BEYOND HORIZONTALLY LAYERED SOILS: SITE EFFECTS IN THREE-DIMENSIONS

Abstract eng:
We present an overview of our recent research on topography effects in strong ground motion, which ranges from semi-analytical solutions of wave propagation in elastic infinite wedges to numerical simulations of the seismic response of complex topographic features on the surface of nonlinear soils. Starting from the simplest idealized feature, we first present a semi-analytical solution to the response of an infinite wedge, which has been traditionally used in studies of topography effects due to its similarity with features like mountain roots and crustal discontinuities. We use the semi-analytical results as guidance for the parameterization of topographic amplification in convex features, as well as for validation of our numerical simulations. We then present results of simulated topography effects for more complex features. Of particular interest are our analyses of topographic amplification on shallow granular soil deposits overlaying bedrock, which we also validate by comparison with centrifuge experimental data. Our results suggest that the inelastic response of pressure-dependent soils near the surface has a unique impact on the amplification intensity and frequency characteristics of topographic amplification, and could thus differ substantially from viscoelastic model predictions traditionally used in studies of such effects. Results from viscoelastic and nonlinear simulations show that near surface velocity contrasts result in amplification higher than otherwise predicted by the superposition of topographic amplification and 1D site response. We attribute this discrepancy to coupling effects between wave reverberations and ray focusing at sites with irregular surface geometry and pronounced soil layering. Our results have implications in ground motion predictions for mountainous regions, as well as in the response of man-made geotechnical systems like retaining structures and gravity dams.

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