  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.
