INTRODUCTION
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.
Along the coast of Norway, about 30 subsea tunnels have been constructed the last 20 years. Most of these are road tunnels (2 lanes/cross section about 50 m 2, or 3 lanes/cross section about 70 m2), but some are also for water, sewage, or oil and gas pipelines (cross section mainly about 25 m ?, but occasionally more than 70 m=). All tunnels this far are drill and blast. The locations of some key projects, and tunnels being discussed later in this paper, are shown in Figure 1, and some main figures concerning length and depth in Table 1.
The tunnels are mainly located in hard, Precambrian rocks (typically granitic gneisses). This also is the case also for the deepest tunnel so far; the Hitra tunnel (260 meters below sea level at the deepest point). Some of the tunnels are, however, also located in less competent Paleozoic rocks like shale and schist. This is the case, for instance, for most of the longest tunnel this far; the Northcape tunnel (6.8 km).
At the Norwegian University of Science and Technology (NTNU) and the Foundation for Scientific and Industrial Research at the university (SINTEF), research on various aspects of the subsea tunnels has been much in focus the last 10-15 years. Initially, a comprehensive review was conducted with main focus on investigating the usefulness of preinvestigations to predict actual tunneling conditions, and on studying the effect of the saline environment on rock support materials. More recently, particular emphasis has been placed on evaluating stability and rock cover. Results of this research are described in Dah16 & Nilsen (1994) and Nilsen (1989, 1990 and 1998).
The last few years, very difficult ground conditions have been encountered in several subsea tunnels. The problems emphasize the need of a better understanding of the key factors determining stability and water leakage of such projects. In this paper the issue will be discussed based on the experience from completed projects, and with particular reference to a recent study of the Fröya subsea tunnel (Nilsen, Palmstr0m & Stille, 1997), which is currently under excavation, and where very difficult rock conditions are expected.
