## 2 Underwater Rock-Plug Blasting Experiments
### Preparation of the Experimental Model and Experimental Set-Up  
The rock-plug model--a polyvinyl chloride cylinder with a diameter of 100 mm to be filled by the block particles--was prefabricated. The reserved space was neatly filled with cubic blocks of cement mortar with a side length of 10 mm. The block particles were prepared according to a (C30) cement-sand-water ratio of 2:1:0.65. The prepared rock-plug model and blocks are shown in Figure 2. Furthermore, standard cylindrical samples were prepared for uniaxial compression testing (Table 1).  
The most important characteristic of rock-plug blasting is that it aims to penetrate a rock plug and form a tunnel through blasting under conditions in which there is water on one side of the rock plug and no water on the other. This research aimed to investigate the motion of block particles in the rock plug subjected to the same charge with different heads of water and opening angles. To this end, the experimental set-up was designed with three parts (Figure 2): an upstream water storage tank measuring 250 mm \(\times\) 260 mm \(\times\) 460 mm, a rock-plug sample in the middle, and a downstream water tank for collecting broken blocks (hereinafter referred to as the downstream water tank) measuring 380 mm \(\times\) 240 mm \(\times\) 240 mm. The opening angle of the rock plug was denoted as \(\beta\) and the distance from the center of the upstream end face of the rock plug to the water  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mass (kg)** & **Density (kg/m\({}^{3}\))** & **Compressive Strength (MPa)** & **Young’s Modulus (GPa)** & **Poisson’s Ratio** & **Longitudinal Wave Velocity (m/s)** \\ \hline
0.4138 & 2107 & 32.33 & 21.2 & 0.25 & 2234 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mechanical parameters of concrete.  
Figure 2: Experimental device and samples.  
surface was the head of water, denoted h. The main part of the experimental set-up was supported by an angle-iron frame and the opening angle of the rock plug could be changed by adjusting the positions of some of the angle irons.  
Considering the safety of the experiment, black powder was used as the explosive. Figure 3 shows the cylindrical charge of 0.7 g, which had a length of 30 mm and diameter of 8 mm and was detonated by excitation with high-voltage pulses.  
The outer shell of the rock-plug sample was a polyvinyl chloride cylinder with a diameter of 100 mm. According to engineering practice, the length-diameter ratio in the design of rock plugs generally needs to be larger than or equal to 1, so the block-filled area in the rock-plug sample was 100 mm long. As shown in Figure 2, a length of 25 mm was reserved on two sides of the polyvinyl chloride cylinder to ensure that the rock-plug sample could be firmly connected to the upstream and downstream water tanks, so the total length of the rock-plug sample was 150 mm. When filling the sample with block particles, the prepared cylindrical charge of 0.7 g of black powder (Figure 3) was also embedded in the sample. Considering the water pressure on the upstream end face of the rock plug, the center of the cylindrical charge of 0.7 g was positioned 40 mm from the upstream end face and 50 mm from the downstream end face of the rock-plug sample. To avoid the sliding and ensuing collapse of the block particles filling the rock plug before blasting, plaster was smeared on the outermost blocks on two ends of the rock-plug sample, such that the blocks did not drop down after the solidification of the plaster even if the rock-plug sample was placed vertically.  
The experimental process, which was recorded using a GX-8 high-speed camera at a frame rate of 2000 FPS, a resolution of 832 \(\times\) 600 pixels, and a shutter speed of 249.6 \(\upmu\)s, is shown in Figure 4. The experimental cases for the underwater rock-plug blasting are listed in Table 2. A fabric background with orthogonal lines spaced at 100 mm intervals was pasted on the inner wall of the water tank.  
Figure 4: Test system.  
Figure 3: Preparation of explosives.