By Bridget McCrea
Introduced in 2005, the Clean Rivers Project in Washington, D.C., is a massive infrastructure endeavor focused on reducing combined sewer overflows into the district’s waterways. The project, managed by the District of Columbia Water & Sewer Company, also known as DC Water, is designed to capture and clean water during heavy rainfalls. Once completed, it will reduce CSOs annually by 96% throughout the system.
The first major component of the work was the construction of the 24,200-foot-long Blue Plains Tunnel. As the largest of four major tunnels to minimize CSOs into the Potomac and Anacostia Rivers, the work – led by a joint venture consisting of Traylor Brothers, Skanska and Jay Dee – included tunneling four, 150-foot-deep shafts up to 25 feet in diameter. The entire tunnel was lined with precast concrete.
Precast: The natural choice
The Blue Plains Tunnel work was awarded to Traylor-Technopref. According to Bryce Scofield, P.E., project manager for Traylor Precast, the project’s engineers specified precast as the material of choice for the tunnel lining.
“It was soft ground, so there was no way to do an open-face cut, CM tunneling or any other type of tunneling,” he said. “It had to be a tunnel boring machine, which naturally lends itself to using concrete-reinforced liners.”
The nearly 25,000-foot-long, single-bore tunnel was made using an earth pressure balance tunnel boring machine that requires precast concrete tunnel liners. Each liner has a 23-foot interior diameter and is 14 inches thick and 6 feet long.
A successful outcome
For the Blue Plains Tunnel project, the precaster produced 4,030 rings comprising 28,200 total segments of steel-fiber reinforced precast concrete. Scofield said the use of steel fibers affected the design approach.
“We got involved somewhat with the design and specifying just how much fiber would be required to achieve the required flexural strength for the tunnel lining,” he said. “We had to come up with a system that would deliver the loose fibers to the mix in a very well-distributed manner.”
Technopref’s Louis Charette said the project kicked off in 2012 and that the precaster completed casting in April 2014. Lady Bird, the TBM, mined and constructed the 4.5-mile-long tunnel in 23 months. Half of the tunneling was predominantly in Potomac clay, which can be stiff and sticky, but was well suited for earth pressure balance technology.
Now completed, the Blue Plains Tunnel collects sewage and mitigates water runoff during storm events.
“This is a terrific milestone for DC Water’s Clean Rivers Project,” said DC Water CEO and General Manager George S. Hawkins (1). “We are fortunate that the tunneling went so smoothly, finishing on time and on budget, and I applaud our DC Water staff as well as Traylor/Skanska/Jay Dee and everyone else who took part in this successful dig.”
Bridget McCrea is a freelance writer who covers manufacturing, industry and technology. She is a winner of the Florida Magazine Association’s Gold Award for best trade-technical feature statewide.