# 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
### The Internal Relationship between Stress Release Rate and Displacement Release Rate  
Tunnel excavation will disturb the surrounding rock and remove the initial stress field constraints so that it can produce material and energy exchange with the outside world, and then the stress in the surrounding rock will gradually be released with the passage of the excavation surface. Tunnel excavation will also cause stratum movement, and the deformation of the surrounding rock gradually increases with the passage of the excavation surface and finally converges under the action of support. It can be seen that the macroscopic stress release during tunnel excavation is the displacement release of the surrounding rock. Tunnel excavation is essentially the accumulation of stress release and displacement release. Stress release and displacement release are the time-space effect problems caused by tunnel excavation. The two have a very close internal relationship. Many scholars have carried out some research and application on the relationship between the stress release rate and displacement release rate: [PERSON] [28] and [PERSON] [29] used model tests and numerical simulations, respectively, to draw the time history curves of stress release and displacement release, and pointed out that the change trend of the two curves was similar. [PERSON] et al. [30] replaced the stress release rate with a more easily controlled displacement release rate and carried out numerical simulations to analyze the deformation and stress characteristics of the supporting structure of tunnel excavation under elastoplastic conditions. Based on the assumption that the stress release rate is approximately equivalent to the displacement release rate when the tunnel excavation face is advancing, [PERSON] et al. [31] proposed a semi-analytical solution for the deformation and stress of surrounding rock with time-varying mechanical properties. Based on the actual project of the Dongmachang I tunnel, this paper analyzes the stress release rate and displacement release rate, respectively. The change trend of the relationship curve between tunnel deformation and excavation surface shown in Figure 5 and the relationship curve between the stress release and excavation surface shown in Figure 8 is basically the same. The law of displacement release (Figure 8) and stress release (Figure 12), measured in actual monitoring, further verifies the rationality of the assumption that the stress release rate is approximately equivalent to the displacement release rate. Because it is impossible to monitor the target section without excavating the surrounding rock in the actual project, the stress and displacement of the section can only be measured after the tunnel excavation surface passes through the target section. In order to further explore the relationship between stress release and displacement release, this paper converts the calculation of the measured results into the expression of the displacement release coefficient and stress release coefficient. When the distance between the excavation surface and the target surface is more than three times the diameter of the hole, the release has not yet started or has been stable, corresponding to the release  
Figure 12: K71 + 882 monitoring surface time-stress curve. coefficient of 0 and 100% in Figure 13, respectively. The measured release coefficient was fitted with the function proposed by [PERSON] et al. [32]:  
\[C(n)=\frac{C_{0}}{2}[\tanh(an-b)+1] \tag{3}\]  
where \(C\) is the release coefficient; \(C_{0}\) is the final release coefficient; \(n=L/D\), \(L\) is the distance between the excavation surface and the target surface, and \(D\) is the tunnel diameter; \(a\), \(b\) are parameters. When \(n\) tends to be negative infinity, the release has not yet begun, and the corresponding release coefficient \(C=0\); similarly, it can be obtained that when \(n\) tends to be positive infinity, it is fully released at this time, corresponding to the release coefficient \(C=C_{0}=1\).