## 5 Failure Process of Fissured Sandstone under Uniaxial Compression
### Failure Process  
The failure process can be categorized into three stages: crack coalescence, initial cracking at the outer fissure tip, and final failure. High-speed photography is employed to observe the failure process in three states using a 45\({}^{\circ}\) inclined sample as an example, aiming to comprehend the compression failure process of fissured sandstone. Due to the time span between crack initiation and final failure exceeding the recording capacity of the high-speed camera, the analysis of the failure process is conducted after some time following crack coalescence.  
(1) Dried state  
Figure 12 illustrates the process of compression failure in dried fissured sandstone. Stage I: a rock bridge between fissures cracks and coalesces. Stage II: (i) the outer tips of fissures 1 and 3 crack, resulting in a wing crack (first crack); (ii) with an increase in axial load, a shear crack (second crack) and anti-wing crack appear in the outer tip of fissure 1. Stage III: The shear crack in the outer tip of fissure 1 propagates to the top-right direction of the specimen, causing exfoliation damage along with an anti-wing crack. The wing crack in the outer tip of fissure 3 propagates to the bottom of the specimen, causing a complete separation into two parts along the penetrating crack. Subsequently, fissure 1 undergoes nearly complete closure.  
(2) Saturated state  
Figure 13 illustrates the compression failure process of fissured sandstone in the saturated state. Stage I: a wing crack appears at the inner tip of fissure 1 and propagates towards the upper tip of fissure 2; simultaneously, a wing crack initiates at the bottom tip of fissure 2 and propagates to the upper tip of fissure 3; additionally, a wing crack appears at the inner tip of fissure 3 and propagates in the direction of fissure 2. Stage II: (i) Exfoliation damage occurs as cracks initiate at the outer tips of both fissures 1 and 3, and a tensile crack propagates in the direction of fissure 2; the rock bridge between fissures 2 and 3 is fractured. (ii) With an increase in axial load, a shear crack (second crack) appears at the outer tip of fissure 3, resulting in fracture within the rock bridge connecting fissures 1 and 2. Stage III: the exfoliated damage from fissure 1 extends towards the top-right direction on specimen; meanwhile the shear crack in the outer tip of fissure 3 propagates to the bottom of the specimen, ultimately causing a division into two parts with slip occurring along the penetrating cracks. As a result, almost complete closure is observed for both fissure 1 and fissure 3.  
(3) Frozen-saturated state  
Figure 14 illustrates the compression failure process of fissured sandstone in the frozen-saturated state. Stage I: the rock bridge between fissures undergoes cracking and coalescence. Stage II: (i) the outer tip of fissure 1 cracks and a shear crack appears; (ii) with an increase in axial load, the shear crack in the outer tip of fissure 1 propagates upward  
Figure 12: Failure process in the dried state. The numbers (1–3) represent fissure numbers, as illustrated in Figure 1.  
and a wing crack appears in the outer tip of Fissure 3 and propagates to the bottom of the specimen; (iii) a wing crack appears in the outer tip of fissure 3 and propagates in the direction of fissure 2. Stage III: the widths of the shear crack in the outer tip of fissure 1 and the tensile crack in the outer tip of fissure 3 increase; consequently, the entire specimen is divided into two parts that slip along a penetrating crack; fissures 1, 2, and 3 almost close.