Ontario Line Leslieville and Gerrard Station

Project Summary

Secant Wall Monitoring for Transit Station Construction

The construction of Toronto’s new Leslieville and Gerrard stations as part of the Ontario Line early works required deep excavations up to six meters below grade to accommodate the lowered rail alignment for future transit operations. IBW Surveyors was retained to provide specialized structural monitoring of the newly constructed secant pile retaining walls that would support these critical excavations adjacent to existing urban infrastructure and active rail corridors. This project demanded precision monitoring capabilities that could detect the smallest angular or positional shifts in these rigid structural elements, as any lateral movement in secant walls could have immediate consequences for both construction safety and the integrity of surrounding infrastructure. The challenge extended beyond basic displacement tracking to require sophisticated data analysis that could validate structural performance throughout the most critical phases of station foundation construction.

Technical Details

Rolling Window Analysis and Unified Web Interface Integration

The technical monitoring approach for the secant walls required IBW to develop specialized data processing methodologies distinct from standard displacement monitoring protocols. Our team implemented advanced “rolling window” analyses within the Trimble 4D Control platform to visualize displacement trends over time, enabling structural and geotechnical teams to understand wall behavior patterns throughout different excavation phases. A significant technical achievement was the integration of secant wall monitoring data into a unified web interface that allowed project stakeholders to independently assess station wall performance separate from rail corridor metrics. This segregated data presentation utilized custom charts and scatter plots analyzing lateral displacement versus excavation depth, providing structural engineers with the precise information needed to validate design assumptions and confirm that the retaining structures were performing exactly as engineered throughout the deep excavation sequence.

Equipment & Technology

Trimble S9 High-Precision Network with Custom Structural Alarm Configuration

IBW leveraged its established automated monitoring infrastructure, strategically extending the network to specifically target the new secant wall structures at both station sites. The monitoring system utilized Trimble S9 Robotic Total Stations selected for their exceptional angular accuracy requirements when detecting minute structural movements in rigid retaining systems. Robust monitoring prisms were securely clamped to the tops of specific secant piles and capping beams using specialized mounting hardware designed to withstand the continuous vibrations of active construction environments. All monitoring data flowed through the Trimble 4D Control platform and Trimble Business Centre, where IBW configured custom alarm thresholds specifically tailored to the structural tolerances of secant wall systems rather than generic displacement limits. The platform’s advanced “Sensor Grouping” capabilities enabled our team to segregate secant wall sensors from rail monitoring sensors, ensuring that structural alerts were routed directly to the appropriate geotechnical and structural engineering teams for immediate assessment and response.