Rock salt formation has been widely considered as a desired geological repository system for nuclear waste disposal. While the crystalline structure of rock salt has been often incorporated phenomenologically in macroscopic constitutive models, the microstructural effects in the thermo-hydro-mechanical responses are dedicated by the nature of polycrystalline rock salt. This paper presents relevant modeling techniques designed for polycrystalline rock salt in terms of its grains and grain-boundaries. We will provide an overview how the multi-phase-field approach combines single crystals into multi-grain systems considering their interfaces. In particular, an interface constitutive model is presented to capture frictional behavior of both grain-boundary and intra-granular facture surface that is approximated by a diffusive transition zone via the phase-field method. Special emphasis is placed on introducing nonlocality into the interface constitutive model based the micromorphic regularization associated with a classical plasticity. This treatment further allows to take the size effects or rock salt into account by adopting a characteristic length for grain boundaries.

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