CSO - NORTH AMERICA | PROJECTS
Citizens and legislators are unhappy with this
spreading pollution. In the US, for instance, CSOs are subject to the National Pollutant Discharge Elimination System (NPDES) permitting program, overseen by the Environmental Protection Agency (EPA). As violations become more frequent and severe, many cities are enlarging and extending their combined sewer systems.
BIGGER SEWERS FOR MODERN CITIES Combined sewers are a logical way to handle domestic sewage, industrial wastewater and rainwater runoff, given that separate drainage systems are not usually practical. During normal rainfall or snowmelt, sewers store the extra water volume until it can be processed by wastewater treatment plants sized to handle average flows. Yet extreme weather conditions are increasingly
common. Many cities have outgrown sewers that were installed in the 1930s or even before, so CSOs may result from routine blockages and breakdowns as
well as heavy rain. As a result, sewers are overloaded and discharges of untreated sewage have become distressingly frequent. Mitigating the CSO problem requires building new
sewers and enlarging existing ones. Diameters vary according to the required capacity, while tunnel lengths per project range up to 20km and more.
CHALLENGES FOR TUNNELERS The great length of many combined sewer tunnels increases the chances of encountering a range of ground conditions, so multi-mode and variable-density tunnel boring machines (TBMs) may be necessary. As much of the work takes place in urban areas, shaft
access is often restricted, and spoil disposal may be complicated. The need to match existing infrastructure also means that a single upgrade project may involve tunnels of several different diameters. Successful sewer upgrade projects therefore require tunneling experts with the resources and experience
PROJECT
Herrenknecht TBMs for the Clean Rivers Project in Washington, D.C. TBM TYPE
Potomac River CSO Tunnel Potomac River CSO Tunnel
Washington North East Boundary Tunnel Washington First Street Tunnel Washington Anacostia River Tunnel Washington Blue Plains Tunnel
Variable-Density-TBM Single Shield/Mixshield EPB-Shield EPB-Shield EPB-Shield EPB-Shield
TBM DIAMETER 6,360 mm 6,360 mm 8,010 mm 6,950 mm 7,910 mm 7,995 mm
Note: The top two machines to have their factory acceptance in 2025. The lower four tunnels were completed between 2015 and 2021
FAT 2025
End 2025 2018 2014 2014 2012
2021
RTB & 2015 2016 2015
Breakthrough
Summer 2025 | 19
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