Effect of the lay distance of a stiff support on the applicability of the Convergence-Confinement method

Abstract

The use of the classical ConVergence ConFinement (CV-CF) method which is a simplified tunnel design tool is very extended nowadays. A major limitation of this plane-strain approach is encountered when it is applied to the study of tunnels with a stiff lining near the tunnel face. Under this configuration, the pressure exerted by the ground over the lining can be underestimated. The reason stems from the fact that this method is not able to simulate the tridimensional arch effect taking place between the ground and the lining. To account for this ground-support interaction, some authors have proposed to enhance the classical CV-CF method by resorting the so-called implicit methods. However, these approaches still show some limits for very stiff lining. In this paper, the applicability domain of the existing CV-CF methods when applied to the design of full-face excavated circular tunnels with a stiff support system is discussed. The results of the ground-lining equilibrium state obtained with the plane-strain approaches are compared with the equilibrium state obtained with an axisymmetric numerical model which can properly capture the arch effect taking place at the vicinity of the tunnel face. A sensibility analysis with a focus on the support lay distance has been carried out. Finally, a simple chart that clarifies which CV-CF method is more adapted to each tunnel configuration is proposed.