Red mudstone fill material (RMF) exhibits high water sensitivity which indicates that the performance of the subgrade is dependent on the water content. A series of mechanical and microstructure tests including 41 cyclic triaxial tests

5 mercury intrusion porosimetry (MIP) tests and 3 scanning electron microscope (SEM) tests were carried out to investigate the dynamic properties of RMF

with particular concentration on water content effects. Results show that the equivalent Young’s modulus and damping ratio are correlated to the strain amplitude

which can be modeled by Hardin-Drnevich’s hyperbolic equation. As the cyclic stress amplitude increases

a transition from stable to unstable permanent deformation is observed. The permanent deformation and critical cyclic stress vary with water content. Specifically

under the same cyclic stress amplitude

the permanent deformation of RMF on the dry side is less than that on the wet side. The number of loading cycles needed to achieve the target permanent deformation is higher for RMF on the dry side compared to the wet side. The critical cyclic stress is greater on the dry side and increases with initial suction

as characterized by the VG model. From the microstructural perspective

the pore size distribution changes from single peak mode to bi-modal structure as water content increases. The denser and more stable soil fabric is observed on the dry side

which results in the better performance of RMF on the dry side compared to that on the wet side. The ratio of critical cyclic stress to static shear strength ranges from 65% to 75%

indicating the overestimation of subgrade strength by the static strength design method. It is recommended to use RMF with a water content of 5.0% to 7.0% in the construction of red mudstone subgrades.