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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).