Recently, the "Shencheng No. 1" shield machine successfully launched on the section from Jiangyang South Road Station to Aihui Road Station of Phase II of Shanghai Rail Transit Line 18. With the smooth advancement of the shield machine, the main line tunnel entered the segment assembly stage, marking another solid step forward in the promotion and application of the new embedded socket-type segment technology in Shanghai rail transit engineering.

According to the Shanghai Metro construction authority, traditional metro tunnel construction usually adopts bolted-joint segments, and the assembly process requires workers to tighten bolts to fix the segments into a ring and complete tunnel formation. Bolted-joint segments have the characteristics of clear force transfer and good economy. However, under long-term external additional loads, water leakage may sometimes occur at individual locations, requiring more operation and maintenance resources in the later stage. The socket-type segments currently being promoted and applied in Phase II of Line 18 represent another transformative technological application in the history of domestic metro construction. Socket-type segments were first piloted in the entrance section of Phase I of Line 18, and were subsequently further expanded in sections with suitable geological conditions such as the western extension of Line 13.

"This segment technology is similar to the mortise-and-tenon structure in ancient Chinese architecture, adopting a concave-convex connection method to tightly interlock the segments together, eliminating the need for manual bolt tightening, thereby forming a tunnel frame that is rigid in the ring direction and flexible in the longitudinal direction," said the technical personnel. Compared with traditional bolted assembly, socket-type segments have achieved a qualitative leap in construction precision, further advancing the unification of tunnel segment production standards and operating procedures. The operator controls the segment erector, and in aspects such as connector design and assembly operations, precise matching and standardized operations can be achieved, which not only saves manpower but also further improves ring formation precision and reduces the occurrence of water leakage and other issues. In addition, it can improve the tunnel's deformation resistance and enhance the stress performance of the tunnel ring, greatly reducing the pressure of later tunnel operation and maintenance.

It is reported that the section from Jiangyang South Road Station to Aihui Road Station has a total length of 1,586.694 meters. It is the longest section tunnel in which embedded socket-type segments have been applied to date, as well as the section tunnel with the smallest curve radius and the largest longitudinal slope on the line. In addition, the geological conditions along this section tunnel are complex, and during construction the shield machine must pass through soil layers with uneven hardness and soil layers containing confined water; moreover, environmental protection requirements are high, and the shield machine must pass under railways, power tunnels, and raw water pipes, and pass alongside high-voltage transmission towers. Therefore, the use of embedded socket-type segments in the section from Jiangyang South Road Station to Aihui Road Station is the first application of embedded socket-type segments in main line tunnel construction featuring long distance, small radius, large longitudinal slope, confined water involvement, and continuous crossing of important structures, making construction extremely challenging.

The contractor, China Railway Shanghai Engineering Bureau Group, has repeatedly carried out discussions on technical issues that may arise during shield construction such as upward floating and segment misalignment, and has carefully prepared construction plans for shield advancement, segment assembly, and construction management measures. A full-scale test platform was set up at the shield R&D center base for simulated assembly tests, and pre-construction practical training was conducted in advance for actual engineering personnel to improve workers' construction operation capabilities.

Shanghai Metro stated that in subsequent construction, it will continue to strengthen technical guidance, continuously summarize construction experience, promote refined shield tunnel construction from the source, improve tunnel engineering quality, and lay the foundation for the further expanded application of embedded socket-type connecting segments.