## 2 Underwater Rock-Plug Blasting Experiments
### Experimental Results and Analysis
#### 2.3.4 Discussion of Block Motion  
The block motion in the rock-plug blasting experiment can be divided into two stages: in the first stage, the blocks moved out of the openings on the two sides along the axis of the rock plug under the effects of the blast load. The block group flowing out from the downstream end face accumulated at the bottom of the downstream water tank after colliding with the sides of the tank, while that from the upstream end face finally fell to the bottom of the upstream water tank after its vertical velocity decreased to zero under the effects of the frictional resistance imposed by the water and gravity. In the second stage, block particles flowed to the downstream water tank as a result of the scouring action of the water. In the first stage, blocks driven by the blast load were separately thrown out of the upstream and downstream end faces and their initial kinetic energy was related to the charge and the charge position. The same charges and charge positions were used in the experiment, so the above assumption was proved by the similar times taken for the block particles to reach the 100 mm gradation under the four conditions.  
Figure 9: The effects of rock-plug blasting with different opening angles and heads of water.  
In the first stage, the four conditions mainly differed in terms of the motion process of the block particles flowing out from the upstream end face as a result of the driving action of the blast load. When the opening angle of the rock plug was 30\({}^{\circ}\), the time taken by the block particles flowing out from the upstream end face under the effects of the blast load to reach the 100 mm gradation was also prolonged with increasing heads of water. This indicated that changes in the applied head of water altered the hydrostatic pressure on the upstream end face, and the head was directly proportional to the hydrostatic pressure. Therefore, the head exerted a significant inhibitory effect on the motion of the block particles flowing out from the upstream end face. After adjusting the opening angle to 40\({}^{\circ}\), the angle of ejection of the block particles from the upstream end face was larger than with an opening angle of 30\({}^{\circ}\), so some of these particles returned to the rock plug during their descent before flowing to the downstream water tank with the water.  
In the second stage, the effect of scouring on the block particles remaining in the rock plug was the key to penetrating the rock plug. All the block particles remaining in the rock plug were treated as a whole (Figure 10). A blasting crater was formed on the downstream end face under the effects of the blast load, which was a free face, and the upstream end face was subjected to hydrostatic pressure Pw, its self-weight G, and friction force Fs. When the head of water increased, the water pressure on the upstream end face also increased; as the head increased to 305 mm, the water pressure on the upstream end face reached a level at which it overcame the frictional force, so the rock plug was obviously penetrated. More block particles were observed to flow to the downstream water tank under the effect of scouring with increasing heads of water, which further enhanced the penetration achieved during rock-plug blasting.