冻土未冻水含量的低场核磁共振试验研究

Experimental study of unfrozen water content of frozen soils by low-field nuclear magnetic resonance

采用低场核磁共振技术测试了冻融循环过程中不同土质、不同Na Cl离子浓度饱和试样的未冻水含量,结合T2分布曲线从微细观角度分析了冻融过程中未冻水在孔隙赋存分布情况。试验结果表明:冻结过程可分为过冷度段、快速下降段、稳定段3个阶段,而融化过程仅存在稳定段、快速融化段,并不存在与过冷现象对应的过热现象。冻结时大孔隙的水首先冻结,而融化时孔隙水的增加却是从小孔隙开始的,这是由水分热动力学势能的差异导致孔隙水冻结和融化在时间上的有序性。并且分析了冻融循环中土质类型、离子浓度对未冻水含量的影响,以及探讨了冻融过程出现的滞后现象的原因。

Unfrozen water contents of saturated soil with different soil types and different concentrations of NaCl are measured by low-field nuclear magnetic resonance （NMR） in freezing-thawing cycles. The distribution of unfrozen water in soil is discussed combining with T2 distribution curve from the microscopic viewpoint during freezing and thawing process. It is concluded from the experimental results that the freezing process can be divided into three stages： super-cooling stage, rapid decline stage and stable stage; while the thawing process can only be divided into two stages： stable stage and rapid melting stage, and there is no overheating phenomenon in thawing process. During the freezing process, water in large pore freezes firstly. In a sharp contrast, the pore ice in small pores melts firstly. Thermodynamic potential difference of pore water results in the sequence of phase change of pore water during the freezing-thawing process. In addition, the influence of soil type and ion concentration on unfrozen water and hysteresis during freezing-thawing process is analyzed and possible mechanisms for the hysteresis are discussed.