DISCUSSION ON THE MATRIX PROPAGATOR METHOD TO ANALYZE THE RESPONSE OF SATURATED LAYERED MEDIA

Seismic response analysis of horizontal layered soil is the precondition of the seismic wave scattering problem. The previous studies mostly focused on the dry soil condition, but in practical engineering, below the underground water level, the soil is filled with fluid. So the saturated porous media model should be considered. Based on the Biot model, considering the attenuation caused by the relative motion of solid and liquid, the steady-state response of saturated layered soil subjected to oblique incident seismic wave is obtained by using the Thomson-Haskell propagator matrix method. The time-domain transient response can be obtained by inverse Fourier transform. This method is validated by a numerical example that SV wave is incident from the bedrock to the overlying saturated soil layers. Two kinds of contraries to law of causality are found in numerical analysis, and their reasons are explained preliminarily:(1) When the incident angle of SV wave is larger than the critical angle for the reflection P wave in the bedrock becoming non-uniform, it leads to noncausality; (2) Due to the attenuation of P2 wave, when the attenuation coefficient associated with the parameters such as permeability, the thickness of the soil layer and the frequency of the incident wave is beyond the computer accuracy, it leads to noncausality either. The range of parameters necessary for causality is obtained accordingly, which can be used as an upper bound in practice. This work provides guidance for the analysis of free field response of saturated soil layer.