Effect of hydraulic hysteresis on shear strength of unsaturated clay and its prediction using a water retention surface

A vadose zone may exist within man-made or natural slopes under the condition of a deep water table. During repeated cyclic infiltration (wetting) and evaporation (drying), the suction and degree of saturation may change within this regime. Thus, it is crucial to have a thorough understanding of the effects of a cycle of wetting and drying on the shear strength of unsaturated soils to assess the long term stability of these slopes. Three series of laboratory tests are presented in this paper, from which the water retention and shear behaviors of an unsaturated clay are studied. A three-dimensional main wetting surface is proposed and a unique failure envelope expressed in terms of Bishopʹs stress can be used to characterize the shear strength of unsaturated soil subjected to different hydraulic histories. The measured shear strengths are compared with those predicted by the proposed main wetting surface and the drying curve under no confining stress. It is found that a better prediction is obtained by using the main wetting surface for unsaturated specimens subjected to different hydraulic histories.