Summaries
Study into the behaviour of the foundation
mattress of the Oosterschelde barrier
The foundation mattress under the piers of the
Oosterschelde barrier consists of three graded
filter layers, which must remain separated
under all conditions. To this end, the mattress
has been composed of three separate layers of
steel wire mesh frames, separated from one
another by synthetic fabrics or synthetic
diaphragms. The three filter layers are
supported by a 'Jumbo' mattress, which has
been reinforced with steel cables. Steel 'locker'
bars pin the whole construction together.
After fabrication, the mattress is rolled over a
drum, towed to its position and rolled out over
the sea bed. The strength of the mattress has
been tested extensively under all conditions.
Strain and stresses on the mattress during the
process have been thoroughly checked to
ensure that the mattress will land safely on the
sea bed. However, the reinforcement has made
the mattress rather rigid, which could mean
that voids could occur under the mattress and,
as a consequence, sediment could be flushed
out. Tests have shown, however, that this
phenomenon will not occur directly under the
pier, provided the sea bed has been levelled
sufficiently and the mattress compacted. In the
transitional zone towards the aprons, the
sediment will, at the initial stages be moved,
but after the mattress has gradually adjusted
itself to the sea bed, this transport will cease.
Anchorage of the equipment in the closure
gaps of the Oosterschelde barrier
The bed-protection constructed on both sides
402
of the Oosterschelde barrier will prevent the
normal anchorage of the many working vessel:
when the channels are closed. Therefore in
1978, 59 fixed anchor piles were driven into
closure gaps to which another 18 had to be
added in 1981.
The site of the anchorages will be studied in
advance in a plotting room, located at the
Roompot-harbour, where the actual position of
the anchorages will be kept up to date on
maps. Changing the position of the
anchor-hawsers takes approximately one hour
and, as various combinations of working
vessels require 16 anchor-movements such
operations are highly time-consuming, in
particular, as these actions can only be executed
at speeds of 1.5 m/s. or less, i.e. in less than
half the tidal period.
A new, specialised, vessel for these anchoring
activities, the 'Area', will simplify this task. The
vessel has been equipped with more powerful
engines and can remain active during higher
flow velocities and it speeds up the anchorage
procedures.
Centrifugal and frictional tests for the
Oosterschelde barrier
To be able to predict the behaviour of the
barrier during a heavy storm, profound
knowledge of the local soil mechanics is
required. One of the places where the condition
of the soil can be investigated is the
centrifuge-centre at the University of
Manchester, where test models are centrifuged
at high speeds equal to the stress levels in the
actual situation. During this process, a great
variety of measurements can be taken.
