GNSS monitoring and stability evolution mechanism of the S-J1 deformation body in the lower reaches of the Jinsha River

Catastrophic landslides triggered by reservoir impoundment represent one of the most destructive geohazards in the field of hydropower engineering, posing significant threats to human lives, property, and the long-term operational safety of critical infrastructure. A large hydropower station on the lower Jinsha River, ranked as the world’s second-largest hydropower project, with an annual water-level fluctuation of up to 60 m, creates extreme geological conditions that make reservoir bank stability a particularly critical issue. Based on a comprehensive study of the S-J1 deformation body on the reservoir bank of the reservoir area, this paper systematically analyzes its deformation characteristics, formation mechanism, and stability through field investigation, GNSS monitoring, inclinometer measurements, and stability calculations. Results indicate that the deformation body represents a large-scale potential landslide triggered by reservoir impoundment and periodic water-level fluctuations, with an identified sliding surface at depths between 22 m and 34 m. GNSS monitoring records ongoing slow deformation, accompanied by significant local vertical displacement. Stability calculations demonstrate that the slope remains generally stable to marginally stable under natural and rainfall conditions, but becomes unstable under seismic and rapid drawdown conditions, suggesting a high failure risk. Reservoir impoundment is confirmed as the initial triggering factor, while drawdown-induced seepage is identified as the primary driver of ongoing progressive deformation. Mitigation measures including anchor cable reinforcement, surface drainage, and crack sealing are recommended, supported by a long-term monitoring and early warning system. These findings provide theoretical and practical references for identifying and preventing reservoir-induced landslides under analogous engineering geological conditions.