Home >> Encyclopedia-britannica-volume-7-part-2-damascus-education-in-animals >> Dormer to Drente >> Dredgers and Dredging

Dredgers and Dredging


DREDGERS AND DREDGING. Dredging is the name given to that branch of excavation which deals with the process of removing materials lying under water and disposing of them according to the requirements of the work in hand. The machines employed by engineers to that end are termed dredgers (dredges in America) and the term dredging plant covers not only the dredging machines themselves but much auxiliary apparatus em ployed in connection therewith. The ceaseless struggle of man's efforts against the mighty forces of nature involves an infinite variety of attack and a vast expenditure of wealth. Spurred by the demands for increased facilities for shipping, and assisted by the great advances of mechanical science, dredgers have developed rapidly from primitive tools to powerful and costly appliances.

Applications of Dredging.—Although dredging machinery is, from the nature of its duties, mounted on floating craft in the great majority of instances, the term does not exclude sub aqueous excavation by plant mounted on land. Dredging may be divided broadly into two classes according to the object for which it is employed, viz., (a)' as a means of removing material for the purpose of increasing existing depths of water or reclaiming low lying ground, and (b) as a means of mining for making available earths useful in themselves or because they contain precious con stituents ; but its field covers a multitude of purposes within the bounds of these two main divisions, and the following uses to which the plant may be put will illustrate the wide scope of dredging operations.

Material Shifting Dredgers.—The creation and maintenance of satisfactory depths of water in harbours, docks, rivers and canals for the purposes of navigation ; the removal of material for the foundations of marine and river works and for the preparation of sites for floating docks ; the deepening, widening and straighten ing of rivers to increase or conserve their discharging capacity; the cutting of drainage and irrigation canals; the removal of ma terial for the reclamation of low-lying land or the formation of dykes, levees or dams.

Mining Dredgers.—The recovery of gold from rivers or placer deposits, also platinum, tin and other heavy metals; the raising of gravel, sand and clay for building and industrial purposes.

Characteristics of Materials Dealt with.—Dredgers have been constructed to deal, in varying degrees of efficiency, with anything from mud to rock. The materials when removed from the bottom are termed "spoil" and naturally occur in endless variety of kinds and mixture of kinds. In general, however, the natural basins of alluvial deposits forming the rivers and bays in which harbours and other marine works are mostly sited, are the situations peculiar to dredging operations. Sand and mud are the most common materials met with in dredging, forming the bulk of obstruction to navigation and of accumulations in and about harbours and river channels. Materials other than sand and mud are more difficult to dredge. Clay and loam are classed as loose materials but can scarcely be regarded as "free-getting" as they may be somewhat difficult to dislodge from the apparatus when being discharged. Further along the scale of compactness comes a wide and rather indefinite class of mixtures of loam and boulders in various degrees of cementation. They are resistant to penetration and cause difficulty owing to size of the boulders. Gravel is often found in dense layers strongly compacted with sand, and this together with indurated sand, is difficult to disin tegrate, but once penetrated removal is comparatively easy. Con glomerates, consisting of pebbles held together by natural forms of cement, and solid rock will require either the application of excessive power in the dredging tool or previous disintegration by percussion or blasting, with subsequent dredging of the debris.

Classification of Dredging

fall into two main classifications according to the conditions under which they operate, viz., sea-going vessels and vessels for inland water serv ice. The former are generally ship-shape vessels constructed as far as possible in the accepted manner for withstanding sea con ditions and ocean voyages, and carrying their own propelling ma chinery. The latter class is of lighter construction partaking more of the box or pontoon form and generally lacking propelling ma chinery. There are, of course, intermediate types, and in general it may be said that dredging vessels of whatever type require to be more strongly constructed to withstand the working stresses to which they are subject than vessels for commercial purposes. Mining dredgers as a class are of the inland type. A further clas sification is possible according to the system on which they dispose of the dredged materials. "Stationary" dredgers discharge their spoil into barges brought alongside, or to river banks or shore either directly or through long shoots or pipes. "Hopper" dredgers receive the spoil into a specially formed hold or hopper in the hull of the dredger and, when full, proceed to sea or other con venient site to "dump" their load. The inland dredger class as a whole is of the stationary type, but sea-going dredgers are of either type according to the particular requirements of the work and situation; and not infrequently hopper dredgers include ar rangements which make them capable of working on either sys tem.

Basic Types of

basic types of dredging machine are the bucket ladder dredger, the suction or hydraulic dredger, the dipper dredger and the grab dredger. In addition, dredging often necessitates the employment of a variety of auxiliary plants, and there are a number of appliances which, although not falling strictly within the definition of a dredger, in that they do no more than loosen without raising the spoil to the surface, yet serve to carry out in some measure with the help of moving water, the work of dredging.

The Ladder Dredger.

The main feature of the ladder dredger is a continuous chain of buckets running round a rigid frame called the ladder extending down from the ship to the bottom of the water, as shown in fig. 1. The buckets run empty down the underside of the ladder, dig into the material as they turn round the end, and return full along the top side of the ladder. On reaching the top each bucket, as it turns over, discharges its contents into a shoot which conveys the spoil to the vessel's own hopper, or to a barge, or in some cases to the shore. The revolv ing members at each end of the ladder are called tumblers and the top tumbler drives the bucket chain. The ladder of the typi cal ladder dredger is hinged at the top to a high structure on the vessel, the height of which is regulated by the requirements of the discharging operation, and extends downwards at an angle not exceeding 45° through an open recess or well in the hull, the lower end being supported by rope tackle which serves to adjust it to the dredging depth required or to raise it up entirely when it is desired to steam away.

The Suction Dredger.

On the other hand the suction dredger, shown in fig. 2, depends on quite different agencies for its operation. Whereas the ladder dredger removes and raises the material mechanically, the suction or hydraulic dredger, as its name implies, relies on suction to lift the material to the surface. The device consists essentially of an air-tight suction pipe, one end of which is lowered down to the bed of the sea or river, and the other connected with a centrifugal pump. The latter comprises a casing within which an impeller or fan is re volved at high speed and by its action expels the contents, whether air or water, outwards from the centre through an outlet on the periphery or rim of the casing, thereby causing a partial vacuum at the centre, to which the suction pipe is attached. The atmos pheric pressure on the water outside proceeds to repair this vacuum by forcing water in through the only available route, viz., the suction pipe, and the water in its mad rush into the pipe agitates and carries with it a proportion of solids. In some situa tions, means are provided at the suction pipe end, or nozzle, of disintegrating or actually cutting the compact materials to enable them to come under the influence of the so-called suction.

Dredgers and Dredging

The Dipper Dredger.

The dipper and grab dredgers are also of the bucket type, but more intermittent in operation than the ladder dredger. The dipper dredger is illustrated diagram matically in fig. 3. The principal features of the dipper dredger are the bucket and the arm to which it is attached, the boom which supports and guides the arm and which is mounted on a turntable so that dredging can be undertaken round a wide arc and the load deposited to either side, the hoisting rope which passes over a sheave at the boom head and gives the required excavating movement to the bucket, and the backing rope by means of which the vessel may be turned and moved forward using the bucket resting on the bottom as an anchor. The bucket has a hinged bottom, the catch of which is released by a hand rope to dump the load. To counteract the heavy digging reactions the vessel is made to rest on the bottom partly independent of water support on two anchor spuds, and a guide or poling spud is provided at the stern.

The Grab Dredger.

The grab dredger employs a slewing crane to operate its bucket which latter is in two parts hinged together, and controlled by levers and chains or ropes. The bucket is dropped to the bottom in an open condition by means of one rope, digs in partially by virtue of its own weight, and com pletes its bite by means of the strong leverage effected by the other rope on the two halves when the crane begins to hoist. By hold ing on the hoist or "holding" rope and releasing the "closing" rope the bucket opens and discharges its contents. Some grabs are specially arranged to operate with one rope only, discharge being effected by means of a tripping device suspended from the jib head.

Auxiliary Plant and Miscellaneous Appliances.—The auxiliary plant often to be found in connection with dredgers is of two types, one of which is employed before dredging and the other after. Examples of the former are rock-breakers and rock drillers used to disintegrate rock by hammering or prepare it for blasting so that it may be dredged. The rock-breaker consists of a pointed heavy steel ram which is dropped through a guide repeatedly on the same spot, being raised by means of a rope operated by a special winch. Auxiliary plant used in the disposal of the dredged spoil comprises barges, either self-propelling or with tugs ; reclamation vessels for delivering the spoil ashore ; pipe lines, floating and on shore ; and shoots. The miscellaneous appli ances comprise snag pullers, grapnels, rakes and harrows, mechani cal stirrers, water jets and other devices for placing the material into suspension so that it may be carried away by river flow or by the ebb tide. Being for the most part makeshift devices and not capable of extended applications they do not exhibit any marked degree of development.

The Mining Dredger.

The mining dredger for the recovery of precious metals is generally an adaptation of the Ladder Dredger. It is often to be found at considerable distances from a river, having cut its way through the land and deposited the spoil behind it, in effect carrying with it the pool in which it floats. A typical tin dredger is shown in fig. 4. The earth brought up by the buckets is discharged into a perforated cylindrical revolving screen which disintegrates it and allows the heavier tin-laden soil to drop through the perforations into a distributing box, while the large rough materials pass out of the screen and are discharged over the stern by a belt conveyor. The soil from the distributing box, together with an ample supply of water, is led by shoots into pans in which it is agitated by mechanically operated jigs. This operation allows the heavy tin to sink to the bottom of the pans while the lighter soil remains in suspension in the water and passes overboard through a tail shoot. The metal is collected from the pans at intervals.


Movers.—Steam is the universal motive power used on dredging plant, being particularly well adapted to the rough and fluctuating character of the work. Steam plant is comparatively simple and well understood, while its inherent flexibility is an invaluable characteristic for dredging operations, enabling it to accommodate itself conveniently to the constantly varying power requirements and, if need be, to sustain a heavy pressure on the tool when the latter has been brought to a dead stop by the severity of the work. The internal-combustion oil engine is find ing some application in situations where economy of space and fuel are vital considerations, but, although suitable for driving centrifugal dredging pumps, is not yet the equal of steam plant for the majority of dredging applications. In general, its lower range of flexibility and inferior capabilities of sustained power at low speeds, together with the high pressures possible, necessitate the introduction of safeguards which operate to reduce the econ omy and convenience to be expected from its higher thermal efficiency and self-contained character. The diesel-electric drive in which the oil engine operates under favourable conditions as a prime mover for the electric secondary machinery, which drives the various motions, has also been applied and possesses char acteristics more suited to many dredging machines than the direct oil engine drive, although with the added cost and complication of electrical equipment. The electric drive pure and simple is in use in the tin-mining areas where many dredgers are supplied through cables from a central generating station.

Selection of Dredging Plant.

It will be evident that the selection of dredging plant to satisfy the requirements of different localities and different circumstances needs the exercise of sound judgment. The ladder dredger is eminently adapted for dealing up to great depths with all classes of material from mud to the softer varieties of rock, including small boulders, and of preparing a level bottom. Its mechanism tends to make it very heavy and cumbrous, with correspondingly heavy capital cost and upkeep charges. It may, however, be regarded as the universal large dredging tool and is preferred in a great number of cases where accuracy of work in a wide variety of materials is required and it can be kept fully employed, provided its bulk and widespread moorings are not of grave consequence. Seventy feet is the maximum dredging depth in sea works and this the ladder dredger can tackle. Some of the mining dredgers, however, work down to Too feet and more. In more restricted situations and for less extensive projects the grab dredger will generally prove more economical in first cost and upkeep, and is well suited to maintenance work and work of an intermittent nature or in awkward situations. The dipper dredger is as capable as the ladder dredger of dealing with all classes of materials and as a tool for really difficult individual tasks at depths up to some 35ft. has no equal, owing to the great force which can be exerted with the cutting edge of its bucket and the large capacity, some times up to i 5cu.yd., of the latter. The grab dredger is not so powerful but can dredge at great depths exceeding any other type, and is eminently suitable for pioneer inland work. The suction dredger succeeds when large quantities of soft materials have to be dealt with, provided there is no objection to the large admixture of water. Quantity not quality is its leading characteristic, for it effects its object in free-flowing materials by excavating large holes in the bottom. The irregularities are subsequently lev elled down by sea or river action, thus effecting an increase in the depth of water. In the more compact sands a series of high pressure water jets around the nozzle serve to break up the mate rial sufficiently, but in clay and similar substances a rotary cutter is required and serves to increase the accuracy of dredging to as great a degree as any dredger can attain. By reason of the large quantity of water, often 90% of the total volume pumped, the suction dredger is relatively inefficient, but the operation is con tinuous and has the valuable property of enabling, when required, the spoil to be discharged direct through a pipe system to con siderable distances without expensive re-handling. The majority of all types of dredgers have their excavating member so located in advance of the hull that the vessel can cut its own flotation.

Methods of Dredging.

In the case of the bucket dredger practically only one general method of dredging is pursued. The vessel is moored by means of six anchors and chain cables set out as shown diagrammatically in fig. 5. The head and stern moor ings are much longer than the side moorings, and the vessel swings to or pivots about the head mooring A and is dragged slowly transversely by one pair of side moorings C and D. On reaching the limit of the cut to that side the vessel is advanced about six feet by pulling on the head mooring, the stern line being paid out to correspond, and the vessel is dragged across to the other side of the cut by side moorings E and F, the other side moorings C and D being paid out the while. This operation goes on until the head chain becomes too short and the angles of the side chains tend to become ineffective, when the vessel is pulled back by the stern mooring B to commence a fresh series of cuts at a lower depth or, if the required depth has been reached, dredging is stopped for the time being and the whole of the anchors taken up and relaid in a fresh position. The depth of cut is on the average about 18 inches and the handling of the moorings is effected by steam winches. The excellent control of operations which this method gives permits of very accurate dredging. In reality the dredged area would at first present the appearance of a ploughed field with exaggerated furrows, but sea or current action soon levels out the ridges.

Influence of Tide and Weather.

In a sea-way the whole operation is highly skilled, as the dredging master must adjust the depth of his ladder to the varying tide level so as to keep the work uniform, and it will be appreciated that inclement conditions will make it exceedingly difficult to maintain a level bottom. In practice, work is not generally permissible when the swell attains some two feet in height, owing to the structure being exposed to immense stresses from the ladder bumping on the bottom. It will be apparent that this system of dredging constitutes a serious obstruction to shipping in a narrow waterway, more especially when barges are employed to remove the spoil, and that the operation of lifting and relaying moorings on a fresh field is a heavy undertaking. A hopper dredger will present less of an obstacle but loses, for dredging, the time occupied in transporting the dredgings to the dumping site and returning to its work. When the hopper is full, the three forward chains are coupled together and let go, and similarly with the three stern moorings, the ends being attached to mark buoys. The operation of picking up these moorings again and connecting them to the winches is also one of considerable moment.

Suction Dredging.

The suction dredger operates generally on the same principles as the bucket dredger, but in some cases the six anchor moorings are dispensed with in favour of two "walking" spuds, placed abreast some distance apart at the stern, together with two forward side or "breast" moorings, consisting of wire ropes and anchors. In operation under this system one spud is let down to the bottom to act as an anchor and the for ward end with suction pipe is swung by one breast mooring to the limit of the cut. Their relative positioning now brings the other spud in advance of the pivot spud; the former is let down to act as pivot, the latter raised, and the dredger swung on a new advanced arc back to the opposite side of the cut by the other breast mooring. The two spuds thus act as legs and the dredger "walks" forward. Fig. 6 illustrates this method.

Bar Dredging.

A particular case in which a different method is employed is the dredging of a navigable channel through a sand bar blocking a river entrance. Here large quantities require to be removed under arduous open sea conditions, precluding in most cases the employment of such a rigid structure as a bucket ladder, or in some cases even the use of moorings. The suction dredger is eminently adapted to this work as its suction pipe can be made flexibly jointed, thus relieving the ship of any bumping stresses. The dredger simply steams to the dredging position, lowers the suction pipe, maintains position roughly with the propellers, and takes on board into her hopper a load of sand in a comparatively short space of time. The power is arranged so that this operation is generally performed in some 3o to 4o minutes even with the largest hoppers up to 4,00o tons or more capacity. Without further ado the dredger steams out to sea, deposits the load through the doors arranged in the bottom, and returns to carry out the same procedure. In those cases where the sand will not run freely to the suction pipe, the latter requires to be made somewhat more rigid and is fitted with a drag head or cutting edge. The dredger steams slowly ahead, and the drag head slices off the material to a depth of i8 inches or two feet to bring it under the influence of the suction pipe. This is of course a rough operation, as the result is simply the cutting of a series of longitudinal fur rows. In such situations the bar is often of sufficient width to permit of a full load being obtained in one cut. The drag method is also employed successfully where the area is large and the depth to be removed small, and it is capable of more accurate work than ordinary suction dredging.

Stationary Dredging.

In the case of the dipper dredger which relies on spuds, no external moorings are necessary. The dredging machinery being arranged on a turntable, cuts are taken round an arc of some 90° or more while the vessel remains sta tionary. The anchor spuds are then lifted and the vessel moved up to suit the next series of cuts by means of the backing rope and the stern spud acting after the manner of a punt pole. The anchor spuds are then re-set and dredging operations continued. A development of the suction dredger employs a suction pipe mounted on a turntable of ter the manner of a dipper dredger, and operates on a similar method, the hull being kept stationary during the progress of each arc of cut either by moorings or by spuds. The grab dredger is another example of this system of operation, but requires some three or four moorings only to retain and adjust its position at its work, the dredging reactions being purely vertical.

Determination of Work Done.

The work done by dredgers is measured either "in the solid" or "in the loose" as the two methods are termed. Measurement in the solid involves the de termination by soundings taken before and after dredging of the levels of the excavation. Measurement in the loose is based on a determination of the amount of spoil actually issuing from the dredger, either from the known capacity of the barges or hopper used, or in the case of pipe-line work, by survey of the deposit. Rough and ready methods based on known data are sometimes employed, such as by counting the rate at which the buckets travel on a ladder dredger, the number of dips or grabs per hour, or estimation of the velocity of flow and percentage of solids in a pipeline. Spoil, originally lying closely compacted in its natural state on the bottom, has been dug into, stirred up, and mixed with water by the action of the dredging tool, hence a swelling, gener ally taken as 30% in the absence of definite data, is observable in "loose" or "barge" measurement, as compared with "solid" or "place" measurement.

Disposal of Spoil.

The disposal of spoil demands close con sideration as it has an important effect on the cost of dredging. Where it is desired simply to get rid of the spoil, as in the im provement and upkeep of navigable channels, it is usually requisite to transport it considerable distances out to sea and deposit it in deep water whence there is little likelihood that it will find its way back under the influence of currents or sea action. For this purpose a fleet of barges, either self-propelled or towed, may be employed or the dredger itself conveys to sea the spoil which it ex cavates. In each case the hopper bottom is formed of a number of doors which can be opened to allow the spoil to drop out. The conditions determining the method to be adopted vary almost with every project. Obviously it is advisable to keep such an expensive item of plant as a dredger fully employed if at all possi ble, and the use of hopper dredgers is only resorted to when at tendant barge traffic is either impracticable on account of inclem ent conditions or likely to cause serious additional obstruction to navigation in a busy and restricted waterway. The hopper type is useful also in the smaller sizes for general maintenance work as a self-contained unit. In those cases where the distance to the dumping site does not exceed a mile or two or where the material is required for reclamation purposes, towed barges are used, while for long distances, large speedy self-propelling barges holding up to 1,200 tons are necessary. Large quantities have been dredged from the Suez Canal by high tower ladder dredgers and deposited directly on the banks through long shoots, but this direct method is more generally applicable to the suction dredger for wide waterways and to the dipper or grab dredgers for narrow channels. For the important work of reclaiming land the suction dredger and a long pipe-line are generally employed so long as free-getting materials are to be dealt with. The portion of the pipe-line from the dredger to the shore is made in sections 3o to Soft. long, flexibly connected together by ball and socket joints so as to permit the dredger its necessary range of movement. Each section of the pipe is carried on a pontoon and the floating pipe line connects to a shore delivery pipe-line laid directly on the ground or mounted on trestles. If harder materials must neces sarily be used or the conditions of exposure or obstruction to navi gation preclude the use of a pipe-line, it becomes necessary to employ a fleet consisting of a cutter-suction or ladder dredger and a special reclamation vessel with barges as the connecting links. The reclamation vessel is in most respects equipped like a suction dredger except that its suction pipe is arranged over one side so as to dip into a barge; it is moored to a wharf or staging on the other side and its discharge pipe is flexibly connected direct to the shore pipe-line. By this means low-lying areas situated up to 6,000 feet from the water-front can be reclaimed, and at still further distances by the employment of boosting pumps.

Those desirous of pursuing the subject further will find the following useful:—C. Prelini, Dredges & Dredging (1912) ; T. C. Earl, Gold Dredging (1913) ; Sergeant, Centrifugal Pumps & Suction Dredgers (1918) ; A. B. McDaniel, Excavation Machinery, Methods & Costs (1919) ; F. L. Simon, Dredging Engineering (1920) ; G. B. Massey, The Engineering of Excavation (1923) ; P. M. Dekker, Dredging & Dredging Appliances (1927).

The writer is indebted to the many firms in Europe and America who have furnished particulars of plant, of which special mention may be made of the following British constructors: Messrs. Wm. Simons & Co., Ltd., Renfrew ; Lobnitz & Co., Ltd., Renfrew ; Fleming & Ferguson, Ltd., Paisley; Ferguson Brothers (Port Glasgow), Ltd., Port Glasgow ; and Priestman Brothers, Ltd., Hull. A number of the examples have been taken from the writer's own practice. (A. T. C.)

dredger, suction, ladder, water, pipe, spoil and materials