BOSTON HIGHWAY TUNNELS

This article describes the central artery/third harbour tunnel project, which will double boston's highway capacity at its critical central bottleneck. An underground, widened central artery will be constructed, together with a new four-lane tunnel under Boston harbour from South Boston to logan airport to East Boston. The project consists of 12 km of mainline highway, of which 7.3 km will be underground; it will require the excavation of about 10.3 million cubic metres of material. The job will be difficult, because of boston's complex geology, noted for frequently occurring adverse ground conditions, especially soft, compressible layers and high groundwater levels. To keep the existing six-lane steel viaduct central artery in service while building the new highway underground, the new road will be constructed by cut and cover under the viaduct, using slurry walls to form each side of the section. During the tunnelling under an existing subway station, the station will be supported on a jacked pipe roof, fixed by transfer girders to piers founded in bedrock. The immersed tube method will be used to construct the 1.2 km tunnel under Boston harbour in stages designed to minimise impacts to shipping; it will also be used to construct the difficult 270 m tunnel under the fort point channel which will pass over underground railway tunnels. Major construction will begin in 1991 and proceed into the mid-1990s; one tunnel will open to traffic in 1994 and the whole highway is due to open in 1998.