Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/485

465 DREDGING 465 on the Suez Canal, not, however, by using pumps, but simply by running the stuff to the banks on steeply inclined shoots, which were supplied with water when the material raised did not contain sufficient water to cause it to runfreely. It is obvious, however, that these arrangements can only be applied in situations where the material to be excavated is of a very soft nature, and where the place of deposit is close at hand. In keeping clear the Suez Canal such appliances may be very useful, as the soft deposit of the canal has only to be raised and projected over the banks on either side. American Dredges. Dredging in Canada and the United States is done by what are called Dipper and Clam-shell dredges, the bucket dredge being seldom used. The dipper dredge consists of a barge, with a derrick- crane reaching over the stern, suspending a large wrought- iron bucket which brings up the dredged material. To the bucket is attached a pole 6 inches by 4 inches in cross section, by which means it is guided while being drawn along the bottom; it is then raised, and its bottom being made to drop open, the contents fall into the barge moored alongside of the dredge. The bottom cf the bucket is kept closed by a catch, which, by means of a rope, can be withdrawn at the proper moment. The dam- shell is a box made of two similar pieces of wrought iron hinged together at one end ; by a simple arrangement of the gearing the clam, mouth open, drops down and sinks into the bottom, and the first effect of heaving up is to close it, thus imprisoning a quantity of material which is raised and deposited as in the case of the dipper. Both kinds of dredges are worked by a steam-engine, and rough as they appear to be, they are extensively employed in deepening and widening river channels, making or deepening canals, and other such works. This is not the place to discuss the merits of different apparatus, which perhaps can only be settled by the actual performance of different arrangements when fully tested by practice. Having thus briefly noticed them, a few practical observations on dredging, as more im mediately applicable to British rivers, have still to be mentioned. Longitudinal and Cross 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 dredg ing ; and the writer invariably finds cross dredging to leave the most uniform bottom. To explain the difference between the two systems of dredging it may be stated that in either case the dredge is moored from the head and stern by chains about 250 fathoms in length. These chains in improved dredges 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 per forming the excavation, so that a longitudinal trench is made in the bottom of the river. When the dredge has proceeded a certain length, it 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 two additional moorings, one on each side ; and these chains are, like the head and stern chains, wound round barrels wrought by the engine. In commencing to work by cross dredging we may suppose the vessel to be at one side of the channel to be excavated. The bucket frame is set in motion, but, instead of the dredge being drawn forward by the head chain, she is drawn to the opposite side of the river by the side chain, and, having reached the extent of her work in that direc tion, 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 opposite chains to the side whence she started. By means of this trans verse motion of the dredge a series of cross furrows is made ; she takes out the whole excavation from side to side as she goes on, and leaves no protuberances such as are found to exist between the furrows of longitudinal dredging, even where it is executed with great care. The two systems will be best explained by reference to fig. 1, where A and B are the head and stem moorings, and C and D the side moorings ; the arc ef represents the course c FIG. 1. 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. Blasting combined with Dredging. In some cases, how ever, the bottom is found to be too hard to be dredged until it has been to some extent loosened and broken up. Thus at Newry, Mr Kennie, after blasting the bottom in a depth of from 6 to 8 feet at low water, removed the material by dredging at an expense of from 4s. to 5s. per cubic yard. 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 cubic yard. Sir William Cubitt also largely employed blasting in connection with dredging on the Severn, of which an instructive account is given in the Minutes of Proceedings of the Institution of Civil Engineers, from which the following particulars are taken : &quot; It appears that a succession of marl beds, varying from 100 yards to half a mile in length, were found in the channel of the Severn, which proved too hard for being dredged, the whole quantity that could be raised being only 50 or 60 tons per day, while the machinery of the dredgers employed was constantly giv ing way. Attempts were first made to drive iron rods into the marl bed, and to break it up ; a second attempt was made to loosen it by dragging across its surface an instrument like a strong plough. But these plans proving unsuccessful, it was determined to blast the whole surface to be operated on. The marl was very dense, its weight being 146 Ib per cubic foot ; J and it was determined to drill perpendicular bores, 6 feet apart, to the depth of 2 feet below the level of the bottom to be dredged out. The bores were made in the following manner, from floating rafts moored in the river. Pipes of T th inch wrought iron, 3 inches diameter, were driven a few inches into the marl. Through these pipes holes were bored, first with a 1 inch jumper, and then with an auger. The holes were bored 2 feet below the proposed bottom of the dredging, as it was expected that each shot would dislocate or break in pieces a mass of marl of a conical form, of which the bore-hole would be the centre and its bottom the apex ; so that the adjoining shots would leave between them a pyramidal piece of marl where the powder would have produced little or no effect. By carrying the shot holes lower than the intended dredging, the apex only of this pyramid was left to be removed ; and in practice this was found 1 Clay weighs about 109 R&amp;gt;, and sandstone about 155 Ib per cubic foot. VII. - 59