By Lynda Myer
Construction of the new stormwater tunnel, that will be placed 21m beneath the ground, is a part of US’s planning for developing and improving its Central City Tunnel System.
Due to increasing stormwater run-off, that means the available 1930s-built deep tunnel now runs at full capacity and at alarmingly high pressures, possibility of a public safety hazard such as flooding and flying manhole covers is really high in these areas. Moreover, in order to reducing damages such as holes and cracking of the unreinforced concrete liner and subsequent erosion of the sandstone immediately behind it, that is due to repeated pressurization during intense rainfall events, repetitious and expensive repairs involving void location, filling with grout and repairing the cracks is required.
Engineering consultancy CDM Smith (CDMS) was commissioned by the City of Minneapolis to develop a conceptual tunnel solution. This comprises a 1.3km-long parallel tunnel located in St Peter Sandstone bedrock, and ranging in diameter between 3m and 3.65m.
In accordance to CDMS’s idea in initial design report, digging with roadheaders chosen as the most appropriate method for the smallest diameters. Because this method of excavation would attain a cathedral-shape that best exploits the properties of the sandstone, especially the 62° angle of the tunnel crown that would match the natural friction angle of the sandstone to provide the most stable tunnel shape. While the intended tunnel is going to outlet into the Mississippi River, two access points will be constructed for construction staging.
Minneapolis Central City Tunnel System, at the moment is working on the Design and engineering phase of this project, although the determined date for construction tentatively is in late summer or early fall 2021 and be completed by May 2023.
