The new approach has been further employed to develop ‘design charts’ to estimate the face collapse pressure of TBM tunnels excavated in weak or very weak HB rock masses with a poor quality. To limit the applicability of the approach, however, it has be reminded that the HB failure criterion assumes an isotropic rock mass behaviour, and that it should only be applied when the structuresize, relatively to the spacing between discontinuities, makes it possible to consider the rock mass as a ‘continuum’ instead of a blocky structure [42]. Therefore, there sults presented here in are only applicable to heavily fractured rock masses where these assumptions are valid and, forinstance, they are not applicable to unstable blocks or wedges defined by intersecting structural discontinuities. Similarly, assolutions of the limit analysis problem, solutions presented herein do not consider the deformations at the tunnel face and,for instance,they do not account for squeezing failures associated to high deformations of the material when subjected to high stresses [43]. As a consequence, the use of the design charts presentedin Fig.12 should be made in a wider design context that consider so ther factors that could influence the success of the project