ABSTRACT
Soft soil subgrades under cyclic rail loading face significant challenges, including excess pore water pressure (EPWP) accumulation, plastic strain development, and fluidization. These issues compromise bearing capacity and result in mud pumping, ballast degradation, and excessive settlement. This study investigates the undrained cyclic behavior of soft clay through a series of laboratory triaxial tests under varying cyclic stress ratios (CSR). A critical CSR threshold is identified, beyond which EPWP and axial strain rise sharply. Two constitutive models based on the Modified Cam Clay (MCC) framework are proposed to predict this behavior, incorporating soil structure degradation and dynamic loading effects. Moisture and fine particle migration toward the surface is shown to drive localized fluidization. To mitigate these effects, the effectiveness of prefabricated vertical drains (PVDs) and geosynthetics is evaluated. Combined systems significantly enhance drainage, reduce EPWP, and limit fines migration, improving subgrade stability. The results offer a predictive and practical framework for designing more resilient railway foundations on soft ground.
