Extracting Nonlinear Shear Stiffness and Damping Ratio from Resonant Column Tests for Seismic Site Response Analysis

When incident seismic waves reach shallow surface layers, their amplitude can increase due to the reduction in shear wave velocity. In the presence of soft soils, resonance of these layers can further amplify motion, making such sites particularly vulnerable to high-frequency earthquakes, even at lower shaking intensities. The behaviour of these near-surface materials during such events is quantified through seismic site response analysis (SRA). In this context, the shear modulus and damping ratio are key parameters governing the response to seismic excitation. Despite their importance, current approaches often rely on simplifying assumptions when determining these properties. A widely used method is the Resonant Column Test (RCT), which measures the frequency response of a soil specimen to infer strain-dependent stiffness and damping. However, traditional interpretations of RCT results neglect nonlinear effects—such as multiple solution branches, superharmonic resonances, and softening—and rely on linear methods to estimate nonlinear properties. This research introduces a nonlinear dynamics framework to enhance the RCT methodology and enable a more accurate characterisation of soft soils. Preliminary experimental results show that the identified nonlinear features are not only observable in measurements but can also be reproduced numerically, providing valuable information for parameter estimation. A corresponding dynamic model of the test is developed to match experimental and simulated response curves. The overarching goal is to establish an integrated approach—from targeted testing and parameter identification to constitutive modelling and nonlinear site response prediction. The implications of the improved framework are demonstrated by comparing traditional and enhanced SRA approaches.