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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.

The object of the research is an intergranular friction model for use in discrete-element modeling of the mechanical behavior of granular materials under static and dynamic conditions. As in this approach grains are modeled by independent elements interacting through contact forces, the selection of contact force models, and in particular for the tangential component (friction), represents the most important task in obtaining the most realistic macroscopic behavior. There are many friction models that work well in dynamic regimes, but fail to model mechanical behavior in static or quasi-static regimes. In this work, an intergranular friction model is proposed based on Coulomb’s regularized friction model, which takes into account the stick-slip phenomenon that appears at low sliding speeds at the contact. Three different examples are designed and modeled in order to demonstrate the robustness of the model in different situations including static, quasi-static and dynamic regimes. The first is a basic example consisting of the translational motion of a grain on a planar surface with a relatively low constant velocity. This example allowed to capture the stick-slip phenomenon. The second represents a grain subjected to its own weight and supported essentially by frictional forces. This example shows that the model works well in both quasi-static and static regimes. The third example consists of a grain sliding on a plate and subjected to accelerated motion. It showed the effect of friction velocity on the occurrence of stick-slip, as well as the evolution of friction force with sliding velocity. The obtained results demonstrated that the model effectively captures shear behavior in the different regimes. It could therefore be used in discrete element modeling of granular materials under both static and dynamic conditions. As in this work, the model is formulated in 2D, it would be interesting to develop a general 3D formulation so that it can be easily applied in 3D modeling. [ABSTRACT FROM AUTHOR]

The object of study is the vertical wave propagation in a sand deposit. This paper is aimed at analyzing the vertical wave propagation in a sand deposit through micromechanical modeling that inherently takes account of intergranular slips during deformation. Such a problem, which is part of the general framework of wave propagation in the soil, has long been analyzed using continuum models based on approximate behavior laws. For this purpose, a 2D Discrete Element Method (DEM) model is developed. The DEM model is based on molecular dynamics with the use of circular shaped 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. A model of rolling friction is incorporated in order 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. Using this model, the fundamental and resonance frequencies of the deposit are first determined. The former is determined from the low-amplitude free vibration and the latter by performing a variable-frequency excitation test. It is noted that there is a significant gap between the two frequencies, this gap could be attributed to the degradation of the soil shear modulus in the vicinity of the resonance. Such degradation is well proven in classical soil dynamics. The effects of deposit height and confinement on resonance frequency and free-surface dynamic amplification factor are then investigated. The obtained results highlighted that the resonance frequency is inversely proportional to the deposit’s thickness whereas the dynamic amplification factor Rd increases with the deposit’s thickness. In the other hand, when the confinement increases the deposit becomes stiffer, which results in reducing the amplification. Such result is in accordance with theoretical knowledge which states that the most rigid profiles such as rocks do not amplify seismic movement. [ABSTRACT FROM AUTHOR]

This paper deals with the numerical simulation of the quicksand phenomenon using a coupled Discrete Elements - Lattice Boltzmann hydromechanical model. After the presentation of the developed numerical model, simulations of ascending fluid flow through granular deposits are performed. The simulations show that the quicksand actually triggers for a hydraulic gradient very close to the critical hydraulic gradient calculated from the global analysis of classical soil mechanics, that is, when the resultant of the applied external pressure balances submerged weight of the deposit. Moreover, they point out that the quicksand phenomenon does not occur only for hydraulic gradients above the critical hydraulic gradient, but also in some cases with slightly lower gradients. In such cases, a more permeable zone is first gradually built at the bottom of the deposit through a grain rearrangement, which increases the hydraulic gradient in the upper zones and triggers the phenomenon. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]