Multiscale modeling of granular dynamics on flowslide triggering and runout

A hierarchical multiscale modeling framework is proposed to simulate flowslide triggering and runout. It couples a system-scale sliding-consolidation model (SCM) resolving hydro-mechanical feedbacks within a flowslide with a local-scale solver based on the discrete element method (DEM) replicating the sand deformation response in the liquefied regime. This coupling allows for the simulation of a seamless transition from solid- to fluid-like behavior following liquefaction, which is controlled by the grain-scale dynamics. To investigate the role of grain-scale interactions, the DEM simulations replace the constitutive model within the SCM framework, enabling the capture of the emergent rate-dependent behavior of the sand during the inertial regime of motion. For this purpose, a novel algorithm is proposed to ensure the accurate passage of the strain rate from the global analysis to the local DEM solver under both quasi-static (pre-triggering) and dynamic (post-triggering) regimes of motion. Our findings demonstrate that the specifics of the coupling algorithm do not bear significant consequences to the triggering analysis, in that the grain-scale dynamics is negligible. By contrast, major differences between the results obtained with traditional algorithms and the proposed algorithm are found for the post-triggering stage. Specifically, the existing algorithms suffer from loss of convergence and require proper numerical treatment to capture the micro-inertial effects arising from the post-liquefaction particle agitation responsible for viscous-like effects that spontaneously regulate the flowslide velocity. These findings emphasize the important role of rate-dependent feedback for the analysis of natural hazards involving granular materials, especially for post-failure propagation analysis.