Shear properties of unsaturated red mudstone fill material over a wide range of suction

Red mudstone fill material for high-speed railway subgrade is naturally in unsaturated state, while the water content varies significantly due to environmental influence, which could lead to subgrade settlement or even failure. This study aims at investigating the suction effect on the shear strength of red mudstone fill material, and proposes its unsaturated strength prediction formula over a wide range of suction. Results show that the cohesion exhibit typical "peak effect", initially increasing and then decreasing, while the internal friction angle gradually decreases with increasing degree of saturation. Moreover, the significant decrease in shear strength of red mudstone fill material at saturated state is mainly caused by the significant reduction in cohesion. In terms of the microscale investigation, the pore size distribution of the red mudstone fill material transforms from monomodal to bimodal with the water content increases, and then back to monomodal distribution under saturated state. Informed by integrative insights from macro to micro-scale experimental findings, a strength prediction formula for unsaturated red mudstone fill material over a wide range of suction was established, considering the effective degree of saturation and the significant change of internal friction angle. The strength prediction formula of unsaturated red mudstone fill material can provide a theoretical basis for evaluating the performance of subgrades across a broad range of suction, thereby advancing the resource utilization of red mudstone waste in high-speed railway construction.