Initiation of immersed granular avalanches

P Mutabaruka, J-Y Delenne, K Soga & F Radjai

Depending on its packing fraction, a granular bed immersed in a viscous fluid and inclined above its angle of repose is either unstable or stabilized by a negative overpressure induced by slow creep and expansion of the bed due to dilatancy. In this paper, we use a 3D coupled DEM/LBM algorithm with appropriate boundary conditions to investigate the spatiotemporal process of slope failure in this configuration. Our findings are in quantitative agreement with the available experimental results. We analyze the evolution of shear strain, packing fraction and pore overpressures for different values of the initial packing fraction and slope angle. We show that the time evolutions of shear strain and packing fraction scale excellently with a characteristic time extracted from a model based on the balance of granular stresses in the presence of a pore overpressure and the relation of the latter with dilatancy due to darcian drag forces. The cumulative shear strain at failure is found to be approximatively 0.2, as in experiments, irrespective of the initial packing fraction and slope angle. The triggering time and packing fraction at failure are correctly predicted by using this shear strain as failure criterion. We also analyze the evolution of the contact network during creep. Remarkably, the network deforms by distortion at a nearly constant connectivity, and slope failure is triggered when the anisotropy saturates. This work clearly demonstrates the feasibility of realistic numerical simulations for immersed granular materials and opens in this respect quite far-reaching perspectives in this field.

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