Underwater tunnel

An undersea tunnel is a tunnel which is partly or wholly constructed under a body of water. They are often used where building a bridge or operating a ferry link is impossible, or to provide competition (or relief) for existing bridges or ferry links. There are many reasons for building an undersea tunnel as opposed to the construction of a bridge or establishment of a ferry link.

Advantages

Compared to bridges

One such advantage would be that a tunnel would still allow shipping to pass. A low bridge would need to be an opening or swing bridge to allow shipping to pass, which can cause traffic congestion. Conversely, a higher bridge that does allow shipping may be unsightly and opposed by the public. Bridges can also be closed due to harsh weather such as high winds. Another possible advantage is space: the downward ramp leading to a tunnel leaves a smaller footprint compared to the upward ramps required by most bridges.

Tunneling will generate soil that has been excavated and this can be used to create new land, as was done with the soil of the Channel Tunnel.

Further information: Tunnel –Choice of tunnels vs. bridges

Compared to ferry links

As with bridges, albeit with more chance, ferry links will also be closed during adverse weather. Strong winds, or the tidal limits may also affect the workings of a ferry crossing. Travelling through a tunnel is significantly quicker than travelling using a ferry link, shown by the times for travelling through the Channel Tunnel (75–90 minutes for Ferry and 21 minutes on the Eurostar).

Disadvantages

Compared to bridges

Tunnels require far higher costs of security and construction than bridges. This may mean that over short distances bridges may be preferred rather than tunnels (for example Dartford Crossing). As stated earlier, bridges may not allow shipping to pass, so solutions such as the Øresund Bridge have been constructed.

Compared to ferry links

As with bridges, ferry links are far cheaper to construct than tunnels, but not to operate.

List of notable examples

• Thames Tunnel The oldest underwater tunnel in the world. (0.4km) (1825 - 1843)
• Severn Tunnel One of the oldest underwater tunnels in the world (3.62 km) (1873 - 1886)
• Chesapeake Bay Bridge Tunnel (23 mi), connecting Virginia Beach with the Eastern Shore of Virginia (1960 - 1964).
• Transbay Tube (5.8 km) (1965 - 1974)
• Seikan Tunnel, world's longest undersea railway tunnel (53.9 km), when non-undersea portions of the tunnel are also measured (1971 - 1988)
• Sydney Harbour Tunnel (2.8 km) (1988 - 1992)
• Channel Tunnel, world's longest undersea portion railway tunnel (37.9 km) (1988 - 1994)
• Bømlafjord Tunnel, a road tunnel (7.8 km) (1997 - 2000)
• Marmaray, connecting Asia and Europe
• Tokyo Bay Aqua-Line, world's longest undersea portion road tunnel (9.6 km)
• Eiksund Tunnel (7.7 km), world's deepest undersea road tunnel
• Cross Harbour Tunnel, Hong Kong, a busy road tunnel
• New Elbe Tunnel, Hamburg, Germany, 8-lane road tunnel crossing the Elbe river
• Vardø Tunnel (2.9 km), connecting the small island community of Vardø in northern Norway to the mainland.
• North Cape Tunnel (6.8 km), The tunnel goes under the Magerøysundet strait between the Norwegian mainland to the large island of Magerøya and the North Cape, Norway.

Proposed

Rail

• Helsinki to Tallinn Tunnel