Investigation of directional fracturing mechanism induced by instantaneous expansion in tunnel excavation

The operational risks and rock damage associated with explosive blasting in tunnel excavation necessitate the development of non-explosive alternatives. This study proposes a novel Instantaneous Expansion (IE) technology that integrates directional energy-focusing tubes and eco-friendly fracturing agents to achieve precise rock breaking. Through field experiments and RHT model-based numerical simulations, the performance of Conventional Blasting (CB), Shaped Charge Blasting (SCB), and IE in full-face excavation is systematically compared. Results demonstrate that IE generates directional tensile fractures along predefined paths, reducing over-excavation by 65% (shaping rate: 7.63%) and minimizing the excavation-induced damage zone (average depth: 1.7 m) compared to CB and SCB. In-situ stress analysis reveals that confining pressure suppresses radial crack propagation while guiding fracture orientation, as evidenced by a 10.3% reduction in fractal dimension under high lateral stress (k = 2). The proposed IE mechanism leverages gas-driven tensile stress concentration, aligning with the "strong-in-compression, weak-in-tension" behavior of rocks. These findings provide critical insights into low-damage excavation for deep energy/resource projects, such as underground gas storage and geothermal reservoirs, while highlighting the need for optimized charge configurations in anisotropic stress fields.