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.

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.

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.

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