The increased complexity of the stress distribution, however, does not seem to improve the results in other cases with better rock mass properties, when the computed critical pressures are almost equal to the uniform distribution case. In addition, to validate the new failure mechanism, 22 test-cases corresponding to rock masses with low quality, as indicated by their GSI value, have been employed to compare our limit analysis results with results of three dimensional simulations conducted with FLAC3D. Two aspects have been compared: (i) the numerical value of the collapse pressure; and(ii) the shape of the failure mechanism. The obtained results suggest that the limit analysis approach proposed herein successfully approximates the FLAC3D numerical results but with a significantly reduced computational cost, so that it could be applied for fast, and relatively reliable, estimations of the pressure needed for face support in shallow tunnels excavated in heavily fractured rock masses.