Analysis of the evolution of road tunnels equilibrium conditions with a convergence–confinement approach

Since both lining structure and rock mass exhibit delayed behaviour, tunnel equilibrium conditions evolve with time. After discussing existing work done on different aspects of long-term tunnel behaviour, the aim of this paper is to “understand” the influence of rock mass and lining degradations on the long-term stability conditions of the tunnel by means of the convergence–confinement method. In order to represent the effects of degradation on tunnel long-term conditions, specific degradation models are selected according to the disorders identified during principal inspections of road tunnels in Switzerland. By simulating the reduction of the mechanical properties of both the rock mass and the lining or by using creep models, it is possible to assess the influence of the main degradation processes on tunnel stability. The results are interpreted in terms of tunnel safety factor. The presented approach for the determination of the long-term behaviour of tunnels, although valid for simple tunnel geometry and field stress conditions, allows to roughly estimate the influence of significant degradation processes that affect the rock mass and the supporting structure. Though this approach results in some simplification, it may be generalised and adopted with more refined numerical analyses for improving the assessment of tunnel long-term conditions.