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Issue link: http://wateronline.epubxp.com/i/672151
By Carlton M. Ray O ver the course of the past 10 years, the District of Columbia Water and Sewer Authority (DC Water) has been planning, designing, and constructing the DC Clean Rivers (DCCR) Project to reduce combined sewer overflows (CSOs) to the District's receiving waters and to mitigate chronic flooding in its historic neighborhoods. The DCCR Project consists of a series of large underground storage and conveyance tunnels, drop shafts, diversion sewers and chambers, overflow structures, and a tunnel dewatering pumping station located at DC Water's Blue Plains Advanced Wastewater Treatment Plant. The backbone of the project is a 23-foot-diameter tunnel system that spans the length of Washington, D.C., from the southwest to northeast quadrants of the city. The tunnels, constructed in soft ground, will total over 13 miles in length and be located approximately 100 feet underground. The project is anticipated to be complete by 2022, three years before its associated consent decree deadline. DC Water's Tunnel Boring Machines Since the entire tunnel system is being constructed in soft ground, DC Water is employing state-of-the-art earth pressure balance tunnel boring machines, or TBMs, to build the reinforced concrete tunnel. The tunnels are being constructed in geologic stratigraphy consisting of 65-million-year-old clays and sands, referred to locally as the "Potomac Group Soils." To date, DC Water contractors have procured three TBMs, which have been diligently digging under the nation's capital since 2013 and are named after prominent District women (see sidebars). These machines are, in essence, underground factories that are approximately 26 feet in diameter and longer than a football field. The 80-ton cutting wheel is driven by a dozen motors. Behind the cutting wheel, the 1.3 million pound TBM shield is backed up by over 300 feet of support equipment — "the trailing gear" that provides the electrical power, hydraulics, ventilation, pumping, grout, instrumentation, and ground conditioning necessary to keep the TBM mining. The 24,243-foot-long Blue Plains Tunnel was excavated by a TBM named Lady Bird. The 12,483-foot- long Anacostia River Tunnel is currently being excavated by a TBM named Nannie. The Blue Plains and Anacostia Tunnels are primarily storage tunnels that will provide approximately 115 million gallons of the system's 157 million gallons of total CSO storage volume. The 2,722-foot-long First Street Tunnel was recently completed by Lucy just prior to Christmas, 2015. The First Street Tunnel and the future Northeast Boundary Tunnel will increase the capacity of the existing sewer system in the District to current design standards, significantly 20 wateronline.com n Water Innovations Digging The District's 21st Century Sewer System Spearheaded by three strong "ladies" and plenty of vision, DC Water's Clean Rivers Project creates massive tunnels to rid the city of combined sewer overflows. Lady Bird was named after "Lady Bird" Johnson (wife of President Lyndon B. Johnson). When her husband became president in 1964, she made it her mission to preserve and protect the environment. She encouraged her husband to declare the Potomac River "a national disgrace," which drew attention to the declining health of America's waterways and was a catalyst for the eventual Clean Water Act of 1972. Nannie is named after Nannie Helen Burroughs, who was an African- American educator and civil rights activist in the District. Lucy was named after Howard University's first dean of women. Figure 1. DC Clean Rivers Project tunnel system