A series of large-scale shaking table tests was conducted on a pile network composite-reinforced high-speed railway subgrade. The displacement, peak acceleration amplification factor, dynamic soil pressure, and geogrid strain data were used to investigate the dynamic characteristics. The Hilbert-Huang transform spectrum, marginal spectrum, and damping ratios were used to study the seismic energy dissipation characteristics and damage evolution mechanisms of the reinforced subgrade. The results indicate that the graded loading of seismic waves induces a global settlement phenomenon within the subgrade, the displacement phenomenon of the slope is more evident, and the reinforcement effectively mitigates the amplification effect of the peak acceleration along the elevation. The peak and cumulative residual dynamic soil pressures were most significant near the bedding layer, and the upper and middle parts of the subgrade exhibited superior stabilization performance. The geogrid reduced the local vibration variability and enhanced the overall stability. The damage evolution in the middle part of the subgrade was relatively gentle, whereas the slope exhibited a multistage development trend. The internal damage of the subgrade grows slowly at 0.1-0.2 g, faster at 0.2-0.6 g, and rapidly at 0.6-1.0 g.

It is necessary to fully understand the settlement of high-speed railway subgrade induced by train loading to ensure the operation safety of high-speed trains. A 1:7 reduced-scale model test was designed to investigate the settlement of subgrade under two loading methods: continuous and intermittent cyclic loading. The testing results show that an increase in load amplitude enhances the load transmission effect to the bottom of the subgrade. After 105 cycles of continuous loading, the cumulative settlement of the subgrade at depth of 0, 20, and 40 cm directly below the loading range is 3.247, 1.05, and 0.09 mm, respectively, showing significant decreases with depth. A significant rebound can be observed when the applied load is removed during the intermittent loading process, which is quite different from the results under condition of continuous loading. Thus, the intermittent effect of train load on the cumulative deformation of the subgrade cannot be ignored. In addition, to better predict the cumulative settlement of the subgrade, a prediction method based on the state evolution model was proposed and used to quantitatively analyze the testing observations. Based on the state evolution model, the predicted cumulative strains at depths of 0, 20, and 40 cm were 1.218%, 0.457%, and 0.047%, respectively, which are in good agreement with the experimental results of 1.099%, 0.48%, and 0.045%, indicating that the theoretical model can accurately predict the cumulative strain of the subgrade caused by train load. Additionally, the parameters of the state evolution model can be updated in a timely manner by applying the updated monitoring data to enhance the prediction accuracy. The current work provides an alternative method for predicting the long-term cumulative settlement of subgrade induced by the train loading, and also a basis for the optimization of high-speed railway subgrade design.

The construction of high-speed railway in Southwest China must traverse extensive regions of red mudstone. However, due to the humid subtropical monsoon climate in Southwest region, the red mudstone is often exposed to a high-water content or saturated state for extended time, and the poor mechanical properties under such condition cannot satisfy the requirements of high-speed railway subgrade. This paper proposes the use of lime and cement to improve the saturated unconfined compression strength (UCS) of the red mudstone fill material. Comprehensive tests, including UCS tests and scanning electron microscopy, were conducted on cement-lime modified red mudstone. Results show that lime stabilisation can significantly enhance the UCS and elastic modulus with the increase of dry density and modifier content. For the specimens with 4% lime and 6% cement, both peak strength and elastic modulus of the modified samples are more than 10 times higher than those of the untreated ones. The modulus exhibits nonlinear degradation with the development of shear stress, but the degradation can be improved with the increase of dry density and modifier content. At 60% of initial tangent modulus, the corresponding stress for untreated soil, lime stabilised and cement-lime modified filler are 0.74, 0.92 and 0.99. As for the energy evolution, the increasing dry density can enhance elastic and dissipated energies through denser particle arrangements, while a higher modifier content raises total energy. When the cement content is 6%, the total energy is more than 8 times higher than that of the untreated material, reflecting increased brittleness to a sudden fracture. The improvements are attributed to the formation of acicular and platy hydration products, which can tighten the pore structure. The study underscores the importance of lime and cement in ensuring subgrade stability for high-speed railways in Southwest China's red bed regions.

Viable remediation of the large lateral deformation of high-speed railway (HSR) subgrades, especially in soft soil areas, is still absent. An integrated rectification scheme of high-pressure jet-grouting (HPJG) combined with stress-release techniques was conducted to rectify the large lateral deformation of an operational HSR subgrade in the soft soil area of China, but without the refined design and mechanical analysis before rectification due to the emergency. The objective of the study is to post-evaluate the rectification effects utilizing field monitoring data and numerical calculations. The monitoring data showed that the maximum lateral deviation of the subgrade was 69.1 mm, which almost met the expected correction requirements. However, the influence of the excess pore water pressure (EPWP) dissipation on the correction deviation was not considered in the scheme. Therefore, a numerical model was established to further investigate this effect and corresponding mitigation methods. The calculated results revealed that EPWP in the foundation dissipated mostly within six months after rectification, and the deformation loss accounted for 30.7% of the total deviation. Prolonging the interval of two-row pile construction can be a plausible approach to mitigate the deviation loss. The findings provide a feasible method for correcting large lateral deformation of HSR subgrades.

Uneven frost heave is frequently encountered in the subgrade-bridge transition zones (SBTZ) in seasonally frozen soil regions, which could lead to the deformation of track and even jeopardize running safety of vehicles. To this end, this paper conducts dynamic analysis of a vehicle-track coupled system accounting for the effect of frost heave deformation. Initially, the finite element method is used to obtain the relationship between rail irregularity and frost heave deformation. Then, a vehicle-track vertically coupled dynamics model is established, and its accuracy is validated by the measured data, published results and existing model. The time-domain dynamic responses of a vehicle-track coupled system under typical frost heave are analyzed. Afterwards, parametric analysis of frost heave deformation is conducted. Finally, the control threshold of frost heave is proposed from aspects of vehicle running safety, comfort, and track deformation. Numerical results indicate that the allowable amplitude of frost heave should be respectively restricted to 5, 20, and 25 mm for frost heave wavelengths less than 10 m, between 10 and 15 m, and greater than 15 m. The research findings offer theoretical support for the maintenance and operation of track in the SBTZ in seasonally frozen soil regions.

Concrete slab tracks help shield the supporting railway earth structure from external water ingress. However, the inevitable cracks that arise during its lifespan provide a pathway for water penetration, leading to changes in the degree of saturation of the underlying support. This can affect the dynamic response of the structure, however is challenging to model due to the computational requirements of three-phase unsaturated soil simulation. To address this, this paper presents two main novelties: 1) an efficient moving frame of reference approach for railway ballastless tracks on unsaturated earthworks subject to train loading, 2) new findings into the effect of degree of subgrade bed saturation on ballastless track dynamics. First the model is presented, including formulations for vehicle-track interaction and unsaturated subgrade dynamics. Considerations for numerical stability are then discussed and the model is validated, before investigating the role of subgrade bed saturation on pore water pressure and displacements. It is shown to have a high impact on pore water pressure generation, but a limited impact on deflections. The effect of train speed is then investigated and it is found that higher train speeds induce higher pore water pressures. Track irregularities are also investigated and it is found that they play an important role in pore water pressures.

In deep-buried long tunnels, train derailment accidents pose a serious threat to the stability of the tunnel lining structures and the safety of personnel along the line. To address the impact damage to the secondary lining caused by high-speed train derailments, a three-dimensional nonlinear dynamic analysis model of the Electric Multiple Unit (EMU) - lining - soil system was established. The advantages of this model include: it fully considers the complex streamlined design of the EMU front end, the nonlinearity of lining materials, and the M-C elastic structural model of the soil, allowing for accurate simulation of the contact and deformation between the EMU and the lining. The results indicate that the first 30 ms of the collision process are extremely intense, primarily involving the first three train vehicles. Among these, the head vehicle experiences the greatest reduction in kinetic energy and plastic dissipated energy, resulting in the most severe plastic deformation of the vehicle body. The impact load exhibits a distinct multi-peak characteristic, mainly composed of lateral impact force components. The area of displacement change in the lining expands continuously along the direction of the train, with peak displacements stabilizing after 30 ms. The lining primarily suffers from tensile failure, with multiple tensile cracks appearing in areas distant from the collision, while compressive damage is mainly concentrated at the point of direct impact. As the collision angle increases, the range of compressive damage along the longitudinal direction becomes narrower. The ratio of tensile damage area to compressive damage area is mainly influenced by the collision angle. In the design of tunnel structures for impact resistance, special attention should be paid to the lateral impact resistance and tensile failure capacity of the tunnel structure.

The subgrade structure of high-speed railways is an important foundation for the safe and smooth operation of high-speed trains, and the scientific design of the subgrade structure provides a fundamental guarantee of its durability and technical economy. As, in the development of high-speed railways in China, higher speeds are being pursued, more requirements have been put forward for the dynamic stability of subgrade structures. To address this issue, this article focuses on the control requirements for the long-term stability of subgrade deformation, and various design methods for high-speed railway subgrade structures are presented. Considering the energy dissipation and dynamic stability characteristics of subgrade filling materials, the dynamic performance of coarse-grained soil filling materials in the bottom layer and graded crushed stones in the surface layer are revealed. The methods for determining the values of dynamic parameters such as the dynamic modulus and damping ratio are provided. Based on the dynamic shakedown theory, the stress-strain hysteresis characteristics of fillers and the variation law of dissipated energy are revealed. The correlation between unit volume dissipated energy and shakedown state under cyclic loading conditions is identified. A criterion for determining the critical shakedown state of high-speed railway subgrade structures based on equivalent unit volume dissipated energy is proposed, and a method for determining the design threshold of dynamic stress and dynamic strain is also proposed. The results show that the shakedown design critical values of equivalent unit volume dissipated energy in the bottom and surface layers of the foundation were between 0.0103 similar to 0.0133 kJ/m(3) and 0.0121 similar to 0.0149 kJ/m(3) , respectively. The critical dynamic strain range was 0.8 x 10(-3)similar to 1.3 x 10(-3). On this basis, a high-speed railway subgrade design method based on energy dissipation and dynamic shakedown characteristics was developed. The results can provide theoretical support for the design of high-speed railway subgrade structures with different filling material alternatives and control standards.

Collapsible loess has special sensitivity to water, and its engineering mechanical properties deteriorate significantly after immersion in water, causing the foundation to sink, which seriously threatens the safety and stability of the high-speed railway subgrade under train vibration loading. Studying this effect is essential to prevent and control the disasters of high-speed railway subgrades. In this study, a model with the function of simulating foundation settlement is established to conduct disaster testing of high railway subgrade under train vibration loading. The results indicate that when different foundation shapes are settled, the surface of the subgrade under static load is gradually settled in a short time, and the settlement value of the track surface is lower than that of the corresponding subgrade surface. Under train vibration load, the maximum dynamic settlement occurs at the middle of the subgrade slope, which is smaller than the corresponding settlement under static load. The number of stabilization times required from different monitoring positions on the subgrade surface is different under different excitation forces, and the number of stabilization times required is more in the middle of the subgrade slope and the slope shoulder. The influence of train speed on subgrade has a critical respond speed that increases with increasing vibration times. There are horizontal, vertical and 45 degrees angle cracks in the middle of subgrade slope. It is qualitatively assessed that the slope of the high-speed railway subgrade in the collapsible loess area is unstable under the effect of train load. The data and rules provided in this document provide some reference values for the construction of a high-speed railway in the collapsible loess area.

In this study, the dynamic response and damage mode of a pile-geogrid composite reinforced high-speed railway subgrade under seismic action were investigated based on a unidirectional shaking table test. Various seismic waves were applied to the subgrade, allowing for an analysis of acceleration, dynamic soil pressure, displacement, and strain responses. The displacement field of the subgrade was visualized using particle image velocimetry (PIV). The study shows that changes in peak ground acceleration (PGA) amplification factors become evident with height due to the presence of geogrid layers. The increase in peak ground motion causes a redistribution of dynamic soil pressures inside the subgrade. The transverse and longitudinal ribs of the geogrids provide an anchoring effect. The peak strain of the piles in the center is greater than that of the piles on the sides. The direction of soil particle displacement is closely related to the damage patterns observed in the subgrade. Damage begins to occur once the peak ground motion exceeds 0.4 g, characterized by collapse at the bottom of the subgrade.