Tunnelling reaches halfway point on major Canadian subway project
A major milestone has been reached beneath the streets of Toronto, as tunnelling on the Scarborough Subway Extension passes the halfway mark—signalling strong progress on one of North America’s most significant transit infrastructure schemes.
Led by Metrolinx, the project will extend Line 2 of Toronto’s subway network প্রায় 7.8 km into Scarborough, improving connectivity and access to jobs, education and services for tens of thousands of residents.
Halfway underground
At the heart of the scheme is a single, large-diameter tunnel being excavated by tunnel boring machine (TBM). Unlike traditional twin-bore subway construction, this tunnel—measuring approximately 10.7 metres in diameter—will house both tracks, making it the largest subway tunnel of its kind ever built in Canada.
Recent updates confirm that more than half of the tunnel has now been excavated, marking a significant construction milestone.
Progress has included the installation of thousands of precast concrete segments to form the tunnel lining, ensuring structural integrity as the TBM advances southwards along the alignment.
Overcoming ground challenges
The journey to this point has not been without difficulty. Early phases of tunnelling encountered challenging ground conditions, which slowed progress and required intervention to maintain safe and efficient operations.
Despite these setbacks, tunnelling has now resumed at a steady pace, demonstrating both the resilience of the project team and the adaptability of modern TBM operations in complex urban geology.
A single-bore approach
One of the defining engineering features of the Scarborough Subway Extension is its single-bore design. By accommodating two tracks within one large tunnel, the approach reduces the need for twin excavations and can streamline construction logistics.
However, it also demands precise control of ground movement and careful segmental lining installation—particularly in urban environments where settlement risks must be tightly managed.
Wider project progress
Alongside tunnelling, work is advancing across multiple fronts. Construction is underway at key station sites, including Scarborough Centre, where crews are installing shoring systems and preparing excavation for station boxes.
The extension will ultimately include three new stations and is expected to significantly increase transit use in the corridor, with improved access to rapid transit for local communities.
Looking ahead
With the TBM now beyond the halfway point, focus will increasingly shift toward completing the remaining drives and preparing for breakthrough at the extraction shaft.
For the tunnelling sector, the project highlights several key trends:
- the growing adoption of large-diameter single-bore tunnels for metro systems
- the importance of managing variable ground conditions in urban environments
- and the continued scale-up of TBM technology in North American transit projects
As work continues beneath Scarborough, the milestone marks not just progress for Toronto’s transit network, but another step forward in the evolution of modern urban tunnelling. For further information in the tunnelbuilder archive click here. 13/06.
