Shear band propagation mechanisms and precursory signatures in closely-spaced tunnels under compressive-shear loading: Insights from physical modelling and DEM simulation

Small-clearance tunneling in high-stress environments faces critical stability challenges due to compressive-shear instability, often inducing rockbursts and structural failures. This study systematically investigates the mechanical behavior and precursor characteristics of closely-spaced twin tunnels (CSTT) under compressive-shear loading through physical modeling and discrete element method (DEM) simulations. A customized servo-controlled shear testing system was employed to analyze the deformation and failure processes of CSTT with a clear spacing of 0.5D (D = tunnel diameter), compared to standard-spacing twin tunnels (SSTT). Key findings demonstrate CSTT exhibits a four-step failure sequence: shear compression → micro-crack propagation → fracture coalescence → frictional sliding. Stress concentration in the rock bridge zone accelerates tensile-shear hybrid crack propagation, causing 23 %–41 % faster rock bridge penetration than SSTT. Acoustic emission (AE) monitoring reveals CSTT generates high-energy bursts during failure stages, with a 37 % broader b-value fluctuation range under high normal stress, enhancing precursor detection sensitivity. DEM simulations elucidate micromechanisms: compressive stress concentrates in the rock bridge zone, triggering earlier bond breakage and accelerated shear band propagation. The study establishes a novel early-warning framework based on b-value thresholds and energy dissipation rates, offering actionable insights for stability control in complex stress environments. The findings advance shear failure prediction in closely-spaced tunnels and provide actionable guidelines for stability control in deep underground engineering.