Playing in the sandbox as kids, many of us dug holes, 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 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 now. 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% faster between Kennedy Road and Mount Dennis.

Route Map of the Eglinton-Scarborought Crosstown LRT (Scarborough RT portion is still variable), image courtesy of Metrolinx

Through densely built-up Midtown Toronto, the LRT will be tunneled for 11 km, avoiding Eglinton Avenue's narrowest and most congested section. Tunneling 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 carefully chosen.

Ground make-up for the majority of the Eglinton LRT route, image courtesy fo Munro Ltd.

The two prevailing techniques for buildings transit tunnels are the cut-and-cover and tunnel boring method. 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, like on the Crosstown LRT and Spadina Extension Project, crews are using 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. This method produces less disruptions to life on the surface, but can cost more. This Metrolinx video explains how tunnel and construction is sequenced.

For the Crosstown LRT, Earth-Pressure-Balance Tunnel Boring Machines (EPB-TBM) built by Catepillar Inc. (formerly Lovat Inc.) near Pearson Airport are being used. From the Western Launch Shaft at Keelesdale Park, on June 5, 2013 the first of four TBMs set off westbound to Yonge Street.

Western TBM Lauch Shaft at Keelesedale Park, image courtesy of Metrolinx

The EPB-TBMs use large rotor heads mounted on cylindrical chassis to chew out the earth. As the machine moves forward, custom concrete lining segments are set in place to create tunnel rings. The machine pushes itself forward against these rings, and the operation continues. The dirt excavated is dumped into specially sized rail cars and transported to the launch shaft to be hauled away. To ensure the tunnel face doesn't collapse, crews can vary the drive speed or the speed at which dirt is moved away from the drill head to maintain an equal pressure against the soft tunnel drill face. This video below provides a detailed explanation of the workings of an EPB-TBM.

Building an LRT station requires and equal amount of finesse. Before the TBM passes through the station site crews first construct the station headwalls. The two main headwall construction methods used are Secant Piling and Jet Grouting. Secant Piling is perhaps the most familiar because it is largely used in many building foundations. To construct a Secant Pile Wall, a set of columns is drilled into the earth with a space in between them and filled with concrete. A second set of columns is then drilled into the spaces between filling the gaps with concrete and a rebar cage.

Secant Pile Wall diagram, image courtesy of Secantpile.com

Secant Pile Driving Rig (left) and a temporary Grout Mix Plant (Right), image courtesy of Metrolinx

The Jet Grouting process, instead of removing from the earth, adds to it. First, a rotating drill head outfitted with a nozzle bores vertically into the ground. On site, a temporary mix plant produces a grouting material. The grout is then pumped into the drill head and sprayed out the nozzle at high pressure into the surrounding earth. As the drill head is retracted towards the surface, columns of grouted earth solidify producing a soft concrete wall. The video below illustrates how jet grouting works.

The TBMs then easily cut through these station walls allowing the tunnels to be constructed without delay. Crews can now dig out the station and install the necessary components. To minimize traffic disruptions, Metrolinx is borrowing an idea from Los Angeles' transit construction: once a sufficient depth is reached, road decking supported by steel girders will be installed allowing excavation to continue underneath traffic. The decking will be periodically removed to haul away muck and for equipment purposes. When complete, the 150 metre long stations will be serviced by multi-vehicle LRV trains produced by Bombardier in Thunder Bay, Ontario.

Road Decking to be used along Eglinton Avenue, image courtesy of Metrolinx

With Toronto's population rising, and increased strain on our transit system, the Eglinton Crosstown LRT can't come soon enough. To learn more about some of the Crosstown LRT Stations, check out our dataBase listings, linked below. Leave a comment in the space provided on this page, or join the discussion by clicking on the associated station Forum threads, or our main Crosstown LRT conversation here.