冻土中气态水迁移及其对土体含水率的影响分析

Vapour transfer and its effects on water content in freezing soils

现有文献几乎尚未系统分析冻结条件下气态水对不同土性含水率的影响。基于热力学平衡理论及水热耦合理论，提出了未冻水含量和冰体积分数的计算方法，建立起新的耦合模型。该模型中最大未冻水含量和冰体积分数仅与水力参数和温度有关，具有明确的物理意义，与砂壤土的冻结试验结果对比也验证了新模型。模型分析结果表明：冻结条件下的气态水迁移主要受温度势而非基质势的作用，粉土和砂土中的气态水迁移是不能忽略的，而黏土中几乎没有气态水迁移；初始体积含水率、冻结温度、冻结时间及地下水位高度等都会对气态水的迁移有影响。总的来说，气态水对于粉土等冻胀敏感性土，即使较小的水分增加仍然能够产生显著冻胀，因此实际工程必须重视气态水的作用。本文分析加深了对“锅盖效应”的理解，也验证了“锅盖效应”通常发生在覆盖层下的粉土区域，而非砂土或黏土。

The effects of vapour on water content in different unsaturated frozen soils have not been systematically analyzed in the literatures. Based on the thermodynamic equilibrium theory and coupled water-heat theory, a new method for calculating unfrozen water content and ice content is obtained. A new model is then established by importing this method to the coupled heat and mass transfer theory. The unfrozen water content and ice content in this new model are only related to the hydraulic parameters and temperature, which have specific physical meanings. The comparisons between the simulations and the test results of sandy loam validate the new model. The simulated results also show that the temperature is the major factor to vapour transfer instead of the suction. And the vapour transfer in silt and sand cannot be neglected with freezing except clay. The initial water content, freezing temperature, freezing time and ground water table can all affect the vapour transfer in freezing soils. In a word, even though the water content increament is low, remarkable frost heave will also occur due to the vapour in susceptible frost heaving soils such as silt. Therefore, the vapour in unsaturated frozen soils must be paid more attention to in practical engineering. This study strengthens the understanding of canopy effect and also validates that the canopy effect usually occurs in covered freezing silt instead of sand or clay.