篇篇学之5：Hydraulic tunnels

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.

WORDS:
hydraulic tunnel 水工隧洞 approximately 近似地 sufficient 足够的 gradient 梯度
drainage 排水 ventilated 通风的 shaft 柱状（通气孔） exhaust 抽风机
excavation 挖掘 NATM（NEW AUSTRIAL TUNNEL METHOD)新奥法 TBM(TUNNEL BORING MACHINE)隧道钻机
boring machine 钻机 blast 爆破 Norwegian 挪威的