Delayed and swelling behaviour of damaged/fractured Callovo-Oxfordian claystone

For the geological storage of radioactive waste, the French National Radioactive Waste Management Agency (Andra) built an underground research laboratory (URL) at Bure, France, in the Callovo-Oxfordian (COx) claystone formation. The excavation of drafts and drifts in URL results in fractures inside the host rock around the openings. In the fractured zone, the hydro-mechanical properties of the host rock were significantly affected. To ensure the long term safety of repository, it is necessary to investigate the hydro-mechanical behaviour of damaged/fractured COx claystone. The present study focuses on the compression, swelling and creep behaviour of damaged/fractured claystone. The preparation methods of damaged/fractured claystone were first developed in the laboratory. Then, oedometer tests were conducted on fractured claystone. Results show that the fracture can significantly affect the swelling, compression and creep behaviour. The self-sealing of fractured claystone upon wetting is caused by the filling of the swollen clay minerals. The compression results show that the fracture effect is dependent on its orientation and the stress level. Besides, the possible grain breakage effect was identified during compression. To clarify such effect, a series of oedometer tests were conducted on Teguline clay and artificial sand-clay mixture. Results confirm the effect of grain breakage on the compressibility of soils, depending on the fine fraction. In addition, suction effects on the swelling and compression behaviour of damaged COx claystone were investigated through oedometer tests with controlled suction. It is observed that the compressibility decreases and the yield stress increases with the increase of suction. The rate-dependent behaviour of damaged COx claystone was investigated through constant rate strain compression tests. Based on the results, the related mechanical parameters were determined, allowing characterization of the time-dependent behaviour of damaged COx claystone. Finally, accounting for the damage effect, a constitutive model was proposed allowing description of the viscoplastic behaviour of unsaturated COx claystone. The damage mechanism of COx claystone is clarified, allowing definition of a damage coefficient. The model is validated based on the results obtained in this study and some results from literatures. A good agreement is obtained between the simulations and the experimental results, showing the performance of the model proposed. (author)