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.

This study investigates the mechanisms controlling multiphase landslide reactivation at red soil-sandstone interfaces in subtropical climates, focusing on the Eastern Pearl River Estuary. A significant landslide in September 2022, triggered by intense rainfall and human activities, was analyzed through field investigations, UAV photogrammetry, and geotechnical monitoring. Our results demonstrate that landslide evolution is governed by the interplay of geological, hydrological, and anthropogenic factors. Key findings reveal that landslide boundaries are constrained by fractures at the northern trailing edge and granite outcrops in the south, with deformation progressing from trailing to leading edges, indicative of a creep-traction failure mode. Although the landslide is stabilizing, ongoing deformations suggest disrupted stress equilibrium, emphasizing the risks of future reactivation. This work advances the understanding of progressive landslide dynamics at soil-rock interfaces and provides critical insights for risk mitigation in subtropical regions.

Ancient landslides with platform geomorphology occasionally reactivate, posing serious geohazards. On September 9, 2021, persistent heavy rainfall triggered the reactivation of the Dahekou ancient landslide within a gently sloping geomorph0logy at the core of Zhangjiantan syncline in China's western Qinling-Daba Mountains. This event caused one death, damaged 80 houses, and blocked the Yushui River. This study reconstructs the sliding process of the Dahekou landslide and deciphers the complex landslide initiation mechanisms through field surveys, unmanned aerial vehicle (UAV) imagery analysis, drilling, electrical resistivity tomography (ERT) and small baseline subset-interferometric synthetic aperture radar (SBAS-InSAR) monitoring. We divide the sliding process of the Dahekou landslide into three stages. Two new landslides (#1 and #2) occurred at 18:30 on September 9, 2021. Subsequently, the ancient landslide (#3) slid in the 230 degrees direction at approximately 20:30 on September 9, 2021, then changed the direction to 170 degrees-240 degrees at 22:30 on the same day, and moved in the direction of 300 degrees at 10:00 the next day. Finally, the reactivated ancient landslide (#3) formed two partially sliding masses, with volumes of approximately 158x10(4) m(3) and 160x10(4) m(3), along the directions of 170 degrees-240 degrees and 300 degrees, respectively, damaging 80 houses and blocking the Yushui River. Field surveys suggest that new landslides #1 and #2 are rock landslides and soil landslides, respectively, with volumes of approximately 230x10(4) m(3) and 7.49x10(4) m(3). Compared with the InSAR data, the new landslide #1 thrust the ancient landslide #3, with an uplift velocity rate of 22.68 mm/a at the rear edge, from September 2020-September 2021. An analysis of drill hole data reveals that the bedding in the landslide area has complex geological conditions, comprising mudstone prone to slipping with different degrees of weathering. Notably, the core of the Zhangjiatan syncline sits on the sliding bedding of the ancient landslide, contributing to a change in the sliding direction. This comprehensive study reveals that the landslide #1 loading and thrusting, the persistent and heavy rainfall, and the complex geological conditions influenced the reactivated ancient landslide. Considering the intricacies of landslide failure mechanisms, we advocate for giving more attention in the future to the zone of potentially slip-prone strata located at the edge of ancient landslides.

Understanding the reactivation causes of ancient landslides is imperative for the prevention of landslides. However, the reasons for the reactivation of thick loess-mudstone ancient landslides and evolutionary mechanisms are unclear. This paper investigates the Gaojiawan thick loess-mudstone ancient landslide as an example using field investigation, InSAR time series analysis, and laboratory testing methods to analyze the reactivation deformation characteristics and reactivation causes of the thick loess mudstone ancient landslides, which were and verified by numerical simulation. The results show that fault fracture zones and groundwater primarily control the reactivation of Gaojiawan's thick loess-mudstone ancient landslide. Due to the fragmentation of rock mass and the development of structural planes in the fault fracture zones, as well as the excavation and unloading zone formed by the surrounding rock of the tunnel, it is beneficial to the enrichment of groundwater. It intensifies the interaction of groundwater-rock-fault fracture zones, especially for the red mudstone with more clay mineral content. The strength degradation is significant after encountering water, resulting in an imbalance in the stress state in deep strata and the reactivation of the landslide.

Fracture (fault) reactivation can lead to dynamic geological hazards including earthquakes, rock collapses, landslides, and rock bursts. True triaxial compression tests were conducted to analyze the fracture reactivation process under two different orientations of Q2, i.e. Q2 parallel to the fracture plane (Scheme 2) and Q2 cutting through the fracture plane (Scheme 3), under varying Q3 from 10 MPa to 40 MPa. The peak or fracture reactivation strength, deformation, failure mode, and post-peak mechanical behavior of intact (Scheme 1) and pre-fractured (Schemes 2 and 3) specimens were also compared. Results show that for intact specimens, the stress remains nearly constant in the residual sliding stage with no stick-slip, and the newly formed fracture surface only propagates along the Q2 direction when Q3 ranges from 10 MPa to 30 MPa, while it extends along both Q2 and Q3 directions when Q3 increases to 40 MPa; for the pre- fractured specimens, the fractures are usually reactivated under all the Q3 levels in Scheme 2, but fracture reactivation only occurs when Q3 is greater than 25 MPa in Scheme 3, below which new faulting traversing the original macro fracture occurs. In all the test schemes, both epsilon 2 and epsilon 3 experience an accumulative process of elongation, after which an abrupt change occurs at the point of the final failure; the degree of this change is dependent on the orientation of the new faulting or the slip direction of the original fracture, and it is generally more than 10 times larger in the slip direction of the original fracture than in the non-slip direction. Besides, the differential stress (peak stress) required for reactivation and the post-peak stress drop increase with increasing Q3. Post-peak stress drop and residual strength in Scheme 3 are generally greater than those in Scheme 2 at the same Q3 value. Our study clearly shows that intermediate principal stress orientation not only affects the fracture reactivation strength but also influences the slip deformation and failure modes. These new findings facilitate the mitigation of dynamic geological hazards associated with fracture and fault slip. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

An ancient landslide located in Xichang, China, that was reactivated by mining excavation and rainfall was investigated in this study. The volume of the reactivated landslide was approximately 1200 x 104 m3, thus posing a major threat to the mining area's safety. Field surveys, drilling, on-site monitoring, laboratory studies, and numerical analyses were performed to investigate the landslide deformation characteristics and reactivation mechanism. The reactivated landslide was divided into four zones: the leading-edge collapse, the sliding, the uplifting, and the traction sliding zones. X-ray diffraction and ring shear tests indicate that the sliding zone soil exhibits significant strength-weakening characteristics when exposed to water, and the residual cohesion and internal friction angle decrease by 26.9% and 28.9%, respectively, as the moisture content increases from 15 to 24%. Additionally, a three-dimensional numerical simulation was conducted to quantitatively analyze the stability evolution of the landslide. The results showed that the topographic, stratigraphic lithology, and sliding zone soil properties provided the basic conditions for landslide formation, while mining excavation and concentrated rainfall triggered landslide reactivation. Furthermore, a conceptual model characterizing the reactivation process was constructed, and the reactivation process was divided into five stages: leading-edge collapse, sliding, extrusion and bulging, deformation expansion, and accelerated creep deformation. This study provides a basis for understanding the reactivation mechanism of ancient open-pit mine landslides.

The reactivation events of old landslides in the Three Gorges Reservoir area occur frequently, making it imperative to study the water softening characteristics and reactivation mechanism. An old clay landslide was selected as the focus of the research, and a segmented water injection permeable sliding surface was designed to simulate the formation and evolution of the old sliding zone during the process of groundwater rise. Volumetric water content sensors, pore water pressure gauges, high-speed camera devices, and Geopiv-RG digital image processing technology were used to obtain data on multiple physical fields. The analysis results indicated that the decrease in shear strength of the sliding zone soil and the sudden increase in pore water pressure on the sliding surface were important factors in the reactivation of old landslides. The surface deformation exhibited prominent zoning characteristics, primarily categorized into zones of strong deformation, weak deformation, and traction deformation. The failure mechanism involved shear sliding at the front edge, tensile cracking and failure at the trailing edge, and shear creep in the middle section. The development of multi-stage secondary sliding zones in old landslides can be categorized into three types: parallel to the original old sliding zone, partially overlapping with the original sliding zone to form a layered landslide, and completely overlapping with the original sliding zone, indicating overall reactivated deformation.

There are a vast number of large-scale ancient landslides in the east Tibetan plateau. However, these landslides have experienced reactivation in recent years and resulted in increasingly serious casualties and economic losses. To study the reactivation mechanism and early identification of ancient landslides on the eastern margin of the Tibetan Plateau, high-resolution remote-sensing interpretation, field survey, interferometric synthetic aperture radar (InSAR) monitoring, laboratory and in situ geotechnical tests, physical modeling tests, and numerical simulations were used, and the main results obtained are as follows. The development and distribution of ancient landslides on the eastern margin of the Tibetan Plateau were clarified, and an efficient identification method was proposed. Reactivation characteristics, triggering factors, and typical genesis patterns were analyzed. Second, the macroscopic mechanical properties of gravelly slip-zone soil and their strength evolution mechanisms at the mesoscale were revealed, and then the strength criterion of gravelly slip-zone soil is improved. Third, combined with typical cases, the reactivation mechanism of ancient landslides under different conditions is simulated and analyzed, and a multistage dynamic evolution model for the reactivation of ancient landslides is established by considering key factors such as geomorphic evolution, coupled endogenic and exogenic geological processes. Finally, an early identification method for ancient landslide reactivation was proposed, enabling rapid determination of the evolutionary stage of ancient landslide reactivation. These findings provide new theoretical and technical support for effectively preventing the risk of reactivation disasters of ancient landslides on the Tibetan Plateau.

The reactivation mechanism of multi-slide landslides entails high complexity, and the shear mechanical properties of high groundwater-level landslides are crucial for analyzing the formation mechanism of reactivated landslides. Taking the K39 landslide of Wenma Expressway in Yunnan Province as the research object, we identified the geological and hydrogeological conditions of the landslide, the physical and mechanical properties of the slip zone soil, and the landslide deformation law using geological mapping, geotechnical engineering, indoor testing, and in situ monitoring. The results show the landslide exhibited alternating acceleration and deceleration movements under seasonal heavy rainfall and high groundwater levels. The shear strength of the soil in the deep sliding zone was greater than that of the soil in the shallow sliding zone. The deep and shallow sliding zone soils showed a decrease in shear strength with increased water content. Moreover, the residual strength of the deep sliding zone soil displayed a negative rate with an increased shear rate. In contrast, the residual strength of the shallow sliding zone soil exhibited a positive rate. Furthermore, under different shear rates, the residual internal friction angle and cohesion of the deep sliding zone soil decreased with increased water content, whereas only the residual internal friction angle of the shallow sliding zone soil followed this pattern. Finally, we performed a sensitivity analysis using the GA-BP neural network for the ring shear test parameters of the deep and shallow sliding zone soils, which included consolidation pressure, water content, and shear rate. Our analysis revealed that the residual strength of deep sliding zone soils is most affected by water content, whereas the residual strength of shallow sliding zone soils is most affected by consolidation pressure. Furthermore, it was found that the effect of water content on residual strength is much greater than the effect of shear rate on residual strength for both deep and shallow sliding zone soils. The study results contribute to a unified understanding of how shear rate affects residual strength mechanisms, support research on shear mechanical properties for multiple landslide revivals, and inform engineering practices and policies in landslide-prone areas.

Ancient landslides tend to reactivate along pre-existing slip zones that have reached a residual state. On the eastern margin of the Tibetan Plateau, previous research has indicated that the slip zone of ancient landslides is primarily composed of clayey soil with gravel, known as gravelly slip zone soil. However, the relationship between the macromechanical behavior of gravelly slip zones and the mesostructure of the shear surfaces affected by gravel is still unclear. Herein, ring shear tests and reversal direct shear tests were performed on gravelly slip zone soil, and the 3D morphology and shear surface roughness were quantitatively characterized by using 3D laser scanning technology and the power spectral density method. The results showed a significant correlation between the friction coefficient of the shear surface and its roughness. Gravel played a crucial role in influencing the macromechanical behavior of slip zones by altering the mesomorphology of the shear surfaces. By analyzing the mechanical properties of the contact unit on the shear surface, the residual strength of the gravelly slip zone was found to be jointly controlled by the basic strength of the fine-grained soil and the undulations caused by the gravel. Finally, a residual strength model was developed for the gravelly slip zone considering both the strength of the fine-grained soil and the shear surface roughness caused by the gravel. The reactivation of ancient landslides has caused serious casualties and economic losses. Field investigations have revealed that the slip zones of ancient landslides commonly contain gravel. However, we still have limited knowledge regarding the effects of gravel on the behavior of slip zones. We carried out shear tests on gravelly slip zone soils and quantitatively characterized the shear surface morphology. Our results showed a strong correlation between the friction coefficient of the shear surface and its roughness. We found that the presence of gravel significantly influenced the macromechanical behavior of the slip zone by altering the mesostructure of the shear surface. Based on our findings, we developed a residual strength model for the gravelly slip zone that considers both the strength of the fine-grained soil and the roughness of the shear surface caused by the gravel. Our study provides valuable insights into the behavior of ancient landslides along pre-existing slip zones and improves our understanding of the role of gravel in influencing their macromechanical behavior. The friction coefficient of the slip zone is positively correlated with the shear surface roughness The gravel controls the macromechanical behavior of the slip zone by altering the morphology of the shear surface A residual strength model for the gravelly slip zone soil considering the shear surface roughness caused by gravel is proposed