Underground Eglinton Crosstown Lrt Stations Take Shape with Unique Mining Technique

In fact, it's the future site of Laird station, one of 25 planned stops on the Eglinton Crosstown LRT.

Laird will be one of 15 underground stops on the Crosstown, the $5.3-billion provincially-funded light rail project that will stretch 19 kilometres across Toronto's midtown, and it's one of only three stations that are being built using a unique mining process called the sequential excavation method (SEM).

The station is still years away from completion, but progress is already apparent.

"It's always exciting. When I came here, it was just a parking lot, nothing else. Every time you're seeing it at each stage, it's changing, and it's impressive," said Aleix Giralt, Laird project manager for Crosslinx, the consortium hired to build the LRT.

Underground stations are more typically built using the "cut-and-cover" method, which involves digging up a large area, constructing the station underground and then covering it back up.

With SEM, only a small surface area is excavated to create a shaft down to the station level, and then the station site is hollowed out from below.

The method was chosen because at 489 metres, Laird will be a particularly long station that will accommodate not only passenger platforms, but also a track crossover to allow trains to reverse direction, and a pocket track to store vehicles.

Cut-and-cover would have required digging up nearly 500 metres of Eglinton Ave., which would have been even more disruptive than the Crosstown construction has already been.

SEM is more common in Europe, but according Crosslinx, it has never been attempted in Toronto before.

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Once in the tunnels, workers gradually expand them outward in order to create a space big enough to build the station. It's a complex process because workers must constantly stabilize the tunnels as they enlarge them in order to prevent a cave-in.

First, the crews use a massive machine called a "jumbo" to drill 18-metre long pipes horizontally into the clay. Spaced close together, they create a "pipe roof" that supports the ground above.

Then, using an excavator they dig out the material below the pipe roof, including demolishing the tunnel lining previously installed by the tunnel boring machines.

Lastly, the surface of the newly enlarged tunnel is sprayed with special concrete known as shotcrete. The shotcrete solidifies so fast it's safe to work under within about 10 minutes.

The workers repeat those three steps at every section of the tunnel, excavating about 1.5 metres a day.

Eventually, the twin tunnels will be expanded until they merge to create a single open space. But in the current phase, they have been expanded upward into soaring galleries with vaulted arches. The arches are the best shape to support the weight of the ground above, but the effect is to make the site appear like some kind of subterranean church.

The SEM process is expected to be completed by July 2019, and the Crosstown is set to open by September 2021. To meet those timelines, two crews of ten people each work around the clock, seven days a week.

"You try to keep the ball rolling," said Steven Dubeau, site superintendent for the project. "This never stops."