Deep below the streets of downtown Minneapolis, one of the city’s most significant investments in public infrastructure took place. The Central City Parallel Storm Tunnel expanded a stormwater system that had served the city for over a century.
Much of the original tunnel system dated back to the 1930s, with some sections built as early as the late 1800s. Over time, the system faced increased pressure during heavy rainfall events. Stormwater surcharge and localized street-level flooding became growing concerns as development and rainfall events intensified. To address these challenges, the Central City Parallel Storm Tunnel was developed by the City of Minneapolis and the design team of CNA Consulting Engineers (now part of TKDA), Hatch, and Sambatek.
Stretching roughly 4,200 feet underneath Chicago Avenue and Washington Avenue and located about 80 feet below ground, the 10- to 15-foot equivalent diameter tunnel was built to carry stormwater from downtown to the Mississippi River below the Stone Arch Bridge more efficiently and reliably at a construction cost of $56M. It runs parallel to portions of the existing system from Portland Avenue to Nicollet Avenue, adding capacity where it was needed most and replacing older sections that no longer met current demands.
Working with the Region’s Geology
The unique geology of Minneapolis is made up of layers of sandstone, shale, and limestone that influenced how the tunnel was designed and built. For the downstream section, crews excavated wide, flat back sections using a rock drum cutter attachment on a Brokk demolition robot to excavate the soft sandstone and take advantage of stronger limestone located at the top of the excavation for roof support. This section ranges from 8.5 to 16 feet in height following the base of the limestone, and from 10.5 to 22 feet in width. The upstream section was constructed entirely in sandstone because the bedrock contains a monoclinal fold, where the bottom of the limestone raised 30 feet in elevation. The 11.5 feet tall by 10-feet wide arched, cathedral-shaped profile provides stability in the softer sandstone. This flexibility allowed the project team to adapt to underground conditions rather than apply a single design across the entire alignment, working with the natural environment rather than against it.
Connecting a Complex Urban System
Prior to this project, downtown Minneapolis already had an extensive network of stormwater tunnels. Different segments ran underneath major roadways and connected to a main tunnel that carried water to the Mississippi River. In the new tunnel, the project team developed multiple connection points that allowed existing tunnels and drop shafts to feed into the new tunnel system. These connections helped redistribute flows and reduced pressure in older sections. By using this approach, the system was improved without the need for a complete rebuild.
Building Underground in a Busy Downtown
Construction occurred continuously in one of the most active urban neighborhoods in Minnesota from September 2021 to June 2024. Work progressed beneath streets, utilities, and buildings while daily life continued above. The contractor, PCiRoads, accessed the tunnel through three work sites: an open-cut access at the portal and two shafts. The team also coordinated activities to reduce impacts on traffic, businesses, and nearby residents. The work sites were wrapped in informational graphics about the project, and noise blankets were installed to reduce the sound of the construction to adjacent properties. The crews removed around 67,800 tons of earth and placed 15,100 cubic yards of 5000psi reinforced concrete liner for these sites.
Construction crews also managed groundwater conditions to keep the excavation dry. Deep wells lowered groundwater levels by up to 15 feet temporarily, allowing crews to work within the sandstone while maintaining stability in the surrounding area. At the river, a temporary cofferdam created a dry work area so crews could rebuild the outlet structure and ensure a smooth transition of water into the Mississippi River. These efforts required careful planning, coordination among agencies, and a deep understanding of local conditions.
Delivering Long-Term Community Benefits
Minneapolis is an evolving city, and at the same time, rainfall has intensified due to shifting weather patterns. By adding a parallel conveyance route, the new tunnel can handle larger storm events without overwhelming the system. Infrastructure like this creates the foundation for everything built above it, and this work translates into fewer disruptions for residents, businesses, and visitors.
The tunnel increases the overall capacity of the downtown stormwater system, but the benefits extend far beyond drainage. A more reliable stormwater system helps protect property, reduces maintenance demands, and improves public safety during major weather events. The tunnel is buried deep below the surface, but its impact is felt across neighborhoods, businesses, and waterways every time a storm passes without flooded streets or overwhelmed systems.
Craig Eckdahl
Senior Registered Engineer
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