Impact of Liquefiable Layers on Site Response Analysis

This study evaluates the effect of a liquefiable layer in one dimensional site-response analysis to understand the propagation of ground motion through soil deposits. The evaluation was directed using an analytical method based on the application of a simplified hyperbolic constitutive model termed MRDF+u (modulus reduction and damping curve fit using a reduction factor and including porewater pressure generation, u) to integrate the Dobry and GMP PWP generation models accounting for ground softening (liquefaction). The methodology used 15 spectrally matched input motions selected from the PEER database, which were applied to a generic 30-meter sand profile with varying positions of the liquefiable layer. The soil profile was discretized into 20 sublayers, and the dynamic properties were kept constant within each layer. The analysis included equivalent linear total stress, nonlinear total stress, and nonlinear effective stress computations using the MRDF+u model. Corresponding calculations indicated that the liquefiable layer plays a major role in affecting the results of the site-response analysis that can lead to severe ground deformation and failures during seismic events. The effect of the liquefiable layer was demonstrated in the increment of porewater pressure values - by as much as 80% - and the large shear strain values by up to 15% observed in this layer. However, discrepancies in response spectra suggest that current models might underestimate the effects of soil softening and modulus degradation under high PWP conditions. The results obtained also suggested that the MRDF+u constitutive model is effective to capture the main soil response features, and it could be used in regular engineering practice. The research contributes to developing techniques in engineering design and public safety by providing more reliable geotechnical models to build earthquake-resistant structures. Correctly accounting of liquefiable layers facilitates the design of robust foundations and structures and reduces the risk of failure during earthquakes. Future research and engineering applications should focus on investigating the effect of different soil types, varying seismic conditions, and the application of multi-dimensional analysis methods to expand our understanding of liquefiable layers and improve site response predictions.