Size effects of the tensile mechanical behavior of basalt containing hidden joints investigated by DFN–FDEM modeling

Basalt, which is a geological medium used for engineering construction in Southwest China, contains defect structures at various scales. In particular, the widespread presence of mesoscale hidden joints significantly affects the mechanical properties of basalt and the stability of engineering structures. However, research in this specific subject has been limited. Hence, this study focused on basalt found in the Baihetan Hydropower Station and systematically explored the size effect of the tensile mechanical behavior of basalt with hidden joints. The investigation was based on a synthetic rock mass (SRM) model that combines the micro-discrete fracture network (μDFN) method and finite-discrete element method (FDEM), providing in-depth insights into the multiscale fracture mechanism of basalt. The results showed that: (1) the μDFN-FDEM model generated based on the statistical geometric parameters of hidden joints could accurately consider the real meso-structural characteristics of basalt, successfully replicating the mechanical behavior observed in laboratory tests and engineering sites. (2) The representative elementary volume (REV) size of basalt rock blocks containing hidden joints was approximately 0.5 m. With an increase in the sample size, the stress-strain curve characteristics under Brazilian disc splitting transitioned from a single-peak shape to a zigzag shape and then to a multi-peak shape, and the failure modes also shifted from single-center splitting failure to local structure-controlled failure and ultimately to a multicenter splitting failure. (3) The failure mechanism of basalt containing hidden joints evolved with increasing sample size, progressing from high stress-induced tensile failure to local structure-controlled failure, and finally to tensile failure resulting from stress-structure coupling. These findings can help further enhance and refine fundamental theories and technical methods for multiscale analyses in geotechnical engineering, providing a robust scientific and technological support for the safe construction and operation of deep underground engineering projects.