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A tunnel can be defined as an underground passage for the transport of passengers, goods, water, sewage, oil, gas, etc. The construction of a tunnel is normally carried out without causing much disturbance to the ground surface.
The history of tunnels is very old. The first tunnel was constructed about 4000 years ago in Babylon to connect two buildings. The first railway tunnel in the world was constructed at the end of the nineteenth century to connect Switzerland and Italy. The cross section of the tunnel was in the shape of a horseshoe and its length was about 20 km. On Indian Railways, the first tunnel was constructed near Thane on Central Railways known as the Parsik tunnel. It is the longest railway tunnel of India with a length of about 1317 m.
Necessity/Advantages of a Tunnel
The necessity of constructing a tunnel may arise because of one of the following considerations.
(a) A tunnel may be required to eliminate the need for a long and circuitous route for reaching the other side of a hill, as it would considerably reduce the length of the railway line and may also prove to be economical.
(b) It may be economical to provide a tunnel instead of a cutting, particularly in a rocky terrain. Depending upon various factors, a rough calculation would indicate that for a small stretch of land the cost of constructing a tunnel is equal to the cost of a cutting in a rocky terrain.
(c) In hills with soft rocks, a tunnel is cheaper than a cutting.
(d) In metropolitan towns and other large cities, tunnels are constructed to accomodate underground railway systems in order to provide a rapid and unobstructed means of transport.
(e) A tunnel constructed under a river bed may sometimes prove to be more economical and convenient than a bridge.
(f) In the case of aerial warfare transportation through tunnels provides better safety and security to rail users compared to a bridge or deep cutting.
(g) The maintenance cost of a tunnel is considerably lower than that of a bridge or deep cutting.
However, the construction of tunnels is also disadvantageous in certain ways, as enumerated here.
(a) The construction of a tunnel is costly as it requires special construction machinery and equipment.
(b) The construction of a tunnel involves the use of sophisticated technology and requires experienced and skilled staff.
(c) It is a time-consuming process.
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