Cross Section Effects on Convergence-Confinement Method in Multi Stage Tunnel Excavation

Dimensionless coefficient () in convergence confinement method shows the relaxation of stress in the wall of the tunnel at different excavation movements. This factor is considered a constant number in previous studies and tunnel geometric characteristics (such as depth, cross-section shape, radius, soil material, etc.) are not included in its determination; however, ignoring these effects can cause significant errors in the analysis of tunnels. In this study, the effect of excavation pattern and tunnel cross section shape on stress reduction factor has been investigated. For this purpose, at first, by considering the conventional cross sections of the tunnel (circular, horseshoe, and double arch), using finite difference numerical simulation software FLAC 2D and FLAC 3D, the type of excavation (full face and multi face) was investigated in estimating the stress reduction factor by considering depth, radius, and different points around the tunnel. Finally, studies were carried out between the 2D and 3D analyses and the experiments conducted at Karaj Subway to verify the results obtained from the above 2D analyses. The results of this study show the important effect of tunnel cross-section on determining stress reduction factor. Using variable stress reduction factor in the all-around of tunnel cross section can lead to more realistic simulations of complex behavior of tunnels during the excavation. Moreover, this method can be used for the analysis and design of tunnel due to its time saving nature and at the same time sufficient accuracy.