Numerical model for shaley rock masses displaying long-term time dependent deformation (TDD) behavior and its application to a pedestrian tunnel constructed under Lake Ontario

Abstract

Southern Ontario is known for very high horizontal in-situ stresses and time dependent behavior of shale formations, which has resulted in continued rock deformation and distress in final linings in several previous projects in the region, sometimes years after installation. Billy Bishop Toronto City Airport Pedestrian Tunnel is a passenger access tunnel under the Western Channel of Lake Ontario. This paper discusses a series of numerical approaches used for predicting time dependent deformations, both during, as well as after the completion of the tunnel, to determine the optimal timing of the placement of the final lining, such that the remaining time-dependent movements can be sufficiently accommodated within the structural capacity of the tunnel lining. The paper also discusses the design process using a series of numerical analysis and construction sequencing considerations pertaining to the long-term time dependent deformation of the shaley rock formation, the instrumentation and monitoring program, and a comparison of the predicted against the actual movements during construction. Results from a custom-made FLAC sub-routine are provided in addition to the details of the numerical analysis - both the problems that were identified during the analyses as well as their solutions - throughout the discussion.