Part 27

_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 lb. 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.

_Sand-pump Dredgers._--Perhaps the most important development which has taken place in dredging during recent years has been the employment of sand-pump dredgers, which are very useful for removing sandy bars where the particular object is to remove quickly a large quantity of sand or other soft material. They are, however, apt to make large holes, and are therefore not fitted for positions where it is necessary to finish off the dredging work to a uniform flat bottom, for which purpose bucket dredgers are better adapted. Pump dredgers are, however, admirable and economical machines for carrying out the work for which they are specially suited.

In the discussion upon Mr J. J. Webster's paper upon "Dredging-Appliances" (_Proc. Inst. C.E._ vol. 89) at the Institution of Civil Engineers in 1886, Sir John Coode stated that he had first seen sand-pump dredgers at the mouth of the Maas in Holland. The centrifugal pump was placed against the bulkheads in the after part of the vessel, and the sand and water were delivered into a horizontal breeches-piece leading into two pipes running along the full length of the hopper. The difficulty of preventing the sand from running overboard was entirely obviated by its being propelled by the pump through these pipes, the bottoms of which were perforated by a series of holes. In addition, there were a few small flap-doors fixed at intervals, by means of which the men were able to regulate the discharge. On being tested, the craft pumped into its hopper 400 tons of sand in 22 minutes. The coamings round the well of the hoppers were constructed with a dip, and when the hopper was full the water ran over in a steady stream on either side. The proportion of sand delivered into the hopper was about 20% of the total capacity of the pump. The dredger was constructed by Messrs Smit of Kinderdijk, near Rotterdam. In the same discussion Mr A. A. Langley, then engineer to the Great Eastern railway, gave particulars of a sand pump upon the Bazin system, which had been used successfully at Lowestoft. The boat was 60 ft. long by 20 ft. wide, and the pump was 2 ft. in diameter, with a two-bladed disk. The discharge pipe was 12 in. in diameter. The pump raised 400 tons of sand, gravel and stones per hour as a maximum quantity, the average quantity being about 200 tons per hour. The depth dredged was from 7 ft. to 25 ft. The pump was driven by a double-cylinder engine, having cylinders of 9 in. diameter by 10 in. stroke, and making 120 revolutions per minute. An important improvement was made by fitting the working faces of the pump with india-rubber, which was very successful and largely reduced the wear and tear. The cost of the dredging at Lowestoft was given by Mr Langley at 2d. per ton, including delivery 2 m. out at sea. The quantity dredged was about 200,000 tons per annum.

One of the earliest pumps to be applied to dredging purposes was the Woodford, which consisted of a horizontal disk with two or more arms working in a case somewhat similar to the ordinary centrifugal pump. The disk was keyed to a vertical shaft which was driven from above by means of belts or other gear coupled to an ordinary portable engine. The pump within rested on the ground; the suction pipe was so arranged that water was drawn in with the sand or mud, the proportions being regulated to suit the quality of the material. The discharge pipe was rectangular and carried a vertical shaft, the whole apparatus being adjustable to suit different depths of water. This arrangement was very effective, and has been used on many works. Burt & Freeman's sand pump, a modification of the Woodford pump, was used in the construction of the Amsterdam Ship Canal, for which it was designed. The excavations from the canal had to be deposited on the banks some distance away from the dredgers, and after being raised by the ordinary bucket dredger, instead of being discharged into the barges, they were led into a vertical chamber on the top side of the pump, suitable arrangements being made for regulating the delivery. The pump was 3½ ft. in diameter, and made about 230 revolutions per minute. The water was drawn up on the bottom side and mixed with the descending mud on the top side, and the two were discharged into a pipe 15 in. in diameter. The discharge pipe was a special feature, and consisted of a series of wooden pipes jointed together with leather hinges and floated on buoys from the dredger to the bank. In some cases this pipe was 300 yds. long, and discharged the material 8 ft. above the water level. Each dredger and pump was capable of discharging an average of 1500 cub. yds. per day of 12 hours. Schmidt's sand pump is claimed to be an improvement on the Burt & Freeman pump. It consists of a revolving wheel 6 ft. in diameter, with cutters revolving under a hood which just allows the water to pass underneath. To the top side of the hood a 20 in. suction pipe from an ordinary centrifugal pump is attached. The pump is driven by two 16 in. by 20 in. cylinders, at 134 revolutions per minute, the boiler pressure being 95 lb. per sq. in. This apparatus is capable of excavating sticky blue clayey mud, and will deliver the material at 500 to 650 yds. distance. The best results are obtained when the mixture of mud and water is as 1 to 6.5. The average quantity excavated per diem by the apparatus is 1300 cub. yds., the maximum quantity being 2500 cub. yds.

Kennard's sand pump is entirely different from the pumps already described, and is a direct application of the ordinary lift pump. A wrought iron box has a suction pipe fitted at the bottom, rising about half way up the inside of the box; on the top of the box is fitted the actual pump and the flap valves. The apparatus is lowered by chains, and the pump lowered from above. As soon as the box is filled with sand it is raised, the catches holding up the bottom released, and the contents discharged into a punt.

Sand-pump dredgers, designed and arranged by Mr Darnton Hutton, were extensively used on the Amsterdam Ship Canal. A centrifugal pump with a fan 4 ft. in diameter was employed, the suction and delivery pipes, each 18 in. in diameter, being attached to an open wrought-iron framework. The machine was suspended between guides fixed to the end of the vessel, which was fitted with tackle for raising, lowering and adjusting the machine. The vessel was fitted with a steam engine and boiler for working and manipulating the pumps and the heavy side chains for the guidance of the dredger. The engine was 70 h.p., and the total cost of one dredger was £8000. The number of hands required for working this sand-pump dredger was one captain, one engineer, one stoker and four sailors. Each machine was capable of raising about 1300 tons of material per day, the engines working at 60 and the pump at 180 revolutions per minute. The sand was delivered into barges alongside the dredger. The cost of raising the material and depositing it in barges was about 1d. per ton when the sand pumps were working, but upon the year's work the cost was 2.4d. per cub. yd. for working expenses and repairs, and 1.24d. per cub. yd. for interest and depreciation at 10% upon the cost of the plant, making a total cost for dredging of 3.64d. per cub. yd. The cost for transport was 3.588d. per cub. yd., making a total cost for dredging and transport of 7.234d. per cub. yd. Dredging and transport on the same works by an ordinary bucket dredger and barges cost 8.328d. per cub. yd.

Two of the largest and most successful instances of sand-pump dredgers are the "Brancker" and the "G. B. Crow," belonging to the Mersey Docks and Harbour Board. Mr A. G. Lyster gave particulars of the work done by these dredgers in a paper read before the Engineering Congress in 1899. They are each 320 ft. long, 47 ft. wide and 20.5 ft. deep, the draught loaded being 16 ft. They are fitted with two centrifugal pumps, each 6 ft. in diameter, with 36 in. suction and delivery pipes, united into a 45 in. diameter pipe, hung by a ball and socket joint in a trunnion, so as to work safely in a seaway when the waves are 10 ft. high. The suction pipe is 76 ft. long and will dredge in 53 ft. of water. The eight hoppers hold 3000 tons, equivalent when solid to 2000 cub. yds.; they can be filled in three-quarters of an hour and discharged in five minutes. Mr Lyster stated that up to May 1899, the quantity removed from bar and main-channel shoals amounted to 41,240,360 tons, giving a width of channel of 1500 ft. through the bar, with a minimum depth of 27 ft. The cost of dredging on the bar by the "G. B. Crow" during 1898, when 4,309,350 tons of material were removed, was 0.61d. per ton for wages, supplies and repairs. These figures include all direct working costs and a proportion of the charge for actual superintendence, but no allowance for interest on capital cost or depreciation. On an average, 20% of the sand and mud that are raised escapes over the side of the vessel. Mr Lyster has, however, to a considerable extent overcome this difficulty by a special arrangement added to the hoppers (see _Proc. Inst. C.E._ vol. 188).

At the Engineering Conference, 1907, Mr Lyster read a note in which he stated that the total quantity of material removed from the bar of the Mersey, from the Crosby channel, and from other points of the main channel by the "G. B. Crow" and "Brancker" suction dredgers amounted to 108,675,570 tons up to the 1st of May 1907. "In the note of 1899 (he added) it was pointed out that the Mersey was a striking instance of the improvement of a river by dredging rather than by permanent works, and the economy of the system as well as the advantage which its elasticity and adaptability to varying circumstances permit, was pointed out.... The most recent experience, which has resulted in the adoption of the proposal to revet the Taylor's bank, indicates that the dredging method has its limitations and cannot provide for every contingency which is likely to arise; at the same time, the utility and economy of the dredging system is in no way diminished.... Having regard to the ever-increasing size of vessels, a scheme for new docks and entrances on a very large scale received the authority of parliament during the session of 1905-1906 In this scheme it was considered necessary to make provision for vessels of 1000 ft. in length and 40 ft. in draught, and having regard to this prospective growth of vessels it has been determined still further to deepen and improve the outer channel of the Mersey. No fixed measure of improvement has been decided on, but after careful survey of existing conditions and a comparison with probable requirements, it has been determined to construct a dredger of 10,000 tons capacity, provided with pumping power equivalent to about three times that of any existing dredgers. By the use of this vessel it is anticipated that it will be possible to deal with very much larger quantities of sand at a cheaper rate, and to 10 ft. greater depth than the existing plant permits."

The vessel in question was launched on the Mersey from the yard of Messrs Cammell, Laird & Co. in October 1908, and was named the "Leviathan." Her length is 487 ft., beam 69 ft., and depth 30 ft. 7 in. Her dredging machinery consists of four centrifugal pumps driven by four sets of inverted triple expansion engines, and connected to four suction tubes 90 ft. long and 42 in. in internal diameter. Her propelling machinery, consisting of two sets of triple expansion engines, is capable of driving her at a speed of 10 knots.

Another powerful and successful sand-pump dredger, "Kate" (Plate I. fig. 7), was built in 1897 by Messrs Wm. Simons & Co. Ltd. for the East London Harbour Board, South Africa. Its dimensions are: length 200 ft., breadth 39 ft., depth 14 ft. 6 in., hopper capacity 1000 tons. The pumping arrangements for filling the hopper with sand or discharging overboard consist of two centrifugal pumps, each driven from one of the propelling engines. The suction pipes are each 27 in. in diameter, and are so arranged that they may be used for pumping either forward or aft, as the state of the weather may require. Four steam cranes are provided for manipulating the suction pipes. Owing to the exceptional weather with which the vessel had to contend, special precautions were taken in designing the attachments of the suction pipes to the vessel. The attachment is above deck and consists of a series of joints, which give a perfectly free and universal movement to the upper ends of the pipes. The joints, on each side of the vessel, are attached to a carriage, which is traversed laterally by hydraulic gear. By this means the pipes are pushed out well clear of the vessel's sides when pumping, and brought inboard when not in work. Hydraulic cushioning cylinders are provided to give any required resistance to the fore and aft movements of the pipes. When the vessel arrived at East London on the 18th of July 1897, there was a depth of 14 ft. on the bar at high tide. On the 10th of October, scarcely three months afterwards, there was a depth of 20 ft. on the bar at low water. Working 22 days in rough weather during the month of November 1898, the "Kate" raised and deposited 2½ m. at sea 60,000 tons of dredgings. Her best day's work (12 hours) was on the 7th of November, when she dredged and deposited 6440 tons.