Main challenges related to deep subsea tunnels - a review based on experience from Norwegian projects

Deep subsea tunnelling involves considerable challenges regarding ground investigation, tunnel design, excavation and rock support. In this paper such challenges are discussed based on experience from Norwegian hard rock subsea tunnels. Since the early 1980’s around 50 subsea rock tunnels have been built along the coast of Norway. Most of these are road tunnels, with the 14 km long twin-tube Ryfylke tunnel, going down to 290 m below sea level as the currently longest and deepest. At the pre-construction investigation stage, seismic refraction surveying and core drilling (with directional drilling commonly used today), in addition to conventional engineering geological mapping, are the most important methods for identifying potentially adverse rock mass conditions. At the construction stage, engineering geological mapping, probe drilling ahead of the face (percussive as well as core drilling) and MWD (Measurement While Drilling) are continuously carried out. For the most recent Norwegian subsea tunnel projects, reflection seismic sounding (TSP) to investigate the conditions ahead of the face has also been used. During excavation, grouting ahead of the tunnel face is carried out whenever required according to the results from probe drilling. Sealing of water inflow by pre-grouting is particularly important before tunnelling into a section of poor rock mass quality. When excavating through weakness zones, a combination of spiling bolts, short blast rounds and reinforced ribs of sprayed concrete (RRS) close to the face is commonly applied. The basic aspects of ground investigation methodology, excavation, stability evaluation and rock support design are discussed and illustrated by experiences from recent Norwegian cases. Major faults/weakness zones and large water inflow have caused challenging conditions in most of the Norwegian subsea tunnels. Most commonly such zones have low permeability due to high content of clay minerals (including swelling clay), but in some cases major zones with clay minerals have also considerable water seepage. Such situations have been particularly difficult to cope with, and have generally represented the greatest challenges for Norwegian subsea tunnels.