The TBM tunnelling technique was initially focusing on soft rock environment. As the technique developed towards stronger machines, more powerful motors and larger cutters, hard rock TBM tunnelling became a routine task. Today, there are few ground conditions where the TBM technique is not feasible. A certain, and in some cases serious limitation, is however represented by varying ground conditions, such as dykes, faults, weakness zones or even soils/soft rock, combined with hard rock. This is commonly referred to as mixed-face conditions. Variable geological conditions may call for customised machine design, and in the development of the design, geological knowledge is crucial. An identification of scenarios which may cause a threat for the successful implementation of the TBM-method is more important than for conventional drill and blast tunnelling, as the latter has a greater flexibility. Reliable prognostication on project cost and time schedules are crucial for all sub-surface projects, and not at least for TBM-tunnelling. A method commonly used for prediction of TBM performance is the so-called NTNU-model. The NTNU-model does not take fully into account the effect of mixed-face conditions on TBM performance. Such conditions have shown to be highly influential at several recent projects. In his Siv.ing.-thesis at NTNU, Steingrimsson (2000), used geological data from Iceland as an example in an approach aimed at providing an universal correction factor for mixed-face conditions. This paper presents the essentials of this approach and the use of the suggested "mixed-face correction factor".