Investigation of the effects of sediments inhomogeneity and nonlinearity on aggravation factors for sedimentary basins

The prediction of the seismic response of sedimentary basins and its incorporation into seismic building codes is of prime interest to the engineering community, since highly populated areas and infrastructures are often located within such geological configurations. The additional effect of the two-dimensional (2D) basin response above the corresponding one-dimensional (1D) response, expressed as the ratio between 2D and 1D acceleration response spectra at the basin surface and commonly referred to as “aggravation factor”, might be an engineering feasible way to incorporate such effects into building codes. The majority of the studies are based on numerical simulations for modeling the seismic behaviour of complex subsurface geology and thus for the estimation of aggravation factors, usually taking place under the assumption of homogeneous viscoelastic sediments, ignoring the fact that sedimentary deposits may be very heterogeneous in both the horizontal and the vertical directions, as well as the effect of soil nonlinearity, which for strong seismic excitation may be crucial. The present work is an extension of our work [1], in which extensive numerical analyses of the linear viscoelastic response of homogeneous sedimentary basins were performed, in order to explore the potential additional effects of sediments inhomogeneity and nonlinearity on aggravation factors. To this end, a generic relationship between shear wave velocity and depth was developed based on experimental shear wave velocity data from well-documented sites worldwide. To further model the nonlinear behaviour of sediments material, a nonlinear kinematic hardening constitutive model was used, combined with the Von Mises failure criterion and an associated plastic flow rule. All analyses were conducted for trapezoidal sedimentary basins using the generic finite element code ABAQUS. The results of the analyses revealed that the replacement of an inhomogeneous soil with an equivalent homogeneous might lead to underestimation of aggravation in the vicinity of the edges and overestimation of aggravation at the flat part of the basin, while consideration of soil nonlinearity may result in a decrease of aggravation factors for the flat part of the basin. These outcomes may be useful for the proposal of simple recommendations to the engineering community for the introduction of basin effects in the seismic design of structures.