3D simulation of TBM excavation in brittle rock associated with fault zones: The Brenner Exploratory Tunnel case

Abstract

Brittle fault zones represent a major challenging geological environment for TBM tunnelling in deep Alpine tunnels, particularly when the faults are near parallel to or cross the tunnel axis at a low angle. This is the case of the Brenner Exploratory Tunnel in Italy. Serious local instabilities occurred at the left side wall during TBM drive in the granitic rocks associated with a sub-vertical fault zone, parallel to the tunnel axis. The segmental lining was collapsed at a distance of more than 2D (D is tunnel diameter) behind the face, without any evidence. The deformation and failure then propagated intensively to nearby, previously stabilized sections with a length of approximately 60m in the longitudinal direction, leading to a subsequent damage of the shields and grippers of the machine and to a stoppage of the excavation in almost 4months.To deal with these severe geotechnical problems encountered when tunnelling through a fault zone, a realistic 3D numerical simulation based on a site investigation and characterisation of the fault zone, can provide a helpful decision aid as they give a quantitative assessment of the potential mode of failure. In the case of the Brenner Exploratory Tunnel, the behaviour of the rock mass is neither ductile nor brittle, but governed by the combination due to the presence of the brittle fault zone. This paper focuses on the 3D simulation of such complex failure evolution. Special emphasis is placed on the modelling of the fault zone and of the TBM excavation process. The results demonstrate the role that the local rock mass condition and the complex interaction between the rock mass, the TBM components, and the tunnel support play on the characterization of this instability phenomenon.