The need for an upgrade became apparent in 2012, when four major storms hit the city. The sewage system was overwhelmed, causing extreme flooding, private property damage, and the health risks associated with exposure to sewage.
This led to a multi-agency response and community calls for answers and action. The Mayor’s Task Force accelerated the First Street Tunnel project and made it a high priority. Work began in October 2013, and the project was completed this month.
“The First Street Tunnel was designed to relieve flooding and sewer overflows, which previously caused the streets to pond knee-deep with sewage during bad storms,” said Ravi Jain, WSP USA deputy project manager and one of the lead designers. “At times it was so bad, manhole covers would just pop up.”
WSP was the designer for the $170 million design-build project, working on behalf of the contractor, Skanska and Jay Dee Contractors (SKJD) and the owner, the District of Columbia Water and Sewer Authority (DC Water).
Constructed 180 feet below street level as part of the DC Water Clean Rivers Project, the 2,700-foot, four-shaft tunnel extends along First Street NW and across seven city blocks in the Bloomingdale and LeDroit Park areas, from the McMillan Sand Filter site at Channing Street NW to Rhode Island Avenue NW.
During a severe storm, the 20-foot-diameter First Street Tunnel can store up to 8 million gallons of combined sewer overflow that previously had nowhere to go but up to the surface. The tunnel will utilize the Rhode Island Avenue pump station, which will pump the overflow back into the sewer system once the storm has subsided and flows return to normal.
To dig the tunnel, a 23-foot-diameter tunnel boring machine (TBM), nicknamed “Lucy Diggs Slowe,” was launched from the Channing Street mining shaft located at the northern end of the project. The TBM then headed south, curving from the shaft towards First Street before hitting a straight drive to the intersection of Rhode Island Avenue NW and First Street NW. TBM mining was completed in December 2015.
A temporary bulkhead was established near the southern end of the tunnel, near the pump station shaft. The bulkhead is unreinforced concrete with circumferential steel sections cast-in.
The bulkhead is a temporary structure that will remain until the completion of the next DC Water Clean Rivers Project – the construction of the Northeast Boundary Tunnel. The Northeast Boundary Tunnel, currently in the bid stage, is targeting a 2022 completion, at which time the pump station will be decommissioned and storm water from the First Street Tunnel will flow by gravity to the DC Water’s Blue Plains Advanced wastewater treatment plant.
The project, constructed in an urban residential neighborhood, included innovative construction methods such as multiple-location ground freezing, utilizing a centralized freeze plant to minimize community impact.
WSP proposed the idea of ground freezing as a way to provide temporary support necessary during excavation, with minimal disruptions above the surface.
Ground freezing is a low-risk solution that minimizes noise, dust, and vibration during construction and results in ground improvement that is needed to excavate tunnels and shafts in difficult ground and groundwater conditions. It not only provides support, but also eliminates the need for dewatering.
“Chilled brine from the central ‘freeze plant’ was distributed to various project sites through a network of pipes,” Jain said. “Within each site, dozens of freeze pipes were drilled from the surface to create a subterranean network of pipes in the areas to be frozen.”
The method circulates salt water brine through pipes, capturing the heat within the ground, similar to the operation of a domestic freezer. The brine freezes and solidifies the ground, making it secure for excavating.
Jain said he was grateful to have the opportunity to follow the First Street Tunnel project from start to finish.
“This is the first project of my career that I worked on from proposal phase to project close-out,” Jain said. “It took over three years from start to finish, but gave me a great sense of accomplishment, especially working under the guidance of project manager David Smith, who managed the job really well, both technically and from a leadership perspective.”