# 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  
### Engineering Background  
Dongmachang No. 1 Tunnel (Figure 1) is a control project of the Huaping-Lijiang Expressway. The starting point mileage of the tunnel is ZK70 + 130, and the stopping point mileage is ZK75 + 335. The total length of the tunnel is 5205 m, and the maximum buried depth is 613 m. The elevation of the tunnel area is 1870-2512 m, and the relative height difference is 643 m. The terrain is rugged, and many bedrocks are exposed, and the soft rock accounts for a large proportion. The tunnel intersects the Chenghai-Binchuan active fault zone at K72 + 164, which is a Holocene active fault. The width of the fault zone is about 95 m. The fault occurrence is 310\({}^{\circ}\)/72\({}^{\circ}\), and the angle with the tunnel is 41\({}^{\circ}\), which belongs to the compressive reverse fault. Affected by the fracture, the lithology of the tunnel entrance section and the exit section is quite different. The entrance section is dominated by argillaceous siltone, and the exit section is dominated by dolomite limestone.  
The entrance section of the tunnel is excavated using the three-bench method (Figure 2). However, due to the poor stability of the surrounding rock, the problem of large deformation of soft rock is extremely prominent. The tunnel shows the characteristics of large deformation, high deformation rate and difficult convergence, long deformation duration, and uneven deformation and failure. In the construction process, large deformation diseases of the surrounding rock, such as large deformation of initial support, invasion limit of initial support, and uplift of inverted arch, have appeared one after another (Figure 3). Combined with the actual situation of the project, the tunnel entrance section K71 + 810-K71 + 910 is selected as the typical section. The surrounding rock of this section is mainly argillaceous siltone, which is divided into active fault influence sections after expert demonstration.  
[PERSON] et al. (2018) used the hydraulic fracturing method to monitor the in situ stress and established a three-dimensional finite element model. The least squares method was used for regression inversion to obtain the in situ stress value of the rock mass in the whole area. The in situ stress inversion results along the tunnel are shown in Figure 4. Combined with the relevant in situ stress test and inversion results, it can be seen that the stress state of the stratum in the selected typical section is complex. Taking section K71 + 882 of the tunnel entrance section as an example, the in situ stress inversion results are shown in Table 1, and the tunnel passing through it is affected by the large horizontal tectonic stress.  
Figure 1: Longitudinal profile of Dongmachang I tunnel on the Huaping-Lijiang highway.  
Figure 4: Inversion results of the ground stress field in the tunnel site area [25].  
Figure 3: Typical large deformation diseases in engineering sites: (**a**) Initially branch the block peeling off; (**b**) inverted arch uplift.  
Figure 2: Tunnel site schematic diagram.  
The tunnel excavation method has a great influence on the stability of the tunnel and is one of the key factors to control the deformation of the surrounding rock. Therefore, it is of great significance to research the excavation method. Since tunnel excavation is essentially a process of stress release and displacement release, the selection of tunnel construction methods is closely related to the two. Therefore, based on the selected research section, the appropriate excavation method for the tunnel is explored from the perspective of stress release considerations. The research results can provide a reference for subsequent construction.