Abstract: Permeability coefficient is a main parameter to reflect the permeability of soil. Existent studies about influence of particle size and gradation on permeability coefficient of soil mainly focused on experiments. It is difficult to change a parameter completely independently because of the interaction of various variables in the experimental study, which results in considerable differences between the conclusions from different researchers. Under the condition that other parameters remain unchanged, numerical simulation can basically change a certain parameter independently to make up for the shortcomings of experimental methods. In this paper, the particle discrete element method is used to generate soil of porous media with different particle sizes, different porosities and gradations by adjusting the size of the control particles. Lattice Boltzmann method is used to simulate seepage in soil at meso-pore fluid scale. The results show that the lattice Boltzmann method can simulate soil seepage accurately and effectively. Main passageway phenomena and large particle size effect are found in the seepage process. The simulation results are in good agreement with those by Kozeny-Carman (K-C) formula, demonstrating the K-C formula is applicable to porous media with different gradations. The permeability coefficient increases with the increase of uniformity coefficient and the curvature coefficient. A formula for calculating permeability coefficient with gradation parameters is given. The formula is nearly equivalent to K-C formula, but the parameters in this formula can be measured easily.

Key words: lattice Boltzmann method, permeability coefficient, particle size, gradation, soil