Army tunnels underground under Mpls. To expand the rainwater drainage system

Every morning, Eric Foby and his colleagues taunt themselves in a red cylindrical cage that is slowly lowered into a hole in the ground.

As you sink far below downtown Minneapolis, the sunlight is swallowed up. The cage vibrates as it makes its way down a metal-lined shaft into the ground, down the street, past a layer of limestone and into the sandstone below. Traffic noise quickly fades away and is replaced by the hum of ventilation air.

Finally, about 80 feet underground, a large cave chamber opens. Bright white lights reveal what looks like a massive, round building made of light-coloured sand. At the end of a short road, huge tunnels open to the left and right, large enough to accommodate a school bus. A team of workers creates a new underground world.

“I’ve been in the tunnels a lot during my career here,” Kevin Danin, a Minneapolis-based sewer operations engineer, said after a recent tour of the new $57 million Central City tunnel project. “But this one took my breath away. That was an amazing sight.”

Invisible to most of the city above, crews are constructing a large rainwater tunnel down downtown that runs parallel to a tunnel located down Washington Street. Work began in September and is due for completion in June 2023.

Why are they building this?

The project was designed to alleviate flood concerns in downtown Minneapolis.

When the current rainwater drainage system was built in the 1930s, downtown still had homes, dirt streets, and grassy courtyards to soak up the rain. Now, concrete dominates the landscape, and some skyscrapers occupy entire city buildings. Excess water has nowhere to go except in the aging system.

“The tunnel system is under stress right now because we’re trying to put a lot of water in it… It’s actually breaking down the concrete,” Danin said. Repairs will cost up to $600,000 annually.

Storm Tunnel Upgrade in Minneapolis

Deep in the streets of downtown Minneapolis, a network of tunnels built in the 1930s to carry rainwater into the river bursts at the seams when it rains. A $57 million project near Washington Street will ease the pressure.

1. Storm tunnels divert water into the river, while sanitary tunnels send water for purification in St. Paul.

2. The effect of age and increasing torrential rains on the current rainwater tunnels. The city is repairing the existing tunnel and building another, larger tunnel next to it.

3. The new 4,200-foot sewer tunnels will be twice the size of the old system and will be able to handle runoff from a 100-year storm.

Source: Minneapolis • By Mark Boswell, Star Tribune

Beneath the streets of Minneapolis, 830 miles of sanitary sewers carry wastewater from homes to a treatment plant. There is also a system of more than 500 miles of storm tubes and 12 miles of deep tunnels to carry rainwater from street entrances to nearby lakes or rivers. There are approximately four miles of tunnels down downtown.

The current downtown tunnel is 6 feet in diameter. When the new tunnel is complete, which is about twice that size, the two tunnels will work together.

“This is just a good example of us reinvesting in our infrastructure and trying to be proactive about the changing weather conditions we are going through,” Danin said.

What is the average day?

The 46-year-old Fobbe, from Buffalo, has worked in tunnels for 12 years. When he goes to his job under Minneapolis, he wears a hard hat, face mask, and googles. He uses a lunchbox-sized remote control to control a bright yellow-orange demolition robot called Brokk. Grinders are used at the end of a long arm to expel free pieces of sandstone from the walls.

The green laser draws the outline, so Fobbe knows what and where to grind.

His “style” is to sculpt the shape of a cathedral to allow the sandstone to collect over itself at an arched point for additional support. Not every system that is drilled will have the same look. Another part of the tunnel goes into the denser limestone, which can support the shape of the box.

Other workers use bulldozers – the size of a large lawn mower – or an excavator to remove the ground sandstone. It has the feel of chalk or baby powder. At the end it is chambered in a basin which is lifted by a crane from the tunnels and thrown into the street. Then workers load it into a dump truck, and transport it to a landfill or elsewhere for reuse.

Freshly cleared tunnel walls are sprayed with sodium silicate to harden the sandstone overnight. The work crews then put in rebar nails, nets and rocks to hold the sandstone in place. It works from 7am to 6pm almost every day.

The fine, light brown sandstone is what makes this project unique to Bruce Wagener, a CNA tunnel engineering consultant, who has worked on more than 300 tunnels.

“It’s easier to dig but it just takes more support to keep it from collapsing,” he said.

It’s not a bad place to work, as temperatures are kept at around 50 degrees – which makes it look like a basement. Yellow pipes run along the roof and top of the shaft to provide ventilation.

Fresh air is exchanged every three minutes. However, face masks are worn to protect against inhaling toxic silica dust from sandstone grinding.

current progress

Construction in the middle of the road. The total project of Central City covers about 4200 feet. The new tunnel and the existing tunnel will connect at the link structures. Two of the four buildings were built.

Once the excavation is completed, workers will line the tunnels with concrete. Both tunnels will carry rainwater into the Mississippi River, where it enters near the Stone Arch Bridge.

Weather conditions may cause some roadblocks. In the event of heavy rain, workers will evacuate the existing tunnel and wait for the water to be expelled, snowstorms may affect workers arriving at the site or delay excavation.

“All underground construction involves a lot more unknowns than building above ground, because when you work above ground, obviously you can see everything you’re dealing with,” Danin said. “When you’re underground, things can change.”

Eventually, the large machinery and other supplies underground would be lifted by a shaft, the same way they—and the workers—went in.

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