1. Key Technologies for Underground Engineering Excavation  

In the process of underground space development in China, very mature underground engineering technologies have been developed, among which the following three technologies are representative.

Open-cut technology (foundation pit technology). With the development of underground space in China, the number and scale of underground engineering construction projects have also increased rapidly, generating a large number of deep foundation pit projects and forming various types of foundation pit retaining and excavation technologies with a complete range. In terms of support technology, various technologies such as gravity type, braced type, soil anchor type, and soil nail type have been formed and developed. In terms of retaining methods, various methods such as simple retaining wall, timber sheet pile, steel sheet pile, steel pipe pile, cast-in-place pile, diaphragm wall, and top-down method (also called cover excavation method) have been formed and developed. The design theories and calculation methods of foundation pit engineering have also been continuously improved, and the design codes for foundation pit engineering have also developed to a certain extent.

Subsurface excavation technology (represented by tunnel design and construction technology). In China, a large amount of linear underground space is formed by subsurface excavation. Among them, two key technologies—the shield method suitable for soft soil (rock) areas and the New Austrian Tunneling Method (and mining method) suitable for hard soil (rock) areas—have both been widely applied and developed in China.

Shield method. Since China began researching and developing shield technology in the 1960s, shield construction technology has been successfully used to build dozens of vehicular and hydraulic tunnels. In terms of design methods, it has developed from the mining method based on loose rock pressure control design to the New Austrian Tunneling Method based on rock deformation pressure control design, and various computer numerical methods can be used to calculate and predict the deformation and stress mechanisms in rock masses relatively accurately. In terms of construction technology, it has developed from the drill-and-blast method to the TBM method, with an increasingly high degree of construction mechanization. Shotcrete-anchor and concrete support technologies have made great progress, and excavation cross-sections are flexible and diverse. At the same time, other auxiliary technologies, such as rockburst prediction technology and detection technology for rock properties and groundwater ahead of the excavation face, have also made tremendous progress.

Underpinning technology Due to the density of Chinese cities and the shortage of space, the development and utilization of underground space involves a large number of facility conflicts, spatial intersections, and situations requiring protection of cultural relics and the environment, which has promoted the development of underpinning technology. Underpinning technology is a general term for technologies that solve the problems of foundation treatment and foundation reinforcement required for existing buildings, as well as the problems of constructing underground engineering beneath the foundations of existing buildings and building new projects nearby that affect the safety of existing buildings.

2. Prospects for the Development and Utilization of Underground Space

With the development of China's economy and science and technology, the improvement of urbanization levels, and the implementation of urban sustainable development strategies, the development and utilization of urban underground space is showing increasingly huge benefits and potential. The development and utilization of urban underground space in China will surely move toward modernization, internationalization, and scientific development. Specifically, the development and utilization of urban underground space in China will show the following development trends:

The trend toward comprehensive development and utilization. The development and utilization of urban underground space will no longer satisfy a single function; it will be based on the overall construction and functional requirements of the city and will integrate and accommodate multiple urban functions, such as large-scale complexes that meet transportation, commercial, supply, and environmental needs. At the same time, it will no longer be an isolated spatial form, but an organic and rich spatial whole formed by the flexible combination and interconnection of various forms such as points, lines, surfaces, and volumes.

Development of planning and design theories. Based on the strategic thinking of urban sustainable development and three-dimensional urban development, underground space will be regarded as one dimension of three-dimensional urban development. Underground space planning and design theories will be gradually enriched and improved, guiding cities to extend scientifically underground.

Development of development technologies. China's current civil engineering technologies for underground space development have approached or reached the world's advanced level, but in some key auxiliary equipment and technologies, such as machinery technology, computer and electrical control technology, automation technology, etc., there is still a large gap compared with the world's advanced level, which will affect the scale and cost of underground space development. In the future, with the digestion and absorption of imported technologies and increased investment in research and development, these gaps will gradually narrow.

Regulations and management and maintenance will become increasingly complete. There will not only be complete regulations, policies, management measures, and advanced maintenance technology levels, but also a complete set of entities and management departments that promote the comprehensive development and utilization of underground space.

Environmental requirements and environmental control will receive more attention, and the corresponding technologies and processes will become increasingly mature. Unmanned urban underground space facilities will be safer and more efficient, and manned urban underground space facilities will be more comfortable and beautiful. Technologies such as landscaping, illusion effects, and ground environment simulation in the underground space environment will develop greatly. At the same time, urban underground space will be developed and utilized more from the perspectives of environmental protection, urban landscape protection, and historical relic protection.

New processes and new materials will continue to emerge. In order to reduce the cost and difficulty of urban underground space development, adapt to the combination of various forms of underground space, and meet the intersection and coexistence of multiple facility functions, efficient and economical construction processes will continue to be produced, especially with greater progress in mechanical excavation technology and construction automation technology. At the same time, new building decoration materials, especially materials for underground waterproofing and environmental improvement, will also continue to emerge.