15 August 2002
following paragraphs describe types of floating caissons, their operation, and materials
Requirements. Caissons must allow reasonably rapid opening and
closing of the dock entrance at any tide stage, be stable transversely and longitudinally
at any draft, have strength and other characteristics to resist water pressure, and
provide watertight seal when seated.
Types. Floating caissons are usually one of the four types described
Ship Type Caisson. This type has a faired shape similar to the lines of a
ship. The required curvatures for frames and plates make the construction
uneconomical, and it is now outmoded.
Hydrometer Type. This type has a narrow width over as much of the
height as possible between its waterline at extreme light draft when afloat and its
waterline at mean high water when seated. This reduces the amount of adjusted water
ballast during lowering and raising for seating and unseating operations.
Box Type. This type is constructed in the shape of a box, with vertical or
sloped ends to suit the seat shape. The girder ends project from the box to bear
against the seats built in the dock side walls.
Rectangular Type. This type is similar to the box type. It has simplified
internal and external welded construction. All body girders and breasthooks are
completely enclosed by the plating. All modern designs have been of this type.
Ballast Control. The draft of caissons is adjusted for seating, unseating,
and towing by flooding or pumping out water ballast. (Refer to section 9-2.7 for
machinery for ballast control.).
Operation. Operation is functional as applied to the design objectives.
Detailed operational instructions must be supplied for each installation but are, in
general, as described in sections 9-2.5.1 and 9-2.5.2.
Opening the Entrance. To open the entrance, the procedure is as
In preparation for lifting the caisson from its seat, the water level inside the
graving dock must be raised to the water level outboard the caisson, by
Caisson dewatering pumps are started; then with the discharge valves
open and sufficient water ballast removed, the caisson floats off the seat.
Pumping is continued until desired draft is obtained. The caisson in
operation is very rarely pumped up to light draft. For air operated