Modelling instabilities in triaxial testing on unsaturated soil specimens

The paper presents a theoretical approach to deal with mechanical instabilities in unsaturated soils. Towards this goal, the concept of test controllability is extended to a hydro-mechanically coupled framework. A constitutive approach based on the introduction of hydraulic generalized stress-strain variables is first adopted, in order to better describe suction effects on the mechanical behaviour of soil. The mathematical consequences of hydro-mechanical coupling are presented next and two indices are defined to identify the onset of an instability. Possible instability modes linked to saturation processes are discussed. It is shown that the way in which the hydraulic variables are controlled in tests on unsaturated soil specimens is the key factor for the possible occurrence of instabilities and the consequent loss of test controllability. It is shown in particular that unsaturated soil specimens are prone to instability when an externally controlled water flux is injected into the specimen (inundation). This result could in part explain the sudden collapse of soil specimens subjected to wetting under constant applied stresses, which is observed both in the laboratory and in the field.