## 3 Fracture Characteristics
### Fracture Characteristics after Freeze-Thaw and Triaxial Compression
Figure 4: Fracture failure diagram of fissured sandstone with various dip angles under different freeze–thaw temperatures. The failure diagram and failure sketch of quasi-sandstone samples with different angles of fissure after thawing at \(T_{f}=-20\)\({}^{\circ}\)C reveal that the intact specimen and single-fissured specimens under triaxial compression all underwent tensile-shear mixed failure. There are three wing cracks (1), one branch crack (4), and one oblique crack (5) along the shear crack in the middle of the intact sample. When \(\theta=0^{\circ}\), there are three wing cracks (1) and one secondary inclined crack (2) at both ends of the prefabricated fissure. With the continuous decrease in freezing temperature, the local damage effect of frost-heaving force at the tip of the prefissure increases, but the horizontal coplanar frost-heaving crack has little effect on the crack coalesce-fracture mode of quasi-sandstone with a \(0^{\circ}\) angle prefissure. When \(\theta=45^{\circ}\), the main crack leading to final failure is the same as that at \(T_{f}\) = \(-10\)\({}^{\circ}\)C because there is only coplanar propagation cracking, but there are two cracks: inclined (5) and wing (1). This is because, with the decrease in freezing temperature, the frost-heaving force generates a stress concentration at the tip of the prefissure to form new cracks, thereby weakening the mechanical properties of fissured quasi-sandstone. When \(\theta=75^{\circ}\), freezing-thawing does not produce macroscopic cracks. The secondary coplanar cracks (3) at both ends of the prefissure extend to a certain extent, deflect toward the orientation of the maximum main stress, and propagate to the end face, leading to the quasi-sandstone through fracture. There is also a wing crack (1) near the upper end face. When \(T_{f}=-30\)\({}^{\circ}\)C, under triaxial compression, the intact sample experienced a tensile-shear mixed failure, and the single-fissured specimen had a shear failure. In addition to a vertical wing crack (1), a secondary coplanar crack (3) with oblique section penetration also formed in the intact quasi-sandstone after thawing because of the large damage of the internal particles of the sample caused by the frost-heaving force during the freezing process at \(T_{f}=-30\)\({}^{\circ}\)C. Then, the damage particles close to the transverse distribution under the action of deviatoric stress penetrate and form a fracture surface. The quasi-sandstone sample with a fissure dip angle of \(0^{\circ}\) after thawing developed a fracture cross section along the length direction of the prefissure under triaxial compression. This is because of the long secondary coplanar propagation cracks at the tip of the prefissure under the action of frost-heaving force. These cracks further develop and expand under the action of deviatoric stress. Finally, the main fracture surface is formed, leading to the instability and failure of the sample. After thawing, the failure mode of quasi-sandstone with a fissure dip angle of \(45^{\circ}\) is still a typical shear-slip failure, and the upper crack is the frost-heaving crack further expanding along the quasi-plane of the prefissure. Compared with \(T_{f}=-20\)\({}^{\circ}\)C, the particles around the secondary coplanar cracks (3) are more broken, and there is no inclined crack (5) in the middle of the pre-fissure. Compared with the quasi-sandstone sample with a fissured dip angle of \(75^{\circ}\) and \(T_{f}=-20\)\({}^{\circ}\)C, in addition to the two secondary coplanar cracks (3) forming the main sliding surface along the pre-fissure, there are also one secondary inclined crack (2), one inclined crack (5) and two branch cracks (4) at the end of the frost-heaving cracks. This shows that, with the decrease in the frozen temperature, the mechanical properties of the quasi-sandstone sample with a single fissure after thawing are weakened, and the block after loading is more broken.