A Treatise On The Principles And Practice Of Dock Engineering

276 DOCK ENGINEERING. exposed tidal way, with its attendant ground-swells and rapid currents, the risk is sufficiently great to render other methods préférable. Concrete Bag Work, introduced in 1865 by Mr. P. J. Messent for the purpose of repairs at Tynemouth, and developed into a system of subaqueous construction about the year 1870 by Messrs. Cay and Barton at Aberdeen and Greenore respectively, consists in filling jute bags with fluid concrete and depositing them immediately in situ with the aid of divers. If the work be carried out expeditiously, before the concrete has had time to set, the bags will adapt themselves to the inequalities of the surface upon which they are laid, and so ensure a complète and uniform bearing for each successive course. The size of the bags used in various instances, ranges from a capacity for 5, to one for 100 tons of concrete, or even more. The material used is jute sacking, weighing from 25 to 30 ounces per superficial yard. The bags, after being filled at the mixing station, are conveyed to their respective positions and lowered in wrought-iron skips, through the hinged bottom of which they are discharged. Adjustment and flattening is performed by the divers. As there is a tendency for the exposed ends of the outermost bags to break away under heavy wave action, it is advisable to construct the work slightly wider than the nett width desired. Bag work forms an admirable method of dealing with irregular foundations too in- durated for dredging, such as hard rock and clay containing massive boulders. Concrete Block Work is an adaptation of the principles of masonry on a large scale to concrete construction. The blocks are prepared on shore in the ordinary way, by means of wooden moulds of the shape required. For foundation and interior work the rectangular or square form is the most usual. The blocks are of any convenient size, ranging from 5 tons to a weight limited only by the power available for lifting and depositing. In order to facilitate setting, each block is sometimes constructed with two vertical or slightly inclined perforations, through which are passed iron bars with T or angle ends, capable of engaging against the under side of the block when turned through a right angle. These are, of course, removed after the block has been set. Other appliances for lifting and depositing are illustrated on p. 114, ante. Setting operations may be carried out by a floating crane by a traveller running upon a temporary staging, or by a crane traversing the portion of the work previously constructed and able to set a block some distance in front of its leading wheels. Except in the case of very smooth water, the traveller and the land crane constitute by far the steadier agents. The blocks are set on the outer faces of the structure, and are ranged as closely as possible in order to admit of being connected by cramps and joggles. Where the circumstances render such a process feasible, the joints may be pointed in cement, or, if too wide for this, the openings may be made good with brickwork in cement. The interior of the work will then be filled with blocks, arranged so as to break joint, and well bedded in concrete grouting, which may be run through a pipe under a considérable head after the blocks are set.