Canopy effect caused by vapour transfer in covered freezing soils

Canopy effect in this paper refers to the phenomenon whereby moisture accumulates underneath an impervious cover. Field observations reveal that the canopy effect can occur in relatively dry soils where the groundwater table is deep and can lead to full saturation of the soil immediately underneath the impervious cover. However, numerical analysis based on existing theories of heat and mass transfer in unsaturated soils can only reproduce a minor amount of moisture accumulation caused by an impervious cover, particularly when the groundwater table is relatively deep. In an attempt to explain the observed canopy effect in the field, this paper proposes a new mechanism of moisture accumulation in unsaturated freezing soils: vapour transfer in such a soil is accelerated by the process of vapour-ice desublimation. A new approach for modelling moisture and heat movements is proposed in which phase changes during the evaporation, condensation and desublimation of vapour flow are considered. A laboratory device is designed specifically to study vapour transfer in unsaturated freezing soils. The test results reveal that vapour transfer can lead to significant moisture and ice accumulation near the freezing end of a soil specimen. The computed results show that the proposed model can indeed reproduce the unusual moisture accumulation observed in relatively dry soils. The results also demonstrate that soil freezing fed by vapour transfer can result in a water content that is close to full saturation. Because vapour transfer is seldom considered in geotechnical design, the canopy effect deserves greater attention during construction and earth works in cold and arid regions.