Analysis of the mechanism underpinning the evolution of rockburst-collapse compound hazards in a deep-buried drilling and blasting tunnel

Two consecutive rockburst-collapse compound hazards that occurred during the drilling and blasting excavation of a deep-buried tunnel in southwestern China were investigated. A systematic engineering geological survey and on-site monitoring were conducted in the hazard-affected zone and adjacent tunnel sections. Utilizing microseismic (MS) monitoring technology and tunnel seismic prediction (TSP), the macroscopic damage of the compound hazards, the spatio-temporal evolution of MS events, the fracture mechanisms, and the inelastic deformation of the source region were characterized. Numerical simulation methods were further employed to reveal the stress-induced mechanisms of the hazards and the rock mass response. The findings are as follows: (1) Rockburst-collapse compound hazards are more likely to occur in areas with abrupt changes in the rock-mass integrity, where the integrity of rock masses in the hazard zone is significantly lower than that in adjacent sections; (2) MS monitoring reveals that the hazard evolution process exhibits a “long quiet period–short active period” pattern, with a sudden increase in high-energy MS events during the active period serving as a critical precursor to the occurrence of a hazard; (3) In the three days preceding the hazard, the inelastic deformation in the source region intensified rapidly, with significant local stress concentration triggering energy release; (4) The mechanism of MS events is characterized by predominantly tensile fracturing in the evolution period, while alternating shear-tensile fracturing occurs during the hazard, with tensile fracturing dominating, revealing the evolutionary path of complex fracture modes; (5) Numerical calculations indicate that zones with significant differences in the integrity of rock masses, the intersection of structural surfaces, and their intersections with the tunnel profile are sensitive areas of stress concentration, prone to forming local high-energy release zones, thereby inducing compound hazards. This study systematically analyzed the mechanisms underpinning the formation of frequent rockburst-collapse compound hazards in deep-buried tunnels, and proposed dynamic excavation adjustment and hazard warning strategies based on MS characteristics and geological conditions. The results offer theoretical support and practical reference for the identification and prevention of similar hazards in engineering practice.