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Built on the former bed of a massive prehistoric glacial lake, Toronto's unique geological history has shaped the way we build in this city. Formed in alternating layers over eons, the dense clay and sand glacial deposits common in the region trap layers of water that add challenges to the deep excavations required for high-rise buildings. While many dewatering solutions are designed to manage single layers of trapped groundwater, fewer options exist to address multi-layered aquifers common to the Toronto area. This has become more of an issue over the last decade or so as developments in the region require deeper and deeper excavations, facing conditions traditional technologies are not as well-equipped to handle.
One megaproject where this challenge was encountered was at Metrolinx's Eglinton Crosstown Light Rail Transit (LRT) project. With multiple impermeable clay layers running across several of the excavations, the underlying aquifer was divided into many pockets, creating complications for the enormous 25-metre-plus-deep excavations. In the face of mounting complications that standard dewatering systems created when integrated into the typical pile and lagging shoring systems, specialists WJ Groundwater Canada were brought in by Crosstown joint venture Crosslinx Transit Solutions Contractors (CTS) to help decipher and manage the ancient glacial deposits and their trapped water layers.
Surface-based ejector (or eductor)-type dewatering systems were originally employed before WJ's involvement. Most eductor systems, drilled straight downwards around the perimeter of an excavation and drawing from a single point like a drinking straw, are only able to attack one layer of trapped water at a time. With continued groundwater issues proving to be a challenge, WJ's solution was to work around the existing dewatering system and drill inclined through the shoring walls using a specialized wellpoint system where multiple pockets of groundwater could be targeted collectively by individual wells. WJ Groundwater was eventually engaged to manage groundwater on several additional Crosstown stations facing similar groundwater issues, keeping the critical infrastructure project moving.
After focusing the efforts of their Canadian division entirely on the complex Crosstown project, WJ Groundwater has since moved on to work in all construction sectors. Most residential builders are not familiar with the emerging solutions that have already shown great promise in these local infrastructure projects, but with residential high-rise developments being built in record numbers across the Toronto region, and with even deeper garages being built, excavations across the GTA are regularly encountering complex glacial deposits and associated groundwater issues similar to what was seen at Crosstown sites.
Two shoring systems are common in Toronto. Waterproof caisson walls, and the less-costly but permeable pile and lagging shoring systems. Builders can often save money without compromising the end product by using cheaper pile and lagging walls in conjunction with dewatering systems. Until now, dewatering has been seen as a gamble in some situations, as the cost of groundwater management can often expand well beyond initial estimates as excavation uncovers buried glacial deposits not identified during exploratory drilling.
Until recently, general contractors and engineers have had limited options for dewatering solutions with pile and lagging shoring walls, with the market previously monopolized by rental-based business models and methods that would be considered “traditional” by WJ’s standards. These rental-based models can put a developer or general contractor at the risk of being ultimately held responsible for the bill when unexpected changes to projected cost are encountered.
Wellpoint and eductor systems such as those used by WJ to overcome the challenges faced at Crosstown excavations are set to become a regular feature in construction sites across the region, utilizing wells to draw groundwater from all elevations of excavations. WJ can also access individual water-bearing layers through shoring walls much easier than vertical drilling. This is accomplished using specialized 360-degree drilling rigs that can bore into shoring walls horizontally or at angles, for simpler dewatering.
The diagram below, showing angled holes drilled from the base of an excavation, illustrates how these systems can lower the water table below the base of an excavation as the dig continues downward. High-pressure "water rotary flush" methods allow the drilling rigs to inject water into boreholes, washing out cuttings and debris, and allowing a filtering layer of sand to be installed that enables a good hydraulic connection to the native ground
In sites where angled drilling through shoring is not as feasible, vertically-drilled ejector/eductor dewatering systems made by WJ offer an advanced solution better suited to the local geology than the traditional jetted eductor well systems commonly seen in the GTA. WJ's eductor wells feature systems with screens that can draw water from the full length of a well bore. These 4" eductor wells offer as much as quadruple the flow of traditional 2" well systems, and offer maintenance and replacement solutions not commonly available to high-rise builders before WJ's arrival in the market.
Packed in sand as a filtering medium, these wells reduce total suspended solids (TSS) in discharge water. This results in potential space-saving advantages, with WJ's eductor systems not requiring the on-site sedimentation tank rentals common to traditional eductors, instead offering a built-in option that can be hung from shoring walls. This is especially useful for constrained urban high-rise development sites, often built to lot lines and requiring costly lane closures or temporary work platforms to accommodate the space needs of traditional systems.
The diagram below shows WJ's technology (left) versus the traditional method (right) employed at a site with pile-and-lagging shoring walls, with green-shaded sections representing water-saturated pockets and brown areas representing layers of clay. While the traditional jetted eductor well system on the right is sucking water from the lowest layer of groundwater screening only at the base of the well through the five-foot-long yellow-marked section, the trapped layers above remain untouched. On the left, WJ's system can be seen drawing groundwater from all layers simultaneously.
Even with all the technology at play, systems like these have the potential to actually lower dewatering costs for builders by freeing up space, minimizing disruption to other aspects of construction work, and offering better insight and more accurate predictions regarding the geological conditions the builders may encounter.
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