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Construction

The Ontario Line - 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 Ontario Line.

Although the alignment will evolve throughout the detailed design process, the Ontario Line will use a mix of below-grade (tunnelled), at-grade (ground level) and above-grade (elevated) structures. Subway tunnels can be constructed using: tunnel boring, cut-and-cover and mining construction techniques.

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. Using the sequential excavation method, a ‘roadheader’ mines the tunnel in sections and a support canopy is installed simultaneously to keep the ground stable.

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

For certain portions of the elevated track, such as the section across the Don River, bridges must be built to minimize impacts on sensitive environmental areas below. To stand upright, bridges must balance forces of tension and compression, while carrying the load of the subway and resisting environmental forces. Tension is a pulling force. Compression is a pushing force. Bridges channel the forces into chords or horizontal segments and piers or towers.

First, a foundation is set into the ground for piers to be built upon. Then, towers are built upward from the base, complete with a support system to connect the towers, cables and abutments (ends of the bridge). Sections called ‘superstructures’ are then built from end to end at even intervals and secured to the support system.

To complete the bridge, the deck/ track surface is assembled and furnished with electrical and lighting systems. When all elements are complete, the bridge undergoes final testing to ensure it meets quality standards before operation.

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. Whether it is above or below ground, constructing a station is a multi-step process.

Rending of Caledonia Station
Caledonia Station on Eglinton Crosstown rendering

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.

Rendering of Burquitlam station
Burquitlam station is an attractive part of Vancouver’s Evergreen Line. (Photo by Andrew Latreille, courtesy of Perkins & Will)

All stations will have similar elements in the public areas such as architectural design treatments, information displays, lighting, signage, security monitoring devices and other design elements. At-grade stations are generally less expensive and quicker to construct because less time is spent excavating soil, meaning building can begin and end sooner than tunnelled stations.

When conducting any construction, we must consider the infrastructure that is already in the area. Construction can involve the removal, replacement, relocation and upgrading of utilities. This means third party utility (e.g., gas, hydro and telecommunications) and city infrastructure (e.g., water, sanitary sewer and stormwater) may need to be relocated and in some cases upgraded to make room for the Ontario Line.