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.

This study investigated the hydraulic and mechanical behaviors of unsaturated coarse-grained railway embankment fill materials (CREFMs) using a novel unsaturated large-scale triaxial apparatus equipped with the axis translation technique (ATT). Comprehensive soil-water retention and constant-suction triaxial compression tests were conducted to evaluate the effects of initial void ratio, matric suction, and confining pressure on the properties of CREFMs. Key findings reveal a primary suction range of 0-100 kPa characterized by hysteresis, which intensifies with decreasing density. Notably, the air entry value and residual suction are influenced by void ratio, with higher void ratios leading to decreased air entry values and residual suctions, underscoring the critical role of void ratio in hydraulic behavior. Additionally, the critical state line (CSL) in the bi-logarithmic space of void ratio and mean effective stress shifts towards higher void ratios with increasing matric suction, significantly affecting dilatancy and critical states. Furthermore, the study demonstrated that the mobilized friction angle and modulus properties depend on confining pressure and matric suction. A novel modified dilatancy equation was proposed, which enhances the predictability of CREFMs' responses under variable loading, particularly at high stress ratios defined by the deviatoric stress over the mean effective stress. This research advances the understanding of CREFMs' performance, especially under fluctuating environmental conditions that alter suction levels. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Red mudstone is highly sensitive to water content variations. Lime treatment is recommended when using red mudstone as subgrade fill material. The mechanical properties of lime-treated red mudstone fill material (LRMF) degrade due to wetting-drying (WD) cycles caused by seasonal environmental effects. A series of WD cycle tests, unconfined compression tests, and bender element tests were conducted to investigate the degradation of strength and small strain stiffness of LRMF. Combining with the successive water-dripping scanning electron microscope (SEM) tests, the microstructure disturbance of LRMF after WD cycles was examined. Swelling of specimens on both the wet and dry sides was observed during low-amplitude WD cycles. For high-amplitude WD cycles, swelling on the wet side was also observed. On the dry side, initial volume shrinkage was recorded, followed by swelling in successive cycles, even though the water content was significantly lower than the initial state. Swelling results in the degradation of strength and small strain stiffness. Volumetric shrinkage increased strength, but small strain stiffness was still reduced due to crack propagation. A unified model is proposed to identify the degradation of strength and volumetric strain, while the small strain stiffness for dry specimens under large-amplitude WD cycles is significantly below the degradation line. The degradation rate of small strain stiffness is significantly higher than that of strength. After water exposure, the LRMF generally retains its initial microstructure. However, loosened aggregates, slaking, and crack propagation are clearly seen in water-exposed specimens. Degradation of the mechanical properties of LRMF can be attributed to damage to the soil fabric.

Backfilling mining technology provides an efficient and environmentally-friendly solution for treating additional products (such as tailings, coal gangue and other solid waste) in coal mining process, which are filled into the underground goaf area, thus reducing surface subsidence, roof strata damage, and associated geological disasters. In this study, a novel backfill material called lean cemented gangue backfill material (LCGBM) is introduced, and various experiments, including uniaxial compression tests, creep tests, CT-SEM scanning and reconstruction are carried out to evaluate its mechanical properties and engineering practicability. The test results indicate that the strength and volume fraction of self-compacting cement (SCC) slurry and solid waste aggregate jointly control the uniaxial compressive strength and failure mode of LCGBM, reflecting the bucket effect under the influence of two components. Although the volume fraction of aggregate and SCC slurry is intentionally reduced to control the cost, this cemented granular material shows excellent compressive strength, overall stiffness and long-term bearing capacity in laboratory and field tests. In the process of engineering application, the uniaxial compressive strength of the LCGBM reaches 14 MPa, and the initial setting time is 120 s, which reduces the consumption of gangue aggregate by 35% and greatly reduces the construction time. In addition, a calculation method of the optimal mixing ratio of raw materials is proposed, which can adjust the strength and volume fraction of SCC slurry according to the optimal mixing range, so as to maximize the utilization of the strength of LCGBM, and reduce the construction time and aggregate of backfilling. The research findings emphasize the potential of the LCGBM in promoting clean and sustainable coal mining practice.

With the frequent occurrence of natural disasters, the problem of dam failure with low probability and high risk has gradually attracted people's attention. This paper uses flume model tests to systematically analyze the overtopping failure mechanisms of concrete face rockfill dam (CFRD) and identify its failure modes. The tests reveal that the longitudinal erosion of the CFRD breach progress through stages of soil erosion, panel failures, and water flow stabilization. Meanwhile, the cross- breach process involves the evolution of breach size in rockfill materials, including traceable erosion, lateral broadening, and breach morphology stabilization. The fracture characteristics of the water-blocking panel are primarily evident in the flow-time curve. By analyzing the breach morphology evolution processes in longitudinal and cross sections, the flowtime curve can be subdivided into stages of burst flow formation, breach expansion with flow increase, rapid increase of breach flow discharge due to panel failures, and stabilization of breach flow and size. The primary damage process of the CFRD occurs in a cyclical stage of breach expansion, flow increase, panel failure, and rapid discharge. The rigid face plate and granular body structure contribute to partial dam failure, showing a tendency for gradual expansion of the breach. The longitudinal illustrates dam failure resulting from panel fracture and rockfill erosion interaction, while downstream slopes exhibit failure due to lateral intrusion of rockfill and cyclic instability. These research results can serve as a reference for constructing a concrete CFRD failure prediction model and conducting disaster risk assessments.

In southwest China, red mudstone fill material (RMF) is widely used in constructing railway subgrades to substitute the conventional unbound granular materials (UGMs). Besides the strain-level dependent dynamic properties, RMF significantly depends on loading cycles. However, such an effect has yet to be incorporated into the current design method, which would lead to a considerable misprediction in dynamic responses of the RMF subgrade during the operation period. This paper presents a comprehensive study of long-term dynamic properties of RMF (a silty clay) over a range of water contents and cyclic stresses. The objective is to establish a normalization framework of dynamic properties that considers the effect of large numbers of cyclic loading. With this emphasis, 40 cyclic triaxial tests with 50000 loading cycles were conducted on RMF specimens compacted at various water contents. Two-stage behavior has been identified in equivalent Young's modulus and damping ratio evolutions. An exponential model is thus proposed to capture the two-stage pattern. The proposed normalization procedure showed a competent availability for the characterization of equivalent Young's modulus and damping ratio at different loading cycles. Soil fabric also played a decisive role in evaluating RMF's dynamic responses. Evidence of microfabric effect on the dynamic responses of RMF was strengthened by the Mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) analysis.

During the operational phase of pumped storage hydropower stations, rockfill materials within the dam experience cyclic loading and unloading due to water level fluctuations. This cyclic behavior can result in the accumulation of irreversible deformation, posing a substantial threat to dam safety. However, there is an absence of a constitutive model capable of accurately capturing the low-frequency multi-cycle hysteresis behavior of rockfill materials due to the constraints of conventional laboratory test methods. In this study, we employed the combined finite and discrete element method (FDEM) to investigate the mechanical characteristics of rockfill materials and develop an improved constitutive model capable of effectively capturing their hysteretic behavior. The results demonstrate the FDEM accurately reproduces the mechanical behavior of rockfill material under shear and cyclic loading and unloading conditions. The hysteresis loop exhibits a discernible densification trend with increasing cycles. And the variations in elastic modulus and strain primarily occur within the initial five cycles. The plastic strain increment exhibits a strong positive correlation with stress level, while its relationship with confining pressure is comparatively less pronounced. The proposed constitutive model successfully captures the complex low-frequency multi-cycle hysteresis characteristics of rockfill material with few parameters, showing substantial potential for practical applications.

The utilisation of red mudstone waste as subgrade fill material after lime stabilization can meet the requirements of green development. This study aims at investigating the lime stabilization effect on the changes in mechanical properties and microstructure of compacted red mudstone fill material, with particular emphasis on the curing time effect. Red mudstone fill material were stabilized by 4 % lime, and the unconfined compressive strength, direct shear strength and microstructure of the lime-stabilized fill material specimens were determined at various curing times. Results show that the lime stabilization can significantly improve the unconfined compressive strength, modulus and direct shear strength and effectively mitigate the water sensitivity of the red mudstone fill material. The density function curve of the saturated red mudstone fill material shows monomodal characteristic, but the saturated lime stabilized fill material still maintains bimodal characteristic even with only one day of curing. The microstructure modification induced by lime stabilization mainly results from the rapid flocculation of soil particles and the formation of hydration product. During curing, the percentage of the nano-pores increases, mainly attributes to the formation of C-A-S-H which fills the micro-pores gradually.

The effect of the geocell layer and the geocell with geogrid layer on the settlement Behavior of sand under static and cyclic loads is investigated in this experimental study. The parametric experiments were conducted to examine the effects of reinforcement type and spacing between reinforcement on footing settlement Behavior. Additionally, to ascertain how settlement varies under various circumstances, the load-settlement Behavior of square footings installed over both reinforced and unreinforced sand beds was investigated. According to the experimental investigation, the foundation resting over geocell-reinforced sand showed an improvement of about 150% in the sand bed's bearing capability. The reinforced sand bed sustained loading cycles twice that of the unreinforced sand. The results showed that construction and demolition waste performed best among the infill materials that were tested. When construction and demolition waste is used as an infill material, there is about a 94% increase in elastic uniform compression coefficient value and about a 30.4% increase in sand bed load-carrying capability. For waste foundry sand and a mixture of construction and demolition waste with waste foundry sand (50% each), there is about an 8.86% and 16.11% increase in the value of load carrying capacity as compared to when geocell is infilled with sand. For the geocell-geogrid reinforced sand bed, the maximum increase in the elastic uniform compression coefficient value is 17.9% with a 28.61% decrease in settlement value compared to when geocell is used alone in the sand bed.

Under the high stress of a 300-m dam, the particle breakage patterns of rockfill material may differ from those under low-stress levels. The existing studies on the particle breakage of rockfill material under ultra-high dams are relatively rare. In this study, by performing a series of large-scale triaxial shear tests under different relative densities and confining pressures, the stress-strain relationships and particle breakage characteristics of a sandstone rockfill material were investigated. The development of four particle breakage indexes before and after the triaxial test, the evolution of the gradation curves, and the applicability of three gradation formulas to the data of this study were analyzed. Based on the distribution of one relative breakage index, its relationship with strength and compressibility was established. Finally, three failure modes for the sandstone rockfill material after the triaxial test were given. And the relationships among failure modes and confining pressure, and particle size were discussed.