Deep-Seated Large-Scale Toppling Failure: A Case Study of the Lancang Slope in Southwest China

An old large-scale landslide with a volume of 4.6 × 106 m3 located on the right bank of the Lancang River, Southwest China, was formed by the deep-seated toppling failure of a rock mass. The rock mass located downstream of the landslide toppled intensely with a maximum toppling depth exceeding 200 m and a volume over 1.5 × 108 m3. We studied the formation mechanism of the landslide and determined the most likely future instability range of the toppled rock mass. The results show that the toppled rock mass located downstream of the landslide could be classified into four zones, namely, highly toppled, moderately toppled, weakly toppled and normal rock mass, from the surface to the deep-seated rock mass along the slope according to three factors: the unloading intensity, variation in the occurrence of the rock layer, and toppling fracture zone. The bottom boundary of the highly toppled rock mass was limited by the depth of strong unloading of the rock mass. The landslide deposits mainly originated from the highly toppled rock mass, and the slip zone was formed based on the toppling fracture zones. The highly toppled rock mass in the rock slope located downstream of the landslide will be the most likely area of instability in the future.