| The number of tunnels constructed at a severe condition such as urban tunnel is increasing upon improvement of mountain tunnelling technology in recent years. Short bench cut method is generally adopted as a conventional method in order to prevent collapse of tunnel face by dividing section of heading. However, with progress of auxiliary methods, full-face method with reinforcing heading is increasingly applied even in poor ground by means of immediate ring closure in such way that convergence and subsidence of ground surface are restrained by closing support ring immediately after excavation. Although immediate ring closure is empirically known as a useful excavation method, the mechanism of restraint effect on subsidence and the means of quantitative evaluation for the effect have not been clarified. In this paper, 3D numerical analysis where excavation process is modelled faithfully is performed to compare behaviours of tunnel and its surrounding ground between full-face method adopting immediate ring closure with auxiliary bench and conventional short bench cut method, assuming shallow tunnel in poor ground at urban area. Moreover, applicability of 2D numerical analysis is examined to reproduce the behaviour of tunnel excavated by full-face method with auxiliary bench. |
