Minneapolis weathers the storm with a new parallel tunnel

The existing Central City storm tunnel system was constructed between 1939 to 1940

The proposed tunnel would significantly minimize tunnel maintenance and repair costs

Overall construction bid of approximately $75 million

Challenges

Minneapolis' existing Central City storm tunnel system was designed and constructed in the early 1940s to support the storm water drainage requirements of the time. Land development in the time since has significantly increased the amount of storm water the system handles.
Over pressurization has degraded the tunnel infrastructure, leading to cracks and failures in the tunnel lining, infiltration of groundwater, and build-up of ice and sediment.
To minimize ongoing repairs and increasing maintenance costs, the City of Minneapolis decided to construct a new storm tunnel running parallel to the existing, 80-year-old tunnel.

Solutions

Operated as a subconsultant in partnership with CNA Engineers, Inc. to support design, bidding, and construction phase services for the new parallel tunnel and expansion of the existing tunnel.
Project tunnel design included a 12-ft equivalent diameter, cathedral-shaped tunnel with four box section junction structures to connect with the existing tunnel infrastructure.
Provided cost and schedule estimation, tunnel system hydraulic engineering, tunnel junction structure design, tunnel cross-connect design, and outlet structure concept design.

Highlights

Increased intervals between tunnel inspections and significantly reduced maintenance and repair costs, and the overall operating budget for the existing tunnel system.
Extended overall service life of the tunnel system, mitigating critical public safety hazards including tunnel failure, over pressurization, and utility access hole cover blow-off.
Reduced the risk of surface flooding during heavy rainfall and large storms and overflows from the historically connected sewer system.

Project numbers

3.9 miles of deep drainage tunnels in the downtown Central City storm tunnel system
70 to 80 feet below street surface
$75 million, the approximate overall construction bid