The aim of the present report is to:
• Introduce a common technical language and a simplified classification on the main geotechnical hazards that may be encountered during the excavation of long and deep tunnels crossing difficult rock conditions;
• Analyse the influence and the consequences of these phenomenon regarding the various TBM types;
• Provide recommendations for the design, for the selection of the TBM type and for the mitigation measures to be implemented on-site.

The Appendix of the report is a worldwide date base collected by the members of the ITA Working Group 17 on the TBM tunnelling experience in difficult rock conditions gained over the last 20 years. The present report focuses on hazard scenarios that are associated – or their
magnitude is by far increased – by the high overburden. However, as for more shallow tunnels, other hazard scenarios may be encountered during the tunnel construction of long and deep tunnels (e.g. rock falls, swelling, packing of fines around the shield, karstic phenomena, gripper bracing problems, environmental aspects as gas, radioactivity or asbestos etc…), and so have to be additionally considered in the design phase of a project. In the same idea, the following nongeological related hazards are not handled in this report: fatigue of the  workforce due to long travel distances, logistic difficulties in long tunnels, excessive wear and replacement needs for equipment and materials due to long construction times, maintenance needs for the already constructed parts of the project, and personnel fluctuation.