Construct Tunnel-Riser Connection in Challenging High Groundwater Zones on the Ashbridges Bay Treatment Plant Outfall Project

Abstract

One of the main challenges in underground construction projects is unanticipated ground conditions. To mitigate significant cost and schedule impacts on projects due to unanticipated ground conditions, innovative technical solutions need to be developed during construction. These innovations are rarely restricted to one tunnel project and generally introduce a significant advancement in the tunnelling industry.

The Ashbridges Bay Treatment Plant Outfall (ABTPO), which is being constructed under Lake Ontario in Toronto, is one of the largest outfalls in Canada. The outfall will convey up to 3,923 million litres per day of treated and disinfected effluent from the ABTP into Lake Ontario through a 3,500 m long, 7 m internal diameter tunnel, and disperse the effluent along a 1 km diffuser section through 50 risers connecting from the tunnel to the lakebed.

The 50 m long, 1m internal diameter risers were successfully installed offshore in advance of tunnelling. The bottom elevation of the pre-installed risers was about 1 m above the excavated tunnel crown. Due to a high rate of groundwater inflow encountered in the tunnel during tunnelling (up to 400 lpm), one of the most challenging aspects of the project was constructing the 50 tunnel-riser connections including excavation from the tunnel crown at each riser location, forming, and placing concrete. The Contractor, Engineer, and Owner developed an innovative and practical design and construction procedure to overcome the challenging groundwater inflow conditions. This paper reviews the innovative design and construction methodology to overcome the challenges of constructing the tunnel-riser connection in areas of high groundwater inflows under the lakebed.