For the first time in TTC history, a single 12 metre outside diameter tunnel will be used for the 5.8 kilometre tunnelled portion of the Line 2 East Extension. The single tunnel will accommodate subway tracks in both directions.
Other recent TTC subway projects, including the Line 1 Extension and Line 4 Sheppard, were constructed using twin tunnels (about 6 metre diameter each). Line 5 Eglinton (Metrolinx project – currently under construction) also has twin tunnels (6.5 metre diameter).
Single large diameter tunnels have been constructed for several subway lines in Europe, Asia and the Middle East. In Canada, this type of tunnel was used for the Vancouver Evergreen Line (9.84 metre diameter).
The key benefit of the single large diameter tunnel is that it minimizes the areas that must be constructed using cut-and-cover construction, such as crossovers, resulting in reduced construction disruption at the surface level.