Investigation of liquid and vapor migration in coarse-grained soil during open-system step-freezing test

In this study, the characteristics of vapor migration and migration of liquid coupled with that of vapor in the step-freezing mode were investigated, and the results for soils with different initial water contents were compared. The findings clarified the effect of the freezing mode on external water intake in coarse-grained soil (CGS) in high-speed railway (HSR) subgrade in cold regions. Moreover, the mechanisms of vapor migration and liquid and vapor coupling migration in HSR subgrade CGS fillings were examined through a comparison of the characteristics during constant and step freezing. The results show that vapor migration is the main form of external water intake in the step-freezing mode. The migration of liquid water has a considerable impact on the distribution of water in an unfrozen zone but has negligible effect in a frozen zone. As the initial water content and freezing depth increase, the amount and migration rate of water taken in from external sources decrease, and the maximum water content during step freezing also decreases. Compared with the ice layer formed by vapor migration and liquid and vapor coupling migration in the constant-temperature freezing mode, the one formed in the step-freezing mode is distributed more evenly. This study highlighted the need to pay significant attention to frost-heave deformation caused by vapor migration in CGS fillings in the design of HSR subgrade, provided that the engineering environment is considered. •Real-time tracking of liquid and vapor migration is achieved by fluorescein and water separation layer.•Vapor is the main form of liquid and vapor coupling migration in CGS under step-freezing mode.•The external water intake, especially vapor migration, decreases dramatically with an increase in initial water content.