The cable way
The French company Neyrpic was brought in to design and build the cable way, together with the Ministry of Waterways and Public Works. On the shore of Goeree-Overflakkee, a changeover station was built, a pier was placed in the middle of the closing gap as a point of support, and the Island of Oude Tonge was provided with a changeover and filling station. Adjustable counterweights on the Island provided an almost constant counter tension to the cable. The cradles would run on top of the cables, instead of being hung under them. Each had its own engine and was driven by a chauffeur. They weighed about 100,000 kilograms each and could carry about 10,000 kilograms of stones. The speed limit of the cradles was 32 km/h, but the average speed was 18 km/h. These speeds were reached using an engine of 200-240 horsepower.
On the 3rd of August 1963, the first cradles drove over the cables, which were ninety-two centimetres thick. By 24th of August 1964, the schedule was a lot more efficient, with the cradles able to run throughout the night. It took each cradle twenty minutes to drive from the filling station to the site the material needed to be unloaded. At any one time, there were always ten cradles driving over the cable way, which together plunged about three hundred tonnes of material into the Grevelingen every hour. In total, 190,000 tonnes of material needed to be laid.
On the Island of Oude Tonge, a storage area was built for sand, cement, stones, and boulders. In the work area, there were about 55,000 tonnes of stones and 80,000 tonnes of boulders. In addition there was a backup reserve of approximately 60,000 tonnes. Thus, in total there were more than 195 million kilograms of stones available to finish the Grevelingen dam. That is the same as the combined weight of 39,000 elephants! Cradles started their journey at the Island of Oude Tonge station. The net which was hung under the cradle was lowered into a loading platform, where a truck filled it with rocks. The filled net was then fixed to the cable way.
Plunge it down!
The first layer of stones that was plunged consisted of coarse gravel, weighing between 10 and 300 kilograms per piece. These were too small, however, for the mesh of the net and fell through into the water before the cradle had reached the correct position. A solution had to be thought up on site, because the cable way had never been used for such a large-scale project. Initially, pieces of canvas were put into the nets, but these pieces broke up quickly. After that, strips of conveyer belt were placed in the nets, instead of the canvas.
The costs were very high, because almost all of the stone had to be bought from abroad. For this reason, a number of methods were considered to use the sand of the Island of Oude Tonge.
1. The first method involved mixing a large silo of sand with water. The wet mud that was created was squirted into a large bag via a tube. These bags, which consisted of natural and synthetic fibres, weighed 2,500 kilograms each.
2. The second method was to replace the water by asphalt. The additional costs for the asphalt were compensated by the reduced costs of a different type of bag.
3. The third method was a variation of the second: so much asphalt was used, that a bag was not required. In fact, the material was nothing but a giant piece of asphalt, mixed with sand.
4. The last method involved simply filling the bags with sand. Consequently, the air pressure in the bags was reduced, and as a result the bags became rock-hard.
During the construction of the dam, measurements had to be carried out from time to time to ensure everything proceeded according to plan. Rapids were created, for instance, to make sure the bottom did not wash away. The stones that were plunged could move during strong rapids. To gather as much information as possible, the measurements were taken in an area of about one square kilometre. The size of the area was then reduced when the construction of the dam progressed. The risk that the dam would move was reduced as construction of the dam progressed. The recording of measurements increased however: from the beginning of 1962 the measurements were carried out monthly, whereas from July 1964 they were carried out weekly.