Weather changes induce wetting and drying cycles in soil and consequently affect the matric suction and may create residual effects in soil, which is referred to as hydraulic hysteresis. The results of some researches in the literature indicate that effective stress parameter is changed under the influence of hydraulic hysteresis. In this regard, it is important to study the simultaneous effect of void ratio and hydraulic hysteresis on effective stress parameter in a wide range of suction. Therefore, in this research an experimental approach involving suction-controlled triaxial experiments was used to investigate the variations of effective stress parameter and degree of saturation versus suction at different initial void ratios along drying and wetting paths. The results of the experiments show that hydraulic hysteresis due to suction variation affects the effective stress parameter and changes the value of this parameter value during wetting and drying paths. Furthermore, the value of this parameter cannot be taken equal to the degree of saturation. In addition, effective stress parameter at a given value of suction depends on void ratio. The variation of effective stress parameter obtained from the experiments was compared with available models and their capabilities in predicting the soil behaviour were evaluated.
