Face stability conditions with earth-pressure-balanced shields

Earth pressure balanced shields provide continuous support to the tunnel face using the freshly-excavated wet soil, which under pressure completely fills up the work chamber. EPB-shield tunnelling has been successfully applied worldwide in recent years. Under extremely unfavourable geological and hydrogeological conditions, however, face instabilities may occur. In this paper, the mechanism efface failure is analysed under drained conditions; Accordingly, a distinction is made between water-pressure and effective pressure in the chamber. The stability of the tunnel face is controlled through the combined effects of these two entities. The effective pressure can be visualized as a ''grain to grain'' contact pressure between the muck and the ground at the face. The water pressure in the chamber reduces the hydraulic head gradient in the ground and, consequently, the seepage forces acting in, front of the face. The face is thus stabilized both by the direct support of the pressurized muck and by the reduction of the seepage forces in the ground. The greater the head-difference between ground and chamber, the higher the effective support pressure will have to be. A high effective support pressure has, on the other hand, operational disadvantages such as excessive cutter wear, high torque etc.. It is of great practical interest to investigate quantitatively the relationships between the effective support pressure required and the hydraulic head in the muck for a given geotechnical situation. In this contribution, normalized diagrams are provided which allow the assessment of tunnel face stability.