The MWMO worked with the City of Minneapolis to remove decades’ worth of accumulated sediment and debris from the Old Bassett Creek Tunnel (OBCT), which runs beneath Downtown and North Minneapolis. The multi-phase project is designed to prevent accumulated sediment and its pollutants from being mobilized and flushed into the Mississippi River. This second phase of the project, completed in 2020, removed 800 tons of sediment from an 853 foot section of the tunnel.
Constructed in the early 1900s, the OBCT was built to convey water from the historic Bassett Creek to the Mississippi River. A new tunnel was constructed in the 1970s, and the OBCT has a separate drainage area and conveys flow from Bassett Creek only under heavy rains and high (stream) flow conditions. The MWMO has been working together with the city since 2012 to study and ultimately remove the sediment from the tunnel. You can learn more about the project background and the investigations of the pollutants in the tunnel on our planning initiative page.
This Phase II project resulted from a 2017 assessment and cleanout plan prepared by Barr Engineering Co. for the City of Minneapolis and MWMO to remove deposited sediment and debris within the OBCT. The study found approximately 4,500 tons of sediment, 2,500 pounds of phosphorus, and hundreds of pounds of metals accumulated within the 7,915 linear feet of tunnel. The first phase, totaling 1,080 feet, was completed in 2018. That phase removed approximately 935 tons of sediment and debris. Phase II totaled approximately 853 linear feet, from Fourth Avenue North to Olson Memorial Highway in North Minneapolis. The sediment depths ranged from 5 inches to 27 inches. This was identified as the section with the most accumulated sediment in the OBCT. The MWMO funded a contractor to remove, haul, and dispose of accumulated sediment and debris at an approved waste facility.
Initiating Phase II required the City of Minneapolis to install a new access hatch under Fifth Avenue North, just east of Van White Memorial Boulevard. The invert of the tunnel in this location is approximately 14 feet below ground, and takes various shapes as parts of the tunnel have been built or replaced over time. Some of the original sections are made out of brick and curved, with a maximum height of 7.5 feet, so the contractor was limited to using walk behind skid steers as equipment that was lowered into the tunnel to push the sediment towards the entrance. A crane lifted the sediment out of the access point and it spread out to dry before hauling it away.