Abstract: A two-stage conjoint analysis method is proposed for safety analysis on tunnel structures under unloading. In the first stage, a three-dimensional (3D) stratum-structure model is established according to the unloading condition above tunnel structures, and the earth pressure and earth equivalent stiffness around tunnel structures are calculated. In the second stage, a 3D structure-load model is established, and segment stress, bolt stress, convergence deformation and joint opening are analyzed based on the earth pressure and earth equivalent stiffness calculated in the first stage. The analysis method is applied to a tunnel with a foundation pit above. The results show that the vertical deformation and
convergence deformation of the tunnel structures under unloading can be rationally estimated, and the cracking and falling of concrete in the joints between segments and the inner side of the waist of tunnel structure should be paid attention to, where the steel ring reinforcement should be carried out when necessary. By using the two-stage conjoint analysis method, the advantages of the stratum-structure model and the structure-load model are fully played, which can provide a new idea for evaluating the safety status of tunnel structures under unloading.

Key words: tunnel structure, upper unloading, foundation pit, stratum-structure model, structure-load model, two-stage
conjoint analysis