SUMMARYThe stability analysis of a tunnel excavated in a water-saturated frictional soil is investigated in the light of a failure design approach. The soil strength properties being classically formulated in terms of effective stresses, it is first shown how the effect of seepage flow generated by the excavation process, may be accounted for in such an analysis by means of driving body forces derived from the gradient of an excess pore pressures distribution. The latter is obtained as the solution of a hydraulic boundary value problem, in which both water table evolution and soil deformability can be neglected. A variational formulation of this hydraulic problem in terms of filtration velocities is then presented, leading through appropriate numerical treatment, to a search for the minimum without constraints of a quadratic functional (hybrid formulation), which is formulated by a finite element method. Some numerical examples are given, which provide ample evidence of the crucial role played by seepage forces in the tunnel face stability, since the factor of stability may be divided by as much as three. The influence of such parameters as the tunnel relative depth or soil anisotropic permeability is finally discussed, thus offering a first illustration of the various capabilities of this numerical tool.