Ottawa Trillium Line – Caissons Project

As part of the City of Ottawa’s Stage 2 Trillium Line South Extension project, significant infrastructure upgrades were required to support the expansion of the O-Train network from Greenboro Station to Limebank Road in Riverside South. The transit expansion added approximately sixteen kilometres of new track and included the construction of eight new stations serving southern Ottawa communities including Manotick, Findlay Creek, Greely, and Osgoode.

To accommodate the new rail alignment, multiple rail-over-road structures were constructed to safely carry the transit line above existing roadways and infrastructure corridors. Marathon Underground was engaged to complete deep foundation and shoring works required to support these structures, including the installation of large-diameter caissons and engineered roadside protection systems.

Caissons, also referred to as drilled shafts, were selected as the preferred foundation solution due to their high load-bearing capacity and superior lateral resistance compared with conventional H-piles or pipe piles. These characteristics reduced the number of foundation elements required while meeting the structural and seismic performance requirements for the new transit infrastructure.

The caissons installed for the project ranged from approximately 1500 mm to 1800 mm in diameter and extended to depths of up to 30 metres below grade. Construction conditions presented several technical challenges, including sandy soils with potential buried boulders, artesian groundwater pressures, and the close proximity of active roadways and live traffic corridors.

Artesian groundwater conditions created the potential for ground instability, sediment infiltration, and concrete washout during installation. To manage these risks, Marathon Underground’s experienced field crews maintained controlled head pressure throughout casing advancement and concrete placement operations. Marathon’s in-house engineering team also reviewed geotechnical data and recommended specialized drilling tooling capable of handling difficult subsurface obstructions and variable soil conditions.

Due to the close proximity of excavation activities to major roads and traffic infrastructure, Marathon Underground designed and implemented a sheet pile roadside protection system to minimize the risk of settlement or movement during construction. Continuous monitoring measures were maintained throughout the project to ensure the stability and safety of adjacent infrastructure.

Through detailed engineering coordination, specialized drilling expertise, and proactive ground management strategies, Marathon Underground successfully completed the caisson and shoring works within the required project schedule while maintaining a strong commitment to safety, quality, and environmental protection.