15 August 2002
STRUCTURAL TYPES OF DRYDOCKS
DESIGN AND CONSTRUCTION. This section deals with the influence of
foundation requirements on shape and the reasoning leading up to specific type
Basis of Type Designation. Type designations are determined by the
structural requirements necessary to neutralize the pressure of the water that surrounds
Hydraulic Pressure. Hydraulic pressure can be resisted by the
employment of sufficient weight and strength to resist the full pressure potential, or can
be diminished by absorbing the flow so as to lower the hydraulic gradient under the
drydock. The degree to which the water pressure is relieved determines the type
terminology: (1) full hydrostatic; (2) fully relieved; and (3) partially relieved.
Piles. Each of the three types of drydocks may be built with or without
piles. For the full hydrostatic type, piles may be used to engage soil beneath the
drydock to contribute to the holddown weight. For the fully and partially relieved types,
piles may be used to improve the elastic modulus of the foundation or to reinforce the
soil at locations of excessive soil pressure; for example, beneath the toes of walls or
under ship blocking.
Methods of Construction. Since a drydock is constructed on the shore
of or extending into water, there is always the problem of excluding water from the
construction site. To define completely the structural type of a drydock, it is generally
necessary to state its method of construction.
Water Exclusion. On sites where water exclusion is feasible, a
cofferdam, which is in itself of a nature of a drydock, should be used. Since it must be
deeper and wider than the finished drydock, a cofferdam often presents technical and
engineering problems more difficult than for the structural design of a drydock itself.
The structural type and shape of the drydock may be influenced by the method used to
solve the cofferdam problem. On sites where water exclusion is not feasible, a drydock
must be constructed by underwater methods, in which case the method used also
influences the structural type and shape.
TYPES DICTATED BY FOUNDATION CONDITIONS
Full Hydrostatic. A drydock is classed as full hydrostatic unless there is
a relief drainage system that lowers the natural hydraulic head on the walls or floor. No
material, even rock, can be considered impervious in the sense that it will prevent or
decrease the hydraulic pressure on the structure. The full buoyancy of the drydock
must be resisted by one or more of the following factors: (1) weight of concrete; (2)
weight of soil below the dock engaged by holddown devices; or, (3) weight of earth