The effect of temperature on the volumetric strains of the clay skeleton and clay – water system has been examined experimentally. Heating tests at (various) constant effective stress and isotropic loading tests at (various) constant temperatures were performed on low-porosity clays under drained conditions. The results indicate that the thermal expansion of adsorbed water in these clays is significantly lower than that of free water. Depending on the effective pressure, the clay skeleton may undergo either compression or expansion. Key words: nuclear waste isolation, clay, low porosity, thermal strain, adsorbed water.

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Argillaceous masses considered for potential nuclear waste repositories may exhibit significant space variability in their carbonate content. This may affect mechanical clay properties, such as strength or maximum apparent preconsolidation stress known to strongly depend on carbonate content. This paper investigates experimentally the dependence of thermo-hydro-mechanical behaviour of clays on carbonate content. The properties investigated are thermal strains, thermally induced over-consolidation, strength changes, destructuration, and thermally developed water pressure in undrained conditions. The experimental data are analysed in terms of a thermo-elasto-plasticity theory for clays, being an extension of Cam-clay model, modified to include the variability of the carbonatic content.

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Long-term changes in mechanical properties of clays induced by heating are investigated based on findings at Orciatico site of natural analog of nuclear waste disposal. To quantify the extent of such changes, a chemo -thermo-plastic model for illitization and re-smectitization of clays [Int. J. Plast. (2000)] is used. The model is an extension of Cam-clay model. Identification of parameters is presented for the material from one peripheral borehole where temperatures developed in clay near the contact were in the analogy range. Short-term heating experiments on previously thermally unaffected specimens were used to identify thermal softening characteristics. Apparent maximum past precompression stress was determined throughout the borehole. Hypothetical history of the clay in the borehole including pre-loading, heating, cooling and mineralogical reaction was reconstructed, and the apparent maximum past precompression stress data were used to identify the characteristics of chemical softening function describing the long-term clay transformations. D

SUMMARYAn empirically established rule of Wroth' for the dependence of the shear modulus on the mean effective pressure and the overconsolidation ratio in clays is investigated within the framework of non-linear elasticity. The resulting isotropic-deviatoric coupling is derived and compared to experiments.

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