Planning and Design of the New Ashbridges Bay Treatment Plant Outfall for the City Of Toronto

Abstract

The Ashbridges Bay Treatment Plant (ABTP) Outfall project by the City of Toronto involves construction of a new tunneled outfall that will convey treated effluent (water) from the ABTP into Lake Ontario. This new outfall will be built to allow cessation of operations of the existing outfall which is reaching the end of its service life and has limited hydraulic capacity.

Outfall construction will include mining a 3,500 m long, 7 m internal diameter tunnel through rock beneath the lakebed. The project will initiate at an 85 m deep, 14 m internal diameter onshore shaft, to be constructed adjacent to the shoreline and downstream of the future UV Effluent Disinfection Facility. Treated effluent will flow by gravity from the plant through connecting conduits to the shaft and tunnel out into the lake. Vertical in-line risers will be constructed along the last 1,000 m of tunnel to connect the tunnel and convey flows to the lake. Tunneling operations will be supported from the onsite shaft and the risers will be drilled from over-water barges. The investigations and design phase of the project work is planned through the end of 2017, tendering in Q1/Q2 of 2018 and construction starting in the third quarter of 2018 and extending to the end of 2023.

The paper presents the planning and design of the project including discussions of challenges such as optimization of the risers, meeting regulatory requirements, addressing underground construction in rock known to exhibit time dependent behavior and riser construction up to 48 m in length through 38 m deep soft ground lakebed deposits overlying the bedrock.