Recent research in soil dynamics is directed toward a better understanding of the seismic behavior of soil within the framework of the principles of nonlinear continuum mechanics. The objective of this paper is to study the nonlinear behavior of a volcanic hill situated in Azores islands (Faial) with a three dimensional model and compare results from the analytical model with motion recorded during an aftershock of the July 9, 1998 earthquake. And, in particular, to see the influence of increasing modulus of deformation and of the angle of friction of soft soil, along the height of the hill, on the maximum values of the horizontal component of accelerogram. A finite element three-dimensional representation using the Drucker-Prager plasticity model is employed in the study.
Several investigators have recently carried out studies of the seismic response of some soils, using different theoretical methods and models. These approaches are summarised below. Pease et al. (1997) made an evaluation of the transient shear strain and strong motion characteristics of liquefaction sites in Treasure Island. The SHAKE (1991) Code was used to model the one-dimensional dynamic site. Borja et al. (1999) investigated the non-linear ground response of the hysteretic and viscous material behavior on a test site in Lotung, Taiwan. They used a non-linear Finite Element constitutive model based on three-dimensional bounding surface plasticity theory with a vanishing elastic region. The soils behave as an elasto-plastic material (Chen et al. 1990). For practical purposes, it is better to use a smooth surface in stress space in order to get an approximation of the yield surface in elasto-plastic finite element analyses. The model that has this smooth surface is the Drucker-Prager perfectly plastic model. The Abaqus Code (Hibbit et al. 1997) is used within this paper.