A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER CONSTRUCTION. 167 Hable to bürst. This, of course, could be remedied by strengthening and improving the sacking. Moreover, the bags cannot be brought to a per- fectly level surface ; neither can they be compacted very closely in suc- cessive rows ; and further, they are liable to work loose and be sucked out by the sea, or, failing that, the ends may be broken off by waves. These defects, however, are not vital; careful setting will go far to minimise them; and many breakwaters in existence have been partially, or wholly, constructed of bagwork. The jute sacking generally used for the purpose weighs from 25 to 30 ozs. per superficial yard. Block-making'. —The makiug of blocks for breakwater building calls for little explanation. The blocks are built of concrète in moulds at a block- yard adjacent to the site. The weights range from 5 tons upwards, accord- ing to the capacity of the setting machine. There is, however, practically no limit to size, since huge monoliths may be deposited by special means. At Dublin a wall has been built with foundation blocks weighing 350 tons each, while the caisson blocks at Zeebrugge weighed no less than 4500 tons each. Blocks should be allowed to mature for at least a couple of months before depositing in position, though they may be removed from the moulds at the end of a fortnight. The season of the year and the temperature produce variations in the time of maturing. In order to facilitate the placing of blocks, they are usually constructed with two vertical or slightly inclined perforations, through which are passed irou bars with T or angle ends, capable of engaging against the underside of the block when turned through a right angle. When the blocks are very heavy, the T heads should be provided with hard wood or iron-bearing surfaces to prevent the concrete from suffering damage. Bond. ______The problem of bond in breakwater construction is a difficult one. Theoretically, the effect of introducing a system of interlocking is to materially strengthen the breakwater by binding together, in close association, the separate elements of which it is composed. Practically, there are the con- séquences of unequal settlement to be considered, whereby the sinking of any part of the breakwater will probably fracture the blocks connecting that part with the portions adjoining. The evils attaching to such a contingency can only be averted by discarding the idea of bonding horizontally. Vertical or sloping joints then become inévitable. The breakwater can be eonnected longitudinally, wherever this is done, by means of dowel- or joggle-joints. Such connections offer no resistance to settlement. A dowel^oint consists of a square-shaped aperture, set diagonally, one-half or a V-shaped portion being cut out of each of two stones. When these are joined together, the aperture is filled with a piece of stone of diamond section or with concrete. A yoppZe-joint differs only from a dowel-joint in that the connection is formed by a projection on one piece fitting into the aperture in another. It is the stronger method of the two, but more expensive, because a con-