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The correlation between contact forces and the texture of a packing of rigid particles subject to biaxial compression is analyzed by means of numerical simulations. Four different aspects are investigated: stress tensor, dissipation due to friction, angular distribution of forces, and fabric tensor characterizing the anisotropy of the texture. All of them provide evidence that the contact network can be decomposed unambiguously into two subnetworks with complementary mechanical properties.

We analyze particle velocity fluctuations in a simulated granular system subjected to homogeneous quasistatic shearing. We show that these fluctuations share the following scaling characteristics of fluid turbulence in spite of their different physical origins: 1) Scale-dependent probability distribution with non-Guassian broadening at small time scales; 2) Power-law spectrum, reflecting long-range correlations and the self-affine nature of the fluctuations; 3) Superdiffusion with respect to the mean background flow.PACS numbers: 83.80. Fg, 45.70.Mg The key role of fluctuations in quasistatic (QS) flow of granular media has been noted by several authors refering basically to stress fluctuations in time or the inhomogeneous distribution of forces in space [1,2]. Amazingly, few studies have been reported about the fluctuations of particle velocities under homogeneous strain conditions. These fluctuations have been observed to occur in a correlated fashion, though their scaling properties have not yet been analyzed [3,4]. Other recent studies concern mainly Couette flows where the strain is localized in the vicinity of the inner rotating cylinder [5].Analogies may be drawn with driven multiphase media dominated by steric interactions at the microscopic scale. Rheologically oriented studies of foams, for example, reveal large-scale cell rearrangements and vortex-like structures of the velocity field that control the complex flow behavior of foams [6,7]. However, the (capillary) elasticity of cell walls is an important factor that discourages a closer comparison with systems composed of stiff elements such as granular materials. Concentrated suspensions provide a closer analogy in this respect. There exists now convincing evidence based on particle tracking that non-Brownian particles in a sheared fluid at small Reynolds numbers undergo a (kinematic) diffusive motion beyond a concentration-dependent set-in time [8,9].A much more remote comparison can be made with the field of turbulence. There obviously the physics is fondamentally different from that governing granular media. Nevertheless, the rich body of work devoted to the statistical analysis of the fluctuating part of the velocity field in turbulence provides a suitable framework that can be applied in order to characterize the analogous fluctuating part of the velocity field in a granular medium. To be more concrete, let us consider the following key aspects of turbulent fluctuations[10]: 1) Non-Gaussian broadening of the probaility density functions (pdf's) of velocity differences as a manifestation of small-scale intermittency, 2) Multiscale organization of the velocity field reflected in its power-law spectrum, and 3) Anomalous diffusion pertaining to the Richardson regime. Although deeply rooted in fluid dynamics (Navier-Stokes equations, inertia regime), these scaling features may, in principal, prove to be relevant as well within a different physical context such as granular flows.Following this route, the objective of this paper is to show that parti...

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.