On the hydro-mechanical behaviour of unsaturated damaged Callovo-Oxfordian claystone

Abstract

During the excavation of drifts within the French Underground Research Laboratory (URL) in the Callovo-Oxfordian claystone (COx) formation, an excavation damaged zone (EDZ) was formed, which is expected to have adverse effects on the long-term safety of the radioactive waste repository. In this study, high pressure oedometer tests with controlled suction were performed to investigate the hydro-mechanical behaviour of damaged COx claystone. The hydraulic conductivity was measured under different vertical stresses. The suction and damage effects were analysed in terms of swelling, compression, hydraulic conductivity and creep. Besides, the evolution of micro-cracks inside the damaged claystone sample was also analysed through X-ray computed tomography (CT) and Mercury intrusion porosimetry (MIP). The oedometer results indicate that the damaged claystone has a lower swelling strain than the intact one, due to the effect of macro-pores created by damage. The damaged claystone with a higher suction exhibits a lower compressibility and larger yield stress. There is a linear correlation between hydraulic conductivity and vertical stress at zero suction. Compared with the intact claystone, the damaged one presents a lower compression index Cc. Besides, step compression index Cc* and secondary consolidation coefficient Cα both increase with the increase of vertical stress and with the decrease of suction, indicating the dependence of the compression and creep behaviours on stress and suction. A satisfactory linear relationship is observed between Cα and Cc* at each suction and the corresponding slope depends on suction. Microstructure observations from CT and MIP tests show that the micro-cracks inside the damaged sample after loading are more significant with the increase of suction, suggesting that the micro-cracks were closed more easily by loading at a lower suction. A damaged coefficient was identified to quantitatively analyse the evolution of micro-cracks.