A turbomachine casing assembly includes a first casing element locatable radially outward of one or more rotating aerofoil elements of a turbomachine and a second casing element located radially distal to the first casing element, with a void being defined between the first and second casing elements. Each of the first and second casing elements includes, at a first end thereof, respective co-operating first and second interface portions. In the event that an aerofoil element becomes detached from the turbomachine and impacts the first casing element, the second interface portion; slides along the first interface portion and the first end of the first casing element enters the void.