Work moves ahead – and deep under – to prepare for construction of Scarborough Subway Extension

Before you really dig into a project like the Scarborough Subway Extension, there’s lots of things to understand and do. Here’s how that important advance work is unfolding, and how crews have been digging in to make sure communities avoid disruptions down the line.

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.

Image shows a map.
Scarborough Subway Extension project map. (Metrolinx image)

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.

Image shows crews working beside a road.
Telecommunication services being relocated at Eglinton Avenue East and Danforth Road. (Metrolinx photo)

“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.

A tractor digs a hole.
Utility relocation at Midland Avenue and Eglinton Avenue East. (Metrolinx photo)

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.

Crews work next to a road.
Geotechnical investigation at McCowan Road and Meldazy Drive. (Metrolinx photo)

Collecting samples from different depths provides a detailed understanding of what is just below the surface to several feet below the proposed tunnel. The depth of water found below where the subway tunnel will run, called groundwater, is particularly important to engineers.

“The information about the type of soil and the groundwater table are critical to design the structure of the tunnel, as well as to identify the right amount of pressure that must be applied by the tunnel boring machine to tunnel below the surface,”said Lionzo. “So these investigations we’re carrying out help us account for unique site conditions along the project route.”

Specifically, the investigations are critical in determining how deep subway tunnels should be built, the type of materials needed to support the tunnels, and any innovative design solutions that may be needed to lower the pressure of the groundwater depending on how close it is to the proposed tunnel.

The majority of boreholes for the Scarborough Subway Extension are located directly in line with the project’s route. A number of them have also been drilled on neighbouring properties, but that doesn’t necessarily mean that Metrolinx will be building a tunnel or station there. Samples are collected from these areas to learn about how stable the surrounding soil is for tunnel construction and to learn about any potential contaminants in the surrounding soil so they aren’t released and are safely managed during construction.

Crews have also taken advantage of reduced traffic volumes this year to speed up this work.

“With this increased productivity, we’ve been able to shorten timelines for our investigative activities and this has reduced the overall duration of impacts to the community,” said Lionzo.

Metrolinx recognizes that a convenient, reliable rapid transit system is important for the Scarborough community. Even though it’s been a challenging year, a lot of preparatory work has been completed and continues to be planned to keep the Scarborough Subway Extension moving forward.

As this much-anticipated transit expansion gains even more momentum going into 2021, when the tunnelling contract will be awarded, Metrolinx is hosting an online engagement to update the community about the progress being made and what to expect when tunnelling does get underway. Visit here to take part in the discussion and help move this important project forward.

Story by Joshua Patel, Metrolinx senior advisor, Subways.