A tunnel is a horizontal passageway located underground. While erosion and other forces of nature can form tunnels, in this article we'll talk about man made tunnels -- tunnels created by the process of excavation. There are many different ways to excavate a tunnel, including manual labor, explosives, rapid heating and cooling, tunneling machinery or a combination of these methods.
Some structures may require excavation similar to tunnel excavation, but are not actually tunnels. Shafts, for example, are often hand-dug or dug with boring equipment. But unlike tunnels, shafts are vertical and shorter. Often, shafts are built either as part of a tunnel project to analyze the rock or soil, or in tunnel construction to provide headings, or locations, from which a tunnel can be excavated.
The diagram below shows the relationship between these underground structures in a typical mountain tunnel. The opening of the tunnel is a portal. The "roof" of the tunnel, or the top half of the tube, is the crown. The bottom half is the invert. The basic geometry of the tunnel is a continuous arch. Because tunnels must withstand tremendous pressure from all sides, the arch is an ideal shape. In the case of a tunnel, the arch simply goes all the way around.
* Tension, which expands, or pulls on, material
* Compression, which shortens, or squeezes material
* Shearing, which causes parts of a material to slide past one another in opposite directions
* Torsion, which twists a material
The tunnel must oppose these forces with strong materials, such as masonry, steel, iron and concrete.
In order to remain static, tunnels must be able to withstand the loads placed on them. Dead load refers to the weight of the structure itself, while live load refers to the weight of the vehicles and people that move through the tunnel.
There are several excellent sites about tunneling, including information about Boston's Big Dig Tunnel system: