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.

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.

Pavements and light structures constructed on expansive subgrade layers have experienced severe damage due to volume changes. These layers have been exposed to climatic changes such as freeze-thaw (FT) cycles. Accurate estimation of design parameters regarding heave/settlement is essential for sustainable performance. This work study the instantaneous and long-term effects of successive FT-cycles on the volume stability, swelling, and compressibility characteristics of natural and lime-treated expansive subgrades. Volume changes were traced during successive 15FT cycles. Swelling and consolidation characteristics were studied immediately after FT-cycles. The long-term effect of FT was tested at different recovery periods around year. FT-cycles significantly affects volume changes and compressibility, this effect is proportional to soil type and limited up to a certain number of FT-cycles. During the long-term recovery, a considerable part of underwent deformation is a permanent and not recovered, even after year the soil still memorizes this effect. Due to the propagated cracks, the plastic deformation increased with increase in FT-cycles.

Expansive soil is known for its ability to undergo significant volume changes in response to changes in moisture levels. Several investigations have been conducted to explore the stabilisation of expansive soils, encompassing a variety of reinforcement techniques and stabilisation approaches. Analysing dynamic properties such as shear modulus and damping ratio in expansive soil is imperative as they provide vital insights into the soil's response to dynamic loads. The use of lime and fibre treatment for stabilising expansive soil offers a comprehensive solution that addresses both chemical and physical stabilisation. This approach enhances soil properties and reduces the risk of damage to structures. To understand the complex behaviour of soil systems under varying conditions, cyclic triaxial tests on expansive soil were conducted using fibre-lime treatment. The variations in dynamic shear modulus and damping ratio with respect to shear strain amplitude, as well as the dynamic shear stress-strain curves, were studied across various confining pressures. The paper also presents the small strain shear modulus and modulus reduction curves. Besides, for a comprehensive understanding of the microstructural reactions of the fibre-lime-treated soil, specimens subjected to cyclic loading were examined using SEM analysis. A comparative analysis, examining the dynamic properties of soil under varying lime percentages, with and without 0.5% polypropylene (PP) fibre, as well as comparing scenarios with different lime contents between 0 and 0.5% PP fibre conditions has been presented. The experimental findings suggests that the fibre-lime treatment led to an enhancement in the dynamic shear modulus and damping ratio, with the increase of confining pressure. From the SEM analysis of the fibre-lime-treated soil, the microstructure displays a fabric-like pattern with cementitious gel connecting soil particle clusters. The images also show clay particles bonding together, forming a compact structure.