UrbanToronto: How to Dig a Light-Rail Tunnel
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UrbanToronto: How to Dig a Light-Rail Tunnel

Metrolinx is digging transit tunnels underneath some of Toronto's most densely populated neighbourhoods. Here's how it's being done.

The development, design, and history of building projects, brought to you by UrbanToronto.ca.

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The western launch shaft for the tunnel boring machine at Keelesdale Park. Photo courtesy of Metrolinx.

Playing in the sandbox as kids, many of us dug holes and built sand bridges, moats, and castles. Moulding the earth to suit our needs and desires doesn’t have to end with childhood, though: if you just can’t get enough of it, engineering just may be the job for you! One of Canada’s largest infrastructure projects is putting the expertise of many engineers to use underground. The Eglinton-Scarborough Crosstown LRT is the marquee project in Metrolinx’s $8.4 billion light-rail plan for Toronto. When complete and opened by 2020, the new 19-kilometre line will move passengers 60 per cent faster between Kennedy Road and Mount Dennis.

Through densely built-up Midtown Toronto, the LRT will be tunnelled for 11 km, avoiding Eglinton Avenue’s narrowest and most congested section. Tunnelling through built-up areas—and under two subway lines—is no small task. Toronto’s geology changes throughout the city: in some places the underlying Dundas shale bedrock lies close to the surface and in others, many layers of soil, silt, clay, and gravel extend down before reaching the bedrock. Multiple traffic, construction, and community concerns must be taken into consideration, and the right technology for each task needs to be chosen carefully.

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Ground makeup for the majority of the Eglinton LRT route. Image courtesy of Munro Ltd.

The two prevailing techniques for building transit tunnels are the cut-and-cover and tunnel-boring methods. In cut-and-cover, crews dig out massive trenches along the route, pour the concrete tunnel forms, install the tracks and electrics, and finally back-fill the trench with dirt to recreate the surface. This kind of construction was largely used on the Yonge-University-Spadina and Bloor lines. For tunnel boring, the technique employed on the Crosstown LRT and Spadina Extension projects, crews use large tunnel boring machines (TBM) to chew out the dirt from beneath the surface. Stations and TBM launch and extraction shafts are dug out via the cut-and-cover method where required. Keeping cut-and-cover to a minimum produces fewer disruptions to life on the surface, but can cost more.

Head over to UrbanToronto.ca to learn more about the project.

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