Project

Ontario Line Urban-Rail Extension – Ongoing

Description of Services

Support of Excavation Design, Geotechnical Engineering, Tunnel Alignment Options and Schedule Analysis, Coordination with Stakeholders, Planning and Optioneering, Constructability Evaluations

Reliance is part of the Owner’s Technical Advisory Team, providing services to Metrolinx as a sub-consultant to HDR on this major urban rail expansion project. The proposed Ontario urban rail project involves the construction of approximately 13 km of dual mainline Light Rail Transit (LRT) corridor, including underground, at-grade, and elevated guideway sections, 400 meters of new track for the Maintenance and Storage Facility, and fifteen new metro stations. The alignment and profile of the new LRT line require extensive underground construction, including nearly 10,000 meters of tunneling and eight of the fifteen stations located in mined cavities, ranging from 30 to 40 meters below grade.

In the project’s early planning phase, Reliance provided conceptual engineering services for tunneling, geotechnical, geo-structural support of excavation (SOE), mining, and high-level project schedule analysis to finalize the alignment and profile. As part of the Technical Advisory team, Reliance is responsible for the portal sections of the South Civil contract, interfacing with the GO Line at Exhibition Station and in Don Yard.
The new tunnel alignment traverses an ancient, buried deep river valley filled with very soft and highly variable alluvial soils and other soft deposits. As part of the project’s risk mitigation strategy, the section through this valley is included in an early construction package procured as a design-bid-build section within the P3 project. During the P3 development stage, Reliance provided geotechnical engineering, SOE design, tunnel design, and schedule analysis for the planning stages of this large urban rail project, undertaken by a public rail transportation authority. This project involves extensive coordination with numerous stakeholders and adherence to requirements from multiple agencies. Services included project planning and optioneering, multidisciplinary engineering (geotechnical, structural, geo-structural), SOE design, review of existing geotechnical data, alternative alignment and profile evaluations, constructability assessments, and preliminary construction schedule analysis.

Reliance provided conceptual through final detailed design of the SOE system through the buried river valley. The alignment of the river valley, skewed to the tunnel alignment, resulted in a non-symmetric rock and overlying soil profile, as shown in the geotechnical profile developed by Reliance.

Key Challenges and Solutions

Challenge 1: Adjacent Active Rail Tracks and Stringent Ground Movement Criteria

The presence of four active rail tracks adjacent to the excavation required strict limitations on ground movements to ensure continued train operations. Ground movement was required to be limited to less than 5 mm (3/16″), allowing trains to operate at normal speeds.

Challenge 2: Geotechnical Complexity of the Buried River Valley

The tunnel alignment traversed an ancient, buried deep river valley with highly variable soil conditions, including very soft alluvial deposits, resulting in geotechnical asymmetry.

Challenge 3: Soil Liquefaction, Groundwater Seepage, and Bottom Stability

The presence of soft soils and high groundwater levels posed risks related to soil liquefaction, bottom blowout, and uplift forces.

Challenge 4: Complex Stakeholder Coordination

The project required extensive coordination with multiple agencies and stakeholders, including transit authorities, municipal entities, and rail operators.

Solution: Advanced Monitoring and Precision Engineering

To meet these stringent requirements, the SOE design incorporated secant pile walls with tie-back anchors to provide maximum stiffness. Continuous ground movement monitoring and phased excavation sequencing were implemented to mitigate risks. Advanced numerical modeling was also utilized to predict settlement behavior and optimize structural performance.

Solution: Asymmetric Earth-Retaining Structure Design

The final SOE design was tailored to address the asymmetric soil profile by incorporating secant piles, tie-backs, and localized ground improvements. Engineering refinements accounted for significant variations in rock depth and uneven rail loading on either side of the excavation.

Solution: Ground Improvement and Bottom Plug Formation

To address these concerns, a bottom plug was created using deep soil mixing and jet grouting techniques. This improved soil stability, mitigated seepage risks, and provided sufficient bearing capacity for the excavation support system. Additionally, dewatering wells were strategically placed to control groundwater levels during construction.

Solution: Proactive Communication and Integrated Planning

Reliance facilitated stakeholder coordination through regular technical meetings, alignment workshops, and early engagement with all parties. By integrating multidisciplinary engineering teams and conducting iterative design reviews, potential conflicts were addressed proactively, ensuring smooth project progression.

Through innovative engineering, advanced numerical modeling, and careful stakeholder coordination, Reliance has played a key role in advancing this critical urban infrastructure project while mitigating complex geotechnical and constructability challenges.