## 4 Numerical Simulation Analysis of Slope Instability
### Analysis of Finite Element Simulation Results
#### 4.4.2 Slope Instability Analysis  
Figure 12 shows the displacement distribution characteristics after excavation, and Figure 13 shows the surface displacement vector diagram.  
Figure 11: Distribution characteristics of initial displacement. (**a**) Displacement along slope. (**b**) vertical displacement.  
Figure 10: Initial ground stress distribution.  
Figure 12: Displacement distribution after excavation. (**a**) Resultant displacement. (**b**) Displacement along slope direction. (**c**)Vertical displacement.  
Figure 13: Displacement vector diagram after excavation. (**a**) Slope surface displacement at the beginning of excavation. (**b**) Slope surface displacement after balance after excavation.  
According to the finite element calculation results:  
1. After the completion of the excavation, due to the influence of excavation unloading, the settlement and deformation of the left bank slope recover rapidly. Relative to the initial in situ stress state, the displacement is mainly vertical and upward, indicating that under excavation unloading, the left bank slope has rebounded in the excavation range and nearby soil. Due to the poor mechanical properties of the rock mass in the upper strongly weathered area, the slope in the strongly weathered area is mainly deformed along the slope direction, and the excavated slope in other parts is mainly deformed by unloading and rebounding toward the excavation free face. At the initial stage of excavation, the surface deformation of the slope mainly occurs downward, and the displacement of the slope before excavation is significantly greater than that after excavation. In the later stage of excavation, the whole body moves upward, and the displacement within the excavation range is significantly greater than that of the other parts.
2. When the excavation is completed, and the whole slope is balanced, the incremental deformation of the slope is between 3-160 mm, and the maximum value mainly occurs in the block with an elevation of 465-480 m and is surrounded by faults FZ8 and F15. The deformation of the front edge of the excavated slope is significant, and the rear rim is small.
3. It is worth noting that in the displacement along the slope, there is a prominent area with large displacement at an elevation of about 615 m above the upper edge of the excavation. The displacement along the slope is about 1.5-2.5 times that of the excavation platform at an elevation of 590 m. This phenomenon may be caused by rebound deformation of the foundation soil due to excavation unloading. However, due to the existence of fault F1, the deformation is not coordinated, and the fault interior becomes loose. This may be one of the geological causes of the landslide on 19 July 2016.  
The slope stability is analyzed through the shear strain concentration area caused by rainfall after excavation. Figure 14 shows the slope plastic area distribution characteristics after recess, and Figure 15 shows the shear strain rate concentration area development after rain.  
Figure 14: Distribution of slope plastic area.  
The results show that with rainfall infiltration, the shear strain concentration area first appears at the internal structural plane of the slope and extends along the structural plane to the front and back edges of the slope. When the excavation is completed and balanced, the shear strain concentration area penetrates the entire excavation slope. The plastic area of the slope is dominated by shear yield. Tension shear yield exists locally at the structural plane and the excavation-free face. After the excavation is completed and balanced, the rock mass above the fault has entered the plastic yield state, and the slope is close to the critical state at this stage.