Failure-oriented acoustic emission diagnosis of tunnel surrounding rock under blasting disturbance using h-value and shear crack proportion

Distinguishing transient blasting-induced damage from progressive instability is essential for failure-oriented assessment of tunnel surrounding rock. Conventional acoustic emission parameters can describe cracking activity, but cannot simultaneously characterize event-scale energy intensity and cracking-mechanism evolution. This study proposes a coupled acoustic emission framework using the H-value and shear crack proportion for damage diagnosis and instability assessment. The H-value is defined as the average energy released per acoustic emission event within a statistical window, while the shear crack proportion is determined from the RA–AF method to represent the contribution of shear-related cracking. Laboratory acoustic emission tests on intact red sandstone, PFC3D mesoscopic simulations under different lateral stress levels, and field monitoring in the Second Jiaozhou Bay Tunnel were conducted for validation. The results show that the H-value identifies the transition from distributed low-energy microcracking to localized high-energy fracture, whereas the shear crack proportion captures the transition from tensile-dominated cracking to tensile–shear mixed damage. Their coupled evolution supports a three-stage identification model comprising stable, metastable and unstable states. Field monitoring revealed synchronous anomalies in absolute energy, ring-down count, cumulative crack number, H-value and shear crack proportion during blasting, indicating concentrated crack activation, local shear slip and enhanced energy release. After blasting, these indicators gradually stabilized, suggesting short-term localized damage enhancement rather than progressive instability. The acoustic emission diagnosis was consistent with the cumulative deformation measurements and field inspection, supporting the engineering applicability of the proposed energy–mechanism framework for tunnel surrounding rock damage assessment and early warning.