A turbomachine casing assembly includes a first casing element locatable radially outward of one or more rotating aerofoil elements of a turbomachine, a second casing element located radially distal to the first casing element and one or more arcuate fence elements positioned between the first and second casing elements. Following impact of a detached fan blade, a radially outward movement of the first casing element causes the or each fence element to project into the air flow passing through the turbomachine.