The stability of a tunnel face with a free span and a non-uniform support

We investigate the effects of the support pressure distribution and of the free unsupported span on the stability of the tunnel face by means of limit equilibrium computations, which are based upon the so-called method of slices. Design equations and diagrams are introduced for the estimation of the necessary support pressure taking into account these effects assuming a linear pressure distribution. Furthermore, the errors introduced by the common assumption of support pressure uniformity are discussed for the typical distributions of face support pressure in tunnelling by slurry shields, EPB shields and compressed air shields. The error is less than 10% in the case of closed-shield tunnelling but may be significant in the case of face reinforcement.