A double-porosity formulation for the thm behaviour of bentonite-based materials
- Autores: Ramon Barboza de Vasconcelos
- Directores de la Tesis: Antonio Gens Sole (dir. tes.), Jean Vaunat (codir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2021
- Idioma: español
- Materias:
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- Resumen
The thermo-hydro-mechanical (THM) behaviour of expansive clays has been extensively studied in the last decades due to the potential use of bentonites as components of engineered barrier systems (EBS) in deep geological repositories for high-level and long-lived radioactive wastes. Since the early post closure period, the EBS is subjected to changes in temperature, moisture content and stresses due to the coupled THM processes expected to occur in such an environment. The different structural levels found in unsaturated expansive clays requires the use of constitutive models that considers the explicit distinction of these pore-structure levels in their mathematical formulation in order to reproduce the development of the fabric of bentonite materials subjected to the complex THM paths taking place during the lifetime of a nuclear waste repository. A coupled THM formulation that represents the expansive soil as two overlapped but distinct structural media has been developed in the framework of classical and generalized plasticity theories. In such a double-structure approach, the THM behaviour of the expansive soil is characterized by constitutive laws formulated to account for the relevant processes affecting each porous medium and for the interaction mechanisms relating the deformation and the saturation states of the active clay particles to the structural arrangement of the clay aggregates and to the water potential in the larger interconnected pores. In addition, the mechanical response of the porous medium to any THM loading is intrinsically related to the compressibility of the clay minerals. The irreversible changes in the soil fabric are attributed to the loading-collapse (LC) mechanism and to the micro-macro structural coupling (ß-mechanism). Thermal effects are incorporated into the mathematical formulation of the double structure model, which has been implemented in a finite element code (CODE_BRIGHT) able to solve, in a fully coupled way, the system of partial differential equations arising from the governing equations (balance equations). An explicit and robust integration scheme with automatic sub-stepping and error control has been employed to update the stress tensor and the internal (history) variables. The capabilities of the implemented double-porosity model to predict the expected response of expansive clays under isothermal and non-isothermal scenarios have been checked by the performance of constitutive analyses following a number of prescribed THM paths under confined and unconfined conditions. In addition, sensitivity analyses have been carried out in order to verify the dependence of the local expansive response on the initial conditions and on the sequence of load application. Special attention has been placed on the role played by the pore-water mass transfer between the two pore-structure levels in the development of the swelling potential of the expansive porous medium. The performance of the model in reproducing the actual THM behaviour of laboratory-scale tests has also been examined by means of the modelling of the hydration of two heated columns made of granular bentonite materials, selected as potential buffer materials in the construction of engineered barriers. The comparison between the available experimental data and the model results has shown the ability of the current double-porosity formulation to simulate the main observed features of the THM behaviour of the expansive material when subjected to complex loading paths.
