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:
“The lichen on the rocks is a rude and simple shield which beginning and imperfect Nature suspended there. Still hangs her wrinkled trophy.”
—Henry David Thoreau (1817–1862)