You might have noticed that teams have fine-tuned the tunneling approach for the Scarborough Subway Extension by going from two tunnel launch shafts to one. Here’s a behind-the-scenes look at what went into that decision, and why it’s better for the community.
Planning for a large transit project means having an eye for a million or so details.
And being able to accommodate for everything – including the workings of a local hospital.
There’s been great progress – and thought – given to the design and construction methods behind the Scarborough Subway Extension (SSE). The details are leading to innovative solutions for the 7.8 km route, which will extend the TTC’s Line 2 from Kennedy Station to Sheppard Avenue and McCowan Road.
It’s been more than a year since Metrolinx released the preliminary design business case for the extension, which is expected to attract approximately 105,000 daily riders. While procurement for the project marches on, planners and engineers at Metrolinx continue to refine countless details.
Mark Ciavarro, who leads Metrolinx’s Scarborough Subway Extension team, is on a mission to build a world-class subway extension and is committed to building it in the best way possible, with the least disruption to the community.
“It’s really exciting,” said Ciavarro, the project director. “I’ve been at Metrolinx for over a decade and every role I’ve been in has touched upon finding a solution to bring more reliable and convenient rapid transit to Scarborough.”
The initial construction proposal from Metrolinx planners and engineers included launch shafts at the northern end (Sheppard and McCowan) and southern end (Midland and Eglinton) of the route to start the work of two tunnel boring machines into the ground.
The machines – often called ‘TBMs’ – are powerful construction diggers. They would move toward each other from either end and would then be extracted from a shaft that would be built at Lawrence Avenue East and McCowan Road, which is also the site of a future station. Since tunnelling and stations are being delivered in two different contracts and phases for the extension, planners proposed the launch shafts and extraction shafts be constructed on sites of future stations, wherever possible. This would help reduce construction footprints and property impacts for the community.
As planners and engineers began gathering more data from across the corridor over the past year, including surveys, geotechnical investigations and identifying utility lines, they were also trying to improve construction solutions.
For planners, every solution creates more possibilities. But they also look for things that could become hurdles for both builders and the community.
Scarborough Health Network’s General Hospital
The intersection at Lawrence Avenue East and McCowan Road, where the extraction shaft was proposed, is also home to Scarborough Health Network’s General Hospital, an important healthcare facility in the area. A new subway station near the hospital will significantly improve access to both healthcare and employment opportunities at the hospital. This area was initially chosen as the location for the launch shaft because it is in the middle of the route. But that didn’t sit well with planners.
“We were concerned about the impacts and disruptions that the construction and operations of the extraction shaft may have on the hospital, especially at a time when healthcare services are an even bigger priority to the community,” said Ciavarro.
“We began working closely with the hospital on our plans and we also started looking for different solutions to reduce impacts.”
The data collected from surveys and geotechnical investigations at the intersection also made Metrolinx’s planning team aware that the excavation depth required for this extraction shaft would have to be 40 metres deep, the height of an average nine-storey building. This meant that that the excavation work itself would take quite a long time and would be more disruptive to the surrounding area.
At the same time, Metrolinx was also thinking about how to reduce impacts at Midland and Eglinton, an area that is very familiar with major construction and disruption as Crosstown’s Kennedy Station is being built nearby. Apart from construction starting earlier than other parts of the extension, a launch shaft in this area would also mean many impacts to the local community– roads would have to be reconfigured because of the size of the launch shaft, many utilities would have to be relocated and there would be a lot of noise from trucks delivering materials for the tunnel boring machines and launch shafts. Truck traffic is not a major concern for the launch shaft at Sheppard and McCowan because it is located near the highway, reducing impacts to local roads.
“We were really looking at every way possible to reduce our construction footprint in the Midland and Eglinton area to provide some relief to the community,” said Ciavarro.
As procurement continued on the advance tunnel contract and teams thought more and more about how work could be carried out, finding a solution to the issues at Lawrence and McCowan and Midland and Eglinton stayed top of mind. Eventually, teams landed on a solution to tunnel using a single tunnel boring machine, requiring only one launch shaft and preserving the project delivery date of 2029-30.
“Metrolinx and Infrastructure Ontario have been refining the procurement process to better address feedback and ideas we’ve been getting from our bidders,” said Ciavarro. “We were very pleased to find a solution that would reduce impacts to the community and still deliver the project on time and on budget.”
The refined construction plan now includes one launch shaft at Sheppard and McCowan and one extraction shaft at Midland and Eglinton. This means that one tunnel boring machine will now begin tunnelling at Sheppard and McCowan and make its way towards Eglinton and Midland, where it will be extracted. One of the reasons this approach doesn’t affect the project’s timeline is because the deep excavations needed at the Lawrence and McCowan would have taken a lot of time, meaning that the second tunnel boring machine wouldn’t have been able to start tunnelling until much later in the project schedule.
Moving the extraction shaft from the hospital to Midland and Eglinton means there will be fewer disruptions in this important area at a very critical time, with construction for the station slated to begin in 2024.
This new plan also reduces community impacts to the Midland and Eglinton area. Extraction shafts require much smaller construction areas than launch shafts because launch shafts require a lot more space for the tunnel boring machine to be assembled and to store materials that will be used to build the tunnels. The extraction shaft also does not need to be ready until tunnelling has started from the launch shaft so this means that construction of the extraction shaft can start later. There will also be less truck activity because less materials need to be brought into the site, which means less noise and congestion in the community overall.
“Refining these construction designs for the project is a solid reflection of how we’re working really closely with the people who are going to be delivering these projects to deliver it in the best way possible,” said Ciavarro.
As construction inches even closer, Metrolinx will reach out to the community to share details on what to expect and how they will be supported throughout construction impacts. As always, the latest project information and updates can be found here, where you can also sign up to receive email updates from the project.
Story by Joshua Patel, Metrolinx senior advisor, Subways.