Paula Wallis, Reporter

Route of the new water supply facility

A bid of $112.5 million is the apparent low bid for the Washington Suburban Sanitary Commission's (WSSC) Bi-County water tunnel. The bid, submitted by Renda/Southland/SAK JV, beat the Engineer's Estimate by $17.3 million and beat out two competitors at the bid opening earlier this week (Tuesday, March 24).

The deep 5.3 m (8.5km) water tunnel in the Washington, DC area will help meet the growing demand in Montgomery and Prince George's Counties and provide a dependable, high-pressure clean water source during high-use periods, droughts and emergencies.

"This is only the second tunnel that the WSSC has built," said John Mitchell, Design Project Manager for WSSC. "It is also the single largest contract in terms of dollars in the company's history."

The general tunnel alignment will follow the I-270 and I-495 Capital Beltway around Washington, DC through a densely populated area of mostly single family homes and condominiums notorious for rush hour gridlock.

Black & Veatch designed the deep rock tunnel and Jacobs Associates is lead partner in a joint venture with EA Engineering, Science, and Technology to manage construction for the WSSC.

The 5.3m (8.3km) tunnel will lie in competent bedrock 150 to 300ft (46- 91m) underground with a10 to 12ft (3-3.6m) excavated diameter and a 7ft (2m) finished diameter, lined with steel pipe.

"It's a fairly straightforward job," said Phil Chandler, project manager for Jacobs Associates. "But it's a long, narrow excavation that will pose a challenge for muck handling and ventilation. Conveyors are favored over rail for muck handling, although another option is to excavate a larger diameter tunnel in order to accommodate trains."

Tunnels run up to 91m below ground surface

Excavation by TBM will occur in two separate drives. The design calls for three 150ft (48m) deep shafts excavated through overburden and rock. The main working shaft located on a former I-495 construction site, off Connecticut Avenue is easily accessed from an I-495 off ramp. The first drive will extend 4,124ft (1,257m) east to the Stoneybrook Shaft near the intersections of Beach and Stoneybrook Drives in Kensington.

The second drive will extend 4.5m (7.2km), without any intermediate access shaft, from the Connecticut Shaft west to the Tuckman Shaft just north and east of where Tuckerman Lane passes under I-270 in Rockville.

A tight construction schedule allows one year for shaft excavation and TBM procurement, one year for tunnel excavation, with a final 15 months of tunnel lining.