Rainfall-induced landslide in loess area, Northwest China: a case study of the Changhe landslide on September 14, 2019, in Gansu Province

A large-scale loess landslide occurred in Xiaozhuang Village, Changjiahe Town, Tongwei County, Gansu, at approximately 11:00 am on September 14, 2019. A volume of 774 x 10(4) m(3)of a historical sliding mass was reactivated by rainfall and slipped down along a weak interface. The sliding mass accumulated in the valley of the Kushui River, blocking the channel for a distance of similar to 1000 m and destroying several roads and the Yangpo Bridge. This landslide is a typical case of rainfall-induced revival of a historical seismic landslide in the loess region of Northwest China. On the basis of site investigation, Google satellite image, unmanned aerial vehicle photography, and numerical simulation, the deformation and failure characteristics of this landslide are described in detail, and the possible inducing factors and failure mechanism are preliminarily assessed. The Tongwei earthquake in 1718 and the Haiyuan earthquake in 1920 caused the slope to slide multiple times in history. Due to continuous rainfall, the existing softened layer (previous sliding surface) was highly saturated due to the perched water on the top of the relatively impervious mudstone and formed the critical weak zone. Therefore, previous sliding mass in the lower and middle slope was reactivated along the softened layer on September 14, 2019. We suggest that the perched water mainly came from the dominant seepage channels such as sinkholes and underground rivers. In addition, sinkholes on the previous sliding body are the key factors controlling the boundary of the landslide. The movement of the landslide lasted for 12 h, and the main landslide lasted for only 60 s, with a runout distance of 60 m and an average velocity of 5 m/h. Study of the formation conditions and failure mechanism of the Changhe landslide is of great significance for the early identification and risk prevention of this kind of landslide in the loess region of Northwest China.