Of the thousands of visitors who will immerse themselves in the rich heritage of Alexandria, Va., this summer, only one will have the job of helping the city just south of Washington, D.C., make some history of its own.

The new arrival is a 380-ton tunnel boring machine (TBM) that will play a key role in Alexandria’s largest infrastructure project to date—the $665-million RiverRenew initiative to remediate longstanding combined sewer overflow (CSO) issues and improve the quality of city waterways.

The U.S. Environmental Protection Agency says RiverRenew will cut CSO frequency from 70 events a year to fewer than four, also reducing the average volume of waterway overflows from 140 million gallons to less than 16 million gallons.

The 14-ft, 8-in. tall TBM, which will stretch 465 ft when fully assembled, will dig a two-mile-long, 12-ft-wide tunnel designed to collect about 120 million gallons of sewage annually from four combined sewer outfalls during heavy rain events. Sewage stored in the tunnel will gradually be released to the city’s Water Resource Recovery Facility for treatment and subsequent discharge to the Potomac River. The machine was designed and fabricated by Herrenknecht in Schwanau, Germany.

With a cutterhead speed of 4.5 rotations per minute, the TBM will creep at a pace of 20 ft per hour when its approximately year-long journey 100 ft below the surface begins later this summer. The process is designed to be completed with no disruption to existing sewage collection and treatment operations that serve more than 300,000 people in Alexandria and adjacent parts of Fairfax County.

Design-Build Joint Venture

The design-build joint venture of Traylor Bros Inc. and J.F. Shea Co. developed the tunnel project with support from Jacobs and Corman Kokosing Construction Co., with Brown and Caldwell serving as owner’s representative for Alexandria Renew Enterprises, the city’s wastewater treatment utility. 

Other elements of the $454.4-million tunnel project include 75-ft and 40-ft deep TBM launch and recovery shafts that subsequently will be converted into high-capacity pump stations to empty both the tunnel and a new mile-long, six-ft-wide interceptor line.

Alexandria implemented RiverRenew under a 2017 state mandate to fix overflows from the city’s 240-mile combined sewer network, parts of which have been in place since the 1800s. Due for completion by July 2025, the program is funded by state and federal sources, including a $321-million EPA loan. Pump station capacity upgrades and treatment facility improvements were completed last year in preparation for the tunnel work.


Named For Environmental Justice Pioneer

The mole is named Hazel in recognition of environmental justice pioneer Hazel Johnson, who died in 2011. Alexandrians chose her for the naming in an online poll that included five others who made outstanding contributions to the city, the environment, engineering or construction fields.

Johnson worked to improve living conditions in Chicago's public housing system, including organizing a grassroots effort to address landfill-related environmental and public health impacts. She also played a key role in efforts that led to President Bill Clinton's 1994 executive order addressing environmental justice disparities affecting minority and low-income populations.