Liquid water–vapour migration tracing and characteristics of coarse-grained soil under high-speed railway train loading in cold regions

Although high-speed railway (HSR) subgrades in cold regions are filled with coarse-grained soil (CGS), they still undergo frost-heaving displacement. The liquid water–vapour migration mechanism can explain the frost-heaving process; therefore, the characteristics of liquid water–vapour migration in CGS are investigated in this study. The results show that in CGS, vapour migration is dominant under unidirectional freezing and dynamic loading. The water redistribution in the frozen zone and freezing front is primarily caused by vapour migration, whereas that in the unfrozen zone is primarily caused by liquid water migration. The final migration height of liquid water and the amount of vapour migration are positively correlated with the amplitude and frequency of dynamic loading. Continuous pore-structure changes and soil-particle movement provide favourable conditions for vapour migration, which are amplified by increasing the dynamic loading amplitude or frequency. A pumping action produced by dynamic loading exists inside CGS. In the design and maintenance of HSR subgrades in cold regions, vapour migration merits careful consideration because it induces more than 50% of the frost-heaving displacement.