DREDGING the south side of Blyth, about a mile from the harbour entrance. The bottom of the river at this place was formed of a large patch of rock, which had long been a serious obstruction to dredging operations, and which it was necessary to remove in order to provide the required depth. The rock was 700 ft. long by 139 ft. 6 in. at its widest part, a width of 85 ft. being dry at low water of spring tides. The material consisted of yellow sandstone, rocky shale, and clay, lying in horizontal beds; there was also a considerable proportion of very hard quartzite in the form of large boulders and irregular beds. The shoal was of very irregular character and the clay layers were the worst feature, as they rendered effective blasting difficult. The means employed were, first, boring and blasting; secondly, dredging. The boring was effected by hand labour, from rafts 25 ft. by 13 ft., made of yellow pine logs decked over with planking. The bore-holes were 2| in. diameter. The drills were chisel-pointed, 2| in. broad, the shanks being in. diameter. Boring tubes 3 in. diameter were firmly driven into the ground after the raft was moored, and drills were worked within them. The tubes prevented the sand from getting into the bore-holes, and enabled cartridges to be rammed home without the aid of a diver. Four men were required at each drill. The average speed of boring was about 3 lin. ft. per hour, and the cost for labour was Is. per lin. ft. At first the work was carried on as tide-work, but afterwards by day-work irrespective of the tide, which was found to be cheaper. It was originally intended to do the operation in one lift, but the dredger was not powerful enough, so the operation was effected in two lifts, the first 9 ft. deep and the second to the finished depth of 15 ft. below water at spring tides. The bore-holes were arranged in diagonal squares of 6 ft. 3 in., the upper series being bored to 9 ft. 10 in. and the lower to 15 ft. 6 in. below low water. The blasting charges were contained in water-tight tin cases 2 in. diameter, closed at the top with wood plugs 2 in. deep. Bickford’s double-coated gutta-percha fuzes and Nobel’s detonators were used, and there were few misfires, except in frosty weather, when the dynamite was liable to burn. No tamping material was employed. After blasting, the material was removed by dredging, the greater portion being loaded into hopper barges and deposited about three miles from the site of the works. The dredger employed belonged to the harbour authorities, and was 100 ft. long by 25 ft. beam by 10 ft. deep. Thirty-two buckets and claws, capable of dredging 90 tons per hour of soft sand and mud from a depth of 23 ft., excavated only 6 tons of rock per hour. The buckets had a capacity of 2-82 ft. and weighed 5 cwt. The hoppers were each of 120 tons capacity. The Priestman grab was occasionally used, and was found effective tor lifting large loose pieces of rock up to 2 tons in weight. When the blocks exceeded this weight, they were lifted by a floating steam crane in conj unction with a diver. Dredging was carried on both by day and night, two crews working alternately in twelve-hour shifts. Small ridges and patches about 12 in. high, missed by the dredger, were removed by a diver with pick and bar. In this way 24,500 cubic yds. of rock, with a large quantity of mud and sand, were removed; 4500 in. holes were fired; and after deducting the gunpowder which was not efficacious, 11,820 lb of explosives, principally nitro-glycerine compounds, were used, being 0-482 lb of explosives per cubic yard. The cost of the explosives per cubic yard was Is. 4d., of boring Is. 9d. per cubic yard, and of dredging 3s. per cubic yard, including repairs, but nothing for the use of plant. The whole cost worked out to 6s. Id. per cubic yard on the average. Dredging in the Diver Scheldt below Antwerp.—This dredging took place at Krankeloon and the Belgian Sluis under the direction of Mr 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 very difficult to raise. A suction dredger being unsuited to the work, a bucketladder dredger was employed. The dredging was commenced at Krankeloon in September 1894 and continued to the end of 1897. A depth of six metres (19-68 ft.) was excavated at first, but was afterwards increased to eight metres (26-24 ft.). The place of deposit was at first, on lands acquired by the State, 2-17 miles above Kran-
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keloon, 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 6 "56 ft. to 22 "96 ft. 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, bince 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 bucketladder dredgers were employed upon the work, and a vessel called Scheldt I. used for discharging the material from the scows. Four tug-boats 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 cubic ft. capacity. The output per hour was 10,594 cubic 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 cubic 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 tripleexpansion 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 tankg 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 one mile. When the dredged material is discharged into the Scheldt, a quantity of 5886 cubic yards has been raised and deposited in a day, the mean quantity being 4700 cubic yards. "When the distance of transportation is increased to 2i miles, six voyages were made in a day, and the day’s work amounted to 3530 cubic yards. The work was let to the contractor at the following prices : For excavation, transportation, and deposit 2-17 miles in Polder of Borgerwoort 4-5d. per cubic yd. F or excavation, transportation, and discharge in disused parts of the Scheldt . . . . . 2‘0d. For excavation and leading into a scow 1 -2d. the transportation by water per kilometre being .... -15d. For discharge of scows in the Scheldt .‘ -55d.’’ For dredging into scows and delivering on to the embankments in Polder of Borgerwoort .... 2"0d. ,, The extra price for dredging heaps of stone found in the bed of the river was l-75d. „ ,, The output and cost of dredging and delivering were as follows from the different dredgers :—
