# Dynamic Response and Rock Damage of Different Shapes of Cavities under Blasting Loads
## 3 Experimental Results  
### Specim Fracture and Break  
Due to the frame rate of the high-speed camera being set at 20,000 Hz, the interval between each image is 50 us. Key time-point images were selected for an overall analysis of the damage characteristics of the samples containing cavities under blasting loads. The damage and destruction process is shown in Figure 3. Generally speaking, the process of damage and failure of the rock mass under blasting loads is as follows: Firstly, the electronic detonator initiates the explosive charge, causing the PVC pipe in the blasthole to be propelled out by the blast wave, but no cracks were observed on the side of the sample. Subsequently, a vertical crack appeared at the position closest to the blasthole, and the crack extended further until it pierced through the entire sample, with a branching crack also forming at the middle of the side. As the damage progressed, the branching crack pierced through the sample, with the entire crack forming an \"X\" shape. At this point, the explosive gas began to be ejected, and the blocked sand and earth were continuously thrown out. New cracks continued to form at the top of the side and gradually expanded towards the left free surface. An additional branching crack also formed in the middle of the \"X\" crack, expanding towards the right. Finally, as the action of the explosive gas intensified further, the width of the cracks kept increasing, and no new cracks were produced. Figure 4 shows the crack propagation process of the control group (i.e., with no structural components) under blasting loads. From the figure, it can be observed that with the significant displacement of the PVC pipe, obvious deformation was also seen on the upper surface of the sample. Subsequently, a noticeable crack propagation was observed, and the propagation speed was relatively fast. After the basic formation of cracks, the width of the cracks continued to increase. Throughout the explosion process, the emission of explosive gases from the blast lasted for a considerable period. When no new cracks were produced, the process of width increase began to see the efflux of explosive gases and debris. This indicates that the action time of the explosive gas was prolonged, and the energy of the explosive was fully utilized, which is conducive to the fragmentation of the sample. The results of the aforementioned research indicate that under the intense shock wave generated by the explosion of the explosive in the rock mass, the shock and compression of the surrounding rock mass cause local pulverization. After passing through the crushed zone, the compressive stress wave continues to propagate outward, but its intensity has decreased to the point where it can no longer directly cause rock fracturing. When the  
Figure 4: Rock fracture process of intact control specimen and clay-containing specimen under blast loading. Figure 3: Fracture process of different shapes of cavities under blasting loads. compressive stress wave reaches the free surface, it reflects from the free surface as a tensile stress wave. Since the tensile strength of rock is significantly lower than its compressive strength, if the strength of the reflected tensile waves exceeds the dynamic tensile strength of the rock, the rock will begin to produce layer-by-layer tensile fracture damage from the free surface [24]. This mode of fracture is also known as \"falling in pieces\". As the reflected wave propagates towards the explosion source, the falling in pieces continues until the rock within the explosion range is completely pulled apart. High-speed camera images clearly show that the free surface cracks are primarily generated in the area closest to the blasthole, as the stress waves arrive there first, producing a tensile effect and concentrating stress, which in turn leads to crack formation. Subsequently, under the action of the explosive gases, the cracks continue to propagate, and their widths keep increasing.