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Shear strength of wet granular materials: Macroscopic cohesion and effective stress

Abstract : Rheometric measurements on assemblies of wet polystyrene beads, in steady uniform quasistatic shear flow, for varying liquid content within the small saturation (pendular) range of isolated liquid bridges, are supplemented with a systematic study by discrete numerical simulations. The numerical results agree quantitatively with the experimental ones provided that the intergranular friction coefficient is set to the value µ 0.09, identified from the behaviour of the dry material. Shear resistance and solid fraction ΦS are recorded as functions of the reduced pressure P * , which, defined as P * = a 2 σ22/F0, compares stress σ22, applied in the velocity gradient direction, to the tensile strength F0 of the capillary bridges between grains of diameter a, and characterizes cohesion effects. The simplest Mohr-Coulomb relation with P *-independent cohesion c applies in good approximation for large enough P * (typically P * ≥ 2). Numerical simulations extend to different values of µ and, compared to experiments, to a wider range of P *. The assumption that capillary stresses act similarly to externally applied ones onto the dry granular contact network (effective stresses) leads to very good (although not exact) predictions of the shear strength, throughout the numerically investigated range P * ≥ 0.5 and 0.05 ≤ µ ≤ 0.25. Thus, the internal friction coefficient µ * 0 of the dry material still relates the contact force contribution to stresses, σ cont 12 = µ * 0 σ cont 22 , while the capillary force contribution to stresses, σ cap , defines a generalized Mohr-Coulomb cohesion c, depending on P * in general. c relates to µ * 0 , coordination numbers and capillary force network anisotropy. c increases with liquid content through the pendular regime interval, to a larger extent the smaller the friction coefficient. The simple approximation ignoring capillary shear stress σ cap 12 (referred to as the Rumpf formula) leads to correct approximations for the larger saturation range within the pendular regime, but fails to capture the decrease of cohesion for smaller liquid contents.
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Michel Badetti, Abdoulaye Fall, François Chevoir, Jean-Noël Roux. Shear strength of wet granular materials: Macroscopic cohesion and effective stress. European Physical Journal E: Soft matter and biological physics, EDP Sciences: EPJ, 2018, 41 (5), ⟨10.1140/epje/i2018-11677-8⟩. ⟨hal-01980048⟩

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