46 Book of Engineering 



machinery. The Greathead Shield was an 

 invention that followed quickly in action after 

 the boring of the Severn Tunnel, and it pro- 

 vided a means of boring tunnels at depths 

 which were formerly thought quite imprac- 

 ticable. 



Simply described the Greathead Shield is 

 a section of steel tube which can be lowered 

 down a shaft. At the bottom the cutting 

 machinery is set in action on the face 

 of the proposed tunnel. As the cutters 

 work their way forward the shield follows 

 upon some temporary rails. The " spoil " 

 is passed backwards to trucks which are 

 already in position, and these run upon 

 the temporary track to the bottom of the 

 shaft where it is raised to the surface and 

 disposed of. 



As the shield goes forward the steel tube, 

 surrounded by a concrete coating, is built 

 into position ready for the laying of the 

 permanent track. In practice a tube is 

 bored from several shafts, and a shield is 

 working in either direction from the foot 

 of a single shaft. 



