Abstract: With the constant progress of science and technology, tunnels constructed by shields are becoming increasingly prevalent in the development of urban underground space. In this paper, the authors, on basis of the characteristics of shield tunneling, explore the development and technological innovations associated with shield tunneling; the authors summarize the latest applications of shield tunneling in the construction of metros, municipal roads, urban railways, urban water conveyance tunnels, and urban utility tunnels. The authors address the technical challenges associated with tunnels constructed by shields, including varying geological conditions, increasing instances of river/sea crossings, diverse structural crosssections, complex ground surface conditions, and the durability of the tunnel structures. The authors present the technological innovations in shield tunneling, including shields with extralarge diameter, replacing cutters of largediameter slurry pressure balanced (SPB) shields under atmospheric pressure, multimode shields, construction of tunnels using quasirectangular shields, construction of cross passages by mechanical method, shield tunneling in ground containing very hard material and very weak material, earth pressure balanced (EPB) shields for waterrich sand/gravel strata, seismic mitigation/isolation and earthquakeresistant design for tunnels constructed by shields in high seismic intensity areas, and extralongdistance shield tunneling. The authors also analyze the progress in intelligent shield tunneling technologies, such as the identification of unfavorable geological conditions, realtime perception of equipment status, synchronous shield advancing and segment erecting, automatic shield driving technology, and intelligent shield control platform. Additionally, the authors elaborate on the trends of shield tunneling in the development of urban underground space, i.e., facilities such as metro tunnels, urban roads, utility tunnels, and urban water supply and drainage systems shall be upgraded constantly and shall form corresponding networks, which requires the constant progress of the shield tunneling technology and the intelligentization of shield tunneling. The authors also make prospects on the development trend of the shields: diversified shields characterized by "multimode, cutterfree operation, and suitability for extraterrestrial space"; intelligent shields characterized by "multisource information perception, intelligent segment erection, geological detection and prediction, and decisionmaking based on autonomous perception during tunneling"; shields with extralarge diameters, shields for extralongdistance tunneling, shields for tunneling under extrathick covers, and shields for tunneling under extrahigh water pressures.

Key words: underground space, metro tunnel, municipal road tunnel, urban railway tunnel, urban water conveyance tunnel, urban utility tunnel, tunnel constructed by shield, shield with extralarge diameter, ground containing very hard material and very weak material, intelligent tunneling technology, synchronous shield advancing and segment erecting