The Second World War presented British marine engineers with their biggest challenge this century. In 1943, they were ordered to build the Mulberry Harbours, artificial havens to shelter ships arriving for the Normandy invasions of 1944.
Among their tasks was to produce 213 precast reinforced concrete caissons in secret and inside seven months before floating them across the Channel to Normandy.
The caissons - typically 60m long, 18m wide and 18m high - were produced at Tilbury, Goole, Middlesbrough and at London’s East India and Surrey Docks. When complete, they were floated across the Channel to Arromanches and St Laurent and then sunk on to the seabed where they - along with strategically scuttled block ships - acted as breakwaters creating the huge temporary harbours forming the landing points for British and US troops.
Within the harbours, engineers had to establish 23 floating jetties, each 1.2km long resting on 670 pontoons. The pier heads were jack up barges connected to the shore by modular pontoon bridges. The jack up barges’ legs stood on the sea bed but the barges themselves were able to float up and down with the tide, allowing ships to moor at all times.
All of these components had to be prefabricated in England - a task which involved 300 firms and 15,000 workers.
The whole project used 20,000 workers, employed by contractors including Balfour Beatty, McAlpine, Bovis, Nuttall, Costain, Taylor Woodrow Mowlem and Henry Boot. Consultants involved included Rendel Palmer & Tritton, Oscar Faber, Sir Alexander Gibb Mouchel and Halcrow.
The harbours were vital to the success of the Normandy invasions, delivering 220,000 soldiers and 39,000 vehicles. Establishing the harbours on the exposed Normandy coast also wrong-footed the Germans, who had expected an invasion near Calais, which is much closer to the British mainland.