LAUSR

Abstract A deep-seated flexural toppling fracture situated on the left bank of the Lancang River, Southwest China, was used as a case study of the fracture mechanism and relations with the rapid and continuous downcutting of the Lancang River. The toppling fracture body has a maximum horizontal distance to the slope surface exceeding 150 m and a volume of approximately 6 × 106 m3. Based on surface and adit surveys, the toppled rock mass was divided into three zones: highly toppled, moderately toppled and weakly toppled zones. The formation of the deep-seated toppling failure in the steeply dipping layered rock mass was the result of the intense unloading of rock mass caused by the continuous and rapid downcutting of the Lancang River. The intense unloading caused the rock mass to be relaxed and cataclastic, and formed a deep stress relaxation zone in rock mass on both banks. The toppling failure of the steeply dipping layered rock mass occurred in the stress relaxation zone under the action of gravity, and developed inwards and downwards to the lower parts of the slope with the river further downcutting. The horizontal distance to the slope surface of highly toppled and moderately toppled rock mass was limited by the boundary of strongly unloaded rock mass.