## 3 Results and Discussion
### Study on the Support Effect of Bolts in Soft Rock Tunnels  
#### 3.4.1 Comparative Analysis of Surrounding Rock Deformation  
To investigate the support effects of the two types of bolts after the excavation of a soft rock tunnel, a plane strain model of the tunnel, as shown in Figure 7, was established for excavation and support simulation. For comparative analysis, three scenarios were simulated: no support, conventional bolt support, and yielding bolt support. In all three scenarios, the surrounding rock mass deformed toward the tunnel, and the displacement cloud maps in the vertical and horizontal directions, along with the resultant displacement vectors, are depicted in Figures 17-19.  
Figure 16: The bending moments varied at several bolt positions. These positions refer to the distances from the measuring points to the bolt head.  
Figure 17: Displacement nephogram and resultant displacement vector diagram of the surrounding rock without supporting after excavation of the tunnel: (**a**) vertical direction; (**b**) horizontal direction.  
After excavation, maximum deformation occurred at the arch crown. In Scenario One, which was without any form of support, the stabilized settlement of the arch crown reached 11.40 cm, with a deformation of 8.89 cm on the right sidewall. In Scenario Two, which was with conventional bolt support, the stabilized settlement of the arch crown was 9.79 cm, and the sidewall deformation was 8.16 cm. For Scenario Three, which was with yielding bolt support, the arch crown settlement was 9.42 cm, and the sidewall deformation amounted to 7.40 cm. Compared to no support, the yielding bolt support increased the control of crown settlement and sidewall deformation by 17.37% and 16.76%, respectively.  
Resultant displacements at the monitoring points are shown in Figure 20, clearly reflecting the rock surface deformation under the three scenarios. It can be inferred that employing the bolt support after tunnel excavation effectively constrained the deformation of the surrounding rock mass. Furthermore, when yielding bolt support was used, the deformation of the rock mass was less compared to using conventional bolt support, indicating that yielding bolts are more effective in controlling rock mass deformation during tunnel excavation.  
Figure 19: Displacement nephogram and resultant displacement vector diagram of the surrounding rock with yielding bolt supporting after excavation of the tunnel: (**a**) vertical direction; (**b**) horizontal direction.  
Figure 18: Displacement nephogram and resultant displacement vector diagram of the surrounding rock with the conventional bolt supporting after excavation of the tunnel: (**a**) vertical direction; (**b**) horizontal direction.  
As shown in Figure 21, it can be seen that, regardless of the support scenario, the positions where the surrounding rock deformation was relatively large were the arch crown and arch base. It can be known from the curve with the error bar that the standard deviations of the surrounding rock deformation at the arch crown and arch base were larger, which indicates that the supporting effect of the yielding bolts was the most obvious at these two locations. This indicates that, within a certain deformation range, the greater the deformation of the surrounding rock, the more suitable it is to adopt yielding bolt support. On the other hand, even after the use of yielding bolt support, the deformation of the surrounding rock at the arch crown and arch base was still greater than that at the other monitoring positions without support. This indicates that reducing or avoiding the deformation of the surrounding rock in soft rock tunnels cannot rely solely on bolt support, it is also necessary to combine it with other technologies.