## 3 Results and Discussion
### Absolute and Relative Spacing  
It is well-known that the spacing between parallel twin tunnels significantly influences the dynamic characteristics of the adjacent tunnel [31; 32; 33]. Figure 11 compares the \(D_{\text{R}}\) and \(V_{\text{R}}\) of adjacent tunnel liners and surroundings under varying spacing conditions to investigate the effects of absolute and relative spacing. Figure 11: Comparison of \(D_{\text{R}}\) and \(V_{\text{R}}\) in adjacent tunnel liners and surroundings under B\({}_{1}\)–B\({}_{3}\) conditions: (**a**) \(D_{\text{R}}\); (**b**) \(V_{\text{R}}\). Figure 10: Comparison of coordination in Tunnel 2 surroundings at various angles under A\({}_{1}\)–A\({}_{5}\) conditions. As shown in Figure 11a, the maximum and minimum differences of \(D_{\text{R}}\) of liners and surroundings under B\({}_{1}\)-B\({}_{3}\) conditions occur at 180\({}^{\circ}\) (closest to T1), confirming that proximity to the vibration source dominates displacement patterns. The maximum liner-surroundings \(D_{\text{R}}\) differences for B\({}_{1}\), B\({}_{2}\), and B\({}_{3}\) are 0.49 mm, 0.33 mm, and 0.12 mm. Such discrepancies possibly destabilize the surroundings due to insufficient structural support. The liners \(D_{\text{R}}\) ranking is as follows: B\({}_{2}\) > B\({}_{1}\) > B\({}_{3}\). For the surroundings, all other monitoring points align with the same ranking of the liners except at the vault (90\({}^{\circ}\)), where the \(D_{\text{R}}\) of the small-diameter tunnels is slightly lower than that of the 0.25 \(D\) super-large-diameter tunnels. This indicates that super-large-diameter tunnels exhibit greater static-dynamic responses under the same absolute spacing while showing slightly lower responses under relative spacing equivalence. Additionally, the granular nature of the surroundings leads to distinct displacement patterns at the vault compared to other monitoring points, necessitating focused stability assessments in this region. As shown in Figure 11b, the \(V_{\text{R}}\) of liners and surroundings for the 0.25 \(D\) super-large-diameter twin tunnels (B\({}_{3}\)) is significantly smaller than those under B\({}_{1}\) and B\({}_{2}\) conditions. This indicates that the dynamic characteristics of super-large-diameter tunnels with equivalent relative spacing (B\({}_{3}\)) are markedly lower compared to small-diameter tunnels (B\({}_{1}\)) and super-large-diameter tunnels with 0.25 \(d\) (B\({}_{2}\)). Additionally, the \(V_{\text{R}}\) of surroundings and liners align closely across all monitoring points, suggesting that the surroundings effectively support the tunnel structure, placing the super-large-diameter tunnel in a safer state. Comparing B\({}_{1}\) and B\({}_{2}\) conditions, significant \(V_{\text{R}}\) differences exist between surroundings and liners. In the 45-135\({}^{\circ}\), the \(V_{\text{R}}\) in surroundings is smaller than the value in liners, while in the 180-270\({}^{\circ}\), the \(V_{\text{R}}\) in surroundings exceeds the value in liners. This opposing dynamic behavior in the upper-right and lower-left regions of Tunnel 2 disrupts the support mechanism between surroundings and liners, potentially leading to liner cracking, leakage, or instability. The \(V_{\text{R}}\) in liners for super-large and small-diameter tunnels overlap at most monitoring points, with minor differences at 0\({}^{\circ}\), 45\({}^{\circ}\), and 315\({}^{\circ}\). However, the \(V_{\text{R}}\) in surroundings exhibits substantial disparities. In the 0-135\({}^{\circ}\) interval (upper half), super-large-diameter tunnels (B\({}_{2}\)) show higher \(V_{\text{R}}\) than small-diameter tunnels (B\({}_{1}\)), while in the 180-270\({}^{\circ}\) interval (lower-left corner), small-diameter tunnels (B\({}_{1}\)) exhibit higher \(V_{\text{R}}\).