15 August 2002
appropriate allowance for decreased upward and lateral water pressures in accordance
with the degree of lowering of hydraulic gradients.
Drydocks Built by Underwater Methods. When completed, these
drydocks are always of the full hydrostatic type. In some cases, however, this method
of construction involves loadings not encountered with construction in the dry. These
cases occur when walls are built entirely in the dry in cofferdams set on slabs previously
constructed underwater. Under these conditions, when wall cofferdams are unwatered,
the partially completed structure does not have the benefits of wall and finish floor slab
weight to overcome the buoyancy of the cofferdams, or the full sidewall thrust to
overcome the tension in the slab. For examples of two such conditions, see Figure 5-3.
These drydocks are frequently associated with the use of holddown piles,
necessary if the weight of the floor slab is insufficient to overcome the total buoyancy
including that of the empty cofferdam or cofferdams. Note in (a) of Figure 5-3 there is a
tendency to develop tension in the slab bottom without benefit of axial compression
from a sidewall thrust. In (b) of Figure 5-3, there is a tendency to develop tension in the
slab top with a side thrust that is much smaller than will be developed against the walls
of the completed dock.
MATERIALS AND DESIGN. This section contains special provisions
applying to concrete for drydock walls, floors, and general cross section.
Classes of Concrete. Recommend using concrete as specified in UFGS
03311 Marine Concrete. Use minimum 24,132 kPa (3,500 psi) estimated 28 days
compressive strength cast in place concrete for the main body of the dock. Classes of
greater strength may be used in accordance with structural requirements. Do not
specify mortar intrusion concrete (see Glossary) for permanent drydocks.
Admixtures. Admixtures may be used to produce air entrainment, higher
strength, greater durability and better workability, up to maximum percentages detailed
in the project specifications.