machine for driving tunnels in soft ground, especially under rivers or in water-bearing strata. The problem of tunneling under a river had defied the engineering imagination for centuries because of the difficulty of preventing mud and water from seeping in and collapsing the tunnel heading. In 1818 Marc Isambard Brunel, an emigre French naval officer in England, observed the action of a tiny marine borer, the shipworm, whose shell plates permitted it to bore through timber and push the sawdust out behind it. Brunel built a giant iron casing, or shield, that could be pushed forward through soft ground by means of screw jacks, while miners dug through shutter openings in the face
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