A Thermodynamics-Based Constitutive Model for Clays Accounting for Double Porosity

Clays are natural materials characterised by a complex internal microstructure consisting of particles often arranged to form aggregates. Recent advances in experimental techniques, such as mercury intrusion porosimetry (MIP) combined with digital image correlation analysis from scanning electron microscopy (SEM), make it possible to identify at least two levels of porosity, one related to the intra-aggregate pores and another one related to the inter-aggregate voids, and to track their evolution upon loading. In continuum mechanics, constitutive modelling of clays can benefit from the aforementioned experimental evidence to identify measurable micro-inspired internal variables to back-predict the observed macroscopic response. In this perspective, in this study we propose a constitutive model for clays developed within the framework of thermodynamics with internal variables, in which we have selected two scalar internal variables, intra-aggregate and inter-aggregate porosities to track the clay response. The formulation takes advantage of a new strategy of initialisation of the inter-aggregate and intra-aggregate porosities based on the results of the MIP and the analysis of SEM images to reproduce the whole mechanical response of clays, from the early irreversible response to failure.