冻结过程中盐渍土冰分凝特征及盐冻胀机理研究

Sulfate and chloride saline soil are widely distributed in cold and arid regions of Northwest China. Detrimental frost heave and salt expansion occur in these areas due to harsh natural conditions, and then cause great damage to the stability of the engineering structures. In this study, saline soil on Hexi Corridor is taken as the research object, ice segregation characteristics and the mechanism of salt expansion/frost heave in freezing process are investigated: (1) The phase change processes of saline soil in different concentration are researched. Furthermore, the variations law of water super-cooling point and salt supersaturation in pore solution are investigated, then the porosity change of soil and the effects on subsequent transition are analyzed with salt and ice accumulation. (2) The moisture migration driving force of saline soil with different salt content are investigated under water desorption and freezing process. Then the effects of salt crystals on the soil water potential are obtained. (3) Taking sodium sulfate saline soil as the research object, ice segregation characteristics and the deformation of salt expansion/frost heave are studied. A theoretical model is established to describe the water and salt migration, where the influence of ice and salt formation are considered. The result of this study will further enrich the related theory of saline soil in cold area, and provide theoretical reference for further understanding of the frost heaving mechanism of saline soil.

在我国西北的寒旱区，硫酸盐和氯盐渍土分布广泛。由于气候条件恶劣，土体发生着严重的盐胀和冻胀的工程病害，这对修筑在其上的工程建筑物的稳定性造成了极大地危害。本课题以河西走廊的盐渍土为研究对象，研究在冻结过程中盐渍土的冰分凝特征及盐冻胀变形机理：（1）研究不同含盐量的土体在冻结过程中孔隙溶液的相变规律，探讨降温过程中水相变的过冷度和盐相变的过饱度问题，分析冰晶体和盐结晶在累积过程中土体孔隙率的变化以及其对后续孔隙溶液相变过程的影响规律。（2）研究不同含盐量盐渍土在脱湿和冻结过程中水分迁移驱动力的变化规律，得到盐晶体对土体土水势的影响规律。（3）以硫酸钠盐渍土为研究对象，研究在冻结过程中盐渍土冰分凝规律及盐冻胀变形规律，建立考虑冰、盐结晶综合影响的水盐迁移的理论模型。本研究成果将进一步丰富寒区盐渍土相关理论，为深入了解盐渍土的盐冻胀机理提供理论参考。

寒区盐渍土具有普通盐渍土和普通冻土的双重性质，当外界环境变化时，水盐相变造成土体性质显著改变，同时水分和盐分的迁移聚集引起土体产生盐胀和冻胀病害。本项目借助室内试验、理论分析等手段研究盐渍土在冻结过程中的相变规律，探究盐渍土在冻结过程中的冰分凝特征及盐冻胀病害机理。主要研究成果为：1.采用一系列降温试验，研究了不同含水量和含盐量土体孔隙溶液的相变规律。结合水盐相图理论，分析了盐渍土孔隙溶液和自由溶液相变的区别和联系，发现土体孔隙溶液的二次相变温度并不是一个定值，而是随初始含盐量的不同而动态变化。结合物理化学相关理论进一步提出盐渍土二次相变温度预测的数学模型，基于该模型分析了盐渍土在降温过程中冰盐结晶量对二次相变温度的影响规律。2.研究了盐渍土液态水含量随温度的变化规律，结合多孔介质理论，建立了盐渍土液态水含量计算模型，模型能够较好预测盐渍土在两次相变过程中液态水变化过程。另外，利用PFmeter传感器测试了盐渍土在降温过程中基质吸力的变化规律，探讨了水盐相变对基质吸力的影响，并讨论了盐渍土在降温过程中的水分迁移驱动力。3.研究了高含盐量硫酸钠盐渍土的水盐迁移规律，结合相图理论，推导了盐、冰、未冻水计算公式，并建立了考虑盐分结晶的盐渍化正冻土多物理场耦合数学模型，模型不仅能够较好预测盐渍土在冻结过程中的水热盐变化规律，同时对盐渍土变形也能有较好的反映。以上研究对深入了解寒区盐渍土的相变特征及变形机理提供了理论依据，对研究盐渍土路基水盐迁移特征及病害防治具有重要现实意义。