Abstract: Staggered zones have the properties of “multiple shear dislocation in history, wide range, easy to soften when exposed to water and low mechanical strength”, which cause rock masses with staggered zones to easily induce varying degrees of deformation and failure under excavation unloading. It seriously affects the stability of the underground caverns. In order to study the collapse of rock masses with staggered zones in the underground caverns on the right bank of Baihetan hydropower station, we adopted microseismic (MS) system to study the microseismicity in the process of collapse, including spatiotemporal evolution of MS events and distribution characteristics of magnitude and apparent stress. We analyzed the fracture mechanisms (tension, shear or mixed) of rock masses through moment tensor theory, and summarized the evolution process: surrounding rock tension fracture → fractures gradually extend deeper → fractures initiate and expand in the direction of staggered zones (mainly tensile fractures, accompanied by shear or mixed fractures) → the fractures merge with the staggered zones, then unstable wedges are cut → the rock masses with staggered zones collapse under the blasting vibration or gravity. This paper provides references for the excavation and support of rock masses with staggered zones in underground intersecting chambers under high geo-stress, and also provides guidance for the construction of similar projects.

Key words: underground intersecting chambers, staggered zones, collapse, MS monitoring, spatiotemporal evolution, fracture mechanism