Numerical modeling of columnar jointed basalt with a dual irregular joint network and its unloading response under true triaxial conditions

The irregular columnar structure and internal hidden joints of Baihetan columnar jointed basalt (CJB) lead to its prominent anisotropy, and developing an appropriate model and accurately understanding its unloading responses are crucial for engineering safety. Based on the natural structure of Baihetan CJB, a dual irregular joint network numerical model, including both columnar joints and internal hidden joints, was proposed to investigate its anisotropic mechanical characteristics and damage failure mechanisms under true triaxial loading and unloading conditions. The results demonstrate that the dual irregular joint model with an irregularity coefficient of 0.47 effectively captures the cross-sectional geometry and internal structure features of the actual CJB. After a two-stage parameter calibration, the numerical and laboratory uniaxial test results are relatively consistent, and cracks exhibit four typical propagation modes. Under true triaxial loading and unloading conditions, both the deformation and strength behaviors of CJB show significant anisotropy. The unloading of minimum principal stress weakens the mechanical properties of CJB and increases its anisotropic level. The failure of the model with 15° inclination angle under unloading condition helps explain the column fragmentation observed at the side walls after excavation, which aligns with stress-structure controlled failure characteristics. The progressive damage of CJB involves three stages, and unloading the minimum principal stress or reducing the restriction of intermediate principal stress will accelerate the internal damage rate. The research results provide valuable guidance for the rational design and safe construction of related projects.