Abstract: The stability of the surrounding rock of the high sidewall draws great attention during the construction of the super large underground cavern. Based on geological conditions, geophysical prospecting, monitoring data and construction materials, the deformation evolution mechanism, spatial distribution pattern and failure mode of the surrounding rock are analyzed by using rock mechanics theory and FLAC3D numerical simulation. The research shows that in the vertical direction, the unloading response of the surrounding rock under 10 m of the excavation surface is intense. The surrounding rock deformation accounts for 50%-55% of the total deformation during this period. In the axial direction of the cavern, the influence area by excavation is concentrated in the range of twice the span of cavern, and the displacement increment caused by excavation takes up about 97% of the total unloading displacement change. The difference in the deformation of the surrounding rock is large at different depths due to the geological structural surface. Because of the soft plasticity of the weak interlayer, the strain energy release after the confining pressure is a slow process, and the aging shearing characteristics are obvious. During the layer-by-layer excavation process, the stress state of the surrounding rock in high sidewalls has undergone four stages: the accumulation of strain energy, stress release, stress adjustment, and stree stabilization. These research results provide significant references for similar projects.

Key words: interlayer staggered zones, underground powerhouse, deformation characteristics, Baihetan hydropower station