## 1 Introduction  
Splitting failure usually occurs in tunnels and other underground structures, and many previous experimental, numerical, and theoretical results proved that the occurrence, distribution, and scale of the failure regions are mainly controlled by in situ stresses and the section shape [1; 2; 3; 4; 5]. Furthermore, researchers have paid attention to the splitting failure procedure and rock-burst characteristic of tunnel-surrounding rock under true triaxial compression conditions, and the true triaxial compression test provided powerful support to the related academic and practical achievements. [PERSON] et al. [6; 7] carried out experimental tests on cubic rock samples containing circle- and D-shaped tunnels, and simulated the progressive splitting failures under biaxial or triaxial conditions. [PERSON] et al. [8] studied the failure characteristics of cube granite containing a circle hole through a true 3D triaxial compression test, and separated the failure process into four typical stages (calm period, rock debris ejection, splitting failure forming, and rock burst). [PERSON] et al. [9] monitored the development of visible secondary cracks, strain field, and AE signals in sandstone with a circle hole under biaxial compression conditions, and tensile cracks were initiated at the arched position and then two V-shaped failure faces were formed. [PERSON] et al. [10] focused on the splitting failure and rock burst shown in a red sandstone sample under dynamic true 3D compression conditions, which proved that the dynamic loading contributed to faster rock ejection, deeper V-shaped failure, and mixed tensile-shear failures. [PERSON] et al. [11] tried to find the transformation characteristics of secondary failures (from splitting failure to rock burst) by changing the length of the tunnel sidewall in the true 3D triaxial compression test. As the sidewall length decreased, the distribution area of the V-shaped splitting failure shrank, while the rock burst became more active. [PERSON] et al. [12] carried out a tunnel excavation (internal unloading) test under biaxial compression conditions, and the 'High initial stress & Internal unloading & Stress adjustment' simulation path is closer to the actual engineering conditions. Compared with the traditional test (prefabricated circular hole, high initial stress and stress adjustment), the internal unloading test contributed to a smaller initial failure strength and more severe splitting failure in the tunnel sidewall. In addition to the experimental method, the numerical analysis method is a good tool for simulating the mesoscopic failure behavior in rocks. Based on discrete element theory, the Particle Flaw Code (PFC) has been applied to simulate the fracture behavior of rocks, and the displacement vector field has been utilized to estimate an accurate mode of secondary cracks [13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Rock samples containing prefabricated holes and flaws were usually selected as objects in previous studies. [PERSON] et al. [23] studied the crack development characteristics in a rock-like sample containing twin cavities. The particle displacement trend was applied to analyze the failure mechanism at the mesoscale, and cluster analysis was used to identify the dominant secondary failures in the sample. [PERSON] et al. [24] conducted experimental and numerical studies on a cube rock sample with a central circular hole and parallel non-persistent flaws. The PFC2D simulation results indicated that the tensile cracks played the dominant role in the failure coalescence, and the slipping failure in the sidewalls was mainly formed by the shear cracks. Although the experimental and numerical methods for the true triaxial compression test have matured, a special condition is rarely mentioned in previous studies: the potential influence of neighboring caverns on the safety of tunnel-surrounding rock. Caverns commonly appear in the geological body when fracture structures are widely distributed and groundwater shows strong power in the local region.