# Influence of Deep Foundation Pit Excavation on Adjacent Pipelines: A Case Study in Nanjing, China
## 3 Analysis of Measured Data  
To ensure the safety of the underground pipelines during the excavation of the foundation pit, monitoring points were set up and monitored for two pipelines--a water supply pipeline and a gas pipeline--located on the south side of the pit. As illustrated in Figure 2, these monitoring points, named P1-9 and P10-18, were placed approximately 10 m apart. For ease of analysis, a rectangular coordinate system was established, with the central axis line in the direction of the south side of the pit as the x-axis and the edge of the west side of the pit as the y-axis. The horizontal distance of the pipeline settlement monitoring points from the west edge of the pit was represented by the x-coordinate. The final settlement data from all monitoring points, along with the monitoring data for the entire excavation process at characteristic points, were selected. These data were used to plot the time history curve of vertical displacement at characteristic points and a comparison chart of the final vertical displacement of the pipeline. Figure 3 depicts the time history curves of the vertical displacement for characteristic points along two pipelines. The vertical displacement at these monitoring points generally showed a downward trend, as shown in the figure. Moreover, as the foundation pit excavation progressed, the amount of settlement gradually increased. The settlement amount increased as the monitoring points' x-coordinates increased (i.e., as they became closer to the center of the pit), but the rate of increase diminished over time. Upon completion of the excavation, the maximum settlements of gas pipeline P1-8 (diameter \(\varphi\) = 1200 mm, wall thickness \(t\) = 5 mm) and water supply pipeline P9-16 (\(\varphi\) = 1000 mm, \(t\) = 5.25 mm) were 33.4 mm and 29.2 mm, respectively. The settlement of pipeline P1-8 exceeded the deformation warning threshold. A comparison of the settlement data at the same x-coordinates for both pipelines at identical time points revealed that the gas pipeline consistently experienced greater settlement than the water supply pipeline. This discrepancy was attributed to differences in the location, burial depth, and material properties of the pipelines. Figure 2: Typical cross-section of the excavation (Section B-B). The final settlement of the pipelines following the completion of the foundation pit excavation is illustrated in Figure 4. As can be seen from the figure, for the monitoring points with the same horizontal coordinates, the final settlement of the gas pipeline (P1-8) was larger than that of the water supply pipeline (P9-16), and the closer to the middle of the pit, the larger the difference, with a maximum difference of 9.6 mm. With the excavation of the pit, the pipeline experienced uneven settlement. After the pit excavation was completed, the pipeline finally was groove-shaped, with large settlement in the middle and small settlement at both ends. The difference in the settlement of the monitoring points in the range of 40-140 m in the horizontal coordinate was small, while the final difference in the settlement of the monitoring points in the range of 0-40 m was larger, which is consistent with the conclusions in the literature [17]. It can be seen that the largest uneven settlement occurred in the 20% of the excavation length at the end of the foundation pit. During the excavation process, pipelines located near the ends of the pit required special protection. Common protection methods included relocation, suspension, tying, and isolation. Typical cases of the impact of pit excavation on neighboring pipelines were selected [18,19], and the pipeline monitoring data from this project were compared and analyzed. In addition to the different strata of the pit, other factors such as pipe material, pipe depth, and pipe spacing from the pit were basically the same or similar in the three cases.