**6. Numerical Simulation of Impact Damage on Heading Face**

A coal-rock dynamic disaster refers to a strong dynamic phenomenon in which the surrounding coal and rock masses in an underground mining space are rapidly destroyed and a large amount of energy is suddenly released [30]. The HJC constitutive model parameters for briquette samples shown in Table 8 were applied in the numerical simulation of coal-rock dynamic hazards caused by rock bursts or blasting impact disturbances. Taking Yongcheng Cheji Coal Mine as an example, the impact damage surface of the excavation face was established. A numerical simulation model was used to verify the applicability of the HJC constitutive model parameters for briquette.

#### *6.1. Model Establishment*

The coal-rock layer was too thick or the size was too large, which increased the difficulty of meshing and the solution time. Therefore, the model was simplified. In the impact damage model developed for the tunnel face, the length and height of the roof and floor rock layers were considered to be 20 and 4 m, respectively. The length and height of the coal seam were 20 and 3 m, respectively (Figure 17), and the roadway size was 8 m. In order to simplify calculations, the rock layers of the top and bottom plates adopt an elastoplastic model with a density of 2500 kg/m3, an elastic modulus of 18 Gpa, and a Poisson's ratio of 0.3 [31]. Coal seam was set based on the HJC constitutive model parameters for briquette shown in Table 8.

Considering the influence of ground stress on underground roadways, the maximum vertical ground stress of the roadway at 450 m depth for Yongcheng Cheji Coal Mine was measured to be 15.22 MPa. Therefore, during the simulation, a vertical static load was applied to the coal seam. The stress wave disturbance generated by excavation blasting was simplified into a semi-sinusoidal pulse, and based on the dynamic stress–strain curve obtained from the briquette SHPB experiment, 20 MPa was taken as the peak value (*pmax*) for a half-sinusoidal pulse.

**Figure 17.** Establishment of a two-dimensional coal and rock mass model including a roadway.

#### *6.2. Numerical Simulation of Impact Damage on Heading Face*

In underground blasting operations, the shockwave generated by blasting releases huge amounts of energy to the surrounding area, which may cause the deformation, cracking, and even destruction of roadways and other structures. Explosion energy can loosen or destroy coal rock masses in the process of shockwave release and propagation. The numerical simulation of the impact damage of the tunneling face predicts the power of the disaster caused by the head. Numerical simulation results are shown in Figure 18.

Figure 18 clearly shows the destruction process of the coal seam in the roadway. As can be seen, at t = 158.61 μs, the spreading stress wave contacted the coal seam and triggered its failure. When t = 227.31 μs, the coal seam was cracked by the action of the compressive stress wave, which continued to propagate deep into the coal seam. At t = 278.52 μs, the crack gradually expanded and the coal seam was notched. When t = 416.76 μs, the size of the gap generated in the coal seam was increased with by the propagation of stress wave. Finally, when t = 536.62 μs, a large cavity was formed deep in the coal seam as the gap was increased.

The stress time-history curve of the coal seam in a two-dimensional model is shown in Figure 19. As can be seen, first the coal seam was cracked and destroyed by the propagation of the stress wave. The curve first rose sharply to the yield point and then began to fall. Then, as the stress wave propagated deep into the coal seam, the curve began to rise again and the coal seam was destroyed.

(**a**) Numerical Simulation Results when time is 158.61 Ɋ

(**c**) Numerical Simulation Results when time is 227.31 Ɋ

(**e**) Numerical Simulation Results when time is 362.62 Ɋ

(**g**) Numerical Simulation Results when time is 464.57 Ɋ

(**b**) Numerical Simulation Results when time is 185.59 Ɋ

(**d**) Numerical Simulation Results when time is 278.52 Ɋ

(**f**) Numerical Simulation Results when time is 416.76 Ɋ

(**h**) Numerical Simulation Results when time is 536.62 Ɋ

**Figure 18.** Excavation surface impact damage process (*pmax* = 20 MPa).

**Figure 19.** Time-history curve of coal seam stress in a 2D model.

#### **7. Conclusions**

Through SHPB experiments of briquette combined with experimentally measured HJC constitutive parameters and ANSYS/LS-DYNA software results, the following conclusions can be drawn:


**Author Contributions:** B.X. conceived and designed the research; Z.Y. performed the experiment and wrote the manuscript. Y.D. undertook data curation; Z.Z. was responsible for software; X.Z. reviewed and edited the manuscript.

**Funding:** This research was supported by the National Natural Science Foundation of China (51404277, 51274206) and the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) (WS2018B08). This support is greatly acknowledged and appreciated.

**Acknowledgments:** Y.Y., J.J.Z., and S.Z.F. are acknowledged for their valuable technical support.

**Conflicts of Interest:** The authors declare no conflict of interest.
