Minneapolis gets ready to seal sewer system cracks

For a 128-year-old, the 10th Avenue storm tunnel outlet is in pretty fair shape.

The tan and red bricks that line its floor are polished by more than a century of rain and melting snow as it drains into the Mississippi River near the University of Minnesota. Its horseshoe-arched concrete ceiling is more than a foot thick.

But it was built for an era when more rain soaked through unpaved streets and yards. Urbanization means more water rushes into the catch basins that feed the tunnel. It just wasn't built for that kind of pressure.

The $3.5 million or so that the city's been spending annually to repair tunnels like this one isn't doing the job, according to an updated assessment. So the City Council is poised to double what it had planned to spend on repairs to the 14.7-mile tunnel system in 2011-2015. The money will come from added borrowing and storm water fees assessed to homeowners.

On Friday, the council authorized a down payment on that job, upping its 2010 storm tunnel repair budget by $5.2 million. Starting later this month, the 1882 outlet to the 10th Avenue tunnel will be widened, as the first project in that work. More repairs will be made as funding accelerates.

This oldest 462-foot section of a mile-long tunnel is a bottleneck for bigger, newer sections upstream. So when water roars through after a rain, the constriction means that bigger sections built in the 1930s undergo hydraulic pressure that can crack their unreinforced concrete and even blow-out sections.

Deep beneath our feet

He and two others walk them periodically to record and map their deterioration. Their job is to guard a system of infrastructure that few think about because it's 65 to 100 feet beneath them, in easily tunneled St. Peter sandstone.

They've rated nearly a quarter of the storm tunnel system as being in poor condition, meaning there's advanced deterioration, mainly cracks that allow water to eat into and undermine the sandstone that supports the tunnel. Another half of the tunnel system is rated as fair.

Storm tunnels drain about 27 percent of the city. Some were built by the state when its freeway construction severed existing city tunnels. All drain to the river.

The rest of the city is served by shallower pipes that drain to lakes or creeks.

Parts of the tunnel network once did double duty, draining sewage to the river as well. But sewage and storm pipes largely are separate now, with the former draining to the metro sewage treatment system.

They look for new cracks, monitor old ones to see if they've widened, and try to peer through them to see if escaping water has scoured the enveloping sandstone.

One easy giveaway is sand on the floor of the tunnel. That means that water has pushed out and drained back, carrying sand with it.

"It's not a good thing, but it's good for us to see it," Danen said. "It indicates a problem upstream."

Avoiding voids

The city makes some repairs by spraying concrete onto sections of tunnel. For other areas, it's considering installing strapping to keep a crack from widening until a more permanent repair is made. It injects grout to fill some small voids behind cracks. For larger voids the city is considering lining the widened space to add to the tunnel's capacity.

That fee is scheduled to rise to $12.41 monthly by 2015 under a budget proposal recommended by Mayor R.T. Rybak and the city's citizen capital advisory committee. The accelerated program will add roughly a nickel more per household per month, starting in 2012, but that will be supplemented by borrowing that will be repaid over a much longer period.

Danen can testify to just how quickly water pressure can force repairs. He was below 2nd Avenue S. downtown in June, and he noticed slight cracking.

When he descended again in August after a heavy rain, he could see that a 50-foot-long section of the pipe's crown had shifted 3 or 4 inches. That allowed hemorrhaging water to break into an adjacent sanitary pipe, through which it escaped.