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subway construction techniques

Tunnelling Process

Tired of the same old topics of conversation when you get together with family and friends? Next time, why not impress them with your newly acquired knowledge of how the vivaNext subway extensions will be constructed?

Surprisingly, few people know that there are many different elements to consider when building a subway, including which construction technique to use. As it turns out, that mostly depends on where one is digging.

Now for the boring stuff. Tunnel boring, that is. It’s one of the two main techniques used to build a subway. The second one is cut-and-cover.

The tunnel boring technique is generally used for deeper applications such as between stations. Because the tunnels are literally bored out below the surface, there is no disruption at the ground level except at the entry and exit points of the machinery.

Cut-and-cover, on the other hand, is the technique generally used for shallow applications such as station locations. In this case, there is some disruption at the ground level and temporary decking is installed when necessary to accommodate vehicular and pedestrian traffic.

In addition to tunnels and stations, the construction of a subway involves building a number of other special structures located above and below ground. These include crossovers, triple track structures, tail tracks, electrical substations, emergency exit buildings, vent shafts, bus and commuter rail terminals, parking lots, passenger pick-up and drop-off facilities, and pedestrian entrances.

spadina subway

Yonge Subway Extension

Yonge Subway Extension

The planned Yonge subway extension will extend 7.4 kilometres north from Finch Station to Highway 7. This critical rapid transit link will include five stations at Cummer/Drewry, Steeles, Clark, Langstaff/Longbridge and Richmond Hill Centre. Intermodal terminals will be located at Steeles and at Richmond Hill Centre and 2,000 commuter parking spaces will be at Highway 407.


  • The Yonge Subway Extension is ready to move to full engineering and construction.
  • In April 2009, the Ontario Ministry of the Environment unconditionally approved the Environmental Project Report [and addendum] for the Yonge subway extension.
  • In 2012, the Conceptual Design Study that looked at specific engineering elements was completed and approved by the Toronto Transit Commission and The Regional Municipality of York.
  • In 2015, Metrolinx released the findings of a Yonge Relief Network Study, and recommended advancing the Yonge Subway Extension with preliminary engineering.
  • In June 2016, the Province of Ontario committed over $55 million to advance design work to 15%.
  • Engineering and construction will take approximately 10 years, and during preliminary engineering a more detailed schedule will be confirmed.

ridership & capacity

Metrolinx’s Yonge Relief Network Study analyzed options for crowding relief to the existing Yonge Subway line by examining new local and regional travel opportunities and improving mobility across the GTHA.

Key findings include:

  • Today, the Yonge Subway line is operating at +11% over capacity.
  • Significant relief to the Yonge Subway line will be achieved through already committed transit improvements, including the TTC’s automatic train controls [adds 36% capacity], new signals [adds 10% capacity], six-car trains [adds 10% capacity], the Toronto-York Spadina Subway Extension [adds 8% capacity] and Regional Express Rail/SmartTrack.
  • The Yonge Subway Extension can be built – with the capacity improvements noted above added during the 10 years of engineering and construction, the Yonge Subway line will be running under capacity by 2031.
  • The Downtown Relief Line has become a long-term project that will not be needed until after 2031.
  • A recommendation to “Direct staff to work in consultation with York Region, City of Toronto and the TTC to advance the project development of the Yonge North Subway Extension to 15% preliminary design and engineering.”
  • Only a subway can service the potential future daily ridership of 165,000 [two people per second] – even today, over 2,500 bus trips per workday are needed to serve the current demand.


  • Completes a critical missing link in the regional rapid transit system of the GTHA, making an easy and seamless connection between regions. To read about why the Yonge Subway extension to Highway 7 is considered a top priority, see The Missing Link.
  • Fuels a proposed 48,000 residents and 31,000 jobs at the Richmond Hill/Langstaff Urban Growth Centre at Highway 7 and Yonge Street.
  • Saves over 28 tonnes of greenhouse gas (GHG) per workday by replacing the 2,500 bus trips currently servicing this segment of Yonge Street.
  • Produces a lasting economic stimulus and creates jobs.
  • Aligns with plans for growth and rapid transit in the GTHA, integrating with Regional Express Rail, building on the existing investment of more than $3 billion in York Region’s rapid transit, and meeting the Province’s smart growth objective of a transportation hub at Yonge and Highway 7.
  • Almost eliminates the 2,500 bus trips per workday currently serving the demand between Finch and Highway 7.


Tunnelling Process

Subway Construction Techniques

Get the scoop on what goes on underground when building a subway.

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