冻融过程中盐分迁移和相变作用下盐渍土细微观结构破坏机理

Numerous engineering problems including frost heave and thaw settlement, as well as salt heave and dissolve settlement caused by temperature change and salt transport occur commonly when constructions build in seasonal saline frozen soil regions. Microstructure transformation is thought substantially to be reason for the failure mechanism of saline soil. Focused on the saline soil with sodium sulfate in seasonal frozen regions, the proposal utilizes a comprehensive approach that combines macroscopic and microscopic, experimental and theoretical methods. Based on freeze-thaw cycles, CT scanning, MIP and SEM tests, water and salt transport characteristics and the mesoscopic damage evolution law of saline soil are obtained. The pore size distribution characteristics are analyzed to calculate crystallization pressure and induced effective stress and then the model of crystallization pressure of saline soil is established. According to the microstructural characteristics, the particles’ contact and sliding model is formed considering freeze-thaw cycles and salt transport, and it is used to calculate the macro deformation, and predict the variations of salt and frost heave. The objective of this project is to provide theoretical guidance to the design of bearing capacity and the measurements of preventing subgrades from salt and frost heave in seasonal saline frozen soil districts.

在季节性盐渍化冻土地区工程建设中，温度的变化和盐分的迁移将引起冻胀融沉和盐胀溶陷等工程病害问题，盐渍土的破坏机理从本质上可认为是土体颗粒内部细微观结构的改变。本课题以季节性冻土区硫酸钠盐渍土为研究对象，采用宏观与细微观结合、试验与理论相结合的方法,通过冻融循环试验、CT扫描、MIP及SEM测试等方法，得出盐渍土的水盐迁移特征及细观损伤演化规律；根据孔径分布特征，推导结晶压力及有效应力表达式，建立盐渍土结晶压力模型；结合土体微观结构特征，建立考虑冻融与盐分迁移的颗粒接触滑移模型，以此模型来计算土体的宏观变形，并预测盐胀与冻胀规律。本研究有望为盐渍土路基承载力设计及防盐胀冻胀措施的选取提供理论依据。

季节性盐渍化冻土区的盐冻胀破坏严重影响工程运营安全，温度变化下的水盐迁移特征及盐冻胀破坏机理对于保障工程安全具有理论意义。结合青藏高原盐渍化冻土的基本物理热学性质，建立冻结温度与未冻水含量的关系模型。研究的主要结论如下：(1)在正温区间，未冻水含量基本保持不变，负温区间，随温度降低，未冻水含量迅速降低之后变缓。 (2)采用径向劈裂法测试冻结缘温度范围内粉质黏土的抗拉强度，抗拉强度与盐分浓度整体呈线性负相关，温度越低，土体的抗拉强度逐渐增大。(3)基于有压单向冻结和冻融循环试验，得到土体内部水盐迁移特征及盐冻胀破坏特征。温度初期迅速降低，之后逐渐趋于稳定。水盐分布主要集中在冻结缘区域，随盐分浓度的增大，冻结深度减小。应力初始为正值，随时间增加，逐渐变为负值，随时间增长，应力逐渐增大。(3) 采用压汞和电镜扫描等对冻融循环后的粉质粘土进行微观结构分析，得到不同含盐量的粉质粘土的孔径分布特征、表观形貌及孔隙率、分维数等参数特征。随含盐量增加，孔隙体积和表观孔隙率先增加后减小，孔径分布特征呈双峰型，分维数基本不变。研究结果为季节性盐渍冻土区的承载力设计及防盐胀冻胀措施的选择提供理论依据。