Analysis of Local Site Effects on the Seismic Response of Soil Profiles Using a 1-Directional 3-Component Propagation Model

Abstract eng:
The seismic response at the surface of alluvial deposits is influenced by several parameters such as dynamic soil properties, impedance contrast between soil layers and the wavefield polarization. Local site effects on the dynamic response to strong seismic motions of surficial multilayered soil are analyzed through a one-directional three-component (1D-3C) wave propagation model. The three components (3C-polarization) of the incident wave are simultaneously propagated into a horizontal multi-layered soil. A 3D nonlinear constitutive relation for dry soils under cyclic loading is implemented in a quadratic line finite element model. The soil rheology is modeled by mean of a multi-surface cyclic plasticity model of the Masing-Prandtl-Ishlinskii-Iwan (MPII) type. Its major advantage is that the rheology is characterized by few commonly measured parameters. Previous studies showed that from one to three component unidirectional propagated wave, the soil shear modulus decreases and the dissipation increases, for a given maximum strain amplitude. The 3D loading path due to the 3C-polarization leads to multiaxial stress interaction that reduces soil strength and increases nonlinear effects. Nonlinearity and coupling effects between components are more evident with decreasing seismic velocity ratio in the soil and increasing vertical to horizontal component ratio of the incident wave. This research aims to compare computed ground motions at the surface of soil profiles in the Tohoku area (Japan) to 3C seismic signals recorded during the 2011 Tohoku earthquake. The 3C signals propagated along soil stratigraphies had been recorded at rock outcrops or downhole in the Tohoku area. Both boundary conditions, of absorbing soil-bedrock interface and of borehole imposed motion, may be assumed in the model, relating to different study cases. The 1D-3C approach is compared with the combination of three separate one-directional analyses of one motion component propagated independently (1D-1C approach), evidencing the influence of the 3D loading path. 3C motion and 3D stress and strain evolution are evaluated all over the soil profile.

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