Abstract: Unsaturated soil mechanics has become an active researching area in recent years. However the understanding of some fundamental problems is inconsistent and some basic concepts are confusing. This paper aims at the fundamental problems, e.g. the choices of state variables and effective stress of unsaturated soils, the definition of suction, the limitation of axis-translation technique and the characterization of unsaturated soil structure. In these problems, the choices of state variables and effective stress of unsaturated soils play an important role in the theory of unsaturated soil mechanics and constructing constitutive models. Firstly, this paper briefly reviews expressions of effective stress of unsaturated soils and discusses their advantages and disadvantages. Secondly, this paper points out that there does not exist such matric suction beyond a certain limit in real site because of cavitation of capillary water. However the laboratory axis-translation technique conceals the possibility of cavitation and modifies soil behavior. Thus the validity of theories of shear strength and deformation of unsaturated soils based on it is a question worth study. Thirdly, the paper suggests that the characterization of unsaturated soil structure should include the distributions of pore water and pore air, the interaction among all phases (e.g. capillary and physical-chemical forces), except for the elements of saturated soil structure, e.g. fabric and inter-particle actions. Finally, this paper further combs the confusable concepts. The purposes of these discussions are to attract attention of domestic scholars on these problems, to set up correct understanding about them, and to promote development of unsaturated soil mechanics.

Key words: unsaturated soils, suction, effective stress of unsaturated soils, limitation of axis-translation technique, unsaturated soil structure