Brunel's shield, rectangular in plan, was successfully employed in driving the world's first underwater tunnel, under the Thames (Thames Tunnel) at London, 1825–42. In 1865 Peter Barlow of London patented a much simpler shield of circular cross section, 8 feet (2.5 m) in diameter, with which James Henry Greathead (Greathead, James Henry) drove a small-bore tunnel under the Thames in less than a year at modest cost. Simultaneously, Alfred Ely Beach (Beach, Alfred Ely) of New York City devised a shield, also circular in cross section, which he used to drive a short experimental subway under Broadway. In the 1880s Greathead successfully used compressed air behind his shield in a London subway tunnel to prevent flooding while the lining was being installed. The combination of shield and compressed air made tunneling possible under the largest rivers.

Modern tunneling shields are essentially the same as the Greathead design; that is, powerful steel cylinders shoved forward by hydraulic jacks. A diaphragm, or curtain, in the front has a door that may be opened to permit men to work in front of the shield, or it may be closed when the shield is shoved through very soft ground. In front of the diaphragm the cylinder is prolonged by a circular cutting edge that projects farther at the top, forming a protective hood for those working in front of the shield. Behind the diaphragm an erector arm, an adjunct of the shield, constructs the tunnel lining by successively setting segments of steel rings in place. The steel is later covered over with masonry. Hydraulic jacks to advance the shield are braced against the end of the completed lining.