Influence of Site Amplification and Ground Improvement on Seismic Demands

Abstract

Ground improvement techniques are widely employed in seismically active regions to prevent ground failure or mitigate the amount of settlements transmitted to buildings. In many cases, these solutions substantially modify the subsoil conditions and the surface ground motions intensities accordingly. This thesis assesses the capability of ground response analysis (GRA) models to predict site amplification effects and quantify the influence of the degree of ground improvement and geometry of the improved zone on the seismic demand of buildings. In the first part of this research, a new frequency-dependent equivalent linear (FDEL) algorithm is proposed to predict ground motions propagating through soil profiles for total stress GRA. When simulating strong ground motions that exhibit higher levels of soil deformation, the frequency content of ground motions processed within this FDEL procedure is substantially improved as compared to the traditional equivalent linear method, with results more consistent with those from nonlinear time-domain analyses. The new FDEL method is less sensitive to the parametrization of nonlinear material curves compared to other methods. However, all GRA methods tend to under-predict the spectral accelerations at high frequencies when compared to historical downhole array records, with substantial discrepancies across the model predictions. In the second part, the effects of ground densification and stiffening on seismic site amplification factors are investigated through a series of parametric GRAs for total and effective stress conditions. When a densified crust is present with unimproved soft soil layers underneath, the ground surface accelerations are generally reduced. However, the levels of de-amplification decrease as the depth of ground densification increases. It was found that the reduction of foundation displacements across all improved sites is accompanied with an amplification of the seismic base shear developed in structures. The increase in the shear-wave velocity of the densified layers provides a poor correlation when the densified crust is underlain by liquefiable layers. However, the ratio of improved zone thickness to the bottom depth of soft/liquefiable layers correlates well with the increase in the seismic demand. Examples of possible correlations that could be developed to better anticipate the effects of ground densification on the seismic demand of buildings are presented.