Ashbridges Bay Treatment Plant Outfall

Completed in 2024 by Southland Contracting, the Ashbridges Bay Treatment Plant (ABTP) Outfall Project consisted of a new outfall designed to increase discharge capacity and improve the conveyance of effluent wastewater from the ABTP into Lake Ontario, as well as to enhance operations under high-flow conditions, especially during rainstorms. The backbone of the project was a 2.2-mile long, 23-foot inside diameter (ID) precast concrete segment-lined tunnel, excavated with a shielded Tunnel Boring Machine (TBM) in shale beneath Lake Ontario. The TBM was launched from a 46-foot ID, 280-foot-deep launch shaft onshore. Since a retrieval shaft under Lake Ontario was impractical, the TBM was partially disassembled and pulled back to the launch shaft for removal once mining was completed. The project also involved extensive marine construction that required underwater divers to perform specialized work. This work included the installation of 50 vertical risers by driving an 8-foot diameter containment casing into the lakebed; installing a 5-foot diameter steel casing through overburden; excavating soils using pile grabs and a hydraulic dredger; excavating rock with a reverse circulation drill (RCD) with a 5-foot diameter cutting head; raising and lowering 50 stainless steel pipelines, each 3 feet in diameter and 164 feet long; conducting tremie grouting and concreting the shaft annulus; and removing temporary casings. The average water depth at the construction site was 60 feet, and the average total excavation depth through the ground was 161 feet.