基于孔隙尺度的非饱和土蒸发机理研究

Evaporation from unsaturated soil is main reason for causing geotechnical disasters, nonetheless, less attention on evaporation was taken from geotechnical view, the mechanism of evaporation is not totally demonstrated yet, and the computing methods of soil evaporation are still belongs to empirical formulas. Considering the nondeterminacy of parameters in evaporation model of normal scale, this research focuses on the pore scale mechanism of soil evaporation and its parameterization, which would be conducted from a combined approach of experimental investigation, theoretical development and numerical simulation. The specific content is as follows, firstly, evaporation test will be conducted on glass bead of different sizes, sandy soil and silt, such that, critical point on evaporation curve will be defined by appropriate soil parameter, decline law of Stage 2 of evaporation will be revealed and parameterized, the effect of salt precipitation and osmatic suction on evaporation rate will be investigated by evaporation test under solution of different species and concentrations. Secondly, a pore scale model for soil evaporation will be established on basis of porous media theory, which aims to demonstrate how the evaporation rate relations to matric suction, water content and particle size etc. Finally, the computing method of considering osmotic suction with clear physical background will be presented from micro to macro. The outcomes of this research would benefit the disciplines of geotechnical engineering, agriculture, and hydrology when handling with soil evaporation problems, meanwhile, great value in theory and practical significance is considered to be generated from this research.

土中水分蒸发是许多工程病害的重要诱因，但岩土领域围绕非饱和土蒸发的研究很少，其计算方法处于经验模型阶段。鉴于宏观尺度模型参数的不确定性，本项目针对孔隙尺度的非饱和土蒸发机理及其计算方法，拟采用室内试验、理论建模与数值解析相结合进行系统研究。首先结合微细观观测技术进行不同粒径的玻璃微珠、砂土和粉土的蒸发试验研究，定义表征蒸发过程转折点的土性参数，明确第二阶段蒸发下降规律和控制参数；然后进行玻璃微珠在不同浓度、种类盐溶液下的细观蒸发试验研究，探明盐晶析出及溶质吸力对蒸发的影响规律；基于试验成果和多孔介质理论，建立非饱和土蒸发的孔隙尺度模型，实现数值解析，探索蒸发速率与基质吸力、含水率、颗粒粒径的定量关系；进而通过建立细观和宏观参量的联系，提出考虑溶质因素、物理意义明确的蒸发计算方法。研究成果可为岩土、农业和环境领域在控制或计算土的蒸发问题上提供科学依据，具有重要的理论价值和实际意义。

土中水气相变是许多工程病害的重要诱因，但岩土领域围绕非饱和土水气相变的研究很少，其理论方法处于经验模型阶段。为揭示非饱和土蒸发机理，基于指数形式非饱和土水特征曲线和渗透系数模型，提出了考虑任意初始状态含水率分布的非饱和土蒸发计算模型。模型揭示了瞬态蒸发速率和非饱和土水分重分布变化规律，建立了蒸发速率与基质吸力、含水率、颗粒粒径的定量关系。基于开发的气候控制蒸发试验系统，展开系列室内试验，验证了所提蒸发模型的有效性。本项目重点研究了非饱和冻土水气迁移与相变诱发“锅盖效应”的机制，阐释了“锅盖效应”的机理、影响因素及研究意义，建立了综合考虑水气3个相变过程的非饱和冻土水热气耦合迁移的数学模型，数值再现了“锅盖效应”的发生发展及致灾过程。另外本项目提出了“锅盖效应”的新的系列试验装置及方法, 阐释了“锅盖效应”区别于传统冻害的特点：a. 冻结，相同；b. 水源：从冻结相反方向提供水源，不同；c. 土性：砂土及粗粒土不冻胀，粘土难冻胀，粉土最易冻胀，不同；d. 冻胀往往引起破坏性更强的融沉，不同。揭示了初始含水率、土性、干密度、边界温度、补水条件、试验周期等对“锅盖效应”的影响规律。