Rethinking the Strength Properties of Soils in the Greater Toronto Area

ABSTRACT: An extensive laboratory testing program was carried out for recent transit expansion projects in the Toronto area. The testing included routine characterization index testing - moisture content, grain size distribution, Atterberg Limits and unit weights - and a suite of advanced testing consisting of approximately 181 triaxial compression test sets (in the order of about 540 specimens) to define the strength properties (friction angle, φ’, and the cohesion, c’) of the soils for use in design. In this paper, we present a summary of the characterization and triaxial compression results, classified using the physical properties of the TTC Soil Groups. Two different sampling methods were used to obtain triaxial test samples conventional PQ coring and the faster Sonic coring method. A comparison of the results indicates that the sampling method (PQ or Sonic) had minimal impact on friction angle results, with the Sonic soil samples generally indicating a slightly lower friction angle when compared to those soil samples obtained using PQ methods. A review of strength parameters recommended for various new developments and transit infrastructure improvement projects was carried out and compared to the results obtained from the triaxial compression testing. Based on this review it appears that geotechnical design engineers may be underestimating the strength of the soils in the Greater Toronto Area favouring to use more conservative values for friction angle and cohesion.