DIGGING BOSTON OUT OF A JAM

This article describes the formidable geological and other challenges presented by the construction of the Central Artery in Boston, USA, where a major highway is being built through the city's downtown area; it includes a route map. Much of the natural harbour in the area, which was originally several small inlets, were backfilled to allow Boston to grow. This led to a highly variable mixture of fill, sands, silts, and peat overlaying the Boston Blue Clay; there is also a high water table. One part of the project is the replacement of the deteriorating elevated section of the I-93 highway by an underground expressway, from eight to ten lanes wide, along the same alignment. It is being built top-down between diaphragm walls under the overpass. Two new crossings over the Charles River will be built, the larger of which will be one of the world's widest cable-stayed bridges, carrying ten traffic lanes. One of the most complex parts of the project, and the most challenging geotechnically, is the South Bay interchange, which will be on six levels, two of them underground. This involves large tunnel jacking with ground freezing, to pass under main line railway tracks, and immersed tube tunnelling and cut-and-cover techniques to cross the Fort Point Channel and existing subway lines.