Turning an abandoned airport into a new residential community is a novel idea. But it requires a lot of work, and plenty of new infrastructure.
For 95 years, Downsview Airport was a fixture in north-central Toronto. The airport opened in 1929, when the de Havilland Aircraft Company of Canada set up a production facility and airfield on the site. Downsview closed in April 2024 when de Havilland’s successor company, Bombardier Aerospace, moved to a state-of-the-art manufacturing facility at Toronto Pearson International Airport in Mississauga, Ontario.
With Bombardier gone and Downsview’s midcity open land too valuable a location for developers and city planners to resist, the 2.1-square-mile site is being redeveloped as a multipurpose community with 115,000 new residents and 52,000 workers expected to move in over the next 30 years.
To cope with the area’s growth, the city of Toronto is conducting a substantial upgrade of the infrastructure.
The Black Creek Sanitary Trunk Sewer Relief System is at the heart of this upgrade. It’s a massive underground infrastructure project designed to solve three issues in the Downsview and adjacent Rockcliffe areas.
“The primary objective of this project and this trunk sewer is to reduce surcharge levels into the existing Black Creek sanitary trunk sewer, which contributes to both surface and basement flooding during extreme weather events,” says Simon Hopton, director of design and construction, major infrastructure for the city of Toronto. “It’s also to reduce combined sewer overflow discharges to the Black Creek water course. And thirdly, it is to accommodate sanitary flows for the projected 2041 population and employment growth and beyond.”
Three major components
The Black Creek project is divided into three major components, all of which will use large tunnel boring machines and microtunneling equipment.
The largest component is the Keele Relief Sanitary Trunk Sewer, which is an 8.1-mile, 9.8-foot-diameter sewer pipe running along Keele Street. Its depth will range from 131 to 230 feet deep.
“We are going to dig this sewer using two TBMs, one from the northernmost shaft and one from the intermediate shafts moving towards the southern end,” says Ajay Puri, the city of Toronto’s manager for major tunnels, design and construction. “Each TBM is going to mine approximately 3.7 to 4.3 miles.”
“The sewer’s tunnel will be constructed using precast segments, and those are being installed immediately behind the TBM,” Hopton adds. “We have also sunk a large-diameter shaft. This is used both at the start to lower the tunnel boring machine to commence the mining and to remove the tunnel spoil as the TBM advances.”
The second component is the Jane Relief Sanitary Trunk Sewer. This is a 71-inch trunk sewer being constructed via microtunneling. The third element is the Rockcliffe CSO Storage Trunk Sewer. This is a microtunneled system with two legs (North-South and East-West pipes, both 71 inches in diameter) specifically designed to target local flooding.
The Black Creek Sanitary Trunk Sewer Relief System is designed to integrate the Downsview redevelopment seamlessly into the city’s existing wastewater and stormwater infrastructure. This is being done in line with the Downsview Master Environmental Servicing Plan.
In doing so, the relief system will ease the strain on the city’s existing water treatment infrastructure. The new sewer is designed to store stormwater during extreme weather events to prevent downstream infrastructure such as the Humber Sanitary Trunk Sewer and treatment plant from being overwhelmed. Once the storm surge passes, the relief system will release its stored water in a controlled manner; this prevents a large surge of water washing away beneficial bacteria at the Humber treatment plant.
A difficult dig
Without a doubt, the 8.1-mile Keele Relief Sanitary Trunk Sewer is the most difficult part of the project. The two TBMs have to cut through hard shale bedrock and mixed ground to reach their targets, each of which presents different tunnelling challenges.
“We are doing an extensive geotechnical borehole program to map out this route, but there are always unexpected anomalies that you can potentially face underground such as groundwater seams and changes in ground conditions,” Hopton says. “That’s the biggest challenge when you’re dealing with mining at that depth. We are also going to intercept a number of combined sewer overflows as we add in the new pipes. Whenever you are intercepting and connecting a significant infrastructure to an existing system, that requires a balanced, delicate and comprehensive plan to ensure that the work does not present any problems during construction.”
“The most fascinating part of this project is the extensive work going on underground,” Puri says. “The public does not see much going on, but if you go to the shafts and see how much material we have coming out of the ground and how it’s coming out using the mud train locomotives and the muck train — that’s a very fascinating part.”
Minimizing impact
Despite the massive scale of the Black Creek Relief System, the project has been designed to leave the aboveground area relatively untouched, to preserve traffic flow and public access to local amenities like Downsview Park. For instance, the shafts are located outside of municipal rights of way and travel lanes to eliminate road closures. They are being protected from intruders using heavy concrete and 10-foot wooden hoardings.
“We were able to find some good properties where we could sink the shafts without having a major impact on the community,” Puri says. “The construction compound is planned in such a way that there is no interruption to the park users and the trails will stay open.”
Ahead of development
Mindful that the developers want to start work on the Downsview site as soon as possible, the city of Toronto is doing what it can to move the Black Creek Relief System along. It has budgeted CA$800 million (about 587 million U.S. dollars) to pay for the project, and is hoping to tender it in mid-2026. If all goes to plan, the Black Creek project contract will be awarded in late 2026 or early in 2027. “We then expect construction to commence shortly after that in the beginning of 2027, with final completion in 2031,” Hopton says.
All told, the Black Creek Sanitary Trunk Sewer Relief System is one of the most ambitious sewer projects that the city of Toronto has ever executed. But then again, turning an airport into a multipurpose land development isn’t something that happens every day in any city.
It is a monumental aboveground transformation, made possible only by a masterpiece in underground engineering.
Continue reading for free
