The dam from Hellegat to North-Brabant
The caisson dam was built in a canal within which heavy shipping traffic moved between Rotterdam and Antwerp. So that the shipping would not be hindered by the placement of the caissons, a lock was built. The locks were built on reclaimed land, close to the fortified city of Willemstad. The building dock in which the works took place was about 350 metres wide and 850 metres long. The building excavation would, among others, consist of the following elements:
1. A building and storage site, which were situated outside the ring dike of the building excavation.
2. An entry canal which connected the work site with the Hollandsch Diep. This entry canal would be widened in a later stage of the works and would then become part of the northern port. This was started in 1968 and it was finished in 1969.
3. A road connection to the construction site, which had to be made suitable for heavy traffic.
4. The entry roads to the bridge which would lead the traffic over the locks in the future.
The placement of the caissons
The gap that remained open after the building of the lock, would be closed by fourteen caissons. The placement of these caissons took place following a fixed pattern. The caissons were made to float when the water was pumped out, and they were transferred to an area close to the threshold on top of which they would be placed. At the moment when the rate of flow was sufficiently low, the caissons were taken above the threshold. However, the caissons needed to be linked to each other. To accomplish this, each caisson was shipped aslant into the other and consequently turned into the correct position at the turning point of the tide. When the tide turned, the caissons were filled with water, and as a result they sank and were placed on the threshold. The basins on top of the caissons were filled with sand, and stones were dumped on both sides made that the caissons could not move.
Storm and flow
The influences of nature during the construction of this part of the dam had to be considered. The flow of the water and the power of the wind could not be underestimated. The season during which the dam was built, could influence the success of the project. The caissons would be placed between the 8th and 25th of April, because this period had the least amount of storms expected. More importantly, the drainage of the Maas and the Waal was smaller in April than in March. Waiting until the summer would mean a high chance that the dam would not be finished before the autumn. The turbulent sea should not get the chance to disturb the work of the engineers. To prevent the bottom of the dam being swept away, the caissons were built on top of a threshold. The bottom was covered anyway to prevent too much sand flowing away in the long term. Without the threshold, the caissons would have sunk. It was learned from the experience with the Veerse Gat dam that the caissons should be placed as soon as possible after one another. The smaller the last remaining gap, the stronger the flow and the greater the damage on the threshold. Many tests were carried out on the threshold in the Hydrodynamic Laboratory.
The last placement was planned on 25th April and would take place during slack water. Slack water occurs twice a month, when the difference between high and low water levels is the smallest. During spring tide, the sun and the moon are in a line in relation to the earth. During slack water, however, the sun and the moon are at right angles to each other. Consequently, their combined influence on the tides is the smallest at these moments. In total, twenty days were reserved for the whole operation. This time was divided up into 16 days for sinking the caissons, and 6 days were set aside for catching up. The six days for catching up could be necessary if a number of caissons could not be placed in the correct position. Besides the data, the points of time also needed to be taken into account. A caisson had to be placed an hour before sunset, otherwise the work could not be finished before darkness. On the 25th April 11:27 AM, the last caisson was placed on the threshold. This was earlier than expected, because one had not been able to leave the port for a couple of days.
The sailing schedule
During the sinking of the caissons, the points of time were very important. The first moment was the departure from the dry dock to the position downstream of the threshold. The second moment was the departure from this ‘parking place’, towards the closing gap itself. The third moment was the turning of the caisson. Lastly, but most importantly, was the opening of the valves, resulting in the sinking of the caisson. The exact moment at which the caissons would be lowered was unknown, because this depended on the water flow.
The following times were known:
1. It took twenty minutes to sail the caisson from the dry dock to the parking place in the middle of the canal.
2. Next, it took another twenty minutes before the caisson was manoeuvred above the threshold.
3. The insertion and turning of the caisson lasted another fifty minutes.
4. When the caisson was placed in the right position, it took six minutes to sink the caisson.
A caisson needed to leave the dry dock two hours before it was placed. The turning of the tides which was necessary for the placement would take place two hours after high water. The caissons would therefore have to leave the dry dock during descending water. There was a chance that the caisson would get bogged down in the canal from the dock to the parking place. To overcome this, the caissons left an hour before high water. The caissons spent this extra hour on the parking place in front of the dam.
After the left-over gaps were filled with stone, 192 slides and 12 drain caissons were lowered on the 28th April at 9:15 AM. The Volkerak dam was then closed. Another difficult job had to be carried out now: sand had to be raised against 5.6 million square metres of dam. The first million square metres were finished after three weeks, thanks to three enormous sand suckers: the Queen of Holland, the Concorde, and the Versde.