Ashbridges Bay treatment plant outfall – Designing for time dependent deformations in precast concrete tunnel lining

Abstract

The City of Toronto is planning the construction of a new 3.5 km long outfall at the Ashbridges Bay Treatment Plant (ABTP) to convey treated effluent from the ABTP into Lake Ontario. The project comprises of a single construction shaft (14 m inside diameter), a single outfall tunnel (7 m inside diameter), 50 diffuser risers, effluent conduits and on-shore surface works adjacent to the construction shaft. The outfall tunnel is approximately 85 m underground and will be constructed entirely in the Georgian Bay Shale (GBS) formation. Single-pass precast concrete tunnel lining (PCTL) will be used to support the tunnel.

Previous tunneling experience in shales (including GBS) in southern Ontario indicates continuation of rock convergence (i.e. time-dependent deformation (TDD)) into the tunnel long after completion of the construction. This TDD was found to be the result of volume change (swelling) of rock due to diffusion and dissolution of calcite that is inherent to the shale formation upon introduction to fresh water. When the TDD is resisted by the stiffness of the tunnel lining, additional deformation and stresses will be developed in the lining and must be accounted in the lining design.

This paper discusses a methodology that can be used to determine the magnitude of TDD and to assess its impact on the design of the tunnel lining. Factors affecting the TDD behaviour will be discussed. Finally, predicted impact of TDD on the tunnel lining used for the ABTP Outfall Project will be presented.