Flowslide High Fluidity Induced by Shear Thinning

Abstract Rainfall‐induced flowslides and the subsequent debris flows are the most recurrent and destructive natural hazards due to their high velocity and long‐distance runout. It is widely accepted that shear‐induced excess pore pressure controls the high fluidity in flowslide movement. Nevertheless, flume experiments demonstrate that flowslides can exhibit high fluidity even in the absence of excess pore pressure, demonstrating that even saturated granular matter has an intrinsic shear‐velocity‐dependent fluidization mechanism. Herein, drained ring shear experiments were conducted to obtain a further understanding of the fluidity of the saturated granular material. By means of a transparent shear chamber, the shear‐zone thickness and the velocity gradient within samples could be directly observed and thus its rheology could be well determined. The granular soil showed substantial shear‐thinning even in the absence of excess pore pressure, which could well explain the high fluidity in flowslide dynamics in addition to excess pore pressure, especially during rapid shear. Acoustic emissions (AEs) in the ring shear device provided additional evidence of the rheological mechanism. The macroscopic apparent viscosity of the granular system was weakened by rapid shear accompanied by increasing AE power. We propose that increasing grain pressure associated with increasing shear strain rate weakens intergranular frictional contacts between granular layers, thus weakening the apparent viscosity, and vice versa. By using AE measurements combined with dimensionless number analysis, we suggested that shear‐thinning is one of the most pivotal mechanisms accounting for the ultra‐high fluidity in flowslides.