15 August 2002
Backfill. Backfill may be placed in the water or in the dry next to sidewalls
and abutments. For tremie docks, it may be expeditious to place backfill in the wet. In
the dry, backfill should be placed in shallow layers and each layer thoroughly
compacted. Backfills comprise foundations for roadways, laydown area, and other
structures built directly on them, and must have sufficient strength to support such
Construction and Use. Entrance closures (caissons) are usually built of
welded steel construction, but may also be built of reinforced concrete. Construction
may be by a naval shipyard, but is usually done by a private contractor. Being
adaptable for towing, a caisson may be built some distance away, in a yard particularly
suited to economical prefabrication, and subsequently towed to the graving dock site.
Dimensions. It is important to accurately check drydock seat dimensions
against those of the proposed closure, particularly for clearances. This is essential in
the case of new caissons for existing docks.
. Permanent ballast necessary for proper operation and stability
a caisson is usually provided prior to an inclining test, and additional ballast, if
necessary, is added at testing time.
Launching. Caissons may be constructed in a graving or floating drydock
or on shipbuilding ways. Some small caissons can be built on piers and placed, by
crane, in the water.
Steel Caissons. Methods and operations used in prefabrication,
assembly, and launching of steel caissons are very similar to those of other welded
steel floating vessels.
Concrete Caissons. Several prototype concrete caissons were built
during World War II, using practices developed for construction of concrete vessels and
ships. These practices differ from standard concrete construction methods in the
unusual care needed to achieve a watertight, durable, and structurally sound caisson.
The service record of concrete caissons should be evaluated prior to proceeding with
Closure Tests. Four types of tests are required as described in sections
10-3.7.1 through 10-3.7.4.
Watertightness Test. Watertightness must be made prior to launching,
either by filling a caisson with water or by use of high-pressure air.
Seating Test. After launching, caissons must be seated and then raised
several times, to check their operation.