## 4 Damage Stability Assessment of Nearby Underground Tunnel Structures Due to Hydrogen Gas Explosion
### Influence of Tunnel Depth on Damage  
Figure 17 shows the results of the damage analysis performed under three different tunnel depths (H = 8, 12, and 16.4 m) at the same horizontal separation distance (L) of 5 m. In terms of the minimum separation distance between the explosion source and the underground tunnel structure, Case 1 had the closest separation distance (5 m), followed by Cases 4, 5, 2, and 3 (see Table 6). The damage patterns in Cases 1 and 4 were noteworthy. In terms of the minimum separation distance, Case 1 is slightly closer to the explosion source with a difference of 1 m, but the two analysis results show totally different damage patterns, and it appears to be more dominant in the distribution pattern of the damage owing to the tensile deformation for Case 4. As mentioned above, this can be attributed to the close  
Figure 16: Geological boundaries on the underground tunnel near the explosion source. Figure 15: Damage assessment results of a concrete tunnel lining subjected to explosions with different separation distances. relationship between the damage distribution and dynamic characteristics of the stratum in the area where the underground tunnel structure is located. Figure 18 shows the geological boundary and explosion wave directions based on the tunnel location in the stratum model. The shape and distribution of the tensile damage zone in Case 4 clearly corresponded to the stratum boundary, and symmetric damage patterns for the propagation direction of the explosion stress waves propagating from the explosion source were also observed. Case 5 in Figure 17 shows the damage behavior when an underground tunnel structure exists in a single medium. Here, the shortest separation distance between the explosion source and tunnel structure is 6.5 m, which assumes an explosion near the field. It can be confirmed that the overall compression damage occurred in the direction in which the explosion stress wave propagated. A spherical tensile damage zone was observed at the point where the wavefront of the explosion stress wave and the underground tunnel structure met a tangent line, which is believed to be due to the spalling phenomenon covered in the general blasting theory. When evaluating the overall analysis results from a conservative standpoint, it was concluded that the safety of the tunnel structure can be ensured when a minimum separation distance of at least 10 m is maintained, assuming an underground hydrogen storage explosion scenario with specifications such as the analysis conditions. In addition, it was confirmed that when an underground structure is situated in multilayered ground, the structural and dynamic characteristics of the ground layers are very important factors in assessing the stability of the adjacent structure against a hydrogen gas explosion. The results of this analysis suggest that not only simple separation distances but also the dynamic properties of the ground materials comprising geological structures are crucial factors that should be fully considered when designing disaster responses for underground hydrogen storage. Therefore, by incorporating accurate ground properties into the simulation model through in situ ground investigation and dynamic property characterization, a more reliable and safer design of the storage facility can be achieved, minimizing damage in the event of a hydrogen gas explosion. Figure 17: Damage assessment results of a concrete tunnel lining subjected to explosions with different tunnel depths. Since this study, a specific scenario-based analysis and evaluation were conducted focusing on the target structure, without considering the application of additional techniques for ground vibration mitigation. Consequently, further exploration is needed, including an analysis of more general cases and a review of additional techniques for ground vibration reduction. For instance, research by [PERSON] et al.