Results of a program of instrumentation involving a precast segmented concrete-lined tunnel in clay

Abstract

In Thunder Bay, Ontario, a 2.16 m diameter tunnel is nearing completion in a soft to firm clay using a full-face tunnel boring machine and an unbolted precast segmented concrete lining. The lining consists of rings composed of four trapezoidal sections of unreinforced concrete, 1 m in length and 11 cm thick. Each ring is assembled within the protection of the tailpiece of the tunnel boring machine and clay grout is injected behind the ring as the machine advances by thrusting on the completed lining.Because of the uniqueness of this method of construction, an extensive field instrumentation program was undertaken to evaluate the tunnelling procedure and the performance of the completed tunnel. The first phase of instrumentation involved a 60 m length of the tunnel and included observations of ground surface deformation, soil deformation (as measured by slope indicators, magnetic extensometers, and deep settlement points), total pressure on the lining, and deformation of the lining. The information also included tunnelling procedure and weight of spoil removed as the tunnel boring machine passed through the instrumented area.These observations are presented and discussed. Some problems of instrumentation are considered, particularly as they related to the second array of instrumentation which was designed to provide detailed information on spatial deformations and pore pressure changes.