In the above articls,we have described the types of dams.Here we will discribe the tunnels.
A tunnel is an underground passage usually made without removing the overlying rock or soil.Although the tunnels are approximately horizontal.they must be built with sufficient gradient for proper drainage.tunnels may be ventilated by shafts leading to the surface or by exhaust fans at the ends.
Tunnels are dug in various types of materials,from soft clay to hard rocks.Depending on the types of soil,a method of excavation is chosen.When digging soft clays,the New Austrian Tunnelling method or NATM is used.When digging in weak rocks a tunnel boring machine or TBM is used.In hard rocks blasting is usually the fastest method,as in the Norwegian tunnelling method.
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