In order to understand the unsaturated host medium of the high-level radioactive nuclear waste disposal repository in deep geological formation,a series of hydration and dehydration tests were performed on the hard clay which has been considered as the potential host medium for storage of the high-level nuclear waste. A suction control device was developed and the effects of suction cycles and suction gradients on the hard clay were investigated. The results showed that the suction gradient had little effec...