Strain-rate effect on the hydro-mechanical behaviour of unsaturated damaged Callovo

Strain-rate effect on the hydro-mechanical behaviour of unsaturated damaged Callovo–Oxfordian claystone

Wang, Hao / Cui, Yu-Jun / Vu, Minh Ngoc / Talandier, Jean

An underground research laboratory (URL) was built at Bure, France, for conducting in situ experiments, which aims to demonstrate the feasibility of constructing and operating a High Level Waste disposal facility in the Callovo–Oxfordian (COx) claystone formation. During the URL construction, excavation of the galleries unavoidably induced a damaged zone, namely excavation damaged zone (EDZ). The ventilation within the gallery makes the EDZ under partially saturated condition. For the assessment of the long-term safety, it is necessary to investigate the time-dependent behaviour of damaged and unsaturated COx claystone. In this study, intact COx claystone was first artificially damaged in the laboratory, then its strain rate-dependent volume change behaviour was investigated through one dimensional CRS (constant rate of strain) compression test with controlled suction. The results from suction equilibration indicate that the damaged claystone exhibits a higher water content and a larger void ratio. Further examination shows that the damaged claystone has a slightly lower degree of saturation at the same suction of 9 MPa. This is due to the pre-existing micro-cracks inside the damaged claystone. The results from the CRS tests show that the volume change behaviour (compression index and yield stress) is suction-dependent. Moreover, at the same suction, the yield stress increases with the increase in strain rate, which can be described with a linear correlation in a log–log plane. This phenomenon is consistent with the observations on other soils. A suction-dependent viscous parameter CA is thus obtained, which corresponds to the slope of this linear correlation. Furthermore, the compression and viscous parameters were determined and analysed for the damaged claystone, allowing a loading-collapse curve to be defined, which accounts for the effects of strain rate and suction.