Anisotropy of partially saturated granular soils with generalised plasticity constitutive model

Soil anisotropy has been widely studied for saturated conditions, and its importance is generally accepted. Analysis of anisotropy for unsaturated soils is a less explored topic, both from experimental and constitutive standpoints. This works focuses on the analysis of experimental tests on very loose partially saturated granular soils for different stress paths, namely triaxial compression and extension, and proposes a methodology for its constitutive modelling within the framework of generalised plasticity. Its application for partially saturated conditions requires the implementation into the model of the volume change due to collapse of pore-air bubbles and the observed variation of maximum deviatoric stress between compression and extension. The former is approached by its intrinsic relation to the partially drained behaviour of soils, considering both fully undrained and drained limit states, whereas for the latter an additional hypothesis of the bonding function, based on experimental analysis, is presumed. The current proposal introduces no additional constitutive parameters into the model and predicts reasonably the experimental tests available in literature. Thus, a better understanding of geotechnical problems for unsaturated conditions, where anisotropy plays a relevant role, can be gained, as is the case for liquefaction of seabed slopes formed by gassy sediments.