Frost heave and thaw settlement are primarily caused by water migration. Variations in liquid water and its components during freezing and thawing are crucial variables for investigating water migration. Here, a novel soil layer-scanning method based on magnetic resonance imaging cryogenic soil-moisture analysis (MRI-CSMA) was applied to measure dynamic changes of liquid water and its components in nine layers (each 11.1-mm thick) of four loess samples with different initial water contents (9.2%, 15.7%, 20.0%, and 30.0%) in freezing–thawing experiments under a closed system. The ice–water phase transition and water migration were key factors that controlled the liquid water and its components. During freezing, the liquid water content of each layer (except layer 7) decreased. In the frozen zone (layers 1–6), the liquid water content decreased with decreasing temperature owing to the water–ice phase transition. The lower the temperature, the higher the fraction of water that transitioned to ice. In the unfrozen zone (layers 8–9), parts of the liquid water migrated upward owing to the suction generated at the freezing front. The farther away the freezing front, the smaller the suction and quantity of water migration. During thawing, the liquid water content of each layer increased owing to the ice–water phase transition in the frozen zone and infiltrated down to the unfrozen zone. The contents of the free and adsorbed water exhibited similar change trends to the liquid water content during freezing. However, the change degree of free water was much larger than that of adsorbed water because the phase transition and migration of free water is easier than that of absorbed water. Notably, the liquid water content near the freezing front (layer 7) increased in the early stage of freezing, owing to the combined effect of soil compression and the water–ice phase transition. These results provide a reference for research on water migration in the soil during freezing–thawing and validate the advantage of MRI-CSMA when characterising water migration in large soil samples.

中文翻译：

---

使用新的核磁共振技术研究黄土冻融作用过程中未冻结的水及其成分

冻胀和解冻沉降主要是由水迁移引起的。冻融过程中液态水及其成分的变化是研究水迁移的关键变量。在这里，基于磁共振成像低温土壤水分分析（MRI-CSMA）的新型土壤层扫描方法被应用于测量四个黄土样品的九层（每层11.1毫米厚）中液态水及其成分的动态变化。不同初始含水量（9.2%、15.7%、20.0%和30.0%）的密闭系统冻融实验。冰水相变和水迁移是控制液态水及其组分的关键因素。冷冻过程中，各层（第7层除外）的液态水含量下降。在冰冻区（第 1-6 层），由于水-冰相变，液态水含量随着温度的降低而降低。温度越低，水转化为冰的比例就越高。在未冻区（第 8-9 层），由于冻结前沿产生的吸力，部分液态水向上迁移。越远离冰冻锋，吸水量和迁移量越小。在解冻过程中，由于冻结区的冰水相变，各层的液态水含量增加，并向下渗透到未冻结区。游离水和吸附水的含量在冷冻过程中呈现出与液态水含量相似的变化趋势。然而，游离水的变化程度远大于吸附水，因为游离水的相变和迁移比吸附水容易。值得注意的是，由于土壤压缩和水-冰相变的共同作用，在冻结初期（第 7 层）附近的液态水含量增加。这些结果为研究冻结过程中土壤水分迁移提供了参考。–解冻并验证 MRI-CSMA 在表征大型土壤样品中的水迁移时的优势。