The success of Toronto's upcoming Crosstown LRT line will be wholly dependent on the success of the various builders that are participating in the project.
One of those is Entuitive, a consulting engineering practice with, it says, "a vision of bringing together engineering and intuition to enhance building performance." Entuitive is completing the structural design of three underground Crosstown stations. Its design scope includes below-grade stations, pedestrian and ventilation tunnels and surface structures for access, substations, and ventilation equipment. Tunnel interfacing and excavation support to protect structures and utilities were also key design requirements.
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Recently, UrbanToronto had the opportunity to talk to three members of the Entuitive team to discuss their work on two of those stations—Fairbank (at Dufferin Street) and Oakwood. The company has also worked on engineering and designing several other stations on the line. In an earlier article, the team discussed its work at Eglinton and Cedarvale stations.
Building both stations relied on Entuitive’s advanced engineering knowledge and skill—and on construction methods that are unconventional in Toronto: "top-down" construction at Fairbank, and sequential excavation method (SEM) construction or "mining" at Oakwood. They also highlighted the need for in-depth co-operation and collaboration with other consultants.
Mike Meschino, a principal at Entuitive, described the usual process. "Typically, when building stations, you set up temporary decking to maintain the traffic flow on the roadway, then you dig down to the depth of the station structure and you build it from the bottom and work your way back up to the top.
"You have this temporary decking and road detours happening for the whole duration of the station construction. And, when that happens you end up disturbing the people, the traffic, the businesses that are adjacent to all this. So, there's a fair bit of pain for the people that are in the area where this is taking place, but that's the typical way that this is being done."
He then discussed the specifics of the top-down construction method for Fairbank Station.
"At certain locations, it's sometimes advantageous to look at top-down construction. What do we mean by that? We put in shoring along the perimeter and we dig down just enough to put in the roof slab of the structure. And we build the permanent roof slab to sit on the shoring system along the perimeter. And, when that's done, we can reinstate much of the surface—the road and the sidewalks—while we're continuing to dig down to the bottom level of the eventual permanent station structure. What that allows us to do is reinstate the road surface faster, so there's less overall duration of the disruption to the community while we're building the station, so that's the primary advantage of that top-down construction.
"Basically, you dig down just enough to put the roof in, support the roof on the shoring system, and then you backfill up and reinstate the road surface while you continue to dig down below… and that construction happens underneath the road system that is now reinstated, with less disruption."
"Top-down construction has been used around the world”, he continues. “However, it hasn't really been used here. It's not only the roof slab that’s supported by the excavation support system. As you dig down, the lower-level floors are also connected to the excavation support system... So, you would dig down below the roof until you got to the concourse-level slab, you would build that slab, also on the ground at that level and also connect it to the shoring system. And then you'd dig below that until you got to the invert slab, pour that on the ground but connect it again into the shoring system."
Mike added that when Entuitive originally bid on the project, their bid design was based on traditional "bottom-up" construction and they had completed 30 per cent of the station design, using that method of construction.
"But, in that initial design period, the construction group was concerned about managing some of the risks associated with utilities and business and detours in these areas, and so they asked us to take part in a task group—which we were one of the key components of. We were asked by Crosslinx to explore whether we could use top-down construction at a number of these stations and, if so, how would we do it? What would the system look like? And, what were the complications that were involved in that? And so we spent about a month and a half with this task group, exploring different ways to get it done, analyzing how it could work, and what kind of details we would need to make it happen.
"We found that it was feasible to do top-down at Fairbank and we developed a system to do it and once we had done that, the focus-group team presented it to the rest of the structural engineering consultants on the Crosstown project, that were getting involved in designing their stations using that top-down approach that we were developing. And then we went away and did our station like that."
As a result of that consultation, other engineering firms were able to design three other Crosstown stations using the top-down method.
Jonathan Ho, who managed much of the Entuitive work at the station told us about some of the finer details of the construction. "At Fairbank we have entrances and a service building that's off the roadway. And those are built with conventional construction. They started excavating those areas first. Then they shifted to construction of the roof slab for the top-down construction. Construction of the entire roof slab took about a year and a half—from mid-2018 until the end of 2019.
Jonathan continued by explaining some of the challenges of building the station top-down.
"Because we were restricted as to how much of the roadway of Eglinton could be closed, the roof slab was actually constructed in six segments along the length of Eglinton. Obviously, we didn't close the entire roadway and traffic needed to be maintained on Eglinton... We would excavate down, install the SOE [support of excavation – or shoring], construct part of the roof slab, then we would do all the waterproofing that was necessary for the long-term durability and protection of the roof. And then we would backfill as soon as the roof was completed.
Mike added, "What you're really talking about is digging down through the earth and you get to the level of the underside of the roof slab – you actually pour the roof slab on the ground, so there's no form work there. Once the roof is complete, they start to excavate underneath, take away that dirt and start to build within the cavity."
Jonathan explained, "So, if we're cutting up the structure into pieces… there's a lot of co-ordination that's required with the contractor in determining which pieces they want to pour, where they want to cut up the structure and how we need to take into account all these additional joints that they're putting into the structure. So, they're constructing the station in smaller pieces—if you were doing a conventional construction, you'd be pouring much larger sections of the slab at one time—the loads on the structure will actually vary, and you have different loading scenarios you have to consider. Those were some of the major complexities… the staging of the structure and how we could assist the contractor in finding the optimal way of constructing it. Their goal was to minimize impact to the roadway, minimize the effect on traffic, while being able to accelerate construction as much as possible.”
He continued, "In terms of the analysis of the design of the station, when you construct top-down, because the roof slab is initially sitting on the ground, and you backfill over it, the roof slab is essentially carrying no load until you begin to excavate for the station-box construction and it goes from a scenario when it's in ground with no load to essentially having to carry the full load of the roadway above. It's about an 18- or 19-meter clear span from the support of excavation wall to the other support of excavation wall. It's pretty impressive when you get to go down there and you see that we've built a giant cavern without any of the portions of the station yet."
According to Chongsong Yu, a Senior Associate at the firm, a unique thing about the project, is that the design incorporated the SOE system—usually a temporary element of a project—as a permanent load-bearing part of the station wall.
Jonathan clarified further, "In terms of design there are some differences between designing a temporary structure and a permanent one. Because the SOE wall has now become a permanent structure, designed to support the load of the roof slab, we had to do extensive co-ordination with the SOE designer to ensure that they're following the requirements of the design for a permanent structure.
Mike explained, "Picture a cross-section of the station. You have the roof slab spanning all the way across onto the excavation support system at the perimeter. We've dug down actually below that and we've started to build the bottom slab of the station—the invert slab. And we're also building walls inside the excavation support-system walls. You've got the excavation support system, then there's a layer of waterproofing and then you have perimeter station wall, as well. So the load from the earth and the water on the outside are being supported by a combination of the excavation support system and the permanent wall that we're building inside… We've designed the perimeter slabs and walls and the contractor's shoring engineer was designing the excavation support system. We had to co-ordinate the two designs. That's because, as you move down—as you're excavating—and as you move up—building the new station—the forces on those walls change. And Jonathan and his group really had to capture the changes in forces as you were going through those different stages."
Jonathan talked about some of the other challenges of the Fairbank Station project. "Seeing that we're going with top-down construction, we would have a bit of a thicker roof slab. The contractors wanted us to put a lot of the electrical conduits embedded in the roof slab. That required a certain amount of co-ordination with the electrical engineer in verifying the amount of conduit and analyzing how all these conduits being put in the roof slab impacts the strength of the roof slab.
"Another interesting challenge is the main entrance of the structure. Because we had to provide access from the main station box to the entrance, we actually have ten- to 12-metre openings in the SOE wall. One opening is for user access to get from the station to the concourse level and to access the platform to get on the light rail vehicles, and the other opening is for mechanical and vent-shaft space. As you can imagine, we're building this roof slab and near the entrance there's these large gaps where we don't have SOE support structure. We had to design these very large and deep beams to be able to support all that soil load and all that roof structure over these openings.
"Because the entrance structures and the service building are off the roadway, they're built with the conventional bottom-up construction method. What that results in is that you have very different settling behaviours between the bottom-up and the top-down structures. At the interface of the two structures, you have different settling behaviours, there's actually very large forces when you connect the structures together and they try to settle, they move differently. There's a lot of detailing required to make sure that the joint structure there can accept the load and can transfer the load between the two structures.
As for Oakwood, Mike told us, "The driving factor… in changing it from a conventional bottom-up to a mined station was a major sewer line running along Eglinton Avenue that's fairly deep and big and fairly sensitive because of the age in which it was constructed. In order to try and not touch that, the whole idea was to do this mined system, where we would get down below that. The station is actually located below that sewer. So we'd get down there and start mining our way in and mining along underneath until we reached the other end of the station. "
In that station, Entuitive's primary role was designing the cut-and-cover entrance and exit structure. Mike elaborated that this was "basically the main and secondary entrance structures that include the fan rooms, and all of the escalators and the elevators—all the equipment to basically access down to the platform level. So you picture those entrance buildings—those are built off the roadway—where the buildings are along Eglinton Avenue. Picture a big box that you're designing to house all of that equipment and it also initially forms the hole in which the excavation equipment is dropped down and allows it then to mine its way in."
But, similar to the Fairbank project, building components of the station with two different methods presented its own complexities, Mike said.
"What happens is that you dig in and remove soil away from one side of this entrance box, so you have full earth pressure and water pressure on one side of this box and on the other side you have this big hole that you've just dug for the station. So, there's a tendency for that entrance box to slide laterally toward the station structure – and what it causes is… a transfer of loads or interaction between this cut-and-cover entrance structure and the mined station structure. There's a fair amount of collaboration between ourselves and the consultant that was designing the mined part of the station because we had to transfer forces between the two. One part of the structure relied on the other part to transfer those horizontal forces across the joints. There was concern about differential settlements—one part of the structure would settle differently than the other part, because they're built at different times and they're carrying different forces. So, how you deal with those differential settlements across that interface was also pretty critical.
"Then, of course, because we're in the ground and the water levels along Eglinton are fairly high, there's always this concern about leakage and having to somehow keep the water out of the station, and that meant—how do you deal with these joints from a waterproofing perspective? That was complicated at Oakwood, particularly, because we have this interface between a cut-and-cover box and a mined station, so those two types of construction have different demands. The waterproofing interface details between the mined station and cut and cover sections were tricky and required a lot of thought.”
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We will continue our conversation with the Entuitive team in an upcoming article, where we discuss the firm's role in transit-oriented development projects.
You can learn more from our Database files for the project, linked below. If you'd like to, you can join in on the conversation in the associated Project Forum threads, or leave a comment in the space provided on this page.
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