Technical Articles
Britannica: Tunnels and Underground Excavations
Encyclopedia Britannica article: extensive information on history and technology of tunnels and underground excavations.
Tunnel
Encarta online.
Tunneling: Mechanics and hazards
A tunnel is a long, narrow, essentially linear excavated underground opening, the length of which greatly exceeds its width or height. (Walhstrom, 1973). In contrast, a cavern is an underground opening whose length and width are roughly similar. These two
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