Tunnel face stability under seepage flow conditions

The present paper investigates the problem of the stability of the tunnel face under seepage flow conditions based upon the so-called “method of slices”. This computational model improves the limit equilibrium method of Anagnostou and Kovári (1996) by treating the equilibrium in the wedge consistently with the overlying prism, i.e. without an a priori assumption concerning the distribution of the vertical stresses. Furthermore, it shows that tensile failure of the wedge may be more critical than shear failure, if the gradient of the hydraulic head in the ground ahead of the face is high. For an approximate distribution of the hydraulic head in the ground around the tunnel face, we derive a closed-form solution for the necessary face support pressure. In addition, we provide normalized diagrams, which allow for a quick assessment of the stability of the tunnel face.