Key factors determining stability and water leakage of hard rock subsea tunnels

The many hard rock subsea tunnels having been completed along the coast of Norway the last 20 years offer excellent opportunities to study the key factors determining stability and water leakage of such projects. This paper is discussing the issue based on research having been performed at the Norwegian University of Science and Technology (NTNU) on the rock engineering aspects of the Norwegian subsea tunnels. Particular emphasis is put on results of a recent study of the Fröya subsea tunnel, which is currently under construction, and where very difficult rock conditions are expected. It is concluded that distinct faults or weakness zones generally represent the most difficult rock conditions. Major water inflows seldom are directly connected to the major zones, and are difficult to prognosticate.