The closing gap of the ‘Veerse Gat’ was 320 metres wide. The threshold, on which the caissons stood firmly, was situated 11 metres below Amsterdam ordnance zero and was 100 metres wide. This threshold was constructed, following the so-called method of filter construction, a way to help protect the bed from erosion. Another solution to prevent erosion is the placement of a fascine mat. This is often a construction of osier, which is sunk using stones. Because of the labour-intensive character of osier revetments (which come from osier-beds), geo fabric is mostly used today. Geo fabric is a polyester fibre, like many clothes,
sails, curtains and sleeping bag fillings. These fibres are wear-resistant, very elastic, and resistant to acids. The filter construction which is used for the threshold of the ‘Veerse Gat’ dam contained many layers of stone of different grain sizes. The grain diameter of the top layer and the bottom is determined by means of the standards for sufficient current resistance. The top layer of the threshold of the ‘Veerse Gat’ dam was made of such large stones, that the threshold kept its position, even during the strongest currents (that is, just before the placement of the last caisson).
The caissons needed to meet many quality requirements. Firstly, the dam had to be strong and firm. A ‘firm’ dam means that there is a little chance that the dam will be affected by the pressure of the water. The second quality requirement was that the caissons initially needed to be able to let water through. The caissons needed to be stable during the transfer and the sinking phases. Lastly, the caissons needed to have a good floating ability. Otherwise, it would have been impossible to move them into the right positions. To meet these requirements, the standard caissons were modified in a number of ways. Firstly, the caissons were given taller and stronger sidewalls.
Secondly, only the concrete areas that water would flow through were reinforced. Also, the concrete itself was left out. A basin was fixed on top of the caisson, which would be filled up with sand, to ensure that the caisson would keep its position after the placement.
The construction of the ‘Veerse Gat’ dam would have been much more complicated, without the use of the new caissons. Drain caissons are enormous hollow basins which consist of float partitions and raiseable slides. Each one was the size of a seven floor flat. Because they were hollow, it was relatively easy to transfer them to their relevant positions. The caissons were sunk into position by filling them with water, and the float partitions were removed. Consequently, the slides were raised, after which the water of the North Sea could flow through unhindered. When all the caissons were in place, the slides were let down simultaneously. This happened at the very moment when the ebb changed to flood. Finally, the caissons were filled with stone.
To be sure that the tides would not move the caissons, a layer of sand was put on top as an extra precaution. Eventually, a layer of asphalt was laid on top of the sand and stones. A road was built on the side of the lake of Veere, and the side of the North Sea was given a wide beach. In the nineties, the whole area was raised with sand, after which beach grass was planted. It was a condition that the asphalt dam had to look as much like a dune as possible.