Effect of temperature on the mechanical behavior of Callovo-Oxfordian claystone

The Callovo-Oxfordian claystone, as potential host rock for the storage of radioactive waste in France (Bure, Meuse/Haute-Marne), is subjected to coupled processes, such as stress variations during excavation, changes in saturation, thermal variations caused by exothermic waste, and chemical interactions. To assess the feasability of geological storage in the short and long term, it is essential to develop coupled THMC (Thermo-Hydro-Mechanical-Chemical) models and experimental characterizations. These approaches, which are fundamental to geo-engineering applications, allow for a more precise understanding of associated risks.

This study focuses on evaluating the thermal effects on the mechanical behavior of Callovo-Oxfordian claystone through triaxial tests that simulate in-situ storage conditions. The tests are conducted in triaxial compression cells equipped with heating systems to examine the material at temperatures ranging from 20 to 90°C. Deformations are measured using strain gauges. Our experiments are focusing on the influence of parameters such as confining pressure (4, 8, 12 MPa), temperature (20, 45, 70, and 90°C) and orientation (parallel and perpendicular to the bedding plane). To reduce data dispersion, all tests are conducted on cores extracted from the same borehole, ensuring a homogeneous calcite content (approximately 20%) and particular attention is given to the initial saturation degree of the samples. Each sample undergoes preliminary 2D X-ray imaging to visually evaluate initial cracks. This step is critical for selecting the least initially damaged samples, thereby reducing biases caused by pre-existing microcracks. Only the samples with minimal cracks are further scanned in 3D, both before testing (initial state) and after testing (final state). These scans are analyzed with VGStudio MAX software (Volume Graphics GmbH) to evaluate deformation mechanisms occurring during deformation.

Our tests reveal that, for both orientations (parallel and perpendicular), the heating phase generates an overpressure of interstitial water (due to thermal expansion), likely inducing microcracks parallel to the bedding planes. This results in a slight reduction of the peak strength of the Callovo-Oxfordian claystone, which increase with increasing temperature due to thermo-hydro-mechanical damage caused by heating. Furthermore, regardless of orientation or confining pressure, an increase in the heating rate enhances the decrease in peak strength.

This research is essential for understanding the impact of heating on the mechanical properties of the host rock in order to optimise the design of the disposal and improve its long-term safety.