A Treatise On The Principles And Practice Of Dock Engineering

88 DOCK ENGINEERING. A turntable for such temporary purposes would, of course, be impracticable on grounds of expense. The waggons also need reversing at intervals, as there is a tendency for the flanges of the wheels to wear unequally when the curves of the roads have one prevailing direction. This can be done by a crane. Waggons are of three kinds—ballast or permanent way, side-tipping, and end-tipping. Ballast waggons have fixed bodies, and thus, being steady in travelling, are employed for the conveyance of spoil to great distances. The contents, about 5 cubic yards of material each, have to be discharged by hand, unless the waggons be lifted bodily and overturned, as is some- times done. Side-tipping waggons generally have their bodies supported on rockers formed by curved channel bars bearing upon short cross rails. They are temporarily secured by pins and catches, upon releasing which tilting becomes possible and the contents are shot out. End-tipping waggons have bodies hinged at one end to longitudinal bearers. They can be lifted in order to discharge, but are usually driven with some impetus against a wooden log fixed as a buffer upon the rails. The abrupt stoppage causes the tail-end of the waggon to jump up. The method involves, as can readily be imagined, considérable wear and tear. Tipping waggons contain rather less than ballast waggons, say, from 31 to 4 cubic yards of material. Dredgers and Dredging Plant.—All opérations involving the removal of material under water are comprehended in the term dredging, whether the mode of action be dragging, sucking, or digging. As a primary distinction all dredgers may be included in one of two classes :— Compound hopper-dredgers. Simple dredgers with attendant hopper barges. The hopper-dredger is self-contained and complété in itself, being pro- vided not only with apparatus for raising material, but also with compartments for its reception when raised. The dredger loads itself conveys its load to the assigned position, discharges it there and returns all under its own engine power. An obvious disadvantage is the discontinuity of its dredging operations with the attendant repetition of mooring manœuvres. Where new works are being carried out there is a corresponding loss of time, which is a matter of serious importance from several points of view. For maintenance works and minor undertakings the objection has possibly not so much weight; but, in either case, the drawback is emphasised by the possibility of the dredger being weatherbound and unable to leave a sheltered position in order to proceed to sea and discharge. On the other hand, the combined hopper dredger costs less in initial expenditure and subsequent upkeep than a separate dredger and hopper with corresponding or even greater capacity. It also monopolises less valu- able water space in restricted areas, such as the interior of docks. Only one crew is required to carry out all duties; the working expenses are less,