Safety factors to detect flowslides and slips in unsaturated shallow slopes

Abstract

Unstable movements in slopes can be the outcome of various soil failure processes, each associated with a different degree of post-failure mobility. This note uses plasticity theories for unsaturated soils to derive safety factors for shallow slopes that can be readily used in spatially distributed models of landslide susceptibility. For this purpose, a simple suction-dependent constitutive law has been used to derive initiation conditions for two scenarios: frictional slips and flowslides. Despite its simplicity, the model enables the consideration of properties usually not included in landslide hazard studies (e.g. suction-hardening, liquefaction potential and retention curve). The results of the analyses show that these properties affect the mobilised friction angle at failure, up to a level at which the triggering of wetting-induced flowslides can anticipate the onset of frictional slips. The theory has been tested against data from flume tests on volcanic soils from Campania (Italy), thus validating it against physical systems with a failure kinematics similar to that of infinite slopes. The analyses show that the safety factors capture satisfactorily the dependence of the failure mechanism on suction and porosity. In addition, they have been used to derive charts quantifying the sensitivity of each failure mode to the slope angle, thus offering a tool to calibrate soil parameters against landslide inventories.