Effect of geotextile on ballast fouling under cyclic loading in seasonal freeze–thaw conditions

Ballast fouling during spring thaw is a recurrent challenge in seasonally frozen regions and can seriously compromise track stability. This study combined one-dimensional freeze-thaw cycling and cyclic loading model tests to investigate moisture-driven deterioration at the ballast-subgrade interface and the mitigation effect of geotextile separation. Freeze-thaw cycling under a top-down thermal gradient drove upward moisture migration, forming a moisture-enriched upper soil zone. Based on the post-cycling water-content profiles, representative upper-layer water contents were selected to simulate progressively wetter spring-thaw conditions in cyclic loading tests on ballast-silty clay specimens. The results showed depth-dependent pore-pressure responses and staged water migration, with persistent upward seepage in the upper zone and limited movement in the lower zone. Geotextile installation reduced pore pressure in the lower layer and weakened the upper-zone hydraulic gradient, but also delayed pore-pressure dissipation near the interface, resulting in higher residual pore pressure in the top layer at the end of loading. The final difference between reinforced and unreinforced specimens increased from 4.2 to 20.1 kPa as the upper-layer water content increased from 22% to 34%, while the geotextile reduced the final volumetric water content in the top layer by 1.9%-4.7%. Post-test observations and layered sieve analyses showed that higher moisture intensified ballast penetration and fine intrusion in unreinforced specimens, whereas the geotextile prevented silty clay intrusion and largely preserved the initial ballast gradation. Overall, geotextile separation mitigated spring-thaw-induced interface degradation by suppressing fine migration while altering the interfacial hydraulic response.