The problem of mud pumping in saturated subgrade seriously affects the safe operation of trains on railways. There are relatively few research results on the characteristics of subgrade mud pumping, and those that do exist dispute the precise mechanism of the mud pumping. In this paper, a new test model is designed to study the important characteristics of subgrade mud pumping. The model can monitor not only the evolution of subgrade mud pumping but excess pore water pressure and dynamic stress in soil as well. In particular, we study the mud pumping of Lean Clay. Our results show that with the increase in the number of cycles, the axial strain of samples increases rapidly and then slowly. The axial strain increases with the increase in cyclic loading amplitude and decreases with the increase in loading frequency and initial dry density of Lean Clay. We also find that the excess pore water pressure first increases rapidly and then decreases slowly with the increase in the number of cycles. Furthermore, with the increase in cyclic loading amplitude, excess pore water pressure increases, and with the increase in the initial dry density, the excess pore water pressure decreases. We find that the loading frequency has little effect on excess pore water pressure. After the test procedure, we find that an increase in cyclic loading amplitude aggravates the degree of Lean Clay subgrade mud pumping and that an increase in loading frequency and increase in initial dry density of subgrade soil reduces the degree of mud pumping. We further find that the upward migration of fine particles driven by excess pore water pressure gradient is the main mechanism of subgrade mud pumping. However, the generation of an interlayer can also promote the occurrence of subgrade mud pumping.

The internal structure of sandy cobbles strata is sensitive to disturbances in the urban underground environment, but the structural evolution process under coupling hydraulic and dynamic loads remains unexplored. This paper presents a detailed investigation into the migration patterns and mechanisms of fine particles in sandy cobbles induced by coupled hydraulic and dynamic loading. A sandy cobble specimen with a typical particle size distribution (PSD) was designed and tested using an apparatus that included a constant inlet water head control system and an eccentric-vibrator-based dynamic loading system. Based on physical modeling tests, a numerical model was constructed to reproduce the internal structural evolution under hydraulic and dynamic loading by calibrating the time history of local permeability. The test results indicate that the application of dynamic load can instantly disrupt the stable internal structure of sandy cobbles under static seepage, imparting kinetic energy to fine particles that detach from the skeleton structure and migrate along the seepage direction. Significant fine particle loss occurs near the seepage outlet, but due to energy loss during migration, fine particles far from the seepage outlet are recaptured by the skeleton pore throats and clogged again in the migration path. As the intensity of the dynamic loading increases, the migration path for fine particles becomes longer, and the amount and size of fine particles lost significantly increase. The changes in the internal structure of the soil are reflected in hydraulic parameters as a transient increase in local flow velocity, an increase in local pore water pressure due to clogging, and a decrease in the overall permeability coefficient with the loss of fine particles. These findings enrich the knowledge of internal erosion in urban underground environmentand will be meaningful for future geotechnical engineering design and analysis.

Due to rainfall, the soil-rock differential weathering interface of spherical weathered granite soil slopes is prone to evolve into a dominant seepage channel and undergo seepage suffosion, which accelerates the deformation and instability of these slopes. However, little research has been carried out on the characteristics of seepage suffosion and the migration of fine particles. Based on the unsaturated seepage theory of porous media, a numerical calculation framework is established to accurately describe the seepage suffosion process at the soil-rock interface, considering the coupling relationship between the fine particle migration, suffosion initiation response and unsaturated seepage. The finite element method is used to construct a seepage suffosion model for unsaturated granite residual soil under the effect of dominant flow. Based on the seepage suffosion process of homogeneous soil columns, the suffosion characteristics of dominant flow under three typical soil-rock interface burial states are systematically investigated. The results show that the soil-rock interface and the matrix permeability of spherical weathered granite soil slopes are highly variable, with the wetting front forming a downward depression infiltration funnel, and the degree of depression of the wetting front becomes more pronounced as rainfall continues. The degree of fine particle loss is related to the burial state of the soil-rock interface, in which the dominant flow potential suffosion of the under-filled soil condition is the most significant, and even excess pore water pressure occurs at the interface, which is the most unfavorable to the stability of this type of slope. The research results can provide a scientific basis for accurately evaluating the stability of spherical weathered granite soil slopes under rainfall conditions.