Tunnel is a concept that comes from the English word tunnel. It is an underground path that is usually artificially opened to allow communication between two places. In some cases, the tunnels are created by the action of nature.
For example: “The provincial government is building a tunnel on the hill to facilitate access to the municipality of Pucalpé”, “They must improve the signaling of the tunnel to avoid accidents”, “One of the most important works for the coming years is the construction of a sub-fluvial tunnel that connects the two provinces”. Tunnels allow the transport of goods and people, although their characteristics are varied. There are tunnels that are only suitable for pedestrians, others that allow the passage of cyclists and some that are suitable for motor vehicles (such as cars, trucks, etc.). There are even tunnels specially designed for the passage of the train.
Tunnels are sometimes built in undeveloped natural terrain to improve communication. A tunnel, in this sense, can be built in a mountain to facilitate the passage of vehicles without the need to overcome the elevation or scale its surface. The development of the tunnel, of course, requires an exhaustive study to analyze its feasibility. In cities, tunnels are used to bury the railway or for underground transport services such as the subway, to name two possibilities. If we focus on football, finally, the game that consists of passing the ball between the opponent’s legs is known as tunnel, pipe or cassock. There is a construction method called cut-and-cover tunnel used in works that begin with a surface excavation of all or almost all of the space that will be used for the tunnel, to carry out the construction in the open and then cover it once it is finished. In this case, it is necessary to have a system capable of supporting the structure and its respective loads. It is possible to distinguish two variations of this method, depending on the order in which the works are carried out: * bottom-up (“bottom-up”): as indicated above, the first step is to excavate to reveal the area that will occupy the tunnel. Then the interior work is carried out, manufacturing the concrete there or loading prestressed concrete, corrugated or prestressed steel arches, or brick (the most common material in the first cut-and-cover tunnels);
* from top to bottom (“top to bottom”): unlike the previous method, it consists of digging a trench and pouring it from the surface to build the walls of the tunnel, continuing with the upper slab once finished. In this way, only the hole necessary for mounting the slab on the walls must be dug. Finally, the surface must be rebuilt, even before all the internal work has been done. It is worth mentioning that the top down method is currently the most used for the construction of tunnels in urban areas. Among its advantages over others is the fact that it does not require the use of specialized machines. On the other hand, it also allows the construction of tunnels with more than one floor, through the use of intermediate slabs. To automate the process of creating tunnels, tunnel boring machines are used, which are associated with direct and inverse systems. There are several types of tunnel boring machines, each one suited to a different type of job and job site conditions (for example, the ground may be crumbly or dense rock, among other possibilities, and this brings several challenges) .
