The hysteresis effect of unfrozen water during freeze-thaw cycles greatly influences the hydrothermal properties of soil. To better understand the hysteresis behavior of unfrozen water in the soil, this study utilized frequency domain reflectometry to measure the unfrozen water content variations in silty clay under both stepwise and rapid temperature change modes. The hysteresis effect of unfrozen water in soil was analyzed, also the underlying mechanism was revealed. The results indicate that unfrozen water content variations are consistent across the two temperature change modes, with hysteresis observed in both scenarios. This effect was more noticeable during the rapid temperature change mode, and soil samples with higher initial moisture content froze earlier and thawed more slowly in this mode. The hysteresis phenomena are mainly influenced by the ice crystal metastable nucleation, the blockage effect of pore ice crystallization, and the pore water pressure changes during phase transition. The main cause of unfrozen water hysteresis in soil during the initial freezing phase is the metastable nucleation process. In the later stages of freezing, the hysteresis effect is primarily driven by changes in capillary water curvature, induced by the blockage effect of pore ice crystallization, and shifts in pore water pressure during the ice-water phase transition. Also, a hysteresis model was proposed and validated against experimental data and existing models, demonstrating good performance and accurately predicting unfrozen water content under varying temperature conditions. This research enhances the understanding of the mechanism responsible for the hysteresis effect of unfrozen water content in frozen soil.

BackgroundAt approximately 4:00 PM on 18 July 2023, a heavy rainstorm lasting one hour triggered a significant mudstone landslide in Dongping, Weiyuan County, Gansu Province, Northwest China. The landslide resulted in the burial of houses, the fracturing and destruction of roads, and posed a serious threat to 16 households. The estimated economical loss from this disaster reached 3.2 million yuan. This study presents a detailed field investigation of the Dongping landslide, focusing on the deformation and failure characteristics through a multi-layered analysis of sliding strata, rock mass structure, slope configuration, and failure mechanism. Moreover, the study explores the key triggering factors of the Dongping landslide, with particular attention to the roles of seismic activity, rainfall, and preferential flow in the development of large-scale mudstone landslides.ResultsThe stratigraphic profile of the Dongping landslide reveals a two-layer structure, consisting of overlying loess and underlying mudstone, with the sliding surface primarily located within the underlying Neogene red mudstone. The initiation location of the Dongping landslide is situated at the rear of the slope, while the main slip-resistant is located in the middle of the landslide, exhibiting a predominantly thrust-sliding. After encountering resistance in the middle section, the front part of the sliding mass continued to move, leading to the formation of secondary landslides. The overall movement of the Dongping landslide is characterized by rotational sliding, with the sliding mass remaining relatively intact.ConclusionsThe initiation of the large-scale mudstone landslide in Dongping was driven by multiple factors. The heavy rainfall served as the direct triggering factor for the landslide occurrence. However, some historical factors, including seismic activity and previous sliding surface, had already weakened the slope structure by degrading the mechanical properties of the landslide mass and creating preferential flow channels, thereby setting the stage for the Dongping landslide. Structural fractures in the landslide area, along with sinkholes formed by a combination of tectonic joints, soil properties, and human activities, constituted preferential seepage pathways for water within the slope. These pathways provided the hydraulic conditions necessary for rainfall-induced landslides, making them the primary controlling factors in the occurrence of the Dongping landslide.