Résumé
By means of 3D coupled molecular dynamics/Lattice Boltzmann simulations, we analyzethe destabilization process of a granular bed of spherical particles inclined above its angleof repose and immersed in a viscous fluid [1]. Extensive simulations were performed fordifferent values of the packing fraction and slope angle. We study the evolution ofmacroscopic observables such as shear strain, packing fraction and excess pore pressure.We then analyze the contact network anisotropy. Two regimes are evidenced as inexperiments [2,3]: a loose regime where the slope fails spontaneously and a dense regimewhere the failure is delayed as a result of negative excess pore pressure built up inreaction to the dilation of the bed. The two regimes belong to the packing fractions belowand above 0.59, respectively. We focus in more detail on the creep-like deformation of theinclined bed in the dense regime. The time evolution of the packing fraction and shearstrain scale with a characteristic time obtained from a model based on the balance ofgranular stresses in the presence of a pore excess pressure and the relation of the latterwith dilatancy controlled by Darcian drag forces. The cumulative shear strain at failure isfound to be around 0.2, close to the experimental value [2], irrespective of the initialpacking fraction and inclination angle. In the same way, the time and packing fraction atfailure are correctly predicted by the model. A noticeable finding is that the networkdeforms by distortion at nearly constant connectivity. The contact network anisotropygrows with shear strain, and slope failure is triggered when the anisotropy levels off. Theanisotropy thus appears as an internal variable, reflecting the distortion of the contactnetwork. The independence of the internal friction angle with respect to the initial packingfraction and its dependence on the slope angle were studied and shown to be aconsequence of slope stabilization by the cohesive-like effect of negative excess porepressure. It is also interesting to note that the transition from stable equilibrium to inertialflow in the presence of a fluid is accompanied by large fluctuations. As soon as thecapacity of volume change by distortion is nearly exhausted, slope instability is triggeredby the fluctuations and amplified by lubrication forces as the avalanches proceeds.