## 3 Test Results
### Spalling Failure Process
From 1381.68 s to 1482.48 s, \(\alpha_{\mathrm{z}}\) increased from 59.14 MPa to 62.00 MPa, causing more fine particle spalling in the lower-right shoulder. Two intersecting macroscopic axial cracks formed: one in the lower-right-shoulder corner and one at the right sidewall (Figure 11d). During the period from 1482.48 s to 1501.20 s, \(\alpha_{\mathrm{z}}\) = 62.00 MPa, allowing for internal stress adjustment around the tunnel. At 1486.44 s, a rock slab from the right sidewall toppled. At 1501.20 s, the rock behind the upper-left-shoulder corner showed buckling and deformation (Figure 11e). Between 1501.20 s and 1563.12 s, \(\alpha_{\mathrm{z}}\) was first loaded from 62.00 MPa to 64.00 MPa, causing buckling in the upper-left-shoulder corner along the negative X-axis, with a macroscopic axial crack forming at 1528.92 s. At 1561.32 s, \(\alpha_{\mathrm{z}}\) = 64.00 MPa, and another rock slab toppled from behind the lower-right-shoulder corner, and a small section of rock from the upper-left-shoulder spalled in the form of fine particle ejection (Figure 11f). At 1562.40 s, violent failure occurred when the rock slab ejected from the right sidewall, resulting in significant damage. Notably, the spalling on the right sidewall occurred in two layers, with the first rock slab being relatively small (Figure 11g). At 1563.12 s, the rock slab toppled, and spalling penetrated axially, prompting the cessation of loading (Figure 11h). Figure 10: Failure process of specimen “S-0-0”. Figures 12 and 13 show the failure processes of specimens \"S-0-25\" and \"S-0-100\", respectively. By comparing the failure processes of sandstone sidewalls under different fissure-penetration conditions, it was found that the failure progression was generally similar across varying conditions. The rectangular-hole sidewalls experienced a calm phase, followed by fine rock particle spalling, the generation and propagation of cracks, and ultimately sidewall spalling failure, during which an obvious V-shaped notch formed. However, with increasing fissure penetration, some differences were observed. As fissure penetration increased, the initial failure location moved from the left to the right sidewall. Additionally, the duration of each phase and the extent of final sidewall failure varied with the penetration degree. The time spent by sandstone specimens with different fissure penetration degrees in each stage is listed in Table 3.  
Figure 11: Failure process of specimen “S-0-50”. Figure 12: Failure process of specimen “S-0-25”. It is evident that overall failure time decreased as fissure penetration increased, and the time spent in the calm stage initially increased and then decreased with increasing fissure penetration. This trend was also reflected in the time points of failure and initial failure stress, as the loading path was consistent in this experiment. During the fine particle spalling stage, the time spent in this stage first decreased and then increased as fissure penetration increased. Possible reasons include the following: A shorter calm period means less time for elastic strain energy to accumulate, so when the failure enters stage 2, energy is transferred in the surrounding rock due to fine particle spalling, preventing sufficient energy accumulation to cause new damage. This cycle repeats, causing specimens with shorter calm periods to spend more time in stage 2. Prefabricated fissures weaken the sandstone specimens. As fracture penetration increases, the overall failure time decreases, shortening the time in stage 2. Under identical stress conditions, rock strength is higher at low fissure penetration, the duration of sidewall damage is longer, and the overall degree of damage is lower. With fewer paths for strain energy transfer in the surrounding rock, initial sidewall failure occurs earlier, accumulating more strain energy. This results in more intense slab cracking, manifesting as rockburst phenomena.