A Treatise On The Principles And Practice Of Dock Engineering

KÖLLING CAISSONS. 355 rubbing pieces above, and tilting is prevented by the adjustment of the water-ballast, and by rollers on the underside of the camber girders. The caisson is stopped automatically at the end of its course into or out of the camber, and buffers are placed in the recess opposite the carnber, in case of a failure of the automatic stopping gear. The maximum tensile strain on the plating of the caisson does not exceed 64 tons per square inch, when one side of the caisson is dry and the water is up to deck B on the other side. The keels and stems are greenheart, 10J by 8 inches, and the rubbing pieces and fenders are American elm. Two sluice valves, 31 feet in diameter, and 1| feet above the deck floor, furnish an auxiliary means of filling the dock. A 4-inch hånd-pump serves to remove water from the air-chamber. The hauling arms can be readily moved when the caisson has to be floated out of place.” In Rolling Caissons, as the epithet implies, the sliding ways are replaced by rollers which are attached either to the underside of the caisson or to the pathway. This method of guidance obviates an impediment to movement, due to the slight side clearance between a caisson and its sliding ways. Often while travelling, the projecting portion of such a caisson cornes under the influence of the wind, which results in its getting jambed diagonally. There is also less friction with rollers than with sliding surfaces, and, consequently, less abrasion. There is the risk, however, that the rollers themselves may get out of order, in which case any advantages they may have are more than counterbalanced by the trouble and difficulty of effecting repairs. At the same time, it is only fair to admit that, from experience of many cases, it has been found that the likelihood of such a contingency is remote. As an example of a rolling caisson, the following description of one constructed at the sea-lock of the new Bruges Canal,* within the past few years, may be useful (see figs. 332 to 335) : — The caisson is a steel framework, with plating of the same metal, of 14j feet uniform width, presenting in elevation the form of a trapezium, whose top and bottom lengths are 80-1 feet and 67)4 feet, respectively. The height of the caisson is 41| feet, the upper surface being 8 inches above the highest tide level. A watertight deck is laid about 16 feet above the keel, and the chamber thus formed is occupied only by the kentledge necessary for preserving equilibrium. The upper part of the caisson, though enclosed, is adapted to the free entry of water from either side, by the formation of a series of orifices, 14 inches in diameter, in each face, at the level of the watertight deck. Under the fluctuations of water level, the volume of displacement remains constant, and, consequently, the weight on the wheels remains unchanged after once being regulated by ballasting. These orifices are opened and closed, as required, by valves worked from the top deck. The caisson is carried on eight wheels, each 3^ feet in diameter, on four axles, * Vide Piens on “Portes à un seul vantail de l’écluse Maritime du nouveau Canal de Bruges,” Seventh Int. Nav. Con., Brussels, 1898.