## 3 Experimental Results
### Progressive Breakage Process  
In the description of the dynamic breakage process of rock, the characteristics of crack initiation, propagation, and formation are essential [34]. The dynamic breakage processes of the AOI after the moment of detonation are summarized in Figures 10-13. Unfortunately,  
Figure 8: Surface strain field of DT4. Figure 9: Surface strain field of DT7. the experiment of DT6 missed its high-speed images due to failure of triggering the cameras. In the images of breakage processes, some arrows are set to indicate the location of the cracks. It should be noted that the rupture of the AOI was completed before 915 \(\upmu\)s. The photographs after 915 \(\upmu\)s are mainly the motion of fragments and were quickly obscured due to the escaped explosive gas, so these are not presented. A compass is set at the top-left of the 0 \(\upmu\)s plot to describe the direction of the crack extension. The borehole is indicated by dashed lines in frame 0 \(\upmu\)s, the explosive charge is indicated by the red solid line, and stemming is indicated by the orange rectangle. Figure 10 shows the AOI breakage process of DT1. A main horizontal crack appeared in the middle of the AOI at 286 \(\upmu\)s. At 543 \(\upmu\)s, the main horizontal crack expanded to the southeast with two branching cracks, horizontal and vertical, at the crack tip. At the same time, a horizontal crack appeared at the top edge of the AOI, and a vertical crack appeared on the right edge of the AOI. Then, a vertical crack located at the left of the AOI penetrated the main horizontal crack and the top horizontal crack. At 915 \(\upmu\)s, the right vertical crack appearing at 543 \(\upmu\)s opened up and intersected with the horizontal branching crack of the main horizontal crack. In the breakage process of DT2 and DT3, shown in Figure 11, horizontal cracks first appeared in the middle of the AOI. Then, many vertical cracks appeared in the AOI, but these vertical cracks did not affect the expansion of horizontal cracks. As time increased, the horizontal cracks crossed through the AOI with numerous vertical cracks on both sides. This indicated that horizontal cracks appeared faster than vertical cracks in the two experiments and dominated the fragmentation of the bench models. Figure 11: The dynamic breakage processes of the AOI: (**a**) the AOI of DT2; (**b**) the AOI of DT3. Figure 10: The dynamic breakage process of DT1. Figure 12 shows the breakage process of the bench slope in DT4, DT5, and DT7. These three experiments had similar breakage patterns. Specifically, vertical cracks first appeared at the minimum burden of the first borehole and extended faster than horizontal cracks. The horizontal and vertical cracks intersected at 286 \(\upmu\)s. Then, the vertical cracks continued to expand, powered by explosive columns, and prevented the expansion of the horizontal cracks at 543 \(\upmu\)s. Simultaneously, new cracks appeared on the right of the AOI. Finally, a crack network formed at the bench slope, and the specimen was fragmented. The results indicate that as the delay time increased, no horizontal cracks passing through the AOI as with DT2 and DT3 appeared in the middle region. Horizontal cracks appeared on the left of the AOI of DT8, shown as the 286 \(\upmu\)s in Figure 13. Simultaneously, vertical cracks appeared on the right of the AOI, which is the minimum burden of the second blast hole. At 543 \(\upmu\)s, the horizontal crack on the left developed gradually in the northeast direction. At the same time, the vertical crack on the right of the AOI branched at its crack tips, and one of the branching cracks intersected at 286 \(\upmu\)s. The horizontal cracks were also observed on the right of the AOI. Figure 12: The dynamic breakage processes of the AOI: (**a**) the AOI of DT4; (**b**) the AOI of DT5; (c) the AOI of DT7. Figure 13: The dynamic breakage process of DT8. with a new horizontal crack on the bottom of the AOI. At 915 us, the opening width of the cracks increased with the bench slope bulging.