A Treatise On The Principles And Practice Of Harbour Engineering

HARBOUR ENGINEERING. 158 sists of a huge cantilever crane with a substantial wheel base. There are two variants in design. In one case the cantilever arm is a girder trussed within its flanges; in the other it is supported by means of tension rods from above. The former obviously lends itself to greater stiffness and steadiness, while the latter is lighter and carries the arm at a lower level for the same over-all height. Apart from their systems of trussing, Titans differ in that some have a fixed base, while others are pivoted upon their carriages. The former class, generally differentiated by the term “Mammoth,” are provided with à carrier having longitudinal and transverse motions; the action of the latter class is radial. The radial machines can command a wider lateral range than the rectilinear machines, but they are not so conveniently adaptable to setting out work, a diagonal movement being less easily regulated to align- ment in dual directions than a direct one. However, radial machines are capable of depositing wave-breakers along each flank of a breakwater to some distance outside, and this is a feature in which they decidedly excel the alternative type. Moreover, with Mammoths, the block has to be run under the machine before it can be picked up, but with Titans this is not the case. This is not unimportant, owing to the moorings. The Titan is served with monoliths by a “Goliath " (fig. 130)—the generic name for an overhead traveller, the carrier of which runs 011 tracks trans- versely to the road of a wheel base of considerable span. The blocks are loaded on to trollies by the Goliath, and so conveyed from the block- yard to the breakwater, there to be set in position by the Titan. There is, however, nothing rigorous about the practice. The yard machine may be,’ and is, in some cases, a Titan. Examples of both these machines are shown in the accompanying figures. The Caisson System is an adaptation of the power of natural buoyancy to transportation purposes. Gigantic boxes of iron framework mcased 111 concrète are fornied in a sheltered recess or inlet on the coast or 111 an inner dock. Wheti built to the required size—which is such that when sunk in position their topmost edges will project slightly above the surface of the sea at low water,—they are temporarily strutted in the interior launched, and towed out to the site they are intended to occupy. Great care has to be exercised 111 aligning these huge boxes and in maintaining their perpendicularity while foundering. When this delicate operation has been successfully performed by admitting water to the interior of the caissons, they are filled with fluid concrete, stone rubble, and small blocks, so as to form ultimately a solid monolith. The method involves some risk, especially on an exposed coast. The caissons are very unwieldy: they call for powerful towing and directing appliances; but once in position and rendered solid throughout, they constitute a most potent defence against breaking seas. As regards cost, it is not apparent that they are more expensive than other forms of break-