# 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
### Displacement Releasing Rate  
Tunnel excavation is accompanied by the deformation and displacement release of the surrounding rock, and the displacement release is too large due to the low strength of the surrounding rock during the excavation of a soft rock tunnel, which easily produces large deformation diseases. In the process of advancing the tunnel excavation face, the surrounding rock begins to settle and deform at a certain distance from any unexcavated face. The radial displacement of the surrounding rock does not converge until the excavation face passes a certain distance from the target section. The relationship between the deformation of the tunnel and the position of the excavation face is shown in Figure 5, \(u_{\text{(x)}}\) is the radial displacement corresponding to the target section when the excavation face is in any position. The displacement release rate at any section around the tunnel can be obtained by the ratio of the radial displacement to the maximum radial displacement when the tunnel is excavated to the section. The displacement release rate is deduced as follows [26]:  
\[\lambda=\frac{u_{0}}{u_{\text{max}}}\times 100\% \tag{1}\]  
where \(\lambda\) is the displacement release rate; \(u_{0}\) is the radial displacement of any section of the tunnel from the beginning of deformation to the excavation; \(u_{\text{max}}\) is the maximum radial displacement of the same section after the deformation is stable. A ring tunnel deformation monitoring section is set up every 5 m in the research section. A total of seven measuring points are set up along the inner wall of the tunnel in each monitoring section, three measuring points are set up for the vault subsidence, and one deformation measuring point is set up on each side of the spandrel and arch waist position. In order to study the on-site displacement release of the tunnel, the pile number K71 + 882 at the entrance section of the Dongmachang I tunnel was selected as a typical monitoring section to monitor the vertical settlement (G1, G2, G3) and horizontal convergence (BC, DE) at the top of the tunnel during the construction period. The layout of monitoring points is shown in Figure 6. The total station was used as the displacement  
Figure 5: Relationship between tunnel deformation and excavation face position. monitoring equipment, and the non-contact three-dimensional displacement observation method was used to monitor the deformation of surrounding rock, as shown in Figure 7.  
The tunnel displacement monitoring frequency was once a day, and the final displacement of the tunnel was recorded every day. The deformation time history and deformation rate curve of the K71 + 882 monitoring section tunnel are drawn, as shown in Figure 7. Because the tunnel can be regarded as being excavated at a uniform speed of one meter per day in practical engineering, the excavation face advancing distance can be characterized by time, and the displacement change curve with time is equivalent to the relationship curve between the displacement and excavation face position, and the displacement release change rule is basically consistent with Figure 5.  
Figure 8 shows that the deformation of the surrounding rock lasts for a long time, and the deformation rate decreases logarithmically, showing obvious creep characteristics. After the excavation of the tunnel monitoring section, the displacement around the tunnel increases sharply, and the maximum deformation rate is about 62 mm/d. With the excavation of the tunnel, the deformation rate shows a decreasing trend. After about 25 days of construction of the monitoring section, the deformation rate of the tunnel tends to be stable at this time, which is basically stable below 10 mm/d. It shows that the application of double-layer support can effectively control the displacement around the tunnel and ensure the safety of tunnel construction. The deformation around the tunnel is dominated by vault settlement.