# Research on Excavation Method for Soft Rock Tunnel Based on Stress Release Rate
## 2 Model Test Method of Soft Rock Tunnel Excavation Based on Stress Release
### Stress Release Rate  
The displacement release of the tunnel is accompanied by stress release. When the excavation face is a certain distance away from an unexcavated section, the stress redistribution begins to occur in the target section. The constraint stress generated by the surrounding rock on the proposed excavation boundary of the section is defined as the initial stress. With the continuous advancement of the excavation face, the constraints on the excavation boundary of the section are continuously reduced until the tunnel face passes through the section and has a certain distance. The constraint is reduced to 0, which is the stress release process accompanied by tunnel excavation. The relationship between the stress release and the position of the excavation face is shown in Figure 9. \(\sigma_{(x)}\) is the constraint stress release value corresponding to the target section when the excavation face is at any position. The definition of the stress release rate at any point around the tunnel can be obtained by the ratio of the reduction in the constraint stress to the initial stress. The stress release rate is deduced as follows [27]:  
\[\eta=\frac{\sigma_{0}-\sigma_{d}}{\sigma_{0}}\times 100\% \tag{2}\]  
where \(\sigma_{d}\) is the constraint stress generated by the surrounding rock after the tunnel target section is excavated; \(\sigma_{0}\) is the initial stress; \(\eta\) is the stress release rate.  
In order to study the stress release of the tunnel site, the K71 + 882 section of the entrance section of the Dongmachang I tunnel was selected as a typ  
Figure 8: Monitoring data of K71 + 882: (**a**) Displacement; (**b**) displacement rate.  
Figure 9: Relationship between stress release and excavation face position.  
After the tunnel was excavated and the steel arch was arranged, the pressure cell was installed between the primary support and the surrounding rock to monitor the contact pressure, as shown in Figure 10. The contact stress between the initial support and the surrounding rock at the tunnel vault (A), left and right arch shoulder (B, C), and left and right arch waist (D, E) during the construction period was monitored, respectively. The layout of the monitoring points is shown in Figure 11.  
The variation curve of the tunnel stress with time is shown in Figure 12. According to the definition of stress release rate, the contact stress between the surrounding rock and the initial support is the result of the release of the constraint stress of the surrounding rock on the boundary of the section to be excavated. Because the tunneling speed is constant, the stress change curve of Figure 12 can be equivalent to the relationship between the stress release of the tunnel and the position of the excavation surface. At this time, both Figures 9 and 12 can represent the relationship between the stress release coefficient and the advance distance of the excavation surface, and the shape of the two curves is similar. It can be seen from Figure 12 that the stress change in the tunnel's surrounding rock lasts for a long time. In the early stage of monitoring, the stress of the tunnel vault increases sharply and reaches the peak value of 588.05 kpa after the excavation of the lower bench (14 th day). The stress at the arch shoulder and arch waist increases steadily with time, and the growth rate decreases gradually. After 100 days, the stress change range is small, which can be regarded as stable. At this time, the stress at the right arch shoulder is the largest, which is 806.05 kpa, and the stress at the left arch shoulder is the smallest, which is 318.36 kpa.  
Figure 11: Stress measuring point layout diagram.  
Figure 10: Pressure cell installation.