423 dead. A double-curvature arch dam failed when uplift water pressure on a left-abutment shear plane exceeded the rock's shear strength.
After heavy rains filled the reservoir to capacity, water seeped through a previously-undetected gneiss fault, building uplift pressure on a buried shear plane in the left abutment. The pressure exceeded the rock's frictional resistance; a 30-million-tonne block of abutment slid out, releasing the dam.
Geological mapping had missed the fault. Modern permeability testing + uplift checks would have flagged it. Buoyancy-equivalent uplift checks (similar to basement floatation analysis) would catch the same condition under a basement raft today.
Buoyancy stability — hydrostatic uplift vs structure dead weight (FoS ≥ 1.2).
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