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During the excavation by TBM of the Aicha Exploratory Tunnel along the Brenner Base Tunnel in Italy, in hard brittle granitic rocks, an instability phenomenon occurred at chainage 6+151. The left-hand sidewall lining partially collapsed for a length of approximately 60m. This led to a stoppage of the TBM lasting about 4 months. With the reconstruction of the geological model, a sub-vertical fault, not forecast at the design stage, was identified parallel to the direction of the excavation. In this paper, a three-dimensional (3D) model, which simulates the complex interaction between the rock mass, the TBM and its system components, and the tunnel support, is presented. The model accounts for the main machine components: shields, cutterhead, grippers, pea gravel and lining. The brittle failure of a thin granite diaphragm between the fault zone and the tunnel wall is reproduced and the results are discussed in terms of failure zone and horizontal displacements.

For long deep tunnels as currently under construction through the Alps, mechanized excavation using tunnel boring machines (TBMs) contributes significantly to savings in construction time and costs. Questions are, however, posed due to the severe ground conditions which are in cases anticipated or encountered along the main tunnel alignment. A major geological hazard is the squeezing of weak rocks, but also brittle failure can represent a significant problem. For the design of mechanized tunnelling in such conditions, the complex interaction between the rock mass, the tunnel machine, its system components, and the tunnel support need to be analysed in detail and this can be carried out by three-dimensional (3D) models including all these components. However, the state-of-the-art shows that very few fully 3D models for mechanical deep tunnel excavation in rock have been developed so far. A completely three-dimensional simulator of mechanised tunnel excavation is presented in this paper. The TBM of reference is a technologically advanced double shield TBM designed to cope with both conditions. Design analyses with reference to spalling hazard along the Brenner and squeezing along the Lyon–Turin Base Tunnel are discussed.