## 3 Experimental Procedures
### Experimental Results  
#### 3.3.1 Crack Propagation Characteristics  
Figure 4 shows the evolution of a caustic spot in the medium containing a vertical joint under blast loading. Due to the high stress gradient at the blast wave front, series of ring-shaped fringes were generated from the borehole after the detonation of slit charge, as can be seen in Figure 4b. When the blast wave encountered the vertical joint at the time of 20 \(\upmu\)s, a significant pseudo-caustic spot was formed at the surface of the joint, whereas no obvious stress concentration was generated at the end of  
Figure 3: Schematic diagram of the experimental model: (**a**) specimen with a vertical joint; (**b**) specimen with an oblique joint.  
the joint. This indicated that the impinging of the dilatational wave is not conductive to the wing crack propagation. At the same time, the reflected wave was generated and propagated towards the borehole. Then, at the time of 40 \(\upmu\)s, the reflected wave began to interact with the blast-induced crack, lead to a significant shape distortion of the caustic spot, and further altered the crack propagation behavior. Afterwards, the caustic spot became smaller when the crack propagated near the vertical joint, and the blast-induced crack arrested when the crack encountered the vertical joint. Shortly, an obvious caustic spot began to increase at both ends of the vertical joint, and the wing cracks initiated when the caustic spot reached its critical value. In addition, the propagation direction of the wing cracks at both ends of the vertical joint are basically consistent with the propagation direction of the blast-induced crack.  
Figure 5 shows the evolution of a caustic spot in the medium containing an oblique joint with 45\({}^{\circ}\). It can be seen that the propagation behavior of wing crack at the end of oblique joint is quite different than that at the end of the vertical joint. Due to the existence of the oblique joint, the reflected wave propagated obliquely to the crack propagation direction and induced a significant stress concentration at the end of the oblique joint during the interaction between the blast wave and the reflected wave. At the time of 80 \(\upmu\)s, the wing crack at the closer end of the oblique joint began to be initiated and propagated toward the borehole due to the influence of the local stress field, which is generated from the tip of the blast-induced crack. Whereas, the win crack at the far end of the oblique joint initiated and continued to be propagated forward after the blast-induced crack connecting with the oblique joint. This implies that the initiation of a wing crack at the far end of the oblique joint is mainly attributed to the instantaneous energy release at the tip of main crack when it coalesces with the vertical joint, whereas the initiation of a wing crack at the closer end of the oblique joint is mainly owing to the interaction between the local stress around the main crack and the oblique joint.  
Compared with the crack propagation behavior in Figures 4 and 5, it can be seen that under the influence of the local stress field around the tip of main crack, the wing cracks at the ends of the oblique joint is more easily to initiate than it at the ends of the vertical joint, because strong stress concentration is formed at the end of the oblique joint. In addition, for the medium with a vertical joint, the size of the caustic spot at the tip of wing crack decreased significantly after being initiated, whereas it changed a little for the oblique joint. This indicates that more energy is consumed in the initiation of the wing crack at the vertical joint than in the oblique joint. Thus, the wing crack at the vertical joint arrested shortly after it initiated. Therefore, for the actual blast engineering, the oblique joint should be escaped in order to obtain a desired contour of the roadways. While the distance between boreholes should be shortened in the medium containing vertical joint.