Tunnels are underground passages used for underground transportation.They could be used for carrying freights and passengers,water,sewage etc..
The methods involved are underground operations known as tunnel driving and the surface is not disturbed
It is understood that first tunnel was constructed by Egyptians and Babylonians about 4000 years ago.It was built to connect two buildings in Babylon.The length ,width and height of this tunnel , were 910 m, 360 cm , 450 cm respectively
Advantages of tunneling
Tunnels are made more economical than open cuts beyond certain depths
Tunnels avoid disturbing or interfering with surface life and traffic during construction.
Tunnels prove to be cheaper than bridges or open cuts to carry punlic utility services like water,sewer and gas
If tunnels are provided with easy gradients the cost of hauling is descresed
In case of aerial warfare and bombing of cities ,the tunnels would grant better protection compared to bridges.
When depth of cut is more than 18 m, tunnelling is advisable from point of view of economy in construction, maintenance etc.
Selection of Route of tunnel
There are problems like water table,position of fractured rocks etc are to be tacked,
The site of tunnel should be selected in such a way that the least difficulty is ecperienced for various environmental factors such as disposal of exhaust gas,groundwater etc,,