This paper is to discuss TBM tunnelling in difficult ground conditions, when problems are met which may reduce dramatically the average progress rates and practical consequences may be such as to pose serious questions on the use of mechanised TBM tunnelling versus drill and blast and other so-called traditional excavation methods. Following a few remarks on rock TBM tunnelling in relation to the selection and dimensioning of the machine, the attention is posed on the limiting geological conditions which may be envisaged with respect to the use of TBM tunnelling and on the importance of geological and geotechnical investigations, in order to derive an appropriate understanding of the rock mass conditions along the line of the tunnel. The discussion is centered upon the relatively more important or difficult ground conditions including borability limits, instability of excavation walls, instability of excavation face, fault zones and squeezing. Whenever available to the authors and based on project experience, the point of view is illustrated by case examples, which give the opportunity to underline specific difficulties encountered and recommendations.
TBM excavation represents a big investment in an unflexible but potentially very fast method of excavating and supporting a rock tunnel (Barton, 1996). When unfavourable conditions are encountered without warning, time schedule and practical consequences are often far greater in a TBM driven tunnel than in a drill and blast tunnel. The unfavourable conditions can be produced by either a rock mass of very poor quality causing instability of the tunnel or a rock mass of very good quality (i.e. strong and massive rock mass) determining very low penetration rates. A prominent example is given by the recent (from 1995 to 2000) construction of the one tube 24.5 km long Laerdal Tunnel in Norway, the world's longest road tunnel.