Moulay Saïd El Youssoufi

Shear strength properties of wet granular materials.

We investigate shear strength properties of wet granular materials in the pendular state (i.e., the state where the liquid phase is discontinuous) as a function of water content. Sand and glass beads were wetted and tested in a direct shear cell and under various confining pressures. In parallel, we carried out three-dimensional molecular dynamics simulations by using an explicit equation expressing capillary force as a function of interparticle distance, water bridge volume, and surface tension. We show that, due to the peculiar features of capillary interactions, the major influence of water content over the shear strength stems from the distribution of liquid bonds. This property results in shear strength saturation as a function of water content. We arrive at the same conclusion by a microscopic analysis of the shear strength. We propose a model that accounts for the capillary force, the granular texture, and particle size polydispersity. We find fairly good agreement of the theoretical estimate of the shear strength with both experimental data and simulations. From numerical data, we analyze the connectivity and anisotropy of different classes of liquid bonds according to the sign and level of the normal force as well as the bond direction. We find that weak compressive bonds are almost isotropically distributed whereas strong compressive and tensile bonds have a pronounced anisotropy. The probability distribution function of normal forces is exponentially decreasing for strong compressive bonds, a decreasing power-law function over nearly one decade for weak compressive bonds, and an increasing linear function in the range of tensile bonds. These features suggest that different bond classes do not play the same role with respect to the shear strength.

Micro-scale study of rupture in desiccating granular media

Capillary bridges between two, three, and multiple fixed glass spheres are examined experimentally during their natural evaporation. The key variables of the process of evolution are measured using a calibrated balance recording and digital image processing with still and high-speed cameras. The calculations of Laplace pressure, as well as suction and surface tension resultant components of the interparticle force are made for two-grain systems. Evolution, properties and failure of evaporating liquid bridge are controlled and induced by decreasing liquid volume. For the two grain configuration, tests show a gradual decrease of suction down to zero and into a positive pressure range before a two step failure occurs, including a formation of a water wire according to a Rayleigh instability pattern followed by a simultaneous rupture at two points of the lowest (negative) total (Gauss) curvature of the bridge surface. For more complex systems, a thin-film pinching instability is shown to result from two-dimensional cavitation of water, leading to a re- configuration of the water body into separate bridges between individual pairs of grains, which then rupture as described above. Water body instability generated dynamic penetration of air may also provide an imperfection for the granular system potentially leading to cracking.

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.

A three-scale cracking criterion for drying soils

Cracking is a most unwanted development in soil structures undergoing periodic drying and wetting. Desiccation cracks arise in an apparent absence of external forces. Hence, either an internal, self-equilibrated stress pattern resulting from kinematic incompatibilities, or a stress resulting from reaction forces at the constraints appear as a cracking cause, when reaching tensile strength. At a meso-scale, tubular drying pores are considered in the vicinity of a random imperfection, inducing a stress concentration in the presence of significant pore suction. This approach allows one to use the effective stress analysis, which otherwise, away from the stress concentration, usually yields compressive effective stress and hence a physically incompatible criterion for a tensile crack. Recent experiments on idealized configurations of clusters of grains provide geometrical data suggesting that an imperfection as a result of air entry deep into the granular medium penetrates over 4 to 8 internal radii of a typical pore could yield a tensile effective stress sufficient for crack propagation.

From liquid to solid bonding in cohesive granular media

Abstract We study the transition of a granular packing from liquid to solid bonding in the course of drying. The particles are initially wetted by a liquid brine and the cohesion of the packing is ensured by capillary forces, but the crystallization of the solute transforms the liquid bonds into partially cemented bonds. This transition is evidenced experimentally by measuring the compressive strength of the samples at regular intervals of times. Our experimental data reveal three regimes: (1) Up to a critical degree of saturation, no solid bonds are formed and the cohesion remains practically constant; (2) The onset of cementation occurs at the surface and a front spreads towards the center of the sample with a nonlinear increase of the cohesion; (3) All bonds are partially cemented when the cementation front reaches the center of the sample, but the cohesion increases rapidly due to the strengthening of cemented bonds. We introduce a model based on a parametric cohesion law at the bonds and a bond crystallization parameter. This model predicts correctly the phase transition and the relation between microscopic and macroscopic cohesion.

Rupture of an evaporating liquid bridge between two grains

The study examines rupture of evaporating liquid bridges between two glass spheres. Evolution of the bridge profile has been recorded with the use of high-speed camera. Geometrical characteristics of the bridge were then used to calculate evolution of the variables during the process: Laplace pressure, capillary force, and surface tension force. For the purpose of reference, the bridge evolution is followed also during kinematic extension. During both processes the diameter of the neck decreases, with an acceleration of about 1–2 ms before the rupture. Two distinct rupture modes are observed, depending on the bridge aspect ratio. After the rupture, the mass of liquid splits, forming two separate oscillating drops attached to the spheres, and a suspended satellite droplet. Just before the rupture, an increasing repulsive Laplace pressure, and decreasing negative surface tension force develop. Capillary force follows the trend of the surface tension force, with an accelerating decline. Duration of the whole process and liquid mass stabilization is from 10 to 60 ms.

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.

Influence of Temperature on the Water Retention Curve of Soils. Modelling and Experiments

We investigate the influence of temperature on the retention curve of soils. This curve represents the constitutive relation between water content w and suction s, for a given temperature T and a given void ratio e. We present a model based on the differential of suction as a function of T, w and e. When adjusted for a retention curve obtained at a given temperature, this model enables to predict this curve for any temperature. In parallel, we carried out experiments on a clayey silty sand by using a pressure cell immersed in a thermostatic bath. The model was validated by several tests on the clayey silty sand at 20 and 60˚ C. The application of the model to data found in the literature confirms its predictive power for a wide range of porous materials. These results allow us to plot the retention surface, from experimental tests obtained at a given temperature and from modelling. It can be considered as a generalization of the classical retention curve. Finally, we discuss the influence of the void ratio variation during experiments on the curve predicted by the model.

Effect of Capillary and Cemented Bonds on the Strength of Unsaturated Sands

The cohesive interactions between grains play a prevailing role in the mechanical behaviour of unsaturated granular materials such as fine sands. These interactions are generally bonds of various natures that evolve according to the surrounding hygrothermic conditions. We study the case where the liquid present in the material is a water solution saturated with sodium chloride. The bonds are then of capillary type and the cohesive interactions are mainly attractive. In this case, the mechanical strength in an unconfined compression test is relatively low. At low relative humidity, the phase change of water involves a crystallization of salt at the contact points between grains generating thus bonds of solid type. The mechanical strength of the material is thus enhanced. An experimental study of the variation of the mechanical strength during the crystallization of salt allowed us to show two distinct cohesive regimes: capillary and cemented. The transition between these two regimes does not seem to be correlated with the mass of the crystallized salt, but rather with the residual degree of saturation. An analysis of these results is proposed by comparison with numerical simulations based on the discrete element approach.

Collapse phenomena during wetting in granular media

We present an experimental study of identification and analysis of the collapse phenomenon in granular media, using a sample made of glass beads. The collapse potential of a soil depends on several parameters such as liquid limit, matric suction, dry apparent density, initial water content and the amount of small particles. In this study, we investigate the liquid effect during the wetting process, without external mechanical stress. Experimental tests at different initial water contents were made for different grain sizes. The equipment used is a triaxial device for unsaturated soils, instrumented with local axial and radial displacement sensors. The experimental results determined a critical water content of collapse, as well as the influence of initial water content, matric suction and grain size.