## 5 Analysis and Discussion of Numerical Simulation Results  
### Study on Mechanical Behavior Characteristics of the Surrounding Rock at Stratigraphic Boundary  
#### 5.1.1 Study on the Distribution Characteristics and Evolution Law of the Stress Field  
Since the model has a symmetry, the maximum principal stress field distribution and evolution characteristics of the surrounding rock on the right side of the roadway were recorded, and the numerical simulation results are shown in Figure 11. When the roadway crosses C1d (limestone), the stress state of the surrounding rock around the excavation profile surface changes significantly (three-directional stress state becomes two-directional or one-directional stress state) due to the excavation unloading effect, which breaks the initial stable state of the rock mass, and the properties of the rock mass gradually deteriorate. When the disturbed stress on the rock mass exceeds the ultimate strength of the rock mass, the rock mass undergoes instability damage and gradually transitions to the residual flow stage, and the bearing capacity of the rock mass is substantially weakened. Therefore, it can be seen from Figure 11 (excavation step 1) that the rock mass near the excavation profile appears to have a large stress release zone transferred toward the deep rock mass. As the tunnel face advances, the distribution pattern of the stress field of the surrounding rock around the roadway tends to be stable, and there is an obvious local stress concentration area near the arch top and arch bottom at a distance of about 3 m from the excavation-free surface. When the roadway crosses the stratigraphic boundary (excavation step 12), a local abnormal stress zone is formed in the lower plate of the stratigraphic boundary (D3 zg). The stress concentration phenomenon in the area is significant, and the maximum principal stress reaches 68.5 MPa. There are differences in the distribution characteristics of the surrounding rock stress field of the roadway in different formations when crossing the stratigraphic boundary. To quantitatively investigate the relationship between the two, monitoring point A (vault) on monitoring section X (C1d formation) and monitoring point A (vault) on monitoring section Y (D3 zg formation) of the roadway were selected to analyze the variation curves of the secondary stress field of the surrounding rock when traversing different formations, as shown in Figure 12. Monitoring point X-A (monitoring point A on monitoring section  
Figure 11: Distribution and evolution of surrounding rock stress field (unit: Pa). X) reaches the peak at the position of 3 m around the roadway, which is 2 m farther than the peak stress position (1 m) of monitoring point Y-A (monitoring point A on monitoring section Y). However, the maximum principal stress at monitoring point X-A is 38.5 MPa, which is 28.8% lower than that at monitoring point Y-A (49.6 MPa). It can be seen that the region of stress concentration in the C\({}_{1}\)d formation is relatively far from the excavation surface, but the stress level is lower. In contrast, the stress concentration region in the D\({}_{3}\)zg formation is closer to the excavation face, but the stress level is higher. In addition, the concept of Stress Concentration Degree (SCD) is introduced to study the stress characteristics at different locations of the excavation profile surface. The SCD expression is shown in (11):  
\[SCD=\frac{\sigma\tau}{\sigma}=\left\{ \begin{array}{l}>\)1 stress concentration \\ =\)1 in - situ stress(undisturbed) \\ <\)1 stress release \\ \end{array} \right. \tag{11}\]  
where \(\sigma^{\prime}\) is the maximum principal stress of the rock unit after being disturbed by excavation, MPa; \(\sigma\) is the maximum principal stress of the rock unit in the original rock stress state, MPa. The monitoring scheme is shown in Figure 10, where monitoring section X is located in the C\({}_{1}\)d formation (limestone), and monitoring section Y is located in the D\({}_{3}\)zg formation (dolomite).