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Building a Subway


Two methods are being used to build the Toronto-York Spadina Subway Extension. Much of the extension uses tunnel boring technology to build the subway tunnels while the stations are built using cut and cover construction.

Tunnel Construction

A total of 6.4 kilometres of the Toronto-York Spadina Subway Extension Twin Tunnels are built by tunnelling underground with Earth Pressure Balance tunnel boring machines, a powerful circular cutting machine that bores a tunnel in soil or rock with minimal disruption to the surface above.

Tunnel Boring Machine "Torkie"
Tunnel Boring Machine 'Torkie' at the factory.

The tunnel boring machines are launched from a deep excavated shaft called a "launch shaft" and bores a tunnel to an end location called an "extraction shaft". The material removed by the tunnel boring machine is moved out of the tunnel by a conveyor system and rail cars to the launch shaft and then transported away by dump trucks.

Downsview Park Station Launch Shaft
Tunnel Boring Machine 'Holey' in launch shaft at Sheppard Avenue West, looking west. June 2011

As the tunnel boring machine advances, pre-cast concrete tunnel segments called "liners" are set in place behind the machine to form the tunnel. As they are pre-made off-site, there is some truck activity as the liners arrive on site.

Delivery of Tunnel Liners
Delivery of tunnel liners to Steeles West launch site. Spring 2011.

The tunnel boring machine operates 24 hours a day, 7 days a week. Tunnelling takes place deep underground with minimal disruption to the ground above. Once it finishes a "tunnel drive", the tunnel boring machine is broken down into several parts at the extraction shaft and transported by truck to the next launch shaft to be prepared for its next tunnel drive. Two tunnel boring machines drill in parallel about a month apart, in the same direction, to create the twin tunnels for the subway.

A construction staging area, or worksite, is prepared at the launch shaft prior to the arrival of the tunnel boring machine and the tunnel liners. A smaller extraction shaft area is also prepared before the tunnel boring machine reaches its destination.

Contractor Staging Area at Launch Shaft
Contractor staging area at Lauch Shaft 5

Tunnel Fit-Out

Now that tunnelling is complete, there is a lot of work yet to be done in the tunnels to get them ready for subway trains to ride in them!

We will be posting a series of photos on this page along with a short description of what stages are involved in what we call tunnel fit-out.

Tunnelling complete - Highway 407 to Steeles Ave West - September 20, 2013
Tunnelling complete - Highway 407 to Steeles Ave West - September 20, 2013

Tunnel Surveying

A detailed survey of the tunnels is done once the tunnel boring machine is removed. This detailed survey of the as-built tunnel alignment provides critical information for locating and building the track invert and walkways and other elements in the tunnel.

Surveying tunnel alignment; southbound tunnel south of Steeles Ave W - May 24, 2012
Surveying tunnel alignment; southbound tunnel south of Steeles Ave W - May 24, 2012

Tunnel Stripping

The next step after tunnelling is complete and the tunnel alignment survey is done is to strip the tunnels of all of the equipment used for tunnelling (seen in the photo at the top); rail, conveyor system, ducts and pipes, power, ventilation system must all be removed.

Tunnel stripping complete - northbound tunnel south of Steeles Ave W - Jan. 9, 2013
Tunnel stripping complete - northbound tunnel south of Steeles Ave W - Jan. 9, 2013

Invert Construction

The invert is the flat concrete slab constructed at the bottom of the tunnel on which the track ties and track will sit. The next step in the process is setting the alignment for the invert which is done by installing steel rods shown in the photo below. A concrete pipe is installed to bring the concrete from the entry shaft into the tunnel to pour the inverts.

Photo of concrete pipe and alignment set up

Formwork is installed to lay out the shape and alignment of the invert. The photo shows the centre channel formwork being installed. This channel is needed to drain the tunnels. Almost 12,800 metres of invert must be built.

Photo of installation of tunnel invert formwork.

Concrete is fed through a “slick line” to pour the invert in the new subway tunnels. The “slick line” is fed from concrete trucks at the surface through a shaft at the end of the tunnel.

Photo of concrete placement for tunnel invert.

Once the invert is complete, reinforcing steel bars (rebar) is placed for the concrete curbs. These curbs, similar to but larger than a road curb, will form the trackbed.

Photo of tunnel with completed invert

Curbs are poured into the formwork that has been set up to enclose the rebar. Special machinery carries and pours the concrete.

Photo of concrete pouring into formwork.

The last step before track installation is forming and building the safety walkways. These walkways also contain conduits that carry cables for the many systems that are required to operate the subway including communications, power, signals, and emergency systems.

Photo of tunnel showing conduits.

Installing Track

Once the inverts are poured and the curbs are installed, the next stage to ready the tunnels for service involves the installation of track. A specialized contractor is now performing a series of steps to complete this work.

First, concrete double ties (reinforced concrete pads) each weighing 2,400 kg are delivered and stacked in the tunnels in preparation for installation. Next, special vibration isolation pucks, resembling giant hockey pucks, are pre-fastened to the underside of the concrete ties, separating the ties from the concrete trackbed and forming a cushion to absorb noise and vibration. Then a special machine nicknamed "Gizmo" is used to place the concrete ties in the tunnels. In total, 11,000 concrete double ties will be installed in the new tunnels.

Concrete double ties, ready for installation.
Concrete double ties, ready for installation

Vibration isolation puck.
Standard isolation puck

Installing Ties north of (existing) Downsview Station.
Installing Ties at (existing) Downsview Station

Concrete double ties installed between Pioneer Village and York University stations.
Double concrete ties

Once the ties are secured in place, rail is lowered through a drop shaft, several of which are constructed in various locations along the new subway alignment. The rail is then welded into long strings and pulled farther into the tunnel.

Drop shafts located on Keele Street, south of Finch Avenue West.
Drop shafts on Keele Street

Next, the rail is positioned on and secured to tie plates which are affixed on top of the double ties. A pip-jig supports the rail in the correct position and bolts are then fastened into the holes in the ties with epoxy. A grout pad is then poured under the tie plates, filling the void.

Pip-jig holding rail while the grout pads are poured.
Jig holding contact rail

The contact rail (also called the power rail or, more commonly, the third rail) is secured to rubber and ceramic insulators nicknamed “mushrooms”. Coverboards and support brackets are then attached to the baseplates and finally, reinforced safety coverboards are installed to allow workers to step over the contact rail (as it will carry 600 volts DC).

Running rail, fasteners, contact rail and reinforced safety coverboards are all installed.
Installed track

Completed track installed in Downsview Park Station.
Completed track

At the end of 2014 over 60% of track installation had been completed.


Building the actual station structures is complex and involves the construction of platforms, concourse levels, bus terminals, commuter parking lots and associated entrances. Here the "cut and cover construction" method is used. Cut and cover construction involves excavating a large trench from the surface, building a concrete tunnel or structure and then backfilling and restoring the ground or roadway.

The unique station structures of the Toronto-York Spadina Subway Extension require extensive staging to complete the structure and, like tunnel boring, a large construction staging area will be needed.

Subway station staging area

Staging area for Sheppard West station excavation.

In some situations, installation of a large metal deck over an excavated trench allows for other activities associated with the construction and for neighbourhood traffic to be maintained.

Decking over staging site

Installation of decking at Allen Road south of Sheppard Avenue West for Wilson Yard connection excavation.

Construction traffic and public transportation activities travel over the decking, while excavation of the soil continues underground to allow for the various stages of the construction of the station box. The last major phase of construction involves the removal of the decking and restoration of the surface or street. The track is then installed for the trains along with the other equipment that is required for a fully operational subway.

Subway station box at Downsview Park Station site

Subway station box excavation at Sheppard West Station site, east of GO Rail line. Summer 2011.


Safety during construction is our number one priority. Environmental Coordinators and Construction Safety Officers will be monitoring the construction and visiting affected streets regularly.

We also work closely with our contractors who are committed to achieving excellent safety performance as we strive for continuous improvement with an ultimate goal of zero injuries.


The Toronto-York Spadina Extension project and our contractors will monitor air quality and noise levels during construction activities. We have procedures in place for air quality and noise.

Air Quality 

Dust and debris are controlled through the use of standard techniques, such as:

Cover or wet down dry materials to prevent blowing dust and debris.

  • Preventative measures to control dust from blowing across the site and from leaving the site, in particular frequently wet paved and unpaved temporary roads and excavated areas.
  • Wash down the streets within the work site as required.
  • Securely cover excavated material being removed from the site and all fill materials being delivered to the site to prevent blowing of dust.

Noise levels

Our contractors are required to provide measures to reduce construction noise, such as:

  • Shields or other physical barriers are erected to restrict the transmission of noise.
  • Sound reduction for housings or enclosures.
  • Efficient intake and exhaust silencers are included on air equipment.
  • Efficient intake and exhaust mufflers are affixed to internal combustion engines.
  • Sound deadening lining material is attached to hoppers and storage bins.
  • Controls are in place to minimize the noise from truck loading, unloading and hauling operations.
  • Electric rather than internal combustion power is used (whenever possible).
  • Distancing stationary noise producing equipment in public areas.
  • Abiding by local noise by-laws.


Bus Route Changes


Review the new changes to bus routes in the area to connect to the new subway extension.

Read about it here!

Need Information

To contact us for information about the project:

Call 416-393-4636 (INFO) automated information available 24 hours daily.

Contact the Project

Photo Gallery

Check out the latest construction photos and project YouTube videos.

Click here to view the Photo Gallery.