The Thames Tunnel introduced the tunnelling shield. This consisted of three iron frames mounted on top of one another like an open-backed bookcase. This structure was placed against the area of earth that needed to be excavated and then workers proceeded to dig in its cavities, removing soil in sections. Once the distance of the cavities had been excavated, it was shunted forward by a series of screw braces. This meant that at each stage of the dig the shield supported the walls and ceiling, allowing time for masonry reinforcements to be installed.