Analysis of potential cave-in from fault zones in hard rock subsea tunnels

Summary As a part of a research program on the rock engineering aspects of hard rock subsea tunnelling, analyses of potential cave-in from fault zones have been carried out at the Norwegian Institute of Technology. This is a topic of great importance for the planning of future subsea tunnels, and particularly for the selection of the minimum rock cover of such projects. The paper is divided into three main parts: a) review of cases of instability in Norwegian subsea tunnels, b) evaluation of theoretical maximum sliding, and c) discussion of cases of cave-in in tunnels under land. In theory, a cave-in during subsea tunnelling may propagate far higher than the normal minimum rock cover. Taking into consideration the comprehensive geo-investigations that are always carried out for subsea tunnel projects today, it would, however, be unrealistic to base the dimensioning of rock cover for future projects on worst-case scenarios. Consequently, the main result of this study is to emphasize the importance of comprehensive geo-investigations, detailed tunnel mapping, a high degree of readiness during tunnelling and a thorough quality control.