Engineering Wonders of the World
Volume III

HARBOUR CONSTRUCTION. 69 sheer weight of the bodies placed in the path of the waves. As Mr. Alan Steven- son pointed out sixty years ago, mass rather than cohesion is the quality on which the harbour engineer must depend for the stability of a wall. A single concrete block weighing 40 tons is much more reliable than four blocks of ten tons each bonded and tied together with the utmost human art. A joint means potential weakness. The building of efficient concrete block breakwaters has been greatly assisted by recent improvements in cranes of the “ Titan ” and “ Goliath ” types. The first of these has a large carriage supported on a number of wheels run- Giant Cranes. ... .. ning on a wide-gauge railway laid along the completed portion of the break- water. The wheels are furnished with springs to allow for inequalities in the track. Across the carriage run two large girders braced together horizontally, and pivoted on a pin which is set at the centre of a circle of rollers interposing between a path on the summit of the carriage, and a similar path attached to the underside of the girders. On the short arm of the girders are stationed a movable counterweight and the steam- engine which swings the arm round, operates the hoisting tackle, and, on being connected up through gearing with the track wheels, moves the crane bodily backwards or forwards. The largest Titans have an “ overhang,” measured from the centre of the pin to the extreme limit of which the hoisting carriage “ Overhang;.” can be moved out along the longer arm, of about 100 feet, and so are able to pick up or deposit a block weighing anything up to 50 tons within a circle 200 feet in diameter. A liberal over- hang is of great importance when large blocks are handled, as the blocks are necessarily laid in courses, the outer ends of which form a series of steps. The deeper the water th© DIAGRAM TO ILLUSTRATE THE GANTRY SYSTEM OF LAYING BLOCKS. greater is the number of steps, and the further is the bottom step from the last completed top course. Hence it follows that a crane with a very big reach can lay blocks in a depth of water which would with a crane of smaller reach necessitate the use of smaller blocks. The great advantages of this type of crane are that in stormy weather it can be with- drawn out of reach of the waves, assuming that the breakwater has con- nection with the shore ; and „ * Goliath. that, as it builds its own path, no trestle work or other special structures liable to damage are required. On the other hand the “ Goliath ” or gantry crane, running on tracks supported by rows of piles driven ahead of the block laying, and spanning the area to be covered by blocks, is able to assist in preliminary operations, such as levelling tho surface on to which the blocks will be lowered, as we shall notice later on when dealing with the Dover Harbour Works. Also a long “ working end,” allowing the lowest course to be laid over a considerable area before the upper courses are superimposed, minimizes the cracks and settlements which sometimes occur when the short working end associated with the Titan is used. Coming now to a brief review of some of the most notable artificial harbours, the first place