A Treatise On The Principles And Practice Of Dock Engineering

LAND DREDGERS. 77 are occasions recorded in which the French machine has overturned when working in light, marshy clays. While the principle of the land dredger is identical with that of the sea dredger, there isa difference in the method of working. In the sea dredger the buckets excavate downwards, away from the vessel, whereas in the land dredger the cutting action is upwards, towards the machine. The buckets of a land dredger are much shallower and lighter than those of a sea dredger, but both machines are alike in that they are fitted with two tumblers, about which the buckets revolve, discharging their contents in passing over the upper tumbler. A very important advantage attaching to the use of land dredgers is the saving of a considerable amount of haulage up inclines. The machines not only excavate cuttings to a depth of 15 or 20 feet, but they also deliver the spoil at a level of 6 or 8 feet above the ground upon which they travel. This means, of course, a marked saving in time, as well as in locomotive or winding power. On the other hånd, the first cost of these machines is very great, amounting to about £2,400 each ; and they require much heavier roads than machines of lighter build. Under very favourable circumstances the cost of excavation with these machines has amounted to l|d. per cubic yard excavated, but this figure may very easily be doubled in cases where space is circumscribed and action impeded. Such conditions often prevail in dock construction. French Machine.—A. land dredger constructed by Messrs. J. Boulet et Cie., of Paris, is illustrated in figs. 42 and 43. It was employed in excavating tlie site of Canada Branch Dock No. 2, Liverpool, and formed one of four engaged upon the formation of the Manchester Ship Canal. Expérience shpwed that it is only suitable for use in connection with light soils, such as earth filling, sand, loam, and gravel. It is of no value in stiff clay or in rock, however soft. Being heavy in build (about 80 tons weight), a strong and expensive road is entailed to carry the machine upon the soft ground in which alone it is effective. For this purpose steel rails, weighing 80 Ibs. per yard, are required, resting upon cross sleepers 2 feet apart, and sometimes upon longitudinal sleepers in addition. Another important considération is the faet that a special locomotive is required in attendance upon the machine to move the waggons along under the discharge shoot, as, although the excavator has motive power, it is not sufficiently rapid to keep pace with the rate of filling. About forty men are also required to be in attendance, tending and laying the road. At the Canada Branch Dock the French machine has excavated 770 cubic yards of soft material in a day of ten hours, and its average has been 600 cubic yards per diem, but the area in which it worked was restricted and the material not altogether favourable, so that it did not