## 5 Discussion
### Surface subsidence and Underground Rail Transit Construction  
Subway has now become an important means of transportation in people's daily lives. As of August 2020, Shanghai Metro has 17 operating lines (including maglev lines), 416 stations (including 2 maglev lines), and a total mileage of 705 km (including 29 km maglev lines). At the same time, four rail transit lines are under construction in Shanghai. Given the factors of geological soft soil in Shanghai, the construction of underground rail lines are greatly affected by geology, and abnormal subsidence occurs from time to time. Therefore, it is particularly necessary to monitor the subsidence of the rail transit lines that are under construction and their surrounding areas [54; 55; 56].  
The construction of Shanghai Metro Line 15 was started in August 2016 and the whole line was opened in December 2019, with underground stations. In the time span of this experimental study, the line is in the period of accelerated construction and natural consolidation of the ground, so it is of great engineering significance to monitor its subsidence, and the study on the correlation between surface subsidence and underground rail transit construction is the representative of Shanghai Metro Line 15.  
In this section, Shanghai Metro Line 15 is selected as the research object, and the subsidence field is extracted from the line and the surrounding area within 600 m. In 2016, the construction of Shanghai Metro Line 15 began, and in December 2019, the whole line was successfully completed. The total length of Line 15 is 42.3 km with 30 stations, all of which are underground stations.  
According to the monitoring of this line and its surrounding areas (the subsidence rate figure is shown in Figure 11), two obvious subsidence areas of A and B are found in Metro Line 15 (Respectively shown in a and b in Figure 11). Area a is located in Minhang District, in the south of Route 15, including Zizhu High-tech Zone Station to Jingxi Road Station. The accumulated subsidence of Area a is shown in Figure 12. Two obvious subsidence funnel areas are found on the subsidence trend line. By comparing Figure 11, the two places are the area of Yongde Road Station and the area between Yuanjiang Road Station and Shuangbai Road Station. The maximum accumulated subsidence of the two places is \(-40.2\) mm and \(-37.8\) mm respectively. Area b is located in Changning District, Shanghai, and includes Yaohong Road Station to Guilin Road Station. The subsidence of different degrees appeared in this area, and the subsidence near Wuzhong Road Station was the most obvious, with the maximum accumulated subsidence of \(-28.9\) mm. In December 2019, the two underground construction lines of Metro Line 15 converged on the Guilin Station. The intersection of the two construction lines greatly increases the bearing capacity of the ground, which is the main reason for the obvious subsidence of the area [57; 58]. After the completion of the subway, the natural consolidation subsidence becomes the main reason for the subsidence of Areas a and b [59; 60].  
In Zizhu high-tech area standing nearby, the subsidence phenomenon is more evident. The area of Minxing District of Shanghai Zizhu science park situates many universities and scientific research bases to gather hereat. Residents' living water all come from the exploitation of groundwater, and the consolidation subsidence caused by subway construction becomes the settling of the main reasons.