A tunnel is an underground gallery giving way to a communication route (railroad, canal, road, footpath). Large underground hydraulic works, such as aqueducts, collectors and outfalls intended either for the supply or the evacuation of water from large centers and certain conduits established in connection with dams and hydroelectric plants, are related to tunnels by their method of construction.
Between the moment when the first shovelful of earth is removed, modifying the equilibrium of a mass of ground in place, and the moment when the completed lining offers all its resistance, it is necessary, as much for the safety of the teams at work as for the maintenance of the given dimensions of the excavation, to oppose, by an appropriate device, the more or less intense thrusts which tend to close the created cavity. This is usually achieved by means of propping systems resting on the floor of the galleries, or by a system of anchoring by bolting or temporary hangers. After completion, the tunnel lining is made up either of these anchors associated or not with shotcrete, or of concrete or metal rings which thus constitute a shell.
In the rock, the tunnel can be dug either with a conventional method using explosives or with a mechanized method using a tunnel boring machine.
The first technique is very old; a firing plan is established which determines the order in which the detonators will be set off, as well as the position of the drillings in which the explosives are placed. The evacuation of the spoil (marinating) is usually carried out with dump trucks. Then comes the installation of the support which will consolidate the ground, the waterproofing and the pouring of the concrete coating.
Tunnel boring machines (TBMs), which appeared more recently at the end of the 19th century, are drilling machines well suited to urban areas and long structures. Each machine is a prototype, designed for a specific structure, but which can be reused on another site if the geometry and geology match.
The harder and more homogeneous the rock, the easier the digging. On the other hand, it is difficult to ensure the stability of the ground in fine or powdery soil (for example sand); in water-bearing ground (with a high presence of water as in a water table), it is sometimes even necessary to freeze the ground before digging.