Asymmetric Spalling Failures and Tensile Fracturing Mechanisms in Underground Busbar Chamber Induced by Blasting Excavation

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.