ABSTRACT
A numerical analysis of the Red Line subway tunnels in Boston, MA was performed to evaluate the structural integrity and safety of the tunnels during construction of a freeway which will pass within 1.5 m (6 ft) of the top of the tunnels. The freeway section above the subway tunnels will consist of six immersed tube tunnels (ITTs). Approximately 75% of the existing overburden above the subway tunnels will be dredged to accommodate the freeway. Dewatering of the adjacent ITΤ casting basin will also impact the existing subway tunnels. The analysis was used to evaluate factor of safety against buoyancy, tunnel displacements, existing state of stress in the concrete liners, change in stresses in the liners due to construction loads, and to develop instrumentation requirements. The analysis included modeling the subway tunnel construction sequence and the ITT construction sequence. Results of the numerical analysis agree favorably with subsequent field measurements.
