Behaviour of an unsaturated highly expansive clay during cycles of wetting and drying

Cycles of wetting and drying were performed under isotropic stress conditions on compacted samples of an unsaturated highly expansive clay consisting of a natural sodium bentonite mixed with speswhite kaolin. The results showed that both irreversible shrinkage over wetting-drying cycles and irreversible swelling over wetting-drying cycles can occur on samples of the same soil, and this type of behaviour is not restricted to highly expansive clays. The precise forms of irreversible behaviour were not consistent with existing elasto-plastic constitutive models. An alternative physical explanation for irreversible volume changes is proposed, based on the occurrence of hydraulic hysteresis in the variation of degree of saturation.