Tunnelling shield

When excavating through ground that is soft, liquid, or otherwise unstable, there is a potential health and safety hazard to workers and the project itself from falling materials or a cave-in.

The first successful rectangular tunnelling shield was developed by Marc Isambard Brunel and patented by him and Lord Cochrane in January 1818.

[1] Brunel is said to have been inspired in his design by the shell of the shipworm, a mollusc whose efficiency at boring through submerged timber he observed while working in a shipyard.

In 1868 Beach built a circular shield - a picture of which was printed in a New York news article about his pneumatic tunnel system idea.

Greathead was the first to ever use a cylindrical tunnelling shield, which he did in the course of the construction of the Tower Subway under the River Thames in central London in 1869.

His shield was also used in the driving of the 12 foot 1+3⁄4 inches (3.702 m) diameter running tunnels for the Waterloo & City Railway which opened in 1898.

An original Greathead shield used in the excavation of the deep London Underground lines remains in place in disused tunnels beneath Moorgate station.

A tunnel boring machine (TBM) consists of a shield (a large metal cylinder) and trailing support mechanisms.

Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which are used to push the TBM forward.