Abstract: Based on the non-equilibrium flow theory of multiphase porous media, the thermodynamic mixed theoretical model is proposed by Wei et al[1]. According to the model, the dynamic water-soil characteristic curve of unsaturated soil, which describes the incremental relationship between capillary pressure and saturation, is deduced. Within the context of the dynamic water-soil characteristic curve, an evolution equation of saturation in an unsaturated sample is developed. The equation describes well the variation of saturation with time in a multistep transient outflow experiment provided that the deformation of the matric of the unsaturated sample is ignored. On the basis of the evolution equation and the characteristic of the multistep transient outflow experiment, a new predicted method is proposed to determine the soil-water characteristic curve. For the low liquid limit silt and the low liquid limit clay, by comparing the theoretical simulation with experimental data in the multistep transient outflow experiment, it is shown that the saturation evolution equation can very well describe the constitutive relationship between saturation and time provided that the incremental step of matric suction is small. In addition, the calculated results of the soil-water characteristic curve agree well with the measured results of that measured by the laboratory combined system.

Key words: non-equilibrium flow theory, multiphase porous media, multistep transient outflow experiment, soil-water characteristic curve