## 3 Test Results
### Spalling Failure Process  
To facilitate the description of the internal failure phenomena within the rectangular tunnel, each part of the tunnel was named, as shown in Figure 9. Using the non-fissure sandstone specimen \"S-0-0\" and the sandstone specimen \"S-0-50\" as examples, the failure processes of the rectangular tunnels were analyzed. The complete failure process of specimen \"S-0-0\" is illustrated in Figure 9.  
From 0 to 890.64 s, the specimen remained in a calm period with no significant damage to the sidewalls. At 890.64 s, \(\sigma_{\mathrm{z}}\) = 47.50 MPa and fine particle spalling occurred in the lower-left-shoulder corner, as seen from the camera (Figure 10a). Between 890.64 s and 1563.12 s, \(\sigma_{\mathrm{z}}\) increased to 64.00 MPa, causing the lower-left-shoulder corner to continue spalling along the positive X-axis. Buckling deformation occurred in the upper-left shoulder, accompanied by additional fine particle spalling (Figure 10b). After 94.32 s, \(\sigma_{\mathrm{z}}\) rose further to 66.00 MPa, leading to buckling and the failure of the upper-left shoulder along the negative X-axis, forming a macroscopic axial crack (Figure 10c). From 1657.44 s to 1724.40 s, the vertical stress increased to 68.00 MPa. At 1669.32 s, the first macroscopic longitudinal crack formed from top to bottom, at one-fifth of the distance from the camera. At 1712.52 s, \(\sigma_{\mathrm{z}}\) = 67.71 MPa and a second longitudinal crack formed at one-third of the distance from the camera, accompanied by substantial spalling (Figure 10d). From 1724.40 s to 1742.40 s, three layers of slab formed on the left sidewall. The first slab toppled at 1727.28 s, followed by the second slab at 1735.04 s. The third slab spalled at 1742.40 s (Figure 10e). From 1742.40 s to 1801.08 s, as \(\sigma_{\mathrm{z}}\) continued to rise, the left arch rock near the camera experienced buckling and deformation, with additional slabs toppling. At 1801.08 s, the right sidewall began to fail as \(\sigma_{\mathrm{z}}\) reached 70.00 MPa. Buckling occurred in the lower-right-shoulder conner (Figure 10f). The failure process in the lower-right shoulder mirrored that of the upper-left shoulder, with axial cracks forming along the positive X-axis, accompanied by fine particle spalling. A bottom-up longitudinal crack appeared at four-fifths of the way, and spalling between the crack and the loading block occurred at 1840.32 s (Figure 10g). From 1840.32 s to 1911.96 s, the axial crack in the lower-right shoulder continued to propagate along the negative X-axis, while longitudinal cracks formed bottom-up. As \(\sigma_{\mathrm{z}}\) increased to 72.00 MPa, further spalling occurred between the cracks. At 1902.24 s, numerous cracks  
Figure 9: The names of each part of the rectangular tunnel. appeared on the right sidewall, and spalling completely penetrated along the sidewall (Figure 10h). At 1912.68 s, the specimen experienced violent failure, with a large number of particles ejected inside the rectangular hole, forming a \"white mist\" as the sandstone specimen completely failed. The complete failure process of specimen \"S-0-50\" is illustrated in Figure 11. From 0 to 1188.72 s, the specimen remained in a calm state with no noticeable damage to sidewalls. At 1188.72 s, \(\alpha_{\mathrm{z}}\) = 54.00 MPa, causing buckling and the deformation of rock particles at the center of the lower-right-shoulder corner (Figure 11a). It is important to note that this buckling occurred at the end of the unpenetrated fissure on the right side of the rectangular hole. Between 1188.72 s and 1244.16 s, \(\alpha_{\mathrm{z}}\) increased to 56.00 MPa, and the buckling in the lower-right shoulder extended along the negative X-axis (Figure 11b). From 1244.16 s to 1381.68 s, the buckling deformation in the lower-right shoulder continued to expand in the negative X-direction. At 1381.68 s, with \(\alpha_{\mathrm{z}}\) loaded to 59.14 MPa, fine particle spalling appeared at the center of the lower-right-shoulder corner (Figure 11c).