Summaries
tugged away. The entire deep-well pumping
system served for six and a half years. By thet
time the installation had just about worn out.
Evaluation of the placement of the piers
High demands were made where accuracy was
concerned in the placement of the piers in the
Oosterschelde barrier. Deviations from the
theoretical positions might occur in three
directions: in the length and in the breadth and
also in angle distortion. Actually the deviations
turned out to be extremely small in all three
directions.
The time table was able to be maintained as
only a few tile mats were needed to compensate
for the angle distortion of the pier base. With
regard to the Hammen and the Schaar a couple
of points were altered in tackling the Roompot.
These concerned the weight of the current on
the mooring of the service vessels, and the
erosion and silting underthe 'Ostrea'-'Macoma'-
combination.
Deep-well pumping of the construction pit
Schaar
A construction pit had already been laid behind
the central work island for the manufacture of
the piers and sill beams at an early stage of the
Oosterschelde barrier works. The construction
pit consisted of four compartments which
could be drained and inundated separately.
For this an extensive deep-well pumping
system was necessary. The work floor lay at 15 m
below M.S.L. Previous research had provided
insight into the geological profile and the
necessary water right to be expected. A good
300 deep-wells were necessary for drainage of
the construction pit, of which 25 reached
M.S.L. -61 m. Pumps had to be added in the
partition dikes when the first compartments
were inundated to enable the piers to be
Ice formation and ice movement in the Oost' r-
schelde mouths during the winter of 1984IE 35
There were two periods of hard frost in the
Netherlands in January and February 1985.
Such ice conditions had not occurred since
1963. Bearing in mind the storm surge barrie
developments were followed with great
accuracy. In fact the only precaution necessa y
was the temporary dismantlement of a numl er
of measuring poles.
The ice formation process progresses differer tly
in salt water than it does in fresh water. At fii st
the salt becomes enclosed in the ice crystals It
crystallizes out at temperatures lower than-8C C.
It is the cumulative cooling off which determir es
the ice formation. In tidal waters the ice can e
carried along by the current and even pile up
Much floating ice was observed from aeroplar es
in concentrated channels of the Oosterschelc e.
In the second period of frost ice formation
developed far more rapidly as a result of the
water already being colder. This was a very ry
period, the ice became harder and after the
onset of the thaw it persisted for a relatively
long time.
Piping
A drop with a hydraulic construction causes a
water current which may also transport with it
riverbed sand. This threatens the stability of
the construction. The scouring then usually
runs in meandering channels. In the case of he
storm surge barrier drop is provoked by
differing levels and wave movement. Proble n
areas are located under and in the near
proximity of the piers and at a transitional
construction further up. A foundation mat h s
been laid under the piers so as to check for
piping. If this is laid on an uneven ribbed
surface then piping can still occur. Moreove
the fabric of the mat can fall apart with age.
Much the same applies to the upper mat.
The answer to this problem of piping has b( en
found in levelling the riverbed by suction ar d
sucking clean the mats, and in the compact jn
of the mats with a vibration beam or sheet. The
problems are less in the case of the transiti nal
constructions.
The Roompot lock
During the execution phase of the storm su ge
274
