Mechanized tunnelling is evolving dramatically and every year constraints are reduced and TBM tunnelling is applied in new geological situations. Progressing from homogeneous, stable rocks through heterogeneous formation and unstable rocks, today the positive control of the face, or dealing with squeezing ground, constitute a great advance. Tomorrow we can expect the same TBM for any type of ground: the universal TBM. In conventional tunnelling, adjustments can be made even in the case of significant unforeseen changes of the geological conditions. On the contrary, mechanized tunnelling depends subversively on the geological conditions and the choice of the correct machine and operating mode is critical. Geological factors are discussed regarding both the type of geomaterial and its development in space, and the ground water conditions. The key issues for the choice of the appropriate TBM are the stability of the tunnel walls, the stability of the face and the control of surface settlements (for urban tunnels). The discussion on the selection covers all types of full face machines: open rock TBM, shielded TBM (single or double shield), TBM with pressurized face (notably slurry or earth pressure balance). Particular geological conditions are given special attention, such as alternations and severe changes of the geological material with radically different quality along the alignment, fault zones, karstic conditions, squeezing ground, rock bursting of strong rock under high overburden. In the final choice possible adjustments and arrangements of the operational elements of the machine can be considered to deal with site specific geological conditions. A number of case histories and the way problems from unforeseen geological conditions were faced will be presented.
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