A tunnelling shield is a protective structure used in the excavation of tunnels through soil that is too soft or fluid to remain stable during the time it takes to line the tunnel with a support structure of concrete, cast iron or steel. In effect, the shield serves as a temporary support structure for the tunnel while it is being excavated.
Read more about Tunnelling Shield: History, Manual Shield Tunnelling, Modern Tunnel Boring Machines, Lining, Shields in Japan
Famous quotes containing the word shield:
“Vice is its own reward. It is virtue which, if it is to be marketed with consumer appeal, must carry Green Shield stamps.”
—Quentin Crisp (b. 1908)
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