Quote:
Originally Posted by someone123
This makes me think I don't really know of a clear distinction between bridges and tunnels. From an engineering perspective I think of a tunnel as a path cleared through dirt or rock where the challenge may be to keep the surrounding material from caving in.
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There seems to be some variance as to the point that an underpass becomes a tunnel.
According to
wikipedia:
Quote:
The definition of what constitutes a tunnel can vary widely from source to source. For example, the definition of a road tunnel in the United Kingdom is defined as "a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more."[1] In the United States, the NFPA definition of a tunnel is "An underground structure with a design length greater than 23 m (75 ft) and a diameter greater than 1,800 millimetres (5.9 ft).
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Tunnels don't necessarily have to be bored through existing terrain (i.e. the mountain scenario), they can simply be a trench dug into existing with structure built over it and backfilled to allow the upper surface to be used for something else, while the roadway continues beneath it.
A tunnel vs bridge is much more simple to define, whereas a tunnel is generally an underground structure to allow passage (usually) below the surface of existing terrain, a bridge is a structure to allow passage above the surface of existing terrain.
I would tend to call the structure we are discussing a long underpass, but could also see it loosely described as a tunnel, depending upon which definition you adhere to, and whether you consider the roadway above to be existing terrain or not.