Page:EB1911 - Volume 08.djvu/587

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564
DREDGE AND DREDGING


buckets afterwards. This system of subaqueous rock cutting plant, on Messrs Lobnitz’s patent system, was effectively employed in deepening the Manchester Ship Canal, and removed a considerable length of rock, increasing the depth of water from 26 ft. to 28 ft. at a cost of about 9d. per cub. yd. A full and illustrated description of this plant, and of a similar plant supplied to the Argentine Government, was published in Engineering of August 17, 1906. An illustration of a bucket of 54 cub. ft. capacity constructed by Messrs Lobnitz & Co. is given (Plate II fig. 11), from which some idea of the size of dredging machinery as developed in recent practice may be obtained. In regard to the depth of water that can be obtained by dredging, it is interesting to note that the dredger “Diver,” constructed by Messrs. Hunter & English for Mr Samuel Williams of London, is capable of working in 60 ft. of water. In this vessel an ingenious arrangement was devised by Mr Williams, by which part of the weight of the dredger was balanced while the ladder itself could be drawn up through the bucket well and placed upon the deck, enabling a long ladder to be used for a comparatively short vessel. The “Tilbury” dredger, also constructed by Messrs Hunter & English, was able to dredge to a depth of 45 ft. below the surface of the water.

Hopper Barges.—To receive the materials excavated by barge-loading dredgers, steam hopper barges are now generally employed, capable of carrying 500 tons or more of excavation and of steaming loaded at a speed of about 9 m. per hour. These hopper barges are made with hinged flaps in their bottoms, which can be opened when the place of deposit is reached and the dredgings easily and quickly discharged.

Good examples of these vessels are the two steam hopper barges built for the Conservators of the river Thames in 1898. The dimensions are: length 190 ft., breadth 30 ft., depth 13 ft. 3 in., hopper capacity 900 tons. They are propelled by a set of triple expansion engines of 1200 i.h.p., with two return-tube boilers having a working pressure of 160 ℔. Special appliances are provided to work the hopper doors by steam power from independent engines placed at the forward end of the hopper. A steam windlass is fixed forward and a steam capstan aft. The vessels are fitted with cabins for the officers and crew. On their trial trip, the hoppers having their full load, a speed of 11 knots was obtained, the coal consumption being 1.44 ℔ per i.h.p.

Methods of Dredging.—In 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 dredging; and cross dredging leaves a more uniform bottom. In either case the dredger is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredgers 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 performing the excavation, so that a longitudinal trench is made in the bottom of the river. After proceeding a certain length, the dredger 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 four additional moorings, two on each side, and these chains are, like the head and stern chains, wound round barrels worked by steam power. In cross dredging we may suppose the vessel to be moored at one side of the channel to be excavated. The bucket frame is set in motion, but instead of the dredger being drawn forward by the head chain, she is drawn across the river by the starboard chains, and, having reached the extent of her work in that direction, 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 port chains to the side whence she started. By means of this transverse motion of the dredger a series of cross cuts is made; the dredger takes out the whole excavation from side to side to a uniform depth and leaves no protuberances such as are found to exist between the furrows in longitudinal dredging, even when it is executed with great care. The two systems will be understood by reference to fig. 1, where A and B are the head and stern moorings, and C, D, E and F the side moorings. The arc e f 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.

Britannica Dredge and Dredging 1.jpg
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