## 3 Numerical Simulation
### Result Analysis
#### 3.3.2 Pipeline Settlement Analysis  
The reaction of the aviation oil pipeline adjacent to the foundation pit for soil excavation was analyzed. A total of 20 points were set along the pipeline in the \(x\)-axis direction (see Figure 3) at intervals of 20 m, with the end \(O\) of the foundation pit as the starting point. Figure 8 shows the variation pattern of pipeline settlement in the four stages during excavation. It can be found that the distance between the foundation pit and the pipeline had a significant impact on pipeline settlement. The maximum settlement of the pipeline occurred in the section adjacent to the foundation pit. Upon completion of the excavation, layered excavation resulted in pipeline settlement of 3.2 mm at minimum and 8.1 mm at maximum. The final pipeline settlement in bench excavation showed a similar pattern to that in layered excavation, with a large settlement in the middle and a small one at both ends. Bench excavation caused a pipeline settlement of 3.4 mm at minimum and 12.5 mm at maximum, with the amount of variation reaching 9.1 mm. The overall pipeline settlement caused by bench excavation was larger than that caused by layered excavation. The settlement of pipeline caused by bench excavation showed great variation, which is not conducive to the safety of the pipeline. Point A, 70 m away from point O, was the pipeline point nearest to the foundation pit. Figure 9 shows the changes in vertical pipeline settlement at point A. The curve presenting the variation of pipeline settlement with the construction stage of layered excavation was in the form of steps. The vertical displacement of the pipeline experienced several abrupt changes, from 14.41 mm at the initial stage to 5.25 mm at the final stage. These abrupt changes were caused by the advancement of the excavation face of foundation pit soil toward the cross-section at point A. The settlement changes caused by each excavation to point A were 0.73 mm, 2.68 mm, 2.62 mm, and 2.11 mm. In bench excavation, the pipeline settlement curve showed a single abrupt change. At the third excavation stage, when the excavation face reached the cross-section at point A of the aviation oil pipeline, the  
Figure 8: Final settlement of the pipeline. Figure 7: Horizontal displacement after the completion of excavation. pipeline settlement started to rise from 14.20 mm. At the seventh stage, the excavation face passed through the cross-section at point A, and the vertical pipeline settlement fell to a minimum of 6.09 mm. At the eighth stage, the vertical pipeline displacement was 7.60 mm due to soil creep. In the subsequent stages, the vertical displacement remained at 7.60 mm. The maximum amount of variation throughout the excavation stages was 8.11 mm. By comparing the two excavation methods, it was found that layered excavation led to small vertical pipeline displacement in both magnitude and rate of change; therefore, it is more preferable for pipeline safety. According to the analysis of horizontal displacement and settlement of soil near the pipeline, the maximum horizontal displacement of the soil near the foundation pit caused by layered excavation was only 0.5 mm less than that caused by step excavation, while the maximum settlement of the soil near the foundation pit caused by layered excavation was 35.2% (4.4 mm) less than that caused by step excavation. The above shows that layered excavation had slightly better control over horizontal displacement of soil than step excavation, while layered excavation had much better control over settlement than bench excavation. This may be because the step excavation was carried out in the same direction, and the depth of the foundation pit excavated first was larger, while the depth of the excavation later was smaller, resulting in greater settlement of the soil near the larger depth. Layered excavation always maintains a uniform depth along the length of the foundation pit; thus, this excavation method has a great difference in the influence of the nearby soil displacement.