Study on the impact of partial unloading on the structural deformation of prefabricated segmental subway tunnels

To address the issue of deformation in prefabricated segmental subway tunnels caused by partial excavation unloading which affects the safety of subway operational safety. In this article, the subway tunnel of Zhengzhou metro, which partial unloading due to pit excavation on the underground expressway reconstruction project, was taken as an example to study this issue. First, the mechanism of partial unloading of upper bar pit excavation on the mechanical response of lower existing tunnel structure was investigated. Then, a three-dimensional finite element model was developed to simulate the interaction between the excavation pit and the tunnel. The influence of partial unloading caused by strip foundation pit excavation on the mechanical response and deformation characteristics of underlying existing tunnel segment structures were analyzed. Finally, the reinforcement effects of tunnel deformation control measures were compared, and the optimal solution to control tunnel deformation affected by partial excavation and unloading was proposed. Results indicate that the tunnel structure mainly produces overall uneven uplift, which is affected by the partial unloading of the upper bar pit excavation. The vertical displacement at the top of the tunnel is the largest, the vertical displacement near the centerline of the pit is significant, and the maximum vertical displacement usually occurs at the joints. The force of bolts generated in the prefabricated segment structure under partial unloading conditions was tensile force, which was related to the variation trend of the segment joints and the magnitude of the bending moment at the joint locations. After comparing the different methods, portal reinforcement and counterpressure can significantly control deformation generated by partial unloading.