Insight into cracking mechanisms between two parallel flaws: A systematic clarification of crack coalescence types

Abstract

The coalescence behavior of flaws critically governs rock mass stability. Despite established coalescence classification systems, ambiguities in the mechanisms of morphologically similar crack trajectories and limitations in universality and accuracy persist. To address these issues, the present study employed the bonded particle model to generate a comprehensive range of flaw geometries with fine incremental steps and simulate the coalescence process between parallel flaws under uniaxial compression. By tracking the dynamic evolution of force chains and displacement fields, the physical origin of the long-unexplained compression-induced tensile (CIT) crack is identified. Findings reveal that CIT cracks are driven by the accumulation of nested tensile contact forces within dominant compressive forces; the confinement effect within the rock bridge leads to delayed but abrupt energy release, resulting in sudden internal failure. Furthermore, the present study deciphers the divergent mechanical mechanisms underlying various coalescence patterns in overlapping and stepping geometries, resolving controversies where identical crack morphologies arise from distinct stress states. Additionally, results demonstrate that coalescence patterns are governed not only by macroscopic flaw geometry, but also by stress heterogeneity induced by random particle distribution and dynamic stress redistribution during crack propagation. These insights facilitate a refinement of Wong’s classification system that prioritizes mechanical criteria over geometric morphology. Ultimately, this mechanism-driven framework eliminates previous observational ambiguities, establishing a robust foundation for predicting the failure behavior and stability of jointed rock masses.