Instability of unsaturated soils: A review of theoretical methods

The paper reviews a series geomechanical approaches for interpreting instabilities in unsaturated geomaterials, a class of solids involved in numerous geo-engineering problems, such as hazard forecasting, infrastructure management, underground disposal of by-products and energy technologies. The review details the connection between second-order work input, loss of controllability and material failure. Hydro-mechanical problems are addressed, focusing on a specific class of environmental perturbations that can cause sharp changes in both mechanical and hydrologic variables. A procedure to define the second-order work input to an unsaturated soil volume is discussed first. It is pointed out that this energy measure motivates the incremental form of the constitutive laws for stability analyses. It is then discussed how to link the theory of hydro-mechanical controllability with constitutive approaches for unsaturated soils, which are typically based on the framework of strain-hardening plasticity with extended hardening. In doing so, the crucial role of the properties that govern the interactions between soil skeleton and fluid-retention processes is emphasized. The implications of these findings are commented with reference to a specific application: the forecasting of landslide triggering in natural slopes. It is shown that the use of suitable stability indices allows one to differentiate between frictional slips and volumetric collapses turning into flows. These results suggest that geomechanical theories calibrated for site-specific properties can support the quantitative assessment of landslide susceptibility, as well as a number of other engineering applications involving the instability of unsaturated porous media.