The soil water interaction in unsaturated soils can be divided into capillarity and adsorption. The current study of unsaturated soils is mostly limited to lower suction range where capillarity dominates. In practice

due to climatic changes

soils near the ground surface often experience cyclic wetting and drying

and the low water content and high suction conditions

where adsorption dominates. Using the vapor equilibrium technique on the saturated salt solution

the high suctions are controlled for non-expansive kaolin-sand soil under drying and wetting paths respectively. The strength and deformation behaviors under high suction are tested under four confining pressures of 0 (close to unconfined)

25

50

100 kPa. The results show that the soils under high suctions indicate strain softening failure and dilatancy. With confining pressure applied

the failure mode change from the longitudinal cracking to shear failure. It is proved that the modified Bishop's effective stress model is not suitable for describing the peak shear strength at high suction. The peak shear strength can be expressed by the net stress-dilatancy relationship. The peak shear strengths are directly related to specific surface area and depend on the development of aggregates and dilatation.