Behaviour of a large-diameter shield tunnel through multi-layered strata

In engineering practice, a large-diameter shield tunnel frequently extends through multi-layered strata. The behaviour of tunnels is more complex in multi-layered strata, but many designs are still constructed with the assumption that the soil is uniform. In this study, centrifuge tests and FEM analysis are conducted to investigate the mechanical behaviour of a large-diameter shield tunnel through multi-layered strata in terms of tunnel inner force and convergence. Two-layer strata and three-layer strata with different positions, inclinations, and thicknesses of the soft interlayer are considered, along with the relative stiffness of different layers. Both centrifuge tests and numerical simulation indicate that the relative stiffness of different layers and the position of the soft interlayer are the main factors affecting the behaviour of the tunnel in multi-layered strata. An increase in the relative stiffness leads to a significant reduction in the bending moment at the crown and springline. The maximum bending moment tends to appear in the soft layer. The distribution of inner force and convergence are asymmetrical owing to the inclination of the soil interface. Based on the results, some design suggestions are presented for large-diameter shield tunnels in multi-layered strata. Segment joints are discouraged near the soil interface and the ground around the tunnel can be considered to be soft soil when the interface is below the springline. In addition, the bending moment should be concerned carefully when the tunnel extends through the soft interlayer.