Bi-County Water Tunnel breakthrough in Maryland

The USD 168M Bi-County Water Tunnel project in Maryland achieved breakthrough at 8am (EST) on 23 November.

The tunnel will run for 5.3 miles (8.5km) from the main access shaft near Connecticut Avenue to the Stoneybrook/Beach Drive shaft, and lies about 200 ft (approx. 60m) below the surface.

The tunnel, designed by Black & Veatch and constructed by a Renda/Southland/SAK JV, will house an 84in (2.1m) diameter water main, connecting the two existing 96in (2.4m) mains.

Excavation was carried out by a 10ft (3.1m) in diameter TBM. The cutter head was arrayed with 21 carbide cutting heads of 17in (43.2cm) each. A substation was built at the Connecticut Avenue site to provide electricity for the TBM. To meet project requirements, the 17-year-old TBM has been boosted from 800 to 1200 horsepower.

In order to create space for the subterranean assembly of the machine, workers used explosives to excavate two 150-ft (50-m) tunnels from the Connecticut Avenue shaft; the last blast occurring on 23 June.

The cutterhead will be transported back to the main access shaft near Connecticut Avenue by truck while the rest of the setup will be reversed through the tunnel. The TBM will then bore 4.5 miles (7.2km) westwards to the Tuckerman Lane shaft. This process could take two months.

Tunnelling began on 23 July and completion is expected in 2013.