For an analysis of the stability of an underground excavation in two dimensional space, the convergence–confinement tunnel design process is usually applied to investigate the interaction of support and ground of the excavation. In order to achieve the design process, a coefficient of stress release is defined. The stress release process is realized using a numerical software UDEC, which is validated by the closed-form solution of a circular tunnel. The stability of a cavern in rockmass is analyzed based on the convergence-confinement method using UDEC. The displacement around the cavern is investigated. The numerical results show that the magnitude of displacement depends on the stress release, i.e., the installation of support.
During excavating an underground excavation, the distributions of stresses and displacements around the excavation belong to a problem in the three dimensional space. However, the stability of a tunnel is usually analyzed using a two dimensional numerical method, which will cause the inaccuracy in the design of tunnel support. Most two-dimensional numerical analysis for excavation assumes plane strain conditions. However, the plane strain conditions are only applicable to tunnel sections far from the advancing face. Curran et al. (2003) presented that in the numerical simulation of a tunnel, if the tunnel is first excavated and a passive support system installed thereafter, the support system will carry no loads. This is because all deformations would have taken place before the support is installed. On the other hand, if the supported is installed in the model before the tunnel is excavated, the support system will be exposed to the entire induced loading, a scenario that would arise only if the support were to be installed before any displacements whatsoever of the excavation boundary occurred. This would lead to conservative design since in reality some degree of stress relief always occurs by the time support is installed.
If the two dimensional numerical method is used to investigate the stability of a tunnel. The displacements around the tunnel must be estimated before the installation of the support. Hoek (2001) gave a first estimate of tunnel squeezing problems by plotting the curve of convergence against the ratio of rock mass strength to in situ stress. The plot indicated that the permissible convergence of a tunnel was not a constant, but related to the strength of rock and the in situ stress. These evidences illustrate that the pressure of the support is not simply determined by strain levels, but also by rock mass property, field stresses, and so on. The permissible convergence of a tunnel is different due to the different rock mass.
The objective of this paper is to using a numerical method to analyze the stability of a tunnel based on the concept of convergence confinement. The process of stress release around an excavation during excavating is considered. The interaction between the support and displacement is investigated.