The rock mass is susceptible to instability and damage during cavern construction. The blast-induced cracking process of the rock mass contains a wealth of information about the precursors of instability, and the identification of fracture nucleation signals is a prerequisite for effective hazard warning. A laboratory mechanical test and microseismic (MS) monitoring were carried out in the Baihetan Cavern to investigate the fracture nucleation process in the rock mass. MS monitoring shows that pre-existing microcracks were closed or new cracks were generated under the action of high stress, which caused the migration of microcracks. As the crack density increases, the fracture interaction gradually increases. The study of the rock fracture nucleation mechanism helps to reveal the MS sequences during the rock fracture process, and the fore-main shock was found in the MS sequence during access tunnel excavation. This study can effectively provide guidance for the early warning of rock mass failure and the stability analysis of underground caverns under blasting excavation disturbance.

Underground caverns of hydropower project have significant mechanical properties of high stress, high sidewall, large span, complex geological conditions and mutual disturbance of adjacent cavern construction, resulting in very complex deformation and rupture characteristics. Through the laboratory triaxial compression experiments, the mechanical response of deformation and failure mechanism of granite is explored, and the elastic-brittle mechanical properties are also revealed. With the help of high precision microseismic (MS) monitoring system, the inoculation law of the internal fracture initiation, expansion, intersection and penetration of the surrounding rock is captured, and the inoculation mechanism of the rock macroscopic damage is also clarified. The research results of this paper enrich the database of rock mass failure cases of rock cavern, which can provide support for the control of rock mass fracture.

Spalling failure in hard rock caverns poses a major threat to the safety of underground caverns. To clarify the relationship between the macroscopic damage characteristics of spalling and the fracture mechanisms under symmetric cavern geometric configurations and asymmetric excavation-unloading stress paths, we employed a multi-scale approach combining microseismic (MS) monitoring, laboratory testing, field geological investigations, and scanning electron microscopy (SEM). The recorded MS events progressively clustered along the free surfaces of the symmetric busbar chamber, with the highest concentration occurring at the cavern crown. Fracture mechanism inversion based on the S- to P-wave energy ratio quantitatively indicated that tensile fracturing dominated the failure of the busbar chamber portal. Uniaxial compression tests confirmed the pronounced brittleness of hard basalt and its high susceptibility to spalling failure, while SEM observations resolved the associated mesoscopic fracture features. This study establishes a multi-scale link between excavation-induced MS activity, tensile fracture development at free faces and macroscopic spalling damage within symmetrically arranged deep underground structures, providing a theoretical basis for early warning of spalling hazards in deep hard rock caverns.