The Washington Suburban Sanitary Commission’s (WSSC) Bi-County Water Tunnel is an 84-inch (213 cm) diameter water main that meets the growing needs of customers in Montgomery and Prince George’s counties, Maryland. The main provides increased water volume and system redundancy to both counties. McMillen Jacobs Associates was first a subcontractor to Louis Berger & Associates, performing a feasibility investigation of design alternatives and to address horizontal and vertical alignments for pressure tunnels. We then led the construction management effort for the tunnel in a joint venture with EA Engineering, Science, and Technology.
Construction involved excavation of three 150-foot-deep (46.7 m) shafts through overburden and rock, and one production shaft and two shafts for connection of the pipeline to existing water mains. The 5.3 mile-long tunnel was mined using a TBM with a 10-foot (3 m) excavated diameter, at depths between 90 and 280 feet (27.4 m and 54.8 m) below the ground surface. The final lining consists of welded steel pipe, and surface valve chambers were constructed to connect to existing surface main pipelines. The relatively small tunnel diameter versus the tunnel length presented significant logistical challenges for conventional tunneling, most notably muck handling and ventilation.
Excavation by TBM occurred in two separate drives: the first extending 4,124 feet (1,257 m), and the second extending 4.5 miles (7.2 km) without intermediate access shafts. The project was completed in 2013.