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Completing the Tunneling Process on Canada’s Largest Outfall

Robbins TBM in Ashbridges Bay Outfall

The completion process of a record run beneath Lake Ontario, on the Ashbridges Bay Outfall project, has been done using a 7.95m diameter Robbins single shield TBM.

Excavating 3.5km in sedimentary rock for the Ashbridges Bay Outfall in Toronto, Ontario, Canada, this machine 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.

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

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

The operation shift of the machine is in two 12 hours from Monday to Friday, while a Robbins continuous conveyor system, including a 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.

Although the last kilometer of the tunnel, dug beneath a series of 50 risers under Lake Ontario, was challenging but finally successful.

Southland project manager Joe Savage stated: “The team really worked together to overcome some tough ground conditions and high water inflows in the tunnel.”

Acceptance of all-remote machine, which was implemented because of the Covid-19 pandemic, was won accolades from the Tunneling Association of Canada in late 2021. 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.

In order to enable efficient dispersion of treated effluent over a wide area of the lake, the completed Ashbridges Bay outfall will link to the 50 in-lake risers while making it the largest outfall in the country. Replacing a 70-year-old available outfall, the project for the City of Toronto is going to improve the city’s shoreline and Lake Ontario’s water quality.

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