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.

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.

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).

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.

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

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Wikipedia