Seismic dynamic response and instability failure patterns of fractured, loosened, unloaded rock slopes at Lancang River hydropower station

Fractured loosened rock masses are widely developed in the shallow subsurface of alpine valleys in southwestern China and are commonly underlain by toppling-deformed rock masses, forming a distinctive layered slope structure of "upper fractured–lower toppling" that poses a prominent instability risk under strong earthquakes. However, existing studies have mostly focused on single-layer or homogeneous rock slopes, and the dynamic response characteristics and instability evolution mechanism of this composite structure remain insufficiently understood. Taking the SL slope of a hydropower station on the Lancang River as the prototype, this study constructs a refined three-dimensional numerical model using 3DEC coupled with a discrete fracture network and performs a full-time-history dynamic analysis. The results show that: (1) The fractured loosened rock mass is the dominant sensitive unit of dynamic response; the vertical PGA amplification coefficient at the slope crest exceeds 3.0 and the horizontal PGA amplification coefficient on the slope surface exceeds 2.0, exhibiting a vertical zonation pattern of "deep attenuation—middle fluctuation—shallow amplification." (2) The slope instability successively undergoes four stages: surficial scattered rockfall (peak velocity 7.32 m/s), middle-layer tensile cracking and sliding (8.16 m/s), toppling-fracture-induced retrogressive movement (9.83 m/s), and whole-layer disintegration failure (13.28 m/s); the core mechanism is the retrogressive overall instability triggered by flexural fracture of the underlying toppling-deformed rock mass. (3) With increasing seismic intensity, the failure mode evolves in a stepwise manner, and three critical thresholds are identified—structural-plane shear strength (0.2 g), toppling-mass flexural strength (0.5 g), and slope-toe shear-through (1.0 g)—corresponding to the evolutionary sequence of "surficial scattered rockfall → middle-layer tensile cracking and sliding → toppling-fracture-induced retrogressive movement → whole-layer disintegration failure."