In general the behavior of unsaturated soil depends on the micromechanical structure of the three phases - soil skeleton, liquid(water), and gas(air). Thus the behavior of unsaturated soils is much more complex than that of saturated soils. Two stress variables(net stress and suction) are considered as important factors to represent relationships between stress and strain, state surface functions and permeability functions.
The numerical analysis of the mechanical behavior of unsaturated soils requires three governing equations. The governing equations are equilibrium equation, continuity rule for the pore air flow, and continuity rule for the pore water flow. The flows of air and water in unsaturated soils follow Fick's and Darcy's rule, respectively.
In this paper particularly the expansion behavior is considered. The finite element method is adopted to explain the expansion behavior of unsaturated soils. One dimensional and plain strain analyses are considered. The combined formulation of flow and deformation is adopted to simulate the expansion behavior of unsaturated soils. In the formulation it is assumed that the state of pore air is continuous and hence the pore air pressure is not developed. Merely only the pore water permeability is considered.
Particular initial and boundary conditions are also assumed for the expansion behavior of unsaturated soils. Initially negative suction pressures are considered and a constant excess pore water pressure is operated at top and bottom boundaries. From the analysis results, the swelling behavior of unsaturated soils could be reasonably simulated by the numerical formulation. The functions adopted for the pore water permeability and water characteristics are also reasonable for describing the behavior. However, more realistic state surface functions are necessary to be developed through experimental research results to model the in-situ field phenomena.
