Behind the Project: Finch LRT
The Finch Light Rail Transit project in Toronto will provide an east-west, dedicated LRT line on Finch Avenue, starting at Humber College in the west end of the city and moving east until Keele Street, integrating there with Line 1 of the TTC (Finch West station). The 18-station, 11-kilometre line will expand transit options in the northwest segment of the city and is set for completion in 2023.
Entuitive is the Lead Support of Excavation Designer on this project for Keele station and Humber College station, as well as the Construction Engineering Consultant for the rapid bridge replacement at Highway 400 and Finch, and at the CP Rail interface between Emery/Weston and Signet stations. ARUP is the Permanent Structure Designer on this project, with Dufferin Construction serving as the Contractor.
We sat down with Stephen Brown, Principal, and Mohammad Mirmoezi, Associate, to learn more about the interesting challenges behind this project.
Thanks so much for sitting down with us, Stephen and Mohammad. Can you tell us a bit more about our work on this project?
Mohammad: Thank you for having us and absolutely. We have several distinct scopes of work on this project. First, we are supporting the excavations at Keele and Humber College, which are the two terminal stops for the new LRT.
Keele will be an entirely below-grade transit station that connects to the Finch West TTC subway station below it. And Humber College station will be an open-air station that is slightly depressed into the ground. It is technically a below-grade station, but with an open roof.
Stephen: The second scope of our work, as Construction Engineering consultants, involves a rapid bridge replacement at Highway 400 and Finch, and the CP Rail interface work between Weston Road and Signet Drive. At both these junctures, the highway and the CP Rail tracks cross Finch Avenue on a bridge (largely north to south).
These four projects all sound like very complex, challenging parts of the larger LRT project. Can you tell us about the technical challenges at each site?
Mohammad: Of course. At Humber College station, the main challenge was that the LRT shaft (which is depressed into the ground) would cross Humber College Boulevard and the Humber College access road. This meant that these two areas would have to be underground, or covered, so as not to disturb traffic. We solved this by employing top-down construction, meaning we poured the roof slab of the tunnels at those two locations first, then excavated the ground underneath it, connecting the roof slab to temporary piles to keep it in place during construction. This allowed the traffic flow to be resumed on top of the tunnel sections.
It also gave us the opportunity to integrate the support of excavation and permanent structures together to be able to send these sections back to traffic flow and reduce the thickness of permanent walls. In this case, the support of excavation is designed to withstand the pressure from the soil and weight of the roof slab, as well as the traffic above it, for the 100-year lifespan of the project.
Support of excavation.
Another challenge at this station included working around underground utilities. Especially at the Highway 27 and Finch intersection, we have tens of utilities crossing the excavated areas. Our team provided support there, coordinating the positions of our tiebacks so they wouldn’t interfere with the utilities. Further south at Humber College Boulevard and Highway 27, we have the same situation, where we had to support the utilities in place across the excavation to avoid disturbing the distribution of services in the area.
And how about our work at Keele station? What were the particular challenges there?
Stephen: At Keele station, the LRT, which will be underground at that point, crosses above the existing, underground TTC subway tunnel. At this site, we faced several challenges, including ensuring our work didn’t disrupt ongoing TTC operations as well as counteracting buoyancy forces.
Mohammad: To maintain the above-ground traffic flow at this busy intersection while constructing the LRT, we opted again for top-down construction. We also integrated the support of excavation structure with the permanent structure at the main station and entrance shafts. The SOE in these areas are designed to withstand the soil pressures as well as the weight of the roof slab, overburden soil, and traffic above for the 100-year lifespan of the project. Due to the limited space available for the roof slab of the permanent structure, the connection between the roof slab and SOE piles were designed as rigid connections to reduce the thickness of the structure roof slab. The SOE piles are designed to withstand these moments as well.
The second challenge here was that because of how deep the excavation is to build this underground LRT station and the high level of water in storm conditions, we were also dealing with buoyancy forces. What this means is that the station had the potential to float upwards during ultimate storm flooding conditions. Our SOE piles counteract this force by essentially pinning down the structure and preventing it from floating.
The third challenge here, similar to Humber College station, was the huge amount of utilities at this intersection, such as a gas main, storm sewer, water main, and telecoms. Our work ensured the support of these utilities in place across the excavation during construction.
Support of excavation traffic deck and utility support.
Finally, the method of construction for the LRT tunnel before entering the main station shaft, which is located at Finch and Keele, was bottom-up in this instance. Since this would of course massively disrupt traffic at that intersection, we had to place a temporary traffic deck over the open cut area at this intersection.
Wow, there was really no shortage of interesting challenges at these stations, each requiring carefully thought-out solutions and considerations. What about the rapid bridge replacement at Highway 400 and Finch, and the CP Rail interface?
Stephen: The rapid bridge replacement was necessary because of the extremely limited closures permitted for Highway 400. To achieve the replacement on time and with minimal disruption, we worked with the project team to provide the assembly yards, falsework, and movement plan for shifting the bridge structure. The bridge deck itself was built at a remote location, and we went in on the weekends to remove the existing portion of the highway, put in the falsework, and then move the new bridge deck into place using mobile transporters.
This required careful coordination and technical analysis, considering the highway at this location is on a significant vertical profile. Simply put, the vertical profile under the bridge was too steep to work the climbing jacks needed to raise and lower the bridge. The conventional solution was large amounts of fill to reduce the slope. We eliminated that slow procedure by sloping the ground in the yard to compensate for the road. We also controlled for deflections, jacking placement, geometry, controls, falseworks, safety of the structure when it was being moved, and the movement plan.
Over at the CP Rail interface, this is a section of Finch Avenue between Weston Road and Signet Drive where, like Highway 400, the rail tracks cross over top of Finch on a bridge. The LRT tracks will be laid on Finch under the CP bridge structure but the addition of the LRT eliminated space for pedestrians under the bridge and required a new multi-use path to be constructed through the railway embankment. The main challenge here was the limited availability for closure time for the CP tracks to accommodate this new active transportation crossing. The design involved a shoring system and temporary railway trestle bridge that could be installed in a series of very short closures, opening up a gap for construction of the permanent MUP culvert.
Thanks so much for talking to us about this interesting and technically challenging project with lots of moving parts. As a final thought, what was your favourite aspect of this project?
Mohammad: For me, I enjoyed the challenge of integrating our SOE works with the permanent structures. Not only is this type of work an interesting technical challenge, but it also requires deep collaboration with the entire engineering and construction teams, which I really enjoy.
Stephen: I would echo that. We’re also eager to take on technically challenging projects that require deep collaboration with all project stakeholders.
Thank you so much for you time, Mohammad and Stephen. If you’d like to learn more about this project, reach out to Mohammad Mirmoezi or Stephen Brown.