Abstract: The authors provide an overview of the latest progress in five key technologies of shield tunnel engineering in China, including accident analysis of the main bearing during large-diameter shield tunneling and independent R&D of domestic main bearings, muck adaptability improvement technology and supporting materials for efficient tunneling in shield tunnel engineering, safety technology and materials for shield tail seal, new segment connection technology for shield tunnel engineering, and key technologies of  three-mode boring machine in urban tunnel engineering. The overview covers research background and methodology, mechanisms/principles, materials/equipment, technical verification, experimental research, and engineering applications. It is concluded that: (1) In terms of the purity of large rings and rollers, fatigue performance of the large ring, hardness uniformity of the large roller, hardenability and depth of hardening layer of the ring material, as well as the fatigue life and tensile breaking force of the retainer welds, the independently developed main bearings with a diameter of 8.01 m for super large shield are superior to the imported counterparts. The design method of large roller with real logarithmic curve is established, achieving a breakthrough in high-precision large roller processing technology and reaching an internationally advanced level. The high-precision assembly ensures that the dimensional accuracy and geometrical tolerance of parts meet the design requirements. Through product acceptance, installation application demonstrations, and engineering applications, it has been demonstrated that the use of this large-diameter main bearing does not lead to insufficient contact fatigue strength or fretting fatigue wear. Therefore, this kind of main bearing can be used in the super-large-diameter shield tunnel engineering. (2) In order to improve the rock-soil environment, focusing on complex strata such as cohesive strata, sandy strata, and hard rock strata, a series of environmentally-friendly muck conditioners has been developed including polymer foam agents with high arrangement density, polymer antiadhesion agents, gushing prevention agents, and wear-resistant dust suppressant. In addition, the active prevention and control technology for insitu improvement of the shield mud cake, gushing, and wear has been developed, and the technical system for dynamic improvement of adaptability of all-strata muck in shield tunnel engineering has been established, thus ensuring safe and efficient tunneling of the shield machine. (3) In response to the challenging issue of shield tail seal failure during shield tunneling, a new type of shield tail sealing grease with "adhesive-viscous-fluid" properties has been developed. It exhibits low temperature sensitivity (viscosity-temperature coefficient: 0.4) and strong water pressure resistance (maximum water pressure resistance: 3.5 MPa). The overall performance is improved by 1.3 times, providing strong assurance for shield tail safety. Addressing the difficulties of slurry bleeding and segregation of synchronous grouting as well as stratum settlement during shield tunneling, a filling agent for synchronous grouting has been developed. By optimizing the mix ratio of the inert filling slurry, zero  bleeding within 30 minutes as well as inert filling behind the wall were achieved, ensuring the quality of grouting behind the segment wall (i.e., grouting of cavity between excavation face and segment) and the refined control over the stratum settlement. (4) A connection form of circumferential seam insertion + longitudinal seam sliding is adopted for the new segment connection technology, enabling high-standard control of joint opening and dislocation of the tunnel. With high precision in segment fabrication and assembly, it can reduce the waterproofing difficulty of joints, enhance the quality of formed tunnels, eliminate the need for handhole setting and manual fastening operations, as well as achieve high levels of automation and construction efficiency. This approach can save construction costs and improve the assembly environment inside the tunnel. (5) The developed "EPB + slurry + TBM" three-mode boring machine is able to meet the tunneling construction requirements of almost all types of strata, including complex and varied strata, as well as uneven soft/hard composite strata. It can achieve rapid switching of tunneling modes without the need to stop or dismantle equipment. Under the TBM tunneling mode, the screw conveyor and slurry pipeline work together to discharge the muck, effectively solving the problem of difficult muck removal during tunneling in water-rich hard rock strata. The machine features a high level of mechanization and intelligence, guaranteeing construction safety and reliability while also improving construction efficiency. It enhances working conditions for workers, ultimately saving time and costs. China has achieved significant progress in shield tunnel construction, which has driven the development of shield tunnel technology not only in China but also worldwide. However, shield accidents still occur, necessitating further research and breakthroughs in equipment, materials, technologies, and management. In the foreseeable future, shield tunnels in China, especially for large-diameter shield tunnels, will continue to experience rapid development. Construction conditions will become increasingly complex, and the technical challenges will also grow. To achieve fast, safe, and healthy shield tunnel construction, it is crucial to address key issues in shield tunnel technology and make breakthroughs in critical core technologies in this field. The above key technologies still need to be continuously improved, promoted and applied, so as to promote the development of China′s shield tunnel construction towards high adaptability, high intelligence, high reliability, high benifit, high quality, high safety, and low energy consumption, forming new shield tunnel technologies suitable for the "soil" of China.

Key words: shield tunnel, large diameter main bearing, muck adaptability improvement, shield tail seal, segment connection, three-mode boring machine