Usage Limitations
A tunnel is relatively long and narrow; in general the length is more (usually much more) than twice the diameter, although similar shorter excavations can be constructed such as cross passages between tunnels.
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 150m or more.". 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 1800 mm (6 ft).".
Read more about this topic: Tunnel
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