The exposed position of Dover made the building of its huge naval harbour a tremendous feat of engineering, calling for much daring and high organizing skill
LOWERING A CONCRETE BLOCK to the bed of the sea along the line of one of the breakwaters in Dover Harbour. The breakwaters were built of huge concrete blocks, laid on the sea bed and not on a foundation of rubble. The sea bed was first cleared to bedrock with the help of divers working in diving bells such as that shown on the platform to the left.
THE National Harbour at Dover can claim to be one of the greatest examples of such works the world has yet known. It is one of the largest, if not the largest, of artificial harbours. Eleven years and £4,000,000 were spent on it. The harbour has a low-water area of 610 acres, affording accommodation for a fleet of twenty-five first-class warships with the attendant smaller craft such as destroyers and submarines. In addition there is a commercial harbour of 75 acres with the requisite jetties and wharves for dealing with passenger and cargo vessels.
When the National Harbour was opened by King George V (then Prince of Wales) on October 15, 1909, Dover, the chief of the Cinque Ports, took on a new role. She became an important naval base, the Gibraltar of the English Channel.
From time immemorial Dover has been an important strategic point, naval and military. To say when it first became a fortified place would be difficult. In the days of Queen Elizabeth Sir Walter Ralegh called attention to the unique position of Dover, declaring that “no promontory, town or haven in Christendom is so placed by nature and situation, both to gratify friends and annoy enemies, as this town of Dover. Nor is there in the whole circuit of this famous isle any port, either in respect of security or of defence, or of traffic and intercourse, or rather of necessity, to be regarded than this town of Dover”.
Having read this, Queen Elizabeth, we are told, decided, with commendable enterprise, to restore and improve the harbour. The channel, which had previously been only 4 feet deep, was extensively dredged and deepened. Successive sovereigns also gave their attention to Dover, and so things continued until it became desirable to transfer British naval strength to home waters and the North Sea.
It was decided, therefore, to build at Dover a national harbour capable of accommodating part of the British fleet. It was recognized that the building of such a harbour would be an engineering feat of the first magnitude. Dover’s geographical situation is such that it is exposed to all gales between extreme east and extreme west, and the full fury of the seas appears to be concentrated, or at any rate is experienced, at this point. There was no convenient headland or other natural barrier of which advantage could be taken, so that to convert the port into a harbour of refuge easily accessible in all weathers, and completely safe, necessitated elaborate development works.
The national harbour was formed by the building of three walls or breakwaters. The old Admiralty Pier, used by the steamers engaged in the cross-Channel traffic, was extended for about 1,600 feet. This now forms the west arm of the harbour.
From the base of the cliffs to the east of the town, immediately below the old convict prison, a second wall, 2,942 feet in length, was built out into the English Channel to the south-west. This wall is known as the east arm. Then, three quarters of a mile from the shore, and running almost parallel with it, is the third wall, or south breakwater, 4,212 feet in length. If the sea wall along the foreshore is included, an aggregate length of more than 12,654 feet, or nearly two and half miles, of breakwater was built.
AT THE FOOT OF THE CHALK CLIFFS at Dover an area of about 21 acres was reclaimed from the sea. A sea wall, 3,900 feet long and 30 feet deep, was built of 3-tons concrete blocks, about 250 feet from the cliff. Some 600,000 cubic yards of material were required to fill the space. The area reclaimed was used for block-making and other purposes.
Access to the anchorage is secured by a gap, 740 feet wide, between the western extremity of the south breakwater and the Admiralty Pier, and on the eastern side of the south breakwater by another gap, 650 feet in width. By this means the harbour can be entered in any weather, tidal circulation is promoted and silting up within the harbour is prevented. The depth of water at the entrances ranges from about 41 feet at low spring tides, and at high spring tides the water depth is about 60 feet. Within the harbour itself a water depth at low tide of up to 40 feet is assured.
The contract for the undertaking was placed in 1897 with S. Pearson and Son, Ltd. The engineers for the Admiralty were Coode, Son and Matthews. Work on the site began in March 1898. One of the first tasks of the contractors was the reclaiming of a part of the foreshore to the east of the town. Hundreds of men, roped together, attacked the cliffs, drilling holes and blasting down great masses of the dazzling white chalk to form a solid platform well above the sea.
As the shore was at this time exposed to the full violence of the seas, great quantities of the chalk were carried away until a retaining wall of 3 tons concrete blocks had been erected on the sea edge of the reclamation by cranes running along platforms on piles driven on to the cliff side of the wall. The sea wall is 3,900 feet long and 30 feet deep. The space between the wall and the base of the cliff was filled with chalk and rubble.
It proved a tedious undertaking, over 600,000 cubic yards of material being required to fill the huge space. The contractors, however, secured an area of reclaimed land about 3,900 feet long and 250 feet wide, or some 21 acres in all, as a site for a concrete block-making yard, workshops, repair shops, foundries and stores.
64,000 Concrete Blocks
A strong wooden barricade, about 9 feet high, was raised to protect the area from the surf of the waves driven in by south-westerly gales. On the completion of the harbour this area was taken over by the Admiralty and upon it stand various Admiralty buildings - repair shops, stores and so forth, including two protected reservoirs for the storage of petrol for submarines.
While the foreshore was being reclaimed, work was begun on the Admiralty Pier and also on the east arm. Until the reclamation was effected the huge concrete blocks, of which the harbour walls are built, were manufactured at Sandwich, some thirteen miles from Dover. Sandwich was selected because the ballast (sand and shingle) necessary for the manufacture of these blocks exists there in large quantities. During the earlier stages of the work the blocks were conveyed to Dover by water. Later, when block-yards had been established upon the site, the ballast was brought to Dover by rail, partly over the old South-Eastern and Chatham system, as far as Martin Mill Station, and thence over Pearsons’ own railway to the cliffs and lowered down to the yard by a funicular railway. The cement was brought from the River Thames by a fleet of barges.
The walls of the harbour are built entirely of concrete blocks. In this point Dover differs from most other harbours. Its walls rise from the sea bottom and not from a rubble mound, as is the usual method. The blocks of which the walls are composed range from 25 tons to 42 tons in weight. The heaviest blocks measure 14½ feet long, 7½ feet wide and 6 feet deep. About 64,000 blocks, averaging 30 tons in weight, were used in the sea walls - 1,920,000 tons in all. Including the blocks for the retaining wall along the foreshore and the apron blocks laid on the seaward side of the breakwaters, there is a grand total of about 3,000,000 tons, by far the greater portion of which lies below the water.
In addition to the blockmaking yard immediately below the cliffs to the east of the town, another yard was established at the western end of the port. Here blocks were made for the Admiralty Pier extension. This yard, too, was built on reclaimed ground. The eastern yard, however, was by far the larger and here the greater majority of the blocks were made.
At one end of the yard stood the charging platform with its hoppers, into which trucks tipped the material - sand, shingle and cement. These hoppers automatically delivered the material to the mixers running underneath on six sets of elevated rails above an equal number of rows of wooden moulds. The moulds were open at the top and their sides were removable. Immediately after having received its charge of six parts of gravel and sand to one of cement and a due amount of water, the mixer began to travel towards its mould, churning up the contents as it moved along the rails, so that no time was lost.
It was necessary for the mixer to make fifteen or sixteen revolutions, and an automatic clockwork device in the cabin of the machine informed the operator in charge when the desired number of revolutions had been made. Having arrived at the mould, which had been previously well greased, the mixer emptied its load, and then returned to the charging platform. As every charge was tipped, the concrete was well rammed into the mould, and when the mould was full the surface was struck off with a straight edge.
After the moulds had been filled they were left to set. After two days the sides of the moulds were slackened off. In eight days the block would be sufficiently set for removal. Running up and down the yard were two 45-tons Goliath cranes. They carried the blocks to one end of the yard and stacked them there. Here they remained for a period of six weeks until ready for the builders.
For the easy handling of the blocks they were “cored”. Two oblong holes, inclined towards each other, were moulded into the concrete by the insertion of wooden bolt cores, which were removed when the concrete had set. Into each hole was inserted a bar called a lewis bolt, furnished with a T-head at the lower end, which was given a quarter turn to grip the block underneath, where there was a recess somewhat deeper than the head. The shackles on the upper ends of the bolts were then passed over the double crane hook of the Goliath crane. In the side of every block was an indentation in the form of a semicircle. When two blocks were placed in position these notches formed a complete circle. The crevice was filled with concrete in bags just large enough to fill the hole, which, having become solid, helped to bind the blocks together. Those concrete blocks exposed to the action of sea and air were faced with granite, which is unaffected by the weather. Most of the granite used came from quarries in Devonshire.
The surveys made showed that the sea bed consisted of chalk, chalk marl and flints. The foundations were secured and the blocks laid from a temporary wooden pier or gantry. As this staging had to bear the weight of Goliath cranes that weighed 100 tons without load, heavily loaded railway trucks and various machinery, it had necessarily to be strongly built. Then there were tides and currents to contend with and the rough seas in stormy weather.
ADMIRALTY PIER EXTENSION being built in Dover Harbour. The Admiralty Pier was extended seawards for a distance of 1,600 feet, thus doubling its length. The extension was advanced at the rate of about 600 feet a year.
On either side of a line of the future harbour wall sets of six great ironshod wooden piles were driven into the sea bottom by piledrivers operating a 2-tons “monkey”. Three or more such sets or groups of piles were driven fifty feet ahead of one another. The piles were then strongly braced and the groups connected by lattice girders. There was a clear opening between the two groups of piles of 70 feet, marking the course of the sea wall. This was bridged at intervals of 20 feet to 25 feet by lattice girders bolted to those which ran longitudinally from one group of piles to the other. Upon these girders heavy timber platforms were placed. They were 15 feet wide, one at either side of the course of the sea wall. They carried rails upon which ran the Goliath crane, and also the trucks which brought the blocks from the yard and carried away the debris. The opening in the centre left the room necessary for operating the grabs and diving bells by which the foundations were secured and the blocks lowered.
The platforms stood 28 feet above high-water mark. The piles that carried them were 100 feet long and about 20 in. square. At first the contractors used Oregon pine, but it was found unsatisfactory. It was liable to damage by attack from seaworms, and if any of the piles broke loose, as some did in stormy weather, they were a menace to shipping. The Oregon pine was accordingly discarded for Australian blue gum, which, curiously enough, came from a place called Dover, in Tasmania. This wood is so dense that it will not float, and it is also immune from the attacks of sea insects. It was so heavy that it had to be towed into position, tied to empty barrels. Some 1,500,000 cubic feet of timber were requisitioned in the building of the harbour. As the wall progressed, the staging was taken down and the material carried along to the farther end, where it was again erected, this operation continuing until the wall was completed.
To secure the foundations great clamshell grabs with open mouths were lowered to the sea bottom. There they worked their way into the chalky bed, fastening their teeth into it. The larger ones, if they got a good bite, brought up five tons of flint and chalk to fill a railway truck. When the ground proved too hard for the grabs a “breaker” - a solid block of iron with three projecting teeth - was used to pound the sea bed into pieces which the grabs could gather. By this means the upper crust was eaten away until a solid bed was reached. Divers were then sent down in bells to level the bed ready for the blocks.
One of the bells was 17½ feet long, 10 feet wide and 7 feet high; it weighed about 40 tons. It was swung out between the platforms by a crane and gradually lowered into the sea, the water being driven out by compressed air. It was lighted by electricity and was almost as bright as day. The moment the bell touched the ground there was a depth of about 2 feet of water in it. This was quickly driven out by an extra force of compressed air.
OUT INTO THE CHANNEL. The Admiralty Pier was extended seawards in the same way as the east arm and south breakwaters were built. On either side of the line of the future wall platforms were mounted on piles to hold the gantry and other cranes. On one of the gantries was mounted a temporary lighthouse which thus advanced as the wall was built.
The man in charge was able to move his bell where he wished by sending signals up to the man in charge of the crane to which the bell was attached. The floor of the sea was uneven and ragged. Four men worked in a bell under a pressure of 27 lb. to the square inch for three hours at a time, digging up the ground until it was perfectly smooth and level. The material was thrown into a large wooden box, swung in the centre of the bell.
Staff of Eighty Divers
The underwater work was carried out by Siebe, Gorman and Co., whose diving staff totalled eighty men. The dress divers, who donned jacket and helmet, worked from small boats. Their work was important and they were responsible for the setting of the blocks under water. It was imperative for these to be evenly and accurately laid. As the great stones were lowered by
the 60-tons Goliath cranes, the divers informed the craneman above by signals when the block was in its exact position and when to lower it on to its bed. They then gave the lewis bolts the necessary twist to disengage them.
From foundation level up to low water the blocks were “joggled”. In other words, the cylindrical cavities left between the blocks were filled in with concrete in bags. The concrete cemented and bound the blocks together. Where the blocks formed the ends of the breakwaters they were further strengthened by being bound to one another with iron bars. Above low water the courses were bedded and grouted with cement, and the outside blocks above this point were faced with granite, the stones being well bonded into the concrete matrix.
While work was progressing on the eastern arm, attention was being paid to the Admiralty Pier. It was doubled in length, that is, to about 3,200 feet. Because of a revolving gun turret on the old pier head, which it was decided to retain, the pier had to be widened for a distance of some 690 feet to permit the laying of the railway tracks serving the steamship berths. The blocks for this portion of the works were made in the west block yard. The rate of work was necessarily dependent upon the weather. In rough seas nothing could be done. When the weather was calm, operations continued night and day. The proper balancing of the different stages of preparation and construction, so that no one stage should get too far ahead or impede that behind, was the contractors’ first care. It is a testimony to the growth of engineering science that, whereas the old Admiralty Pier, built in 1871, was advanced only 91 feet in a year, the more recently built extension, of equal section, was put together at a yearly rate of 600 feet. In one particular month the wall was advanced 75 feet and 601 blocks were laid. Because of the strong currents and the depth of the water, block laying was impossible except for three hours on each tide, and on rough days not at all.
RECLAIMED LAND AND BREAKWATERS are shown in solid black on this map of Dover Harbour. The east arm or breakwater is 2,942 feet long. After a gap of 650 feet the south breakwater extends for 4,212 feet, coming to an end 740 feet away from the 3,200-feet Admiralty Pier and extension. The Admiralty Pier was widened to accommodate the Marine Station and the railway lines serving the cross-Channel vessels which berth there.
The building of the southern breakwater was among the most difficult of the whole undertaking, because of the exposed position and the great depth of the water. As with the rest of the undertaking the work was carried on from a strongly built staging. The average depth of the water along the line of the breakwater at spring tides is about 47 feet. In some places it is as much as 53 feet. The underwater operations on this section of the undertaking were therefore considerable.
Work was begun on the south breakwater in August 1904, and by December of the same year 430 feet of foundations had been completed and the harbour wall had been brought up to the level of low water.
When the extension on the Admiralty Pier had been completed and the plant there had been transferred to the island section, work was maintained at a much higher speed. In a year 2,000 feet of foundations were completed and in two months alone 1,145 blocks were set in position.
Robbing the Seas of Their Force
Because of the severity of the scour and tidal action, the seaward sides of all the breakwaters are protected by “aprons” projecting 25 feet horizontally from their base. The aprons are built of blocks weighing from 9 tons to 14 tons and 3| feet deep. The bed for the aprons was excavated to a depth of 2 feet by divers. The blocks were laid by powerful jib cranes. At the angles which the east arm makes with the shore are two large semicircular aprons shelving upwards with a gentle slope. These give the incoming waves a circular motion which plays one off against another and robs them of their force.
The width of the harbour walls at foundation level ranges between 52 feet and 57 feet, their greatest height being 90 feet. The width of the east arm at deck level is 47½ feet, that of the south breakwater is 40 feet and that of the Admiralty Pier extension 45 feet. In all the height of the deck level above high water spring tides is the same, namely 10 feet. The Admiralty Pier extension and the east arm are provided on their seaward side with parapets 11 feet and 10 feet wide respectively, the top of the former being 43¼ feet, and that of the latter 39 feet, above low water. On a level with the bottom of the parapet is a “bull nose”, or large blunt lip, bending out towards the sea, to deflect the water and hinder it from washing over.
From 1,500 to 1,800 men were engaged upon the work. Considering its character, the accidents were insignificant in number. No deaths or permanent injuries were recorded in connexion with working in compressed air, either in the diving bells or in diving dresses.
Several exciting incidents occurred. While the south breakwater was under construction, the German liner Deutschland ran into it, seriously damaging her stem and removing a mass of concrete.
Severe delays occurred through gales. The full force of the south-westerly and easterly storms was experienced and waves swept over the works.
The harbour walls at Dover have been built entirely of solid concrete blocks set and joggled together. Although the walls stand 90 feet high, no central filling of loose material and no rubble were permitted. The breakwaters to-day are among the mightiest structures of their kind in the world.
HEAVY TIMBER PLATFORMS were built on rows of piles driven on either side of the line of the breakwaters in Dover Harbour. The platforms were braced together by lattice girders at intervals of about 20 feet. Along the platforms ran the cranes and railways for the handling of the concrete blocks which formed the structures.