Sinking by using a pontoon
 

Waalwijk
Roads and a drainage system were constructed in preparation for future building. This drainage system lies below the level of the ground water. In order to prevent the nearby nature area from drying out 185,000 m² PVC foil was laid during the construction of the drainage system.
 
Foil construction
The foil construction was composed from PVC foil with a thickness of 0.8 mm in strips 2.00 m wide. The foil was prefabricated in sheets of about 2,000 m2 and was then transported on rolls folded up in a zigzag manner. The rolls were placed on the foil pontoon using a cable crane fitted with a balance and welded into one package of about 25.00 m with a total surface area of about 15,000 m2 using channel welding. 

Sinking procedure:
Because the excavations were elongated in shape and had a constant width it was decided to sink most of the foil construction using a pontoon, on the basis of the increased pressure principle.  
 
Adding ballast sand:
After sinking, a layer of sand was added above a sieve construction on to the foil using a hydraulic digger. The sieve construction prevented any sharp parts coming in contact with the foil.
 
Connections of crossing roads:
Because the foil construction was placed in the route of the paving and drainage system complex connections had to be made at the positions of crossroads. By using sludge screens it was possible to start applying the ballast sand earlier and the construction time was considerably shortened. 

The connections were made by allowing the end of the foil section to float temporarily by using a float hose. The foil was then sunk from the pontoon in the crossing section. The foil construction was kept floating at the crossing site, so that it was possible to make the welds at the connections by hot air welding from floating cushions.
 
Connection of phases:
Because of the way the ground will be exploited the customer chose to carry out the laying of the foil construction in phases, with an intermediate period of 2 years. For this reason the foil from the first phase was ended in a layer of chippings, and a protective construction built around this which was then covered with ballast sand. 

Connections were made during the second phase to the foil construction that was already ballasted with sand. Because of pumping problems this was dug out of the wet situation using specialist equipment. The foil, fitted with a float hose, was then folded back using hand winches connected to pulling loops. The excavation of the section was then completed to the required depth and the foil pontoon sailed up to the connection point. The new foil was hot air welded to the existing foil.
 
It was decided to use the principle of sinking under own weight to position about 15,000 m2 of foil at the position of three very complex crossing roads and an excavation that varied widely in width.

The folded foil package was connected at the short end to the existing foil construction and pulled over the water over a length of about 250 metres to be positioned in such a way that all three connections could be made by hot air welding from floating cushions. The foil was then sunk by pumping the water out from under the foil and on to the foil.