Thermo-mechanical behaviour of in-situ heated Callovo-Oxfordian claystone

Abstract:

The Callovo-Oxfordian (COx) claystone, the candidate host rock for the deep geological repository of the radioactive waste in France, is expected to experience thermal loading up to 90 °C due to the decay heat of radioactive nuclides. This seminar focuses on the thermo-mechanical behaviour of in-situ heated COx claystone. Previous works have evidenced a plastic thermal compaction of COx claystone. In our study, we tested in-situ heated samples, which, in principle, should have been already compacted and therefore exhibit thermo-elastic behaviour when reheated in the laboratory. However, our laboratory thermal cycling on these samples showed thermal plasticity during the first heating cycle, and only the second thermal cycle recovered a thermo-elastic response. To investigate this phenomenon further, we applied multiple thermal cycles with intermediate mechanical deconfinement cycles, designed to mimic excavation-induced unloading. Thermal compaction was observed in first thermal cycle, followed by an irreversible plastic swelling during unloading cycle. The subsequent thermal cycle again produced thermal plasticity, demonstrating a strong correlation between thermal compaction and mechanically induced plastic swelling. Finally, we characterised this expansive behaviour in detail: Cyclic unloading under low confinement produced plastic expansion, while reloading revealed plastic compression, indicating that the preconsolidation stress had been reduced by the deconfinement process. Based on these experimental insights, a constitutive model was developed based on the anisotropic elasto-plastic framework previously calibrated with deviatoric tests. The model was extended by introducing a swelling yield limit and incorporating temperature dependency into both the compression and swelling yield surfaces through a coupled hardening-softening law. Validations show that this enhanced model successfully reproduces the thermo-mechanical responses observed in the experiments.

Short bio:

Hello everyone! I began my PhD in October 2022 at Laboratoire Navier and I work with Siavash Ghabezloo, Patrick Dangla and Philipp Braun. Before this, I obtained a Master’s degree in Road Engineering from Southeast University in China and an Engineer’s degree in Civil Engineering from ENPC in France. My PhD research focuses on understanding the thermo-hydro-mechanical behavior of Callovo-Oxfordian claystone, the host rock for the deep geological storage of radioactive waste in France. My aim is to develop the constitutive model, calibrate parameters with laboratory tests and simulate the underground structures to assess the performance of the claystone over the span of the repository.