## 4 Analysis of Progressive Failure Process of Surrounding Rock and Tunnel Structure
### Analysis of Progressive Failure Process  
By integrating the numerical simulation results with field-measured data, the following conclusions can be drawn.  
Figure 20: Real shots of the scene. (**a**–**h**) represents the failure of surrounding rock from different perspectives.  
(1) To visually depict the deformation characteristics of soft rock tunnels, we extracted the deformation process curve and propagation positions from the model section under working conditions. Figures 21 and 22 illustrate that at \(K_{0}=0.5\), deformation propagation occurs in three main stages. Initially, deformation progresses gradually from the surface into the surrounding rock with minimal displacement. In the second stage, the surrounding rock deformation stabilizes. Finally, deformation occurs in groups within the surrounding rock, specifically between 0-320 cm and 320-500 cm, with each group exhibiting nearly uniform deformation rates.  
(2) To analyze the deformation characteristics of soft rock under various working conditions more clearly, we extracted deformations under different lateral pressure coefficients. Figure 23 illustrates that at \(K_{0}=1.25\), deformation progressively extends into the surrounding rock, exhibiting consistent trends across different positions. Surface displacement is uniformly distributed over the affected area. Due to the extensive affected area, the strain in the surrounding rock remains moderate, characterized by 'change but not collapse'.  
(3) From Figures 19-21, it can be observed that when \(K_{0}=0.5\), the grouping deformation characteristics are obvious. The surrounding rock at 6 m is basically not deformed, the relative displacement between the rock strata is 1.7 cm, and the displacement of the separation layer is 7 cm. The relative displacement between rock strata only accounts for  
Figure 23: Deformation process curve—buried depth of 100 m, \(K_{0}=1.25\).  
Figure 21: Deformation process curve (buried depth of 100 m, \(K_{0}=0.5\)).  
Figure 22: The position of deformation propagation.  
(1) It can be seen from Figures 27 and 28 that when \(K_{0}=1.25\), the failure circle develops rapidly at the initial stage of deformation. When the surface deformation is 3 cm, the failure circle develops to the position of 6 m, but the position of the failure circle remains steady after this. However, the local loose circle continues to develop.  
(2) Based on the previous analysis and the field-measured loose circle, it can be concluded that the deformation and failure process of surrounding rock under different stress conditions shows different characteristics. For the specific situation, the surface displacement can be used to infer the location of the expansion of the internal failure zone of the surrounding rock. From the calculation results, in both cases, when the surface deformation is 8 cm, the failure zone develops to the position of 6 m, but the development rate is large when the lateral pressure coefficient is large. Compared with the uniform surrounding rock, the development speed of the failure zone is faster in the case of steeply inclined thin-layered soft rock.