Tunnelling completed on Canada’s largest outfall

25 March 2022

A 7.95m diameter Robbins single shield TBM has completed a record run below Lake Ontario, on the Ashbridges Bay Outfall project.

The machine bored 3.5km in sedimentary rock for the Ashbridges Bay Outfall in Toronto, Ontario, Canada – and set a city-wide record of 30 rings in one day (about 47m of advance), surpassing a previous best day of 21 rings at a project with similar specifications.

Southland/Astaldi JV launched the machine in March 2021 from an 85m deep, 16m diameter shaft from where it began its bore in predominantly shale, with limestone, siltstone and sandstone.

“This is a wonderful type of geology for our machines. During the entire excavation, a total of seven cutters were changed. The wear behaviour is incredible, between 2-5mm, and everyone is amazed by the cutter performance,” said Alfredo Garrido of Robbins Field Service.

The crew operated the machine in two shifts of 12 hours from Monday to Friday. A Robbins continuous conveyor system, including vertical conveyor, transported muck behind the machine.

“Every 25 machine cycles, it was necessary to stop the excavation to probe drill holes in front of the cutterhead to check for possible water. This drilling was done basically every day, stopping the machine for a few hours, but it was very necessary,” said Garrido.

The last kilometre of tunnel, bored below a series of 50 risers under Lake Ontario, was challenging but ultimately successful. “The team really worked together to overcome some tough ground conditions and high water inflows in the tunnel,” said Southland project manager Joe Savage.

The project won accolades from the Tunnelling Association of Canada in late 2021 for its all-remote machine acceptance, implemented because of the Covid-19 pandemic. The machine acceptance, the first of its kind, enabled communication and confirmation between the machine’s assembly location in Mexico, suppliers in the US and those involved in Canada, and it could set an industry trend.

“It was a challenge for all the people involved due to the pandemic travel restrictions; however, through good planning and communication we were able to go through the Acceptance Test successfully. I think this might become quite common in the near future,” said Robbins project manager Javier Alcala.

The completed Ashbridges Bay outfall will connect to the 50 in-lake risers to enable efficient dispersion of treated effluent over a wide area of the lake, making it the largest outfall in the country. The project for the City of Toronto will improve the city’s shoreline and Lake Ontario’s water quality by replacing a 70-year-old existing outfall.