Stability of the tunnels and underground spacesí excavation face has a significant roll in the choice of the proper excavation and stabilization technique. In this paper the stability of the excavation face in the cohesive soil (C-ō) by an upper bound method for limit analysis method is studied and the pressure on the face of the excavation is computed. Considering the 3D aspect of the problem, the failure mechanism is consisted of one or two 3D broken conical blocks which fail from the face into the tunnel. In this study besides overcoming some of the limitations in the determination of the limit pressure on the excavation face, and the results are presented in the form of dimensionless graph.
Key words: tunnel, stability of excavation face, limit analysis, upper bound method