Abstract: Considering the effect of grouting pressure on soil porosity, the grout flowing law is analysed and deduced from continuity equation in porous medium. The simple approximate formula of diffusion radius is obtained. Then two-phase theory is applied to grouting research. With the hypothesis that soil pores are filled with water and grout and they are immiscible, unsteady flow model of grout driving water is established. The solving key lies in getting the relations between relative permeability, capillary pressure and porosity. Two-phase flow module in FLAC software is used to simulate grouting process of a certain dam; and the results are compared with the deduced expression. These two results are approaching in the initial stage. But because of considering capillary pressure, the diffusing rate using two-phase theory decreases faster. The simulation results show that there is a transitive area between grout and water. Grout saturation is varied with time and space. As the time passing, grout permeates farther, but the rate becomes slow gradually. Diffusion radius is not only related with permeability coefficient but also with porosity; and porosity has more influence than permeability coefficient. The greater permeability coefficient makes grout flow faster and the larger porosity lets grout flow farther. Two-phase theory is better to analyse the permeating regularity.

Key words: curtain grouting, diffusion radius, two-phase flow, numerical simulation