Tunnels are defined as an underground route or passage driven through the ground without causing a disturbance to the overlying rock or soil cover. Tunnels are made for different purposes and are categorised accordingly.
Tunnelling has been practised on a large scale over the last two centuries in all major countries of the world for ensuring faster and better communications through roads and railways. At certain places like in high mountains, tunnelling is an absolute necessity to connect two countries or two different parts of the same country for increased access between the two locations.
Metros can also be seen as a version of tunnelling and in fact, may involve a reasonable length of tunnels as the essential component.
Excavations below the ground require very sound knowledge about the rock and soil to be excavated on the one hand and keep the tunnels safe and stable at economically viable costs for the entirety of its lifetime on the other hand. Just like roads, railways, buildings and several other construction jobs, tunnelling projects are among the most important developmental activities of a nation.
Traffic tunnels include all tunnels that are excavated for diverting the traffic load from the surface to subsurface routes for a short length for facilitating the flow of traffic at a certain speed, at minimum cost and maximum convenience. The highway tunnels, pedestrian tunnels and railway tunnels are main subgroups of traffic tunnels. A traffic tunnel is generally adopted as a convenient and cost-effective alternative to providing a direct transportation link between two places separated by immovable obstacles like mountains, water-bodies, hills or even densely populated areas in the metropolitan cities.
The Hydropower Tunnels
During the twentieth century, most tunnelling work has been in connection with hydropower generation. Such tunnels are called hydropower tunnels, and in most cases, they are driven through rocks for conveying water from one point to another using gravity to cross a hill. In such cases, they are known as discharge tunnels.
The other type of hydropower tunnels is the ones which feed water under tremendous pressure to turbines and are distinguished as pressure tunnels.
Public Utility Tunnels
Public utility tunnels include various underground excavations created for specific purposes like disposal of urban waste; for carrying cables, pipes; supplies of oil, water etc. A recent development in public utility tunnels is the construction of underground parking spaces and storage chambers in order to overcome space shortage in metropolitan cities.
Subways and tube railways can technically fall into the category of excavations as well, but they are, in all practical terms, not tunnels in the sense that they are excavations made in the ground and covered from the top later. This method of placing the tracks or tubes is called a cut and cover method and not technically tunnelling. Here the top cover remains intact and undisturbed during the excavation.