非饱和土水气迁移与相变：两类“锅盖效应”的发生机理及数值再现

结合非饱和土水气迁移的物理过程和内在机理,将“锅盖效应”分为两种情形。第一类“锅盖效应”定义为非饱和土水气的冷凝过程,而第二类“锅盖效应”定义为水气迁移成冰过程。相较于前者,第二类“锅盖效应”会造成覆盖层下土体含水率大幅度提高,且现有非饱和土水热气迁移理论无法给出合理解释。综合考虑水分的蒸发、冷凝和冻结3个相变过程,建立了非饱和冻土水热气耦合迁移的数学模型,并通过数值模拟求解,再现了第二类“锅盖效应”的形成过程。另外计算结果表明：温度梯度下的气态水迁移并成冰会造成覆盖层下土体接近饱和含水率;一定表层深度范围内,土体含水率增加存在两个陡升段。由于现行工程设计很少考虑防气隔气,在寒旱地区进行工程建设需对第二类“锅盖效应”引起足够重视。

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.