Tunnel boring machine Hazel has completed her 2.2-mile journey 100 feet beneath the Potomac River and Old Town Alexandria, Virginia.
Her 16-month expedition culminated at 10 a.m. March 13 with the creation of a 12-foot-wide tunnel to prevent sewage from entering the Potomac and nearby creeks. The tunnel will be integrated into an upgraded wastewater treat plant that connects the city’s four combined sewer outfalls that date back to the 1860s.
Hazel emerges into a flooded shaft by the Potomac River to complete the 2.2-mile Waterfront Tunnel.AlexRenew
The project’s goal is to upgrade the city’s overcapacity combined rainwater-sewage system. The system’s sewage-treatment capacity is often exceeded during rain, sending untreated sewage mixed with stormwater to four outfalls that discharge into river and local waterways.
Under state law, Alexandria must fix the pollution-discharge problem by July 1, 2025.
AlexRenew, the wastewater treatment authority for Alexandria and parts of Fairfax County, estimates the tunnel will capture over 120 millions gallons of sewage mixed with rainwater. The effluent will be treated at AlexRenew’s wastewater plant and then returned to the Potomac after it is cleaned, the authority says.
The new tunnel runs along Old Town’s riverfront and extends into the city parallel to the I-495 Woodrow Wilson Bridge. It ends at the AlexRenew wastewater plant.
Hazel's cutterhead and front shield is lowered into place after her 16-month journey.AlexRenew
Building a smaller sewer line along Hooffs Run and a 12-story underground pumping station at the treatment plant remain to be completed, with work expected to last through 2025.
The 380-ton Hazel was shipped to Virginia in 2022. She is 14 feet 8 inches high and 465 feet long. She has a top boring speed of about 4 inches per minute.
To show their gratitude for the tunneling feat, city officials designated April 13, 2024, as “Hazel the Tunnel Boring Machine Day.” Hazel was named after environmental activist Hazel M. Johnson.
“While Hazel is just a machine, she has embodied the heart and grit of our team, and like her namesake, has set a foundation for a healthier environment for our community, wildlife and future generations,” says Justin Carl, general manager and CEO of AlexRenew.
Hazel's path shown by blue lineAlexRenew
