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.