harbours and sea works

harbours and sea works

harbour also spelled  harbor 

      any part of a body of water and the manmade structures surrounding it that sufficiently shelters a vessel from wind, waves, and currents, enabling safe anchorage or the discharge and loading of cargo and passengers.

      The construction of harbours and sea works offers some of the most unusual problems and challenges in civil engineering. The continuous and immediate presence of the sea provides the engineer with an adversary certain to discover any weakness in the structure built to resist it.

Principles of maritime engineering

      The principal objectives of such works fall broadly into two classifications: improvement of transportation, and reclamation and conservancy of land. Under the first fall works directed at providing facilities for the safe and economical transfer of cargo and passengers between land vehicles and ships; fishing ports for the landing and distribution of the harvest of the sea; harbours of refuge for ships and small craft; and marinas for the mooring or laying up of small private craft. Under the heading of reclamation and conservancy come works directed to the protection of the land area from encroachment by the sea, to the recovery and conversion to land use of areas occupied by the sea, and to the maintenance of river estuaries as efficient means for the discharge of inland runoff. In many places, without continuous attention to such maintenance, the coincidence of high tides with heavy rainfall would lead to frequent disastrous flooding of inhabited areas.

      The civil engineering techniques used for either of these objectives are broadly similar, and indeed the realization of both objectives at the same time will frequently be a feature of the same project. An operation of maintaining a river estuary at a depth sufficient for navigation, for example, may at the same time greatly improve its capacity for the drainage of upland floodwaters.

Hydraulic models
      The planning of maritime civil engineering works, whether for transportation, reclamation, or conservancy, has been facilitated by the development of the technique of model studies. Once regarded as scientific toys, such studies are now considered an essential preliminary step to any large-scale redevelopment of a port or coastal area and are useful even for minor modifications or additions.

      Scale models of the area, harbour, or estuary are made so that water can be caused to flow in such a way as to reproduce the various tidal and other streams in the same direction and with velocities equivalent to those occurring on the site. A variety of devices, usually electronically controlled, have been developed to produce both wave and tidal effects.

      The value of these experiments derives from the reduction in the time scale, which has been found to correspond to the reduction in the dimensional scales of the model. Thus, the large model of the Clyde (Clyde, River) estuary of Scotland works on a tidal cycle of about 14 minutes, or about 50 times the actual frequency. The effect of three years of tides following any modification of the profile of the harbour can thus be studied on the model in a matter of three weeks, and any tendency to otherwise unanticipated scour (clearing by powerful current) or siltation can probably be detected. The relative values of alternative positions of breakwaters in affording shelter can be similarly studied using the wave-generating devices available; and the development of secondary, or reflected, waves with undesirable disturbances within the sheltered area may be anticipated and, if possible, forestalled.

Natural and artificial harbours
      In certain favoured points on the world's coastlines (coast), nature has provided harbours waiting only to be used, such as New York Bay, which the explorer Giovanni da Verrazano described as “a very agreeable location” for sheltering a ship. Such inlets, bays, and estuaries may require improvement by dredging and must be supplied with port structures, but basically they remain as nature made them, and their existence accounts for many of the world's great cities. Because such natural harbours are not always at hand where port facilities are needed, engineers must create artificial harbours. The basic structure involved in the creation of an artificial harbour is a breakwater, sometimes called a jetty, or mole, the function of which is to provide calm water inshore. Locations for artificial harbours are of course chosen with an eye to the existing potential of the coast; an indentation, however slight, is favoured. Yet it has often been found justifiable on economic or strategic grounds to construct a complete harbour on a relatively unsheltered coastline by enclosing an area with breakwaters built from the shore, with openings of minimum width for entry and exit of ships.

Sea works for transportation

Classical harbour works
      Improvements to natural harbours and construction of artificial harbours were undertaken in very ancient times. There is no conclusive evidence for the date or locality of the first artificial harbour construction, but it is known that the Phoenicians built harbours at Sidon and Tyre in the 13th century BC.

      The engineers of those days either knew or thought little about conservancy even as applied to the ports they constructed. Evidence is to be seen in the once thriving ports around the shores of the Mediterranean that now are not merely silent ruins but seem so far from even sight of the sea that it is difficult to imagine the presence of seagoing ships at the wharves, the alignment of which can occasionally be traced in the fertile alluvial land now occupying the site. Ephesus, Priene, and Miletus, on the Aegean shores of Asia Minor, are examples of this type of harbour disappearance, the destructive agent in each of these cases being the picturesque Meander (now the Menderes (Menderes River)) River, whose creation of new land from the sea is readily perceivable from high ground adjacent to the river mouth. The formation of further bars is proceeding visibly—and, as there is currently no port in the vicinity whose livelihood can be threatened, it is interesting to speculate how far out to sea this process will ultimately continue in the course of the next millennium or so.

      At Side, facing the island of Cyprus, the remains of an ancient breakwater, built to protect the anchorage, can still be seen, but the area enclosed between it and the advancing shoreline is now not a stone's throw wide. In this case, not only the river in the vicinity but also littoral drift, (the movement of sediments by a current parallel to the coast), which produces and maintains extensive beaches to the east and the west, must be held partly responsible for the scale of siltation.

      Of many of the ancient port structures, no physical trace remains, but knowledge of the fact that they existed and even a measure of technical description has come down through the written word. With these descriptions and the monuments that still remain, some picture may be formed of the work undertaken by the maritime civil engineers of ancient times.

      Given the frailty of the craft for which they were providing, shelter from the weather was the prime consideration; and much effort was devoted to the construction of breakwaters, moles, and similar enclosing structures. Cheap labour was abundant, and the principal material used was natural stone. Surviving structures built in this way are likely to give an appearance of indestructibility, which occasionally attracts favourable comparison with the lighter, more rapidly depreciating modern structures. It is not, however, necessary to credit the engineers of antiquity with a conscious intention to build forever. Given the materials they had to use and the purposes they were implementing, they could do little else; moreover, because there was no rapid pace of advance in the development of ships or land transport, they were undisturbed by the shadow of obsolescence. In the 20th century, far from wanting to build forever, the port engineer has to be careful to avoid saddling posterity with structures that may long outlast their usefulness and turn into liabilities. The modern balance between excessive durability and dangerous frailty is one that the ancients never had to strike.

      Aided by the characteristics of the material they employed, the ancients constructed maritime works on a scale that is certainly remarkable to this day. Interesting technical practices included the use by the Romans (ancient Rome) of the semicircular arch in constructing moles or breakwaters, an arrangement that allowed a measure of ingress and egress by the sea to produce a beneficial scouring action in the harbour. The Romans underpinned their structures with timber piling and frequently resorted to the construction of cofferdams (cofferdam) (watertight enclosures) that they could dewater by the employment of Archimedean screws and waterwheels. This practice enabled them to carry out much of their foundation work in the dry; and the use of their famous hydraulic cement, pozzolana, gave their structures a durability far exceeding that afforded by the lime cement available to their predecessors.

      Among the more interesting harbours of the ancient world are Alexandria, which had on the island of Pharos (Pharos of Alexandria) the first lighthouse in the world; Piraeus, the port of Athens; Ostia, the port of Rome; Syracuse; Carthage, destroyed and rebuilt by the Romans; Rhodes; and Tyre and Sidon, ports of the earliest important navigators, the Phoenicians.

Breakwaters (breakwater)
      Because the function of breakwaters is to absorb or throw back as completely as possible the energy content of the maximum sea waves assailing the coast, they must be structures of considerable substance. The skill of the designer of a breakwater lies in achieving the minimum initial capital cost without incurring excessive future commitments for maintenance. Some degree of maintenance is of course unavoidable.

Breakwater design
      A common breakwater design is based on an inner mound of small rocks or rubble, to provide the basic stability, with an outer covering of larger boulders, or armouring, to protect it from removal by the sea. The design of this outer armouring has fostered considerable ingenuity. The larger the blocks, the less likely they are to be disturbed, but the greater the cost of placing them in position and of restoring them after displacement by sea action. Probably the least satisfactory type of armour block, frequently used because of its relative ease of construction, is the simple concrete cubic, or rectangular, block. Even the densest concrete seldom weighs more than 60 percent of its weight in air when fully immersed in seawater; consequently, such blocks may have to be as much as 30 tons (27,000 kilograms) in weight to resist excessive movement.

      Boulders of suitably dense natural rock are generally much more satisfactory and, in a project completed in the United Kingdom in the 1960s, it was found by experiment, and subsequently confirmed in experience, that armouring of this type could be composed of blocks of as little as six to eight tons to resist the action of waves up to 18 feet (5 metres) in height. The same experiments showed that, to afford the same protection in the same circumstances, concrete blocks of 22 tons would have been necessary.

      In such cases, an intermediate layer of smaller blocks or boulders is inserted between the armouring and the inner core to prevent the finer material in the core from being dragged out by sea action between the interstices of the armour—a process that leads to ultimate settlement and possible breaching by overtopping of the breakwater.

      The increasing cost and frequent unavailability within economic distance of suitable natural rock has provoked considerable thought to the design of concrete armour units that can, by reason of their shape, overcome the disadvantages of the simple cubic, or rectangular, block. One of the most successful has been the tetrapod, a four-legged design, each leg projecting from the centre at an angle of 109 1/2° from each of the other three. Legs are bulbous, or pear-shaped, with the slightly larger diameters at the outer end. These units have the property, when placed, of knitting into each other in such a way that the removal of a single unit without the displacement of several others is almost impossible, while the interstices between them act as an absorbent of wave energy. Weights substantially less than those needed for cubic blocks are adequate in the case of tetrapods in similar storm conditions. The tetrapods can be mass-produced adjacent to the site through the employment of reusable steel forms.

      It is usual to construct some form of roadway along the crest of a breakwater, even when this is not required for any other dockside purposes, to facilitate inspection and access for labour, materials, and equipment for damage repairs.

Solid breakwaters
      In certain circumstances, particularly in parts of the world where clear water facilitates operations by divers, vertical breakwaters of solid concrete or masonry construction are sometimes employed. Some preparation of the seabed by the depositing and leveling of a rubble mound to receive the structure is necessary, but it is usual to keep the crest of such a mound sufficiently below the surface of the water to ensure its not becoming exposed to destructive action by breaking waves. Repulsion of the waves by vertical reflection rather than their absorption is the philosophy of protection in all such cases, but it is not possible to state categorically which arrangement produces the most economical structure.

      This type of breakwater can be conveniently constructed through the use of prefabricated concrete caissons (caisson), built on shore and floated out, sunk into position on the prepared bed, and filled with either concrete or, less frequently, simple rubble or rock filling. A historical example of this arrangement was the Mulberry Harbour (Mulberry), built by the Allies and floated into position for the invasion of Normandy (Normandy Invasion) in 1944. No previous preparation of the seabed was possible, and only partial filling of the caissons had been carried out when the progress of the war rendered further operations unnecessary. Nevertheless, the fact that several of the caissons remained in position basically undamaged for nearly a decade after the invasion on this notoriously stormy coast demonstrated the possibilities of the method.

Floating breakwaters
      Because of the large quantities of material required and the consequent high cost of breakwaters of normal construction, the possibility of floating breakwaters has received considerable study. The lee of calm water to be found behind a large ship at anchor in the open sea illustrates the principle. The difficulty is that, to resist being torn away in extremes of weather, the moorings for a floating breakwater must be very massive. They are therefore difficult to install and subject to such constant chafing and movement as to require substantial maintenance. Another problem arises, especially in areas of large tidal range. The unavoidable—indeed, essential—slack in the moorings may allow the breakwater to ride large waves, so that they pass underneath it carrying a considerable proportion of their energy into the area to be sheltered.

      One approach to the problem is based on the concept of causing the waves to expend their energy at the line of defense by breaking on a large, floating horizontal platform.

Pneumatic breakwaters
      Finally, the pneumatic, or diffusion, breakwater has been widely discussed. Experiment and limited experience have shown that a curtain of air bubbles blown up from the seabed through a row of perforated nozzles acts as a barrier to the movement of waves over the surface. The mechanics of the arrangement appear to be that the rising bubbles generate streams flowing on the surface, outward in both directions, and the flow meeting the oncoming waves can be made sufficient to hold them up. There is reason to believe that jets of water would be almost as effective as air. Although the volume of air or water necessary to restrain completely the waves generated in severe weather over a wide front would require installation of a plant of uneconomical size, the device can be useful for the temporary protection of a short length of shore to allow the execution of specific works. The air or water pipes can be laid on the seabed at the perimeter of the area to be protected and fed from a mobile plant on shore, and the whole body of equipment can be removed after the operations have been completed.

Docks (dock) and quays
      Because the principal operation to which harbour works are dedicated is transfer of goods from one transportation form to another (e.g., from ships (ship) to trucks), it follows that docks, wharves, and quays are the most important assets of a port.

      Ships must lie afloat in complete shelter within reach of mechanical devices for discharging their cargoes. Although in emergencies ships have been beached for unloading purposes, modern vessels, particularly the larger ones, can rarely afford contact with the seabed without risking serious structural strain. The implications of cargo handling, as far as civil engineering works are concerned, do not differ much whether the loading and discharge are effected by shore-based cranes or by the ship's own equipment. In either case, large areas of firm, dry land immediately alongside the ship are required; the engineer must find a way to support this land, plus any superimposed loading it may be required to carry, immediately adjacent to water deep enough to float the largest ship.

      The capital cost of such works probably increases roughly in proportion to the cube of the deepest draft of ship capable of being accommodated; thus the economic challenge posed by the increase in the size of modern ships is considerable. The advent of containerization—the packaging of small units of cargo into a single larger one—has not fundamentally altered this problem, except perhaps to reduce the number of separate individual berths required and to increase greatly the area of land associated with each berth. A figure of 20 acres (8 hectares) per berth is freely mentioned as a reasonable requirement. The problem of land support at the waterline remains the same.

Gravity walls
      The solution initially favoured, and indeed predominant for many years, was that of the simple gravity retaining wall, capable of holding land and water apart, so to speak, through a combination of its own mass with the passive resistance of the ground forming the seabed immediately in front of it. To ensure adequate support without detrimental settlement of the wall, to ensure its lateral stability, and to prevent problems of scour, it is necessary to carry the foundations of the wall below the seabed level—in some cases a considerable distance below. In earlier constructions, the only guide to this depth in the planning stage was previous knowledge of the ground and the acumen of the engineer in recognizing the characteristics of the ground upon seeing it. Many projects were carried out in open excavation, using temporary cofferdams to keep out the sea. In particularly unfavourable or unstable soils (soil mechanics), accidents caused by collapse of the excavation were not unknown.

      In modern practice, no such project is initiated without exhaustive exploration of the soil conditions by means of borings and laboratory tests on the samples. Continuous monitoring of the soil conditions during construction is also considered essential. Even so, accidents caused by soil instability still occasionally occur.

      The material composing the walls is today almost universally concrete, plain or reinforced, according to the requirements of the design. This material has entirely superseded the heavy ashlar (natural rock) masonry at one time used for such construction, when the techniques for the large-scale production of concrete were not so well developed as they are today.

      In some circumstances, particularly those in which the water is reasonably clear or the design and soil conditions do not require very deep excavation into the seabed, the construction of quay walls is adopted by means of large blocks, sometimes of stone but generally of concrete, placed underwater by divers. The economics of this method of construction are influenced by the high cost of skilled divers and by the cumbersome nature of diving equipment. The development of lightweight, self-contained equipment, which leaves the diver considerably more mobile, may relieve this problem.

Concrete monoliths
      The risks and difficulties attendant on the construction of gravity walls have been avoided, in suitable conditions, through the use of concrete monoliths sunk to the required foundation depth, either from the existing ground surface or, where the natural surface slopes, from fill added and dredged from the front of the quay wall on completion. This technique amounts to the construction above the ground of quite large sections of the intended wall, usually about 50 feet square in plan, which are then caused to sink by the removal, through vertical shafts, of the underlying soil. Another lift of wall is then constructed on top of the section that has sunk, more soil is removed, and the process is repeated until the bottom has reached a foundation level appropriate to the required stability. Considerable skill is sometimes necessary in the sinking process to prevent the monoliths (usually provided with a tapered-steel cutting edge to the lowest lift) from listing, an eventuality that can occur if any part of the periphery encounters material that is particularly difficult to penetrate. Differential loading of the high side and special measures to undercut the material composing the obstruction may be necessary.

      The shafts through which the excavated material is removed are generally flooded throughout the operation simply from the intrusion of the groundwater; if necessary, this water can be expelled by the use of compressed air. The excavation of difficult material in detail and in the dry can then be undertaken. It is an operation of some delicacy, because the flotation effect of the compressed air adds a further element of instability to the monolith, and a blow (sudden leakage of air) under the cutting edge may result in flooding of the working chamber. When the bottom edge of the monolith has reached the designed level, the excavation shafts are sealed by concrete plugs. The shafts themselves can then be filled, either with concrete or with dry filling to give the final wall the required mass for stability.

      Success in this form of construction cannot be guaranteed. In the case of the Western Docks at Southampton, Eng., constructed between World War I and World War II, it was found impossible, except at inordinate cost, to get the monoliths to sink through the opposing strata to the depth required for stability as a retaining wall. It was therefore necessary to reduce the thrust involved in this function by cutting the retained material back to a natural slope and spanning the gap between the back of the monoliths and the top of this slope by means of a reinforced-concrete relieving platform, supported along its other edge on reinforced-concrete piles. This arrangement has served well enough as far as the quay wall itself is concerned, but the maintenance of the natural slope, stone-pitched as a protection against erosion, has been a continuing liability. In addition, the presence behind the quay of the relieving platform constitutes a formidable obstacle to further construction work—e.g., warehouses or multistory transit sheds.

      In situations in which the depth from ground level to the final dredged bottom is not excessive and the material available for retention as reclamation is of good self-supporting qualities, quay walls can be constructed of precast concrete caissons floated into position and sunk onto a prepared bed in the same manner as that described for breakwaters. Care is taken to design caissons able to withstand the thrust of the retained material, which is carefully selected for the areas immediately behind the quay wall. The conditions suitable for this form of construction are generally typical of the Mediterranean, where the slightness of the tidal variation keeps the depth required to a minimum. An outstanding example of this kind of construction is the extension to the area of the Principality of Monaco, which is being increased by as much as 22 percent by reclamation retained by this technique. Similarly constructed installations for transportation and ship-repair purposes exist elsewhere in the Mediterranean, in parts of which the earthquake factor is an additional influence on the retaining-wall design.

      In all cases of dock wall construction by concrete monolith or caisson, it is the basic structure of the wall that is provided by these means; the final superstructure, above highest tide level, will depend for its detail on the requirements for dockside services, crane tracks, and other elements.

The piled jetty
      The high cost, difficulties, and possible dangers of providing dock and quay walls of the kind just described have always encouraged a search for alternative solutions that would eliminate the need for operations on or below the seabed. Of these, the earliest and most obvious is the piled jetty—its piles (pile) can be driven from floating craft and the deck and superstructure added thereto, working wholly above water. In regions in which there is a large tidal range, it may sometimes be both advantageous and necessary to take the opportunity provided by extremely low tides to make attachments to the piles for bracing and stiffening purposes. With a reasonable programming of the work, this operation can usually be done without particular difficulty, assuming that the seabed is of a composition reasonably amenable to penetration by piles to a sufficient depth to secure the lateral stability of the structure. Hard rock is not suitable, although some of the more friable rocks can be pierced by steel piles.

      Piles may be of timber, reinforced concrete, or steel. Timber (wood) is a popular choice if there is a large natural supply. Lateral stiffness and stability can be achieved by using a sufficiently close spacing of the piles in both directions and adequate rigid bracing between the tops, timber being a material readily amenable to the workmanship required. Its chief drawback is lack of durability, particularly in the area between wind and water, although a timber jetty with reasonable maintenance can often resist normal operational obsolescence. There are examples of construction in which the piles are connected together by casting a reinforced-concrete slab around the heads, its soffit (underside) just below lowest water level. By this means, the timber is kept continually submerged, a condition under which its durability is prolonged. On the other hand, in tropical or semitropical waters or waters kept warm by industrial effluents, the use of timber may be inhibited by the presence of marine borers. Timber jetties have a considerable advantage in the comparative ease with which repairs to accident damage or deterioration can be effected.

      Reinforced-concrete piled piers and jetties, soundly constructed, exhibit great durability. Attachment to the piles for bracing and similar purposes tends, however, to be more complicated than in the case of timber. This is a disadvantage that applies also to subsequent maintenance and repairs.

The sheet-piled quay

      An extension of the piled jetty concept is a quay design based on steel sheetpiling, the design becoming increasingly popular with improvements in the detail and manufacture of the material. Steel sheetpiling consists in essence of a series of rolled trough sections with interlocking grooves or guides, known as clutches, along each edge of the section. Each pile is engaged, clutch to clutch, with a pile previously driven and then driven itself as nearly as possible to the same depth. In this way a continuous, impervious membrane is inserted into the ground. In most designs the convexity of the trough sections is arranged to face alternately to one side and the other of the line along which the membrane is driven, so that a structure of considerable lateral stiffness is built up. At the same time, a measure of flexibility in the clutches allows some angular deviation so that a membrane curved in overall plan is obtainable, a feature of considerable convenience in developing the layout of a series of wharves or quays.

      The development of steel sheetpiling over the years has largely been characterized by the increasing weight and stiffness of the sections available from the rolling mills. In one design, the clutch is a separate unit from the main structural element, generally of broad-flanged or universal beam section. In this case, the clutch unit appears in a profile of two grooves, or channels, back to back, each capable of embracing the flanges of adjacent beams, which are thus locked together in a continuous sheet, or membrane, of considerable strength. Each universal section is entered, when pitched for driving, into the clutch on the previously driven section and usually carries the clutch for the next section with it. In another design, made economically possible by the advances in the technique of automatic continuous welding, rolled universal beam sections are welded by one flange into the troughs, or pans, of conventional sheet piles, the composite construction producing a unit of unique strength and stiffness.

      The development of steel sheetpiling has kept ahead of the development of hammers (hammer) capable of driving it, probably because the stiffer the section is, the greater the length of pile that can be incorporated in a design. The combination of heavier section and greater length demands a greater proportion of the energy delivered by the hammer being unproductively absorbed in the temporary elastic compression of the pile, leaving less energy to drive the pile further into the ground. Simply increasing the amount of energy delivered, by using a heavier hammer or a higher drop, does not necessarily provide the solution; it may only result in damage to the head of the pile without achieving greater penetration. This difficulty has been in part overcome by the use of high-strength steel piles. Nevertheless, it is not unknown for a pile to appear to be going down with little or no head damage when it is, in fact, sustaining extensive damage below seabed level that gravely compromises its efficiency as a retaining quay wall. This situation, usually the buckling of a pile, can occur particularly where the material of the seabed contains boulders or similar obstacles to penetration.

      The problem has obvious major implications for the construction of quay walls and has provoked much debate among engineers. The skill of the quay designer and the advice of the soil mechanics specialist both contribute to the satisfactory reconciliation of the various conflicting factors outlined in order to achieve the most effective and economical solution.

      In the normal design of sheet-piled quay or wharf wall, the sheetpiling itself forms the quay face, although it is generally found advisable to protect the piles from the impact of ships berthing by timber fenders. Vertical timbers at intervals are generally used. Horizontal walings (wooden ridges) between these timbers can also be employed, but they have a disadvantage, particularly at small wharves and with ships having their own protective belting: on a rising tide the beltings become entangled with the walings, occasioning damage or even minor disaster.

      The upper part of the sheetpiling, being laterally unsupported on the sea side, is generally anchored back to resist the thrust of the retained soil. This resistance is commonly effected by using tie rods secured to anchors buried in the retained soil itself to a depth that, for reasons of overall stability, is beyond the natural slope line of the soil. As often as not, these anchors are themselves composed of lengths of sheetpiling driven, if possible, below the retained soil into the strata beneath. The mild, or alloy, steel tie rods, coated and wrapped against corrosion, can be carried through the exposed sheetpiling of the quay wall with large retaining nuts on the outside or can be secured to welded attachments at the back of the piling. The latter practice is the more commonly favoured arrangement, largely on account of its more finished appearance. The sheet-piled quay just described is completed by casting a reinforced-concrete cope beam to cover as well as contain the exposed heads of the sheetpiling.

      The advantage of this type of quay wall is that the space behind the wall is not occupied—as in the case of the suspended pile-supported deck—by a monolithic, fully structural element, the arrangements of which can be disturbed for subsequent modification of the services layout only at some cost and usually by a potentially complicated design operation. As in the case of a gravity wall, the space can be filled with suitable material that can subsequently be treated, for all intents and purposes, as natural ground in which service ducts can be buried if required. This arrangement is often an advantage in the case of freshwater mains for fire fighting or watering ships because they can thus be protected from frost. Alternatively, it is possible to place concrete-lined service and cable trenches in this material, sometimes conveniently by the use of precast sections, because the ground loads imposed are seldom sufficient to give rise to serious settlement problems.

Structural reinforcement
      Identifiable structural loading—arising, for example, from crane tracks—can be supported on reinforced-concrete beams on piles driven through the filling to the strata beneath. Dockside railways, a decreasing requirement because of the transfer of much shore-to-ship delivery to road vehicles, need not necessarily have piled support, because the loading from these can be spread to remain within the bearing capacity of the filling. Some settlement is bound to take place, and the need for compensating by packing up and releveling of the track has the incidental disadvantage of breaking up the surfacing of the quay, which is almost always provided to facilitate quayside access by road vehicles.

      Sheet-pile quay walls are readily applicable to sites at which only relatively shallow or medium-depth water alongside is needed. As the required depth increases, a sheet-pile section of sufficient strength and stiffness to hold the retained material without further assistance becomes impractical from the point of view of handling and driving. A solution increasingly favoured is the so-called Dutch quay. In this design, after the line of sheetpiling has been driven using one of the heavier and stiffer sections, the ground behind is excavated for a distance determined by the natural slope of the material to be used as filling and taken down as far as possible to lowest water level. At this level, a reinforced-concrete relieving platform is constructed up against the sheetpiling but with independent vertical support from bearing piles driven through the bottom of the excavation to an appropriate depth. Piles for crane tracks are driven at the same time as these—that is, before the construction of the relieving platform.

      Filling material is returned above the relieving platform, and, although the latter now prevents further pile driving in the area, the probability of this being required is remote, whereas the retained load against the sheetpiling is much reduced. The advantages of having filled material behind the sheetpiling for installing services remain. In addition, the relieving platform affords the sheetpiling considerable help in resisting horizontal blows from the impact of berthing ships, and in order to increase this resistance some of the piles supporting the platform are often driven toward the quay face. Reinforced-concrete counterforts between the platform and the sheeting can be an additional help.

      A question that hung over the use of steel sheetpiling in salt water (seawater) in its early years concerned its durability in potentially hostile conditions. The rate of corrosion, particularly at the waterline or within the tidal range, varied from one locality to another according to the state of the water and the effect of such factors as salinity and industrial effluents. Precoating of the pile with a protective film such as tar or a bituminous paint is of only transient value, requiring regular renewal, and is effective only down to the lowest water level.

      The inclusion in the composition of the sheet-pile steel of a very small percentage of copper was tried as a means of increasing its durability, but the effectiveness is doubtful.

      The confirmation of the electrochemical (electrochemical reaction) basis of much of the corrosion affecting steel sheetpiling led to the development of cathodic protection, a process that has wide application in many other fields, especially shipbuilding. Electrolytic (electrolytic cell) corrosion arises from the passage through the piling of electric currents, causing the pile, or part of it, to become the anode, or positive pole, in what amounts to a galvanic cell, or battery. In such a cell, metal is normally removed from the anode and may reappear on the cathode, or negative pole, which remains unaffected. These currents in sheetpiling may arise from stray leakages from adjacent electrical installations or be generated within the pile itself by differences in the electrolytic conditions at differing levels on the pile.

      Cathodic protection is a means whereby cathodic polarity is imposed upon the whole pile, and its operation as an anode (with consequent deterioration) is prevented. This can be done either by supplying from a suitable source—e.g., a battery—an electric current that will overcome and reverse the direction of the naturally generated current or by connecting the piling at intervals to sacrificial anodes of an element—generally aluminum or magnesium—whose atomic relationship to the steel in the piling is such as to generate a current without external assistance. These anodes are buried in the surrounding ground, and care must be taken to ensure full electrolytic continuity between them and the piling to complete the circuit. It is sometimes necessary, in order to ensure electrical continuity between the anode connections in the piling itself, to weld adjacent piles together after driving.

      By whatever means cathodic protection is applied, a small liability for operational maintenance arises, either for the continuous supply of the imposed current or for the periodic renewal of the sacrificial anodes. The considerably increased durability of the structure usually justifies this.

Enclosed docks
      Whenever possible, commercial quays are built open to the tide range to provide maximum freedom for shipping. There are, however, some parts of the world in which the range between low water and high water is so great that the resulting variations in the level of the ship's decks and hatches impose unacceptable disabilities on the handling of cargo. In such circumstances the quay walls may become of such dimensions as to be uneconomical. (The net clear height of the quay walls, disregarding depth of foundations, must span the distance from the lowest seabed level acceptable for navigation at low tide to an adequate freeboard for the coping of the quay wall above the level of the highest high tide. This condition is equally applicable in cases in which only the berths themselves are made to be usable no matter what the stage of the tide.)

      The problem can be met by constructing the quays as enclosed docks in which the water level is kept constant and access to the tidal areas is by means of a lock or locks. An obvious condition for the success of such an arrangement is that the strata of the bed under the enclosed dock area be sufficiently impervious to preclude any significant loss of water through the bottom during low-tide conditions. In this way the tidal range, as a limit on the height of the quay walls, can be eliminated.

      Apart from the fact that they have gates at each end, the structure of maritime navigation locks and the problems involved in their design are very similar to those of dry docks. Although, in normal usage, a lock is never completely dry, it is essential that it should be designed to be capable of withstanding the stresses imposed by this condition so that it may be possible to dewater the lock completely for inspection and maintenance.

      It is common practice to design enclosed docks so that the normal water level maintained is not below the highest likely high tide because the invasion of an enclosed dock by a high tide significantly above the normal water level can be disastrous.

      Although enclosed docks are frequently of such an area that they can supply the lockage water lost when a ship passes through the lock without any drop in level that cannot be made up on the next high tide, it is normal to provide a measure of impounding capacity in the form of pumps for lifting additional water from outside into the dock. Such a provision is essential for situations in which it is required to keep the enclosed dock water level above the highest tide.

      It has sometimes been possible to accommodate ships of larger draft than originally planned for in large but relatively old enclosed docks. This is done by installing impounding pumps for topping up the water level to give an increased depth.

      Enclosed docks generally suffer the operational disadvantage of restricted times of entry and exit because they are subject to a fairly rigid tidal schedule. First of all, the lower the tide level outside, the greater the amount of water lost in the locking operation; and, second, it is seldom economically feasible to maintain full navigation depths in the approach channel to the lock entrance at all levels of the tide. This situation is particularly the case in which enclosed docks are sited adjacent to and operating from a tidal river estuary. The tidal lock at Dunkirk, Fr., opening to allow the passage of the night channel ferry, which runs on a timetable, is an example of a tidal lock operated whatever the state of the tide.

      If possible, the access locks are usually duplicated, lest an accident involving the gates or the structure of the lock put the whole dock area out of operation. Stability calculations of the quay walls within an enclosed dock are important; in older installations such calculations may have been based on the continuing presence of water at the designed normal level, and in the event of a serious failure at the lock—resulting in a considerable drop in the water level—the stability of the quay walls could come into question.

Roll-on, roll-off facilities
      An enormous increase in the use of the roll-on, roll-off technique of loading and unloading developed in the late 1960s. The principle of embarking whole vehicles under their own power was not new. The report of Hannibal ferrying his elephants over the Rhône in the 3rd century BC might be regarded as the earliest example from which the vast amphibious operations of the invasion of Normandy in 1944 were descended. Since the 1960s, however, the spectacular increase in the use of road transport for heavy freight and the increase in handling charges at ports for the loading and discharge of cargo by conventional means have combined to provide the impetus for the rapid commercial development of the roll-on, roll-off technique. In addition, the tendency to assemble machinery at its place of manufacture in larger and larger units has encouraged the development of special transport vehicles, and the economies of moving load and vehicle together from origin to destination can be considerable.

      The principal problem for the port engineer is to provide special berthing for the ferry vessels and means of access for vehicles from the shore to the ship's decks. Railcar ferries, involving somewhat similar problems, have been known for some time, but, because of the severer limits on gradients for such vehicles, there has been a tendency to limit the operation of these services to terminals at places where the tidal range is inconsiderable. For the Dover-Dunkirk ferry, opened shortly before World War II, a special enclosed dock was constructed at Dover in which the water level could be kept constant for loading and unloading, while at Dunkirk the entire dock system is totally enclosed, accessible through sea locks.

      Many of the new roll-on, roll-off terminals for road services are, by contrast, in tidal water; and, where the tide range is large, access bridges of considerable length are often needed to keep the change of gradient between low and high tide within acceptable limits. The change in the ship's trim between conditions of light loading and full loading creates yet another problem.

      At first sight, the solution might appear to be to support the outer end of the link span on a float, or pontoon, so that it would automatically follow the rise and fall of the tide. Several disadvantages of structural detail arise, however, and the system is vulnerable to damage caused by the movement of the pontoon under adverse weather conditions. A means to adjust the height between the span and the supporting pontoon to accommodate changes in a ship's trim is still required; and, therefore, the overall economies of a pontoon are less than might at first be imagined.

      Thus it is almost universal practice to support the outer end of the link span from an overhead structure, either through conventional wire-rope hoisting gear or by means of hydraulic rams. The level of the end of the span can thus be continually adjusted, either automatically or by manual control, to match changes in the level of the ship's deck, whether caused by the tide, by the trim of the ship, or by differences in deck levels between one ship and another. Maximum flexibility of access has become increasingly important with the appearance, on some services, of ships with two independent car decks, both of which must be equally accessible to the link span. This situation has sometimes been achieved by the use of double-decker link spans, a technique that has the effect of keeping the length and—unless the span is intended to carry loads on both decks simultaneously—the weight of the span to a minimum.

      The sudden proliferation of roll-on, roll-off services simultaneously has led to a rather unfortunate development: a number of the terminals have tended to be tailor-made to suit particular ships and to be unable to accept different ships without, in some cases, quite major structural alteration. This feature clearly reduces the otherwise great flexibility of this technique, and an international commission to examine the question was appointed in 1970 by the Permanent International Association of Navigation Congresses.

      Maximum advantage of the roll-on, roll-off technique is gained in relatively short sea passages. On longer voyages, the idle road vehicles make the economies questionable. This problem can be overcome to some extent by embarking only semitrailers and leaving the tractive units ashore; the practice has no effect on the terminal details.

Bulk terminals
      The enormous increase in the marine transit of materials in bulk, with petroleum leading the way, has given rise to the development of special terminals for the loading and discharge of such materials. The principal factor influencing the design of these installations is the still-increasing size of the ships (ship). A single example of the effect of this change on design limits will be sufficient. The “Queen” liners, long the world's largest ships, never drew more than 42 feet of water. Supertankers (supertanker), on the other hand, when fully loaded, draw up to 72 feet. If these ships required berthing structures of the type provided for conventional cargo and passenger liners and if the formula relating the capital costs of such structures to the deepest draft were applied, the cost of building an appropriate berth for such a tanker would reach a figure more than six times the cost of the Queen Mary's old berth. Fortunately, the high mobility of the cargo renders such drastic and expensive measures unnecessary. Heavy capacity access for individual shore-based vehicles to carry away the cargo is not required, nor does the provision of services for the relatively small crews who man these great ships present any problem. The berthing positions can therefore be sited well out from the shore in deep water, and the structure itself can be limited to that required to provide a small island with mooring devices.

      In the case of oil (petroleum) terminals, the link to shore can be a relatively light pier or jetty structure carrying the pipelines through which the cargo is pumped ashore, with a roadway for access by no more than average-size road vehicles, which will probably be used in small numbers or even only one at a time. Because the ship itself carries the pumping machinery for delivering the cargo ashore, heavy mechanical gear for cargo handling is not required.

      In the case of bulk carriers bringing solid commodities, such as iron ore, the problem is more complicated. Hoisting grabs for lifting the ore out of the holds are necessary, even though transit between ship and shore can still be effected by continuous conveyors, corresponding to pipelines. Heavier foundation work is probably necessary at the berthing point to carry this machinery, and, for this reason, ore terminals have not been built as far out in deep water as oil terminals. It seems unlikely that the size of ore carriers will reach anything like the dimensions already attained by supertankers.

      The employment of piled (pile) structures to meet these requirements is almost universal, and a variety of techniques have evolved for handling and sinking into the seabed the long heavy piles required. At the sites likely to be chosen, penetration by piles may not be easy, particularly in places where most of the reasonably accessible deepwater sites tend to be located on the rockier shores.

      One problem that arises is that of shelter in adverse weather conditions. While the ships themselves are reasonably robust, the relatively fragile berthing structures might break up, setting the ship loose, possibly without power immediately available, threatening disaster. As the cost of building breakwaters to protect sites in the depth of water required is likely to be prohibitive, the search has been for natural shelter. In the British Isles the sheltered creeks of the western shores, such as Milford Haven, Wales, have become valuable. Milford Haven had known little shipping other than fishing fleets since the early 19th century, but in the early 1970s it boasted four bulk oil terminals. Two supply refineries were built on the spot; the third pumps to a refinery 60 miles away.

      Another aspect of the terminals is the need for protection against the effects of unavoidable collision impacts. A slight impact from a vessel of these dimensions, by reason of the large kinetic energy of such a mass, can cause considerable damage to the light berthing structure. Much ingenuity and theoretical analysis have gone into devising fendering systems that will absorb this energy. Some systems use the displacement against gravity of large masses of material disposed pendulumwise in the berthing structure as the energy absorbent; others use the distortion by direct compression, shear, or torsion of heavy rubber shapes or sections; still others rely on the displacement of metal pistons against hydraulic or pneumatic pressure. The common feature of all the devices is that at least part of the energy absorbed is not dissipated but is used immediately to return the ship to its correct berthing position. This feature is not exhibited by the older forms of fenders, which relied on the compression and, in extreme cases, on the ultimate destruction of coiled rope or timber to absorb the impact. A major question is the exact ship velocity to be allowed for, the determination of which is primarily an exercise in probability, balancing the economics of designing to a specified velocity against the cost of repairs after impacts at greater velocities. The key factor is the frequency of such impacts, which can be determined only by experience.

Dry docks (dry dock)
      The largest single-purpose structure to be built by the maritime civil engineer is not directly connected with loading, unloading, or berthing but is indispensable to prolonging the life of ships. This is the dry dock, which permits giving necessary maintenance to the underwater parts of ships. The problem of dry-docking is aggravated by the tendency of ships to grow in size by increases in beam (width) and draft (depth below waterline) rather than in length, a process that rapidly renders many of the world's largest dry docks useless for servicing an increasing proportion of the traffic.

      A classic example is the King George V Drydock at Southampton, Eng. Opened in 1933, it was 1,200 feet long and 135 feet wide and was capable of accommodating the largest vessels afloat—namely, the two Cunard liners (ocean liner) Queen Mary and Queen Elizabeth, each more than 80,000 tons deadweight. The later supertankers have deadweight tonnages of 135,000 tons and more, within a length of about 1,150 feet but with a beam of about 175 feet, which precludes them from entering the King George V dock. The lengthening of a dry dock would be a comparatively simple and economical operation; widening, on the other hand, involves at least the complete demolition of one sidewall and its rebuilding to give the increased clear width to the other wall, assuming space can be made available. Increasing the depth would mean a new dock altogether, but, because tankers generally dry-dock in the unloaded condition in which their draft can be considerably less than that of a conventional ship, this problem has not so far been a practical one.

Structural requirements
      Moreover, in a great many cases, the maximum state of stress in a dry dock occurs not when it is carrying the weight of the ship (always considerably less than the weight of the water occupying the dock when flooded) but when it is completely empty and subject to the pressures generated by water in the surrounding ground, particularly under the floor, the support of which may lie at a considerable depth below the level of the adjacent water table. To ensure against any tendency to lift under this pressure, the floor must either have sufficient weight in itself (1 foot, or 300 millimetres, depth of concrete will resist a little less than 21/2 feet head [depth] of water) or be designed as a structural element capable of transmitting this pressure laterally to the walls of the dry dock, which can then be designed to contribute the additional extra weight required. Obviously an operation involving both the complete rebuilding of one wall of a dry dock and the strengthening of the floor to cover an increase in its span as an inverted arch or beam is almost tantamount to the construction of a complete new dock.

      This problem received somewhat tardy recognition, so that, although several large new dry docks were built around the world in the 1960s, only a minority were capable of allowing the entry of tankers of more than 200,000 tons.

      The design of a dry dock probably depends more on ground conditions than does any other engineering structure, with the possible exception of large dams. Mention has been made of the need in many cases to resist upward pressures under the floor. Apart from the simple solution of using the weight of the dock structure itself for this purpose, which is not economical, devices that have been tried include “pegging” the floor to the underlying strata by means of piles or prestressed anchors and extending the floor slab itself beyond the sidewalls, thereby gaining assistance from the weight of the material filling behind the walls, which are designed to act as retaining walls to this filling. Venting of the floor to relieve water pressure can sometimes be of help provided the volume of water so released is not excessive. If it is, continuous pumping to keep the dock dry will be necessary. On sites in which water pressures do not have to be resisted, the design is generally simpler, and sufficient strength and stiffness to spread the loads from the ships' (ship construction) keels over the underlying ground so as not to exceed the bearing resistance of the latter is the controlling floor-design factor.

      The use of dry docks for the building rather than the maintenance of ships is a practice that has been increasingly adopted. Both the building and the launching of a ship in these circumstances can be considerably simplified. The designs of such dry docks are no different from those hitherto described; what is possibly the largest dry dock in the world was completed in Belfast, N.Ire., in 1970. This dock, built along the site of a former channel between two open basins, is capable of accommodating the three Cunard liners Queen Mary, Queen Elizabeth, and Queen Elizabeth 2 simultaneously and is to be used for the building of large tankers. It is spanned by a crane of 400 tons lifting capacity to handle large prefabricated ship sections.

      Dry dock entrances are closed by gates (gate) of different designs, of which the sliding caisson and the flap gate, or box gate, are perhaps the most popular. The sliding caisson is usually housed in a recess, or camber, at the side of the entrance and can be drawn aside or hauled across with winch and wire rope gear to open and close the entrance. The flap gate is hinged horizontally across the entrance and lies on the bottom, when in the open position, to be hauled up into the vertical position to close the dock—a process occasionally facilitated by rendering the gate semibuoyant through the use of compressed air.

      The ship type of caisson gate, a quite separate vessel floated and sunk into its final position across the entrance, is largely out of favour. Although it was comparatively easy to remove for maintenance and had the further advantage that a spare caisson could be kept in reserve in case of damage, the tie-up of capital is usually found unnecessarily expensive merely as an insurance premium.

      The maximum degree of watertightness obtainable between the gate and its seating is essential if continuing and expensive operational commitments for pumping out leakage water are to be avoided. The pressure of the water outside the gate is available to provide a powerful sealing force, but special treatment of the actual contact faces is necessary to make this force fully effective. For a long time it has been held that the only satisfactory arrangement was by the use of a timber lining (generally greenheart) around the contact face on the gate, bearing against stops in the dock structure composed of granite dressed and polished to a high degree of accuracy. The increased expense of such methods and the diminishing number of skilled labourers capable of dressing the granite have led to a search for alternatives. These include such devices as the use of stainless facing bars set in concrete, in place of the dressed granite, and rubber linings on the gates themselves. While these have generally proved effective when first installed, more experience is needed to determine their durability as compared with older methods.

Keel and bilge blocks
      Keel and bilge blocks, on which the ship actually rests when dry-docked, are of a sufficient height above the floor of the dock to give reasonable access to the bottom plates. Such blocks are generally made of cast steel with renewable timber caps at the contact surfaces. Individual blocks can generally be dismantled under the ship to allow access to that part of the plates, if required, and can be reassembled to take their appropriate share of the weight after the operation required has been completed. Most modern ships, particularly tankers, are of nearly square section over a large part of their middle length and can be kept upright in dry dock by the support of the bilge blocks under their bilge keels. In the most up-to-date dry docks, the bilge blocks are provided with mechanical means for traversing them across the dock and altering their height by remote control while the dock is still flooded. This arrangement permits them to be adjusted in their correct position according to the shape of the ship while the latter is still just afloat but in contact with the centre-line keel blocks. The economic advantage of this arrangement is considerable because it allows one ship to be removed and another put into the dry dock on the same opening of the gate, whereas under previous practice it would have been necessary to close the dock and pump it out to reset the bilge blocks to the known profile of the next ship. Apart from the time needed, the power consumed in pumping out a large dry dock is a considerable factor.

      Because of the increasing number of ships suitable for bilge docking, the use of side shores to keep hulls upright in dry dock is a rapidly dying process, and indeed the altars provided for this purpose in dry docks of more old-fashioned design are often an embarrassment to the accommodation of a modern square-sectioned ship. Frequently this situation is remedied by cutting away some altars, an operation that must be conducted with discrimination because the removal of any quantity of material from the sidewalls may have a damaging effect on their stability.

Basic technique
      Dry docks are usually constructed in open excavation in the dry, shutting out the sea by means of a cofferdam. Sometimes it is found convenient to construct the sidewalls first, in trench, next to remove the loose material between them, and then to lay the floor in stages so as not to endanger the stability of the walls before the floor is in position to give them toe support. Extensive pumping, to keep the excavations from filling with water during construction, is generally necessary.

      In one rather unusual case, a dry dock for 240,000-ton tankers was constructed almost wholly under water because large fissures in the rock running through to the sea flooded the site beyond the capacity of any reasonable assembly of pumping equipment. The entire space required for the structure was therefore excavated to formation level by dredging, and the sidewalls were constructed first, using prefabricated concrete caissons sunk into place and filled with concrete. The spaces between adjacent caissons were sealed by filling with concrete in the same way. Stone aggregate, to a depth of 23 feet, was then deposited between these walls and consolidated into a concrete floor by a process of grouting in which colloidal cement grout was forced under pressure between the interstices of the aggregate, subsequently setting to form the whole into concrete. A similar process across the floor at the entrance incorporated a cofferdam of interlocking steel sheetpiling, which allowed the sill and gate hinge to be constructed in the dry. The gate, of the flap variety already mentioned, was floated and stepped into place by divers after the removal of the cofferdam. Only then was it possible to pump out the main body of the dock, which was completed by laying a reinforced concrete topping over the floor in order to provide a satisfactory working surface.

Floating dry docks
      Floating dry docks have the initial advantage that they can be built and fully equipped in shipyard and factory conditions, in which their construction is not subject to unforeseen hazards arising from weather and variations in the ground conditions from those anticipated during design. The floating dock can be towed to the site, moored, and made ready for operation in a comparatively short time. Expenditure on temporary works, often a large fraction of the cost of a fixed dry dock, is also avoided.

      Floating dry docks are usually fully self-contained. The sidewalls provide much of the residual buoyancy and stability required to keep the dock afloat when it has been submerged far enough to allow the entry of a ship into the docking space over the main deck. Most of the machine tools and workshop equipment required for all the normal operations of ship repair and maintenance are also housed in the walls as well as the generating plant (usually diesel driven) to supply power for the operation of the dock and its equipment. Traveling cranes, for handling material off and onto the ship, run on the tops of the sidewalls.

      A floating dry dock can be moved at relatively short notice to another site, should a long-term change in shipping-traffic patterns dictate a change. This advantage may be more apparent than real, because the large work force required to man it may not be so readily transferable.

      Moreover, floating dry docks tend to have large maintenance costs because the steel structure, being continually afloat, requires regular chipping and painting, as the hull of a ship does. The above-water structure presents no particular problem and can generally be given maintenance care without putting the dock out of use. The most vulnerable areas, those immediately adjacent to the waterline, can be reached by careening, a process that involves filling the water ballast tanks along one side to induce a list that lifts those on the other side part of the way out of the water. On completion, the process can be reversed for the other side.

      Methods of underwater scaling and painting, or the use of limpet dams with which small areas can be covered with watertight enclosures inside of which people can work under compressed air, allow a limited measure of attention to be given to the bottom plating outside. Occasionally it is necessary to detach one of the sections of the dock, which is usually constructed in separate sections for this reason, and dry-docking it in the remainder, repeating the process until the whole dock has been renovated. This costly and tedious process is resorted to only for compelling reasons.

      To give a floating dock sufficient depth of water for submerging the docking blocks below the keel of the ship to be docked, it may be necessary to dredge a berth for it. In areas subject to heavy siltation, this dredged area will almost certainly act as a silt trap. Periodic removal of the dock from the berth to allow the latter to be redredged is an additional source of expenditure in such cases. Finally, in places where the tide range is of consequence, special mooring arrangements are necessary to restrain excessive lateral drift of the dock as the mooring chains become slack on low water.

      The arrangement of keel and bilge blocks is generally similar to those described for fixed dry docks.

Sea works for reclamation and conservancy
      An indispensable item of equipment over a wide range of the maritime civil engineer's activities is the dredge with its ancillary units, such as hopper barges, tugs, reclamation units, and servicing craft. There are few navigable harbours or harbour approaches that do not require, at varying intervals of time, removal of deposits of unwanted material, the continuing accumulation of which can ultimately obstruct navigation. With the current trend toward larger ships, dredging is especially important.

      Extensive research has been devoted to the development of dredging equipment. Through more sophisticated techniques—including, in some cases, permanent profile modification of the harbours and waterways—efforts are made to keep the need for dredging to a minimum. Model studies, mentioned earlier, can be of the greatest assistance.

      The material to be removed by dredging operations is usually derived from one of two sources or from a combination of both. In harbours at the mouths of rivers, quantities of silt are carried down in suspension and tend, partly because of the deceleration of the flow in the increased waterway available and partly because of the effects of increasing salinity, to be deposited at the mouth, usually the site of harbour works.

      This process has produced areas of marked agricultural fertility, such as the Nile delta in Egypt. While over a large time span the action is one of great benefit, in the short term it is generally a considerable inconvenience. The skillful employment of modern dredging equipment, however, has indicated possibilities of getting the best of both worlds. The other source of deposited material likely to obstruct navigation is littoral (coastal) drift, especially in areas where there is a sizable tidal range. The incoming tide frequently brings suspended material, some proportion of which settles to the bottom around the turn of the tide when the movement of water is at a minimum. In the absence of any countervailing tendency, an accumulation takes place, which again requires dredging.

      For many years the workhorse of many of the world's dredging fleets has been the bucket-ladder dredge, operating a continually moving chain of open-ended shovels or scoops. At the bottom of the ladder the scoops are pushed into the face of the material and empty themselves as they turn over at the top, the material falling into chutes that divert it into hopper barges for removal. A four-point mooring system enables the craft, and with it the bucket ladder, to be held up to the working face and, at the same time, swung sideways across it in either direction. By this means, an often remarkably level bed to the sea bottom can be closely controlled by adjusting the position of the ladder under the dredge's bottom. The positive action in filling the buckets enables such a dredge to tackle material of considerable stiffness, thereby extending its use to works of dredging and harbour development in which soils other than recently deposited silt or sand have to be excavated. Even some of the softer rocks can be removed in this way if the buckets are provided with hardened and stiffened edges and ripping teeth.

      The principal disadvantage of the bucket-ladder dredge is the need for an elaborate system of fixed moorings. The area that can be covered by one placing of the moorings is limited. Continuous lifting and replacing of the moorings are not only time-consuming but must be carried out in such a way as to offer minimum obstruction to navigation, a requirement that sometimes involves a great number of interruptions in dredging operations.

      In areas in which the deposited silt is highly mobile and accumulates in considerable quantities, it can be economically removed by a suction dredge, which pumps water mixed with silt into open hoppers. By adjustment of the capacity of the hopper to the rate of flow from the pump, the water can be made to remain in the hopper long enough to deposit most of the silt. Careful design of the pumping machinery is required to assume a continuous mixture of maximum silt with minimum water.

      The first suction dredges generally operated from moored positions in the same way as bucket-ladder dredges, but a less elaborate system of moorings generally sufficed because the leveling of the seabed could be left to occur naturally through the mobility of the material. A marked advance was achieved by the elimination of much of the lifting and laying of moorings through the development of the trailer suction dredge. This craft has the capacity to dredge while on the move and cruises up and down the waterway or other area, sucking up silt as it goes. This operation does not eliminate all interference to navigation, because a working trailer suction dredge moves more slowly than a ship under normal steerage way, but the obstruction is markedly less. The dredge's turn at the end of each sweep is usually facilitated by the incorporation of a bow side thrust propeller.

      The growing tendency to use dredged material for reclamation purposes and the suitable condition for such purposes of the spoil as delivered by a suction dredge have encouraged its development. The seabeds and river bottoms in their natural state are often largely composed of relatively soft material and can be deepened by the use of suction dredges operating normally. Where rock or other hard material must be handled, conditions are favourable to the use of the suction-cutter dredge, which incorporates at the suction head a powerful rotating screw cutter that fragments the hard material. The increased dredging stresses arising from the use of a cutter require that a craft so equipped should be operated as a stationary dredge with moorings. Because such operations seldom take place in areas already under use by traffic, the obstruction problem is not often critical. Additionally, in modern equipment, the incorporation of heavy spud legs in the craft to anchor in the seabed reduces the number of separately laid moorings required.

      A useful ancillary piece of equipment to all the above is the grab dredge, either self-propelled or towed to the site. Grab dredges are especially suitable for dredging close up to existing quay walls or other structures with minimum risk of damage, and the grab equipment is often capable of lifting individual boulders. Not infrequently, grab dredges have value for maintenance dredging, particularly in restricted areas and with silt of sufficient mobility to level out the individual holes that are almost inevitably left behind. Although the return fall of the grab takes place with the bucket empty and is, to that extent, nonproductive, with skillful operators this element can be reduced to a minimum, and, with some large craft operating four grabs simultaneously, considerable outputs can be achieved.

      Dredges are characteristically designed to deliver their output either overside into attendant hopper barges or, in the case of self-propelled dredges, into hopper compartments incorporated in their own structure. These hopper compartments are essential in the case of trailing suction dredges, but their value in other cases depends on the circumstances and on the chosen method of disposal of the spoil. When a long journey to the depositing area is involved, it is obviously more economical to leave the dredge continuously at work and to remove the spoil in separate barges.

      When the journey is short and the spoil is to be simply dumped, for which purpose the hoppers are provided with bottoms that fall open, an economical work cycle between dredging area and spoiling ground, using one craft only, can frequently be established.

      A special case is the side-boom dredge, which discharges straight back overside; by making the work coincide with an appropriate state of the tidal current, this arrangement secures the removal of the dredged silt by the tide's operation.

      Dredged spoil is less and less often disposed of by dumping out at sea, a practice that was once almost universal; instead it is used for the reclamation of land from the sea and foreshore. This reclamation process has been stimulated by the rise in the value of the land so created and by the discovery that, in many instances, spoil taken out to sea frequently returns. This phenomenon has been investigated, both on hydraulic models and by mixing radioactive tracers with the dumped spoil in small quantities, permitting its subsequent movements to be followed with Geiger counters.

      A variety of procedures have been developed for the combined operation of dredging and reclamation. Where the area to be dredged and the area to be reclaimed are in close proximity, as sometimes happens, the whole operation can be carried out by a single suction dredge pumping ashore through a floating pipeline. When, as is more often the case, there is a considerable distance between the two sites, transport in hopper barges is more economical. At the reclamation site, the barges either can be pumped out by a suction reclamation unit or occasionally can dump their loads on the bottom; from there the material can be pumped ashore by the unit acting as a stationary suction dredge.

      The layout of reclamation areas is a matter to which adequate scientific investigation should be devoted, covering such aspects as the adequacy and subsequent maintenance of any navigable waterways it is intended to provide through them, the design of the banks required to contain the pump spoil while the solids settle, and the relative positions of delivery and runoff points to obtain the maximum recovery of solid matter. Such schemes for reclamation, carried out in this way, can simultaneously ensure more valuable new land and improve navigation facilities.

The Delta Plan (Delta Project)
      It was noted at the beginning of this section that maritime engineering has two large objectives: improvement of transportation and reclamation and conservancy of land. Outstanding among examples of human ingenuity in the second category has been the long effort of the people of The Netherlands to keep their country, large areas of which are below sea level, habitable and productive.

      The purpose of these efforts has generally been twofold: first to recover, reclaim, and retain more land for occupation; and second to prevent the percolation of seawater into the water table of both the recovered and the original ground—which, if not prevented, would seriously reduce or even altogether destroy the value of the land for agricultural purposes. This second purpose has sometimes been described as “pushing back the salt line.”

      A prime example of the first purpose was the enclosure in 1926–32, by means of a dike some 17 miles in length, of a large inlet known as the Zuiderzee (renamed the IJsselmeer after its enclosure). Considerable areas of this body of water have since been reclaimed by the pumping ashore of dredged sand, and the reclamation of further areas is either in hand or planned for the future. A large proportion of the area will, nevertheless, be maintained as a freshwater lake by the flow of the river IJssel (IJssel River), which takes off from one of the outfalls of the Rhine, known as the Lek, or Neder Rhine, just south of Arnhem. In the 1960s it was found necessary to place a dam across the Lek just below the takeoff of the IJssel to divert an increased quantity of Rhine (Rhine River) water down the IJssel to the IJsselmeer. The growth of shipping traffic on the canal, which connects Amsterdam with the North Sea, the locking operations of which necessarily discharge quantities of salt water into the IJsselmeer, would otherwise tend to nullify the effects of the freshwater flow of the IJssel.

      To maintain navigation in the Lek, in spite of the reduction in water flow, two further dams are provided downstream toward Rotterdam, and all three dams are capable of being opened in the event of excessive floodwater coming down the Rhine.

      The second purpose, that of desalinization, has been at the heart of the Delta Plan, one of the most imaginative civil engineering projects ever undertaken. The incident that triggered the Delta Plan was the disastrous flooding of Feb. 1, 1953, when the notorious North Sea surge brought tide levels higher than ever previously recorded, overtopping many of the existing dikes and causing untold damage and salt contamination of vast areas of agricultural land. The surge also caused considerable flooding and damage on the other side of the English Channel, along the east and southeast coasts of England. Statistical research suggests that tides of this level are to be expected at a frequency of at least once in 300 years.

      The weak points in The Netherlands' defenses against flooding from the sea are the several deep inlets formed at the mouths of the Rhine and Maas rivers (Meuse River), through which the greater part of the water coming down these rivers discharges into the North Sea. Around the shores of these inlets run many miles of dikes, the maintenance of which is a constant burden and the strengthening and heightening of which to prevent a repetition of the disastrous 1953 floods represented a project of considerable magnitude.

      It was considered that the most economical result would be obtained by a major operation of shutting out the sea, more or less at the main coastline, by a series of dams across the mouths of the inlets. By this means some 435 miles of dikes would be cut off from direct sea attack and reduced to a secondary function, whereas the total of the new dams that might still require a measure of maintenance is only 19 miles. By conserving and controlling the vital flows from the Rhine and the Maas, the inlets themselves would be gradually transformed into freshwater lakes, thus greatly contributing to “pushing back the salt line.”

      A secondary effect in this direction will be an increase in the flow of fresh water toward Rotterdam as a result of the raising of the levels in the estuarial inlets, particularly in the northernmost inlet, the Haringvliet. This result should greatly assist desalinization in the Rotterdam area, where the penetration inland of the salt line had reached alarming proportions as a result of the improvement in the navigational approaches to the port, effected by the construction of the channel known as the New Waterway from the Hook of Holland.

      A further benefit to be gained is the great improvement in communications between the mainland and the hitherto somewhat isolated communities on the islands lying between the inlets; the new dams across the inlets will provide foundations for motor roads.

      The Delta Plan construction was scheduled to take nearly a quarter of a century, and the total cost represents a significant percentage of The Netherlands' national budget.

      Although the authors of the plan stress that it is not properly a land-reclamation scheme (little or no extra land will be created by it), there is no doubt that many of the techniques developed for reclamation work are of the utmost value in carrying out the plan, and, conversely, lessons learned in the course of the project will no doubt find useful application in future reclamation work the world over.

      Thus, for the construction of the sluices through the dam across the Haringvliet, which were necessary to provide for the escape of river water in times of flood, a working island was created in what was almost open sea by the continuous depositing of sand on the seabed until the level rose above that of the water. Procedures for the rapid waterproofing of the banks so created have been brought to a high pitch of efficiency. This has been accomplished through the use of nylon carpets or asphalting by special high-speed placing machines. The former take the place of the previously well-tried practice of using fascine mattresses weighted down with stones, for which labour on the scale required to cover large areas with sufficient speed is no longer available.

      The closure of the final gaps in the dams, a hazardous operation because of the large volume of water rushing through the narrow remaining gap at this stage, is effected at the delta by the use of concrete caissons floated into the gap and scuttled in position. The technique has progressed there from the use of solid-walled caissons that had the disadvantage of closing the gap suddenly, with consequent hazard, to caissons incorporating their own sluices, thus allowing the flow of water to continue until all were in place and the sluices could be safely closed.

John Holmes Jellett

Additional Reading
Wolfgang Rudolph, Harbor and Town: A Maritime Cultural History (1980; originally published in German, 1980), includes treatments of port anatomy and historical development. Books on engineering aspects include F.M. Du-Plat-Taylor, The Design, Construction, and Maintenance of Docks, Wharves & Piers, 3rd ed., rev. and enlarged (1949); John F. Brahtz (ed.), Ocean Engineering: Goals, Environment, Technology (1968); A.M. Muir Wood and C.A. Fleming, Coastal Hydraulics, 2nd ed. (1981); Hans Agershou, Helge Lundgren, and Torben Sørensen, Planning and Design of Ports and Marine Terminals (1983); Gregory P. Tsinker, Floating Ports: Design and Construction Practices (1986); Per Bruun, Port Engineering, 4th ed., 2 vol. (1989–90), covering topics such as harbour planning, breakwaters, fishing ports, sediment transport, and geomorphology; Kenneth M. Childs, Jr. (ed.), Ports '89 (1989), conference papers; and Richard Silvester and John R.C. Hsu, Coastal Stabilization: Innovative Concepts (1993). The Dock and Harbour Authority (monthly), is devoted to problems of dock and harbour operation and construction in Great Britain.John Holmes Jellett Ed.

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Universalium. 2010.

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