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­At its most basic, a tunnel is a tube hollowed through soil or stone. Constructing a tunnel, however, is one of the most compl­ex challenges in the field of civil engineering. Many tunnels are considered technological masterpieces and governments have honored tunnel engineers as heroes. That's not to say, of course, that some tunnel projects haven't encountered major setbacks. The Central Artery/Tunnel Project (the "Big Dig") in Boston, Massachusetts was plagued by massive cost overruns, allegations of corruption, and a partial ceiling collapse that resulted in a fatality. But these challenges haven't stopped engineers from dreaming up even bigger and bolder ideas, such as building a Transatlantic Tunnel to connect New York with London.

In this article, we'll explore what makes tunnels such an attractive solution for railways, roadways, public utilities and telecommunications. We'll look at the defining characteristics of tunnels and examine how tunnels are built. We'll also look at the "Big Dig" in detail to understand the opportunities and challenges inherent to building a tunnel. Finally, we'll look at the future of tunnels.

Tunnel Image Gallery

Gotthard Base Tunnel
Image courtesy Daniel Schwen/
used under Creative Commons Attribution-ShareAlike License

The Gottha­rd Base Tunnel, a railway tunnel under construction in Switzerland. See more pictures of tunnels.

Tunnel Basics
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.

A basic tunnel

Tunnel engineers, like bridge engineers, must be concerned with an area of physics known as statics. Statics describes how the following forces interact to produce equilibrium on structures such as tunnels and bridges:

  • 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.

Forces acting on tunnel

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.

We'll look at the basic types of tunnels next.