## 5 Discussions
To more intuitively compare the pipeline settlement caused by the two excavation methods under different soil layer properties, the maximum, minimum, and average values of pipeline settlement under soil layer properties are summarized in Table 4.  
Table 4 shows that, compared with the bench excavation method, the layered excavation method had lower maximum, minimum, and average settlement values of the surface median line under different soil layer properties. Furthermore, the layered excavation method also enjoyed a smaller range of settlement variation than the bench excavation method. In general, the layered excavation method was superior to the bench excavation method in terms of pipeline settlement control. Figure 20 compares the pipeline settlements caused by the same excavation method in different soil layers. According to a comparison of the properties of different soil layer properties in the same excavation method, the pipeline settlement values in plain filled soil and clay were close, while the settlement values in silty clay were larger than those in the other two soil layers. According to a comparison of the physical properties of plain fill and clay, the main property difference is the elastic modulus, with that of plain fill being half that of clay. A lower elastic modulus improves the compressibility of soil and increases the settlement of pipeline. However, in general, the settlement values of plain filled soil and clay were similar, and the elastic modulus exerted little influence on pipeline settlement. By comparing the physical properties of clay and silty clay, under the same elastic modulus, different Poisson's ratios resulted in a great difference in pipeline settlement, as shown in the figure. A larger Poisson's ratio of soil indicates a greater deformation capacity and increased sensitivity to excavation disturbances of surrounding soil, thus resulting in greater pipeline settlement in silty clay than in plain fill and clay. In summary, according to the analysis and discussion of the settlement deformation of different pipeline soil layers and its underlying soil layers under different excavation methods, suggestions are given for adopting different construction methods under different construction conditions. When excavating the foundation pit near an existing pipeline, if the elastic modulus of the soil layer near the pipeline and the underlying soil layer is small and Poisson's ratio is large (i.e., the soil layer has high compressibility and deformation), the layered excavation method should be used for construction. If the elastic modulus of the soil layer near the pipeline and the underlying soil layer is large and Poisson's ratio is small (i.e., the soil layer has low compressibility and deformation), the impact of the two excavation methods on the pipeline is close. In this case, the emphasis of the station construction requirements should be weighed, and the appropriate construction method should be selected on the basis of the construction period, project cost, and other factors. Compared with the characteristics of layered excavation and bench excavation, the bench excavation method has the characteristics of large excavation surface, convenient construction, and fast speed, which is conducive to actual projects. Therefore, it is recommended that the bench excavation method be used for the excavation of soil layers with low compressibility and deformation. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Construction Method** & **Name of Soil Property** & **Plain Fill** & **Clay** & **Silty Clay** \\ \hline \multirow{3}{*}{Layered excavation} & Max. (mm) & 6.62 & 7.16 & 8.97 \\ \cline{2-5}  & Min. (mm) & 3.34 & 3.19 & 2.21 \\ \cline{2-5}  & Avg. (mm) & 5.37 & 5.53 & 6.14 \\ \hline \multirow{3}{*}{Bench excavation} & Max. (mm) & 8.81 & 9.87 & 12.46 \\ \cline{2-5}  & Min. (mm) & 4.07 & 3.51 & 3.36 \\ \cline{1-1} \cline{2-5}  & Avg. (mm) & 6.73 & 7.05 & 8.23 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Pipeline settlement of different soil layers.