The caissons

An overview of the construction of caissons near Bommenede.
Construction caisson Bommenede
While the work on the cableway had started, the plans for the northern canal (from the Kabbelaarsplaat towards Goerree-Overflakkee) were already being executed. On the bottom of the closing gap, a ‘bar’ of stone would be made, standing eight meters below sea level. This would close one third of the canal. The caissons on top of the bar would close another one third of the gap. The water was completely blocked by the closure of the caissons. Research showed that the surface of the closing gap would be 8,000 square metres: the canal was 800 metres wide and 10 metres deep. Consequently, the bar would have to be two metres in height to rise up to eight metres below sea level.

Dimensions

Bird's view of closing of a caisson
Closure of caisson
The current in the lake of Brouwershaven (‘Brouwershavense Gat’) was so strong that one wanted to sink down as small caissons as possible. The caissons must each be placed accurately to their neighbours. In 1968, work began to build twelve caissons, which had a length of 68 metres and a width of 18 metres. Furthermore, two ‘land head caissons’ would be fabricated, to be placed at the beginning and the end of the dam. Moreover, with a ratio of length to width of 3.8 to 1 (length = 3.8 * width), one had  reached a very favourable course of stability. This stability was important during the movement of the caissons to their sinking location.

The placement

Front view of the closing of a caisson
Closing a caisson
Each caisson was 16.2 metres high and had twelve gaps of five metres wide, where the water could flow through until the final closing. During transportation towards the canal, these gaps were closed temporarily by wooden partitions, to prevent the caissons from turning. Once the caisson had reached its destination, the sixteen closures were opened. Twelve of the closures were in the bottom of the caissons, whereas the other four were in the walls. The caissons sunk down when they were completely filled with water.

Each caisson was placed next to one another in this way and then sunk. Because the closures were open, the dam initially had an open character. The sea water could almost freely flow through during ebb and flood tides. The joints between the caissons were consequently filled with sand and gravel. During the transition from ebb to flood, the current is minimal. The hollow caissons were finally filled with sand and stone.

Lock

The removal of units that enabled the caisson to float
Removing float units
The Grevelingen had been enclosed from the North Sea in 1971, and would have become freshwater without human intervention. It was important to keep the Grevelingen salty. For this reason, a lock was built in the dam in 1978. The lock consists of two concrete tubes with a length of 195 metres each and a fish lock of the same length. To install the locks, a part of the Brouwers dam had to be knocked down. This part was repaired later.