# Research on Excavation Method for Soft Rock Tunnel Based on Stress Release Rate
## 3 Model Test Arrangement and Result Analysis
### Experiment Results and Analysis  
(1) Tunnel deformation monitoring  
In order to explore the deformation characteristics of soft rock tunnels under different excavation methods and to make the results more intuitive, this paper converts the tunnel deformation results monitored by the displacement meter according to the length similarity ratio. We obtained the vertical and horizontal directions under different construction methods. The displacement diagram of the load change is also shown (Figure 22). It can be seen that under different excavation methods, the variation law of the displacement around the tunnel is close, which shows that the displacement around the tunnel gradually increases with the increase in load. The deformation around the tunnel is mainly vertical displacement, and the vertical displacement is 5.3-106.4% larger than the horizontal displacement. The confining pressure loading corresponds to the gradual increase in the stress release rate from 0. Combined with Figure 22, it can be seen that there is a certain linear relationship between the displacement of the tunnel and the applied load. The relationship between the two is highly fitted with the straight line, which further verifies the assumption that the displacement release rate is approximately equal to the stress release rate. Figure 22: Displacement changes with load under different excavation methods: (**a**) Three-bench; (**b**) three-bench with reserved core soil; (**c**) three-bench seven-step. In order to more intuitively compare the deformation law of the tunnel surrounding under different excavation methods, the vertical displacement and horizontal displacement of the tunnel under each working condition are compared (Figure 23), and the final displacement value is shown in Table 9. It can be seen from the chart that the final value of vertical displacement is about 420-470 mm, and the final value of horizontal convergence is about 310-360 mm, which is close to the field measurement results (Figure 12). It can be seen that the model test results can better reflect the actual situation. The displacement around the tunnel is greatly affected by the excavation method. Compared with the three-bench excavation method, the final vertical displacements of the three-bench with reserved core soil method and the three-bench seven-step method are reduced by 6.7% and 11.8%, respectively, and the final horizontal displacements are reduced by 6.9% and 12.4%, respectively. It can be seen that the selection of appropriate excavation methods can effectively control the large deformation of the surrounding rock. Among the three excavation methods compared in this experiment, the three-bench seven-step method has the strongest ability to control tunnel deformation. The test always keeps the lateral pressure coefficient \(\lambda=\frac{\sigma_{x}}{\sigma_{y}}=1.25\) for bidirectional loading. It can also be seen from Figure 24 that the overall contact pressure is realized as spandrel > vault > haunch > arch foot > arch bottom. The load in the horizontal direction is large, and with the advancement of the test pressurization process, the stress release rate of the surrounding rock gradually increases. This part of the force is borne by the supporting structure, so the pressure between the initial support and the surrounding rock also increases gradually. Taking the three-bench seven-step method as an example, the contact stress between the vault and the arch foot is large in the early stage of load loading. When the test load gradually increases and approaches the measured ground stress, the contact stress between the arch shoulder and the arch waist increases rapidly. Finally, the contact stress of the arch shoulder is 498.5 kPa higher than that of the arch foot. Figure 25 shows the contact pressure between the first and second primary supports. It can be seen that the contact pressure between the arch foot and the arch bottom is small under different excavation methods, while the pressure at the vault, arch shoulder, and arch waist is relatively close.