Load bearing characteristics of a new support structure for rapid repair of damaged tunnels and related field tests

To address the need for rapid repair and emergency support of traffic capacity in damaged tunnels, this study proposed a new structure combining arches assembled from steel plates and shotcrete with high early strength and performance (hereinafter referred to as the steel-shotcreting structure) based on the concept of "modular assembly, rapid formation, and temporary-permanent combination". It is designed to replace the traditional cast-in-place concrete arch repair technique. Based on the principle of equivalent stiffness, the load borne by the steel arches in the new support structure was analyzed. Numerical analyses were performed to examine the influences of steel arch spacing and the types of I beams and channel steel on the load-bearing capacity of the new structure under varying surrounding rock and tunnel span conditions. The new steel-shotcreting structure was applied in a tunnel in China and its load-bearing capacity was compared with that of traditional steel arch support structures. The results show that its load-bearing proportions were 13% and 18% under the conditions of grade IV and V surrounding rock, respectively. In large-span tunnels with unstable surrounding rock, reducing the longitudinal spacing of plate steel arches, upgrading the channel steel model for longitudinal connectors, and increasing the size of I beams can enhance the load-bearing performance of the whole structure. Compared to traditional steel arch support structures, the new structure is more adaptable, flexible, and stable and demonstrates better support effects. The research findings provide a theoretical basis and technical support for the rapid repair of damaged tunnels.