The current activities of the City of Minneapolis Public Works Department are: building a new storm tunnel parallel to an available tunnel placed beneath Washington Ave. S. among Nicollet Mall and Chicago Ave, extending a portion of the existing storm tunnel, which is located below Chicago Ave. between Washington Ave. S. and the Mississippi River, as well as constructing new tunnel access in Downtown Minneapolis.
Constructed from 1939 to 1940, the present downtown Central City storm tunnel system includes 6.28 km (3.9 miles) of 21.34 m (70 feet) deep drainage tunnels and was designed to handle the storm-water drainage requirements at that time. The remarkable disadvantage of land development is a significant increase in the amount of stormwater that is directed into the tunnel system, resulting in over-pressurization of the system. This over pressurization has led to degradation in the tunnel infrastructure – voids either above or below the tunnel structure, cracking and failure of the tunnel’s liner, erosion of the surrounding sandstone and infiltration of groundwater and sand – and an increase in maintenance spending to inspect and maintain the system.
Decreasing the pressurization in the tunnel system, that consequence in a reduced risk of tunnel failures, raised tunnel system service life, reduction in tunnel repair costs, and a decrease in long term maintenance with the operation of the tunnel system such as surface flooding and blowing manhole covers, are the advantages of constructing this new parallel primary tunnel.