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Excavation projects conducted in urban environments can present numerous challenges at the best of times. As shoring consultants for the second phase of CIBC SQUARE in downtown Toronto, currently under excavation, Grounded Engineering had to contend not only with urban density, but transit tunnels, an active rail line, a heritage building, and other factors which together created a confluence of challenges for the young engineering firm to overcome.
“As shoring consultants we live for the below grade stuff,” said Mike Porco, Principal of the Geostructural Engineering group at Grounded. “So this is really the exciting time of the development process as it relates to our work.”
Both Porco and Chris Elvidge, Managing Principal of Grounded, were involved in the first phase of CIBC SQUARE, the now-completed 81 Bay Street, as project shoring consultants when both were a part of Geotechnical Engineering Firm Terraprobe. That project was constructed in the dense urban environment of downtown Toronto in an area sandwiched between the Gardiner Expressway to the south, and the Union Station Rail Corridor to the north, on the east side of Bay Street, directly opposite Scotiabank Arena.
Grounded Engineering is now involved in the excavation process as shoring consultants for CIBC SQUARE’s second phase, located at 141 Bay Street, across the rail corridor from phase 1 and located in an area bordered by the rail corridor to the south and The Dominion Public Building at 1 Front Street West to the north.
“The challenge with any excavation in an urban environment is the neighbouring conditions,” said Porco.
Porco said a common challenge experienced by both phases of the project was the existence of a nearby TTC light rail tunnel located under Bay Street.
“The tunnel comes out of Union Station and runs south to get to Queens Quay,” said Porco. “TTC is very sensitive as to how much displacement their tunnels can experience before they start to have structural problems and become impacted. So we needed to make sure that our shoring walls don’t move very much, especially where TTC and Metrolinx infrastructure is supported.”
Porco also pointed to the nearby Dominion Public Building on Front Street–which is a heritage structure–as another challenge, saying that it is sensitive to vibrations.
Despite all of this, both Porco and Elvidge said the main challenge was the rail corridor, as the provincial transit agency Metrolinx, which governs the tracks, places great importance on their railway operations and cannot tolerate any train service disruptions.
Not only does CIBC SQUARE consist of two major projects straddling the corridor, but both phases of the project will be joined by a park, which will span over the rail corridor on a suspended steel deck.
“One of the big challenges was the timing of allowing train traffic in relation to pouring caissons,” said Elvidge. “Caisson foundations supporting the deck are located between the rail lines and we needed to work closely with Metrolinx to confirm the cut off times for pouring concrete in order to open the tracks up in time for the morning rush.”
“Doing all this work without impacting the rail service at all was sort of a major logistical challenge which is not common with a lot of SOE (support of excavation) or shoring systems,” said Porco.
Porco said that concrete is typically at 70% of its design strength after seven days and at 100% after 28 days. Additives collectively known as HE (high early) can be added to improve the curing time of concrete. HE concrete can allow for 70% strength to be achieved in three days and 100% in seven days.
“So usually the level of precision with these kinds of discussions about concrete curing is done on the basis of days. In this particular case we would be pouring caisson concrete in the evenings and Metrolinx wanted to resume their rail service hours later. So the conversation came down to hours. How many hours does concrete need to cure before we can have our trains roll past?”
Porco said Metrolinx expressed concern over the plasticity of the concrete considering commuter trains would be rolling past freshly-placed caisson foundations within two to three hours of the preceding night shift, and the use of HE concrete did not satisfy Metrolinx’s concerns.
“Ultimately we had to develop a unique installation procedure whereby a caisson shaft would be drilled but not concreted during the same night shift,” said Porco. “Therefore the idea was to leave an excavated shaft, with temporary steel liners in place to protect the hole from collapse and support the dynamic loading exerted by the trains during the day, before returning on the subsequent night shift to start HE concrete placement immediately as the first action taken in the shift. This was done in order to maximize the curing time of the concrete so that it had at least 6 hours or so before it was subject to any loading from trains rolling past.”
Another challenge related to the rail corridor is the existence of a hydro duct bank on the northwest corner of the site that services a portion of Union Station.
“They (Metrolinx) advised early on in the project’s design phase that it had to be kept active, which is quite unusual for a service running through the middle of a private development site,” said Porco. “Most times if you have utilities in the footprint of a site you would cap and abandon these utilities beyond the excavation footprint and then reconnect them as service connections into the new building.”
Porco said they had to come up with a solution that allowed the concrete encased duct bank to remain in place while excavation was completed around it.
“It’s a utility that is going to be suspended in the air,” said Porco. “Essentially we are going to pour a piece of slab, the future building’s ground floor, hang this utility from this slab, and then excavate beneath it.”
Porco said the unique confluence of challenges associated with the site made it one of the most complex and rewarding projects he has designed in his career.
“When you’ve got two rail lines, one an elevated rail corridor and one a buried subway tunnel, a sensitive heritage structure, and finally a delicate utility spanning across your excavation, that is quite a bit,” said Porco, who added that there are approximately two more months of digging left on the site before it is fully excavated, though foundations for the building core have already started to be placed.
“That’s what makes this project unique and special and challenging, and we love to design solutions for challenging projects,” he said.
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