TBM tunnelling challenges and managing high groundwater inflows on the Ashbridges Bay treatment plant outfall project

Considered the largest outfall in Canada, the City of Toronto’s Ashbridges Bay Treatment Plant (ABTP) Outfall project is building a new tunnelled outfall that will convey up to 3,923 Mega-Litres per day of treated effluent from the ABTP into Lake Ontario. Outfall construction components include an on-shore shaft approximately 85m deep with an internal finished diameter of 14 m; a 3,500 m long tunnel (7m internal diameter); and 50 risers constructed in line with the tunnel along the last 1,000 m of the tunnel extending from the top of the tunnel to the lakebed.

Tunnelling was performed with a single shielded Tunnel Boring Machine (TBM) mined entirely within the Georgian Bay Shale Formation (GBFS). The tunnel lining consisted of a one-pass Precast Concrete Tunnel Lining (PCTL) system. Following TBM assembly and on-site testing, mining commenced in March 2021. The first 2,500 m of tunnel was successfully completed with record setting advance rates reaching 46 m/day. However, the last 1,000 m of the tunnel proved to be extremely difficult as high groundwater inflows (up to 400 liters per minute) were encountered. Significant damages to the PCTL were observed and TBM mining was temporarily placed on standby.

During the high groundwater inflows, several measures were immediately implemented to stabilize the PCTL and allow TBM mining to resume safely and efficiently. These measures included implementation of additional PCTL support, pre-excavation probing and grouting, and implementation of systematic chemical and cementitious grout collar injection around the PCTL. The Engineer, Owner and Contractor met frequently and collaboratively to innovate and implement mitigation measures to overcome challenging groundwater inflow conditions and successfully completed the outfall tunnel.