Coupling between stratigraphic and topographic effects on slopes

In this paper the seismic response of simple slope geometries under vertically propagating in-plane shear waves is assessed through two-dimensional finite element analyses to investigate the amplification of the ground motion induced by soil topography. Topographic horizontal and vertical amplification factors were evaluated through different sets of analyses of slopes overlying a compliant bedrock. The effect of soil non-linear behavior was accounted for through equivalent-linear approximation, varying the range of shear strains in which soil responds linearly; to this purposes several relationships describing the variation of normalized shear modulus and damping ratio with induced shear strain were adopted. Finally, the influence of the amplitude and of the frequency of the input motion was also analyzed. The results of the analyses confirmed that a complex interaction exists between stratigraphic and topographic effects and that the two effects can be neither evaluated independently nor easily uncoupled. Specifically, horizontal and vertical topographic amplification factors were found to depend on the tendency of soil to exhibit a non-linear behavior and on the degree of non-linearity arising in its seismic response. These, on turn, are related to all the factors affecting nonlinear dynamic response: soil stiffness and damping ratio at small strain, characteristics of the input motion and non-linear stress-strain relationship. Finally, the comparison with the results of linear visco-elastic analysis showed that without accounting for soil non-linear behavior, topographic amplification factors may result underestimated.