Tunnels are constructed where they are needed.
In the case of a road tunnel, for example, the rock to be excavated through cannot be chosen; the tunnel has to pass through the rock formations that are «in the way».
During tunnel construction, unexpected rock conditions and water penetration often present considerable challenges for engineers and miners. For this reason, tunnel construction can not be planned down to the last detail even today, despite significant advances in preliminary investigation of the rock formations.
Tunnel boring machines are generally used in hard, compact rock.
Rocks that are soft and strongly water-bearing are more challenging for the tunnelling engineers and are generally excavated by drilling and blasting. The tunnels are secured continually as drilling advances and may be lined with shotcrete.