Sinking by means of pulling over a prefab sheet
 
Aqueduct Langdeel
In contrast to other aqueducts that have been constructed a different sinking technique was used for the first time here on a large scale. The principle rests on sinking a foil construction with the help of its own weight without the need to use a pontoon for the sinking. The construction pit in which the foil was sunk was 800 by 180 metres and about 24 metres deep.
 
A starting point was first constructed on the south bank of the construction pit as preparation. This consisted of a layer of heaped up sand containing a half-sunken drainage system which allowed the foil to be dragged into the water without too much frictional resistance. The complete foil package for the entire construction was built up at this site and made ready including the float and pulling strip. 
     

 

In order to allow the foil to sink water was pumped out from underneath the foil and pumped on top of it. A drainage system was sunk to the bottom of the pit for this purpose over a length of 10 metres. Above water this was coupled by pipes to 20 drainage pumps on the north bank. A winch system with cables was constructed on the north bank for pulling the sheet across. The cables of the winch system were linked to the foil package, the float and the pulling strip on the south bank. 

 

After the whole foil package was ready and the drainage system had been sunk the cables were connected to the pulling strip, and the entire foil package was pulled over the water to the north bank. Because of the great width in the middle of the aqueduct the foil in the middle of the package was first pulled into the water at the start of the sinking operation, after which the sides followed slowly. The foil package was then pulled in one movement over the water. The whole of the front of the foil package reached the north bank at the same time, after which it was pulled up on the bank. About 6 metres excess length was pulled up on the bank and anchored. The rest of the extra length remained lying on the water. There was now about 6 metres anchored foil on the banks of the excavation.        

 

  
The actual sinking began immediately after pulling the foil across. Using the 20 drainage pumps 1 million m3 of water was pumped from underneath the foil and onto the foil construction. By controlling the pumping capacity and the discharge point the water was pumped under control on to the foil construction at the beginning. When the foil was completely under water the pumping capacity was increased to maximum. When the sinking process was completed the depth of water within the foil construction was increased by 0.50 metres in order to press the foil more firmly against the bottom. The foil construction was tested for leaks after 7 days. The thickness of the foil used here was 1 mm and the total surface sunk about 120,000 m2.