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What is the secret to great head shield

what-is-the-secret-to-great-head-shield-photoA later form of shield of rather more compact dimensions was invented by Peter William Barlow, a British railway engineer turned bridge-builder, and patented in 1864. His shield, too, was first used to build a tunnel under the Thames — the Tower Subway, which was to be a small cable-operated railway car running backward and forward, hauled by steam engines at each end and reached by shafts in which there were steam-worked elevators.

The engineer who helped build this tunnel was Henry Greathead, and he developed the shield principle for use in the construction of undergound railway tunnels. The Greathead shield was basically a steel cylinder pushed forward by jacks as workers dug away the earth in front of it. The resulting tunnel was lined with rings of cast iron built up from segments and had an internal diameter of 6 ft 73/4 in. (2.026 m).

The iron rings were slightly smaller than the bore cut by the shield and the space between was filled with liquid cement or grouting. To prevent flooding, the working area could be sealed with an airlock and supplied with high-pressure air.

The same basic principles are used for modern tunnel construction – though mechanical picks and shovels or rotating cutters are used to remove the earth in front of the shield.

Tunnel linings are now often made of reinforced concrete and can be expanded to press securely against the tunnel walls, obviating the need for grouting.