> Scarborough Subway Extension - Construction | Metrolinx Engage

Scarborough Subway Extension

Scarborough Subway Extension - Construction

Building a subway in developed neighbourhoods across a large and busy city is a complex design and engineering undertaking. Here is some general information from other projects to help you understand what teams are considering while they develop more detailed plans for delivering the Scarborough Subway Extension.

How is a subway tunnel constructed?

Subway tunnels can be constructed using: tunnel boring, cut-and-cover and mining construction techniques. Although the alignment will evolve throughout the detailed design process, the Scarborough Subway Extension will use single tunnel construction, limiting disruption at street level because we do not need to dig down to build cross passages. Cross passages are small passageways built to connect two tunnels for maintenance and/or emergency purposes.

Tunnel boring involves the use of a tunnel boring machine (TBM) for the tunnelling excavation process. As the tunnelling progresses, the excavated material is brought to the TBM site through the tunnel using bins mounted on rail cars or a conveyor system.

Cut-and-cover is used for shallow tunnels where a trench is excavated and then an overhead support system strong enough to carry the load of what is to be built above the tunnel is installed.

Once tunnelling between the stations is complete, the tunnel surfaces are finished and the tracks and electrical services are installed.

Tunnel Boring Machines

The tunnel boring machines (TBMs) will operate up to 35 metres underground, 24 hours a day, seven days week, and travel approximately 10 metres per day.

Photo of the Crosstown LRT tunnel boring machines
Crosstown LRT tunnel boring machines

The TBM will move forward with the help of precast segment tunnel liners that are placed on the tunnel walls as it drives through. The TBM will propel itself forward by pushing against the liners.

Crosstown LRT precast tunnel liners
Crosstown LRT precast tunnel liners

How are Stations Constructed?

Stations are designed considering the future customer needs and local neighbourhood environment. Metrolinx considers things like safety, accessibility, access to other transportation and transit, ridership and passenger experience.

Constructing a station is a multi-step process.

Image of a station being constructed underground for the Crosstown LRT

Preparing an underground site for station construction typically begins by protecting or relocating any underground utilities such as power lines, water lines, sewers, gas pipes, cable/telephone lines and storm drains. If the station is being built under a street, a temporary concrete decking is installed once the area is excavated to establish a temporary street surface during construction. The next step is continued excavation and installation of shoring along the edges of the excavation for support.

Once the foundation is complete, construction of the inside of the station begins.

Rending of Caledonia Station
Caledonia Station on Eglinton Crosstown rendering


  • Headwalls are constructed to create the frame for the underground stations.
  • Headwalls for the three proposed stations will be constructed with secant piles, which are overlapping columns made from poured concrete reinforced with steel.
  • Seven emergency exit buildings also require headwalls.
  • Headwall construction at each site will take approximately two to three months.

Photo of headwalls
Photo of headwalls

  • Before we start any construction, we must consider the infrastructure that is already in the area.
  • The proposed alignment significantly reduces impacts by avoiding utilities wherever possible, and because most of the construction will be limited to directly under the Right-Of-Way. However, construction will involve the removal, replacement and relocation of some utilities.
  • The number of utilities will vary by location, but the work will include moving hydro, water mains, sewers, natural gas and telephone lines, traffic lights and street lighting to make room for the extension.
  • Metrolinx is working closely with utility companies to reduce community impacts and to complete any preparatory works in advance of the tunnel contract award.
  • Utility works are underway.

Image of utility relocations underway on the Scarborough Subway Extension corridor

Image of geotechnical investigation underway on the Scarborough Subway Extension corridor

Image of utility relocations underway on the Scarborough Subway Extension corridor

Traffic & Transportation – Work Completed

In developing the Environmental Project Report Addendum, a Transportation Impacts Memorandum was completed to document existing conditions, impact assessment, and potential mitigation and monitoring for:

  • Automobile traffic and transit services.
  • Pedestrian and cyclists.

The Transportation Impacts Memorandum can be found in Appendix B – Technical Reports.

Image of subway train at Kennedy Station
Line 2 at Kennedy Station

Potential Traffic Impacts & Mitigation

Potential Traffic Impacts

Potential traffic impacts during construction may include:

  • Impacts to the road network as a result of the tunnel mobilization sites at the three stations and the launch shaft activities (e.g., at the start and end of the Scarborough Subway Extension alignment) will include temporary lane restrictions partial closure of intersections to prepare for long-term construction operations.
  • These temporary lane closures and intersection closures are required to expedite work and complete work safely.
Traffic Mitigation

Potential mitigation for traffic impacts due to construction may include:

  • Appropriate implementation of traffic staging plans, including design of temporary intersection lane configurations.
  • Temporary traffic signals.
  • Modifications to existing signal timings.

Where possible, minor construction activities may be scheduled during off-peak, weekend or overnight hours to minimize disruptions to traffic.

Monitoring of all transportation infrastructure will take place during construction.

Metrolinx will engage with the community about traffic management plans as soon as they become available.

Noise & Vibration – Work Completed

In developing the Environmental Project Report Addendum, a Noise and Vibration Assessment Report was completed to document existing conditions, impact assessment, and potential mitigation and monitoring. It includes the results of the:

  • Collection of noise and vibration measurements.
  • Identification of noise and vibration sensitive receptors.
  • Determination of noise and vibration mitigation strategies.

The Noise and Vibration Assessment Report can be found in Appendix B – Technical Reports.

Image of construction noise monitoring station, with solar power and remote telecommunications
Long-term construction noise monitoring station, with solar power and remote telecommunications.

Potential Construction Impacts & Mitigation

How will Metrolinx mitigate construction impacts?

Clearly, subway construction cannot be accomplished without impacts. Metrolinx is committed to minimizing and managing these impacts. Key elements of our approach to managing noise and vibration during construction include:

Before Construction

Before construction begins, we:

  • Establish a comprehensive public relations program to inform affected parties about the project’s scope, schedule, noise and vibration management strategies, and communication and complaint resolution protocols.

During Construction

During construction, we:

  • Monitor noise and vibration to inform implementation of additional mitigation measures, as necessary.
  • Address public complaints in a timely manner.

Image of noise logger device used for capturing ambient noise
Noise logger capturing ambient noise.