Water migration in frozen high-speed railway subgrades under traffic vibration: Piston suction versus mud pumping and pot cover effect

Frost heave and thaw settlement in frozen high-speed railway subgrades are governed by coupled water and heat migration in the soil, and may be further intensified by traffic-induced vibration. However, the underlying hydro-mechanical processes in frozen, partially saturated subgrades remain poorly quantified, especially when dynamic loading acts concurrently with freeze–thaw cycles. In this study, a custom one-dimensional freezing apparatus with superimposed cyclic loading was used to investigate water migration in unsaturated frozen soils representative of high-speed railway subgrades. A test matrix of ten soil samples was designed, varying freezing temperature (−5℃ to −20℃), soil compaction (90% vs 95% relative), initial moisture content (10%, 14%, 18%), and soil type (silty clay versus gravelly fill). Real-time measurements of temperature, unfrozen water content, water replenishment, and pore water pressure were obtained throughout the experiments. Substantial upward water migration into the freezing zone was observed in all tests. Lower freezing temperatures markedly increased both the volume and rate of water replenishment, whereas compaction (within 90–95%) had little influence. Soil type and layering controlled the magnitude and timing of upward flux: silty clay induced greater and faster water uptake than Group B fill, and layered profiles showed distinct stagewise behavior. Notably, a vibration-induced piston suction mechanism was identified: cyclic vehicular loading acting on a frozen, low-permeability upper layer generated excess pore water pressure in the underlying unfrozen zone, establishing a sustained hydraulic gradient that pumped unfrozen water toward the freezing front. This mechanism is different from mud pumping and pot cover effects. A unified conceptual framework is proposed that links matric suction, cryogenic suction, and vibration-induced piston suction, delineates their respective domains of dominance, and provides physically based guidance for modeling water migration in frozen high-speed railway subgrades and related cold region porous media.