Water vapor transfer and phase change in unsaturated soils: experimental study on two types of canopy effect

The canopy effect refers to the phenomenon where moisture accumulates underneath an impervious cover. It can lead to full saturation of the soil immediately underneath the impervious cover. A recent theoretical study separates the canopy effect into two types, the first one is caused by the evaporation-condensation in unsaturated soils, while the second one is induced by the freezing-enhanced vapor transfer in unsaturated soils. In order to experimentally validate these two types of canopy effect and to reveal their mechanisms, water vapor migration experiments are carried out through a newly developed laboratory apparatus for unsaturated frozen soils. Six conditions are performed on a calcareous sand with different initial water contents and boundary temperatures. The results show that the water content in the upper position of the sample increases under an upward temperature gradient, and the increment of water content is greater if the soil is subjected to freezing. For the freezing cases, the depth of a peak water content is in line with the freezing front, and the greater the initial water content, the more the water content accumulated at the freezing front. A smaller cooling rate seems to facilitate the vapor migration. For the unfreezing cases, the water content in the upper position of the sample also increases and the increment becomes more apparent with a higher initial water content. The temperature gradient can inhibit the vapor migration. A smaller temperature gradient always results in a more notable inhibition effect. The test results verify the theory of the canopy effect.