TOUGH CONDITIONS, INNOVATIVE SOLUTIONS

Two major innovations in U.S. construction practice are part of the solution to a complex and difficult portion of the Massachusetts Highway Department's $8.5 billion Central Artery/Tunnel project. Deep soil mixing (DSM) and tunnel jacking, both extensions of technologies used overseas, are helping engineers and contractors to work in extremely weak soil conditions in the area. DSM deals with the most perplexing challenge, open excavations 40-60 ft (12-18 m) deep and 200 ft (61 m) wide in the thick deposit of soft clay beneath and adjacent to Fort Point Channel. More than 900,000 cu yd (688,140 cu m) of soft clay and organic soils are being cement-stabilized. These stabilized soils will hold back lateral earth loads and Fort Point Channel tidal waters, resist basal heave in large open excavations, and provide permanent foundation support for cut-and-cover tunnels. Jet grouting creates soil-cement in areas difficult to reach with the large mixing equipment. The second major innovation is the jacking of full-section, multi-lane tunnels beneath the heavily trafficked railroad yard at the busy South Station Transportation Center. Three separate tunnels will advance 35-64 ft (11-20 m) under active tracks. Each of the 180-350 ft (46-107 m) long tunnels is cast in a deep, internally braced excavation adjacent to the tracks. Then, while trains remain in full operation, shield tunneling methods advance one tunnel at a time by incremental jacking and excavation. Special measures, including grouting and ground freezing, are stabilizing the ground before the tunnels advance.