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When shield tunnels are under complex geological conditions, tunnel damage, decreased structural performance, and shield tunnel deformation and destruction may occur. Therefore, a new type of composite segment and combined splicing joint were suggested and experimentally investigated in this study. According to the test results, the new composite segment effectively enhanced the load capacity of the segment, and the yield and ultimate loads of the segment were increased. Compared with the reference specimen, the ultimate load capacities of the proposed segments with different numbers were 23.8%, 50.0%, and 64.3% greater. Moreover, the combined splicing joint enhanced the initial bending stiffness and load capacity of the segment joint, reduced the opening displacement, and improved the rotational stiffness of the segment joint. The addition of a plug-in joint at the segment joint increased its load capacity by approximately 25%. A parametric study showed that the number and posit ion of the plug-in joint clearly affected the load capacity of the segment joint. In addition to the above test results, reasonable theoretical formulas for the new composite segment and combined splicing joint are given in this paper. [ABSTRACT FROM AUTHOR]

Underground space development is a crucial approach to addressing traffic congestion in China's cities, especially through underground construction. However, the traditional pit internal support system, particularly the concrete internal support system, has significant drawbacks. These include high energy consumption and carbon emissions, which are increasingly prominent issues. In this paper, a hollow precast concrete-filled steel tube (H-CFST) internal support system is proposed and a node connection scheme is designed. Through numerical simulation, the load-bearing characteristics of H-CFST with different aspect ratios, hollow ratios, hoop thicknesses, and hoop lengths are investigated, and the optimal design parameters are obtained. Finally, following a field application, monitoring data reveal that the precast H-CFST internal support demonstrates superior load-bearing capacity compared to the concrete internal support, successfully meeting the criteria for replacement of the concrete support. [ABSTRACT FROM AUTHOR]

The traditional segment always experiences gradual performance degradation during its service life in complex geological conditions, which results in numerous problems in the form of large deformations, seepage, and concrete cracking. Therefore, a new type of composite segment was proposed, in which an arc-shaped concrete-filled steel pipe component is embedded. A numerical model of the proposed segment under three-point bending conditions was developed and subsequently validated through laboratory tests. A parametric study was also conducted, encompassing the dimensions and locations of the arc-shaped concrete-filled steel pipe component. The results indicate that the new composite segment can effectively enhance the segment's load-bearing capacity, and both the yield and ultimate loads of the segment can be increased. Additionally, the parametric study revealed that the thickness and diameter of the pipe do not have a significant impact on the segment's load-bearing capacity, and the position of the pipe does not affect the mechanical performance of the segment. The proposed segment can potentially be used to replace traditional segments in complex geological conditions. [ABSTRACT FROM AUTHOR]

When shield tunnels are under complex geological conditions, tunnel damage, decreased structural performance, and shield tunnel deformation and destruction may occur. Therefore, a new type of composite segment and combined splicing joint were suggested and experimentally investigated in this study. According to the test results, the new composite segment effectively enhanced the load capacity of the segment, and the yield and ultimate loads of the segment were increased. Compared with the reference specimen, the ultimate load capacities of the proposed segments with different numbers were 23.8%, 50.0%, and 64.3% greater. Moreover, the combined splicing joint enhanced the initial bending stiffness and load capacity of the segment joint, reduced the opening displacement, and improved the rotational stiffness of the segment joint. The addition of a plug-in joint at the segment joint increased its load capacity by approximately 25%. A parametric study showed that the number and posit ion of the plug-in joint clearly affected the load capacity of the segment joint. In addition to the above test results, reasonable theoretical formulas for the new composite segment and combined splicing joint are given in this paper.

Underground space development is a crucial approach to addressing traffic congestion in China’s cities, especially through underground construction. However, the traditional pit internal support system, particularly the concrete internal support system, has significant drawbacks. These include high energy consumption and carbon emissions, which are increasingly prominent issues. In this paper, a hollow precast concrete-filled steel tube (H-CFST) internal support system is proposed and a node connection scheme is designed. Through numerical simulation, the load-bearing characteristics of H-CFST with different aspect ratios, hollow ratios, hoop thicknesses, and hoop lengths are investigated, and the optimal design parameters are obtained. Finally, following a field application, monitoring data reveal that the precast H-CFST internal support demonstrates superior load-bearing capacity compared to the concrete internal support, successfully meeting the criteria for replacement of the concrete support.

A concrete-filled steel tubular (CFT) yieldable support is proposed as a new passive support system in deep tunnels to improve the stability of soft rock mass, which consists of a CFT arc, a porous metal connector, and a steel tube. The connector provides a new yieldable mechanism; that is, the constant yield resistance and large deformation under compression can be determined by the void ratio and porous length of the connector. The test results indicate that the load–displacement response for the CFT yieldable support can be divided into four stages, namely, elastic stage, yieldable stage I, load transition stage, and yieldable stage II. A numerical model is then developed to investigate the deformation of a deep tunnel supported by the CFT supporting system. The results indicate that a higher concrete strength can lead to a lower deformation of the CFT yieldable support. The deformation of the tunnel roof can be effectively controlled by using higher-strength concrete. The void ratio obviously influences the deformation of the roof, and a higher void ratio tends to lead to a larger deformation. Finally, an empirical approach is suggested to predict the initial resistance and bearing capacity of the CFT yieldable support. [ABSTRACT FROM AUTHOR]