# 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
### Model Test Exavation Method for Soft Rock Tunnel Based on Stress Release  
The above studies have beneficially verified the strong correlation between stress release rate and displacement release rate. In order to further explore the displacement deformation and mechanical characteristics of surrounding rock under different excavation methods of soft rock tunnels with high ground stress, this paper uses a model test to carry out further research. The traditional model test often uses the model box to realize the in-situ stress loading through the loading system and excavates the model soil according to demand to simulate the actual tunnel excavation [28; 33; 34]. This test method can better restore the construction process of the tunnel, but there are still some shortcomings. For example, due to the limitation of site and cost, the traditional model box test usually uses a small-size model to simplify the simulation of the actual tunnel project (Figure 14), which will lead to the inability to accurately restore the three-dimensional effects in the actual project, such as the non-uniformity of the stratum, the complex boundary constraints, and the neglect of the deformation and stress transfer of the vertical direction model soil, which ultimately leads to the inability to reflect the real complex actual project. Figure 13: The relationship between the stress/displacement release coefficient and excavation face position. At present, the mainstream tunnel excavation simulation methods are divided into the stress release method and displacement release method, but it is difficult to achieve displacement release by directly controlling the displacement around the tunnel during simulation excavation. In view of this, based on the stress release, this paper carries out the indoor model test in the form of horizontal loading (Figure 15). Compared with the traditional model test device, the tunnel diameter in the horizontal loading model test is smaller than the size of the whole model soil. The three-dimensional effect of the underground structure can be considered, including stratum deformation, stress transfer, etc., and it has obvious advantages in terms of authenticity and data acquisition. The tunnel model and similar soil material of the test device are placed inside the device. During the test loading process, four jacks arranged in the x direction are used to simulate the horizontal pressure. Four jacks arranged in the y direction are used to simulate vertical stress. The vertical jack and the loading panel are used to constrain the cover plate to control the longitudinal displacement of the test body so that the test body is in a plane strain state during the loading process. The test simulates the different stress release rates of the tunnel model soil by step loading. When the test is in the initial state, and no load is applied, the tunnel model is in the state before excavation, and the stress release rate is 0. When the confining pressure is gradually loaded until it is the same as the initial stress field, the state of the tunnel model is consistent with the actual tunnel after excavation. According to the definition of stress release rate Equation (2), the stress release amount at the tunnel wall should be equal to the initial stress \(\sigma_{0}\), and the stress is completely released. Therefore, the process of loading confining pressure from 0 to the initial stress field step by step corresponds to different stress release rates gradually increasing from 0. Because the stress release rate is approximately equivalent to the displacement release rate, the displacement release rate can be increased synchronously when the stress release rate is increased, which is consistent  
Figure 14: Traditional tunnel model test device [28]. Figure 15: Horizontal loading test device: where \(\eta\) is the stress release coefficient, the \(\sigma_{x0}\) is the initial horizontal principal stress, and the \(\sigma_{y0}\) is the initial vertical principal stress.