Resumen de Hydro-chemo-mechanical model of bentonites applied to swelling processes
This Thesis describes a Hydro-chemo-Mechanical model to characterise the swelling behaviour of bentonites. It is based on the concept of double porosity, in which the voids within the particle aggregates (microstructure) and those between the aggregates (macrostructure) are differentiated. This model, besides describing the hydro-mechanical behaviour of bentonites for different boundary conditions in hydration processes, includes the effect of changes in the salinity of the system.
First, the basis of this model, which adopts a simplified geochemistry of the system, is proposed. This model is initially used to simulate free swelling processes of an MX-80 bentonite under different salinity conditions, obtaining quality results. The mechanical part is based on the Barcelona Expansive Model framework. However, the model also takes into account the macrostructural strain that occurs during swelling processes due to the bentonite destructuration under very low confinement or free swelling conditions. The water exchange between both structural levels is given by the difference in chemical potentials of the water present in both. The analysis of the difference between both chemical potentials results in a definition of the effective microstructural stress, understood as the magnitude that controls microstructural strains.
The hydraulic part of the model takes into account the differentiation of the water content present within the bentonite aggregates, and the free water between the aggregates. Water retention curve data were used to differentiate between the two water contents. In addition, the microstructural model was extended by taking account of the amount of sodium and calcium as exchangeable cations, as well as the cation exchange capacity of bentonite. On the other hand, the water retention curve of the macrostructure was also extended to consider the effect of changes of void ratio on the air-entry pressure. This modification of the model was applied to both MX-80 and FEBEX bentonites, as well as to Boom clay, based on data from water retention curves under free swelling and confined conditions. Likewise, the influence of the formulation of the chemical activity of species present in the macrostructural water was analysed, proving that taking into account the salinity conditions is fundamental to correctly characterise the swelling process.
The proposed model was introduced in the implementation platform Comsol Multiphysics, which allows the solution of partial derivative equations in a multiphysics environment based on the application of the finite element method with Lagrange multipliers. To improve the convergence of the model, two modifications were introduced in the mechanical part. The first one reduces the error due to drift in the calculation of stresses when an explicit integration scheme is used. The second assumes a smooth transition from an elastic behaviour within the yield surface to an elastic-plastic behaviour when yielding occurs. Both modifications allow to improve the computational performance of the numerical model.
Once the characteristics of the model are defined, it is applied to the simulation of different problems, in addition to the free swelling tests already mentioned. First, it was used to predict the results of swelling pressure tests under different salinity conditions. In addition, squeezing tests were also simulated, which have shown that the water extracted in this type of test is a mixture of the solutions present in the macro- and microstructures. In addition, the model was also used to reproduce vertical pinhole erosion tests, both under low salinity conditions and for different concentrations of the salinity solution. Finally, the model was proposed to be adapted for Gaomiaozi (GMZ) bentonites, for which there are numerous experimental characterisation studies. After fitting the model, constant stress swelling and swelling pressure tests were simulated. In all the cases analysed, the results obtained were very satisfactory.
This work shows the usefulness and versatility of the Hydro-chemo-Mechanical model that has been developed. Given its capacity to characterise the swelling behaviour of bentonites, considering different boundary conditions, the model is of special interest for the simulation of the bentonite barriers that will be part of deep geological repositories of spent nuclear fuel.
