A Treatise On The Principles And Practice Of Dock Engineering

SWINGING CAISSONS. 351 x = horizontal distance between verticals), tending either to restore the body to its former position, or to completely overturn it. In fig. 323 a floating body is represented as having undergone a slight displacement. The centres of gravity and buoyancy now occupy relatively different positions, unless the body be a homogeneous sphere. Assuming that it is not, if the centre of gravity lie below the centre of buoyancy, the couple is clearly a righting one. If, on the other hand, the centre of buoyancy lie below the centre of gravity, the couple will not necessarily be an overturning one; its effect will depend upon the following- condition. Premising that the point, in which the vertical through the centre of buoyancy after a slight displacement intersects the vertical through the centre of buoyancy in its former position of equilibrium, is designated the metacentre, the condition for the restoration of equilibrium is that the metacentre shall lie above the centre of gravity of the body, otherwise the latter will tend to depart still further from the position of equilibrium. The two effects are illustrated in figs. 323 and 324. Figs. 322, 323, and 324. In the case of caissons, it is particularly desirable that the metacentre should be well above the centre of gravity, say not less than 2 to 3 feet, but the stability of the caisson will be more completely assured by ballasting it until the centre of gravity falls below the centre of buoyancy. A margin of 18 inches or so will be found sufficient for safe working. If the caisson be fitted with air chambers and a tidal deck, it will certainly be advisable, if not imperative, to adopt the latter précaution. Classification of Caissons.—Caissons are of very diverse design, but they admit of a broad classification into swinging, traversing, and ship caissons. Swinging Caissons have already been referred to, under the name of gate caissons, as forming an intermediate class possessing characteristics common to both gates and caissons. Like the former, they turn or swing upon a vertical axis fixed at one side of a waterway, and they have all the drawbacks attaching to a single leaf gate, in regard to the excessive length of side recess required for their accommodation when out of use. On the other hand, they are built with much broader beam than gates, and this gives them the compensating advantage of a wide roadway for traffic at quay level, whicli would otherwise be impracticable without the aid of a special swing bridge. This feature, however, is more or less common