Mechanism of the 2019 Yahuokou landslide reactivation in Gansu, China and its causes

In this study, we investigated a recent landslide that occurred in Zhouqu County, China. At 18:00 pm Beijing time on July 19, 2019, the Yahuokou landslide was reactivated. About 3.92 x 106 m(3) of debris slumped from the slope, blocking the Minjiang River channel, causing the water level to rise, and disrupting road. Fortunately, there were no injuries. A field survey, remote sensing imaging, borehole core analysis, landslide monitoring, and rainfall data analysis were conducted to examine the geomorphologic and stratigraphic characteristics of the landslide, the reactivation causes, and the dynamic mechanism of the landslide. A model for the reactivation and evolution of the landslide is presented. The field survey of the sliding zone indicates that the landslide has a flow-slide deformation pattern. The landslide mechanism is summarized as follows. Initially, slow plastic flow sliding occurred in the upper sliding body. Then the platform in the middle gentle slope was preloaded by deposits from the upper part failure, resulting in a cutting and filling effect. The failure occurred in the form of creep-tension cracks-debris flow-slide. Finally, the block on the lower part slumped. Thus, the upper, middle, and lower parts of the landslide all transferred stress in the same manner, being activated step by step, and slowly slumped into the river.