Even before construction really begins on the Scarborough Subway Extension, there’s a lot going on far below the surface.

Teams at Metrolinx and Infrastructure Ontario have taken strides this year in moving the three-stop Scarborough Subway Extension closer to construction. Once completed, it will extend the TTC’s Line 2 subway service approximately 7.8 kilometres further into Scarborough, providing seamless, reliable transit to key areas in Toronto’s eastern hub.

After a request for proposals was issued in the summer – for how tunneling work could be carried out for the project – three shortlisted teams have been preparing their bids to be awarded the contract. But before a successful bidder is chosen and tunneling begins, soil is already turning.

Metrolinx has been busy over the past year preparing the corridor for construction and getting a strong understanding of what’s beneath the surface. This has involved completing over a hundred geotechnical investigations and relocating important utilities along the project route to make way for the new subway extension.

Relocating utilities like phone, internet, hydro and natural gas services while procurement is underway clears the path for tunnel construction and minimizes service disruptions to customers during construction.

“We’ve already completed some utility relocations along the Scarborough Subway Extension corridor in 2020, with more being planned going into 2021,” said Alessandra Lionzo, Metrolinx’s technical director for the Scarborough Subway Extension. “We are completing these utility relocations in advance so that construction can begin soon after the tunnel contract is awarded next year.”

Utility relocations involve a lot of work. If the utility is buried, crews need to excavate the area; if the utility is on the surface or overhead like a hydro pole, crews may need to move or replace the structure. Specialized crews for each utility are brought in to move the utility, all while trying to maintain active services for their customers and not disturb the many other cables and pipes that share the space. If a utility service needs to be temporarily disconnected while the relocation is being completed, customers are notified in advance by their utility provider and every effort is made to accelerate work and minimize the disruption.

While crews continue to relocate utilities across the corridor, even more work is happening below the surface in the form of geotechnical investigations. When you conduct a ‘geotechnical investigation,’ that’s a technical way of saying you’re collecting samples of soil and rock by drilling a hole into the ground, called a borehole, to learn more about the geology of an area. These insights help planners and engineers anticipate site conditions and make informed decisions on the design of tunnels, structures and different methods that can be used to deliver the best project for the community.

Information from boreholes is required every 50 to 100 metres along the tunnel route, and from proposed sites such as stations and emergency exit buildings. Metrolinx has drilled over a hundred of these boreholes along the project route since last year. All these samples help planners and engineers better understand what they need to effectively tunnel from one end of an alignment to the next, and to determine the type of soil and structures that may be potentially impacted by construction. Figuring out these details before a team is selected to deliver the project is another way of ensuring the bidder awarded the tunnel contract has the information they need to begin construction as soon as possible.

Borehole samples for the project have been collected at depths ranging from 10 to 60 metres deep, which is about as deep as an average 20-storey building is high. So if you’ve noticed any trucks with tall drilling rigs as you’ve been travelling around your community, now you know what they’re up to – or rather down to.