Experimental characterization of Thermo-Hydro-Mechanical damage in claystones

Predicting and managing thermal and/or hydromechanical damage of rocks in the field of underground engineering represent major challenges that are not yet fully understood. These alterations can lead to serious consequences for the stability of structures, thus jeopardizing the sustainability and resilience of critical infrastructures. This thesis aims to contribute to the understanding of Thermo-Hydro-Mechanical processes inherent in geo-engineering applications to enable a more accurate assessment of associated risks through laboratory experiments. A first series of experiments on the Callovo-Oxfordian claystone focused on studying the influence of temperature on its mechanical properties in the context of geological storage of radioactive waste in France. Triaxial compression tests were conducted under pseudo-drained conditions with deformation measurements at different temperatures (20, 40, 60, 80, 100, and 150 deg. C), confinement pressures (0, 4, and 12 MPa), and sample orientations (parallel and perpendicular to the bedding plane). Analysis of the elastic coefficients indicates that, in all cases, anisotropic damage develops during deviatoric loading due to the opening of axial microcracks. Furthermore, maximum deformation and peak strength increase as confinement pressures increase. The initial heating phase generates a transient overpressure of interstitial water due to thermal expansion, creating microcracks likely parallel to the bedding plane. An overall decrease in maximum strength with increasing temperature (up to 100 deg. C) is observed due to the thermo-hydro-mechanical damage induced by initial heating. For parallel samples subjected to uniaxial conditions, this decrease is most significant, and volumetric dilation occurs during loading at the highest temperatures. For all other conditions, the decrease is more moderate, and there is no dilation because the confinement pressure reduces the creation of initial thermo-induced microcracks, which are also closed when the axial stress increases during compression tests when the orientation is perpendicular. There is no noticeable impact of temperature up to 100 deg. C on the evolutions of the elastic coefficients. The peak strength increases at the highest temperature (150 deg. C) in all cases, due to water vaporization and strong desaturation of the samples, inducing the development of very significant capillary suction. A second series of experiments was conducted on cylindrical samples of Tournemire argillite to study Hydro-Mechanically induced damage. Five hydraulic fracturing tests were carried out in a hollow cylindrical triaxial compression cell with different water injection rates. The samples were subjected to a stress state representative of the in-situ stress state. Damage induced by hydromechanical coupling was analyzed using deformation measurements and acoustic emission monitoring. The results showed that increasing the injection rate tends to increase the wellbore pressure at breakdown. According to the results, the breakdown pressure generally increases with the injection rate. Breakdown pressure is significantly lower than the theoretical value for the lower injection rates and much higher for the highest injection rate. The analysis of the characteristic parameters of the acoustic signals showed that increasing the injection rate generates more tensile cracks, whereas slower injection rates produce more shear cracks. The cumulative damage variable calculated from AE parameters starts to increase significantly when the borehole pressure starts to drop, confirming that the use of a low viscosity fluid induces the propagation of instable cracks initiated almost simultaneously with the pressure drop. The damage rate is faster for higher water injection rates. The analysis of the measured strains and post-mortem visual observation of the tested samples indicate that an anisotropic damage developed during the hydraulic fracturing tests and this damage corresponds probably to the opening of cracks mainly oriented in parallel to the bedding planes, with opening direction mainly perpendicular to these planes. (author)