need $75 million in repairs to prevent such failures, according to engineers studying the problem.

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

"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.

In many places, excessive water pressure has cracked or burst tunnel walls, undermining them, Krumm and other engineers said. If such a tunnel collapsed, the falling debris could block it, backing up storm water and flooding the neighborhood that the tunnel is supposed to protect, engineers said.

The city of St. Paul and the Minnesota Department of Transportation, which own 28 miles of storm water tunnels, face similar problems. Pressure is so high in a tunnel beneath Interstate 35W in south Minneapolis that a geyser erupts from a manhole in the median during some storms. In other places, manhole covers are blown into the sky during heavy storms.

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

Working in the tunnels also carries risk. Two laborers filling cracks in the St. Paul tunnel drowned in July after being stranded underground during a deluge. Minnesota's Occupational Health and Safety Division is investigating the accident.

A view from below

In a storm tunnel 85 feet below downtown Minneapolis, the air is musty but surprisingly warm on a subzero winter day.

The 1930s-era tunnel is just tall enough for city sewer construction engineer Bob Ervin to stand. Sloshing through ankle-deep water, he passed construction floodlights illuminating gaping holes in the concrete walls. For weeks, workers have been shoveling out the eroded sandstone behind it.

"We have a crack in the top and a crack in the side, and the concrete blows out into the sand, allowing water to get in there," Ervin said as he walked along the tunnel.

Over time, heavy storms have sent pressurized water through seams and fractures in the concrete. Once it breached the wall, water further eroded the sandstone with each new storm. Caverns or voids were scoured out of the adjacent sandstone, putting the tunnel at risk.

Just a few yards from where Ervin stands, the tunnel collapsed a few years ago. The accident didn't back up water in the tunnel, which might have flooded part of downtown. Instead, the streaming water burst into an adjacent sanitary tunnel, which carried it away.

Now, new cracks, fractures and voids are undermining the tunnel. "What we are trying to do is to is stop that process by pulling out all the loose material -- the rock and stone -- and filling the area behind the tunnel with concrete," Ervin said.

Beneath Minneapolis, CNA engineers documented numerous tunnel blowouts that have washed away adjacent sandstone. Two large tunnel systems south and east of downtown are in poor condition, with many structural failures and advanced deterioration, the study found.

Engineers say it will take 10 years to fix everything, a pace dictated not only by money, but by weather and safety: January and February are the preferred work months because they pose less chance of washouts. In summer storms, laborers must scramble to the surface before tunnels fill; tools and equipment left below can get washed away.

More money, no more water

The metropolitan area has more than 50 miles of storm water tunnels large enough to walk through. In some, you could drive a car. The oldest, built in the late 19th century, carried sewage until the systems were separated. They are 30 to 150 feet deep, dug beneath the most developed and paved-over parts of the urban area. All angle through soft sandstone toward the Mississippi River.

Even the tunnels built in the 1960s beneath interstate highways are deteriorating. A 5-mile tunnel under parts of I-94 and I-35W in south Minneapolis is cracking, has developed sandstone voids and needs about $10 million in repairs or its condition will worsen, said Beth Neuendorf, an engineer for MnDOT.

Neuendorf said the city is supposed to maintain the tunnel, but it hasn't kept up with the work. Most of the water comes from city streets, filling the tunnel to capacity and creating enormous pressure.

A MnDOT highway camera has repeatedly recorded a geyser blasting from a manhole and flooding I-35W at 35th Street. In 1999, the manhole cover blew off and a pickup truck slammed into it during a heavy storm. Workers modified the manhole so its cover won't fly off, but water still gushes out during deluges. Other pressure-reducing fixes are planned.

Minneapolis officials concede that tunnel maintenance has been a low priority but insist that's changing. "I think the way folks managed in the past was in the reactive mode," said Rhonda Rae, who was appointed director of the city's storm water programs last year. "The checkbook's getting smaller, and as things get older they need more maintenance. So where do we put our priorities?"

Now, Rae and other city officials are trying to answer that question, and it won't be easy. The current repair budget is $2 million a year. Replacing three South Side tunnels would cost an estimated $19 million; rebuilding and replacing 10th Avenue SE tunnels would be another $13 million, according to draft findings from the study. And that would fix only the worst tunnels.

In St. Paul's St. Peter-Rondo tunnels, workers have been patching holes and cracks and reinforcing walls with steel bands to keep them from splitting apart under pressure. The city's next rehab is the 4.7-mile St. Anthony Park tunnel system, where work will begin in 2010, said sewer utility manager Bruce Elder.

What's at stake?

If a tunnel collapses in an urban area, it can mean immediate flooding.

In 1992, an abandoned coal tunnel collapsed in Chicago, flooding downtown office building basements with river water while crews worked frantically to close the breach.

A 2006 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 library's. Melissa Szafran Jones, the library director, was driving home in her sport-utility vehicle when the storm hit, and "I suddenly had water up to my lap," she said. Two men pulled her out before the SUV submerged, she said.

No catastrophic collapses have occurred in Twin Cities storm tunnels, but engineers say the risk exists.

Another risk is that a tunnel failure could scour such a vast cavern in the sandstone that a sinkhole would develop at ground level, caving in streets or undermining buildings. Charles Nelson, a founder of CNA Consulting Engineers and a tunnel expert, said this is a theoretical possibility but has never happened here.

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

David Shaffer • 612-673-7090