Landslide volume and drawdown‑induced stability loss at Shahrchay Dam

Landslides are a critical threat to dam safety, yet reliable volume estimation and stability thresholds remain elusive. This study integrates nine boreholes, triaxial tests, and PLAXIS 2D finite element modeling to assess a landslide mass at Shahrchay Dam (Iran). An inverse calibration reconciles laboratory-field discrepancies, yielding realistic strength parameters (cohesion = 41 kN/m², friction angle = 27°). The slope is marginally stable, with safety factors (SF) as low as 1.03. Drawdown dramatically reduces stability: lowering the reservoir water level from 1581 m to 1551 m decreases SF by 17–30%. Critically, the estimated landslide volume, constrained by the actual bedrock layer, is 326,000 m³ – 2.6 times lower than previous estimates that ignored bedrock. Validation against field-measured failure lengths confirm high accuracy (MAPE < 10%, NRMSE < 0.1). Based on these results, we propose an actionable guideline: a minimum safe water level of 1569 m.