## 3 Test Results and Analysis
### The Structural Dynamic Response Analysis
Comparing with the C30 reinforced concrete test section, the stress reduction effect factor at the arch spring of the high-performance reinforced concrete test section is \(-24.6\%\). The reason is that in the clamped arch stress system, the arch spring first produced a stress concentration. When the peak stress at the arch spring of the C30 reinforced concrete structure reaches 42.31 MPa, the structure produces a compression shear damage, while the high-performance reinforced concrete test section has a high structural stiffness, and the energy of the compression wave is transmitted more in a medium with high stiffness, which makes the stress concentration at the arch spring of the structure more remarkable, with the measured peak stress reaching 52.74 MPa. However, the high-performance concrete materials improve the brittleness of the structure and enhance the fracture toughness and impact resistance of the structure, so that only intermittent cracks form at the arch spring. It is found that damage of the arch spring is mainly caused by the phenomenon of stress concentration at the arch-wall junction caused by the compressive shear load. If the high-performance reinforced concrete test section is damaged, with the continuous propagation of microcracks at the arch spring, the steel fiber will be pulled out of the matrix. As the interfaces between the steel fiber and the matrix are numerous and irregular, the paths of microcrack initiation and propagation are more complex, and the action of bonding and friction between the two interfaces results in the intensifying of the internal energy consumption of the structure. Comparing with the C30 reinforced concrete test section, the peak stress values in the inner layer of the vault, the hance, the straight wall midpoint and the floor midpoint of the high-performance reinforced concrete test section decrease about 30.9%, 41.3%, 24.6% and 27.7%, respectively. Therefore, the high performance reinforced concrete test section improves the structural resistance by enhancing the structural strength and increasing the energy dissipation. Comparing with the C30 reinforced concrete test section, the peak stress values in the inner layer of the vault, the hance, the arch spring, the straight wall midpoint and the floor midpoint of the composite structural test section decrease individually about 61.8%, 73.6%, 46.7%, 55.9% and 38.7%, respectively, which indicates that the foam concrete soft backfill can significantly reduce the peak stress in the inner layer of the structure. The foam concrete, as a porous lightweight material, has a wave impedance much lower than that of the surrounding rock and the C30 reinforced concrete structures. When the stress  
Figure 12: The peak stress tendency curve of the measured points in test sections. waves propagate to the composite structures with mismatched wave impedance, the stress waves at adjacent interfaces form multiple reflections and transmissions, which dissipate certain energy in this process. Furthermore, when the stress wave propagates inside the foam concrete, the free surface formed by the honeycomb holes causes multiple reflections, refractions and superpositions of the stress waves, and the energy from the explosion shock waves further decays. In addition, the measured static compressive strength of the foam concrete is 3.25 MPa; when the explosion stress wave propagates to the energy dissipation layer of the foam concrete, the impact load exceeds the dynamic compressive strength of the foam concrete, and most of the explosion energy is absorbed by the plastic deformation of the foam concrete soft backfill medium, which results in a weakening of the impact load transmitted to the structure. The peak stress tendency curve of the measuring points in the composite structural test section is gentle, as shown in Figure 12. Under the action of a dynamic load, the foam concrete soft backfill layer can slow down the deformation of the surrounding rock reinforcement layer.