After a shaft has been sunk to a certain depth, the final or permanent lining is inserted. This may consist of: brick walling; concrete blocks shaped to the curvature of the shaft; concrete lining put in liquid form behind shuttering; brick coffering; and cast-iron tubbing. The permanent lining is generally built up in sections, during which operation the temporary lining (such as skeleton tubbing) is removed. Concrete is now widely used as a permanent shaft support.