Investigation of parallel joint density thresholds for granite tunnel failure based on physical model experiments and multiscale monitoring techniques

Joint density significantly influences the failure modes of hard rock caverns, yet the underlying mechanisms remain poorly understood. This study systematically investigates the impact of parallel joint density on the failure behavior of hard rock tunnels by integrating physical model tests with acoustic emission (AE), digital image correlation (DIC), and discrete element method (DEM) techniques. The results reveal a threshold effect of joint quantity on mechanical behavior and failure modes, identifying a single joint as the critical control point. Specimens containing only one joint exhibited a 45% higher peak strength and a 33% greater elastic modulus compared to those with multiple parallel joints. These specimens also produced more stable AE signals, marked by a single peak surge at failure. Their failure pattern followed a composite mode governed by stress concentration and discontinuity effects, manifesting as stress concentrations at joint tips or tunnel boundaries, along with large structural blocks and localized stress-induced fragmentation. In contrast, specimens with multiple parallel joints (i.e., ≥ 2 joints) showed diminishing mechanical parameter variations with increasing joint density, eventually stabilizing. These specimens generated unstable AE signals caused by structural damage, displaying two distinct AE surges: one at 60-80% of peak strength and another at failure. Their failure mode was primarily joint-controlled, featuring macro-block structural failure as the dominant pattern and extensive stress-weakened zones due to parallel joint network effects. These findings offer theoretical guidance for stability evaluation and support design in tunnels within multi-parallel jointed rock masses.