Multi-scale description of hydro-mechanical coupling in swelling clays. Part II: Poroplasticity

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This paper presents a micromechanical description of the stress-deformation in partially saturated swelling clays, which accounts for the plastic dissipation induced due to the coupled action of mechanical, swelling and capillary forces that operate at the underlying small scales. This is achieved via a two-stage homogenization procedure based on the average-field theory on a three-dimensional Representative Elementary Volume (REV) of clay which includes clay aggregates and partially saturated inter-aggregate micro-pore space. For the aggregates, which are assumed saturated with an electrolyte solution, the multiscale nonlinear poroelasticity model developed in Part I of this two-part paper is used here, which accounts for swelling forces that develop within the aggregates and span the crystalline and osmotic regimes. The interplay between mechanical, capillary and swelling forces across the material scales leads to the clay aggregates gliding upon each other. The behavior of the interface between adjacent clay aggregates, which describes their sliding, plays a prominent role in the macroscopic behavior of clays. This is manifested in the form of induced plasticity, described here using a simple plasticity law, which is upscaled using principles of continuum micromechanics. This work reconstructs all underlying complex couplings at play to finally arrive at a poroplasticity model at the macroscopic scale, where the macroscopic plastic strain is directly related to sliding action between clay aggregates at the micro-scale. Such a micromechanical description of plasticity in clays is made possible by utilizing the enhanced localization procedure presented in Part I. Finally, the model is shown to be successful in predicting the response of clays in a typical swelling test, when comparing the model predictions against experimental measurements.