Impoundment-induced failure of the large-scale toppling mass at the Cihaxia reservoir bank poses critical stability threats to hydropower infrastructure. It is urgent to gain deeper insights into its failure mechanisms to ensure safe operation of hydropower stations. This study comprehensively investigates the failure processes, strain distribution,hydrological, and displacement responses of the toppling mass during impoundment through physical model tests.Based on above multi-scale data analysis,the toppling failure mechanisms are revealed.The results show that with the increase of lateral depth, the response lag of volumetric water content and pore water pressure increases, and the influence of water level change on the slope diminishes. Strong topplingfragmentation zone (STFZ) shows highest sensitivity to water level changes with the maximum displacement of 8.7 cm (prototype 17.4 m), followed by blocky fracture zone (BFZ) with 5.4 cm (prototype 10.8 m) and Toppling influence zone (TIZ) with 0.13 cm (prototype 0.26 m). The first large-scale toppling failure occurred when water level stabilized at 97 cm (prototype 2925 m)triggered by increased uplift pressure and reduced antisliding force.The failure exhibited a“lever-style” strain distribution (tension above and compression below)with the tension–compression boundary as pivot. The water level has a dual role in the failure: Above it, tensile stresses are induced by uplift pressure and sliding forces. Below it, compressive stresses come from the seepage forces and hydrostatic pressure. Prolonged weathering has altered the rock mass to become soil-like in both properties and size. This change increases hydraulic conductivity and creep tendency, shifts rock mass failure from brittle to ductile, and causes deformation to persist long-term. These results provide valuable insights for engineering practice and risk mitigation strategies related to large-scale toppling mass during impounding.
 

Large-scale toppling mass; Block toppling; Similarity model test; Initial impounding; Failure mechanism