The Central Artery/Tunnel Project, now under construction in Boston, is the largest infrastructure project in the United States. A major element of the project is the new South Bay Interchange for Interstates I-90 and I-93, and includes the extension of the existing I-90 under the tracks leading into the South Station Railway Terminal. Three tunnel sections will be constructed under the South Station approach track system. Several construction methods were considered for the tunnels, including cut and cover techniques involving a sequence of track closures and shifts; mined tunnels, multiple drifts excavated by microtunneling, and jacking methods. The selected approach consists of constructing each tunnel section as a series of full cross-section reinforced concrete boxes and jacking them into place. Ground freezing will be used to stabilize the soft soils in the tunnel face prior to jacking, and during the jacking operation, the frozen ground will be excavated by roadheaders, working in a compartmentalized shield in front of the lead tunnel box section. The features of the various construction methods considered are discussed, as well as the decision process through which the construction approach finally adopted was selected.
