Canopy effect: water vapor transmission in frozen soils with impermeable surface

Frozen ground covers vast areas worldwide and about 75% of the land area in the Northern Hemisphere undergoes seasonal freezing and thawing processes. It has been well acknowledged that freeze–thaw cycles (FTCs) and temperature rise in cold regions have significant effects on the hydraulic, thermal, and mechanical properties and processes of foundation soils of infrastructure, which subsequently affect its stability and durability. Numerous efforts have been devoted to mitigate the capillary rise (transport of liquid water under matric potential gradient) to the freezing front that builds up as ice and leads to frost heave, in addition to reducing temperature rise of the frozen ground beneath foundations in order to alleviate thaw settlement. However, increasing more studies have shown that the “canopy effect” would lead to the frost heaving damage and threaten engineering safety given that the capillary rise is minimized and temperature rise is controlled. The “canopy effect” as a result of water vapor transmission through soils or other porous construction materials during ground FTCs that increases ice content beneath the impermeable layer of infrastructure. At present, there is a lack of in-depth and comprehensive understanding of the processes, magnitude, and significance of canopy effect. Therefore, the objective of this study was to review the progress in research on water vapor transport processes in the soil during FTCs, their dominant factors, experimental setup, and control measures. Pros and cons of available water–heat–vapor measurement methods and mechanisms of frost heaving because of water vapor transmission are discussed. Water vapor transmission models are inventoried, the shortcomings of the currently available research are discussed, and future perspectives on water–heat–vapor transmission are also given.