Stability criteria for unsaturated shallow slopes

This letter focuses on the mechanical conditions governing the initiation of landslides in unsaturated shallow slopes. The goal is to provide mathematical criteria for predicting the onset of slope instabilities and capturing the interplay between saturation conditions and shear failure. For this purpose, the standard scheme of infinite slope is used and the mechanical response of the deposit is represented through simple shear kinematics. This assumption enables the study of different perturbation scenarios using a modelling strategy that: (i) incorporates the constraints describing the interaction with the pore fluids into the control conditions and (ii) allows the analytical derivation of stability indices for each scenario. The theoretical results show that, in unsaturated slopes, the coupling between the solid skeleton and the pore fluid can exacerbate the tendency to undergo mechanical instabilities. This general aspect is clearly disclosed by the analytical expression of the stability index for water-undrained shearing. It is found that, because of hydro-mechanical coupling, the failure mode originated by these perturbations embodies some characteristics of both shear localisation and static liquefaction. As a result, depending on soil properties and initial conditions, the unstable response resulting from its activation can resemble either phenomena and predictive models are necessary to distinguish between the two scenarios.