In the process of expanding ballasted railway capacity, there is a significant increase in train axle load and speed, which leads to significant mud pumping disease under multi-stage/multi-frequency train load-wetting coupling, and its mechanism is still unclear. Mud pumping model tests from ballasted track subgrades under multi-stage/ multi-frequency train load-wetting (MSC-W test/MFC-W test) coupling were conducted. The test results show that in the unsaturated state, the accumulated deformation of MSC-W test is more significant than that of MFC-W test, and the compactness of the subgrade filler is greater without significant particle migration. Under saturated or near saturated conditions, the MSC-W and MFC-W tests produces significant mud pumping by the driving force of dynamic pore water pressure. The amounts of mud pumping, fine particle layer displacement and void contaminant index (VCI) of the MFC-W test are significantly higher than those of the MSC-W test.

Liquefaction behaviors of sand deposits with impervious stratum are quite different from that of homogeneous geological conditions. However, the micro- liquefaction behaviors of the interlayered deposits have been infrequently documented. This study introduces a novel experimental methodology aimed at examining the influence of silt interlayer on the liquefaction mechanisms of sand deposits from both macro and micro perspectives. In the experiments, the Excess Pore Water Pressure (EPWP) was analyzed in conjunction with recorded micro liquefaction images. The migration mechanism of fine sand particles beneath the silt interlayer was revealed. The existence of low permeability interlayer leads to prolonged retention of EPWP beneath the silt interlayer. Substantially, the water film on the base of the interlayer is demonstrated to be the mixture of pore water and silt particles flowing with high velocity under seismic motions, thereby resulting in significant strain localization. An agminated zone of loose fine sand particles is usually generated beneath the silt interlayer after the dissipation of EPWP.

The underestimated risk of contact erosion failure in railway substructures poses a significant threat to railway safety, particularly at the interface between the ballast/subballast and subgrade. The larger constriction size at this interface exacerbates the potential for long-term erosion, necessitating attention to safeguard railway integrity. This study introduces a novel laboratory erosion testing apparatus to evaluate contact erosion at the subballast-subgrade interface under cyclic loading. Subgrade soils with varying fines contents are tested, and the effect of pressure head on erosion is investigated in detail. The results indicate that sandy soil with higher internal stability exhibits a higher critical pressure head for contact erosion. Cyclic loading induces oscillations in pore water pressure within the subballast layer, with higher pressure heads leading to larger amplitudes. Excess pore water pressure is generated in the sandy soil layer during cyclic loading and gradually dissipates over time. Fine eroded particles migrate into the subballast layer, forming mud, while coarse eroded particles accumulate at the base, creating low-permeability interlayers. Notably, the geometric conditions alone may not guarantee effective prevention of contact erosion in railway substructures. The hydraulic conditions for contact erosion are more easily achieved under cyclic loading compared to static loading. These distinctive features of contact erosion in railway substructures, different from those observed in hydraulic structures, provide some insights for the development of remediation strategies and improvements in railway substructure design.

Existing ballasted track subgrades are prone to complex particle migration problems due to intermittent train load-rainfall wetting coupling, which causes mud pumping in severe cases. In this work, a model test on a ballast layer overlying a fine particle layer was conducted under intermittent load-wetting coupling conditions. The experimental results indicate that the coupling effect of intermittent loading and wetting has a significant effect on the increase in the volumetric water content and pore water pressure. The changes in the accumulated deformation, resilient modulus, damping ratio, and particle migration phenomenon mainly occur in the first three loading stages (LS1-LS3 stages), and the changes are most significant in the second loading stage (LS2 stage) because of the high saturation and low density of the soils. During the subsequent loading stages, the changes in the accumulated deformation, resilient modulus, damping ratio, and particle migration phenomenon are not obvious because of the high density of the soils. A low level of resilience occurs during intermittent periods (IS4-IS7). At the end of the test, the ballast fouling index (FI) was 16.4%, reaching a moderate fouling level. Timely replacement and rectification should be conducted for sections that produce mud pumping and ballast fouling.

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 migration of fine particles under hydraulic scenarios is a primary cause of deterioration and even failure in many geotechnical structures. The particle migration test of the two-layer structure of the gravel and sandy-silty mixture under cyclic loading was performed to analyse the properties of particle migration under cyclic loading-hydraulic coupling. The results show that the variation in mud turbidity is influenced by both the fines content and the effective particle size. The particle size distribution of the sandy silt sample exhibits significant changes posttest. The upper layer of the sandy silt sample experiences the most substantial loss, while the middle layer shows a negative comprehensive loss, and the lower layer displays a positive loss. This research enhances our understanding of particle migration mechanisms in saturated soils subjected to cyclic loading, providing crucial insights for the stability assessment of railway substructures.

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.