## 4 Results
### Results of the Evaluation of Geological Factors
#### 4.1.3 Hydrogeological Conditions  
(1) Groundwater Corrosion  
The corrosiveness of groundwater refers to its ability to dissolve or soften underground rocks and construction materials by chloride ions and sulfate ions, thereby reducing the strength of the attached rock mass and soil layers used as construction materials. The corrosiveness of groundwater has a long-term impact on underground space, so attention should be paid to the use of corrosion-resistant materials, the design of protective layers, and the strengthening of monitoring and management of groundwater environments during development. In most areas of Nantong City, the corrosiveness of groundwater exhibits moderate or weak levels. As shown in Figure 9, the corrosiveness in coastal areas of Nantong City is enhanced due to seawater intrusion, presenting moderate to strong levels of corrosion, resulting in poor development suitability. These areas are mainly distributed in Chongchuan District and some inland areas with abundant groundwater resources. The corrosiveness of groundwater in other land areas is weakly acidic or weakly alkaline, making them suitable or relatively suitable for development. Figure 8: Suitability distribution based on subgrade bearing capacity. (2) Density of Water Systems  
During the construction phase, the groundwater level and groundwater flow in areas with concentrated water systems can affect the engineering construction of underground space. In the operational phase of underground space, strong rainfall can cause a sudden increase in groundwater levels in densely watered areas, affecting the normal use of underground space and increasing maintenance costs, rendering them unsuitable for underground space planning and development. Utilizing the Euclidean distance between water systems to reflect their density, the zoning of the suitability for underground space development is conducted. As depicted in Figure 10, areas such as Outang Town, Haian Town, and Chengdong Town in Haian City, as well as Chengdong Street, Xinchengqiao Street, and Hepingqiao Street in Chongchuan District, have densely packed water systems, making them unsuitable for underground space development. Areas closer to water systems will be affected by the rise in groundwater levels, hence they are categorized as less suitable for development. Conversely, towns such as Chae, Yuanzhuang, and Shuangdian have no water system or lake distribution infrastructure and are far away from other water systems, making them suitable or secondarily suitable development areas. (3) Water Content of Water-Bearing Formations  
The water content of water-bearing formations, which represents the ability of underground rock formations to produce water, is one of the most important factors affecting the construction of underground engineering projects. The support structure of underground space interacts directly or indirectly with rock layers and soil, elevating the risk of geological disasters such as surges and sand bubbling in areas with high water content. This, in turn, escalates the challenges in underground space development construction. The water-bearing formation in Nantong City is primarily composed of a Quaternary loose rock pore aquifer which is of a single type, and the maximum water output can reach 2000 m\({}^{3}\)/d. The development suitability grade is divided according to the water content of the water-bearing formation, as shown in Figure 11. The water content of the rock groups along the coastal area of the economic and technological development zone of Haimen City, Haihua Town, and Yinyang Town ranges between 1500 and 2000 m\({}^{3}\)/d,  
Figure 9: Suitability distribution based on groundwater corrosion. making it unfavorable for the development of underground space, and it is an unsuitable development area. The Haian coastal economic zone is a generally suitable development area, with rock group water content ranging between 1000 and 1500 m\({}^{3}\)/d.