The city of Minneapolis is in the midst of a $57 million expansion of a downtown stormwater tunnel system that’s designed to relieve pressure on existing tunnels, reduce long-term maintenance on those tunnels and provide enough additional capacity to handle expected larger rainfalls in the years to come.

The project’s main component is a new 12-foot-diameter, 4,000-foot-long tunnel that’s being bored about 70 feet underground. It runs parallel to an existing 6-foot-diameter main tunnel that’s the backbone of what’s known as the Central City tunnel system, originally built in the late 1930s to drain stormwater into the Mississippi River, says Kevin Danen, a sewer operations engineer in the Public Works Department’s Surface Water & Sewers Division.

The new tunnel will significantly expand the capacity of the Central City tunnel system, which comprises a main line and four roughly 4-foot-diameter, horseshoe-shaped feeder tunnels — 4 miles of tunnels in all, he says.

The project, which broke ground in November 2021, is funded with approximately $38.6 million in bonds floated by the city and an additional $18.4 million in state bonds.

“It’s definitely the largest and single most costly project our division has ever undertaken,” Danen says.

Officials first worked with two consultants, Barr Engineering Co. and CDM Smith, that modeled various ways to alleviate pressure on the tunnel system. Several options were presented, including building a network of rain gardens or demolishing the existing main tunnel and building a new oversized tunnel.

“But there wasn’t enough surface area available to build enough rain gardens to make a dent in the amount of rainwater going into the tunnels,” Danen reports. “So in 2018, we decided to increase the [main] tunnel capacity, which led to building a larger parallel tunnel about 20 feet away from the existing tunnel.” 

The tunnel’s diameter is so large because officials are concerned about the trend toward larger and larger rainfalls in short periods of time. Moreover, land usage has changed over the decades, leading to more and more paved areas and rooftops that shed more and more stormwater.

“We’ve had a couple of occasions where during short but intense rainstorms — maybe 2-inch-an-hour rainfall for, say, 20 minutes or so — we’ve actually seen water ponding by curbs and gutters because the system is surcharged,” Danen says. “So we put some foresight into this project and designed a system that could accommodate a 100-year rain event.”

Stopping the surcharge

Slated for completion in June 2024, the tunnel is expected to reduce stress on the existing downtown tunnels, which typically are fully surcharged — at times under 90 feet of head pressure — and are “literally blowing themselves apart,” Danen says.

“The tunnels were made with 8-inch-thick, unreinforced concrete and weren’t designed to handle this much flow. As a result, when the tunnels reach a pressurized state, the water flow actually cracks the concrete and displaces sections of it, which allows water to get outside the tunnel and erode the surrounding sandstone.”

Sometimes highly pressurized water flow leaks through small cracks in the tunnels with such force that they act like mini water jetters, which scour the surrounding sandstone and create even larger voids than otherwise might be expected. The end result: Stretches of the tunnel no longer are reinforced by the surrounding sandstone, which leads to even further damage.

“We’ve had about a 20-foot-long-by-2-foot-tall section of concrete in a feeder tunnel literally blow apart,” Danen says. “We’ve found voids as large as 20 feet tall, 30 feet wide and 200 feet long.”

Historically, the division has repaired the tunnel walls and filled the exterior voids with grout. But the new tunnel reflects a more proactive, rather than reactive, approach by eliminating the creation of more voids.

“We decided there had to be a better way than to just keep on responding to these tunnel failures,” Danen says.

Miles of stormwater tunnels

In all, the city relies on 15 tunnel systems comprised of 16 miles of tunnels that drain about 39% of the city’s stormwater into the Mississippi. The rest of the stormwater drains either into the Mississippi or creeks, streams and small lakes via gravity-fed stormwater lines.

“We have some unique geography here because the Mississippi drops 50 feet in elevation after it flows over St. Anthony falls [located about in the middle of the city],” Danen says. “Any stormwater that falls south of the falls flows into the tunnels, many of which were built back in the late 1880s as combined sanitary and storm sewers, but have been separated since then.”

Furthermore, as the path taken by the Mississippi changed thousands of years ago, stretches of the old riverbed were transformed into a chain of small lakes connected by creeks. Stormwater in the western and southwestern parts of the city drain into those lakes and a large portion of the stormwater in the southeast quadrant drains into tunnels and then into the Mississippi.

The northern parts of the city drain into the Mississippi via a conventional system of storm sewers.

In total, the city relies on about 830 miles of sanitary sewers, 509 miles of stormwater lines that drain 57 square miles of land, 31 miles of sanitary mains and interceptor lines and more than 400 outfalls. Stormwater is treated with above-ground methods such as grit chambers and rain gardens, Danen notes.

Wastewater from more than 100,000 buildings in Minneapolis is handled at the Metropolitan Wastewater Treatment Plant, which is operated and maintained by Metropolitan Council Environmental Services in neighboring St. Paul. The city of Minneapolis pays fees to MCES, which pay for maintenance of its regional collection system and the direct costs of wastewater treatment.

Monitoring the results

The tunnel project also includes enlarging the last 400 feet of an outlet into the Mississippi, where the new tunnel, the old main tunnel and another tunnel converge. A local contractor, PCI Roads, is doing the tunneling work.

When the project is completed, stormwater from the northernmost of the four feeder tunnels will flow directly into the new tunnel, while the remaining three feeder tunnels will still transport water into the old main tunnel, Danen explains.

When the old main tunnel fills up to a certain height, stormwater will travel into the new tunnel via cross connections between the two channels.

After the new tunnel goes online, officials will rely on 11 pressure meters — two in each feeder tunnel and three in the old main tunnel — to monitor if the system works as designed, Danen says.

“That will tell us how successful we’ve been in alleviating pressure in the feeder tunnels,” he notes.

More work to do

Efforts to rehabilitate the feeder tunnels, which have been suspended during the project, will resume when the new tunnel is finished. Danen says that since around 2008, the city has spent millions of dollars repairing the tunnels.

“We’re about halfway through with our rehab program,” he says.

About 18,000 feet of two feeder tunnels, each located in tunnel systems aside from the Central City system, have been completely rehabbed. Rehabilitation of another roughly 17,000 feet of tunnels in other tunnel systems are scheduled to be completed during the next five to 10 years.

In the end, officials hope the new tunnel will significantly reduce the need for more tunnel repairs and rehab work.

“We’re definitely moving from a reactive position into a more proactive position,” Danen says.  “We’re going through the city step by step and assessing the various needs of our stormwater system.

“We’re not only rehabbing existing tunnels. We’re also adapting the system to new conditions that are much different than when the stormwater system first was built out. And this parallel tunnel is the largest step we’ve taken so far.”

It’s possible another large tunnel will be required in the years ahead.

“At this point, it’s too early to tell. But we’ll keep modeling and assessing things and adjust to what we see.”