Determination of the permeability of the Callovo-Oxfordian clay at the metre to decametre scale

Andra is continuously investigating the Callovo-Oxfordian clay unit as a potential host rock for the deep disposal of long-lived medium and high-level radioactive waste at the Meuse/Haute-Marne underground research laboratory. A large number of hydraulic measurements have been performed in instrumented boreholes. In particular, hydraulic tests have been conducted for the determination of the average hydraulic conductivity and the formation pressure at the decimetre scale. Furthermore, the evolution of porewater pressure was measured for more than four years in test intervals at distances from a few decimetres to 20 m from the drift floor located at 492 m below ground level. These measurements, reflecting in particular the drainage and suction effect induced by the ventilated drift, present a unique set of data for the determination of the permeability of the Callovo-Oxfordian clay at the metre to decametre scale.The pressure data measured since the end of 2005 in four intervals of borehole PEP1001 were analysed by numerical modelling. Two different 2D models (vertical and axial-symmetric) were used considering also the rock desaturation and depressurisation processes due to the climatic conditions in the drift. The suction – related to the relative humidity of the ventilation air – was taken into account according to Kelvin’s equation. Other transient effects, e.g. due to the seasonal temperature variation in the drift were also investigated. Eventually, sensitivity analyses to the key hydraulic parameters were performed.The best fit yielded a horizontal hydraulic conductivity of 7 × 10−13 m/s and a vertical hydraulic conductivity of 4 × 10−13 m/s in the Callovo-Oxfordian clay. The seasonal variation of the climatic conditions in the drift showed only little influence on the general pressure evolution and hence on the determination of the hydraulic conductivity. Further, the model results suggest that the measured pressure evolution in each interval of borehole PEP1001 is superimposed by local effects such as the thermal compressibility of the water in the steel lines, accounting for the seasonal pressure oscillations, and a possible borehole disturbance zone (BdZ) accounting for the relatively high pressure gradient directed towards the drift.

► Hydraulic conductivity (K) measurements of Callovo-Oxfordian clay at decimetre scale.
► Observed pressure (P) at decametre scale.
► Consistency check between K and P by numerical simulation of drainage/suction.
► Impact of borehole disturbed zone and climatic conditions in drift.
► Consistency for 7 × 10−13 m/s horizontal and 4 × 10−13 m/s vertical hydraulic conductivity.