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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. ==History== The first successful tunnelling shield was developed by Sir Marc Isambard Brunel, and patented by him and Lord Cochrane in January 1818. Brunel and his son Isambard Kingdom Brunel used it to excavate the Thames Tunnel, beginning in 1825 (though the tunnel would not be opened until 1843).〔Becket, Derrick (1980). ''Brunel's Britain''. Newton Abbot: David & Charles. ISBN 0-7153-7973-9. Chapter 10: "Tunnels".〕 Brunel is said to have been inspired in his design by the shell of the shipworm ''Teredo navalis'', a mollusc whose efficiency at boring through submerged timber he observed while working in a shipyard.〔 The shield was built by Maudslay, Sons & Field, of Lambeth, London, who also built the steam pumps for de-watering the tunnel. Brunel's original design was substantially improved by Peter W. Barlow in the course of the construction of the Tower Subway under the River Thames in central London in 1870. Probably the most crucial innovation of Barlow's design was that it had a circular cross-section (unlike Brunel's, which was of rectangular cross-section), which at once made it simpler in construction and better able to support the weight of the surrounding soil. The Barlow design was enlarged and further improved by James Henry Greathead for the construction of the City and South London Railway (today part of London Underground's Northern line) in 1884. His system was also used in the driving of the tunnels for the Waterloo & City Railway which opened in 1898; he was joint Engineer to the project until his death. The station tunnels at the City station (now known as Bank) were the largest diameter tunnelling shields in the world at the time. To this day, most tunnelling shields are still loosely based on the Greathead shield.〔John C Gillham, ''The Waterloo & City Railway'', The Oakwood Press, Usk, 2001, ISBN 0 85361 525 X〕 An original Greathead Shield, used in the excavation of the deep London Underground lines, remains in place in disused tunnels beneath Moorgate station. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tunnelling shield」の詳細全文を読む スポンサード リンク
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