Fluidization and Stabilization of Soft Soil Subgrades under Cyclic Rail Loading

Soft soil subgrades subjected to cyclic rail loading are highly susceptible to fluidization, which compromises track stability and increases maintenance demands. This process, driven by excess pore water pressure (EPWP) buildup and upward migration of moisture and fines, leads to excessive settlement, ballast degradation, and loss of bearing capacity. This study combines laboratory testing and theoretical analysis to investigate fluidization mechanisms and assess mitigation strategies. Undrained cyclic triaxial tests were performed under varying cyclic stress ratios (CSR) and loading frequencies to establish critical thresholds for fluidization onset. Results show that once CSR exceeds a critical value, EPWP and axial strain rise sharply, indicating a rapid loss of soil structure. The effects of dry density and kaolin content on fluidization susceptibility were also evaluated. A theoretical framework was then developed to model localized fluidization and instability progression. Finally, ground improvement methods were examined, with prefabricated vertical drains (PVDs) and geosynthetics proving most effective when combined as a geocomposite–PVD system. This integrated approach enhances drainage, prevents fine migration, and improves subgrade resilience under cyclic loading.