Soil stability and flow slides in unsaturated shallow slopes: can saturation events trigger liquefaction processes?

This paper illustrates an application of the theory of material stability to the analysis of unsaturated slopes. The main goal is to contribute to the understanding of rainfall-induced flow slides. For this purpose, a coupled hydromechanical constitutive model is combined with a simplified approach for the analysis of infinite slopes. Simple shear-test simulations are used to evaluate triggering perturbations and investigate the role of both initial suction and stress anisotropy in the activation of slope failures. The numerical simulations clearly show that different mechanisms of activation can be originated. The onset of instability is detected by introducing appropriate stability indices for distinct modes of failure: localised shear failure, static liquefaction and wetting-induced collapse. Critical intervals of slope inclinations are identified, cautioning that the predicted failure mode may change dramatically depending on initial conditions, slope angle and material properties. The numerical simulations demonstrate that, in particular circumstances, saturation of the pore space can be the unexpected result of a volumetric instability. According to this interpretation, a rainfall-induced flow slide can originate from a complex chain process consisting of a sudden volume collapse, uncontrolled saturation of the pores and, eventually, catastrophic liquefaction of the deposit.