High-Speed Railway Tunnel Hood: Seismic Dynamic Characteristic Analysis

When a high‐speed train is passing through a tunnel, micro‐compression wave may be created at the tunnel exit, which will affect the environment around the railway line. Setting hood at tunnel entrance is one of the efficacious ways for solving this problem. While in an earthquake region, in addition to consideration of controlling micro‐com‐ pression wave, the seismic safety of hood structure must not overlook the factor. In this chapter, using finite difference method, several types of hood seismic dynamic character‐ istic were analyzed, and their seismic dynamic respond stress curves were drawn out. As a result, the recommended hood type was determined, which is helpful for hood design in high intensity earthquake zone.


Introduction
In China, high-speed railway technology got quick development and more and more highspeed railway tunnels were built in high earthquake intensity zones. Large amount of postearthquake investigation shows that tunnel entrance is liable to earthquake effect and result in damage. However, for solving train-tunnel aerodynamic effect, tunnel hood has become an indispensable accessory of tunnel structure. So how to improve tunnel hood seismic characteristic is one of the hot point on tunnel seismic research.
To the tunnel entrance seismic character, much amount of research has been made and some important conclusions have been drawn out. Gao et al. (2009) did a thorough tunnel damage field investigation along the Dujiangyan-Wenchuan highway after the Wenchuan earthquake. The inspection results showed that the disaster induced relatively serious damage to the tunnel entrance.
Through collecting and analyzing information of seismic damage of the tunnel portals in Wenchuan earthquake, Wang et al. (2012) summed up the major factors that affected seismic damage extension, and pointed out using fuzzy synthetic evaluation method for the estimation of seismic risk level of mountain tunnel portals. Zheng (2007), using 3D distinct element method, analyzed the tunnel entrance seismic response. Cui (2010), using finite difference software FLAC-3D and experiment method, researched the seismic design calculation method of tunnel shallow-buried portal.
From previous discussion, it can be seen that many researchers have done study on tunnel entrance seismic character, while few topic pointed to the high-speed tunnel hood. In fact, for relief of aerodynamic effect, the hood structure usually set opening on top or side district. So in high seismic area, the tunnel hood will be more fragile compared with common tunnel portal.
In this chapter, using finite element software, Abaqus, the seismic character of two opening hood and nonopening hood was calculated and compared, and the recommended hood structure was given out. The types of hood are side-strip and top combination opening hood, two-side-strip opening hood and two seam opening hood.

Numerical model
In this chapter, finite element software Abaqus was used for simulating the tunnel hood dynamic response under seismic wave. The dimension of the model is shown in Table 1.
For obtaining a good relief microcompression effect, the hood opening parameter must be determined through a large amount of analysis and calculation. Jiang (2014) gave out 20 m length of three types of hood optimum parameter, which is shown in Figure 1. The numerical model meshes of the whole model and hood structure are shown in Figures 2 and 3. For absorption of the seismic reflection wave, the cycle sides of the model is set as infinite element, and the bottom of the model is set as viscous-elastic boundary. The lining of the tunnel is C35 concrete and the surrounding rock is grade IV. Their mechanical parameters are shown in Table 2.  High-Speed Railway Tunnel Hood: Seismic Dynamic Characteristic Analysis http://dx.doi.org/10.5772/66811 The input seismic wave is scaled Wolong wave and is shown in Figure 4, the acceleration pick of which was 0.62 m/s 2 and was set on the bottom of the model for simulating the seismic effect in vertical direction.
The calculation was divided into two steps. First, only setting gravity load, the initial stress condition of the whole model would be obtained. Second, using dynamic implicit model and setting seismic load at the bottom of the model, the dynamic response of the lining was simulated. In the tunnel cross-section, there are nine monitor points on the lining of the tunnel, as shown in Figure 5, for recording the dynamic response in different position. In the tunnel axis direction, the hood was set with four monitor in cross-section. The position to tunnel and hood crossing surface is 1 m (surface I-I), 7 m (surface II-II), 11.5 m (surface III-III), and 15 m (surface IV-IV), respectively, and is shown in Figure 1.

Initial stress condition
Under dead weight of the surrounding rock and lining, the stress status of structure is almost the same to different calculation condition. So only the maximum principal stress contour graphic of side strip and top open combination opening hood was provided (Figure 6). It can be seen that the peak value of the maximum principal stress is 0.67 Mpa.

Stress contour condition analysis
The maximum principal stress conditions of the hood structures at peak period of seismic wave (t = 8.92) were shown in Figure 7 and Table 3.
The simulation results showed that: 1. Under seismic load, the peak value of maximum principal stress is more than 2 MPa. It can be checked out that the tensile strength of C35 is 1.57 MPa, which means that no matter of the setting opening, the seismic load can result in damage to the hood structure.

The stress condition discrepancy analysis
The discrepancy at hood cross-section and axial direction is shown in Tables  The results showed that: 1. Although there are some variations because of the differences in the type of hood, the stress maximum region is almost the same, near to the waist of the hood structure.
2. Except the two seam opening hood, in the hood axial direction, the peak value of max principal stress appeared at the section II-II, which is 7 m to the hood and tunnel cross point.  Proceedings of the 2nd Czech-China Scientific Conference 2016   3. Among these hoods, the peak value on two seam opening hood is the lowest. In some checking point, the max principal stress on its structure is even lower than that on no opening hood as shown in Figure 12 and Table 8.   High-Speed Railway Tunnel Hood: Seismic Dynamic Characteristic Analysis http://dx.doi.org/10.5772/66811