Work has been progressing in a quiet and determined way over the last five years to advance the miles of tunnels needed to intercept combined sewer overflows and protect the rivers of the US capital city, Washington DC from stormwater pollution. Following completion of drives by two TBMs on the Clean Rivers project a third successfully completed its 3.8km (12,500ft) long of 7m (23ft) i.d. segmentally lined Anacostia River tunnel.
TBM Nannie marked the end of its journey this month when the cutterhead was lifted from the 102ft deep shaft to the surface. The TBM began its total 12,255ft (3.7km) journey in November 2015 to connect with the tunnel alignment mined by sister TBM Lady Bird that ends at the DC Water Blue Plains Advanced Wastewater Treatment Plant. When operational in early 2018, the two tunnel segments will relieve combined sewer overflows into the project reaches of the Anacostia and Potomac Rivers.
The Herrenknecht EPBM progressed south from a launch and working shaft near the RFK Stadium breaking through into the reception shaft at Poplar Point on 8 November, almost exactly one year and a day after starting the drive on 4 November 2015. The TBM progressed at up the 30m (100ft) deep through soft waterbearing river silts and clays and passed under the Anacostia River, beneath main railway tracks, and under the Washington DC Metro Green Line to complete the drive.
The tunnel is being constructed by the IHP JV of the Lane Construction Corp, a subsidiary of the Italian Salini Impregilo Group, with its subsidiary SA Healy Co, and JV partner Parsons. Following breakthrough of the TBM, the JV will construct six drop shafts of varying sizes, four diversion structures, and three connecting adits of varying length, to complete the contract by its due date of 13 December 2017.
The tunnel is the latest major work to be completed in the United States by the Salini Impregilo Group, following excavation of metro running tunnel for the Central Line in San Francisco and the Lake Mead water intake tunnel in Nevada.
The Anacostia River tunnel connects directly to the northern end of the 7,193m x 7.1m i.d. Blue Plains tunnel that was excavated by the Traylor/Skanska/Jay Dee JV using another 8m diameter EPBM. The machine launched in July 2013 and broke through in June 2015.
Both tunnels intercept several CSO outfalls along the Anacostia River and will retain and deliver flow to the Blue Plains treatment plant. The Anacostia tunnel was the last major tunnel to be excavated in the program by DC Water, the District of Columbia Water and Sewer Authority, to meet the March 2018 deadline for placing in operation the tunnel system serving the Anacostia River from RFK Stadium south to the Blue Plains treatment plant. More than 38 million gallons of combined sewage can be stored in the tunnel during rain events and before release to the treatment plant.
In addition to the Blue Plains and Anacotia River tunnels, the Clean Rivers Project also includes the 823m First Street Tunnel, which was excavated as a fast-tracked project by the Skanska/Jay Dee JV. Work start on the contract in October 2013 and the refurbished Herrenknecht EPBM completed the 20ft (6.1m) diameter drive in December 2015.
The last major contract on the current program is the 8km long North East Boundary tunnel, which will connet to the First Street tunnel in the north and the Anacostia River tunnel in the south to create a 21km long CSO trunk sewer all the way to the Blue Plains treatment plant in the south (Fig 1). DC Water has issued requests for qualification to bid the project and award of contract and start of construction is planned for 2017.
Once the full project is completed, in 2025, more than 21km of tunnels will have been excavated. The full system will reduce combined sewer overflows by 96% overall and by 98% into the Anacostia River. Plans for a similar interceptor sewer to control CSOs along the Potomac River are being advanced and with green alternatives, that may reduce the length of the tunnel required to achieve the goals, also being studied. Green stormwater soak-away methods are being trialed on the project as an alternative to advancing the original tunneled solution to reducing CSOs into Rock Creek to the north of the project area.
“Our Clean Rivers Project team continues to push forward at an incredible pace, and the completion of this tunnel is another key milestone to celebrate,” said DC Water CEO and General Manager George S Hawkins. “We applaud everyone who has played a part in keeping this work on schedule and ensure a great return on the investment by our ratepayers.”