Failure mechanism of a slow-moving landslide on September 27, 2020, in Chang Nong Village, Guangxi, China

Abstract

A recent slow-moving landslide occurred on September 27, 2020, in Chang Nong Village (24°21′ 25” N, 109°09′ 27″ E), Guangxi Province, China. The precursor activity of the landslide started approximately 5 months before landslides with cut slope construction work at the toe part of a mountain. Given this precursor, engineers set a counterweight berm at the slope’s toe for expanding toe weight to avoid further deformation. However, the measure had a negative effect on the stability of landslides, as the stabilizing berm blocked off natural groundwater channels located at the slope toe, leading to further deformation of landslides. Fortunately, monitoring of the creeping downslope movements of the landslide mass began, and an early warning was issued before the event. Thus, no casualties were caused by the slow-moving landslide; however, 72 houses were destroyed. In order to deduce the Chang Nong Village landslide failure mechanism, the geological characteristics, slope excavation schemes, rainfall data statistics, and landslide deformation patterns were systematically investigated based on unmanned aerial vehicle (UAV) photography, field investigation, displacement monitoring, and laboratory testing. The results show that colluvial stratigraphic structures and poor rock mass properties are the internal factors that induced landslides. The combined effects of prolonged heavy rainfall and the repeated fluctuation of groundwater caused by improper activities, including toe excavation and counterweight filling, are external factors that directly triggered the landslide. This case discusses how improper engineering treatment influences groundwater changes in landslide areas and supports the importance of prevention measures for colluvium landslides.