## 4 Slope Excavation Stability Analysis
### Slope Stability Analysis of Different Excavation Schemes
#### 4.1.1 Scheme Comparison and Selection  
According to the requirements of Code for Design of Highway Subgrade JTG D30-2015, normal working conditions and rainfall conditions should be considered when conducting slope stability analysis. The safety factor is 1.3 under normal conditions and 1.2 under rainfall conditions [29]. The numerical simulation results of the slope stability coefficient of each excavation scheme under normal working conditions are shown in Figure 5a. Figure 5b-d show the comparison of the numerical simulation results of the slope stability coefficient between the normal conditions and the rainfall conditions of each excavation scheme.  
According to the slope stability evaluation standard [30] in the Technical Code for Building Slope Engineering GB 50330-2013, the analysis of Figure 5a shows that the stability coefficient of the sixth-grade slope after excavation in the first scheme is 0.95 under normal working conditions, and it is in an unstable state. The stability coefficient of the second scheme is 1.04 after the excavation of the sixth-grade slope, which is in an unstable state. In the third scheme, the stability coefficient is 2.15 after the sixth-grade slope is excavated, and it is always in a stable state during the excavation process.  
According to the analysis of Figure 5b-d, the stability coefficients of each scheme are reduced under rainfall conditions. After the excavation of the fifth-level slope under the rain condition of Scheme No.1, the stability coefficient changed from 1.03 in the normal  
Figure 5: Slope stability factor. (**a**) Slope stability coefficient under normal conditions of different excavation schemes; (**b**) Scheme No.1 slope stability coefficient under different working conditions; (**c**) Scheme No.2 slope stability coefficient under different working conditions; (**d**) Scheme No.3 slope stability coefficient under different working conditions.  
condition to 0.99 and changed from the under-stable state to the unstable state, and from the sixth-level excavation in an unstable state to the fifth-level excavation in an unstable state. After the fifth-grade slope was excavated under the rainfall condition of Scheme No.2, the stability coefficient changed from 1.1 in the normal condition to 1.05 and changed from the basic stable state to the under-stable state, and from the sixth-level excavation in an under-stable state to the fifth-level excavation in an under-stable state. After the excavation of the sixth-grade slope under the rainfall condition of Scheme No.3, the stability coefficient changed from 2.15 in the normal condition to 2.06, which is still in a stable state.  
From the above analysis, it can be seen that the overall stability of Scheme No.1 becomes worse after the excavation of the fourth-grade slope, the overall stability of Scheme No.2 becomes worse after the fifth-grade slope is excavated, and the overall stability of Scheme No.3 is always better after the slope is excavated. Therefore, Scheme No.3 is the best excavation scheme. However, since the scope of land acquisition for this project has been approved and no new land can be occupied, Scheme No.3 cannot be implemented. The slope stability of Scheme No.2 is significantly greater than that of Scheme No.1, and the number of excavation works does not increase significantly. Therefore, it is finally proposed to adopt Scheme No.2 for excavation.