Piers and slides
The piers were the most important elements of the dam. They were produced in a building excavation with a surface area of about one square kilometre, located 15.2 metres below sea level. A ring-dike kept the sea water outside the excavation. The dry dock consisted of four parts. When the piers of one part were finished, this part would be flooded. The lifting ship then sailed into the dock, lifted the heavy pier and shipped it off to its place in the barrier. Each pier consisted of 7,000 cubic metre of concrete. Therefore, the dock may also be typified as a large concrete factory in which 450,000 cubic metre of concrete was manufactured between 1979 and 1983.
The construction of each pier almost took one and a half years. One started building a new pier every two weeks. This way, thirty piers were in production at the same time. It took an enormous amount of organisation and planning to finish the giant and complex structures in time. People worked day and night, because otherwise the concrete could not harden properly. The sixty-five piers were each between 30.25 and 38.75 metres high and weighed 18,000 tonnes. Two extra piers were built, for safety’s sake. Visitors of Neeltje Jans, the former artificial island, can now climb one of these left-over piers.
When all the piers were finished, the building excavation in which they were built, was flooded. Two ships took the piers to the right place. The ship Ostrea could lift the piers one by one and sailed them to a floating pontoon. This pontoon marked the place where the pier should be sunk. The placement was precision work and could only take place when the current was as weak as possible: during the turn of the tide. The hole between the piers was filled to let the piers link up with the mats perfectly. In order to increase the stability, the piers were filled with sand. Finally, the piers were wrapped up in poured concrete. It was very important that the barrier was absolutely immovable. When, for instance, one slide could not close, the current in the gap would become gigantic. In total, five million tonnes of stone were put among the piers. The stones, which weighed ten tonnes each, were put in their place perfectly by the Trias. Some of the stones were imported from Germany, Finland, Sweden and Belgium, because the amount required was not available in the Netherlands alone. Importantly, a stone with a high density was needed (2.8 to 3.0 tonnes per cubic metre), in order to prevent the tide from moving it.
When the piers stood firmly on the bottom of the Oosterschelde, the construction of the barrier could be finished. The piers were raised with the top-pieces, upon which the slides were fixed. Hollow tubes were placed on the piers, and on top of this came a road. The tubes provided room for the equipment responsible for making the slides move. In fact, the slides are steel tubes, which are provided with sheets on the side of the Oosterschelde. The height of the slides depended on the depth of the gap to be closed. To close the deepest gap, a slide of twelve metres was needed, which weighed 480 tonnes. The slides are driven by hydraulic cylinders, which are operated from the ‘Ir. J.W. Tops house’ (1986).