Vincent Richefeu

Stress transmission in wet granular materials.

Abstract.We analyze stress transmission in wet granular media in the pendular state by means of three-dimensional molecular-dynamics simulations. We show that the tensile action of capillary bonds induces a self-stressed particle network organized in two percolating “phases” of positive and negative particle pressures. Various statistical descriptors of the microstructure and bond force network are used to characterize this partition. Two basic properties emerge: 1) the highest particle pressure is located in the bulk of each phase; 2) the lowest pressure level occurs at the interface between the two phases, involving also the largest connectivity of the particles via tensile and compressive bonds. When a confining pressure is applied, the number of tensile bonds falls off and the negative phase breaks into aggregates and isolated sites.

Short-time dynamics of a packing of polyhedral grains under horizontal vibrations

Abstract.We analyze the dynamics of a 3D granular packing composed of particles of irregular polyhedral shape confined inside a rectangular box with a retaining wall subjected to horizontal harmonic forcing. The simulations are performed by means of the contact dynamics method for a broad set of loading parameters. We explore the vibrational dynamics of the packing, the evolution of solid fraction and the scaling of dynamics with the loading parameters. We show that the motion of the retaining wall is strongly anharmonic as a result of jamming and grain rearrangements. It is found that the mean particle displacement scales with inverse square of frequency, the inverse of the force amplitude and the square of gravity. The short-time compaction rate grows in proportion to frequency up to a characteristic frequency, corresponding to collective particle rearrangements between equilibrium states, and then it declines in inverse proportion to frequency.

Shear Strength of Unsaturated Soils: Experiments, DEM Simulations, and Micromechanical Analysis

We investigate shear strength properties of wet granular materials as a function of water content in the pendular state. Sand and glass beads were wetted and tested in a direct shear cell. In parallel, we carried out molecular dynamics simulations by using an explicit expression of capillary force as a function of interparticle distance, water bridge volume and surface tension. Experiments and numerical simulations are in good agreement. We show that the shear strength is mostly controlled by the distribution of liquid bonds. This property results leads to the saturation of shear strength as a function of water content. We arrive at the same conclusion by analyzing the shear strength from the microstructure and by accounting for particle polydispersity. Finally, we discuss the potentialities of the discrete element approach as applied to unsaturated soils.

Force transmission in cohesive granular media

We use numerical simulations to investigate force and stress transmission in cohesive granular media covering a wide class of materials encountered in nature and industrial processing. The cohesion results either from capillary bridges between particles or from the presence of a solid binding matrix filling fully or partially the interstitial space. The liquid bonding is treated by implementing a capillary force law within a debonding distance between particles and simulated by the discrete element method. The solid binding matrix is treated by means of the Lattice Element Method (LEM) based on a lattice-type discretization of the particles and matrix. Our data indicate that the exponential fall-off of strong compressive forces is a generic feature of both cohesive and noncohesive granular media both for liquid and solid bonding. The tensile forces exhibit a similar decreasing exponential distribution, suggesting that this form basically reflects granular disorder. This is consistent with the finding that not only the contact forces but also the stress components in the bulk of the particles and matrix, accessible from LEM simulations in the case of solid bonding, show an exponential fall-off. We also find that the distribution of weak compressive forces is sensitive to packing anisotropy, particle shape and particle size distribution. In the case of wet packings, we analyze the self-equilibrated forces induced by liquid bonds and show that the positive and negative particle pressures form a bi-percolating structure.

Shear strength and stress distribution in wet granular media

We investigate the shear strength and stress distribution properties of wet granular media in the pendular state where the liquid is mainly in the form of capillary bonds between particles. This work is based on a 3D discrete‐element approach (molecular dynamics) with spherical particles enriched by a capillary force law. We show that the capillary force can be expressed as an explicit function of the gap and volume of the liquid bridge. The length scales involved in this expression are analyzed by comparing with direct integration of the Laplace‐Young equation. In the simulations, we consider a maximum number density of liquid bonds in the bulk in agreement with equilibrium of each liquid bridge. This liquid bond number is a decisive parameter for the overall cohesion of wet granular materials. It is shown that the shear strength can be expressed as a function of liquid bond characteristics. The expression proposed initially by Rumpf is thus generalized to account for size polydispersity We show that thi...

Interaction Between Aqueous Solution Transport and Stress/Strain in a Deformable Porous Medium

Liquid phase transport in heterogeneous media such as gels and biopolymers may induce very large strains. These produces internal mechanical stresses that interact with water transport mechanisms. We analyze transfers in porous media saturated with an ionic solution using the linear thermodynamics of irreversible processes. The interaction between mass transfer and stress/strain is analyzed using free energy. A large number of coefficients appears and we proposed theoretical and experimental method for their determination. A validation of the model is given in the simplified case of osmotic dehydration of Agar gel.