INTRODUCTION
In Norway subsea tunnels have become increasingly popular the last 10 years, particularly for road purposes. This is mainly due to the characteristic topography of the country with a large number of fjords and straits, and the fact that the majority of the population lives on the coast.
The rock conditions in Scandinavia are mainly good. For the actual tunnels, however, this is not necessarily the case, as the locations of fjords and straits are often defined by major weakness zones in the bedrock. The deepest part of the fjord, and hence the most critical part of the tunnel, often coincides with particularly significant weakness zones. Such zones may have widths of 20 30 m or even more and in most cases consist of crushed and weathered rock. The gouge material is often of a swelling type (smectite). Swelling pressures up to 2 MPa have been experienced.
At the Norwegian Institute of Technology in Trondheim (NTH) research on subsea rock tunnels has been going on since 1986. Initially, a state-of-the-art review was carried out in order to summarize and evaluate the experience which may be gained from the completed Norwegian subsea tunnels. Owners, contractors and consultants for the tunnelscontributed actively so that a broad overview was possible.
The initial study included all Norwegian subsea rock tunnels which were completed before 1988. Some information concerning length, cross section, depth, etc. of the actual tunnels is given in Table 1.
Most of the subsea tunnels are related to fjord crossings on the west coast of Norway, and are road tunnels or tunnels for oil and gaspipes. The greatest depth below sea level so far is about 250 m, and the greatest subsea length is approx. 3.1 km (Frierfjord gaspipe tunnel). The tunnels are situated in a variety of geological structures ranging from typical hard rock such as Precambrian gneiss to less competent phyllite and poor quality schists and shales. All tunnels cross significant zones of weakness under the sea.
During the various stages of planning, comprehensive geoinvestigations are carried out. While the total costs of preinvestigations for conventional tunnels in Norway are often less than 1% of the total cost of excavation and rock support, the preinvestigation costs for subsea tunnels are normally between 5 and 10%.
