The relationship between suction and swelling properties in a heavily compacted unsaturated clay

In order to study the hydro-mechanical behaviour of highly compacted unsaturated clays used in engineered clay barriers for nuclear waste disposal, some experimental techniques for controlling suction were extended into the range of high suctions. The technique of control by imposing a given relative humidity was considered for very high suctions (several hundreds of MPa), and calibration data from the literature were examined. Some corrected values were proposed, based on a calibration made with sodium chloride. The extension of the osmotic technique to higher suctions was made, and a value of 10 MPa was attained, enlarging considerably the range of this method. The study of the water retention and swelling properties of the FoCa7 clay under controlled suction and zero applied stress is presented. A good continuity between the two methods of suction control was observed. Fairly reversible responses to suction cycles were observed, in terms of water content and volume change. During these changes, it was observed that the air volume remained constant. The reversibility is related to the predominant role of a saturated microstructural level, strongly influenced by the physico-chemical bonds existing between water and the active clay minerals.