###### Abstract
The above studies are aimed at the phenomenon of the lining structure settlement, dislocation and opening of circumferential joints under the action of soil and water load or earthquake load, but they are not applicable to the calculation of circumferential joint opening after thrust unloading. In the field of mechanical shield docking, the Trans-Tokyo Bay Highway was excavated using the slurry shield method. After the completion of tunneling, the two shields facing each other were docked together under a high water pressure at the depth of about 60 m under the sea level [21; 22]. The test boring, the filling of high-concentration slurry, the freezing and forced thawing, etc., were carried out during underground shield docking. A newly developed relative position measuring system was provided. Applying this system, the docking of two shield machines was performed accurately within a few millimeters of relative distance deviation [23]. In addition, an insertion system, a cylindrical magnifying mortar shield machine (capable of underground mechanical docking of tunnels of different diameters) was developed [24]. In China, the feasibility of applying the freezing method in shield joint of the Qiongzhou Strait tunnel was studied [26]. Using the model test of freezing reinforcement for a shield junction, the influence law of strata deformation caused by the frost heave effect was obtained and the distribution characteristics of the temperature field in the soil stratum during the freezing process were analyzed [27; 28]. In the process of the shield docking of a cable tunnel in Taiwan, broken seal plate bolts caused the ground to collapse. Ground improvement was suggested, even while using the concentric interlace docking method [29]. The possibility of long-distance and high-speed construction using a shield tunneling method was discussed through a case study on the planning of an undersea tunnel project in Osaka Bay. If the mechanical shield docking method were used, it would only take 40 months to construct the tunnel [30]. As a new technology for shield tunneling, the theoretical method of shield docking technology is not mature; in particular, the model and theory of longitudinal stress relaxation between segment rings after shield dismantling and thrust unloading are lacking. Most of the existing studies focus on the docking method and formation reinforcement, but there are few studies on the longitudinal reinforcement of the segment ring in the docking process. Currently, longitudinal reinforcement technology for segments mainly include adding steel channels, welding steel plates and erecting steel arch frames. These reinforcement measures are all empirical methods, and the influence of key parameters on the reinforcement effect has not been studied and summarized in depth. This paper takes the Qiongzhou Strait undersea tunnel to be built in China as the research object. The purpose is to solve the urgent problems, such as the instability of the lining structure and the seawater leakage caused by the longitudinal stress relaxation effect during the docking process, relying on channel steel reinforcement technology. The mechanical mechanism of the longitudinal stress relaxation effect of the segment lining after thrust unloading is analyzed. The theoretical solution for circumferential joint opening with or without steel channel reinforcement is derived using the established mechanical model. Through the numerical simulation of the typical section of the supporting project, the results of the numerical simulation and the theoretical calculation are compared to verify each other. The opening of the circumferential joints is taken as the judgment standard, and then the effect of parameters, such as reinforcement range and the quantity and model of the steel channel, on reinforcement effect is discussed. On the basis of a sensitivity analysis of key parameters, the feasibility of the segmenting reinforcement technology for a steel channel is determined, and a reasonable reinforcement scheme for a steel channel is proposed.