GROUND FREEZING ON THE CENTRAL ARTERY IN BOSTON

As part of the Central Artery / Tunnel Project in Boston, three tunnels had to be installed in a shallow alignment (5-25 ft cover) under the South Station railroad yard. The tunnel alignments pass through a geological profile of historic fill, which contains remnants of many old waterfront structures, building foundations and a forest of 19th century vintage timber piles, underlain by organic sediments and marine clay. The groundwater table is at depths of 5-10 ft. The tunnels with a cross section of 78 x 38 ft and lengths of 167, 258 and 379 ft were installed by means of tunnel jacking. Large-scale ground freezing was used as a safe and reliable means of soil stabilization. The ground was frozen by circulating chilled brine through vertical pipes that were installed in a 7-8 ft grid, covering the full footprint of each tunnel. The freezing system was installed and the ground completely frozen before the start of each tunnel jacking operation. The freezing guaranteed an absolute ground water cut-off, provided face stability that allowed a freestanding vertical excavation face and kept all obstructions solidly embedded in the frozen ground matrix during soil removal. The tunnels were successfully installed in the period October 1999 to February 2001 without affecting any railroad operations.