正冻条件下盐渍土体水盐运移及冻胀盐胀变形机理研究

Under freezing conditions, water and salt accumulate and crystallize towards the cold end in the saline soil of seasonal frozen areas, which causing the salt heave, frost heave in roadbed and other engineering diseases. Until now, few studies have been conducted to investigate the effect of clay surface properties on the crystallization process of pore solution, and how to distinguish the frost heaving and salt heaving in the deformation of saline soil has rarely been considered. The above deficiencies restrict the application of relevant migration model or frost and salt heave model in engineering practice. This project takes the occurrence process of frost heave and salt heave as the main line, and carries out tests on freezing temperature, nuclear magnetic resonance, dynamic evolution process of water and salt migration and frost heave and salt heave deformation. Combining soil-water potential model and thermodynamic theory, the microscopic mechanism of water and salt migration in saline soil under recharge conditions is revealed, and the model of water and salt migration in saline soil is established. According to the microscopic mechanisms of nucleation and growth of ice crystals and salt crystals, the phase transition mechanism of pore solution during freezing is clarified, and the theoretical model of freezing temperature of saline soil is proposed based on the classical nucleation theory.. Furthermore, the prediction model of salt heave deformation in seasonal frozen regions is developed. The purpose of this study is to improve the applicabilities and accuracies of the water-salt migration model and the prediction model of salt heave and frost heave deformation in the engineering application, and to provide theoretical basis for the prevention of engineering diseases of saline soil.

正冻条件下，季节性冻土地区盐渍土中水盐向冷端积聚、结晶，造成路基出现冻胀、盐胀等工程病害。已有研究较少考虑粘粒表面性质对孔隙溶液结晶过程的影响，及如何有效区分盐渍土变形中的冻胀和盐胀，制约了有关运移模型或冻胀盐胀预报模式在工程实际中的应用。本项目以冻胀盐胀的发生过程为主线，开展冻结温度、核磁共振和水盐迁移动态演变过程及冻胀盐胀变形监测等试验；结合土-水势模型和热力学理论，揭示补给条件下盐渍土水盐迁移的细观机制，建立冻结作用下盐渍土的水-盐运移模型；基于经典成核理论，从冰晶体、盐晶体成核、生长的微观机制出发，阐明冻结过程中盐渍土体孔隙溶液的相变机理，建立含复盐盐渍土冻结温度的理论模型；进而构建季节性冻土地区盐渍土冻胀盐胀变形的预报模式。通过本项目的研究，以期提高水-盐运移模型及冻胀盐胀变形预报模式在相关寒区工程中的适用性和应用精确度，为盐渍化工程病害的预防与防治提供理论依据。

我国西北地区气温变化剧烈，且硫酸盐渍土地区多处于季节性或多年冻土区。冻胀的发生导致土体中溶液浓度增大，会加快盐晶体的析出，加剧盐胀的发生，同时，盐渍土中水分的迁移伴随着盐分的迁移，进一步加剧土体的冻胀盐胀变形。随着经济社会的发展，越来越多的工程活动将会在盐渍化季节性冻土地区开展。因此，深入研究冻融作用下季冻区土体盐渍化病害致灾机理及应对措施迫在眉睫。.本项目以河西走廊含盐粉质黏土为研究对象，开展了相关的试验研究与理论分析，探讨了冻融循环作用下水分盐分迁移规律及变形特性，揭示补给条件下盐渍土水盐迁移的细观机制，提出盐渍土冻胀盐胀变形的预报模式。主要内容有：（1）假定土体冻结特征曲线形状依赖于土体的孔径分布，基于土体微结构特征，建立了考虑颗粒粒径分布的土体冻结特征曲线模型。（2）基于土级配参数建立了黏粒含量与残余孔隙率的定量关系；借助VG方程描述土-水特征曲线，提出了一种基于黏粒含量的预测非饱和土含水率的物理-统计方法。（3）引入未冻水含量滞回度与温度间的关系曲线，研究了冻融过程未冻水含量滞后程度随温度的变化规律，得到了冻融滞回曲线和滞后度随孔隙形状的变化规律。（4）发明了一种可区分盐渍土冻胀量与盐胀量的联合测定装置和理论计算方法，对净胀量和温度分别关于时间t的变化曲线进行对比研究，给出了冻胀量与盐胀量的具体计算方法。（5）以粉质黏土、粉土、砂土为研究对象，开展了不同含盐量条件下的冻结温度和融化温度试验研究，基于Pitzer离子模型和相变理论，提出了含盐土-水系统相变温度理论模型。（6）基于土水势理论分析盐分（浓度、复盐浓度比）与孔隙大小对土体界限成核速率的影响，建立界限成核速率与土水势理论的联系。（7）选取了粒子群优化算法优化神经网络的权值和阈值，建立了基于粒子群算法优化后的人工神经网络方法，实现了盐渍土冻胀盐胀变形的准确预报。.本研究成果进一步提高了水-盐运移模型和冻胀盐胀变形预报模式在寒区工程中的适用性和应用精确度，可为盐渍化工程病害的预防与防治提供理论依据。