Moisture transfer and phase change in unsaturated soils: physical mechanism and numerical model for two types of “canopy effect”

Considering the physical process and mechanism of liquid water-vapor transfer in unsaturated soils, this study divides the “canopy effect” into two types. The first is the process of coupling movement of liquid water-vapor and vapor condensation, whereas the second is the vapor transfer facilitated by changing the phase into ice. Differing from the first type of “canopy effect”, the second one can lead to a large amount of water accumulated beneath the impermeable top cover, while the available theory cannot give reasonable predication. This paper attempts to reveal its mechanism by proposing a new theory for liquid water-vapor-heat coupling movement in unsaturated freezing soils, in which the phase changes of evaporation, condensation and de-sublimation of vapor flow are taken into account. On basis of numerical simulation, the proposed model can hence reproduce the unusual moisture accumulation observed in relatively dry soils. The results show that the vapor transfer-induced freezing can produce a water content close to full saturation for soil under the cover, and two remarkable rises can be observed for the water content of soil at certain depth. Since isolating vapor/air is rarely considered in the current engineering design in China, the second type of “canopy effect” needs more attention when performing engineering construction in cold and arid regions.