6 | CONCLUSIONS
The choice of stress state variables has always played a central role in the description of unsaturated soil behavior, which should be based on scientific evidence. It is hence better to select the work‐conjugate variables contained in the work input equation derived from the thermodynamical energy balance equations. In this paper, the work input for unsaturated soils considering interfacial effects is proposed based on the porous media theory. Accordingly, the conjugate stresses and strain increments compliant with the work input equation can be obtained.
Based on the derived work input equation, the principle of effective stress of unsaturated soils and the constitutive law for liquids are discussed. The effective stress of unsaturated soils, conjugated with the displacement of the soil skeleton, is given in Equation 30. The surface tension is included and assumed to be a function of matric suction. The effective stress can be expressed in Bishop's type. It is shown that the effective stress parameter (Equation 34) is not simply equal to the degree of saturation, but relates to both saturation and the interfacial area.
The constitutive law for liquid phase under dynamic condition is presented based on the work input equation. It is indicated that the matric suction depends on both the history and the rate of change of saturation. Only when a new equilibrium is reached and the saturation does not change can the matric suction be equal to the capillary pressure. At equilibrium, a third variable, interfacial area, is added into the soil water characteristic equation. Traditional SWCC curve is in fact the projection of the pc ‐Sr‐awg surface on the pc ‐Sr plane. The interfacial area is very crucial to determine the value of effective stress, which can be calculated from the relationship among interfacial area, saturation, and capillary pressure. Simulation results show that our equations are in good agreement with existing experimental data.
