Milwaukee Metropolitan Sewerage District (MMSD) is going ahead with a 12km long, 6m diameter tunnel capable of holding 89 million gallons of sewage, as part of a wider scheme to construct a system of tunnels with a capacity of 116 million gallons.
It follows a report by the Wisconsin Legislative Audit Bureau criticising a recently constructed network of tunnels under the city that failed to significantly reduce the number of sewer overflows in the Milwaukee area. Known as Deep Tunnel, 15km of tunnel, at depths of up to 99m, were built in the late 80s, early 90s, by contractor CH2M Hill, to contain the billions of gallons of raw sewage that overflows annually into the local waterways, including Lake Michigan.
The report published in July 2002, said that although “the Deep Tunnel has reduced both the number and the volume of sewer overflows… the District has discharged 13.2 billion gallons of untreated wastewater since [it] began.”
Larger than expected storms were partly to blame, but human error was also highlighted. Since June 1999, 107 million gallons of untreated wastewater was discharged during six overflows because the operator had temporarily turned off Deep Tunnel pumps while switching to a lower-cost source of electricity.
Almost $1bn, in total, will be spent increasing the capacity and reducing floods in the district. This includes a $117M tunnel contract awarded to a JV between JF Shea Construction and Kenny Construction Company.
The sewer will stretch from Wauwatosa, to north 91st Street and Denver Avenue in Milwaukee (see diagram), and be bored using a 6.7m diameter Robbins hard rock TBM. The access shaft and starting chamber will be excavated in bedrock using drill and blast, as will the two other access shafts. The sewer will be lined with 300mm thick concrete.
Construction on the tunnel should be completed by early 2006.
Related Files
Diagram showing the route of the 12km long, 6m diameter sewer tunnel under construction in Milwaukee, US
