## 2 Model Experiment Design and Parameters  
The blasting stress wave and blasting gas generated by explosive detonation are the main driving forces for rock fragmentation. Slit charge blasting mainly uses the slit tube to control the release direction of the blasting gas to achieve directional fracture. The essence of the slit charge is a strip-shaped charge. In the unlayered rock mass, the stress wave generated by the blasting of the strip-shaped charge will propagate in the form of a cylindrical wave. As shown in Figure 2, in the layered rock mass, when the angle between the detonation direction of the strip-shaped charge and the layer is 90\({}^{\circ}\), the propagation process of the blasting stress wave is not affected by the layer. When the angle between the detonation direction of the strip-shaped charge and the layer is not 90\({}^{\circ}\), the blasting stress wave will reflect and transmit at the layer, and the propagation process of the blasting stress wave will be extremely complicated. In this experimental design, the angle between the detonation direction of the slit charge and the layer is 90\({}^{\circ}\); that is,  
Figure 1: Structure and engineering application of the slit charge.  
the slit charge is perpendicular to the layer, which avoids the interference of the complex transmission and reflection behavior of the blasting stress wave at the layer on the analysis of the experimental results. Therefore, the pressure relief effect of the blasting gas at the layer will become the main factor affecting the blasting effect.  
The dynamic fracture characteristics of polymethyl methacrylate (PMMA) and rock exhibit remarkable similarities [19; 20]. In the model experiment, multi-layer PMMA plates are used as the layered rock mass specimen. A 5 mm circular diameter borehole is cut in the center of the specimen using a laser. The size of the PMMA plate is 300 mm \(\times\) 300 mm \(\times\) 4 mm, and fourteen PMMA plates of this size are stacked and fixed with clamps as a layered rock mass specimen. Lead azide is used as the model experimental explosive, and lead azide is put into a copper slit tube to make a slit charge. The length of the slit charge is 56 mm; the outer diameter is 5 mm, the inner diameter is 3 mm, and the slit width is 0.8 mm. In order to ensure the structural stability of the slit charge, the slit length is set to 48 mm, and 4 mm length is reserved on both sides of the slit tube without setting slits. The charge length is equivalent to the slit length; that is, the charge length is 48 mm, and the two ends of the slit tube are not charged.  
A high-voltage probe is used to detonate the slit charge, and the detonation point is at the end of the charge. Figure 3 shows the blast-induced crack distribution of the layered rock mass specimen, and Figure 4 shows the slit tube after blasting. After blasting, long directional cracks are formed along the slit direction, and the copper slit tube is not obviously damaged, which provides an effective restraint for the directional release of blasting energy. To facilitate further analysis, the PMMA plates at different positions in the layered rock mass specimen are numbered as shown in Figure 4. The second to thirteenth layers are charging sections, and the second layer is the detonation position. The first layer and the fourteenth layer are non-charging sections, corresponding to the positions of both ends of the slit tube, and there are no slits in such positions.  
Figure 3: Blast-induced crack distribution of the layered rock mass specimen: (**a**) front view; (**b**) side view.  
Figure 2: Detonation propagation and stress wave propagation process of strip-shaped charge in the layered rock mass: (**a**) \(t=t_{1}\); (**b**) \(t=t_{1}+\Delta t\).