The Building of Mulberry Harbour
Vital build-up in reinforcements and supplies of the successful landings in France in June 1944 was made possible by the unique construction methods employed to produce two complete harbours in the British Isles, and to transport them in sections across the English Channel to the French coast. The harbours were designed to be established at Vierville-sur-Mer and at Arromanches, on the coast of Normandy. That for the former, known as Mulberry A, was intended for American use but had to be abandoned after the very severe storm on D-Day + 13 in favour of the French port of Cherbourg which became available earlier than had been expected. The British harbour, Mulberry B, which was in a more sheltered position, was used right through the autumn and winter and into 1945.
At Arromanches, the site chosen for Mulberry B, the sea bead was shallow and sandy, and a small reef – the Calvados Reef – offshore partially sheltered the area. At one time French engineers had proposed a permanent harbour here, but the difficulties were considered insurmountable. The actual planning of Mulberry B was based on French charts, though the final siting was dependent on soundings taken on D-Day.
Very simple in composition, the harbours comprised only three parts, (1) the pierheads and quays, (2) the floating roadways, (3) the breakwaters. The initial decision to construct them was taken at the Quebec Conference in 1942, when instructions for the design of the pierheads and floating roadways were given. At the 1943 conference, also at Quebec, it was decided to proceed with the design and construction of the concrete caissons which formed the breakwaters.
The design and main research work on the pierheads or landing pontoons was undertaken by Messrs. Lobnitz & Co., Ltd., of Renfrew. This firm's association with the scheme came about through the intervention of Brigadier Sir Bruce White, K.B.E., R.E. (director of the scheme for the Quartermaster-General), who had knowledge of a dipper dredger of Lobnitz construction having safely ridden a severe storm in the West Indies several years previously. The problem confronting the designers was summed up by Mr. Churchill this: “Piers for use on beaches: - They must float up and down with the tide. The anchor problem must be mastered – let me have the best solution worked out. Don't argue the matter. The difficulties will argue for themselves.” The Churchillian manner! That was in the autumn of 1942. The prototype was in the Solway Firth by April 1943. This original pierhead, which required over 1,000 tons of steel, was there thoroughly tested for many months, and was found to be entirely satisfactory – an amazing achievement considering the magnitude of the undertaking and the short time available.
The pierheads consisted of a steel pontoon with four steel legs or spuds which were power-operated and when driven hard against the sea bed caused the pontoon to rise. Hence the pierheads could be adjusted to maintain a fairly constant height above sea-level; alternatively, they could float with the tide, sliding up or down the legs.
After completion of the tests, approval was give to about 200 firms throughout the country for the large-scale manufacture of the piers and pierheads. A waste stretch of ground at Leith was very soon transformed into erection berths where the majority of the construction took place. Other sites were at Conway, North Wales, managed by Messrs. Jos. Parkes & Son, of Northwich, and at the Military Port No. 1 at Cairnryan, where the Army entirely undertook the erection.
The roadways were designed so that they could be towed for about 100 miles and be able to stand up to such weather as is common in the English Channel in the summer months. They comprised 80-foot bridge spans, supported on floats. Each span consisted of two 80-foot girders and a 10-foot wide road, and weighed about 30 tons. They were so flexible that the roadway could twist vertically until one end was at an angle of 40 degrees to the other, and yet still carry the heaviest of equipment at a speed of 25 m.p.h. It was composed of separate panels loosely bolted to the cross members. The support for the spans consisted of pontoons of steel plate or reinforced concrete; these were known by the code name of Beetles. Those Beetles which were always to remain afloat, even at low tide, were constructed of precast concrete panels 1˝ ins. thick, and were joined by concrete ribs cast in situ. Other Beetles were built of steel sheeting. Roadway lengths of 500 feet could be towed in one section; that is in six spans made up of five spans and the shore ramp float.
Construction was undertaken with that of the pierheads, contracts for the two schemes combined – known as Whale – being placed in June 1943, through the normal channels of the Ministry of Supply under the Director General, Mechanical Equipment, Sir Geoffrey Burton.
It had been decided at the Quebec Conference in 1943 that the main breakwaters should be composed of reinforced concrete caissons, and that immediate shelter would be provided by means of breakwaters formed by several old ships that would be sunk as block-ships. This line of block-ships was known by the code name Gooseberry. The ships used were old Merchant Navy and Royal Navy vessels, still seaworthy and in some cases still in use. They were to be the first to arrive, and were to be sunk by explosive charges by their Naval and Merchant Navy crews, who were then to return to Britain. This operation was successfully completed in the first five days after D-Day, and provided very valuable initial shelter.
Sole Failure in Vast Programme
A further outer floating breakwater was to be established to give shelter to ships larger than those of the Liberty type, which were the biggest that the port could accommodate. This floating breakwater, the design and production of which was undertaken by the Admiralty, was the only failure in the whole ambitious programme. Its component units, referred to as bombardons, were destroyed in the storm.
The concrete caissons, known under the code name of Phoenix, were designed like ships, because they would be subjected to similar wave action and would require the same stability as ships in calm or stormy seas. Design was as near box-like as the conditions would permit, but to obtain the lowest towing resistance they were provided with swim ends, not unlike Thames barges.
Lack of time precluded prototypes of the caissons being built, but models were made and tested at the National Physical Laboratory at Teddington for towing and sinking and for the the effect of wave action. As a result of these tests no alternation of the design was found necessary. Although the design of the caissons was not commenced until October 1, 1943, construction was actually put in hand on many sites before the end of the month. One hundred and forty-seven units were to be built in six different sizes, with displacements ranging from 6,044 tons to 1,672 tons. Total length of the caissons added up to approximately 5˝ miles. Under a subsequent programme a further 66 units were constructed.
Blitz Rubble Played a Part, Too
The project was undertaken by 24 leading public works contractors, at 14 different sites around the English coast. This task was controlled by a special department of the Ministry of Supply directed by the late Sir John W. Gibson, who received his knighthood for his work in connexion with Mulberry.
Construction took place in the Graving Docks at Middlesbrough, Goole, Southampton, and of the Port of London Authority at Tilbury. Several units were built in an entrance lock at Plymouth, and in the floating dock there. Two London Docks, the East India and South Dock, were de-watered after precast concrete block dams had been constructed in the entrance locks, and 18 units were built on these two sites, where hundreds of thousands of cubic yards of blitz rubble were used to make construction beds on the dock bottoms. Twenty-three units were constructed on slipways built on the foreshore at Stokes Bay, Stone Point and Langston Harbours in the Portsmouth and Southampton areas. Here the caissons were constructed to about 60 per cent of completion and were then towed to jetties where the remainder of the construction was carried out.
At the suggestion of the Ministry of Supply, shallow basins were excavated in open land alongside the River Thames. But here the depth of excavation was limited by the ground water-level, and the units could only be constructed up to a certain height, after which they were floated out and completed. Sufficient of these basins were dug to enable 40 units to be constructed at a time. Altogether, 98 were commenced in these basins, some being completed in other sites. The most suitable dock – the King George V Dry Dock at Southampton – at which many batches of caissons could have been built, was not available as the Admiralty were using it for construction of bombardons.
This vast programme of construction was an outstanding example of co-operation between various Government departments and the British Public Works Contractors. The contractors provided the site organization, key men and supervision, while the Government provided the materials and labour. Men were directed from all parts of the country to the London area and other sites. Where they toiled on objects about which they knew nothing; nor could they find out anything in regard to the ultimate use of what they were working on. Guesses which were numerous, included “A causeway across the English Channel”, “A floating aeroplane base”, and “Blockades for docks and channels”.
In all, 212 caissons were built, of which 147 were ready before D-Day. The remainder, which were used as additional breakwaters and as replacements, were completed before August 1944. This was indeed a remarkable accomplishment, as is further shown by the following quantities of materials involved: 545,000 cubic yards of reinforced concreted; 66,000 tons of reinforcing steel; 9,000 standards of timber; 440,000 square yards of plywood; and 97 miles of steel wire rope.
Grand Tribute to Our Engineering
Maximum number of men employed was about 20,000; this total included 10,500 labourers, 1,000 scaffolders, 5,000 carpenters, and 770 steel fixers. The War Office issued more than 130,000 prints of working drawings dealing with the construction of the caissons.
That Mulberry – code name used to embrace all the details of the great scheme – was constructed and assembled in the British Isles in such a short space of time in the fifth year of the Second Great War was an astonishing performance. That such an operation as the Normandy landings was successful was a glowing tribute not only to thoroughness of the military planning but also to the British engineering industry.
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