Analysis of the Water-Vapor-Heat Coupling Migration of Unsaturated Soil Slope in Seasonal Frozen Regions

An unsaturated seasonal frozen soil slope of the Dandong-Altay highway is used as the research object. This paper is based on vapor-liquid migration and heat conduction multi-field coupling theory. The ice-water phase transition and vapor-liquid transition of water are also considered. The empirical relationship between unfrozen moisture content and ice volume fraction is introduced. A numerical model of the whole freeze-thaw process considering water-vapor mixing migration and heat transfer is established. Based on numerical simulations, we reproduce the periodic process of formation, expansion, thawing, and disappearance of frozen layers on unsaturated seasonal frozen soil slopes. And we reveal the migration law of vapor and liquid water in the slope under the effect of ambient temperature and investigate the effect of vapor migration on the moisture content of the slope before and after freezing of unsaturated soil slopes. The results show that: during the freezing period, the freezing depth increased faster before the temperature dropped to the annual minimum temperature. And before entering the thawing period, the freezing depth is close to the maximum. After entering the thawing period, ice melting continuously absorbs heat from the frozen layer. Under the two-way action of external warming and geothermal heat, the deep freezing depth does not decrease for a long time. But it shows a gradual change law of increasing slowly and then decreasing gradually. In a complete freeze-thaw cycle, the freezing period is about twice as long as the thawing period. The freezing and thawing periods at the top of the slope are the longest, and the freezing depth is the largest. The freezing and thawing periods at the foot of the slope are the shortest, and the freezing depth is the smallest. When the unsaturated soil slope is in an unfrozen state, the engineering hazards of the slope caused by the migration of vapor are relatively small. After it enters the freezing state, the influence of vapor migration on the unsaturated soil slope will increase significantly. The total volume moisture content of the slope in the frozen area without considering the migration of vapor is about [[EQUATION]] lower than that when considering the migration of vapor. As a result, serious engineering diseases may be caused, and enough attention should be paid to them.