July 5, 2022
An underwater tunnel is a tunnel that goes under the sea, a river or a lake. The term "underwater" is also sometimes used for tunnels that go deeper than sea level, even if the tunnels do not cross water. Submarine tunnels are often cheaper to build than bridges, as long as the water depth is not too great. Submarine tunnels are also often chosen where it is considered that bridges will damage the landscape or be inappropriate in other ways. Submarine tunnels are also being built to replace existing ferry connections, and to compete with existing bridges, such as Tromsøysund tunnel. Requirements for sailing height are one of the factors that can cause the tunnel to be chosen over the bridge. In some places, a combination of bridge and subsea tunnel is also chosen. Examples of this are the mainland connection between Giske municipality and Ålesund, the Øresund connection between Denmark and Sweden, and the Chesapeake Bay Bridge Tunnel in the USA. The most common way to build underwater tunnels is to drill / blast the tunnel run from each side, as tunnels in the mountains are built on land. Where the water depth is small and the ground conditions are suitable, subsea tunnels can also be built as large metal pipes that are dug into the seabed / riverbed and welded together into a tunnel. Such immersed tunnels can also consist of concrete elements. Norway's first immersed tunnel is the Bjørvika tunnel in Oslo, which was completed in 2010. Submarine tunnels place additional demands on sealing and safety against water penetration. Submarine tunnels that are drilled and blasted are often equipped with powerful pumps that suck up water that seeps into the tunnel. They are also built with a large water reservoir under the deepest part of the tunnel, so that water that seeps in will collect in the reservoir and not fill the tunnel itself, if the water can not be pumped out fast enough. Submarine tunnels are usually very safe, and it has only happened a few times that water has penetrated into such tunnels in large and uncontrollable amounts. Railway tunnels have clear limits on how much slope they can have. However, road tunnels may have a larger incline. Norwegian underwater tunnels have a rise of up to 10%. However, safety rules introduced by the EU in 2005 set a limit of 5% for new tunnels. If these rules had applied to the Oslofjord tunnel of 7.4 km, the tunnel would have been approx. 1.6 km longer, and cost around NOK 150 million more.  Most Norwegian underwater tunnels have one run with one lane in each direction. However, some underwater tunnels have an extra lane for steep climbs. In foreign underwater tunnels with greater traffic, it is more common with separate runs for each direction of travel.