M. S. El Youssoufi

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.

Compression tests on a sandy silt at different suction and temperature levels.

This paper presents a unified thermo-mechanical experimental study on a remoulded unsaturated sandy silt and brings a contribution to the understanding of the fundamental mechanics of unsaturated soils in non- isothermal conditions. The experimental program was carried out at four temperatures and four suction levels using two thermo-hydro-mechanical (THM) cells, one isotropic and the other oedometric. The effect of suction and temperature on the compressibility and on the apparent preconsolidation pressure of the soil is addressed. Finally, an analytical expression of the evolution of the apparent preconsolidation pressure with respect to temperature and suction is proposed.

Self-stresses and crack formation by particle swelling in cohesive granular media.

We present a molecular-dynamics study of force patterns, tensile strength, and crack formation in a cohesive granular model where the particles are subjected to swelling or shrinkage gradients. Nonuniform particle size change generates self-equilibrated forces that lead to crack initiation as soon as the strongest tensile contacts begin to fail. We find that the tensile strength is well below the theoretical strength as a result of inhomogeneous force transmission in granular media. The cracks propagate either inward from the edge upon shrinkage or outward from the center upon swelling. We show that the coarse-grained stresses are correctly predicted by an elastic model that incorporates particle size change as metric evolution.

Force transmission in highly polydisperse granular media

This work is devoted to the numerical modeling of highly polydisperse granular materials in view of investigating the texture and mechanical behavior in quasi‐static shearing. The polydispersity is modeled in terms of size span and curvature of the particle size distributions. A method is proposed to generate representative size distributions both in number and volume of size classes, and the accessible polydispersity parameters are determined for tractable number of particles. A geometrical method is used to construct large samples and to study the resulting texture in a systematic manner as a function of size distribution. These samples are then subjected to simple shear with periodic boundary conditions by means of the contact dynamics method. We show that: 1) The highest level of solid fraction corresponds to uniform distribution by particle volume fractions; 2) The shear strength in the steady state is independent of polydispersity; 3) A detailed analysis of the texture and force transmission indicat...

Theoretical and experimental study of pendular regime in unsaturated granular media

This article addresses the experimental study of capillary bridge properties with the use of analytical calculation of bridge profile, based on solution of Young–Laplace equation. Using the measurements of some parameters as the contact angle, half-filling angle and the neck radius of the capillary bridge between two spherical particles of radius r, the shape of the bridge is estimated using theoretical solutions of Young–Laplace equation. The corresponding analytical solution is superposed and compared with image data.

Onset of Immersed Granular Avalanches by DEM-LBM Approach

We present 3D grain-fluid simulations based on the discrete element method interfaced with the lattice Boltzmann method and applied to investigate the initiation of underwater granular flows. We prepare granular beds of 800 spherical grains with different values of the initial solid fraction in a biperiodic rectangular box. In order to trigger an avalanche, the bed is instantaneously tilted to a finite slope angle above the maximum angle of stability. We simulate the dynamics of the transient flow for different solid fractions. In agreement with the experimental work of Iverson (Water Resour Res 36(7):1897–1910, 2000) and Pailha et al. (Phys Fluids 20(11):111701, 2008), we find that the flow onset is controlled by the initial solid fraction.

DEM‐LBM Approach to the Onset of Sand Boiling

We present a 3D numerical model applied to study the onset of the boiling phenomenon in a polydisperse granular material by computing the critical hydraulic gradient. The sample is constructed by means of the Discrete Element Method (DEM), then subjected to an upward increasing hydraulic gradient until the intergranular forces vanish. The hydrodynamic forces applied on solid grains are computed by means of the Lattice Boltzmann Method (LBM). To reduce calculation time, bi‐periodic boundaries are implemented in both horizontal directions and for both fluid and solid (grains) materials. The results are in good agreement with the classical values of the critical hydraulic gradient.

Multi-scale Approach for Modeling Desiccation Shrinkage in Granular Soils

This paper aims to identify and quantitatively evaluate various critical mechanisms associated with the processes of desiccation shrinkage and cracking in drying silty soils. A previously developed 1D bundle-of-tubes model is refined to simulate the various stages of drying shrinkage in 2D, using the actual pore size distribution based on Mercury Intrusion Porosimetry (MIP) data. Experimental evidence at a meso-scale has shown that the air entry phenomenon may occur in two possible scenarios: air incursion at the external surface and formation of vapor nucleus in the interior. Further transition of solid-water structural configuration into funicular and pendular states from initially capillary state is simulated.

Micromechanical analysis of water retention phenomenon

We investigate the water distribution and the link between suction and water content in granular media. Firstly, we examine the effect of suction on the shape and the volume of the liquid bridge for different parameters (grain radius, inter‐particle distance, contact angle, surface tension). This local behaviour is then used in a discrete element study of a sample composed of several thousands of grains. We focus our study on the pendular state. The existence of a liquid film around the grains which involves the continuity of the liquid phase is assumed. The water distribution and the water content associated with a given suction are calculated. Then retention curves of the granular media are built. Four different methods are used. The first is based on the local expression of the capillary force coupled with the “gorge method,” the second is based on the Laplace equation, and the third and the fourth are based on the integration of the differential equation that defines the liquid bridge shape. A paramet...