15 August 2002
resting on a ledge formed by the projection of the floor slab beyond the sidewalls or
Selection of Type. Theoretically, a full hydrostatic drydock can be built
under almost any site or foundation condition. However, for large and deep docks,
where the pressure on the floor and sidewalls is great, and especially where it is not
feasible to secure a satisfactory holddown system to the material beneath the dock,
economic considerations may force the choice of another type.
Dry. A full hydrostatic type of dock does not require relief pumping and,
therefore, has the least cost of power and maintenance. If possible, and where all other
local conditions are suitable, it should be constructed in the dry.
Wet. All drydocks constructed in the wet are of the full hydrostatic type
because conditions preclude installing reliable relief systems underwater. Docks of this
type can be constructed with or without pile holddowns. See (a) of Figure 3-3 for the
cross section of a full hydrostatic dock constructed in the dry and (b) of Figure 3-3 for a
dock constructed by tremie methods.
. A fully relieved drydock requires
a drainage system
eliminate or reduce the pressure on the floor and walls so that these elements may be
of minimum size. Lower original cost will be offset to some degree by higher pumping
costs throughout the life of the structure. The pressure relief type may be built for all
types of foundation conditions if the flow of water is naturally cut off by not too pervious
soil, or by natural or manmade means. The exception is for drydocks constructed in the
Drydock in Rock. For this type, it is necessary to line the rock excavation
with concrete and provide weep holes through the floor and sidewall concrete lining.
For an example of this type, see (d) of Figure 3-3.
Drydock in Impervious Soil. Where the soil is impervious, or nearly so,
and the volume of seepage water to be handled is small, provide for this water to be
drawn off through drainage courses placed under the floor and against the walls. This
drainage course may or may not be supplemented by a pipe system to carry the
seepage water into the drydock chamber for disposal by pumping. The volume of
seepage water that must be pumped during the life of a drydock will depend on the
degree of perviousness of the soil.
Drydock in Pervious Soil. For a fully relieved drydock to be built in
pervious surrounding soil, provide a suitable cutoff outside the drydock to stop the
greater part of the general flow.
Sheet Pile Cutoff. A drydock may have an immediate surrounding of
granular material underlain by an impervious stratum. A sheet pile cutoff, perhaps
originally a part of the construction cofferdam and located at a distance from the
drydock, when driven to the impervious layer, can provide the necessary obstruction for