Designed to substantially reduce combined sewer overflows to the waterways of the nation’s capital, the $240-million, 12,300-ft-long hydraulic tunnel traverses a route that includes varying subsurface conditions, fragile utility networks and above-ground environments ranging from rivers and parks to densely populated urban neighborhoods. Expertise for planning and carrying out the project came from around the world as well as nearby communities. Nearly one-third of the work was subcontracted to minority-owned business enterprises.

As a specially configured tunnel boring machine carved a path 100 ft below the surface, an extensive monitoring program ensured that artesian aquifers were protected from adjacent contamination. The complex shapes of the tunnel’s prefabricated concrete segments were designed and placed to accommodate varying diameters, while specially configured drop shafts from the surface control the flow of water into the tunnel during storms. The tunnel, completed on time and $13 million under budget, proved its value shortly after its spring 2018 activation, capturing a total of 830 million gallons of combined sewage from two major rainstorms.

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