Tunnelling shield

A tunnelling shield is a protective structure used during the excavation of large, man-made tunnels. When excavating through ground that is soft, liquid, or otherwise unstable, there is a risk to workers and the project itself from falling materials and/or a cave-in. A tunnelling shield serves as a temporary support structure. It is in place for the usually short time from when the tunnel is excavated until it can be lined with a permanent support structure. The permanent structure may be made up of, depending on the period, bricks, concrete, cast iron, or steel. Although modern shields are commonly cylindrical, the first "shield", designed by Marc Isambard Brunel, was actually a large, rectangular, scaffold-like iron structure with three levels and a solid weight-bearing top — see picture to the right. The structure nevertheless protected the men from cave-ins as they laboured within it, digging the tunnel out in front of themselves.

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 was not opened until 1843). 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 shield was 7 ft 3in (2.2 metres) in diameter.

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