Tunnels are dug in various types of materials, from soft clay to hard rock, and the method of excavation depends on the ground conditions.
Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over. A strong overhead support system is required to carry the load of the covering material.
The Seikan Tunnel in Japan is the longest rail tunnel in the world at 53.9 km (33.5 mi), of which 23.3 km (14.5 mi) is under the sea.
The Channel Tunnel between France and the United Kingdom under the English Channel is the second-longest, with a total length of 50 km (31 mi), of which 39 km (24 mi) is under the sea.
The Lötschberg Base Tunnel in Switzerland is the longest land tunnel, with a total of 34.5 km (21.4 mi).
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