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.

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 problem of subgrade mud pumping under the action of train loads is common and challenging to cure. In order to investigate the occurrence condition and the mechanism underlying the development of mud pumping, the characteristics of 23 groups of soils prone to mud pumping were analyzed. Moreover, findings indicate that most of the soils have the following characteristics: (1) clay content is greater than 2%, and silt content is greater than 20%; (2) the liquid limit ranges between 23 and 75%, and the plasticity index varies between 5 and 42.5; (3) the permeability coefficient is between 3.28 x 10-8 cm/s and 1.39 x 10-4 cm/s; (4) the main mineral components of the mud pumping soils are illite, montmorillonite, and kaolinite; and (5) the saturation of the mud pumping soil is generally greater than 80%. In addition, silty clay was selected to carry out the subgrade mud pumping test. The results show that under cyclic loading, there is an excess pore water pressure gradient in the subgrade soil, which mobilizes the fine particles in the subgrade soil, especially in the upper part of the subgrade soil, to migrate with the water flow, forming mud, and eventually resulting in subgrade mud pumping.

Mud pumping is an undesirable subgrade distress in ballastless high-speed railway, significantly affecting the ride comfort and posing a threat to train operation safety. In this study, a full-scale physical model of the ballastless slab track was developed. A rainfall simulator was installed, and various testing sensors were embedded in the trackbed to investigate the phenomenon of mud pumping in ballastless tracks. The results revealed a three-stage process for the intruded rainwater, including the initial vertical infiltration, the following horizontal infiltration, and the eventual roadbed saturation. A significant excess pore water pressure gradient (PWP) was created vertically in the roadbed due to the moving train loads. Similarly, a small longitudinal gradient was also observed. Both PWP gradients indicated the spatial migration of fine particles within the roadbed. The contact pressure distribution under the concrete base varied notably under different roadbed conditions. In the saturated state, the maximum dynamic soil stress, initially located at side of the concrete base transitioned toward the track center. The findings contribute to a deeper understanding of mud pumping mechanism in ballastless tracks.

The demand for more eco-friendly and efficient solution for soil stabilisation has become highly imperative in recent years, given the burning issue of climate change and environmental degradation. Although numerous efforts have been carried out to develop new methods adopting naturally occurring materials and processes, there are still very limited their successful applications in practice. Among them, the use of biopolymers has shown great potential. As there is still a significant lack of understanding of how this solution can performance under cyclic loading such as road and railway contexts, this study aims to investigate cyclic behaviour of Xanthan Gum (XG)-treated soil. A low plasticity soil prone to mud pumping was collected from the field and mixed with different XG contents (0.5-2% by mass), followed by a series of cyclic triaxial tests mimicking subgrade work condition. The results show that biopolymer can significantly promote cyclic resistance of soil by substantially reducing the accumulated excess pore pressure and minimising ultimate axial strain (<2.5%). With only 0.5% XG content, the soil can mitigate considerably its localised behaviour induced by heavy cyclic loads, thus alleviating the formation slurry at shallow layers of subgrade foundation.

This paper addresses the cyclic behaviour and stiffness degradation of subgrade soils subjected to stress-controlled cyclic loading, with particular emphasis on soils that are prone to mud pumping or subgrade instability. With continuous passage of trains over weak, saturated, low-plastic subgrade foundations, the finer fraction of the soils tends to fluidise (i.e., behave like a fluid) and migrate upwards, thereby, fouling the ballast and hindering the long-term performance of the rail track infrastructure. This leads to significant costs associated with annual track maintenance. Through a series of undrained cyclic triaxial testing varying the cyclic stress ratio (CSR, representing the axle loads) and loading frequency (simulating train speeds), the authors noted a significant upward migration of finer fraction coupled with internal moisture redistribution within the failed specimens. Further analysis revealed the instability of specimens was caused by early softening behaviour, and it is accompanied by a sharp reduction in the specimen stiffness. To tackle this, the stiffness was evaluated in terms of axial dynamic modulus and strain energy per cycle was evaluated to better understand the fluidisation behaviour. A novel quasi-linear relationship between threshold residual strain and number of cycles is proposed to serve as a practical guide.

Railway infrastructure holds a crucial role in a nation's economic development as an affordable mode of transportation and promotes social integration. Mud pumping is one of the major challenges the railway network has faced recently in the soft and mountainous terrain within the ballast layer due to high-speed rail loads. Many researchers found that reinforcement of the ballast and sub-ballast interfaces with geosynthetics is a feasible solution to reduce mud pumping. Still, the demand for sustainable geotechnic solutions is continuously increasing, particularly in countries with abundant natural plant resources. However, studies on the natural soil reinforcement in rail embankment are scarce. In this current study, a numerical investigation was conducted to study the effectiveness of geotextiles and geogrids made from natural plants (Geo-Naturals) in railway embankments. The results showed vertical settlements drastically decreased when Geo-Naturals were included in the poor subgrade. Further, the estimated axial forces, in the lateral direction to the rail length, in single-layer geogrids were found to be the maximum in poor subgrade due to higher compressibility. Geogrids placed close to ballast and sub-ballast were effective in the reinforcement function. Geotextile placed at the subgrade and sub-ballast interface accelerated in-plane drainage and reduced excess pore water pressure, thus preventing mud pumping potential. The outcome of the current study showed the effectiveness of geo-naturals in sustainability, eco-friendliness, and cost-effectiveness in railway embankments.

Considering the impact of the subgrade water level and freeze-thaw cycles, experiments were conducted on ballast track subgrade mud pumping. The study analyzed the migration of water and fine particles, as well as the characteristics of mud formation during the mud pumping process of the ballast track subgrade under cyclic loading. The research findings indicate that, during the initial loading stage at ambient temperature, moisture migrates upwards from the bottom. As dynamic loading is continuously applied, the internal pore water pressure in the subgrade soil gradually dissipates, resulting in a decrease in the pore water pressure gradient and a stabilization of the moisture content in each soil layer. When the water level is positioned in the middle of the subgrade, the upper soil is in an unsaturated state with a relatively low volumetric water content of approximately 26%. Fine particle migration does not occur, and the effective stress at the subgrade surface is much greater than zero, thus preventing mud pumping. When the water level is at the top of the subgrade, particle migration is more pronounced. The effective stress at the subgrade surface rapidly decreases to below 0 under the action of the load, resulting in mud pumping phenomena. Compared to unidirectional freezing, freeze-thaw loading results in a slower descent rate of the freezing front and a greater amount of moisture migration. Under thawing conditions, the upper soil layer of the subgrade melts before the lower soil layer, forming a frozen soil interlayer. Due to the isolation effect of the frozen soil interlayer, the upper soil layer retains a higher moisture content. Under the action of the load, the effective stress at the subgrade surface rapidly develops into negative values, making it more susceptible to mud pumping.

Subgrade mud pumping is a worldwide problem that seriously threatens railway track stability and operational safety. This paper analyzes the influence of Kaolin content (plasticity index) on the characteristics of saturated subgrade mud pumping under cyclic loading using a self-developed test model. Test samples consist of clay mixed with 10%, 20%, and 30% of Kaolin. The results show that the addition of Kaolin can reduce the void ratio (for a given initial dry density) and create a more plastic soil which is less prone to fluidisation under loading, thus reducing the axial strain and excess pore water pressure which helps to mitigate the problem of mud pumping. Based on our test results, we put forward a criterion to distinguish whether mud pumping occurs. In addition, we find that the excess pore water pressure gradient in subgrade soil plays a key role in the migration of fine particles and that the generation of an interlayer under cyclic loading can also promote subgrade mud pumping.