MODELING OF GROUND MOTION IN THE VICINITY OF MASSIVE STRUCTURES

A two-dimensional elastic Chebyshev spectral element method (SPEM) is used to model the seismic wave field within a massive structure and in its vicinity. We consider 2-D models where a linear elastic structure , with quadrangular cross-section, resting on an elastic homogeneous h alf-space, is impinged upon by the waves generated by a surface impuls e at some distance. The scattering of Rayleigh waves and the response of the structure are extensively analysed in a parametric way, varying size, mechanical parameters and shape of the load. Some of the models considered are representative of embankments and earth dams. The simu lation shows that some models resonate, storing part of the incoming e nergy. With realistic parameters, the lowest resonance frequency is du e to pure shear deformation and is controlled by the shear velocity an d height of the load. Flexural modes are excited only at higher freque ncies. The acceleration at the top of the structure may be five/seven times higher than at the base, depending on the mass of the structure. The gradual release of trapped energy produces a ground roll lasting several seconds after the wave front has passed. The ground-roll ampli tude depends on the sturcture's mass and can be as large as 30% of the peak acceleration. Outside resonance conditions, the ground motion is almost unaffected by the presence of the artefact; the horizontal mot ion on top of it is nearly twice the motion at ground level. Similar r esults should be expected when the incident field is an upcoming shear wave. A qualitative discussion shows that the presence of anelastic a ttenuation in the embankment does not significantly alter the precedin g conclusions, unless it is of very low values (e.g. Q < 15). The mode lling results that we discuss indicate that the soil-structure interac tion may substantially alter the 'free-field' ground motion. From a pr actical point of view, the main conclusions are: (1) careful analysis is necessary when interpreting seismic records collected in the vicini ty of large artefacts; (2) seismic hazard at a site may depend on the presence of man-made structures such as embankments, dams, tall and ma ssive buildings.