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Phoenix Caissons, Mulberry Harbours
typical site: Tilbury Docks, London, UK
associated engineer
Oscar Faber
Hugh Iorys Hughes
date  September 1943 - June 1944
UK era  Modern  |  category  Harbour  |  reference  TQ630751
Following the 1943 secret military conference (codenamed Quadrant) in Quebec, at which Oscar Faber was present, the Ministry of Defence instructed some 550 UK engineering contractors to build various elements for the two gigantic Mulberry Harbours used for the Normandy Landings by the British and Americans in World War II. It was an audacious manoeuvre, pivotal to the outcome of the war. All the components of the floating harbours were designed and built from scratch in just one year.
Oscar Faber (1886-1956) attended the Canadian meeting of British and American planning staffs at the direction of the British Admiralty's department called the Directorate of Miscellaneous Weapons Development (DMWD). They met to work on planning for the Normandy Landings, (codenamed Operation Overlord), which were to start the Allied invasion of the continent in response to German aggression, and would be the largest amphibian landing ever undertaken.
Faber travelled to Canada in the bomb bay of a US Liberator plane. In Montreal he transferred to a Dakota for the remainder of the journey. Before the trip, he had been working on a form of floating rubber airbag, stiffened with a concrete keel 180m long. Prototypes, nicknamed "Lilos" had been made and tested in Poole Harbour.
However, the planning staffs instead confirmed a decision to use some form of floating sea barrier made of steel as protection for the troops and material that had to be landed. Two harbours would be built — Harbour A for the American-led armed forces landing on what became known as Omaha Beach, and Harbour B for the British-led forces landing at Arromanches. The steel barriers were referred to as "Hard Lilos" but later codenamed Bombardons.
The idea of the Bombardons was to suppress waves, since the landings would take place on largely exposed beaches and the open water was bound to be rough. They consisted of a series of hollow steel chambers, resembling a Maltese Cross in section. Each measured 60m long and 8m wide, with a draught of 5.7m. The bottom and side chambers of the cross format were flooded with sea water to provide ballast, and the chambers in the vertical arms provided buoyancy.
Inside the ring of Bombardons, a protective assembly of huge concrete caissons were to be placed end to end to provide a breakwater six miles long, along with 'block ships'. All design work was carried out in strict secrecy. Faber had a leading role in the co-ordination of the reinforced concrete design and contracting for the caissons, codenamed Phoenix. Hugh Iorys Hughes (1902-1977), who was working on other elements of the harbours, was a consultant to their construction, deployment and installation.
The construction of the Phoenix Caissons called for 15,000 workmen and 630,000 tons of concrete. One construction site for a part of this work was Tilbury Docks, not far from West Thurrock, where there was a cement works. Faber worked with four contractors, including Costain and Nuttall. Each firm constructed four of the largest Phoenix A1 units, which measured 60m x 17m x 18m high, and displaced over 6000 tons. Other construction sites were dotted around the coast.
At Tilbury, basins were made into the banks of the Thames, but these were limited to 2.4m in depth. The caissons were built up to this height before being floated out of the basins. Construction was completed to full height while they were afloat. Additional dynamic loading was included in the design to accommodate the potential for uneven lifting and flotation.
The idea was to build 146 caissons but in the end 212 were made. Their bottom and external walls were constructed of 380mm thick reinforced concrete, with cross walls 230mm thick. They were open at the top. To make them seaworthy, they had 'swim ends', consisting of 6m x 6m chamfers, roughly shaped across the width at the bottom of the vessel fore and aft.
For the Landings, the caissons were also to be used to form the ends of the harbours, so some would be located in shallow water. They all measured 60m in length but their heights reduced progressively and their width proportionately.
Once constructed, the caissons were sunk at various sites in the English Channel for storage. As D-Day approached, they were pumped out and re-floated. Tugs towed them into place about a mile from the Normandy coast. With the tugs holding them in position, their sea-cocks were opened and they subsided to rest on the sea bed ready to do their job.
The harbour complexes protected a vast array of floating quays and roadways, the designs for which were undertaken by most of the leading British engineers of the day, although none knew about each other's work. The entire project was constructed between October 1943 and D-Day, 6th June 1944, and in the following 10 months enabled the landing of some 2.5 million men, half a million vehicles and untold tonnes of supplies.
Research: ND
"Oscar Faber, his work, his firm & afterwards" by John Faber
Quiller Press, London, 1989
"The Building of Mulberry Harbour" by Michael F Kennard
pp 771-772, April 1947, available at www.thewarillustrated
"The Mulberry Harbours” by A Harris
Royal Engineers Journal, Vol.108, April 1994
"The Secret War 1939-45" by Gerald Pawle
William Sloane Associates Inc, New York, 1957

Phoenix Caissons, Mulberry Harbours