土壤冻融过程水热迁移特性及机理研究

The engineering diseases which caused by the variation of mechanical properties of frozen soil by heat and moisture migration during freeze-thaw process are the main influences on the service safety of high grade traffic infrastructures in frozen earth region. Therefore, to reveal the characteristics and the corresponding microscopic mechanisms of heat and moisture migration of frozen soil during freeze-thaw process is of high strategic importance and scientific and engineering value for constructing the northern part of “the Belt and Road Initiatives”. The proposed project focuses on the change rules of the characteristics of porosity and permeability, the principles of pore water freeze，the mechanism of driving force and resistance of moisture migration of frozen soil during freeze-thaw process. Based on transient dynamics, the influences of microstructure on the movement of soil particle and the distribution of pores are analyzed. Based on molecular dynamics, the formation mechanisms of driving force and resistance of moisture migration of frozen soil during freeze-thaw process are studied. By integrating multi-scale experiment and lattice Boltzmann method, a multi-scale numerical model is established. The impact factors and evaluation criteria of the intensity of the migration is analyzed with the numerical model. The highlights of the proposed project is that it starts with microscopic mechanism, integrating multi-scale experiment and the latest research achievements of lattice Boltzmann method on the heat and mass transfer process in turbid media, adopting “micro to macro” route, systematically investigating the characteristics of heat and moisture migration of frozen soil during freeze-thaw process, offering theoretical basis for the prevention of engineering diseases in frozen soil region.

冻土冻融过程中由于水热迁移造成土壤力学性质改变而引发的病害是制约冻土地区高等级交通基础设施服役安全的主要因素之一。因此，揭示冻土冻融过程水热迁移特性及相应的微观机理具有重要的科学与应用价值。本课题基于瞬态动力学理论，分析微观结构对冻融过程土壤颗粒位移特性及孔隙分布变化规律的影响；基于分子动力学理论，研究冻融过程土壤水分迁移驱动力及阻力的形成机理；结合多尺度实验及格子Boltzmann方法建立冻土冻融多尺度水热迁移数值模型，分析冻土冻融水热迁移强度的主要影响因素，并建立评价准则。结合数值模拟与实验研究，揭示土壤冻融过程渗透特性与结构参数的映射关系，阐明水分迁移驱动力及阻力形成的微观机理并完善数学表达式，建立基于微观物理过程的多尺度水热迁移数值模型，系统评估冻土冻融水热迁移强度的影响因素，为冻土地区工程病害防治提供基础理论依据。

本课题围绕土壤冻融过程中的水热迁移特性及机理开展了实验与模拟研究。基于格子Boltzmann方法，对土壤的微观孔隙水分布特征及冻结渗透特性进行了数值模拟研究，分析了土壤孔隙率、表面亲疏水性、饱和度等对土壤孔隙水分布的影响，建立了考虑未冻水膜的冻结过程土壤渗透率模型。基于Shan-Chen多相流模型及焓的相变模型，对SC模型外力项进行改进，建立了土壤冻结过程孔隙水迁移模型，进行了小尺度孔隙水迁移实验并用于验证数值模型。通过足尺度实验，分析了土壤冻融过程中温度和孔隙水含量分布的变化以及孔隙水迁移的机理，通过小尺度冻胀实验，分析了颗粒尺寸、颗粒级配、冻结温度、供水情况对土壤冻结过程中水热迁移及冻胀特性的影响，通过小尺度融沉实验，分析了颗粒粒径、排水条件、升温速率对土壤融化过程沉降特性的影响。结果表明：(1) 孔隙率的减小和水分饱和度的增加会促进孔隙通道中小气泡的形成，颗粒表面由亲水性向疏水性转变时，孔隙间形成的气泡将逐渐由凝聚于孔隙中心向壁面扩散；(2) 土壤平均粒径减小时，孔隙水过冷度明显增大，颗粒间隙较小点处孔隙水过冷度明显大于间隙较大点处；(3) 土壤冻结过程渗透率的计算应考虑土壤颗粒表面未冻结水膜，基于土壤孔隙水相变温度分布几何计算方法的孔隙尺度土壤冻结过程渗透模型与实验值相关性高达0.97；(4) 对土壤冻结过程孔隙水迁移的分析表明，考虑冻结前沿冻吸驱动力并建立了未冻结孔隙水体积分数与局部孔隙率关系式的数值模型能得到更精确的结果；(5) 在足尺度土壤冻融过程中，随温度由高温逐渐降低到相变温度，未冻结区域土壤水分含量会逐渐增加，并在临近相变点处产生突降现象；(6) 较高小颗粒级配土样的冻胀量和体积含水量的峰值均为最大，吸水量的变化与冻胀量变化基本一致，并显示出滞后性；(7) 浅层土壤的水分在冻结早期被迅速吸收到表层，使表层土壤的总含水量迅速增加，冻结结束后体积冰含量对冷端峰值的贡献较大，冻结缘的总含水量峰值是未冻结水和冰共同作用的结果；(8) 相较大颗粒土，小颗粒土的快速沉降时间更长，位移量更小，不排水情况比排水情况快速沉降时间更短，沉降更小。