Experimental Study of the Effect of Temperature on the Mechanical Properties of the Callovo-Oxfordian Claystone.

To analyse the effect of temperature on the mechanical behaviour of the Callovo-Oxfordian claystone in the context of deep geological disposal of radioactive waste in France, compression tests were performed in a triaxial cell under pseudo-drained condition with strains measurements, at different temperatures (20, 40, 60, 80, 100 and 150 °C), confining pressures (0, 4 and 12 MPa) and samples orientations (parallel and perpendicular to the bedding plane). The analysis of elastic coefficients indicates that in all cases an anisotropic damage develops during the deviatoric loading due to the opening of axial microcracks. In addition, the peak deformation and strength increase with the confining pressure. The initial heating stage generates a transitory pore water overpressure due to thermal expansion, which creates microcracks probably parallel to the bedding plane. Despite the scattering of some results, an overall decrease of the peak strength with increasing temperature (until 100 °C) is observed because of to the thermo-hydro-mechanical damage induced by the initial heating. For the parallel samples under uniaxial conditions, this decrease is the most important and volumetric dilatancy develops during the loading for the highest temperatures. For all other conditions, the decrease is more moderate and there is no dilatancy because confining pressure reduces the creation of initial thermo-induced microcracks, which are also closed when the axial stress increases during the compression tests when the orientation is perpendicular. There is no noticeable impact of temperature up to 100 °C on the evolutions of the elastic coefficients. The peak strength increases at the highest temperature (150 °C) in all cases due to the water vaporisation and then strong samples desaturation, which induces the development of a very significant capillary suction. Highlights: Compression tests were performed on the Callovo-Oxfordian claystone at different temperatures (20–150 °C), confining pressures (0, 4 and 12 MPa) and orientations (parallel and perpendicular). An anisotropic microcracking damage develops during the compression test and the increase of confining pressure increases the ductility and strength of the material. Initial heating stage generates a transitory pore water overpressure due to thermal expansion, which creates microcracks mainly under uniaxial condition. Despite the scattering of some results, an overall decrease of the peak strength with increasing temperature (until 100 °C) is observed due to the initial thermo-hydro-mechanical damage. The peak strength increases at 150 °C, due to the water vaporisation and then strong samples desaturation, which induces very significant capillary suction. [ABSTRACT FROM AUTHOR]