A Treatise On The Principles And Practice Of Harbour Engineering

76 HARBOUR ENGINEERING. break, each layer beiiig well punned and the conerete pressed into all corners, angles, and recesses. The top, or fourth side of the pile, is formed by striking the edges of the box with a straight-edge, so that the conerete just cornes flush with them. The pile is left for a week in the mould, then the mould is removed and the pile allowed to harden, either in water or while constantly wetted. A nionth or six weeks elapses before the pile is ready for driving. To facilitate lifting, a bolt hole is cast near the top of the pile. The bolt and a shackle enable the pile to be swung easily into position. The green pile, however, is not handled in this way, but by means of chain slings passing round the pile, the sides of which are protected by deals at the points of contact. hor vertical moulding, the box is set upright and the metal framework first plaoed in position with the shoe downwards. Conerete is then filled in to the mould and around the metal, as carefully as in the previous case. The pile is built up in a series of layers from 4 to 6 inches in depth, the fourth side of each layer being formed by a batten fixed across the open face by fitting into grooves or being otherwise secured to the box, the whole height being treated in this way. The remaining operations are as before. The materials used for reinforced conerete piles must be the best of their respective kinds. The conerete particularly calls for special attention. The proportions used lie between one part of Portland cement to four or five parts of aggregate, the latter compounded of gravel and sand in the ratio of 2 : 1. In one system (the Williams’) the aggregate consists of clean shingle, which will pass through a f-inch gauge but not through a ^-inch gauge, mixed with half its volume of sand. In Hennebique work the gravel is also sifted through two sieves. The first has apertures 1 inch square ; the other has four un- crossed meshes per linear inch. The residue from the first sieve is thrown against the second, and equal parts taken of that which passes through the second sieve and that which fails to do so. After the pile has been removed from the mould, it is well to give it a coat of pure cement wash. This closes the outermost pores and renders the pile more highly impervious, The non- porosity of a reinforced conerete pile is obviously essential to its durability. It is only by the complété exclusion of moisture from the embedded steelwork that the latter can be maintained in a serviceable condition. Pile-driving’. —Piles are forced into the ground, or driven, by means of piling machines, which are actuated by hand or steam power. The exceptional use of the screw pile has already been noticed (p. 70). The impelling force is commonly a heavy weight or ram, which is allowed to fall within vertical guides from any desirable height. In the hand or ringing machine, the weight rarely exceeds one-third of a ton, and the fall, 4 feet. In other appliances the weight and fall range from 15 cwts. and 10 feet to 3 tons and 4 feet respectively. A heavy weight and a low fall are preferable to a light weight and a considerable fall, owing to the greater oscillation resulting from the latter arrangement and the consequent jar in the delivery of the blow, which thus tends to injure and split the pile. In conerete piles the absence of