| Sumario: | The results of the thermo-hydro-mechanical (THM) study of the FEBEX bentonite performed during FEBEX II are presented. The laboratory test program continued in part with the works carried out during FEBEX I, particularly in activities related to tests aimed to the calibration of the models, the acquisition of parameters by back-analysis and the improvement of the knowledge on the behaviour of expansive clays. But the program has also included tests on new areas: investigations about the influence of the microstructure changes in bentonite, of temperature and of the solute concentration on the behaviour of clay. Besides, several tests were proposed in order to understand the unexpected behaviour observed in the “mock-up” test, towards the end of year 2. Temperature effects on water retention curves in confined and unconfined conditions were determined, and swelling pressure, hydraulic conductivity and swelling and consolidation strains as a function of temperature were successfully measured. Different experimental techniques and equipments were developed to study thermal induced changes under partially saturated states, covering a wide range of suctions. FEBEX bentonite remains suitable as a sealing material in HLW repositories (from the hydro-mechanical point of view) for temperatures of up to 80 °C, as it keeps its high water retention capacity, low permeability and self-healing ability. The extrapolation of results points out to the preservation of properties for at least up to 100 °C. Mercury intrusion porosimetry and environmental scanning electron microscopy provided promising results in order to characterise the bentonite microstructure and to give information about the mechanisms influencing pore size distribution changes on high active clays. The use of digital imaging techniques allowed verifying that at micro-scale level, where chemical phenomena prevail, strains are almost reversible as it is considered in the two-level elasto-plastic models. The swelling strains of clay upon saturation with different kinds of water (deionised, granitic and saline with different salts and concentrations) were measured as a function of initial dry density and overload. The effects of salinity on bentonite stiffness and permeability were also investigated. The swelling capacity of clay, saturated with highly saline water, reduces drastically, osmotic consolidation being even observed. The swelling strains measured when the samples were soaked with different saline concentrations are similar to the strains measured in suction controlled oedometer tests, in which it is applied a final suction equal to the osmotic suction of the saline solutions. The hydraulic conductivity of bentonite increases with saline concentration of the permeant, especially in the case of low densities. The infiltration tests have shown that the gas permeability of dry bentonite is very high. The water intake is higher for the sample tested under room temperature, since the hot zones of the sample tested under thermal gradient remain desiccated for a long time. It has been proven that the combination of infiltration tests and coupled THMC numerical analysis can help to a better understanding of the FEBEX bentonite behaviour under repository conditions. In summary, the behaviour of the clay may be mostly explained taking into account its double structure. The interactions between the two structural levels are responsible of the main features of the mechanical aspects of this behaviour. A deeper insight of the behaviour of compacted bentonite, and of the basic mechanisms controlling it, has been achieved using an elasto-plastic framework that incorporates the interplay between microstructural and macrostructural fabric levels in a simplified manner. On the other hand, temperature induced transfers between intraaggregate adsorbed water –of density higher than that of free water– and inter-aggregate free water could explain most of the features related with the temperature observed. However, more research effort must be dedicated to study certain aspects such as the water flow under low hydraulic gradients, the effect of osmotic and temperature gradients on this flow and the effects of deviatoric stresses on the mechanical behaviour, and, more generally, to relate the actual knowledge about the physico-chemical aspects of clay microstructure to the macroscopic behaviour of compacted bentonite.
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