Deep level underground

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Deep level underground is construction that is 20 m (66 ft) or more below ground and not using the cut-and-cover method, especially train stations, air raid shelters and bunkers, and some tunnels and mines. Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support structure that is strong enough to carry the load of what is to be built above the tunnel.