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Beneath the City – Waterproofing Toronto’s Strachan Avenue Tunnel

To accommodate the transportation needs of Toronto’s growing population, the GO Transit authority (regional public transit service for the Greater Toronto and Hamilton Area) decided to raise Strachan Avenue by almost 6.5 feet. This would allow it to pass over the Go Train rail corridor, providing a smoother traffic flow. Simultaneously, the rail corridor would also be lowered up to 26 feet below Strachan Avenue. Additionally, to accommodate the lowered rail line, they also had to lower the existing sewage tunnel 1641 feet.

For the first phase of the project, the team used a surface-applied system to waterproof the sewage tunnel. In order to pour the concrete in the massive tunnel, the tunnel formwork was de-molded and then moved to the front end on rails, similar to a slip-form system. However, unlike the continuous pouring that a true slip-form system allows, each section was poured separately and allowed to harden before the formwork could be moved ahead for the next pour.

Due to the tight work space that tunnel construction provides, the construction team was using blind side waterproofing. In each section, they would place a surface-applied waterproofing membrane to the tunnel wall, before moving the forms to that section to pour the concrete. With this method of construction, it was difficult and time consuming to apply the membrane. Ensuring that the membrane remained in place and undamaged during the process of moving the forms proved to be a time-consuming and difficult task. The conditions were not ideal for this type of application and the detailing required was labor intensive.

A Better Solution

The team faced a challenge as the project moved into its second phase. They could continue on the waterproofing route they were on, risking further schedule setbacks, or they could find a better solution. They rationalized that finding a solution that would allow them to pour the tunnel and install the waterproofing at the same time would be faster, and may even help them make up for time lost.

To make concrete truly “waterproof”—which means both preventing water passage and resisting hydrostatic pressure—many contractors have embraced a long-term, permanent approach. Using an integral crystalline system, the entire mass of concrete itself can be made the waterproofing barrier, eliminating the need for an external membrane system. A crystalline ssytem works by using the moisture int he concrete to grow crystals, effectively closing off pathways for damaging moisture.

Crystalline-based systems typically come in a dry, powdered form and are hydrophilic in nature. The product is added directly to the concrete itself, eliminating the extra step of applying a membrane ahead of a concrete pour. The crystalline formula allows concrete to self-seal hairline cracks up to 0.02 inch, even years after the original construction. The formula contains no volatile organic compounds (VOCs), and can be completely recycled when demolition occurs.

After careful research, the team ultimately chose to add Kryton’s Krystol Internal Membrane (KIM) Concrete Waterproofing Admixture to the concrete mix. KIM’s extensive third-party testing history and knowledgeable technical support were strong factors in their decision to use KIM. The Kryton team performed multiple site visits and inspections, and offered comprehensive training of the product and systems. Kryton was even able to match the strict warranty requirements that were critical for the job.

Approximately 400 cubic yards of KIM-treated concrete was used in the construction of the approximately 1641 feet below-grade sewage tunnel. Kryton’s Krystol Waterstop System for joints was also used in both the tunnel and tunnel ventilation shafts at Strachan Avenue.

The tunnel is currently in use, with both the KIM and Krystol Waterstop Systems performing extremely well.

Brian MacNeil, with more than 20 years in the construction industry, has for over 10 years been devoted to concrete waterproofing at Kryton International.