Shear strength properties of wet granular materials

Abstract

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.

Collapse of granular media subjected to wetting

“…7). This domain corresponds to low water contents when the capillary cohesion increases with the water content as shown by Soulié et al [11] and Richefeu et al [10]. During wetting, the capillary bridges merge and the liquid phase becomes continuous in the sample.…”

Bonded-cell model for particle fracture

“…In other words, considering only the lengths of the contacts, the weakest "link" in our system is the path of intercell contacts having the shortest length. In this sense, the variability of compressive strength reflects the statistics of all paths (with their different lengths and directions), on one hand, and the heterogeneous distribution of forces, as generally observed in granular materials, on the other hand [56,69,85,86].…”

Discrete simulation of model, loose cohesive powders: plastic consolidation, fractal microstructure and tensile strength

“…The rupture can thus be regarded as brittle -although for low P * c samples may still withstand smaller tensions, as they get progressively torn apart in a strain-controlled test. Classically, the strength of a cohesive material is assessed with the so-called Rumpf formula [9], which amounts in the present case to replacing the average normal force F N by −F 0 in relation F N = 7πaP/(zΦ) (which is nearly exact [10] and directly stems from the classical expression of stresses as sums over contacts involving forces and branch vectors). The Rumpf formula was proposed decades ago [14], based on the assumption that a large population of contacts simultaneously break in tension.…”

“…The present communication investigates those phenomena by discrete element simulations in quasistatic conditions. Such a situation, with loose systems, has hardly been addressed by numerical means, as the recent literature rather explored dynamic compaction [4,5], gravity deposition [6], steady flows [7], or denser materials [8,9]. We introduce a simple model material and report on its consolidation properties, in relation to its microstructure, thus presenting a brief account of studies published in two recent papers [10,11], to which a section with new results on the resistance to tension is added.…”