## 4 Analysis of Rock Fracture Patterns  
### Crack Fracture Morphology  
The macroscopic fracture of rock is closely related to its internal micro-defects and microstructure. By conducting a microanalysis of the fracture surfaces, the relationship between the rock fracture, morphology, and microstructure can be studied, thus explaining the formation of defects and the composition of the microstructure. This analysis of the microscopic mechanisms of rock fracture ultimately builds a bridge between the understanding of microscopic damage mechanisms and macroscopic fracture analysis, carrying  
Figure 12: Curves of \(\Delta\alpha\) variation with in situ stress at different positions. profound theoretical significance and practical value [19]. Scanning electron microscopy (SEM) is an essential method and tool for investigating the morphology of rock fracture surfaces at the micro-scale. Under a 3 MPa in situ stress condition, the fracture surfaces of the sandstone specimens post-blasting were not clear, making the SEM analysis of the fracture morphology unsuitable. Therefore, the fracture morphology analysis was only conducted on sandstone specimens blasted under in situ stress conditions of 0 MPa, 0.5 MPa, 1 MPa, and 2 MPa. Specifically, the SEM samples in the shape of 10 mm square by 6 mm thick rectangular blocks were cut from positions approximately 20 mm from the borehole center in the stemming, charge, and bottom segments of each sandstone specimen group. The selected SEM samples were cleaned, gold-coated, and prepared for electron microscopy observation. However, due to the absence of clear fracture surfaces at the corresponding positions in the bottom segments of the sandstone specimens blasted under 1 MPa and 2 MPa in situ stress conditions, no sampling analysis was conducted for these conditions. By performing electron microscope scanning on the selected SEM samples, the SEM images of the fracture morphologies were obtained, and the representative images were analyzed as follows. The fracture morphology of different segments of sandstone specimens after blasting under 0 MPa in situ stress is shown in Figures 13-15. The fracture morphology in the stemming segment exhibited stepped patterns, mist zone patterns, herringbone patterns, and river patterns. In addition to having similar morphologies as the stemming segment, the charge segment also displayed parallel slip patterns, serpentine slip patterns, and dimple patterns. The fracture morphology of the bottom segment was essentially the same as that of the charge segment, but coupled fractures with both intergranular and transgranular cracking were observed in the bottom segment. The fracture morphology of different segments of the sandstone specimens after blasting under 0.5 MPa in situ stress is shown in Figures 16-18. The fracture surface in the stemming segment showed stepped patterns, mist zone patterns, dimple patterns, and  
Figure 14: Fracture morphology of the charge segment of the specimen after blasting under 0 MPa in situ stress conditions. (**a**) stepped pattern and parallel slip pattern; (**b**) serpentine slip pattern and dimple pattern. Figure 13: Fracture morphology of the stemming segment of the specimen after blasting under 0 MPa in situ stress conditions. (**a**) stepped pattern and mist zone pattern; (**b**) herringbone pattern; (**c**) river pattern. slip patterns. In the charge segment, in addition to the aforementioned morphologies, partial coupled fractures were observed. Due to the presence of developed layered planes within the sandstone, which are relatively weak between layers, and the application of in situ stress, an imbalance between the vertical and horizontal stresses occurred, causing interlayer tearing during the crack propagation, resulting in layered tearing. The fracture morphology types in the bottom segment were not significantly different from those in the charge segment, but intergranular fractures were observed.