The significance of mixed-face conditions for TBM performance

The TBM tunnelling technique initially focused on the 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 that may cause a threat to the successful implementation of the TBM-method, is more important than for conventional drill and blast tunnelling, as the latter has a greater flexibility.