Strong seismic motions estimated from a one direction-three components ("1d-3c") approach, application to the city of rome, italy

Abstract

Strong seismic motions in soils generally lead to both a stiffness reduction and an increase of the energy dissipation in the surficial layers. In order to study such phenomena, several nonlinear constitutive models were proposed and were generally implemented for 1D soil columns. However, one of the main difficulties of complex rheologies is the large number of parameters needed to describe the model. In this sense, the multi-surface cyclic plasticity approach, developed by Iwan in 1967 but linked to Prandtl or Preisach theoretical work, is an interesting choice: the only data needed is the modulus reduction curve. Past studies have generally implemented such models for one-directional shear wave propagation in a "1D" soil column considering one motion component only ("1C"). Conversely, this work aims at studying strong motion amplification by considering seismic wave propagation in a "1D" soil column accounting for the influence of the 3D loading path on the nonlinear behavior of each soil layer. In the "1D-3C" approach, the three components (3C) of the outcrop motion are simultaneously propagated into a horizontally layered soil for which a three-dimensional constitutive relation is used (Finite Element Method). The alluvial site considered in this study corresponds to the Tiber River Valley, close to the historical centre of Rome (Italy). The computations are performed considering the waveforms referred as the 14th October 1997 Umbria-Marche earthquake, recorded on outcropping bedrock. Time histories and stress-strain hysteretic loops are computed all along the soil column. The octahedral stress, the strain-depth profiles and the transfer functions in acceleration (surface/outcrop spectral ratios) are estimated for the 1D-1C and the 1D-3C approaches, evidencing the influence of the three-dimensional loading path.