Three-dimensional numerical simulation of the Shiraz subway second line – influence of the segmental joints geometry and of the lagging distance between twin tunnels’ faces

Abstract

Due to the widespread need for the urban transportation network, construction of underground railway lines by mechanised shield-tunnelling increases in metropolises. The case of the Shiraz (Iran) subway twin tunnel construction is an example of transport network extension. In the present paper, a three-dimensionalnumerical model was developed to investigate the tunnel’s lining behaviour and the ground displacement surrounding the second line of the Shiraz subway tunnels. All the excavation phases of EPB-TBM (Earth pressure balance, Tunnel boring machine) were simulated using the finite difference method. In most numerical simulations, the segmental joints were assumed parallel to the tunnel axis, despite the fact that the longitudinal joints make an oblique angle with the tunnel. Three kinds of lining pattern were then considered: continuous, straight and oblique joints. The numerical results indicated the necessity of using segmental lining with oblique joints pattern to obtain an accurate evaluation of the structural forces applied in segmental lining. The excavation of a new tunnel near to an existing tunnel with high lagging distance between twin tunnels leads to increase in the normal and the longitudinal displacements, in the internal lining forces and to a decrease in the ground surface settlement above the tunnels.