Many cities worldwide are looking for ways to meet their sustainability and climate change goals; Providing reliable, green transportation is a big part of their strategy. This is particularly the case in Toronto, the fastest-growing city in North America. With an exponentially growing population, there is an urgent need for better ways to travel that do not put pressure on the area’s roadways. This has meant prioritizing the expansion of the area’s public transit system, particularly its subway.
Expanding the capacity and efficiency of the subway is key to providing a transportation alternative that is attractive enough to pull commuters out of their cars and onto public transit, effectively freeing up roadway capacity, reducing vehicle emissions and promoting sustainability. It also helps to improve the quality of life for commuters, as stressful drives are likely to decrease.
In every recent extension of the Toronto Transit Commission (TTC) subway system, Golder has played a key role. Most recently, in 2017, this involved an 8.6 km extension, with six new subway stations, to what will become the heart of the fast-growing city of Vaughan to the north of Toronto. The subway also provides better transit access for the 55,000 students and 7,000 staff at York University.
As the principal geo-engineering consultant for the project, Golder’s role was crucial in keeping the project moving smoothly. Because of the wide range of skills required for this project, Golder assembled a global team, which included professionals with a vast experience in subway projects from Canada, the United States of America, Hong Kong, Singapore, and the United Kingdom, while professionals from Golder’s Vancouver office provided seismic expertise. Golder also applied its extensive experience with the ground conditions in the Toronto area, having worked on many local projects since the company’s founding in Toronto, in 1960.
Any project of this size and complexity faces a number of challenges, and Golder’s team rose to the occasion.
1. Avoiding impacts to surface and subsurface infrastructure and buildings
Before construction started, it was important to understand the ground conditions in the area. This helped with project planning and allowed contractors to prepare their bids accurately. It also helped contractors predict how the soil would react during construction, which is particularly important in tunneling through the soil underground. To provide the necessary site information, Golder prepared a Geotechnical Baseline Report (GBR), detailing the ground conditions along the route.
Having accurate site data from the GBR helped the project team avoid some of the problems that can occur with Tunnel Boring Machines (TBM) (a tubular machine with a rotating cutter head that breaks through the soil or rock to make a tunnel). In earth pressure balance TBMs, like those used on this project, the operator applies air or water pressure to support the soil in front of the cutting head. Applying too much pressure at the cutting face can result in an uplifting of the surface and groundwater being forced upwards can cause flooding. Too little pressure can cause soil loss, resulting in subsidence of the ground, possibly causing sinkholes to develop.
Preventing surface and near-surface impacts was particularly important on this project, as the land above the tunnel’s path is crowded with buildings, roads, and underground utilities such as sewer and water lines. Thanks to Golder’s work on the GBR, the contractors had accurate information on soil conditions, helping calculate the amount of face pressure to be applied by the TBM at each point along the way.
2. Navigating a narrow margin between tunnel and a building foundation
One of the most challenging aspects of the project was the section crossing the York University campus, beneath one of its largest buildings which houses the Schulich School of Business. The top of the tunnel was to be just a few metres below the foundation of this structure, with a potential to cause subsidence or uplifting, resulting in damage to the building.
To prevent this, the contractors were prepared to use “compensation grouting,” a method that pumps grout into the ground under the foundation to lift the building up, if there was any sign of subsidence. To test the potential for upheaval or subsidence, Golder designed and implemented an instrumentation program to detect ground responses as the TBMs approached the Schulich building.
After successfully meeting all requirements during the testing phase, tunneling continued beneath the building without the need for compensation grouting, thus reducing cost and schedule delays to the project.
3. Barricading water-bearing sandy materials
Another potential problem was a large, deep deposit of water-bearing sandy materials surrounding the Vaughan Metropolitan Centre Station. Digging down into this soil would produce a fast flow of groundwater into the excavation, lowering the groundwater table, which could potentially damage nearby roads, utilities and building foundations.
To address this concern, Golder devised a plan to keep groundwater out of the excavation. By installing an underground cut-off wall made of bentonite clay cement slurry, which hardened to form a barrier to keep groundwater out of the excavation, the team was able to dig without the risk of harming the surrounding infrastructure.
4. Managing large volume of data
Golder developed and applied a leading-edge way to manage project data, including the vast amount of information gathered about ground conditions, which was collected using sensory devices placed along the route. Rather than keep these records on paper, as was still common at the time, Golder developed an electronic Content Management System (CMS) that provided a graphic display of the project data, which could be easily queried by project stakeholders. The CMS was able to send out alarms if any of the safety parameters were exceeded.
Golder’s subject-matter expertise and innovative approach to navigating the complexities of the project were crucial in keeping the project moving smoothly. The expanded TTC line is now in operation, doing its part to promote sustainable, green transportation that is also fast, reliable, and cost-effective.