A comprehensive series of tests, including dynamic triaxial, monotonic triaxial and unconfined compressive strength (UCS) tests, were carried out on reconstituted landfill waste material buried for over twenty years in a closed landfill site in Sydney, Australia. Waste materials collected from the landfill site were treated with varying percentages of cement, and both treated and untreated specimens were investigated to evaluate the influence of cement treatment. The study examined the dynamic properties of cement-treated landfill waste, including cumulative plastic deformation, resilient modulus, and damping ratio, and also analysed the impact of cyclic loading on post-cyclic shear strength in comparison to pre-cyclic shear strength. The UCS tests and monotonic triaxial tests demonstrated that untreated specimens subjected to monotonic loading exhibited a progressive increase in strength with rising axial strain, whereas cement-treated specimens reached a peak strength before experiencing a decline. During cyclic loading, with the inclusion of cement, a significant reduction in cumulative plastic deformation and damping ratio was observed, and this reduction was further enhanced with increasing cement content. Conversely, the resilient modulus showed substantial improvement with the addition of cement, and this enhancement was further amplified with increasing cement content. The formation of cementation bonds between particles curtails particle movement within the landfill waste material matrix and prevents interparticle sliding during cyclic loading, leading to lower plastic strains and damping ratio while increasing resilient modulus. Post-cyclic monotonic testing revealed that cyclic loading caused the partial breakage of the cementation bonds, resulting in reduced shear strength. This reduction was higher on samples treated with lower cement content. Overall, the findings of the research offer crucial insights into the possibility of cement-treated landfill waste as a railway subgrade, laying the groundwork for informed design decisions in developing transport infrastructure over closed landfill sites while using landfill waste materials available on site.

Aeolian sand along the Hojiakueri Railway in the Taklimakan Desert exhibits poor mechanical properties for direct use as a filler for railway subgrades. Although cemented soil reinforced with single fibers can improve mechanical properties, its limited effectiveness and high cement usage pose significant economic and environmental concerns. This study investigated the improvement of splitting tensile strength (STS) in cemented aeolian sand through hybrid fiber reinforcement. An orthogonal test was designed to evaluate four factors-fiber types (pairwise combinations of basalt, polypropylene, and glass fibers), fiber lengths (3, 6, and 9 mm), hybridization ratios (1:1, 1:3, and 3:1), and fiber contents (4 %o, 8 %o, and 12 %o) - along with their interactions. The performance of cemented aeolian sand reinforced with hybrid fiber (CASRHF) was evaluated through STS tests and scanning electron microscopy (SEM). The results identified the optimal combination as a 1:1 mix of 6 mm basalt and polypropylene fibers with a fiber content of 12 %o. The interaction between hybrid fiber type and fiber length was the most critical factor influencing STS, followed by hybrid fiber type, fiber length, and fiber content. SEM analysis further revealed a linear negative correlation between STS and porosity, providing new insights into the microscopic mechanisms. The findings underscore the importance of optimizing hybrid fiber combinations to meet the performance requirements of railway subgrade beds in aeolian sand regions.

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.

Soil instability and potential failure under principal stress rotation require greater attention than ever before due to increased operation of heavier and longer high-speed trains. This study focuses on the interplay between cyclic vertical stress and torsional shear stress on the failure condition of a low-plasticity subgrade soil, facilitated by a hollow cylinder apparatus. Combined vertical and torsional loading significantly influences strain response, with increasing torsional stress leading to higher strain accumulation. Moreover, the data indicate that an increase in torsional shear stress is generally accompanied by a swift rise in the EPWP and a corresponding decrease in the soil stiffness. In view of this, a novel parameter, the overall stiffness degradation index (delta o) that simultaneously captures both the vertical and torsional shear effects under principal stress rotation is proposed as an early indicator of instability. In addition, a normalised torsional stress ratio (NTSR), which is the ratio of the amplitude of torsional shear stress to the confining pressure, is introduced to assess the impact of torsional shear stress. Whereby, higher NTSR values correlate with premature inception of failure. These experimental results provide new insights for a better understanding of soil instability under simulated railway loading.

In Australia, most rail systems are constructed along the coastal line, traversing soft soil deposits that can cause a series of track instabilities, including excessive plastic deformation and mud pumping. For this reason, the subgrade is considered one of the most critical components of the railway infrastructure, and the accurate prediction of its undrained shear behaviour is of utmost importance to ensure track integrity and safety over time. The subgrade is composed of naturally deposited soil, which may be modified using compaction or consolidation techniques to improve its bearing capacity. In each method, the soil presents a unique arrangement of particles, groups of particles, and voids (referred to as fabric), which govern the response of subgrade soils to the substantial loads imposed by moving trains. For this study, samples of clayey subgrade soil were collected from a site where track degradation had been reported. The soil was reconstituted using slurry consolidation and compaction methods to create different fabrics. A series of undrained cyclic triaxial tests were then carried out to investigate the influence of specimen preparation methods on the shear behaviour of soil. Differences in soil fabric were assessed through X-ray microcomputer-tomography (micro-CT), providing insights into the variations observed in the cyclic response. Based on the findings, it is evident that fabric plays a crucial role in the shear behaviour of subgrade soils and should, therefore, be considered in railway infrastructure design.

A new type of thermally controlled subgrade is proposed to mitigate persistent frost heave issues of railway subgrades in seasonally frozen regions. A dedicated ground-source heat pump system collects low-grade geothermal energy from the stable soil layer near the subgrade, converts it into high-grade thermal energy, and transfers it to the frigid subgrade for active heating and temperature control, thereby eliminating the adverse effects of frost heave. A 20-metre-long test of thermally controlled subgrade was constructed in a frost heave of the Junggar-Shenchi Railway in Shanxi Province, China. During the winter spanning 2021 and 2022, the heating temperature of the heat pump, the thermal regime of the test subgrade and the natural subgrade, the frost depth, and the track heave were measured. The results indicate that the heat pump temperature could reach a peak of 59.4 degrees C, with the average daily heating temperature during intermittent operation reaching 25.2 degrees C or higher, indicating an efficient heat source that plays a favourable role. The freezing period of the natural subgrade lasted for 141 days, while the subgrade in the test was 20 days shorter. The maximum frost depths at the track centre, shoulder, and embankment slope toe in the test were 88 cm, 75 cm, and 58 cm, respectively. These depths were 60 cm, 122 cm, and 78 cm less than those of the natural subgrade, effectively controlling the frost depth within the threshold that may cause potential structural damage. Under natural conditions, the track heave reached a peak of 9.4 mm, leading to a harmful frost heave scenario. In contrast, the track deformation in the test was less than 3 mm, which did not exceed the regular maintenance threshold. The thermally controlled subgrade proves to be an effective method for preventing and controlling persistent frost heave damage in critical locations such as low embankments, cut subgrades, turnout areas, and culvert roofs.

The quality of the railway subgrade is directly related to the fill soil structure, which, in turn, is determined by the local physical and chemical environment. A karst environment, with its frequent rainfall, promotes the dissolution of soluble rocks and underground transportation of solutes, altering the soil structure and performance. To investigate these alterations, we analyzed the properties of underground soil from highly developed karst areas. Fine breccia soil from karst regions was tested to assess its macroscopic mechanical properties and microstructural features, for differing initial water contents and compaction levels. Samples were subjected to simulated rainfall conditions through dry-wet cycles, and then underwent triaxial shear and electron microscopy tests. From these data, a micro-to-macro correlation model and a normalization model were developed. The findings suggest that the resistance of fine angular breccia soil to degradation during dry-wet cycles can be enhanced through high-pressure compaction and by maintaining a moisture content close to 15.6%. Increasing the degree of compaction improves the particle size distribution and the density of the soil skeleton. This is advantageous for minimizing soil particle erosion, thereby ensuring the strong performance of railway subgrades in karst areas with frequent rainfall.

While the fabric of soil can significantly influence its behaviour, the effect of varying fabric parameters on the subgrade shear response is still not well understood. This study creates soil specimens with different fabrics which are then captured through X-ray microscopic-computed tomography scanning and quantified by image processing techniques. A comprehensive laboratory investigation is conducted to understand how the soil fabric affects its monotonic and cyclic shear behaviour. The results indicate that the consolidation method creates a more homogeneous fabric with mainly small-to-medium interconnected pores, whereas the compaction technique creates significantly large and mostly inter-aggregate pores with lower connectivity. In this regard, the consolidated specimens exhibit an elastic-perfectly plastic behaviour, while the compacted specimens show strain-hardening transformation during isotropic monotonic shearing. Under anisotropic conditions, the compacted specimens exhibit a greater strain softening response and excess pore pressure than the consolidated specimens because they have a weaker fabric. Furthermore, the compacted specimens show a smaller threshold strain at a lower critical number of cycles due to the collapse of large pores. These current findings prove the decisive role that soil fabric plays in determining the shear response and failure of subgrade soils.

As an important component of railway track, railway subgrade bears a huge dynamic load from trains. To explore the feasibility and dynamic characteristics of mixed filling of railway subgrade soil with tire debris, a series of large-scale indoor model tests were conducted to compare and analyse the magnitude and distribution of earth pressure, and the cumulative settlement displacement of sleepers, under two different loading conditions (i.e., single-stage cyclic loading and multi-stage cyclic loading). The results demonstrate that under all conditions, the vertical displacement at the sleeper ends is greater than that at the sleeper middle, the vertical displacement at the sleeper end decreases with the increase of static load and frequency, but increases with the increase of amplitude. Additionally, adding tire debris can help reduce the settlement and displacement of the subgrade soil, and the same amount of tire debris has the best effect when mixed in two layers. Moreover, multi-stage cyclic loading can significantly reduce the cumulative settlement of the subgrade soil. Under multi-stage cyclic loading, the settlement displacement at the end of the subgrade soil sleeper is larger than that at the middle of the sleeper, but the difference gradually decreases as the static load increases. It is revealed from the research that tire debris can significantly improve the performance of railway subgrade soil.