Title: Triggering mechanisms of immersed granular avalanches

Author (Talk): Patrick Mutabaruka, Massachusetts Institute of Technology

Abstract:

By means of coupled molecular dynamics–computational fluid dynamics simulations, we analyze the triggering mechanisms of avalanches in a granular bed of spherical particles immersed in a viscous fluid and inclined above its angle of repose [1]. Two regimes are evidenced as in experiments [2,3]: a loose regime where the slope fails spontaneously and a dense regime where the failure is delayed as a result of negative excess pore pressure built up in reaction to the dilation of the bed. The two regimes belong to the packing fractions below and above 0.59, respectively. We focus in more detail on the creep-like deformation of the inclined bed in the dense regime. From detailed numerical data, we explore the time evolution of shear strain, packing fraction, excess pore pressures, and granular microstructure in this creep-like pressure redistribution regime, and we show that they scale excellently with a characteristic time extracted from a model based on the balance of granular stresses in the presence of a negative excess pressure and its interplay with dilatancy. The cumulative shear strain at failure is found to be 0.2, in close agreement with the experiments, irrespective of the initial packing fraction and inclination angle. Remarkably, the avalanche is triggered when dilatancy vanishes instantly as a result of fluctuations while the average dilatancy is still positive (expanding bed) with a packing fraction that declines with the initial packing fraction. Another nontrivial feature of this creep-like regime is that, in contrast to dry granular materials, the internal friction angle of the bed at failure is independent of dilatancy but depends on the inclination angle, leading therefore to a nonlinear dependence of the excess pore pressure on the inclination angle. We show that this behavior may be described in terms of the contact network anisotropy, which increases with a nearly constant connectivity and levels off at a value (critical state) that increases with the inclination angle. These features suggest that the behavior of immersed granular materials is controlled not only directly by hydrodynamic forces acting on the particles but also by the influence of the fluid on the granular microstructure. [1] P. Mutabaruka, J.-Y. Delenne, K. Soga and F. Radjai, Initiation of immersed granular avalanches, Phys. Rev. E 89, 052203 (2014). [2] R. M. Iverson, Landslide triggering by rain infiltration, Water Resources Research 36, 1897-1910 (2000). [3] M. Pailha, M. Nicolas and O. Pouliquen, Initiation of underwater granular avalanches: Influence of initial volume fraction, Physics of Fluids 20, 111701-111705 (2008).