An experimental study of the influence of loess salinity on combined water film thickness based on NMR and nitrogen adsorption technique
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摘要: 结合水膜是一种结合水，对深层黄土渗透与物理力学性质有重要影响，而水膜厚度是其中最为关键的参数。因黄土结构的特殊性及其微观性，结合水膜厚度一般不能通过实验手段直接测得，因此文章基于核磁共振技术与氮吸附试验展开重塑黄土孔隙溶液中含盐量对结合水膜厚度的影响研究。试验结果表明：（1）结合水与自由水的热力学性质不同，低温冻融试验显示黄土内结合水冰点约为-3.3℃，而自由水的冰点则要高于结合水冰点；同时两者在T2驰豫时间上的表现也不相同，通过核磁共振试验分析得到了两者的横向驰豫时间界限-T2截止值为1.65 ms。（2）黄土中结合水膜厚度与土体孔隙溶液内含盐量呈线性负相关，随着含盐量的增加，结合水含量降低，结合水膜厚度不断减小。黄土中含盐量的增加导致结合水膜变薄，水膜厚度的减小又会导致结合水抗剪强度降低，深层黄土渗透性增强。本次试验首次将核磁共振技术与氮吸附法相结合，应用于黄土含盐量与结合水膜厚度关系研究中，试验方法对试样无损伤，且测试快速便捷，具有较高的可靠性，为研究盐渍土中水的赋存状态提供了新思路。Abstract: Bound water film is a kind of binding water, which has an important influence on the permeability and mechanical properties of deep layer loess. Its thickness is an extremely critical factor, and the saturation of the binding water and specific surface area are particularly important parameters in the calculation of water film thickness. In this paper, a new testing approaches for calculating parameters of water film thickness is attempted. Based on the analysis of freezing point of the bound and free water by the NMR technology, a reliable time limit of free and bound water relaxation is obtained, and the bound water saturation in pore fluid samples with different NaCl contents is calculated. The test results show that (1) the combined water and free water have different thermodynamic properties. The low-temperature freezing-thawing test shows that the freezing point of loess internal bound water is about -3.3 ℃, while the freezing point of free water is higher. Meanwhile, the relaxation time of the two in the T2 relaxation curve is also different, and the relaxation time limit (T2 cut-off value) is 1.65 ms. (2) The thickness of water film in loess is negatively correlated with the salt content in pore solution of soil. With the increasing salt content, the content of bound water decreases and the thickness of bound water film also decreases, leading to the decrease of the shear strength of the bound water and the increase of the permeability of the deep layer of loess. In this paper, the Nuclear Magnetic Resonance technology combined with nitrogen adsorption method is used for the first time to study the relationship between salt content and the film thickness of bound water. This method has no damage to the samples, and the test is faster and more convenient with higher reliability, providing a new idea for the microscopic study of water occurrence in saline soil.
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Key words:
- nuclear magnetic resonance (NMR) /
- nitrogen adsorption /
- loess /
- saltness /
- bonding water /
- water film /
- T2 cutoff value
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