ABSTRACT
Aiming at the 5.9 m inner diameter segment structure of a metro tunnel in a coastal city of China, the bearing performance and damage law of segments under extreme unloading conditions are experimentally studied based on the full-scale test loading platform of Tongji University. The test results show that in the design load stage, the horizontal and vertical convergence deformation and internal force of the segment increase linearly with the change of external load, and the increase is less than 18%. The maximum deformation and internal force of the segment mainly occur at the arch waist. After entering the unloading stage, the transverse and vertical convergence deformation of the segment increases obviously. The maximum transverse and longitudinal deformation of the structure in the final failure state is 185 mm and 189 mm respectively. Hence, it means that the tunnel enters the unloading condition when the lateral deformation rate of the tunnel is greater than 20% in the actual project. At this time, reinforcement measures should be taken as soon as possible to intervene against the impact of lateral unloading on the tunnel structure. The corresponding transverse convergence deformation of the first longitudinal seam bolt when yielding is 149 mm, and the corresponding transverse convergence deformation of the first circumferential seam bolt when yielding is 169 mm. In the final failure state, segment cracks mainly occur at the top, bottom and the arch waist of the tunnel structure. At the same time, the convergence deformation of segment structure at these locations is relatively larger. The experimental results can provide a scientific basis for optimizing segment structure design and carrying out structural safety evaluation.
