When her husband became president in 1964 Lady Bird Johnson is said to have made it her mission to preserve and protect the environment. In 1964 she formed ‘The Committee for a More Beautiful Capital’.
She encouraged her husband to declare the Potomac River "a national disgrace", which further drew attention to the declining health of America’s waterways and was a catalyst for the Clean Water Act of 1972.
Nearly 50 years later, the capital, through DC Water, is undertaking a USD 2.6bn mega project to reduce CSOs in the Anacostia River by 98 percent and close to the same in the Potomac River.
There are 53 CSO outfalls listed in DC Water’s existing discharge permit from the Environmental Protection Agency. The water and sewer authority estimates that combined sewers overflow into the Anacostia and Potomac Rivers at least 75 times a year, spilling nearly 1.5bn gallons into the Anacostia and 850 million gallons into the Potomac.
A new 13-mile (20.9m) network of 23ft (7m) diameter tunnels will carry combined storm runoff and sanitary sewage to the Blue Plains Advanced Wastewater Treatment Plant. The first tunnel will be excavated by TBM — one that has fittingly been named, Lady Bird.
This tunnel, the Blue Plains Tunnel (BPT), will be approximately 24,300ft (7,400m) long and run more than 100ft (30m) beneath the eastern shore of the Potomac and Anacostia Rivers, as well as a military facility and numerous utilities and roadways.
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By GlobalDataThe BPT alignment has five large diameter shafts, with the largest some 132ft (40m) in diameter. The tunnel drive begins at the Blue Plains Advanced Wastewater Treatment Plant and heads north to the retrieval shaft located at DC Water’s main pumping station near the new Nationals Baseball Stadium.
The design-build team comprises Traylor, Skanska and Jay Dee (as the builder) and Halcrow (as the lead designer). DC Water awarded the USD 330M contract for the BPT in May 2011. Jacobs Associates has provided program management services for the tunnelling component of the DC Clean Rivers Project. To read more about the history and procurement of DC’s Clean Rivers Project, see Tunnels NA, February 2012, page 26.
Setting up Site
The area where the TBM will launch is a 4.5-acre site at DC Water’s Blue Plains Advance Waste Water Treatment Facility. Located nearby is the Naval Research Laboratory, and that’s just one of the government properties the alignment will pass beneath, including Joint Base Anacostia Bolling Military Installation, National Parks Service land, DC Department of Transportation (DDOT) and DC Water property with main relief sewers more than 100 years old.
Going lower, the ground conditions are fairly mixed with clay, silt and sand. Tunnelling will be done in one heading with two intermediate shafts and a retrieval shaft at the termination point. The TBM is an earth pressure balance machine from Herrenknecht, with expected earth and water pressure up to 3.6 bar on the project.
Excavation of two shafts to used for the launch started in March 2012 and completed by the following August. One, a deep screening shaft, has an inside diameter of 76ft (23m), and is 140ft deep (43m). The other, a dewatering shaft, has a 132ft (40m) inside diameter, and is 170ft deep (52m). The two shafts, connected at a tangent point, create a figure-eight-like shape, and have 45 slurry panel walls, 5ft (1.5m)-thick, and nearly 200ft (61m) deep.
The walls are filled with reinforced concrete with a design strength of 7,000 psi (48.3MPa).
Ground at the launch site consists of approximately 30ft (9m) of fill, underlain by 20 to 30ft of alluvial sand and gravel, followed by stiff to hard clays and very dense clayey sands. For each panel, Bencor Corporation — as a subcontractor to the JV — used a clamshell bucket to excavate to a depth of 15 to 20ft (4.5m to 6m) before switching to a hydromill. Primary panels generally consist of three clamshell ‘bites’. Some pour volumes have approached 1,100cu.yd (841cu.m), taken nearly 13 hours and required up to five simultaneous tremie pipes, according to an update from Jacobs Associates.
Furthermore it says, "Panel installation tolerances are very tight, with no more than 6in (150mm) of play between any two panels. In addition to guidance instruments installed on the hydromill a Koden sonic device was used for additional quality control."
The contract also calls for construction of a 55ft (16.8m)- diameter drop shaft at the main pumping station for the TBM reception, as well as a 50ft diameter drop/overflow shaft and a 55ft diameter junction/drop shaft along the alignment (see map). Slurry walls on the other shafts were completed in early April of this year.
EPB in DC
The 23ft (7m) diameter EPBM arrived on site in late October 2012 from Germany. When Tunnels spoke with Brett Zernich, construction manager with Traylor Brothers, this spring the TBM was on schedule to launch in May.
"The launch will be done using two adjacent shafts, with four of the gantries pre-assembled at the shaft bottom and a fifth and sixth gantry connected with umbilical hose from underground," he explains. "The figure-eight, large diameter shafts support an advanced TBM assembly."
The tunnel will be supported using a single-pass precast segmental liner with a finished inside diameter of 23ft (7m). Segments are being produced in Brandywine, Maryland. Each ring is 14in thick, 6ft-long and 23ft inside diameter and reinforced with steel fiber with guide rods, bolts on the radial joint and dowels on the circumferential.
"We are handling the segments with a special designed dual segment clamps and spreader bar using a 110t gantry crane and lowering them in pairs of half stacks," Zernich explains.
Muck will be removed from the tunnel via a 40in belt conveyor to the shaft and into 35 cubic yard boxes with a specially designed shaft mucking carousel. Once mining starts, there will be three shifts, five or six days per week.
Traylor Brothers, as well as its JV partners, has a lot of experience tunnelling in urban areas including New York. From this experience, particularly in dealing with community concerns such as noise or other disruptions to daily life, Zernich says, "Be a good neighbour and take care of problems as quickly as possible."
The BPT is expected finish in 2015, and there are three more tunnels to construct as part of the Clean Rivers Project. The next tunnel is the nearly 4km-long Anacostia River Tunnel, for which an RFQ was issued in November 2011
