The most challenging rock mass conditions in hard rock subsea tunnels are represented bymajor faults/weakness zones. Poor stability weakness zones with large water inflow can be particularly problematic. At the pre-construction investigation stage, geological and engineering geological mapping, refraction seismic investigation and core drilling are the most importantmethods for identifying potentially adverse rock mass conditions.
The basic aspects of investigation, support and tunnelling for major weakness zones are discussed in this paper and illustrated by cases representing recently completed, very challenging projects (Atlantic Ocean tunnel and T-connection) and projects planned to be built in the near future (Ryfast and Rogfast).