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, mud 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^®. 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.

The waste slag of soil and rock mixture produced by tunnel excavation is gradually being applied as a filler in roadbeds and retaining structures, forming a typical interaction problem between waste slag and concrete interface. A large-scale direct shear apparatus carried out a series of tests on the interface between limestone waste slag and concrete slab. The effects of different vertical stresses and surface roughness on the shear mechanical properties were analyzed from a macroscopic perspective. In addition, the microscopic parameters of the interface were analyzed by the discrete element software PFC^2.0. The results show that the surface roughness of concrete significantly affects the shear stress–shear displacement curves of the interface and the volume change of the sample during the shear process. The rough interface can significantly increase the apparent cohesion, therefore increasing the shear strength of the interface between limestone waste slag and concrete. Notably, the shear strength reaches the maximum when the surface roughness is about 1.59. The grain force chain distribution and the fabric anisotropy variation agree with the variation of macroscopic shear strength. The coordination number can better reflect the vertical deformation law of the interface. More so, the smooth interface shows the characteristic of slip failure, and the shear band is not obvious. On the other hand, the rough interface demonstrates a shear failure pattern that evolves from localized deformation to regional failure. Furthermore, the thickness of shear bands in rough contact surfaces is approximately 6–7 times the average particle size of the slag.

The fluid seepage in local-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, mud 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 local-saturated zone of subgrade considering the effect 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 local-saturated zone of the subgrade are studied. The research results show that due to the continuous erosion and migration of fine particles in local-saturated zone of the subgrade, the volume fraction of fine particles first increases then decreases and finally becomes stable with the increase of time. And 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 increase first and then decrease slightly with the increase of time, and finally tend to be stable. For a 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 in actual engineering, 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.

This article presents a series of cyclic triaxial tests to investigate the particle breakage characteristics of coarse-grained filler under heavy-haul train load. The results show that the main patterns of particle breakage for large-sized particles (the particle size between 22.4 and 31.5 mm) are fracture and abrasion, and the particle breakage makes the outer contour of the particle closer to the sphere. The particle breakage is found in the process of vibratory compaction of specimens, and the particle breakage caused by isotropic consolidation under low confining pressure (no more than 300 kPa) can be ignored. It is also found that significant particle breakage occurs during cyclic loading, characterized by the reduction of the large-sized particles (the particle size between 16 and 31.5 mm) and the increase of fines content. In addition, further particle breakage is caused by the increase in the cyclic stress ratio. Based on the test results, a power function equation of Marsal’s breakage factors and cyclic stress ratio is proposed.