Behind the Project: College Station
College Station is an existing subway station in downtown Toronto, located at the intersection of Yonge Street and College Street. It’s a high-traffic subway station undergoing major renovations in a bustling area. Recently, we sat down with Senior Engineer Nicholas J. Greven and Senior Associate Chongsong Yu to learn more about this exciting project.
Entuitive is providing Structural Engineering services and Stantec is the Prime Consultant on the project.
Thanks for sitting down with us today, Nick and Chongsong. Tell us a bit about this project.
Nick: Thanks for having us. This project involves making College Subway Station more accessible by adding two elevators to its station platforms. For reference, College Street runs east to west, crossing Yonge Street, which runs north to south. It’s a very busy intersection of the city, and the station doesn’t currently have any elevators. The work is part of the TTC’s Easier Access Program to make its stations more accessible to all.
The Yonge and College area in Toronto is very dense, so in order to accommodate the two elevators, a new concourse has to be built below street level on top of the existing TTC subway box.
With such space constraints, there must have been more than a few challenges. Can you tell us about some of the biggest challenges and how you solved them?
Existing Building Conditions
Chongsong: Of course. One of the main challenges was the condition of the existing buildings at this site, since the east and west concourses are being built underneath bustling commercial and retail buildings.
On the west side of Yonge Street at College, the buildings are almost 100 years old. As such, their conditions were challenging to understand, and it took us some time to determine the best approach for building the underground concourse. Where can we put openings? Where can we put struts to support the excavation? How can we minimize the impact on the existing buildings and businesses? These were all questions we had to contend with, and answer.
Another related challenge involved ensuring construction could accommodate the existing below-ground utilities, such as sewers and water mains.
Nick: As an example, the building on the west side of Yonge Street at College is called College Park, a famous building in downtown Toronto. It’s almost 100 years old! And part of the new subway concourse will be accessible from the lower level of College Park. So, we had to connect the new concourse that’s being built to the existing lower level of this building. Challenges here included breaking open the existing College Park structure, strengthening it, and then breaking open the existing TTC structure to provide access to the existing platform. Strengthening those openings and ensuring the existing structures are not overstressed during demolition is paramount since concrete and rebar strengths for these older structures were not what they are today.
At College Park, we specified temporary shoring posts with jacking forces to transfer the floor loads away from the wall being demolished. We then specified a steel beam within the opening with its own jacking forces to ensure proper load distribution.
For the new openings into the existing TTC structure, we went with a reinforced concrete approach and specified dowelling into the structure and created a lintel on top of the existing roof with rebar ties to pick up the tension above the opening. The goal was for the existing structure to behave as closely as possible to its originally intended design, and I think our solution achieves just that.
Nick: Another challenge is the excavation under street level to allow for construction of the concourse. As a reminder, the concourse will run east to west under College Street, crossing Yonge Street. So, we have to excavate underneath that intersection to make room for the concourse. This creates two challenges. The first was that it is critical not to shut down the road since it’s such a busy intersection. The second is ensuring the excavation process doesn’t cause unbalanced loading on the TTC structure.
To solve the first challenge, we designed the excavation to be done in a systematic, staged way, ensuring minimal impact to the road and businesses on either side.
Solving the second challenge tied in with the first. Planning the excavation in a staged way allows us to ensure the TTC structure remains balanced.
Can you elaborate on this a bit? What does it mean for the TTC structure to have to remain balanced? Aren’t we excavating above it?
Nick: Of course. So, the existing TTC subway, the tunnel itself through which the trains travel, are five metres below grade. The concourse will sit on top of this box structure, but still underground, hence the need to excavate those five metres. The challenge is that since we are excavating in stages to minimize traffic disruption, we would be excavating one side of the box structure first. But if you do that, you create an unbalanced load on top of it, whereby one side of the structure has a bunch of soil above it, and the other side does not. This would create an unbalanced load because the structure is meant to share the load of the earth above it.
It sounds counterintuitive because we are removing earth, not adding more onto one side. But even removing or excavating one side can be detrimental to the structure as a whole.
Think of it like when you have several glasses on a tray in one hand. If you’ve ever seen waitstaff serving many drinks, you’ll see they take each glass off the tray slowly and carefully. This is because their hand is continually balancing the tray and adjusting to the load of the drinks. If you take a glass off the tray too quickly, it will topple over because the hand holding the tray suddenly has a new situation, a new load, that it can’t adapt to quickly enough. It’s similar with this project if you think of the ground underneath the TTC box structure as the hand, the box structure as the tray, and the soil on top as the glasses.
To solve this, we designed a clever sequence of excavation in a top-down approach that combined with the construction of the concourse. So, working in sections, we will build a portion of the roof slab of the concourse on top of the soil to be excavated, and then afterwards excavate the soil under that slab. This proceeds repeatedly in stages across College Street.
Further, upon running several models and staging analyses, we realized we can actually excavate a certain amount of earth to maintain most of the balanced loading on the box structure, which fed into our very controlled, staged excavation.
This controlled sequence of excavation is also paramount in ensuring that the businesses on either side of Yonge are not overly impacted by this work.
Collaborating with Stakeholders
Chongsong: That was another challenge. Though I like to see it as part of the collaboration required for these kinds of projects. Of course, we wanted to ensure that the businesses on either side of Yonge Street were not negatively impacted by this construction work. So, we ensured that our staging process and sequence of excavation/construction would allow these store fronts to remain open and not block access to them, or only block access for a very limited amount of time.
Nick: We also specified temporary decking to act as a sidewalk during the construction, and any necessary works are completed overnight so that stores can remain open during the day. That was all part of our staging plan.
Amazing! Sounds like with very careful coordination, collaboration, and creativity you were able to solve each challenge in stride. Can you tell us what your favourite part of the project was?
Nick: The technical challenge! This project involved many moving parts, with challenges every step of the way. But that really was my favourite part! I felt like my engineering career was leading to this point. Evaluating, strengthening, and redesigning the existing structures were all complicated tasks. All these challenges, combined with the complex staging and sequence of construction, made for a very challenging but also very rewarding project.
Chongsong: This was a multidisciplinary project including many different stakeholders, such as the TTC, City of Toronto, businesses, and store owners, and it took a long time to get it right. I’m very proud of all the deep collaboration that took place.
Anything else you’d like to add?
Nick: One final challenge was the sewer! There is an existing 1,200-diameter sewer on the west side adjacent to College Park and the new concourse will be built right above it. So, we had to design the new concourse’s foundation in a way that will not impose any loads on this critical piece of infrastructure. Like I said, every step of the way, there was another challenge to solve. But we thrive on solving complex challenges!
Chongsong: Our main goal at the end of the day is to deliver uncompromising performance to our clients, partners, and stakeholders. We do this by deep collaboration and technical problem solving, and by looking at every challenge as an opportunity.