Out of Sight but Not Out of Mind
January 16, 2024
Minneapolis, seated on the banks of the Mississippi and Minnesota Rivers, is home to a bustling population of more than 425,000 citizens. The largest city in the state, it is supported through extensive underground infrastructure — most of which is decades old. The most pressing concern when it comes to this infrastructure is the deterioration of the stormwater tunnels beneath the city.
PCiRoads, a highway and heavy civil general contractor out of St. Michael, Minnesota, won the bid for a three-year project enlarging portions of the existing stormwater tunnel, constructing a new parallel tunnel and creating new tunnel access in downtown Minneapolis. Excelling in specialized solutions for complex projects, PCiRoads tackles project difficulties like narrow tunnel access, difficult equipment logistics and strict safety codes. The company chose to partner with Brokk, the world’s leading manufacturer of remote-controlled demolition robots, to rework more than 800 feet of stormwater tunneling underneath the heart of Minneapolis and mine 3,600 feet of new parallel tunnel. The final system is set to be 4,200 feet long.
The Old System
Originally built in 1939 and 1940 to move stormwater to the Mississippi River, the tunneling system below Minneapolis was designed to negate flooding based on the drainage needs of the time. However, the rapid growth in population in the last 80 years combined with a 30% increase in rainfall per hour in the last 50 years means the tunnels can no longer keep up with demand. While the city of Minneapolis Public Works Department conducted ongoing repairs to the tunnel lining totaling close to $600,000 annually over the past several years, the fact remains that the system simply was not designed to support today’s vastly different population level. The strain of excessive amounts of water caused cracking, breaking and other concrete deterioration, resulting in an urgent need for repair.
In 2021, the city of Minneapolis Public Works Department approved a $60 million overhaul of the stormwater tunnel system. The Central City Parallel Storm Tunnel Project’s goal is to create a tunnel system that can last another 100 years. The plan is to create a tunnel twice the size of the current system to move stormwater from downtown Minneapolis to the river.
“When the stormwater tunnels start to fail, we see dangerous flooding and other damages to underground infrastructure,” says Nap Scott, estimator and project manager for PCiRoads. “The repair of the current system and addition of the parallel stormwater tunnel will be a huge step forward in handling the increased demands of the city.”
The project includes both repairs to existing tunnels and the new parallel primary tunnel that will reduce the pressure in the current system. This will increase the lifespan of the current system, reduce annual repair costs and provide additional support for rainwater runoff.
The PCiRoads crew found a number of logistical challenges with the Central City Parallel Storm Tunnel Project. Throughout the project, Scott says the most difficult part was the intense planning required at each stage. The tunnels were so small that equipment couldn’t pass one another. If a different piece of equipment was needed, the original piece would need to be removed from the tunnel so that the new equipment could be brought in, significantly slowing down progress. Careful planning and intense equipment choreography helped the team minimize equipment changes and significantly limit the impact such processes had on the overall project timeline.
The first challenge PCiRoads faced with the project was that workers and equipment needed to be lowered by crane 80 feet into the tunnel system. Finding equipment small enough to fit into the existing tunnel system with enough power to get the job done was tough. Traditional equipment, like 5- to 10-ton excavators, couldn’t get into such tight spaces.
“One of the main challenges of the project was the size of the tunneling system and the size of the equipment,” Scott says. “Excavators just can’t work safely in such close quarters without the risk of falling debris and pinch points. We really needed smaller demolition equipment we could power underground where the operators could work at a safe distance from any falling material.”
PCiRoads’ 12 years of experience in the tunneling industry gave it a leg up in choosing the right equipment for such a large project. In the past, the company used Brokk demolition robots for similar, smaller-scale projects.
To complete the project, PCiRoads purchased two new Brokk 520Ds and a used Brokk 400 diesel model. It found that the power-to-weight ratio of these machines gave the best hitting power for their size. A 6-ton Brokk 520D is 5.25 feet wide, with a vertical reach of 23.4 feet. To get the same hitting power as a Brokk 520D, the crew would need to use a 10-ton excavator with a width of approximately 7 feet and a vertical reach of 8 feet. The lighter weight and smaller size of the robots made lowering them down the tunnel’s 80-foot shaft easier and safer than larger equipment.
“The Brokk diesel models are easier to power since they don’t need a generator,” Scott says. “But that means we needed to make sure there was proper ventilation in the tunnels before we could start. Once it was set up, the diesel models offered unparalleled versatility because we didn’t need to worry about generators or cords.”
Increased Versatility and Safety
PCiRoads took advantage of the robots’ versatility by using multiple attachments throughout the project. With remote-controlled operation featuring a range of more than 950 feet, the operator and crew were removed from the danger zone while the equipment worked. For mining the new tunnel, crews used the breaker attachment with the Brokk 520Ds. This part of the process required crews to break the sandstone and shale down to the limestone and remove it from the area to create a box tunnel configuration.
The project required PCiRoads to carve a cathedral shape out of the sandstone using a predetermined template as the tunnel comes to an arch at the top to provide additional support for the tunnel system. Crews used green lasers to draw the template outline, then completed removal with a robot.
The PCiRoads team chose the Brokk Drum Cutter 250 attachments as the best option for mining sandstone after realizing that the breaker wasn’t the best tool for precise shaping. The drum cutters — or road headers — with the Brokk 520Ds meticulously shaved away the sandstone and shale to precisely meet the template shape. Crews then removed the powdered material from the tunnel via skid steer and lifted it out by crane. At the project’s halfway mark, they had already hauled out more than 12,000 cubic yards of sandstone.
Following the removal of the sandstone and shale, PCiRoads moved on to breaking through bedrock at the riverbed. The crew utilized a Brokk 300 with a breaker attachment to tackle the bedrock. During this phase, the team intermittently stopped to install ground control. Additionally, they used a TEI 80 rockdrill to install rock bolts so that mesh could be placed to catch loose rock and debris and protect crew against rock falls and tunnel collapse.
Once PCiRoads completes the bedrock removal phase of the project, new forms will be rolled out for the tunnel, concrete will be placed under said forms and the new tunnel will be tied into the current system.
- Location: Minneapolis, Minnesota
- Duration: ~2 years
- Material Removed: 36,000 cubic yards of sandstone
- 2 Brokk 520D Remote-Controlled Demolition Robots
- 1 Brokk 400D Remote-Controlled Demolition Robot
- 1 Brokk 300D Remote-Controlled Demolition Robot
- 1Brokk Drum Cutter 250s
- 1 Brokk Hydraulic Breaker 705,
- 1 TEI 80 Rockdrill
- Skid Steers
- Various smaller pieces of support equipment