# Influence Law of Foundation Pit Excavation on Stress of Surrounding Tunnel Bolt
## 3 Sensitivity Analysis of Influencing Factors on the Axial Force of Rock Bolt  
### Distance between Foundation Pit and Tunnel  
The following scheme was set as the calculation standard: the depth of the excavation was 15 m, the excavation width was 20 m, and the distance between the tunnel and the excavation was 5 m. When discussing the influence of the distance between the foundation pit and tunnel (denoted as \"\(D_{\text{ft}}\)\") on the anchor bolt axial force, the distance between the foundation pit and tunnel was set as 3, 5, 7, 9, 11, 13, and 15 m, respectively. The left bolt (denoted as \"left bolt\") and the top bolt (denoted as \"top bolt\") of the tunnel were selected as the monitoring objects to record the changes in the bolt axial force during the construction process. When there is a tunnel next to the foundation pit, the tunnel is reinforced with a bolt. After the foundation pit is excavated, the soil around the foundation pit tends to move into the foundation pit. At this time, the soil on the left side of the tunnel also tends to move into the foundation pit, as shown in Figure 3 (Taking \(D_{\text{ft}}\) = 5 m, for instance). After the excavation of the foundation pit, the soil around the tunnel is negative and moves into the foundation pit. At this time, as shown in Figure 4, the bolt on the left is gradually stretched and bears tensile stress, showing a positive value. Moreover, the horizontal displacement at the top of the right side of the foundation pit is the largest. However, the soil around the tunnel within 1-2 times the diameter of the tunnel is affected. It can be seen from the  
Figure 2: Numerical simulation model. (**a**) Numerical model; (**b**) bolt monitoring position. distribution of bolt axial force that along the direction of bolt length, the distribution of axial force is not uniform but shows a trend of increasing first and then decreasing. When the distance between the tunnel and the foundation pit changes, the axial force of the bolt changes accordingly. The stress of the left bolt was recorded, as shown in Figure 5, and it has the same trend as the work by [PERSON], [PERSON] [3], so the results of this paper can be verified. It can be seen from the figure that the axial force of the left bolt is positive; that is, the bolt is in a tension state, and along the bolt length direction, the axial force first increases and then decreases, and the largest axial force is located in the middle of the bolt. When the \(D_{\text{ft}}\) is 3 m, the axial force of the bolt is the maximum, and with the increase in the \(D_{\text{ft}}\), the axial force of the bolt decreases gradually. This is because the larger the \(D_{\text{ft}}\) is, the larger the distance between the foundation pit and the tunnel is, and the smaller the impact of foundation pit excavation is, that is, the smaller the deformation of the soil around the tunnel is, and therefore the smaller the corresponding bolt axial force is, shown in Figure 6.  
Figure 4: Axial force distribution of anchor bolt after excavation. Figure 3: Displacement of soil after excavation of foundation pit. (**a**) Horizontal displacement; (**b**) vertical displacement. In addition, in order to further study the influence of \(D_{\text{ft}}\) on the bolt at the top of the tunnel, the axial force of the bolt at the top under different \(D_{\text{ft}}\) conditions was recorded, as shown in Figure 7. It can be seen that the stress distribution of the top bolt is similar to that of the left bolt, and the axial force increases first and then decreases along the bolt length direction, and the maximum axial force of the bolt occurs in the middle of the bolt. When the foundation pit is not excavated, the anchor rod bears the tensile force, but with the excavation of the foundation pit, the anchor rod is squeezed due to the rebound effect of excavation [39,40], which leads to the decrease in the tensile force.