ABSTRACT
On account of their low permeability, clays respond to tunnel excavation with a considerable delay. This is favourable for the interplay between ground, tunnel support and tunnelling equipment and, inter alia, also for the stability of the tunnel face. Nevertheless, even when tunnelling through practically impermeable clay deposits, the face may fail under certain conditions. The latter represents the subject of the present paper. Specifically, based upon a simple but accurate face stability model and a well-known empirical relationship, this paper provides generic answers to two important questions: which geotechnical conditions would result in an unstable face, thus necessitating, e.g., closed-mode TBM operation? and which conditions would be prohibitive in this respect, i.e. the required face support pressure would be beyond today’s technical feasibility limits, so that soil improvement measures would become indispensable?
