# Dynamic Response Analysis of Parallel Twin Tunnels Under Different Train Loads
## 4 Results and Discussion
### Surrounding Soil Response
#### 4.2.2 Surroundings
Figure 18 compares the maximum strain-yy values in the T2 surroundings under various conditions. Strain, which reflects soil deformation under external forces, is a key indicator for assessing soil deformation and surrounding structure stability. Figure 18a illustrates that the maximum strain-yy values at different angles in T2 soil1 under the A\({}_{4}\) and A\({}_{7}\) conditions are nearly identical. Strain-yy peaks at the top (90\({}^{\circ}\)) and bottom (270\({}^{\circ}\)) of T2, with lower values at the right (0\({}^{\circ}\)) and left (180\({}^{\circ}\)) sides. This suggests that under train loading, the T2 surroundings tend to form a vertically oriented elliptical deformation, which intensifies with higher train loads. Figure 18b-d compare the maximum strain-yy values at 0\({}^{\circ}\), 90\({}^{\circ}\), and 270\({}^{\circ}\) for each layer of the T2 surroundings under various conditions. At the right side of T2 (0\({}^{\circ}\)), strain-yy values for the four layers follow the order: soil3 > soil2 > soil4 > soil1, indicating that strain-yy initially increases and then decreases with distance from T2. Additionally, when  
Figure 17: The \(R_{\text{P}}\) of surroundings in T2 under different conditions: (**a**) soil1; (**b**) 0\({}^{\circ}\); (**c**) 90\({}^{\circ}\); (**d**) 270\({}^{\circ}\). Figure 18: The maximum strain-yy of surroundings in T2 under different conditions: (**a**) soil1; (**b**) 0\({}^{\circ}\); (**c**) 90\({}^{\circ}\); (**d**) 270\({}^{\circ}\). comparing strains at the top and bottom of T2 across different conditions, the vertical strain decreases as the distance from the tunnel increases, with slower reduction rates observed at greater burial depths. Figures 15-18 reveal that the dynamic response of the T2 surroundings mirrors the tunnel structure's behavior: it increases with train load under the A\({}_{1}\) to A\({}_{6}\) conditions, decreases to levels similar to under the A\({}_{4}\) condition under the A\({}_{7}\) condition, and then sharply rises under the A\({}_{8}\) condition. In T1, train-induced vibrations, burial depth, and the twin-tunnel structure create uneven compressive forces in the T2 surroundings, especially in the lower-right and upper-left regions. Overloading exacerbates this issue, potentially leading to the detachment of tunnel liners from the surrounding soil. Thus, in practical applications, the impact of overloading on adjacent tunnel surroundings, especially in the upper regions of T2, demands significant attention. In this paper, the static and dynamic response characteristics of adjacent tunnels of parallel double tunnels under eight different loads (\(F_{i}\) = 62.4-131.5 kN, \(i\) = 1, 2, 3..., 8) are studied by PFC. Compared with the field monitoring adopted by [PERSON] et al. [1], this paper analyzes the influence of different loads on the dynamic responses of the whole tunnel structure and its surroundings. Compared with the finite element method used by [PERSON] et al. [3,30,31], this paper not only find the influence of axle load on the bottom of the tunnel but also analyzes the dynamic response to the surface and finds the amplification effect of the surface. Compared with the scale test adopted by [PERSON] et al. [14,32,33], this paper not only finds that the increase in train load will increase the dynamic response of the tunnel structure and surface but also finds that it will decrease under the A\({}_{7}\) condition.