# Numerical Studies on Rockbursts in Tunnels with High In Situ Stresses and Geothermally Rich Areas
## 5 Discussion
### Effect of the Dip Angle of Faults on Tunnels  
The Sichuan-Tibet railway is located near the plate, and the fault structure is very complex along the line, which also brings great difficulties to tunnel construction. [PERSON] et al. [25] also describes the distribution of faults along the tunnel and the corresponding dip angle. The zone where the tunnel passes through the fault is prone to rock failure and even rockbursts. Based on the above reasons, we establish a numerical model to study the influence of the fault dip angle on the stress of the rock surrounding the tunnel. To study the distribution of the surrounding rock stress and understand the trend of its variation with cooling when the tunnel crosses faults at different angles, a new fault numerical model is built. The size of the model is 200 m \(\times\) 200 m \(\times\) 400 m, the fault is located in the middle of the tunnel, and the other calculation parameters and mechanical and temperature boundary conditions are consistent with those of the previous model. The fault dip angles are set to 45\({}^{\circ}\) and 60\({}^{\circ}\) (see Figure 14). To simulate the influence of the fault plane, a contact element is set between the upper and lower surfaces of the fault model in the process of model construction. The contact element can transfer heat and compressive stress but cannot transfer tensile stress. There is contact friction between the two surfaces of the fault. In this paper, the simulation of faults is relatively simple, and only the influence of closed faults on tunnels is discussed. However, the results of numerical calculation can show the change process of stress, which is convenient for subsequent relevant researchers to understand and analyze. Figure 15a,a' show that when the tunnel has just been excavated at \(t\) = 0 h, the stress is concentrated (red dotted line in Figure 15a,a') on the left side of the junction between the tunnel surface and the fault, and rockbursts easily occurs at this position (see Section 4). At the same time, Figure 15a,a' show that at the location of concentrated stress, the minimum stress (compressive stress) value at 45\({}^{\circ}\) is greater than that at 60\({}^{\circ}\), which indicates that when the tunnel passes through the fault with a small incline, the location of concentrated stress is more prone to rockbursts. After cooling for 24 h, the maximum compressive stress decreases at the stress concentration point (Figure 15b,b'), and cooling unloads stress and inhibits the occurrence of rockbursts. Therefore, for the high-temperature tunnel passing through the fault, timely cooling at the fault junction is conducive to inhibiting rockbursts and ensuring the normal progress of tunnel engineering. At the same time, Figure 15c,c',d,d' show that tensile stress appears on the upper and lower sides of the junction between the tunnel wall and the fault after 24 h of cooling for the two dip angles, and the tensile stress value at the 45\({}^{\circ}\) dip angle is larger than that at the 60\({}^{\circ}\) dip angle. The brief analysis of the simulation results shows that for the high-temperature tunnel crossing the fault, the location of concentrated stress at the junction of the fault and the tunnel should be cooled in the initial stage, which can effectively prevent rockbursts. In the later stage, there is no  
Figure 14: Model and meshing of the tunnel with dip angles of 45\({}^{\circ}\) and 60\({}^{\circ}\). principal stress in the tunnel with a fault (Dip angle = 45\({}^{\circ}\)) when \(t\) = 24 h. (**c**) Maximum principal stress in the tunnel with a fault (Dip angle = 60\({}^{\circ}\)) when \(t\) = 0 h. (**c**) Maximum principal stress in the tunnel with a fault (Dip angle = 45\({}^{\circ}\)) when \(t\) = 0 h. (**d**) Maximum principal stress in the tunnel with a fault (Dip angle = 60\({}^{\circ}\)) when \(t\) = 24 h (The location in red circle is tensile stress area).