ABSTRACT
The stability of the excavation face is one of the main aspects to be studied during the design of tunnels. The excavation at tunnel face causes a change in the existing stresses of the ground, that tent to find a new equilibrium state, transferring the forces to the surrounding ground through a 3D arching effect. The collapse of the tunnel face can occur if the strength of the ground is not compatible with the new stress state, creating risks for workers and equipment in the tunnels and for people and structures above the ground (in the case of shallow tunnels). The failure mechanisms acting at the tunnel face are not yet fully understood as published analytical formulations give sometimes diverging conclusions. The present paper tries to shed light on the problem of the face stability. After a review of some widely used existing analytical methods, the results of parametric studies using 3D finite element method calculations are presented from which a new analytical approach applicable for tunnel under the groundwater level is derived. The case of conventionally excavated tunnels (without the application of face pressure or reinforcements at the face) is considered in the present paper.
