## 5 Conclusions  
By conducting the experimental and numerical tests, the mechanical property, acoustic characteristics, non-uniform displacement, and velocity trend fields, the principal stress redistribution and failure mechanisms of samples are discussed in this study. The main conclusions are as follows:  
(1) Based on the experimental results, the invisible cavern demonstrates a negative influence on the stability of the tunnel and leads to a 25.82% reduction in the peak z-axis load of the specimens. The AE results show that the relatively severe dominant failures mainly occur near the peak stress in all types of specimens, and the speed and intensity of the cavern-existing specimen is significantly greater than that of the cavern-free specimen.  
(2) In both experimental and numerical tests, the cavity-free tunnel shows a mirror-symmetric splitting failure on the left and right sidewalls. However, the secondary cracks appear earlier and show an asymmetrical distribution in the cavern-existing specimen, and the volumes of broken rock blocks in the left and right sidewalls are different. In addition, the cavern directly changes the failure process of the tunnel-surrounding rock (intermediate rock failure is earlier than splitting failure), and the corresponding mechanism of secondary failures.  
(3) The invisible cavern strongly controls the distribution of the principal stress axes. The maximum principal stress axes are consistently parallel to the tunnel free face in the cavern-free sample, and typical stress axes rotation occurs in both sidewalls of the tunnel. On the other hand, because the cavern releases the compressive stresses into the surrounding rock, the stress accumulation effect in the right sidewall is weakened; the intermediate and minimum principal stress axes only present various levels of rotation in the right sidewall.  
Figure 16: Influence of the true 3D cavern on the displacement, velocity, maximum principal stress, and crack distributions in the cavern-existing sample at the final state: (**a**) crack distributions in y-o-z plane; (**b**) velocity trend field in y-o-z plane; (**c**) maximum principal stress axes in y-o-z plane; (**d**) displacement trend field in y-o-z plane.  
(4) The displacement trend fields present a typical centripetal distribution in two kinds of samples at the initial state because of the true 3D compression condition. The increase in the z-axis loading leads to the outward direction of velocity trends in all states, and the uneven velocity field in the horizontal direction indirectly contributes to the splitting failure of the tunnel sidewalls. The true 3D prefabricated cavern causes uneven crack distribution in the y direction (tunnel axis), and the corresponding failure mechanism can only be figured out in the velocity trend field (because a short period of complex displacement fields may be covered by a long-term stable displacement field). It is thus necessary to apply both displacement and velocity trend fields for the analysis of failure mechanisms.  
(5) This work only focused on isotropic material in both the experimental and numerical methods. In the future work, anisotropic materials like granite, shale, and sandstone will be selected as test objects; in addition, the corresponding DEM simulations will be carried out based on various particle shapes and combinations (clusters) [35; 36]. Will secondary cracks cut through grains? How will splitting failure develop and distribute? These are urgent issues that need to be addressed in the next stage of research.  
Conceptualization and methodology, C.C.; software, X.S.; writing--review and editing, Y.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Natural Science Foundation of Fujian Province [grant numbers 2022J01567].  
Not applicable.  
The database obtained and/or analyzed during the current study are not publicly available, but are available from the corresponding author on reasonable request.  
The authors declare no conflict of interest.