###### Abstract  
The fluid seepage in saturated zone of subgrade promotes the migration of fine particles in the filler, resulting in the change of pore structure and morphology of the filler and the deformation of solid skeleton, which affects the fluid seepage characteristics. Repeatedly, the muddy interlayer, muddy pumping, and other diseases are finally formed. Based on the theory of two-phase seepage, the theory of porous media seepage, and the principle of effective stress in porous media, a two-phase fluid-solid coupling mathematical model in saturated zone of subgrade considering the effects of fine particles migration is established. The mathematical model is numerically calculated with the software COMSOL Multiphysics(r). The two-phase seepage characteristics and the deformation characteristics of the solid skeleton in saturated zone of the subgrade are studied. The research results show that the volume fraction of fine particles first increases then decreases and finally becomes stable with the increase of time, due to the continuous erosion and migration of fine particles in saturated zone of the subgrade. The volume fraction of fine particles for the upper part of the subgrade is larger than that for the lower part of the subgrade. The porosity, the velocity of fluid, the velocity of fine particles, and the permeability show a trend of increasing first and then stabilizing with time; the pore water pressure has no significant changes with time. The vertical displacement increases first and then decreases slightly with the increase of time, and finally tends to be stable. For the filler with a larger initial volume fraction of fine particles, the maximum value of the volume fraction of fine particles caused by fluid seepage is larger, and the time required to reach the maximum value is shorter. It can be concluded that the volume fraction of fine particles in the subgrade filler should be minimized on the premise that the filler gradation meets the requirements of the specification in actual engineering. saturated zone of subgrade; fine particles migration; two-phase seepage characteristics; deformation characteristics; volume fraction of fine particles +
Footnote †: journal: Applied Soft  
will form a high-impact transient fluid, which will drive fine particles to seepage in the pores of the filler [1; 2; 3]. In the process of fluid seepage, there are complex effects of erosion and migration of fine particles, which will lead to the change of pore structure and morphology of the filler. At the same time, the action of external loads and the change of the pore structure of the filler will cause deformation of the solid skeleton, which will further change the fluid seepage channel and affect the fluid seepage characteristics, and ultimately induce a series of diseases such as mud interlayer, mud pumping, and so on [4; 5]. It is of great significance to deeply explore the fluid seepage characteristics and solid skeleton deformation characteristics considering the effects of fine particles migration and the change of hydraulic characteristics of coarse-grained soil to analyze the disaster-causing mechanism of the above diseases. At present, there are many studies on the model of fine particles migration and the mechanism of fine particles migration both theoretically and experimentally, and have achieved fruitful results. [PERSON] [6] and [PERSON] [7] have established a classical model for fine particles migration including the conservation equation of fine particles quantity and the kinetic equation of deposition. [PERSON] et al. [8] proposed a fine particles migration model considering the effects of sedimentation and sieving. On the basis of the traditional deep filtration model, [PERSON] et al. [9] applied the law of mass conservation, and established a particles migration model that considers the influences factors of seepage reduction coefficient, the velocity reduction coefficient, the change of porosity, and the suspended particles capture probability.