Running 19km east to west - including a 10km underground stretch - the Crosstown line aims to massively improve Toronto's public transportation system.

The line is set to feature around 25 stations, the largest and busiest of which will be Eglinton interchange. It's here that the Ontario office - formerly Geo-Foundations and now Keller - is constructing a temporary excavation support shoring wall, enabling the construction of a new subway station.

The estimated 2.5-year design-and-build project was awarded by Crosslinx Transit Solutions (CTS - a partnership of four major general contractors) in 2016. The Keller team started on site in April 2017.

"As one of Canada's busiest intersections, this project required a well-thought-out design," said project manager Sean Morrisroe. "We're providing excavation support adjacent to and directly below an existing subway line [the north-south Yonge line], which is remaining in operation while we're on site."

The surface works include a 30m-deep secant pile wall, soldier piles and lagging, 40m-deep structural caissons, and jet grouting where utilities intersect the secant pile wall. It also includes the construction of a traffic deck to maintain vehicle flow throughout the project lifecycle.

The project is presenting numerous challenges. Firstly, the 300m-long site, so close to a live subway, heavy road traffic, pedestrians, utilities and overhead power lines, has required careful safety and logistical planning.

Secondly, movement criteria for the existing subway during the works is extremely stringent: less than 2mm. Keller has involved the country's premier shoring engineering firm to carry out the support of excavation design, and has invested in cutting-edge deviation control monitoring devices for each of the drill rigs, as well as surveying equipment to achieve the tight installation tolerances. This has enabled the piles to be placed with pinpoint accuracy, speeding up the subsequent deck construction.

Once the surface works are completed on both sides of the road, deep excavation will begin. This subsurface work includes lateral bracing and soil anchors adjacent to the existing subway line, in addition to low-headroom vertical and inclined jet grouting, shotcrete and soil nails directly below.