DREDGING. Dredging is the name given by engineers to the process of excavating materials under water, raising them to the surface, and depositing them in barges. It is a process which has been useful from very early times ir works of marine and hydraulic engineering, and it has of late years, by improved appliances, been brought to high perfection.
Bag and Spoon Dredge.The first employment of machinery to effect this object is, like the discovery of the canal lock, claimed alike for Holland and Italy, in both of which countries dredging is believed to have been practised before it was introduced into Britain. The Dutch at a very early period used what is termed the " bag and spoon " dredge for cleaning their canals. It was simply a ring of iron, about 2 feet in diameter, flattened and steeled for about one-third of its circumference, having a bag of strong leather attached to it by leathern thongs. The ring and bag were fixed to a pole, which, on being used, was lowered to the bottom from the side of a barge moored in the canal or river. A rope made fast to the iron ring was then wound up by a windlass placed at the other end of the barge, and the spoon was thus dragged along the bottom, and was guided in its progress by a man who held the pole. When the spoon reached the end of the barge where the windlass was placed, the winding was still continued, and the suspending rope being nearly per-pendicular, the bag was raised to the surface, bringing with it the stuff excavated while it was being drawn along the bottom. The windlass being still wrought, the whole was raised to the gunwale of the barge, and the bag, being emptied, was again hauled back to the opposite end of the barge, and lowered for another supply. This system is slow, and only adapted to a limited depth of water and a soft bottom. But it has been generally employed in canals, and is much used in the Thames. The writer had occasion to use it at the Fossdyke Canal, in Lincolnshire, where 135,000 tons were raised in the manner described.
Dredging hy Bucket between two Lighters.Another plan, practised at an early period in rivers of considerable breadth, was to moor two large barges, one on each side; between them was slung an iron dredging bucket, which was attached to both barges by chains wound round the barrels of a crab wincn worked by six men in one barge, and round a simple windlass, worked by two men in the other. The bucket, being lowered at the side of the barge carrying the windlass, was drawn across the bottom of the river by the crab winch on the other barge; and, having been raised and emptied, it was hauled across by the opposite windlass for a repetition of the process. This plan was in use in the Tay till 1833.
Steam Dredges.In all large operations these and other primitive appliances have now, as is well known, been superseded by the steam dredge, which was first employed, it is believed, in deepening the Wear at Sunderland about the year 1796. The Sunderland machine was made for Mr Grimshaw by Boulton and Watt. Beceiving improve-ments from Mr Hughes, Mr Bennie, Mr Jessop, and others, the steam dredge, as now generally constructed, is a most powerful machine in skilful hands, excavating and raising materials from depths of 15 to upwards of 30 feet of water according to the size of the machinery, at a cost not very different from, and in some cases even less than, that at which the same work could be performed on dry land.
As to the kind of work that may be accomplished by dredging, it may be stated that almost all materials, ex-cepting solid rock or very large boulders, may now be dredged with ease. Loose gravel is probably the most favourable material to work in ; but a powerful dredge will readily break up and raise indurated beds of gravel, clay, and boulders, and even find its way through the surface of soft rock, though it will not penetrate very far into it. In such cases it is usual to alternate on the bucket-frame a bucket for raising the stuff, with a rake or pronged instru-ment for disturbing the bottom. The writer in his own ex-perience has raised boulders weighing upwards of a ton with a powerful dredge of the ordinary construction, and removed disintegrated or rotten rock at least to a limited depth, and he believes that in many cases the surfaces of submerged rocks may, by means of such appliances, be to some extent broken up and removed, so as to obtain in certain situa-tions a considerable increase of depth, without recourse to cofferdams, which involve great expense.
The construction of large river steam dredges is now carried on by many engineering firms. The main feature of the machineis the bucket-ladder, whichis lowered through an ark formed in the vessel till it reaches the bottom. Along this ladder a series of buckets traverse which cut into the bottom at the lower extremity of the ladder and return loaded with the excavated material, which is discharged at the top of the bucket-ladder into a lighter or barge prepared for its reception. The machines are sometimes made with single and sometimes with double ladders, sometimes dis-charging at the stern of the vessel and sometimes at both sides, but it is obviously impossible to give illustrative drawings of the different forms of dredgers in sufficient detail to be practically useful. It may be stated that a first-class dredging machine to work in 30 feet water, and discharge over either side, of 60 horse-power complete, costs at present prices about £16,000 to £18,000. The steam hoppers employed to receive and remove the dredgings carry about 500 tons of excavations; they are 70 horse-power, and steam at about 9 miles per hour. The hopper barges are made with opening hinged bottoms, which can be opened when the place of deposit is reached, and the dredgings easily and quickly discharged. These steam barges cost about £8000. Large dredges, such as those constructed by Messrs Wingate of Glasgow for the Tyne and other places, will excavate at the rate of 460 tons per hour when working on favourable ground.
Hopper Dredge.-Some improvements that have been suggested on the dredging plant hitherto used deserve notice. Among these may be mentioned that of Messrs Simons & Company, Benfrew, who have patented and constructed what they have called a hopper-dredge, combining in itself the advantages of a dredge for raising the material and a screw hopper vessel for conveying it to the place of discharge, both which services are performed by the same engines and the same crew. Messrs Simons have constructed seven hopper dredges on this plan, varying from 200 to 1000 tons of " hopper capacity."
Silt Dredge.Another of the recently suggested improve-ments is that by Mr C. Bandolph, who, in 1870, proposed that, instead of the ordinary dredging buckets, pipes should be lowered until they come into contact with the sand or mud at the bottom. The tops of these pipes were to be in communication with powerful centrifugal pumps, so that the velocity of the in-flowing water through the pipes could be made so great as to carry with it a large percentage of the sand or mud from the bottom ; and when the solid matter, and the water in which it is suspended, were raised to the desired height, they would flow freely to any required place for deposit of the suspended material. It is not known that this plan has been carried into practical opera-tion.
Dredging at Amsterdam and Suez Canals.-Another arrangement is that of raising the material by buckets in the ordinary way, and thereafter receiving it in a vessel and floating it off by pipes to the place of deposit. This, of course, can only be done where the place of deposit is close to the spot whence the material is dredged. Two plans have been proposed for effecting this. One of these has been used in the Amsterdam Canal, where the stuff is dis-charged from the buckets into a vertical cylinder, and is there mingled with water by a revolving Woodford-pump and sent off under pressure to the place of deposit in a semi-fluid state. At the Amsterdam Canal this was done by pipes made of timber, and hooped with iron like barrels. These wooden cylinders were made in lengths of about 15 feet, connected with leather joints, and floated on the surface of the water, conveying the stuff to the requisite distance, like the hose of a fire engine, under a head of pressure, it is believed, of 4 or 5 feet, and depositing it over the banks of the canal. A somewhat similar process was employed on the Suez Canal,not, however, by using pumps, but simply by running the stuff to the banks on steeply inclined shoots, which were supplied with water when the material raised did not contain sufflcientwater to cause it to runfreely. It is obvious, however, that these arrangements can only be applied in situations where the material to be excavated is of a very soft nature, and where the place of deposit is close at hand. In keeping clear the Suez Canal such applianges may be very useful, as the soft deposit of the canal has only to be raised and projected over the banks on either side.
American Dredges.Dredging in Canada and the United States is done by what are called Dipper and Clam-shell dredges, the bucket dredge being seldom used.
The dipper dredge consists of a barge, with a derrick-crane reaching over the stern, suspending a large wrought-iron bucket which brings up the dredged material. To the bucket is attached a pole 6 inches by 4 inches in cross section, by which means it is guid¿d while being drawn along the bottom; it is then raised, and its bottom being made to drop open, the contents fall into the barge moored alongside of the dredge. The bottom of the bucket is kept closed by a catch, which, by means of a rope, can be withdrawn at the proper moment. The clam-shell is a box made of two similar pieces of wrought iron hinged together at one end; by a simple arrangement of the gearing the clam, mouth open, drops down and sinks into the bottom, and the first effect of heaving up is to close it, thus imprisoning a quantity of material which is raised and deposited as in the case of the dipper. Both kinds of dredges are worked by a steam-engine, and rough as they appear to be, they are extensively employed in deepening and widening river channels, making or deepening canals, and other such works.
This is not the place to discuss the merits of different apparatus, which perhaps can only be settled by the actual performance of different arrangements when fully tested by practice. Having thus briefly noticed them, a few practical observations on dredging, as more im-mediately applicable to British rivers, have still to be mentioned.
Longitudinal and Cross Dredging,~Ta river dredging two systems are pursued. One plan consists in excavating a series of longitudinal furrows parallel to the axis of the stream, the other in dredging cross furrows from side to side of the river. It is found that inequalities are left between the longitudinal furrows when that system is practised, which do not occur, to the same extent, in side or cross dredg-ing ; and the writer invariably finds cross dredging to leave the most uniform bottom. To explain the difference between the two systems of dredging it may be stated that in either case the dredge is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredges are wound round windlasses worked by the engine, so that the vessel can be moved ahead or astern by simply throwing them into or out of gear. In longitudinal dredging the vessel is worked forward by the head chain, while the buckets are at the same time per-forming the excavation, so that a longitudinal trench is made in the bottom of the river. When the dredge has proceeded a certain length, it is stopped and permitted to drop down and commence a new longitudinal furrow, parallel to the first one. In cross-dredging, on the other hand, the vessel is supplied with two additional moorings, one on each side; and these chains are, like the head and stern chains, wound round barrels wrought by the engine. In commencing to work by cross dredging we may suppose the vessel to be at one side of the channel to be excavated. The bucket frame is set in motion, but, instead of the dredge being drawn forward by the head chain, she is drawn to the opposite side of the river by the side chain, and, having reached the extent of her work in that direc-tion, she is then drawn a few feet forward by the head chain, and, the bucket frame being still in motion, the vessel is hauled across by the opposite chains to the side whence she started. By means of this trans-verse motion of the dredge a series of cross furrows is made ; she takes out the whole excavation from side to side as she goes on, and leaves no protuberances such as are found to exist between the furrows of longitudinal dredging, even where it is executed with great care. The two systems will be best explained by reference to fig. 1, where A and B are the head and stern moorings, and C and D the side moorings ; the arc ef represents the course
of the vessel in cross dredging; while in longitudinal dredging, as already explained, she is drawn forward towards A, and again dropped down to commence a new longitudinal furrow.
Blasting combined with Dredging.In some cases, how-ever, the bottom is found to be too hard to be dredged until it has been to some extent loosened and broken up. Thus at Newry, Mr Bennie, after blasting the bottom in a depth of from 6 to 8 feet at low water, removed the material by dredging at an expense of from 4s. to 5s. per cubic yard. The same process was adopted by Messrs Stevenson at the bar of the Erne at Ballyshannon, where, in a situation exposed to a heavy sea, large quantities of boulder stones were blasted, and afterwards raised by a dredger worked by hand at a cost of 10s. 6d per cubic yard.
Sir William Cubitt also largely employed blasting in connection with dredging on the Severn, of which an instructive account is given in the Minutes of Proceedings of the Institution of Civil Engineers, from which the following particulars are taken :_
" It appears that a succession of marl beds, varying from 100 yards to half a mile in length, were found in the channel of the Severn, which proved too hard for being dredged, the whole quantity that could be raised being only 50 or 60 tons per day, while the machinery of the dredgers employed was constantly giv-ing way. Attempts were first made to drive iron rods into the marl bed, and to break it up ; a second attempt was made to loosen it by dragging across its surface an instrument like a strong plough. But these plans proving unsuccessful, it was determined to blast the whole surface to be operated on. The marl was very dense, its weight being 146 lb per cubic foot -,1 and it was determined to drill perpendicular bores, 6 feet apart, to the depth of 2 feet below the level of the bottom to he dredged out. The bores were made in the following manner, from floating rafts moored in the river. Pipes of y^th inch wrought iron, inches diameter, were driven a few inches into the marl. Through these pipes holes were bored, first with a 1J inch jumper, and then with an auger. The holes were bored 2 feet below the proposed bottom of the dredging, as it was expected that each shot would dislocate or break in pieces a mass of marl of a conical form, of which the bore-hole would be the centre and its bottom the apex; so that the adjoining shots would leave between them a pyramidal piece of marl where the powder would have produced little or no effect. By carrying the shot holes lower than the intended dredging, the apex only of this pyramid was left to be removed ; and in practice this was found to form but a small impediment. Fig. 2 is a section of the bore-holes, and fig. 3 a plan in which the inner dotted circles repre-sent the diameters of the broken spaoes at the level of the bottom of dredging. The cartiidges were formed, in the ordinary way, with canvas, and fired with Bickford's fuse. The weights of powder used for bore-holes of 4 feet, 4 feet 6 inches, and 5 feet were respectively 2 lb, 3 lb, and 4 lb. The effect of the shot was generally to lift the pipeswhich were secured by ropes to the
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raftsa few inches. Mr Edwards says that not one in a hundred shots missed fire, and these shots were generally saved by the fol-lowing singular expedient :The pointed end of an iron bar, \ inch diameter, was made red hot, and, being put quickly through the water, and driven through the tamping as rapidly as possible, was, in nine cases out of ten, sufficiently hot to ignite the gun-powder and fire the shot.
The cost of each shot is calculated as follows :
Use of material £0 1 0
Labour 0 3 3
Pitched bag for charge 0 0 3
3 lb of powder at 5Jd 0 1 4J
15 ft. of patent fuse at nr of a penny 0 0 9
Pitch, tallow, twine, coals, &c 0 0 4J
Cost per shot £0 7 0
Each shot loosened and prepared for dredging about four cubic yards, so that the cost of blasting was Is. 9d. per yard. The cost of dredging the material, after it had been thus prepared, was 2s. 3d., making the whole charge for removing the marl 4s. per cubic yard."
One of the most recent successful combinations of blasting and dredging was that completed in 1875 by Mr John Fowler of Stockton at the river Tees, to whom the writer is indebted for the following particulars. The chief novelty was in the barge upon which the machinery was fixed. It was 58 feet by 28 feet by 4 feet, and had eight legs which were let down when the barge was in position. The legs were then fixed to the barge, so that on the tide falling it became a fixed platform from which the drilling was done. The holes were bored and charged, and when the tide rose the legs were heaved up and the barge removed, after which the shots were discharged. There were 24 boring tubes on the barge, and that was the limit which could at any time be done in one tide. The surface ovei which the blasting was done measured 500 yards in length by 200 yards in breadth, a small part of that surface being uncovered at low water. The depth obtained in mid-channel was 14 feet at low-water, the average depth of rock blasted being about 4 feet 6 inches. The holes, which were bored with the diamond drill, varied in depth from 7 to 9 feet, the distance between them being 10 feet. Dynamite in tin canisters fired by patent fuse was used as the ex-plosive, the charges being 2 lb and under. The rock is Oolite shale of variable hardness, and the average time occupied in drilling 5-feet holes was twelve minutes.
The dredger raised the blasted rock,the cost for blast-ing, lifting, and discharging at sea being about 4s. per cubic yard, including interest on dredging and other plant employed. The dredger sometimes worked a face of blasted material of from 7 to 8 feet. The quantity blasted was 110,000 cubic yards, and the contract for blasting so as to be lifted by the dredger was 3s. Id. per cubic yard.
Dredging in Exposed Situations.In some cases dredg-ing has to be conducted in exposed situations such as the deepening of the " flats " at Londonderry and the bar at Carlingford. Messrs Stevenson found that dredging at the Foyle could not be conducted when the height of the waves exceeded 2J feet; and Mr Barton at Dundalk so far con-firms this, as he estimates a swell of 2 feet as the highest to work in.
Dredging on the River Clyde.-An important point con-nected with this subject is the cost at which dredging may be done when conducted on a large scale. This, of course, must depend on the character of the stuff to be raised and other circumstances ; but the following information, kindly communicated by Mr James Deas, the engineer to the Trustees of the Clyde Navigation, cannot fail to be both interesting and useful.
Mr Deas says truly that the Clyde Trustees employ pro-bably the largest dredging fleet of any trust in the kingdom, in maintaining and still deepening and widening the river to meet the ever-increasing demands of the shipping trade.
In the year 1871, for example, 904,104 cubic yards, or about 1,130,000 tons, were dredged from the river, of which 689,560 cubic yards were carried to sea by steam hopper barges, and 214,544 cubic yards deposited on land by means of punts. Of this 904,104 cubic yards, 345,209 cubic yards were deposit from the higher reaches of the river and its tributaries, and from the city sewers, and 558,895 cubic yards new material. The total cost for dredging and depositing was £35,448, or about 9*41 pence per cubic yard.
Owing to the difference in power of the dredging machines employed, and the character of the material lifted, the cost of dredging varies much. In 1871 the most powerful machine, working 2420 hours, lifted 430,240 cubic yards of silt and sand at a cost of 2'60 pence per yard; and this was deposited in Loch Long, 27 miles from Glasgow, by steam hopper barges, at 5'46 pence per yard. On the other hand, another dredger, working 2605 hours, lifted only 26,720 cubic yards of hard gravel and boulder clay, at the cost of 20'8 pence per cubic yard, which was deposited on the alveus of the river at the cost of 17'46 pence per cubic yard; another, working 1831-J hours, lifted 122,664 cubic yards of silt, sand, and sewage deposit, at the cost of 5'67 pence per cubic yard, which was deposited on land at the cost of 16 "40 pence per cubic yard; and another, working 2233 hours, lifted 65,160 cubic yards of till, gravel, and sand, at the cost of 5'89 pence per cubic yard, which was deposited on the alveus of the river at the cost of 9"83 pence per cubic yard.
The total quantity dredged from the river during the I twenty-seven years prior to 1872 amounts to 13,617,000 cubic yards, or upwards of 17,000,000 tons. The dredg-ing plant of the Clyde Trust comprises_
6 steam dredges, 14 steam hopper barges, 1 steam-tug, 3 diving-bells, 270 puu.o. and numerous small boats.
The expenditure for wages of crews, coal, and stores amounted in the year 1871 to fully £14,000, and for repairs ¿£10,775. The value of the dredging plant employed is about £140,000.
Mr Deas has also kindly furnished the following tables, from which the reader will see the gradual increase that has been made on the size of the dredging machines to meet the increased depth of water and growing necessity of increased accommodation for the larger class of vessels which now frequent the river :
== TABLE ==
The following are the details as regards the dredgers and barges employed on the Clyde:
No. 8 Dredger.
Length, 161 ft. Breadth moulded, 29 ft. Depth, 10 feet.
Engine, 75 horse power. Cylinder, 48 in. diameter. Stroke, 3 ft.
One bucket ladder, 90 ft. 9 in. between centres.
Size of buckets, 3 ft. 3 in. x 2 ft. 5 in. x 1 ft". 11 in.
When working in sand, can lift 190 cubic yards per hour.
Greatest depth can dredge in, 28 feet.
Working draught, 6 to 7 feet.
Wages per day of 10 hours as under :
Master 7s. Od.
Mate 3 9
Engineer 6 8
Fireman 3 8
Assistant do. and cook 3 4
Bow crabman 3 4
Stern crabman 3 4
Deck hands (3), each 3 2
one at 3 0
Watchman 3 C
Coals 65 cwt
Tallow 2 tb.
Oil (Lard) 16 Rills.
Waste 1£ lb
Steam Hopper Barge.
Length, 145 ft. Breadth moulded, 25 ft. Depth, 11 ft. 9 in. Engines, 40 horse power. Draught light (average), 5 ft. 6 in. Draught loaded, 11 ft. Speed, 8 to 9 miles per hour.
Capacity of hopper, 320 cubic yards, or say 400 tons Average distance run, loaded, 20 miles. Wages per day as under :
Master 7s. od.
Mate 4 6
Engineer 5 10
Fireman 3 6
Deck hands (3), each 3 4
Coals per day of 10 hours 70 ewts.
Tallow 5 ft.
Oil 20 gills.
Waste 2 lb.
Quantity and cost of dredging done by No. 8 Dredger during year ending 30th June 1871 :
2-88 10-00 1
83-19: 3-36 10-20 20SI
Sand, silt, and sewage from Glasgow har-bour.
No. 5 Dredger
No. 7 Dredger
Hard til], gra-vel, and sand, from Erskine Ferry, &c.
Sand, clay, and mud, from Pt Glasgow, <&c.
Hard till and clay from Er-skine Ferry, Elderslie, &c.
Sand, silt, till, and gravel, from Glasgow and Bowling harbours, *fcc.
10 Hopper barges .
Nos. 1, 5, and 7 are punt-loading machines. Nos. 6 and 8 are hopper barge machines.
[Further Reading] Reference is made to the following works :Ency. of Civil Engineering, by Edward Cressy, London, 1847; "The Dredging Machine," Weale's Quarterly Papers, i., London 1843 ; The Improvement of the Port of London, by R. Dodd, Engineer, 1798 ; "Account of Blasting on the Severn," by George Edwards, C.E., Minutes of Proceedings of the Institution of Civil Engineers, vol. iv. p. 361 ; "the River Clyde," by James Deas, C.E., Minutes of Proceedings of the Institution of Civil Engineers, vol. xxxvi. p. 124 ; Principles and Practice of Canal and Paver Engineering, by David Stevenson, 2d ed., A. & C. Black, Edinb. 1872, p. 126. (D. S.)
1 Clay weighs about 109 lb, and sandstone about 155 lb per cubio foot.