Micromechanical Modeling for Analysis of Shear Wave Propagation in Granular Material

This paper presents a numerical study of shear wave propagation in a vertical sand profile through micromechanical modeling. For this purpose, 2D modeling by the Discrete Element Method (DEM), is carried out. The DEM model is based on molecular dynamics with the use of circular elements. The intergranular normal forces at contacts are calculated through a linear viscoelastic law, while the tangential forces are calculated through a perfectly plastic viscoelastic model. Rolling friction is incorporated to account for the damping of the grains rolling motion. Different boundary conditions of the profile have been implemented: a bedrock at the base, a free surface at the top, and periodic boundaries in the horizontal direction. The sand deposit is subjected to a harmonic excitation at the base. The simulations carried out have well reproduced the elastic and damping features relative to shear wave propagation in a vertical soil deposit. The excitation frequency is varied to better understand the phenomenon of wave propagation in granular medium. The conducted simulations highlighted a number of features of soil deposits response subjected to harmonic excitation at the base, including the movement amplification, the resonance phenomenon and the limitation of the displacement at the resonance. The micromechanical analysis showed that the intergranular slips increase with increasing the involved strain level. An inverse analysis is performed to determine a continuum-damped linear elastic model, whose response is similar to that of the discrete-element model. This analysis showed that the wave propagation velocity of the equivalent continuum model decreases with increasing excitation frequency. This finding could be attributed the decrease of shear modulus of the granular material as the deformation level increases.