The Scarborough Subway Extension (SSE)—Large single-bore transit tunneling in Toronto

Author(s): E. Poon, T. Gregor, M. Geary
NAT North American Tunnelling Conference 2018


Hatch is performing preliminary and detailed design for the Toronto Transit Commission’s (TTC) Scarborough Subway Extension (SSE), a 6 km long extension of the existing Line 2. SSE will be delivered by the Design-Bid-Build procurement method and is currently projected for tendering in late 2019. The running structure of the extension consists of a large single bore 10.7 m internal diameter tunnel housing twin subway tracks, mined by earth pressure balance (EPB) or slurry TBM. The large single bore tunnel solution represents a significant departure from the traditional TTC arrangement of twin tunnels for subway. This paper discusses the technical background, design and construction considerations, operational constraints and assessments undertaken that led to the selection of the single bore concept for Toronto’s next major subway expansion.