Face stability of shallow circular tunnels driven under the water table: Anumerical analysis

The stability analysis of a tunnel excavated in a water-saturated frictiona l soil is investigated in the light of a failure design approach. The soil strength properties being classically formulated in terms of effective stre sses, it is first shown how the effect of seepage flow generated by the exc avation process, may be accounted for in such an analysis by means of drivi ng body forces derived from the gradient of an excess pore pressures distri bution. The latter is obtained as the solution of a hydraulic boundary valu e problem, in which both water table evolution and soil deformability can b e neglected. A variational formulation of this hydraulic problem in terms o f filtration velocities is then presented, leading through appropriate nume rical treatment, to a search for the minimum without constraints of a quadr atic functional (hybrid formulation), which is formulated by a finite eleme nt 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 influ ence of such parameters as the tunnel relative depth or soil anisotropic pe rmeability is finally discussed, thus offering a first illustration of the various capabilities of this numerical tool. Copyright (C) 1999 John Wiley & Sons, Ltd.