## 5 Results and Discussion
### Comparison of t2 and T2
#### 5.2.2 Liner  
Figure 15 compares the resultant displacement of the t2 and T2 liner, and the measurement points are shown in Figure 15b,c. The resultant displacement is the arithmetic square root of the \(D_{\rm H}\) and \(D_{\rm V}\). The resultant displacement on the left side of the t2 liner is greater than that on the right side; the maximum value on the left side is about 1.24 mm at 180\({}^{\circ}\), and the minimum value on the right side is about 0.82 mm at 0\({}^{\circ}\). The resultant displacement of the T2 liner is also greater on the left side than on the right side; the maximum value on the left side is  
right side is about 0.25 mm at 0\({}^{\circ}\). Therefore, the resultant displacement of the left liner of t2 and T2 is large, and the values are close to each other. The resultant displacement of the right side of the liner is small, while the difference of the resultant displacement of the right side of the liner in the t2 and T2 is large. The resultant displacement on the right side of the t2 liner is much larger than that of the T2 liner. The possible reason for this is the fact that the distance between the monitoring point of the right-side liner of the T2 and the vibration point of the train in T1 is very large. The energy dissipation of the vibration wave of the train is basically completed. The research results of \(d\) tunnel can not be directly applied to the engineering of the \(D\) tunnel; it is necessary to strengthen the related research of the super-large diameter tunnel. Figure 16 shows the comparison of maximum and average values of resultant velocities between t2 and T2. The maximum and average values of the velocity in t2 and T2 have basically the same variation rule, both of which are larger on the left side of the liner and smaller on the right side of the liner. Moreover, the velocity of the T2 is about 50% of that value of the t2; this indicates that the \(D\) twin tunnel is more stable than the \(d\) twin tunnel, which may be the result of the joint action of the larger space between the \(D\) twin tunnels and the greater water pressure around \(D\) twin tunnels. In addition, different sections of small and super-large diameter twin tunnels may also play an important role. The super-large twin tunnels have been stiffer due to the inner walls (Wall_1, Wall_2, Wall_3, and Wall_4) were added to them. Horizontal stress (\(\sigma_{\rm x}\)) and vertical stress (\(\sigma_{\rm y}\)) of liner in t2 and T2 can be seen in Figure 17. Positive values represent tensile stresses, and negative values represent compressive stresses. The absolute value of \(\sigma_{\rm x}\) at 0\({}^{\circ}\) and 180\({}^{\circ}\) of T2 liner are greater than that of the t2 liner, and the variation amplitude of \(\sigma_{\rm x}\) is smaller in the train vibration process (Figure 17a). The change of \(\sigma_{\rm y}\) at 90\({}^{\circ}\) and 270\({}^{\circ}\)is similar to that of \(\sigma_{\rm y}\) in t2 and T2 (Figure 17b). In addition, special attention needs to be paid to the monitoring points where the tunnel liner exhibits high tensile strength, as the concrete liner has a strong compressive strength and a weak tensile strength. Figure 15: Comparison of resultant displacement of t2 and T2: (**a**) maximum value of resultant displacement; (**b**) measurement points of t2; (**c**) measurement points of T2. Figure 18 shows the cloud of the resultant velocity of the liner in t2 and T2 at some key time points. The resultant velocity at each time point of the tunnel is related to the dynamic load of the train, and the resultant velocity on the left side of the liner is larger than the value on the left side both in t2 and T2. The resultant speed of the super-large diameter tunnel is obviously less than that of the small-diameter tunnel, which means the stability of the super-large diameter tunnel is better than the small-diameter tunnel.