Longitudinal connection effect on initial support steel frames in tunnels—Take the traffic tunnels as examples

Abstract

Most tunnel projects are designed with cross-sectional loads, and the inhomogeneity of the longitudinal forces is ignored. In theory, such a support structure can resist large loads, but in practice, large deformation, concrete cracking, steel frame distortion, and other phenomena often occur in tunnels under poor surrounding rock conditions. Hence, the longitudinal stability of the tunnel must be considered. In this study, the mechanism of longitudinal connecting ribs (LCRs) of tunnels was investigated through element tests, theoretical analyses, and numerical simulations, and the effect of the LCRs was evaluated experimentally. The applicability of the constitutive relations and boundary conditions of the numerical model was verified. The instability mode of the steel frame reflecting the longitudinal stress gradient of the tunnel was analyzed, and the longitudinal surrounding rock pressure and the verified numerical model were applied to analyze the LCR using the load structure method. The results indicate the following: (1) LCRs can effectively improve the ultimate bearing capacity and stability of a structure and reduce the area and degree of damage; (2) Two types of instability modes occur in tunnel steel frames, and the main factor is bending failure caused by the axial force; (3) The distance sensitivity of the LCR in the tunnel is higher than the stiffness sensitivity. For large deformations of tunnels, double rows of rebars with a spacing of less than 1.5 m should be used as longitudinal connections.