Part 28

A large quantity of sand-pump dredging has been carried out at Boulogne and Calais by steam hopper pump dredgers, workable when the head waves are not more than 3 ft. high and the cross waves not more than 1½ ft. high. The dredgings are taken 2 m. to sea, and the price for dredging and depositing from 800,000 to 900,000 cub. metres in 5 or 6 years was 7.25d. per cub. yd. The contractor offered to do the work at 4.625d. per cub. yd. on condition of being allowed to work either at Calais or Boulogne, as the weather might permit. Sand-pump dredging has also been extensively carried out at the mouth of the ports of Amsterdam, Rotterdam and on the north coast of France by sand dredgers constructed by Messrs L. Smit & Son and G. & K. Smit. The largest dredger, the "Amsterdam," is 141 ft. by 27 ft. by 10 ft. 8 in., and has engines of 190 i.h.p. The hopper capacity is 10,600 cub. ft., and the vessel can carry 600 tons of dredgings. The pump fan is 6 ft. 3 in. in diameter by 10 in. wide, the plates being of wrought iron, and makes 130 revolutions a minute. The pump can raise 230 cub. ft. a minute from a depth of 33 ft., which, taking the proportion of 1 of sand to 7 of water, gives a delivery of 29 cub. ft. of sand per minute. The hopper containing 10,600 cub. ft. was under favourable circumstances filled in 40 minutes. The vessels are excellent sea boats.

_Combined Bucket-Ladder and Sand-Pump Dredgers._--Bucket ladders and sand pumps have also been fitted to the same dredger. A successful example of this practice is furnished by the hopper dredger "Percy Sanderson" (Plate I. fig. 8), constructed under the direction of Sir C. A. Hartley, engineer of the Danube Commission for the deepening of the river Danube and the Sulina bar. This dredger is 220 ft. by 40 ft. by 17 ft. 2 in., and has a hopper capacity for 1250 tons of dredgings. The buckets have each a capacity of 25 cub. ft., and are able to raise 1000 tons of ordinary material per hour. The suction pump, which is driven by an independent set of triple expansion engines, is capable of raising 700 tons of sand per hour, and of dredging to a depth of 35 ft. below the water-line. The lower end of the suction pipe is controlled by special steam appliances by which the pipe can be brought entirely inboard. The "Percy Sanderson" raises and deposits on an average 5000 tons of material per day.

_Grab Dredgers._--The grab dredger was stated by Sir Benjamin Baker (_Proc. Inst. C.E._ vol. 113, p. 38) to have been invented by Gouffé in 1703, and was worked by two ropes and a bar. Various kinds of apparatus have been designed in the shape of grabs or buckets for dredging purposes. These are usually worked by a steam crane, which lets the open grab down to the surface of the ground to be excavated and then closes it by a chain which forces the tines into the ground; the grab is then raised by the crane, which deposits the contents either into the hopper of the vessel upon which the crane is fixed or into another barge.

The Priestman grab has perhaps been more extensively used than any other apparatus of this sort. It is very useful for excavating mud, gravel and soft sand, but is less effective with hard sand or stiff clay--a general defect in this class of dredger. It is also capable of lifting large loose pieces of rock weighing from 1 to 2 tons. A dredger of this type, with grab holding 1 ton of mud, dredged during six days, in 19 ft. of water, an average of 52½ tons and a maximum of 68½ tons per hour, and during 12 days, in 16 ft. of water, an average of 48 tons and a maximum of 58 tons per hour, at a cost of 1.63d. per ton, excluding interest on the capital and depreciation. The largest dredger to which this apparatus has been applied is the grab bucket hopper dredger "Miles K. Burton" (Plate I. fig. 9), belonging to the Mersey Docks and Harbour Board. It is equipped with 5 grabs on Morgan's patent system, which is a modification of Priestman's, the grabs being worked by 5 hydraulic cranes. It raised and deposited, 12 to 15 m. at sea, 11 loads of about 1450 tons each with a double shift of hands, at a cost of about 1s. 5d. per cub. yd. of spoil, including the working expenses for wages of crew, fuel and stores. Mr R. A. Marillier of Hull has stated that "the efficiency of these grabs is not at all dependent upon the force of the blow in falling for the penetration and grip in the material, as they do their work very satisfactorily even when lowered quite gently on to the material to be cut out, the jaws being so framed as to draw down and penetrate the material as soon as the upward strain is put on the lifting chain. Even in hard material the jaws penetrate so thoroughly as to cause the bucket to be well filled. The grab is found to work successfully in excavating hard clay from its natural bed on dry land." It is claimed on behalf of grabs that they lift a smaller proportion of water than any other class of dredger.

Since the beginning of the 20th century considerable advance has been made in the use of Priestman grabs, not only for dredging and excavating (for which work they were originally designed), but also in discharging bulk cargo. The first quadruple dredger used by the Liverpool Docks Board had grabs of a capacity of 30 cub. ft., but subsequently second and third quadruple dredgers were put to work in the Liverpool Docks, with grabs having a capacity of 70 and 100 cub. ft. respectively. In discharging coal at Southampton, Havre, Erith, as well as at the coaling station at Purfleet on the Thames, grabs having a capacity of about 80 cub. ft. are in constant use. Perhaps the most difficult kind of bulk cargo to lift is "Narvick" iron ore, which sets into a semi-solid body in the holds of the vessels, and for this purpose one of the largest grabs, having about 150 cub. ft. capacity and weighing about 8 tons, has been adopted. This grab was designed as a result of experiments extending over a long period in lifting iron ore. It is fitted with long, forged, interlocked steel teeth for penetrating the compact material, which is very costly to remove by hand labour. The Priestman grab is made to work with either one or two chains or wire ropes. Grabs worked with two chains or ropes have many advantages, and are therefore adopted for large undertakings.

Wild's single chain half-tine grab works entirely with a single chain, and has been found very useful in excavating the cylinders in Castries harbour. Upon experimenting with an ordinary grab a rather curious condition of things was observed with respect to sinking. On penetrating the soil to a certain depth the ground was found as it were nested, and nothing would induce the grab to sink lower. Sir W. Matthews suggested that a further set of external tines might possibly get over this difficulty. A new grab having been made with this modification, and also with a large increase of weight--all the parts being of steel--it descended to any required depth with ease, the outside tines loosening the ground effectually whilst the inside bucket or tines picked up the material.

_Miscellaneous Appliances._--There are several machines or appliances which perhaps can hardly be called dredgers, although they are used for cleansing and deepening rivers and harbours.

Kingfoot's dredger, used for cleansing the river Stour, consisted of a boat with a broad rake fitted to the bow, capable of adjustment to different depths. At the sides of the boat were hinged two wings of the same depth as the rake and in a line with it. When the rake was dropped to the bottom of the river and the wings extended to the side, they formed a sort of temporary dam, and the water began to rise gradually. As soon as a sufficient head was raised, varying from 6 to 12 in., the whole machine was driven forward by the pressure, and the rake carried the mud with it. Progress at the rate of about 3 m. an hour was made in this manner, and to prevent the accumulation of the dredgings, operations were begun at the mouth of the river and carried on backwards. The apparatus was very effective and the river was cleansed thoroughly, but the distance travelled by the dredger must have been great.

In 1876 J. J. Rietschoten designed a "propeller dredger" for removing the shoals of the river Maas. It consisted of an old gunboat fitted with a pair of trussed beams, one at each side, each of which carried a steel shaft and was capable of being lowered or raised by means of a crab. An ordinary propeller 3 ft. 6 in. in diameter was fixed to the lower end of the shaft, and driven by bevel gear from a cross shaft which derived its motion by belting from the fly-wheel of a 12 h.p. portable engine. The propellers were lowered until they nearly reached the shoals, and were then worked at 150 revolutions per minute. This operation scoured away the shoal effectively, for in about 40 minutes it had been lowered about 3 ft. for a space of 150 yds. long by 8 yds. wide.

A. Lavalley in 1877 designed an arrangement for the harbour of Dunkirk to overcome the difficulty of working an ordinary bucket-ladder dredger when there is even a small swell. A pump injects water into the sand down a pipe terminating in three nozzles to stir up the sand, and another centrifugal pump draws up the mixed sand and water and discharges it into a hopper, the pumps and all machinery being on board the hopper. To allow for the rising and falling of the vessel--either by the action of the tide or by the swell--the ends of the pipes are made flexible. The hopper has a capacity of 190 cub. yds., and is propelled and the pumps worked by an engine of 150 i.h.p. From 50 to 80 cub. yds. per hour can be raised by this dredger.

The "Aquamotrice," designed by Popie, and used on the Garonne at Agen, appears to be a modification of the old bag and spoon arrangement. A flat-bottomed boat 51½ ft. long by 6½ ft. wide was fitted at the bow with paddles, which were actuated by the tide. Connected with the paddles was a long chain, passing over a pulley on uprights and under a roller, and a beam was attached to the chain 14 ft. 8 in. long, passing through a hole in the deck. At the end of the beam was an iron scoop 2 ft. wide and 2 ft. 6 in. deep. When the tide was strong enough it drew the scoop along by means of the paddles and chains, and the scoop when filled was opened by a lever and discharged. About 65 cub. yds. of gravel could be raised by the apparatus in 12 hours. When the tide failed the apparatus was worked by men.

The Danube Steam Navigation Co. removed the shingle in the shallow parts of the river by means of a triangular rake with wrought-iron sides 18 ft. long, and fitted with 34 teeth of chilled cast iron 12 in. deep. This rake was hung from the bow of a steamer 180 ft. long by 21 ft. beam, and dragged across the shallows, increasing the depth of water in one instance from 5 ft. 6 in. to 9 ft., after passing over the bank 355 times.

A combination of a harrow and high pressure water jets, arranged by B. Tydeman, was found very efficacious in removing a large quantity of mud which accumulated in the Tilbury Dock basin, which has an area of about 17 acres, with a depth of 26 ft. at low-water spring tides. In the first instance chain harrows merely were used, but the addition of the water jets added materially to the success of the operation. The system accomplished in six tides more than was done in twelve tides without the water jets which worked at about 80 lb pressure per sq. in. at the bottom of the dock.

Ive's excavator consists of a long weighted spear, with a sort of spade at the end of it. The spade is hinged at the top, and is capable of being turned at right angles to the spear by a chain attached to the end of the spear. The spade is driven into the ground, and after releasing the catch which holds it in position during its descent, it is drawn up at right angles to the spear by the chain, carrying the material with it. Milroy's excavator is similar, but instead of having only one spade it generally has eight, united to the periphery of an octagonal iron frame fixed to a central vertical rod. When these eight spades are drawn up by means of chains, they form one flat table or tray at right angles to the central rod. In operation the spades hang vertically, and are dropped into the material to be excavated; the chains are then drawn up, and the table thus formed holds the material on the top, which is lifted and discharged by releasing the spade. This apparatus has been extensively used both in Great Britain and in India for excavating in bridge cylinders.

The clam shell dredger consists of two hinged buckets, which when closed form one semi-cylindrical bucket. The buckets are held open by chains attached to the top of a cross-head, and the machine is dropped on to the top of the material to be dredged. The chains holding the bucket open are then released, while the spears are held firmly in position, the buckets being closed by another chain. Bull's dredger, Gatmell's excavator, and Fouracre's dredger are modifications with improvements of the clam shell dredger, and have all been used successfully upon various works.

Bruce & Batho's dredger, when closed, is of hemispherical form, the bucket being composed of three or four blades. It can be worked by either a single chain or by means of a spear, the latter being generally used for stiff material. The advantage of this form of dredger bucket is that the steel points of the blades are well adapted for penetrating hard material. Messrs Bruce & Batho also designed a dredger consisting of one of these buckets, but worked entirely by hydraulic power. This was made for working on the Tyne. The excavator or dredger is fixed to the end of a beam which is actuated by two hydraulic cylinders, one being used for raising the bucket and the other for lowering it; the hydraulic power is supplied by the pumps in the engine-room. The novelty in the design is the ingenious way in which the lever in ascending draws the shoot under the bucket to receive its contents, and draws away again as the bucket descends. The hydraulic cylinder at the end of the beam is carried on gimbals to allow for irregularities on the surface being dredged. The hydraulic pressure is 700 lb. per sq. in., and the pumps are used in connexion with a steam accumulator.

An unloading apparatus was designed by Mr A. Manning for the East & West India Dock Co. for unloading the dredged materials out of barges and delivering it on the marsh at the back of the bank of the river Thames at Crossness, Kent. A stage constructed of wooden piles commanded a series of barge beds, and the unloading dredger running from end to end of the stage, lifted and delivered the materials on the marsh behind the river wall at the cost of 1 d. per cub. yd.

_Dredging on the River Scheldt below Antwerp._--This dredging took place at Krankeloon and the Belgian Sluis under the direction of L. Van Gansberghe. At Melsele there is a pronounced bend in the river, causing a bar at the Pass of Port Philip, and just below the pass of Lillo there is a cross-over in the current, making a neutral point and forming a shoal. After dredging to 8 metres (26.24 ft.) below low tide, in clay containing stone and ferruginous matter, a sandstone formation was encountered, which was very compact and difficult to raise. A suction dredger being unsuited to the work, a bucket-ladder dredger was employed. The dredging was commenced at Krankeloon in September 1894 and continued to the end of 1897. A depth of 6 metres (19.68 ft.) was excavated at first, but was afterwards increased to 8 metres (26.24 ft.). The place of deposit was at first on lands acquired by the State, 2.17 m. above Krankeloon, and placed at the disposal of the contractor. The dredgings excavated by the bucket-ladder dredger were deposited in scows, which were towed to the front of the deposit ground and discharged by a suction pump fixed in a special boat, moored close to the bank of the river. The material brought by the suction dredger in its own hull was discharged by a plant fixed upon the dredger itself. In both instances the material was deposited at a distance of 1640 ft. from the river, the spoil bank varying in depth from 2 to 7 metres. The water thrown out behind the dyke with the excavated material returned to the river, after settlement, by a special discharge lock built under the dyke. After 1896 the material was delivered into an abandoned pass by means of barges with bottom hopper doors or by the suction dredger. One suction dredger and three bucket-ladder dredgers were employed upon the work, and a vessel called "Scheldt I." used for discharging the material from the scows. Four tugboats and twenty scows were also employed.

The largest dredger, "Scheldt III.," was 147.63 ft. long by 22.96 ft. wide by 10.98 ft. deep, and had buckets of 21.18 cub. ft. capacity. The output per hour was 10,594 cub. ft. This dredger had also a complete installation as a suction dredger, the suction pipe being 2 ft. diameter. The fan of the centrifugal pump was 5.25 ft. diameter, and was driven by the motor of the bucket ladder. The three bucket dredgers worked with head to the ebb tide. They could also work with head to the flood tide, but it took so long a time to turn them about that it was impracticable. The work was for from 13 to 14 hours a day on the ebb tide. The effective daily excavation averaged 4839 cub. yds. Each dredger was fitted with six anchors. The excavated cut was 164 ft. wide by 6.56 ft. deep. "Scheldt III." was capable of lifting a mass 9.84 ft. thick. The suction dredger "Scheldt II." was of the multiple type, and is stated to be unique in construction. It can discharge material from a scow alongside, fill its own hopper with excavations, discharge its own load upon the bank or into a scow by different pipes provided for the purpose, and discharge its own load through hopper doors. The machinery is driven by a triple expansion engine of 300 i.h.p. working the propeller by a clutch. Owing to the rise and fall in the tide of 23 ft. the suction pipe is fitted with spherical joints and a telescopic arrangement. The vessel is 157.5 ft. by 28.2 ft. by 12.8 ft. The diameter of the pump is 5.25 ft. The wings of the pump are curved, the surface being in the form of a cylinder parallel to the axis of rotation, the directrix of which is an arc of a circle of 2.62 ft. radius with the straight part beyond. The suction and discharge pipes are 2 ft. diameter. A centrifugal pump is provided for throwing water into the scows to liquefy the material during discharge. The dredger, which is fitted with electric lights for work at night, is held by two anchors, to prevent lurching backwards and forwards; it can work on the flood as well as on the ebb tide, and can excavate to a depth of 42.65 ft., the output depending upon the nature of the material. With good material it can fill its tanks in thirty minutes. To empty the tanks by suction and discharge upon the bank over the dyke takes about fifty minutes, depending upon the height and distance to which the material requires to be delivered. The daily work has averaged eighteen hours, ten trips being made when the distance from the dredging ground to the point of delivery is about 1 m. When the dredged material is discharged into the Scheldt, a quantity of 5886 cub. yds. has been raised and deposited in a day, the mean quantity being 4700 cub. yds. When the distance of transportation is increased to 2½ m., six voyages were made in a day, and the day's work amounted to 3530 cub. yds.

_Gold Dredgers._--Dredgers for excavating from river beds soil containing gold are generally fitted with a screen and elevator.

[Illustration: FIG. 2.--Diagram showing Action of Lobnitz Gold Dredger.]

They have been extensively designed and built by Messrs Lobnitz & Co. (fig. 2) and also by Messrs Hunter & English.

The writer is indebted to the _Proceedings_ of the Institution of Civil Engineers, and especially to the paper of Mr J. J. Webster (_Proc. Inst. C.E._ vol. 89), for much valuable information upon the subject treated. He is also indebted to many manufacturers who have furnished him with particulars and photographs of dredging plant. (W. H.*)

Plate I.

[Illustration: FIG. 3.--Barge-loading dredger, "St Austell," constructed for the British Government by Wm. Simons & Co.]

[Illustration: FIG. 4.--Stern-well hopper-dredger "La Puissante," by Wm. Simons & Co. Length 275 ft., breadth 47 ft., depth 19 ft.]

[Illustration: FIG. 5.--Dredger constructed for the Lake Copais Co. by Hunter & English.]

[Illustration: FIG. 6.--Light-draught dredger, with delivery apparatus working round an arc of 210°, by Hunter & English.]

[Illustration: FIG. 7.--Twin-screw sand-pump dredger, "Kate," built for the East London Harbour Board by Wm. Simons & Co.]

[Illustration: FIG. 8.--Twin-screw hopper-dredger, "Percy Sanderson," built for the European Danube Commission by Wm. Simons & Co.]

[Illustration: FIG. 9.--Twin-screw grab-dredger, "Miles K. Burton," built for the Mersey Docks and Harbour Board by Wm. Simons & Co.]

[Illustration: FIG. 10.--Hopper-dredger, "David Dale," with buckets of 54 cub. ft. capacity (see fig. 11) built for the North Eastern Railway Company by Lobnitz & Co.]

Plate II.

[Illustration: FIG. 11.--BUCKETS OF 5 AND 54 CUBIC FEET CAPACITY COMPARED.

The latter, the largest ever made, were for the hopper-dredger "David Dale" (Plate I. fig. 10), built by Lobnitz & Co.]

[Illustration: FIG. 12.--MODEL OF ROCK-CUTTING DREDGER, "DEROCHEUSE."

Built for special work on the Suez Canal by Lobnitz & Co. Length 180 ft., breadth 40 ft., depth 12 ft.]

2. MARINE BIOLOGY

The naturalist's dredge is an instrument consisting essentially of a net or bag attached to a framework of iron which forms the mouth of the net. When in use as the apparatus is drawn over the sea-bottom mouth forwards, some part of the framework passes beneath objects which it meets and so causes them to enter the net. It is intended for the collection of animals and plants living on or near the sea-bottom, or sometimes of specimens of the sea-bottom itself, for scientific purposes.