Hydro–mechanical analysis of embankments stability in partially saturated soils under rainfall events

Road and railway embankments are often constructed in partially saturated conditions and their water content can change over time according to rainfall events and ongoing interactions with the atmosphere. These changes may trigger instability phenomena due to the reduction of suction as the wetting front advances. Conventional slope stability assessments often overlook these changes and assuming either dry or fully saturated conditions, which could lead to inaccurate predictions. In this work, coupled hydro-mechanical analyses were carried out on clayey and sandy soils to investigate the response of the soil during rainfall on the shape of the infiltration profile, which in turn, control the stability of the slope. The analyses were carried out with PLAXIS 2D adopting an elasto-plastic constitutive law, with Mohr - Coulomb strength criterion and Bishop's effective stresses. The water retention curve and the permeability law were described by using the Van Genuchten model. Slope stability was determined using the φ-c reduction method. The results showed that, as expected, the degree of saturation and the suction profiles have a significant influence on the stability of the slope and, therefore, neglecting them can lead to incorrect design and/or prediction of behaviour during the slope service life. Moreover, the observation of the shape of the suction profiles from the numerical analysis led to the validation of paradigmatic infiltration profiles, which can be used to perform uncoupled limit equilibrium analyses accounting for the partial saturation condition in a simplified and yet effective way. A novel design chart is introduced to describe the interplay between the slope's hydromechanical parameters, geometry, groundwater table position, and infiltration profile. Such stability chart correlates the safety factor to the slope inclination and to the mechanical parameters of the soil, as well as to the apparent cohesion induced by the partial saturation in the unsaturated portion of the slope.