ABSTRACT
The presence of air in soil makes its properties very complicated. Firstly, this paper listed the divisions of the air-water patterns by different researchers and showed that discontinuous gas exists in unsaturated soils. It plays an important role on the unsaturated soils, because discontinuous gas will change its pressure during soil deformation. Then it was demonstrated that the gas pressure was needed to be used as another stress variable to describe the soil behaviour based on the work expression with the help of thermodynamics and mixture theory. After that a detailed description of gas behaviour in unsaturated soils is given. In order to take account of the gas effect, the plastic change of gas strain is introduced as an internal state variable to reflect the influence of gas phases. In this way, the dissipation caused by gas phase can be included. A simple model for isotropic stress states are given at last, which could couple the three phases of unsaturated soils.
