Abstract
Subsea tunnels are located deeply under sea. Therefore，they are more difficult in geological investigation，design，construction and operation than land tunnels. In the last 30 years rich experiences have been gained in Norway from constructing 40 subsea tunnels and the“Norwegian subsea tunnelling concept”has formed. In the planning and design stage，careful pre-investigation is of critical importance. Extensive marine seismic refraction survey is the major method for investigating bedrock profiles and detecting fault，weakness zones and depressions，which are the major threats to the tunnel stability. Directional core drilling from onshore passes geological structures and obtains rock cores，therefore，direct evaluation of the rock conditions can be provided. Special rock support methods are needed in order to pass such weakness zones，including reduced blast rounds，bolt-enforced shotcrete rib and dense long spilling bolts. The lining of Norwegian subsea tunnels consists typically of sprayed concrete and rock bolts for support and free standing inner lining for water/frost protection. The lining is designed as a drained structure，sustaining no external water pressure. Cast-in-place concrete is used only in extremely adverse geological conditions. All subsea tunnels in Norway are excavated by drill and blast method. Inflow of sea water is one of the critical issues for subsea tunnels since there is no natural exit for the inflow water，and therefore，it is essential to know the geological and hydrogeological conditions ahead of face. Probe drilling and pre-grouting technique are the best ways to detect the water inflow and reduce it to the acceptable level. Even so，equipments and measures have to be ever ready to meet the emergency situation. The minimum rock cover is an influential factor to the tunnel stability as well as economy and should be evaluated carefully. A minimum rock cover of 23 m has been used，despite the Norwegian road tunnel standard requires more detailed geological investigation and special analysis if the minimum rock cover is less than 50 m. Problems related to in-leaking saline water，such as erosion of shotcrete and metallic parts，are special issues for subsea tunnels and need to be coped with seriously.
