The city of Toronto’s Ashbridges Bay treatment plant outfall – Design innovations

Author(s) A. Solecki, R. McMillan, J. Kempa-Teper, K. Fung
Presented at the 2018 TAC/NASTT-NW tunnelling and trenchless conference - November 8-9, 2018 - Edmonton, Alberta, Canada


Located in Toronto, the Ashbridges Bay Treatment Plant (ABTP) Outfall project will create a new tunnelled outfall that will convey up to 3,923 Mega-Litres per day (ML/d) treated effluent from the ABTP into Lake Ontario. Outfall construction components will include an on-shore shaft, approximately 85 m deep with an internal finished diameter of 14 m; a 3,500 m long tunnel within an internal diameter of 7m tunnelled through shale rock; and risers constructed in line with the tunnel along the diffuser length, extending from the top of the tunnel to the lakebed.

This paper will outline the solutions and innovations used to address some of the challenges identified during detailed design. An innovative instrumentation program for the precast concrete tunnel lining and shaft was developed to collect data to measure the effects of time dependent deformation on the shaft and tunnel lining. A non-destructive test method was selected as a means to accurately and efficiently identify voids in the rock behind the tunnel lining as part of quality assurance testing. Innovative methods were also developed to reduce contractor’s risk by baselining marine conditions and integrating them within the geotechnical baseline report. Finally, innovative use of design aids and technology were implemented for increasing design efficiency and ensuring the project remained on schedule.