Deep-seated toppling deformations at the dam site of the Miaowei Hydropower Station, Southwest China

The high and steep anti-dip slopes at the dam site of the Miaowei Hydropower Station, located in the Lancang River valley in southwest China, are subjected to clusters of deep-seated toppling deformations (DSTDs). Geological mapping, exploratory adits, electron spin resonance (ESR) dating, and kinematic monitoring were conducted on the right bank slopes to explore the causes of failure and kinematic patterns of the DSTDs. The studied DSTDs formed in anti-dip metamorphic rock layers during the youngest terraces owing to the rapid incision of the V-shaped river valley under high tectonic stress. The maximum horizontal depth of toppling rocks on right bank slopes was greater than 200 m. Quantitative indices concerning the dip angle of the toppled rock layers, tensile aperture, and elastic wave velocity of rock masses, as well as qualitative indices including rock weathering and unloading, can be used as a comprehensive indicator system for characterizing the intensity of toppling. Toppled rock layers can be categorized into four zones according to toppling intensity: extremely intense, intense, moderate, and weak, as characterized by complete block detachment, composite tensile-shear fracture, tensile fracture, and reverse slip, respectively. Excavation at the slope toe triggered shallow sliding failures along the toppling-induced cataclinal discontinuities in zones of extremely intense and intense toppling, which seriously affected dam construction.