WASHINGTON DC METRO

This article discusses the design and construction of the 164.8km (103mile) metro at Washington, DC, USA, and especially of its final 21.6km (13.5mile) section. The metro was constructed by the Washington Metropolitan Area Transit Authority (WMATA), which owns and operates all mass transit facilities in Washington. Construction began in 1970, and has continued in stages since then. The final section has an estimated budget of $2.1 billion, of which $1.1 billion is construction costs. About 70.4km (44miles) of the current 143.2km (89.5miles) operating network has been built by various tunnelling techniques, including cut and cover, drill and blast, open-digger shields, earth pressure balance tunnel boring machines (EPB TBMs), immersed tubes, and the New Austrian Tunnelling Method (NATM). The article reports visits to two of the construction sites. At the first site, two open-digger shields are being used in drives of nearly 1km from an existing station. At the second site, the NATM was being used to drill through clays and sands, and provide a surer protection against excessive surface settlement.