Comparing the limit equilibrium method and the numerical stress analysis method of tunnel face stability assessment

Abstract

The aim of this work is to compare two approaches for evaluating the stability of a tunnel face that is reinforced by bolts: the limit equilibrium method of Anagnostou & Serafeimidis (2007) and the numerical stress analysis method. The comparative calculations have been carried out in respect of a dry – frictional soil. The numerical solutions were obtained by employing the finite difference code FLAC3D assuming an elastic, perfectly plastic material obeying the Mohr-Coulomb yield criterion. The paper starts with the relatively simple case of an unreinforced tunnel face, comparing the model predictions concerning the necessary support pressure and discussing the influence of tunnel shape, in situ stress and dilatancy. The calculations show that the limit equilibrium method and the numerical stress analysis method provide results which are close to those obtained by other existing analysis methods. The second part of the paper investigates the reinforcing effect of the bolts. The numerical stress analysis method shows clearly that shear failure of the soil around the bolts governs the bolt-soil interaction ahead of the face, suggesting that the bond strength (i.e. the resistance of the interface between grouted bolt and soil) to be used in the limit equilibrium methods should take due account of the shear resistance of the ground and thus the normal stresses prevailing in the core ahead of the face. As shown in the paper, this can be done reasonably well by means of a simplified procedure.