## 4 Damage Stability Assessment of Nearby Underground Tunnel Structures Due to Hydrogen Gas Explosion  
### Influence of Separation Distance of Nearby Tunnel Structures on Damage  
In this study, to assess the damage to nearby underground structures caused by hydrogen gas explosions, we conducted analyses by adding a tunnel structure to the existing analysis domain and introduced parameters denoted as H (tunnel depth) and  
Figure 12: Vibration value at ground surface to underground hydrogen infrastructure.  
Figure 13: Minimum required distance to hydrogen infrastructure.  
L (separation distance from the hydrogen storage facility). The scenario of a hydrogen gas explosion in a hydrogen storage facility, which acts as an explosion source, assumes conditions of large-scale deformation of the entire area of the protective structure owing to a large-scale explosion, which corresponds to the case in which a hydrogen charging station with a capacity of 200 kg/day causes a complete explosion. Figure 14 shows a conceptual diagram of the simulation model for evaluating the damage stability of nearby underground tunnel structures due to hydrogen gas explosions. Table 6 lists the main variable conditions for each analysis case.  
Figure 15 shows the simulation results depicting the damage behavior of the tunnel lining according to the change in the horizontal separation distance under the same tunnel depth of 8 m (H = 8 m). In all the analysis results, the damage zone owing to tensile deformation tended to be more widely distributed than that caused by compressive deformation, and the degree of damage decreased as the separation distance increased. In particular, the distribution of damage in Cases 2 and 3, with separation distances of 8 and 10 m, respectively, significantly decreased compared with Case 1, where the minimum separation distance was set at 5 m. This trend was consistent with the minimum separation distance discussed in Section 3. In terms of the damage distribution characteristics, although there was a difference depending on the separation distance, damage patterns showing linearity along the axial direction of the tunnel were consistently observed. When these linear damage patterns are compared with the stratum where the tunnel structure is located (see Table 2), it can be seen that damage tends to be concentrated at the boundaries between strata (A, B, C in Figure 16), and this is caused by differences in dynamic characteristics (i.e., characteristic impedance) of the materials (rock, soil, etc.) that make up the stratum. Under the current analysis conditions, the dynamic properties of the materials that make up the strata are different, which can be considered to cause flexural fracture owing to  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Classification**} & **Separation** & \multirow{2}{*}{**Depth,**} & **Minimum** & **Central** & **Angle in the** \\  & **Distance,** & **(m)** & **Distance** & **Interval** & **Direction of the** \\  & **(m)** & & **(m)** & **(m)** & **Explosion Source (\({}^{\circ}\))** \\ \hline Case 1 & 5 & 8 & 5 & 12.5 & 0 \\ Case 2 & 8 & 8 & 8 & 12.5 & 0 \\ Case 3 & 10 & 8 & 10 & 18.5 & 0 \\ Case 4 & 5 & 12 & 6 & 13 & 18 \\ Case 5 & 5 & 16.4 & 6.5 & 15 & 34 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Geometric conditions for numerical simulation cases.  
Figure 14: Simulation with reinforcement tunnel lining. Material: concrete (M30) and steel bar (IS–456).  
the deviation at each boundary, as it causes stress deviation according to the material response characteristics.