Suction effects in deep Callovo-Oxfordian claystone

An experimental programme was carried out with the aim of investigating suction effects in a specimen of Callovo-Oxfordian claystone extracted at great depth (479 m). A specimen that was saturated in an oedometer with limited swelling exhibited significant swelling and desaturation upon quick unloading at constant water content. The microstructure after unloading was compared with the initial microstructure based on mercury intrusion porosimetry measurements. Since the pore size distribution curves before and after loading appeared to be comparable in the range of pore sizes covered by mercury intrusion (entrance diameter between 363ṡ6 μm and 5ṡ5 nm), it was concluded, based on microstructure considerations, that swelling occurred in pores with a diameter less than 5ṡ5 nm, from water transfer within the clay matrix from inter-platelet to intra-platelet pores. This mechanism might occur when excavating galleries in areas of an excavation damaged zone that support instantaneous unloading sequences, as a first desaturation process prior to subsequent evaporation due to the ventilation of galleries.