Support mechanism and effect analysis of an umbrella arch in a super shallow-buried large-span tunnel

When a tunnel passes through soft and broken surrounding rock, the excavation disturbance will cause a large amount of deformation in the surrounding rock. Tunnel advance support measures, such as pipe-roof structures, need to be adopted to control the surrounding rock deformation. The super shallow-buried flood spillway section of the Connecting Line Project of the Shenzhen Eastern Transit Expressway was chosen as the research object. The effect of a pipe-roof structure was studied using numerical calculation and field monitoring methods. The surface settlement, vault settlement, horizontal convergence, tunnel face displacement, vertical displacement, and bending moment of the pipe-roof structure were compared and analyzed. The mechanism of a long pipe-roof structure is discussed. The results show that a long pipe-roof structure in the super shallow-buried section can control the deformation of the surrounding rock to a certain extent; however, the main benefit of a pipe-roof structure is to prevent collapse caused by the excessive deformation of the surrounding rock. In addition, a starting section of one tunnel diameter length should be reserved for the construction of a long pipe-roof structure to obtain the full support effect of such a structure. Combined with the field monitoring data, the proposed numerical model was verified and the mechanical responses of a pipe-roof structure during construction were analyzed. © 2021, Editorial Office of China Civil Engineering Journal. All right reserved.