Tunnel Stability Analysis in Weak Rocks Using the Convergence Confinement Method

Abstract

A support system of weak rock tunnels typically consists of two components, namely an artificial support structure of combined categories and a rock mass surrounding the tunnel. Stability analysis and design methods of the artificial support structure have reached a mature developmental stage, whereas that of the surrounding rock has few progresses, especially when considering the surrounding rock as one of the support components. For this, from the perspective of considering the surrounding rock as one of the support components, an analytical method is proposed to calculate the factor of safety of the surrounding rock component, along with a stability analysis method for the complete tunnel support system (i.e. surrounding rock combined with artificial supports). This analytical method is based on an evaluation approach developed to obtain the stable ultimate deformation of the tunnel’s surrounding rock, along with the minimum internal pressure to stabilize the tunnel. The evaluation approach is on the basis of the generalized Hoek–Brown yield criterion that is considered as the non-linear behaviour of the rock mass, combined with Rabcewicz’s mode of initial instability as well as Carranza–Torres’s solution to tunnel deformation. The validity of the developed evaluation approach is verified by comparing the results to numerical simulation outputs and field data. A comprehensive parametric analysis is also conducted to investigate the effects of parameters involved in the evaluation approach on the stable ultimate deformation and minimum internal pressure. Subsequently, a suggestion is made aiming to revise the notion of ‘optimal installation timing’ of artificial supports for classical support design methods. This research forms a complete set of quantitative analysis tools for the stability and design of the support system of underground tunnels excavated in weak rocks.