Tunnel

A tunnel is an underground passage. Some tunnels are used for cars, and others are used for trains. Sometimes, a tunnel is used for movement of ships. Some tunnels are built for communication cables and some are built for electricity cables. Other tunnels are built for animals.

Tunnels are dug in different kinds of grounds, from soft sand to hard rock. The way of digging is chosen by the type of ground. There are two additional ways of digging : quarry and 'cut and cover'. In quarry, the tunnel path is drilled in a horizontal way.
This system requires a deep tunnel that's built in a firm rock. In the 'cut and cover' system, a tunnel is dug in the ground and, afterwards, a roof is built above the tunnel. This system fits tunnels that are close to the ground like road tunnels and infrastructure.

Building tunnels is a large civil engineering project that could cost very high sums of money. The planning and building of a long tunnel may take many years.

The Channel Tunnel between France and England is one of the longest tunnels in the world. It is 50 kilometers long. The longest tunnel in the world, the Gotthard Base Tunnel, is being dug in Switzerland.

Reasons to build a tunnel 
 A subway is based on a network of tunnels that are dug underground so the trains will not disturb and will not be disturbed by the local transport.
 On the path of a railroad track or a road a tunnel is dug when the lane encounters an obstacle such as a mountain to avoid bypassing the obstacle.
 A tunnel is built sometimes to overcome a water obstacle as a replacement for building a bridge above it.
 A tunnel is built to connect between military posts so the movement between them will not be visible for the enemy.
 A tunnel is built for infrastructure like electricity cables, water, communication and sewerage to avoid damage and disruption above ground.
 Some tunnels are used by prisoners to escape jail.
 Sometimes tunnels are used by criminals to do a bank robbery (e.g. in Brazil, Summer 2005).

Cut-and-cover

Cut-and-cover is a simple way of making hollow tunnels where a trench is hollowed out and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.
Two basic forms of cut-and-cover tunneling are available:
 Bottom-up method: A trench is hollowed out, with ground support when needed, and the tunnel is built in it. In earliest days bricks were used. The trench is then carefully filled and the surface is reinstated.
 Top-down method: Side support walls and capping beams are made from ground level by such methods as slurry walling or neighbouring bored piling. Only a hollowed out excavation is needed to construct the tunnel roof using beams or in situ concrete sitting on the walls. The surface is then remade except for access openings. This allows early recreation of roadways, services, and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is made.

Notes

Books 
 
 Railway Tunnels in Queensland by Brian Webber, 1997, .
 Sullivan, Walter. Progress In Technology Revives Interest In Great Tunnels, New York Times, 24 June 1986. Retrieved 15 August 2010.