Quantitative morphology and stress-scalable AE early-warning of shear failure in twin tunnels integrated experimental and DEM analysis

The escalating depth of resource extraction in high-stress zones has made shear-induced tunnel instability a critical engineering concern. This study presents a holistic investigation into the failure mechanisms and precursor dynamics of twin circular tunnels under compression-shear loading. An integrated methodology—combining physical model tests, acoustic emission (AE) monitoring, 3D fracture scanning, and strain-calibrated particle flow code (PFC) simulation—reveals a four-stage shear-failure progression: elastic compression → stable microcrack propagation → stress-driven rock-bridge penetration → frictional slip. Key quantitative results show that increasing normal stress amplifies the failure dip angle sixfold (from 5.2° to 32.1°) and increases fracture roughness by 27% (from 14.4 to 18.3 JRC). Furthermore, rising stress advances the first rockburst by 55.8 s, collapses the inter-burst interval from 534 s to 0.9 s, and expands the total failure zone by 483%. A dual-parameter AE early-warning framework, tracking the quiescence-to-outburst ring-count surge coupled with an abrupt b-value drop, provides 30–50 s of advance warning. Notably, this precursor window shortens from 50 s to 30 s under higher stress (0.5 → 4.0 MPa), while the outbreak ring count intensifies 9.7-fold and the AE energy amplifies 2.4-fold, establishing a clear stress-scalable warning signature. This physics-informed framework offers actionable insights for the reinforcement design and dynamic risk mitigation of deep tunnels under high-stress conditions.