| Fig. 1.—Diagram showing Moorings for Transverse Dredging. |
Hopper Dredgers.—In places where barge-loading dredgers are inconvenient, owing to confined space and interference with navigation, and where it is necessary to curtail capital expenditure, hopper dredgers are convenient and economical. These dredgers were first constructed by Messrs. Wm. Simons & Co. of Renfrew, who patented and constructed what they call the “Hopper Dredger,” combining in itself the advantages of a dredger for raising material and a scow hopper vessel for conveying it to the place of discharge, both of which services are performed by the same engines and the same crew.
The vessel for this type of dredger is made of sufficient length and floating capacity to contain its own dredgings, which it carries out to the depositing ground as soon as its hopper is full. Considerable time is of course occupied in slipping and recovering moorings, and conveying material to the depositing ground, but these disadvantages are in many instances counterbalanced by the fact that less capital is required for plant and that less room is taken up by the dredger. If the depositing ground is far away, the time available for dredging is much curtailed, but the four-screw hopper dredger constructed by Messrs Wm. Simons & Co. for Bristol has done good work at the cost of 5d. per ton, including wages, repairs, coals, grease, sundries and interest on the first cost of the plant, notwithstanding that the material has to be taken 10 m. from the Bristol Dock. She can lift 400 tons of stiff clay per hour from a depth of 36 ft. below the water line, and the power required varies from 120 i.h.p. to 150 i.h.p., according to the nature of the material. The speed is 9 knots, and 4 propellers are provided, two at the head and two at the stern, to enable the vessel to steam equally well either way, as the river Avon is too narrow to permit her to be turned round.
The hopper dredger “La Puissante” (Plate I. fig. 4), constructed by Messrs Wm. Simons & Co. for the Suez Canal Co. for the improvement of Port Said Roads, is a fine example of this class of dredger. She is 275 ft. long by 47 ft. beam by 19 ft. deep. The hopper capacity is 2000 tons, and the draught loaded 16 ft. 5 in. The maximum dredging depth is 40 ft., and the minimum dredging depth is only limited by the vessel’s draught, she being able to cut her own way. The bucket ladder works through the well in the stern and weighs with buckets 120 tons. The buckets have each a capacity of 30 cub. ft. and raised on trial 1600 tons per hour. The dredger is propelled by two sets of independent triple expansion surface-condensing engines of 1800 i.h.p. combined, working with steam at 160 ℔ pressure, supplied by two mild steel multitubular boilers. Each set of engines is capable of driving the buckets independently at speeds of 16 and 20 buckets per minute. The bucket ladder is fitted with buffer springs at its upper end to lessen the shock when working in a seaway. The dredger can deliver the dredged material either into its own hopper or into barges lying on either side. The vessel obtained a speed of 9¾ knots per hour on trial. The coal consumption during 6 hours’ steaming trial was 1.66 ℔ per i.h.p. hour. Fig. 9 (Plate I.) shows a still larger hopper dredger by the same constructors.
Dredgers fitted with Long Shoot or Shore Delivering Apparatus.—The first instance of dredgers being fitted with long shoots was in the Suez Canal. The soil in the lakes was very variable, the surface being generally loose mud which lay in some places in the sand, but frequently more or less on hard clay. Resort was had to shoots 230 ft. long, supported on pontoons connected with the hull of the dredger. The sand flowed away with a moderate supply of water to the shoots when they were fixed at an inclination of about 1 in 20, but when the sand was mixed with shells these formed a coating which prevented the stream of water from washing out the shoot, and even with an inclination of 1 in 10 material could not be delivered. A pair of endless chains working down the long shoot overcame the difficulty, and also enabled hard clay in lumps to be dealt with. One dredger turned out about 2000 cub. yds. of thick clay in 15 hours, and when the clay was not hard it could deliver 150,000 cub. yds. in a month for several consecutive months.
Shore delivery has been successfully effected by raising the material by buckets in the ordinary way and delivering it into a vertical cylinder connected with floating jointed pipes through which the dredgings pass to the shore. This, of course, can only be done where the place of deposit is near the spot where the material is dredged. Two plans have been satisfactorily employed for this operation. At the Amsterdam Canal the stuff was discharged from the buckets into a vertical cylinder, and after being mingled with water by a revolving Woodford pump was sent off under a head of pressure of 4 or 5 ft. to the place of deposit in a semi-fluid state through pipes made of timber, hooped with iron. These wooden pipes were made in lengths of about 15 ft., connected with leather joints, and floated on the surface of the water. A somewhat similar process was also employed on the Suez Canal.
A dredger (Plate I. fig. 5), constructed by Messrs Hunter & English for reclamation works on Lake Copais in Greece was fitted with delivery belts running on rollers in steel lattice frames on each side of the vessel supported by masts and ropes. It could deliver 100 cub. metres per hour at 85 ft. from the centre of the dredger, at a cost of 1.82d. per cub. metre for working expenses, with coal at 45s. per ton, including 0.66d. per cub. metre for renewal of belts, upon which the wear and tear was heavy.
Another instance of the successful application of shore delivery apparatus is that of a dredger for Lake Titicaca, Peru, constructed by Messrs Hunter & English, which was fitted with long shoots on both sides, conveying the dredged material about 100 ft. from the centre of the dredger upon either side. The shoots were supported by shear-legs and ropes, and were supplied with water from a centrifugal pump in the engine room. This dredger could excavate and deliver 120 cub. yds. per hour at a cost of 1.725d. per cub. yd. with coal costing 40s. per ton. If coal had been available at the ordinary rate in England of 20s. per ton, the cost of the dredging and delivery would have been 0.82d. per cub. yd. for wages, coal, oil, &c., but not including the salary of the superintendent.
An interesting example of a shore delivering dredger is a light draught dredger constructed by Messrs Hunter & English for the Lakes of Albufera at the mouth of the river Ebro in Spain (Plate I. fig. 6). The conditions laid down for this dredger were that it should float in 18 in. of water and deliver the dredged material at 90 ft. from the centre of its own hull. In order to meet these requirements the vessel was made of steel plates 1⁄8 in. thick, and longitudinal girders from end to end of the vessel, the upward strain of flotation being conveyed to them from the skin plating by transverse bulkheads at short intervals. The dredger was 94 ft. long, 25 ft. wide, and 3 ft. deep, and the height of the top tumbler above the water was 25 ft. When completed the dredger drew 17 in. of water. The dredgings were delivered by the buckets upon an endless belt, driven from the main compound surface-condensing engine, which ran over pulleys supported upon a steel lattice girder, the outer end of which rested upon an independent pontoon. This belt delivered the dredgings at 90 ft. from the centre of the dredger round an arc of 180°. The dredger delivered 125 cub. yds. per hour of compact clay at a cost of 1.16d. per cub. yd. or 0.86d. per ton for wages, coal and stores. Another method of delivering dredgings is that of pneumatic delivery, introduced by Mr F. E. Duckham, of the Millwall Dock Co., by which the dredgings are delivered into cylindrical tanks in the dredger, closed by air-tight doors, and are expelled by compressed air either into the sea or through long pipes to the land. The Millwall Dock dredger is 113 ft. long, with a beam of 17 ft. and a depth of 12 ft. The draught loaded is 8 ft. It contains two cylindrical tanks, having a combined capacity of 240 cub. yds., and is fitted with compound engines of about 200 i.h.p., with a 20 in. air-compressing cylinder. The discharge pipe is 15 in. diameter by 150 yds. long. The nozzles of the air-injection pipes must not be too small, otherwise the compressed air, instead of driving out the material, simply pierces holes through it and escapes through the discharging pipe, carrying with it all the liquid and thin material in the tanks. The cost of working the Millwall Dock dredger is given by Mr Duckham at 1.75d. per cub. yd. of mud lifted, conveyed and deposited on land 450 ft. from the water-side, for working expenses only. This dredger is believed to be the first machine constructed with a traversing ladder, as suggested by Captain Gibson when dock-master of the Millwall Docks.
Blasting combined with Dredging.—In some cases it has been found that the bottom is too hard to be dredged until it has been to some extent loosened and broken up. Thus at Newry, John Rennie, after blasting the bottom in a depth of from 6 to 8 ft. at low water, removed the material by dredging at an expense of from 4s. to 5s. per cub. yd. 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 cub. yd. Sir William Cubitt also largely employed blasting in connexion with dredging on the Severn (see Proc. Inst. C.E. vol. iv. p. 362). The cost of blasting and dredging the marl beds is given as being 4s. per cub. yd. A combination of blasting and dredging was employed in 1875 by John Fowler of Stockton at the river Tees. The chief novelty was in the barge upon which the machinery was fixed. It was 58 ft. by 28 ft. by 4 ft., 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. 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 area over which the blasting was done measured 500 yds. in length by 200 in breadth, a small part being uncovered at low water. The depth obtained in mid-channel was 14 ft. at low water, the average depth of rock blasted being about 4 ft. 6 in. The holes, which were bored with the diamond drill, varied in depth from 7 to 9 ft., the distance between them being 10 ft. Dynamite in tin canisters fired by patent fuse was used as the explosive, the charges being 2 ℔ and under. The rock is oolite shale of variable hardness, and the average time occupied in drilling holes 5 ft. deep was 12 minutes. The dredger raised the blasted rock. The cost for blasting, lifting and discharging at sea was about 4s. per cub. yd., including interest on dredging and other plant employed. The dredger sometimes worked a face of blasted material of from 7 to 8 ft. The quantity blasted was 110,000 cub. yds., and the contract for blasting so as to be lifted by the dredger was 3s. 1d. per cub. yd. A similar plan was adopted at Blyth Harbour (see Proc. Inst. C.E. vol. 81, p. 302). The cost of the explosives per cub. yd. was 1s. 4d., of boring 1s. 9d. per cub. yd., and of dredging 3s. per cub. yd., including repairs, but nothing for the use of plant. The whole cost worked out at 6s. 1d. per cub. yd. on the average.