Solid-fluidization transition-induced flow-like events pose significant threats to both ecological systems and human society. This geophysical phenomenon undergoes a continuous and catastrophic solid-fluidization-solid retransition, which often leads to severe disasters. A series of flume and rheological tests were conducted to explore the continuous solid-fluidization-solid retransition mechanism of sedimentary loess. The results showed that the flow distance after phase retransition increased by 39.5% compared with the first flowslip distance. With increasing rainfall intensity, the moisture content during phase transition tended to decrease while the time required for reactivation lengthened. Rheological analyses revealed that the reduction and recovery of storage modulus exhibited by thixotropy is a crucial mechanism in the phase retransition of soil, and they have significant time-concentration dependence. A higher soil water content leads to a longer structural recovery time and stronger thixotropy, which agrees well with the results of flume tests. Our experimental data NSav and NBag showed a positive power-law relationship and had similar fitting coefficients to the field case data, indicating that our experimental results have successfully captured the kinematic and rheological characteristics of real mudflow events. This study suggests that thixotropy can be used to interpret complex phase retransition processes in mudflow and can also help to explain the hypermobility and reactivation of many large geophysical processes, such as pyroclastic flows.

Mudflows are natural phenomena starting from landslides and presenting high impact when they occur. They generate great catastrophes in their path because most of the time there is no indication prior to the failure that triggers them. Understanding how mud is transported is of great importance in infrastructure projects that coincide with hillside areas due to the high risk of occurrence of this phenomenon by cause of the high slopes, which can involve great risks and produce disasters that involve great costs. This work presents the evaluation of mudflows, from the implementation of a laboratory scale experiment in a consistometer with its calibration and validation from numerical models to estimate rheological parameters of the material. Tests were also carried out in an open channel in the laboratory, based on the data previously obtained considering the behavior of the material as a both Newtonian fluid and non-Newtonian fluid. The experiment considered a channel with dimensions of 3 m long, 0.5 m high and 0.7 m wide with slope control, and a mud composition of silty material with 60% moisture. The tests were conducted with slopes of 5%, 10%, 15% and 20%. The numerical models were carried out in ANSYS FLUENT software. In addition, the calibration data of the numerical model were used for a real case study, simulating the slip flow occurred in Yangbaodi, in the southeast of China, occurred on September 18, 2002. The results of the numerical models were compared with the experimental results and show that these have a great capacity to reproduce what is observed in the laboratory when the material is considered as a non-Newtonian fluid. The model reproduced in an appropriate way the movement of the flow at laboratory scale, and for the aforementioned case study, some differences in the final length of deposition were noticed, achieving interesting results that lead the use of the calibrated model towards the estimation of risks due to the mudflow occurrence.

Extreme rainfall events, within the context of climate change, pose a heightened risk of geohazards to mountainous regions. On 22 June 2022, a rainstorm-induced landslide-mudflow occurred in a terraced field in Longsheng County, Guangxi Zhuang Autonomous Region, China. The disaster began as a rotational slide, and mobilized into a mudflow with high mobility and long runout, causing significant damage to the local community. This event served as a wake-up call not only for the safety of mountain settlements, but also for the protection of terraced fields as Globally Important Agricultural Heritage Systems. To elucidate the trigger and mudflow mobilization of the event, field investigation, hydrological and agricultural analyses, and laboratory tests were conducted. It was found that the persistent and record-breaking rainfall directly triggered the disaster by increasing pore water pressure. The transition from paddy terraces to dry terraces was deduced to have contributed to a lack of maintenance in the terrace drainage system, thereby heightening the likelihood of landslides. The mudflow mobilization was attributed to excess pore water pressure generated by soil contraction and an undrained condition maintained by low permeability soil. Soil experiencing sliding may be more susceptible to shear contraction, consequently resulting in long-runout motion. Under conditions of increasing extreme rainfall, greater attention needs to be paid to geo-disaster prevention and terraced field protection in mountainous regions.

This study establishes a foundational framework addressing challenges, implications, and potential remedies related to collapsible soils. Serving as a cornerstone for global exploration, it emphasizes the importance of understanding geological, structural, and mechanical characteristics for early identification and proactive mitigation. The study underscores the significance of preventing structural damages in regions prone to collapsible soils, discussing their diverse types and origins, structural composition, and mechanical behavior. A detailed exploration highlights their prevalence in semi-arid and arid regions, emphasizing distinct geological features associated with their occurrence.