The District of Columbia Water and Sewer Authority’s (D.C. Water) $2.6 billion Long Term Control Plan includes constructing a system of storage tunnels, conveyance pipelines, and structures to divert combined sewerage and storm water flows from entering the Anacostia River, Rock Creek, and Potomac River. The program is driven by a consent decree agreed by the Environmental Protection Agency, the District of Columbia, and the D.C. Water and Sewer Authority. The consent decree requires the entire system to be operational by March 23, 2025.
As part of the Program Consultants Organization, McMillen Jacobs Associates is the general consultant overseeing tunnel and geotechnical engineering. In addition to providing complete preliminary design for all projects and final design for selected projects, cost estimating, procurement strategies, alignment evaluations, permitting support, protection of structures evaluations and constructability reviews, McMillen Jacobs Associates is implementing a geotechnical exploration and testing program and providing other program management–related services.
Ground conditions include varied soil conditions, contaminated soil and groundwater, unexploded ordnance, and flowing artesian pressures. Tunnel alignments are generally in highly populated urban environments and pass beneath rivers, public and private facilities, roadways, bridges, walls, heavy rail, subway tunnels, national parks and numerous critical utilities.
The program includes just over 13 miles of 15- and 23-foot-diameter tunnels and 15 deep shafts ranging between 25 and 132 feet in diameter as well as over 5 miles of 3- to 10-foot-diameter conveyance pipelines constructed by cut-and-cover and trenchless methods. Several of the construction contracts, including the larger diameter deep storage tunnels, are being procured using design-build approach while the remaining contracts are being procured using the traditional design-bid-build approach.
Two of five major tunnels are underway. The Blue Plains Tunnel is approximately 24,300 feet long and 100–130 feet deep, and will be constructed using an earth pressure balance machine. The Anacostia River Tunnel procurement is approximately 12,500 feet long and 100–120 feet deep and will be constructed using a tunnel boring machine. Both will have a single-pass precast segmental liner with a finished inside diameter of 23 feet. Construction of various conveyance pipelines, diversion structures and overflow facilities is also well underway. Engineering for the remaining program components, including two additional deep storage tunnels and the nine remaining diversion facilities, is currently being completed.