Rainfall-induced flowslides of granular soil slopes: Insights from grain-scale modeling

Abstract

Rainfall-induced flowslides in soil slopes remain one of the most destructive natural hazards. To prevent and mitigate the hazard requires a good understanding of soil behavior under the constant shear drained (CSD) stress path. This stress path differs from typical triaxial compression stress paths that have been extensively studied in soil laboratories. Here we present an attempt to explore various behaviors of loose granular packings along different stress paths and the associated micromechanical mechanisms using a robust grain-scale modeling technique. Through systematic simulations of the CSD, constant-volume (CDCV), and conventional drained (CD) shearing stress paths, we show that the behavior under the CSD stress path conforms to critical state concepts at both macro- and micro-scale. The evolutions of the stress ratio (q/p'), coordination number (Z), and fabric anisotropy (ac) along the CSD stress path are highly similar to those along the CD stress path. A substantial plastic strain tends to develop once the second-order work becomes nonpositive along the CSD stress path, while a dramatic drop of shear strength will occur at the vanishing of second-order work along the CDCV stress path. In both cases, the second-order work vanishes at an almost identical stress ratio, which demarcates the lower boundary of the potential instability zone in the stress space. We establish an explicit relationship between this stress ratio and the initial state parameter of the packing. The significance of this relationship lies in its applicability for both CSD and CDCV loading paths and its root in the critical state theory.