Bi-County Water Tunnel

The Washington Suburban Sanitary Commission’s (WSSC) Bi-County Water Tunnel is a new 84-inch (213 cm) diameter water main being constructed to meet the growing needs of customers in Montgomery and Prince George’s counties, Maryland. The new main will ease demands on the existing 54-inch (137 cm) supply main and provide water system redundancy. 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. Now we are leading the construction management effort for the tunnel in a joint venture with EA Engineering, Science, and Technology.

Construction involves excavation of three 150-foot-deep (46.7 m) shafts through overburden and rock; excavation of one starter tunnel and two retrieval tunnels for mobilization and retrieval of the tunnel boring machine (TBM); mining using a TBM of 5.3 miles (8.5 km) of hard rock tunnel 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 will be constructed to connect to existing surface main pipelines. The relatively small tunnel diameter versus the tunnel length presents significant logistical challenges for conventional tunneling, most notably muck handling and ventilation.

Excavation by TBM will occur in two separate drives: the first extending 4,124 feet (1,257 m); the second extending 4.5 miles (7.2 km) without intermediate access shafts. The project must adhere to a tight schedule and is slated for completion in 2012.