Abstract: Relative permeability coefficient is a crucial parameter describing the hydraulic characteristics of unsaturated soil. In researches of traditional relative permeability model for unsaturated soil, the seepage characteristic model of wet phase rather than non-wet phase received much focus. To study the multiphase flow transport model of unsaturated soil, we at first simplified the soil pores into a group of tortuous capillaries obeying the fractal law; by establishing the transport characteristic model of liquid and gas phase in porous media, we obtained the model of relative permeability coefficient of both liquid and gas phase of unsaturated soil in association with the Young-Laplace equation and the Buckingham-Darcy’s law. The model only contains two parameters, with clear physical meanings and can be measured by experiments. Eight groups of test data validated the rationality and applicability of the present model. We hence conclude that the model better describes the multiphase seepage characteristics of unsaturated soil.

Key words: unsaturated soil, relative permeability coefficient model, multiphase flow, fractal theory, pore distribution