## 4 Results
### Results of the Evaluation of Geological Factors
#### 4.1.2 Engineering Geological Conditions  
(1) Soft Soil Thickness  
The thickness of soft soil directly affects the stability of underground space, and in underground engineering projects, soft soil is often a predisposing factor leading to structural deformation and surface settlement. Soft soil is very developed in Nantong City, with its thickness varying greatly, ranging from 10 to 40 m. The burial depth from the top plate is generally between 0 and 20 m. It mainly consists of silty soil, powdery clay, etc., and has the characteristics of low shear resistance, high water content, and large pore ratio. As shown in Figure 6, the soft soil in the study area is mainly distributed in the river and lake phase sedimentary layers in the northwest, southeast, and north coasts, with strong mobility and poor stability. These features have a greater impact on the development of underground space, making it an unsuitable area for development. The areas suitable for  
Figure 4: Suitability distribution based on landform. underground space development are mainly located in Rudong County, Rudong City, and other areas lacking soft soil. (2) Characteristics of Rock and Soil Mass  
The comprehensive characteristics of rock and soil directly affect the bearing capacity and deformation of underground structures. Different types of rock and soil have varying deformation characteristics and permeability, thus influencing the stability of underground space differently. The study area is widely covered by loose sediments of the Quaternary system, with a thickness of 240-350 m. In the upper part of the area, there are loose sandy and clayey soils, accompanied by the distribution of silt, resulting in poor stability. In the middle and lower parts, the water-bearing rock group consisting of coarse sand and  
Figure 5: Suitability distribution based on slope. Figure 6: Suitability distribution based on soft soil thickness. gravel is highly developed. Clayey soil is prone to softening and deformation under force, potentially causing problems such as collapse and soil settlement during the excavation and construction of underground space. In contrast, sandy soil exhibits better permeability and good drainage capacity, conducive to the drainage and ventilation of underground space, thereby reducing the risk of elevated groundwater levels and flooding. Compared to clayey soil, sandy soil is more stable and generally has better bearing capacity. Gravel boasts high bearing capacity and compressive strength, making it suitable for supporting the foundations of underground structures and capable of withstanding larger loads. In the Rugao economic development zone, Rucheng Street, and Xiayuan Town, the distribution of geological strata is uniform, with a high proportion of gravel and sandy soil. In Langshan Township Street, Chongchuan District, and the Five Mountains area, exposed bedrock with no distribution of cohesive soil contributes to relatively stable foundations. These areas are all suitable for development. The second-best suitable development areas are mainly distributed in the hinterland, and their stratigraphic soils are uniformly distributed, with a small amount of pulverized clayey soil. The general suitability of the development area is scattered, and is generally located in the vicinity of the near-shore zones of rivers and lakes. With strong water-rich subterranean rock formations that are greatly affected by groundwater, its development requires attention to the geological hazards caused by landslides and the collapses caused by groundwater. Most of the coastal areas are affected by poorly stabilized soil bodies, such as silt and soft soil, so they are not suitable for the development and construction of underground space, and the suitability of underground space development is poor.