in particular would be subject to serious
damage.
A network of lightning absorbers above the
barrier would be extremely expensive and also
ugly and would leave unsolved the problem of
conduction. Instead of this, lightning which
strikes the cylinders will be conducted via gas
filled lightning arresters, so that no
short-circuiting can occur at unsupervised
spots.
Risk-analysis
The prenatal period of a construction work like
the Oosterschelde barrier can be divided into
two phases: the planning and the execution.
The possibility of risks in both phases should
be well reconnoitred beforehand. The execution
phase hardly allows for the possibility of a
systematic approach.
With modern probabilistic methods of planning
the determination of material qualities and
loads is no longer based on one discreet value
but arises out of a combination of values each
of which has a chance of occurrence. This
method is now also used to analyse the risks
involved in the execution. A workgroup on
execution methodology has been set up, which
has amongst other things drawn up a list of
possible undesirable occurrences for each
work unit, and has prepared preventive and
responsive action to be taken. One important
extra advantage of this is that all the work has
been thoroughly discussed with all involved
before beginning with the execution.
Lift frames for the components
The prefabricated components of the
Oosterschelde barrier are brought into position
by special vessels. The 'Taklift IV' is used for
mounting the sills, upper beams, capping
units, operating mechanisms, gates and road
box girders. The heaviest components which
this trestle has to lift - with assistance - weigh
2700 tons.
The hoisting cables of the 'Taklift IV' are led
through tensible plates and a pressure frame
so as to lessen the pressure on the trestle and
on the construction being hauled. Some of the
components of the barrier can only be brought
into position with the assistance of an extra jib.
This meant that two pressure frames should be
hanging in the pulley-blocks at the same time,
as replacing them in the working harbour is too
time consuming. After long discussion of the
pros and cons, the 'Taklift IV' has been placed
for the purpose of assembly on the
Oosterschelde side of the barrier.
Inspection and maintenance
A separate work group has been formed to set
up an inspection and maintenance plan. This
group is working on a book on maintenance to
be used by the future manager and is also
listing the necessary auxiliary equipment. Each
gate has to make at least one complete stroke
per month, and the whole barrier is closed
once at the commencement of each storm
season. The diesel motors are tested every two
weeks and maintenance servicing is carried out
twice a year on all the components of the
installation. It will be necessary to inspect more
often those components which might display
symptoms of fatigue.
A large inspection car has been designed with
a hydraulic arm of 23m long from which an
inspection hamper large enough to carry two
men is attached. This is to be used to inspect
those parts of the gates and the operating
mechanisms which cannot be inspected from
the barrier itself.
Foto-verantwoording:
Johan Berrevoets 460
Rotterdamsche Droogdok Mij. 469, 472, 479
Rijkswaterstaat 466, 470, 471, 475, 476, 477,
478, 481, 483, 499, 501, 503, 505
Socatri 473,474
Leo van Gelder 482, 484, 489
Bart Hofmeester 512
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