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LEGEND:
SCOURED GULLIES IN FLOODED POLDER
THREE METHODS OF REPAIRING.
HIGH CAISSONS IN THE OLD DIKE FOR
CLOSING TIDAL GAP
NEW DIKE IN THE FLOODED POLDER
LOW CAISSONS ON THE BOTTOM OF THE
POLDER TO CLOSE THE NEW DIKE
NEW DIKE ON FORELAND WITH HIGH
CAISSONS IN THE GAP TO BE CLOSED
POSSIBLE TIDAL GAP IN NEW DIKE
WITH HIGH CAISSONS
The closing of a tidal gap by constructing a horse-shoe
dike round it on the land side method No. 2,
see the accompanying article). On the left the old
destroyed dikeon the right the temporary dike and
on the extreme rightwhere the current is still
runningthe closing of a deep gully by means of a
caisson Situation at Schelphoek).
2. By constructing a horse-shoe dike round the gap on the land side.
3. By constructing a horse-shoe dike round the gap along the shallow
flats outside the original dike.
These three methods of closing dike breaches are centuries old and the
underlying principles have not changed much.
With tidal gaps in which the system of gullies has already developed
far inland it is not always possible to avoid a crossing of the horse-shoe
dike and the gullies. In such a case a combination of two methods must
be employed.
To this end the soil along the course of the horse-shoe dike and in the
gullies to be crossed is first of all protected against erosion by means of
mattresses extending to a width of from 30 to 45 metres. As soon as this
has been done the gullies are closed, whereupon the enclosing dike is built
up across the polder land.
Caissons
Although the actual principles on which the closing of tidal gaps is
based have changed very little, the application of reinforced concrete for
caissons has certainly created new possibilities as regards construction.
The conventional building materials such as brushwood, stone and clay
were again used, but for the very large tidal gaps it was necessary to devise
new methods for constructing the horse-shoe dikes in the fast-flowing water,
since the extent of the disaster was so great that the available skilled
labour was insufficient to ensure the high rate of production required in
respect of the time-devouring manufacture of the brushwood mattresses.
Moreover, the supply of the conventional materials referred to above
was limited.
These circumstances led to the construction of unit caissons of rein
forced concrete, which were used not only for the construction of pier
heads and floating doors for closing the gullies, but also for building up
the horse-shoe dikes in the fast currents across the polder land. The appli
cation of these caissons enabled the use of heavy clay which offers a
high resistance to the current and of brushwood and stone to be reduced
to a minimum.
Without exaggeration it may be said that without the application of
concrete caissons it would not have been possible to close the numerous
gaps in the dikes before winter.
The design of the unit caisson was worked out during the first month
after the disaster. It did not take long to compile a rough survey of the
tidal gaps which would have to be closed by means of caissons and the
tidal rise and fall which had to be reckoned with was known. It was im
possible, however, to decide at short notice on the form and the dimensions
of the closing constructions, since the situation in the gaps was constantly
changing owing to erosion.
The unit caisson was then designed on the following basic principles s
a. the width of the caissons had to vary between 7.5 and 15 metres in
view of the tidal rise and fall to be negotiated, in which connection the
weight of the water or sand-filled caissons had to be such that they
could not be forced out of the gap.
b. the weight of the unit caisson was not to exceed 200 tons, so that it
would be possible to build the units on shore and subsequently to place
them in the water by means of a floating crane.
c. Finally, each unit had to have ample stability when afloat.
On the strength of these basic requirements unit caissons were designed
with a width of 7.5 m, a length of 11 m and a height of 6 m; the total
weight of each unit was 180 tons.
With these units it was possible to assemble caissons of any length
and with widths of 7.5, 11 and 15 m.
Standard sleeves were designed for increasing the height of the caissons.
Later unit caissons were designed along the same lines with heights of
2, 3 and 4 m, which coupled together to lengths of 55 m or 66 m, were
used to speed up the construction of the horse-shoe dikes referred to earlier.
The size of the caisson building programme is evident from the follow
ing data.
For the construction of the units a combination of seven firms of
contractors was set ups in Amsterdam, Rotterdam and near Breda four
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