The water retention properties of the Callovo-Oxfordian claystone

High level long lived radioactive waste might be stored in galleries excavated at great depth in clays or claystones. During storage operations, the walls of the galleries will be successively subjected to desaturation due to ventilation followed by resaturation once the galleries are closed. Partial saturation may also affect laboratory samples, due to the added effects of coring, transportation, storage and trimming. This paper describes an investigation about the water retention properties of the Callovo-Oxfordian claystone (in which the ANDRA Underground research laboratory in France has been excavated) under drying/wetting cycles. A complete description of the changes in water content, volume and degree of saturation from the as-provided state is made. Data show that water content changes between 3% (at 150 MPa of suction) and 12% at zero suction, with a shrinkage of 1.4% (at 150 MPa of suction) and a 7% swelling at zero suction. The data obtained complete existing data by defining the initial state with respect to the main drying and wetting paths and by providing volume changes along the suction cycles. Hysteresis effects are also evidenced. Irreversible volume changes are assumed to be due to micro-crack generation and incomplete recovery.