By Jay Landers

DC Water, the utility responsible for drinking water and wastewater treatment in Washington, D.C., continues to press forward with its Clean Rivers Project, the nearly $3 billion effort to reduce the volume of combined sewer overflows entering three key waterways. In October, the agency approved an $819 million design-build contract for the last of the large-diameter tunnels that form the centerpiece of the Clean Rivers Project.

The contract approval occurred just weeks after DC Water brought online its latest CSO storage tunnel in mid-September.

Massive effort

To be completed by March 23, 2030, the Clean Rivers Project has as its goal the reduction of CSOs into the Anacostia and Potomac rivers and Rock Creek. The massive project is the result of a 2005 consent decree signed by DC Water, the U.S. Environmental Protection Agency, the U.S. Department of Justice, and the District of Columbia.

Under the terms of the consent decree, DC Water is required to construct a roughly 18 mi long system of deep tunnels as well as sewers and diversion facilities (see map below). These features are designed to capture flows that otherwise would discharge from D.C.’s combined sewer system into the environment. The tunnel system conveys the flows to the Blue Plains Advanced Wastewater Treatment Plant. The consent decree also calls for sewer separation and the installation of green infrastructure to help reduce CSOs to the Potomac River and Rock Creek.

Final tunnel

On Oct. 6, DC Water announced that it had awarded the $819 million design-build contract for the 5.5 mi long, 18 ft internal diameter Potomac River Tunnel to a joint venture comprising Civil and Building in North America — a member of the international construction company Bouygues Construction — and the engineering and construction services company Halmar International.

DC Clean Rivers Project location map (Map courtesy DC Water)
DC Clean Rivers Project location map (Map courtesy DC Water)

In a year of average rainfall, an estimated 654 million gal. of combined wastewater and stormwater enter the Potomac River by means of outfalls from D.C.’s combined sewer system. The new PRT and its associated diversion facilities are expected “to reduce CSOs to the Potomac River by 93% by volume and reduce their frequency from approximately 74 events to 4 events in a year of average rainfall,” according to the DC Water website.

Geotechnical challenges

Of the five CSO storage tunnels that comprise the Clean Rivers Project, the PRT is the “most complex,” says Nicolas Duchemin, CBNA’s vice president for North America development. The complexity results mainly from the challenging geotechnical conditions encountered along the planned tunnel alignment, which begins in Georgetown and extends either along or under the Potomac River until connecting to an existing shaft on the Anacostia River Tunnel. (Designed to reduce CSOs to the Anacostia River, the ART conveys flows south to the Blue Plains Tunnel and on to the treatment facility of the same name.)

Mixed ground conditions along the alignment of the PRT will necessitate the use of two different types of tunnel boring machines, Duchemin says. “The northern tunnel section will be mostly in hard rock, schist predominantly,” he says. “The southern tunnel will start with low rock cover, then transition pretty quickly into a mixed ground condition, with alluvium at the crown and rock at the invert. The final 2.5 miles will be in varying soft ground conditions, mostly in dense silty and clay sands.”

To accommodate these conditions, one TBM will travel in a northern direction for a length of 12,800 ft, while another TBM will travel southward for 16,600 ft. For the northern section, a hybrid TBM capable of operating in open mode and slurry mode will be used to tunnel mostly through rock but also a section of former riverbed known as a paleochannel, Duchemin says. For the southern section, a hybrid version of an earth pressure balance and slurry mode TBM known as a variable density TBM will be used to tunnel initially through rock and then soft ground.

Average depths of the PRT will range from 80 to 100 ft, Duchemin says. The gravity-fed tunnel will extend downward at a slope of 0.1%.

Drop shafts

The project includes five large drop shafts, two of which will have a 50 ft diameter and be used to launch the TBMs. Two shafts, one having a 45 ft diameter and the other having a 42 ft diameter, will be at the site of large CSO outfalls. Finally, a 45 ft diameter shaft will be constructed near Georgetown University to retrieve the northbound TBM.

For the southbound TBM, “there is no retrieval shaft,” Duchemin says. Instead, the TBM will connect to an existing shaft on the ART, an arrangement that poses certain complications, Duchemin notes. “That shaft has not been designed and built considering the arrival of a TBM. It's a fully reinforced structure. There is no soft eye within it where we can connect.”

As a result, the project design calls for constructing by means of jet grouting a concrete plug beside the shaft, at the point where the PRT will connect. After the TBM arrives at the plug, all but the machine’s shield will be removed from the inside, and the sequential excavation method will be used to excavate the concrete plug and arrive at the shaft. “Then we are going to cut precisely into that shaft structure and reconstruct a collar to actually make that connection,” Duchemin says.

Sensitive sites

“Another big engineering challenge” of the project is its location near prominent landmarks, Duchemin says. “We are talking about sites that are in front of the Kennedy Center, in front of the Watergate (Hotel), in Georgetown by the waterfront, and in front of Georgetown University,” he notes. “There's a lot of very sensitive sites.”

The project will take steps to ensure that it does not negatively affect adjacent buildings. “There are a lot of structures that require instrumentation and monitoring,” Duchemin says. “There will be a full instrumentation and monitoring program along the alignment and at each shaft location to make sure that the adjacent structures are not affected by the works. That's quite an important challenge.”

The large volumes of muck generated during the tunneling operations will be hauled away by truck and not barge because the Potomac River is too shallow. “It would require extensive dredging,” Duchemin says. “And there are no real disposal sites” along the waterway, he notes. To limit disruption to adjacent neighborhoods and commercial areas, major trucking operations will not occur overnight or during peak traffic hours.

Construction is expected to begin in May. Previously, DC Water installed temporary and permanent electrical power at several of the project sites at a cost of $13.9 million.

“The Potomac River Tunnel will complete projects on the Potomac River,” says Hadiah Jordan, a senior public outreach coordinator for DC Water. “Other projects are also being implemented concurrently to control CSOs in Rock Creek,” Jordan says. “These are green infrastructure projects and a tunnel to control” CSO discharges to a local stream named Piney Branch, Jordan says.

Operations begin

The Northeast Boundary Tunnel, which began operations on Sept. 15, is part of the larger ART System. Phase 1 of this system was a 2.4 mi long, 23 ft diameter tunnel that began operating in March 2018, providing a storage capacity of roughly 100 million gal.

Delivered by design-build by a joint venture of Webuild S.p.A.  (formerly known as Salini Impregilo) and the Lane Construction Corp., the NEBT took five years to construct at a cost of $583 million. Approximately 5.1 mi long and 23 ft in diameter, the new tunnel reduces the chance of flooding in its service area from approximately 50% to 7% in a given year.

“The addition of the NEBT adds another 90 million gal. of storage, for a total of 190 million gal. for the Anacostia River Tunnel projects,” Jordan says.

Phase 1 of the ART “has provided a 91% reduction in CSO overflow volume (to the Anacostia River),” Jordan says. “The Northeast Boundary Tunnel will increase this to 98% on the Anacostia River in an average year of rain,” Jordan says. “Systemwide, DC Water has reduced CSOs by 77%, and this will increase to 96% by 2030 with completion of remaining controls on the Potomac River and Rock Creek.”

This article is published by Civil Engineering Online.