Instability mechanisms in partially saturated coarse-grained soils: Implications for rainfall-induced flowslides

This paper deals with the wetting-induced instabilities that can occur in unsaturated shallow slopes and cause the evolution of slides into flowslides. In particular, it examines the instability phenomena that manifest in the unsaturated regime prior to shear failure, focusing on the triggering mechanism and predisposing factors. A theoretical index to capture instability initiation is derived in a general form that is valid for all single-surface isotropic hardening plasticity models, regardless of the assumption on the flow rule and the type of perturbation applied. The criterion is then specialised to the case of the WR2-Unsat model and used to monitor the stability of a soil element belonging to an ideal infinite slope during wetting processes. The results of the parametric study suggest that particular attention should be paid to relatively steep slopes with a shallow cover of no more than a couple of metres, consisting of highly collapsible and/or compressible coarse-grained soils with a low fine fraction susceptible to liquefaction.