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.

Small strain properties of subgrade fill material are essentially required for the accurate estimation of deformation behavior of railway subgrade. Many attentions have received on small strain properties of soils under the isotropic stress state or low shear stress level. The high level of shear stress and stress ratio induce reduction in small strain stiffness and thus present the potential challenge to the deformation stability of the subgrade. However, there is not much attempt to investigate the small strain properties under high stress ratio. This paper explores the effects of stress path and stress state on small strain stiffness Gmax and Poisson's ratio v of heavily compacted fully weathered red mudstone (FWRM) under a broad range of stress ratio, via a series of stress-controlled triaxial and bender element tests. Three stress paths, named as constant stress ratio (SSP), constant confined pressure (VSP), constant axial stress (HSP) with stress ratio up to 33.0 were considered. Low level of shear stress slightly promotes Gmax, while a significant reduction of Gmax is triggered as the stress ratio exceeds a critical value. A unified correlation between the critical stress ratio and confined pressure is developed. The evolution of Poisson's ratio is also described by a unified three-dimensional feature surface, which influence of stress path is identified by the location and shape of the surface.

Red mudstone is a problematic soil that is easily subjected to weathering, disintegrating, and swelling. In this study, a series of large-scale cyclic triaxial tests were performed to investigate the cumulative deformation behavior of red mudstone clay mixed with weathered red mudstone gravel as an improved coarse-grained red mudstone soil (IRMS). The influences of compaction moisture content and confining pressure were investigated. The cyclic loading was applied from 25 to 225 kPa with an increment of 25 kPa and 1,000 or 2,000 cycles for each stage at a frequency of 2 Hz. The experimental results indicate that the strains at the onset of failure are approximately 1% for the optimal moisture content (OMC) with the number of cycles N = 14,000-16,000, and the strains are approximately 1% for the moisture content 2% dry of OMC with N = 12,000-14,000, while the strains exceed 10% for the moisture content 2% wet of OMC with N = 3,000-4,000. The cumulative strain decreases with increasing confining pressure from 20 to 50 kPa, but the influence becomes more significant under higher dynamic stress. A prediction model is proposed for the evolution of cumulative strain under cyclic loading. The IRMS could be used as a construction material for railway subgrade with proper control of field compaction moisture content.

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.

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.

Assessing the stability of embankments during railway operation is paramount for ensuring safety of railway. However, directly measuring the strength of fill materials can be challenging when the railway is in use. A strong correlation has been observed between soil shear strength and small strain stiffness. By establishing a robust correlation between shear strength and small -strain stiffness in the laboratory, considering various of factors, and combining it with field measurement of in -situ soil small stiffness might be an effective way to this problem. This study focuses on a type of filling materials commonly used in southwestern parts of China for railway construction: fully weathered red mudstone (FWRM) and its lime -treated counterpart (LFWRM), as the objects. A series of triaxial and unconfined compression tests were conducted to examine the effects of water content, confined pressure, and lime treatment on the shear strength and small strain stiffness of FWRM and LFWRM. The results show that the strength and stiffness of FWRM significantly decrease with increasing water content, while LFWRM specimens demonstrate good resistance. All LFWRM specimens displayed a brittle shear behavior. Empirical correlation was established for FWRM and LFWRM. The relationship for LFWRM is water content independent, meanwhile for FWRM is strongly dependent upon whether soil is saturated or not. The ratio of small strain stiffness to strength (E max /q max ) for FWRM decreases substantially after saturation, whereas it remains almost constant for LFWRM. The reduction in strength and stiffness can be attributed to the degradation of the soil fabric due to increasing water content, where the pore size distribution (PSD) of FWRM changes significantly with increasing water content due to aggregate swelling. However, for LFWRM, the PSD remains bimodal, which is due to the cementation bonding observed between lime -treated aggregates that explains the stable structure and improved performance of LFWRM.