Multiscale Poromechanics: Fluid Flow, Solid Deformation, and Anisotropic Plasticity

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Natural geomaterials such as fissured rocks and aggregated soils often exhibit pore size distributions with two dominant porosity scales. In fractured rocks the dominant porosities are those of the fractures and rock matrix, whereas in aggregated soils the micropores and macropores comprise the two relevant porosity scales. When infiltrated with fluids this type of materials may also exhibit two permeability scales. In this paper we present a framework for so-called ‘dual porosity-dual permeability’ materials that covers both steady-state and transient fluid flow responses. The formulation revolves around a thermodynamically consistent effective stress previously developed for porous media exhibiting two porosity scales. Apart from the aspect of multiscale poromechanics, some geomaterials such as shale also exhibit pronounced anisotropy in their mechanical behavior due to the presence of distinct bedding planes. A transversely isotropic constitutive model is appropriate for this type of material behavior. Anisotropic plasticity models can easily be integrated into the aforementioned dual porosity-dual permeability framework.