The Washington Suburban Sanitary Commission (WSSC) is understood to be on the verge of advertising a call for bids for the 5.3-mile (8.5km) long Bi-County Water Tunnel between Montgomery and Prince George’s Counties in Maryland.
Plans originally called for the tender to be advertised in the first quarter of 2008, but WSSC project manager John Mitchell told T&TNA that, although a number of contractors had already expressed an interest, permit acquisitions had pushed the timetable back and it was hoped the call could be advertised in June.
The 12ft (3.7m) diameter deep rock tunnel will be constructed within solid bedrock between 90-280ft (27.4-85.3m) below the surface. An 84in (213cm) diameter pipe will be installed inside it to carry water.
Designed to connect two existing water mains, the tunnel is part of WSSCs commitment to providing a constant, reliable water supply to all of its customers and meeting all projected future water demands.
After studying the options for the new pipeline, it was decided that a deep tunnel was preferred over an open-cut pipeline as it would “minimize community, historical and environmental impacts, be easier to maintain, and would not cost much more than other methods”.
Construction is scheduled to take four years with a main drive shaft at Connecticut Avenue (S3), with Tuckerman Shaft (S1) at the western limit and Stoneybrook Shaft (S4) at the eastern limit of the tunnel reaches. S1 and S4 will primarily be retrieval shafts, but additional small shafts to facilitate grouting and water pipe installation have not been ruled out. S3 will be 35ft (10.7m) diameter and S1 and S4 will be 25ft (7.6m) diameter. All will be excavated by drill and blast. Any grouting shafts will be between 12-48in (30.5-122cm) wide and excavated by caisson drilling.
As the scheme design is finalized, there may also be a requirement for riser shafts excavated upwards from within the tunnel to facilitate connections or ventilation.
Mitchell told T&TNA that changes were made to access at one of the sites and, although it was favorable to the community and would simplify the construction process, it had delayed the permitting process.
He added: “As a result of the delay, we will not be fast tracking the project. Funding is not impacted because this is a ‘growth’ project for WSSC. Growth projects receive funding from our Systems Development Charge (SDC). The SDC is a fee WSSC charges to new development so that growth pays for growth. The charges are placed in a separate fund and can only be used for growth projects.”
Contractors interested in being placed on a list of potential bidders should contact the Commission’s Construction Project Manager, Steve Pinault, by email at spinaul@wsscwater.com
Alignment of the deep rock tunnel and open cut vs tunnel option comparision
