The construction of a railway dry
dock involves several radically different skills so that it is
rare that a single contractor can undertake the entire project
with his own forces. The work is usually divided into two basic
parts - the water construction and the land construction.
The contractor undertaking the water construction is generally
responsible for dredging and excavation underwater, driving the
pile foundation or putting in concrete footings, building the
timber track, and placing it on the piles or installing the steel
Usually the construction of the underwater track and foundation
is carried out in the wet, using divers to cut off wooden piles
and to place the track to line and grade; rarely this work can
be carried out in the dry.
In the past we used timber construction entirely for this underwater
portion of the track, since timber was available and economical
and had indefinite life in the seawater when adequately protected
against marine borers. More recently many designs have called
for steel track because suitable timber has become more difficult
to procure and has risen in price disproportionately.
In either case, the practice is to prefabricate the track in
the dry to assure the necessary precision of alignment and then
Crandall ahs experienced diving crews to carry out pile cutting,
place the track to the specified lien and grade, and do any replace
shimming. The kill and know-how of doing track work underwater
are vital to achieving economy and speed of construction. Once
properly built and installed, a timber track underwater has from
30 to 40 years of life before any major maintenance is required;
a steel track ahs a similar life expectancy, damaged more by salt
water than fresh.
The land portion of the work may be undertaken by another contractor
or the shipyard itself. This work consists of construction of
the concrete track and machine foundation, installation of the
machinery, installation of the roller system on the track, and
erection and fitting out of the cradle.
The structural steel cradle is generally fabricated and painted
by a steel fabricating shop, and when possible, the same shop
is asked to do the erection so that the responsibility for the
work is under one contract.
Construction of a railway dry dock requires substantial time,
and procurement of the cradle and machinery is a long lead-time
activity. Planning and early specification of the elements are
therefore very important to get the job done in the least time
at the best cost. Crandall is set up to furnish the machinery,
roller frames or only their castings, chains, bilge block equipment,
chain gear, and other miscellaneous parts that are special and
not readily available on the open market. Refinement of our designs
over the years has resulted in superior products for the purpose,
which combine our design and specification of suitable materials
with quality control inspection so that everything fits properly
and functions with no problems in the very hostile environment
of the sea.
Next: Restoration and enlargement