## 5 Discussion
### Influence of Regional Stratagraphic Structure on Surface Subspace  
Shanghai, which is located in the southeast front of the estuary of the Yangtze River Delta, is one of the world's famous mega cities located on soft soil. It belongs to the coastal plain landform type, and its geological age is Holocene Q4 and Upper Pleistocene Q3. As the longest river in Asia, the main stream of the Yangtze River flows through 11 provinces and cities before reaching the Huangpu River in Shanghai and entering the sea. The size of soil particles carried by the Yangtze River changed significantly in the process of flowing through various regions. Shanghai has suffered from the long-term impact of the Yangtze River estuary, and a huge amount of land was lost every year. According to the \"Shanghai Water Resources Bulletin in 2019,\" the total soil erosion area of Shanghai in 2019 is 2.89 square kilometers, which accounts for 0.05% of the total land area of the city. Moreover, the sediment carried by the Yangtze River to Shanghai is mainly concentrated in particles of 0.005 mm to 0.075 mm. The particles, which are typical clay particles and accumulate to form silty soil, are relatively small. To analyze the influence of geological structure in Shanghai on regional subsidence better, we obtained the geological data of two Quaternary Geological Boreholes (the green pentagram in Figure 3) in Shanghai area. Borehole a is located near Hongqiao Central Primary School, Minhang District, Shanghai, with a final depth of 300.44 m. The borehole b is located in the south of Yuanjiang Road, Zhongchun Road, Beiqiao Town, Minhang District, Shanghai, with a final depth of 336.29 m.  
In terms of Quaternary geology, the soil layer in Shanghai is evenly distributed. The upper layer is silty powdery clay, and the lower layer is silty clay, which together constitute the typical soft soil in Shanghai. In the soft soil layer, the soil moisture content is higher, and the hardness is relatively soft. Geological layers in Shanghai are mainly divided into artificial filled soil, cohesive soil, silty clay, and powder silt soil. The specific soil layer distribution of boreholes a and b are shown in Figure 7. The soil layer in Shanghai is mostly composed of soft clay soil, which has poor engineering geological properties, such as high water content, large void, and high compressibility. According to existing research, Shanghai is dominated by clay. The natural pore ratio of the soil is 0.7-1.4, the natural water content is between 26.9-49.7%, and the compression modulus of the soil is between 2.56-6.43/Mpa(Es 0.1-0.2) [37; 38]. There exists a certain linear relationship among the three, which reflects the characteristics of high-water content, large voids and high compressibility of the soils in Shanghai. With the development of surface urbanization and excessive development of urban underground space, the soil pressure in Shanghai is increasing. In addition, due to the characteristics of the soil, the bearing capacity of the area is low, and the overall subsidence and uneven subsidence are prone to occur [39; 40]. Under the influence of geology and underlying structure, excessive subsidence occurred at the construction site of Shanghai Rail Transit Line 16 in 2011, which resulted in collapse. In 2012, the construction site of Metro Line 12 collapsed and have caused serious harm to people's property and safety. In Shanghai, the soft soil layer composed of soft clay soil is relatively thick. In terms of engineering construction, subway tunnels in Shanghai area generally pass through the soft soil layer, and viaducts and highways are all affected by the soft soil layer, thereby resulting in subsidence. Therefore, the influence of soft soil layer on regional engineering construction and daily maintenance cannot be ignored. In terms of deep bedrock, the bedrock borehole data used in this paper is about 300 m below ground.