A tent needs pegs to keep from being blown away, a ship anchors to keep from drifting. And a house? You'd think that would be solid by itself. But a good foundation is indispensable to prevent houses from sinking into the ground, especially in a former peat swamp like Amsterdam.
It used to be a popular nursery rhyme in primary schools:
'Amsterdam, that big city It is built on piles If that city fell over Who would pay for that?'
At first glance it seems sheer nonsense; Anyone walking through the center of Amsterdam will not see any stilt houses towering above them. The canal houses, De Bijenkorf, the Royal Palace on Dam Square:they all stand firmly on the ground. And yet the song hits the nail on the head, because without poles the whole of Amsterdam could not have existed. The piles are only not above, but below the ground. A complete jungle of wooden and concrete piles is rising in the top twenty meters of the Amsterdam soil.
There will be no immediate talk of falling over, but subsiding all the more. The subsoil of Amsterdam is remarkably soft:there used to be a peat swamp on the site of the capital, with the result that the soil now largely consists of organic sludge with plant roots in it. When water seeps out of that peat package, the ground subsides, causing the buildings above to subsidence. Geologists think it is crazy to build a complete city on such a jelly pudding these days. But yes, the city is there and that is thanks to two layers of sand that lie under the peat.
The deepest sand layer is now about 20 meters below the surface and was deposited during the last ice age, the Weichsel (about 110,000 – 10,000 years ago). Above it is a layer of clay and another Weichsel sand layer on top, followed by a thick layer of peat. Subsequently, another layer of clay was deposited.
It is the sand layers in which contractors anchor their piles. The deepest sand layer is the most reliable, so piles of about 20 meters are soon driven into the Amsterdam soil. This is done according to the old-fashioned pile-driving method:a steel pile driver of about 1500 kilos is lifted up and down in no time by a winch within a steel tube; with each blow, the underlying post is rammed a little further into the ground. The pile driver actually functions as an automatically driven hammer.
Foundations are not always accompanied by pile driving. In places where the subsoil is solid (thanks to load-bearing sand layers on the surface, for example) there is only foundation on steel. There is no steel involved:the word comes from the Old Germanic 'stal', which means 'permanent place'. Herman Keijer, senior geotechnical advisor at Fugro Ingenieursbureau:'It is sometimes claimed that cowhides were laid under buildings in the past, as a substrate, but that is an Indian story. Skins used to be far too expensive for such a purpose.' In practice, foundation on steel means that walls rest directly (or at most with a few beams in between) on the ground. In that case, the walls at the bottom are a bit wider, so that the building stands more firmly. In the province of Utrecht (sand) foundations are partly made on steel; in Groningen (clay) and The Hague (peat) piles are used.
In addition to the subsoil, a few other factors are important with regard to piling policy:high buildings, such as flats, are often placed on piles. Piles are also used as a precaution in earthquake-prone areas (Roermond).
A so-called probing always takes place prior to pile driving. A thin, steel rod is pushed into the bottom to see how much resistance the bottom provides. A soil layer with a lot of resistance has a large bearing capacity and therefore forms a good foundation layer. A pile must be able to hold thousands of kilos. With a special device, contractors nowadays even test the piles with forces equal to 1600 tons (equivalent to about 1600 cars).
In the past, all piles were made of wood. The disadvantage is that wooden posts can fall prey to bacteria (so-called pile pest) and rot when they come into contact with oxygen. The latter happens when the groundwater level drops (for example due to drought) and the piles run dry. If wooden posts are still used today, it is often sturdy spruce, from spruce. Herman Keijer of Fugro Ingenieursbureau:'Unfortunately, even spruce posts are affected by bacteria. The deterioration usually proceeds more slowly than with pine piles, but in Amsterdam-Centre we have found serious damage in various spruce foundations.' The advantage of wood over concrete is that the wooden piles act as CO2 -storage:in the ground, below the water level, they do not rot and there is no CO2 free.
Today, piles usually consist of steel pipes, which are filled beforehand or on site with a mixture of reinforcement and concrete. Grout is used as a lubricant to make it easier to get the piles into the ground:aqueous cement. Sometimes thin horizontal pipes are also laid in the ground, so that extra grout can be injected if the soil threatens to subsidence. 'A kind of microsurgery', Ad Tissink describes it in his book The foundations of the city. 'Every wrinkle that threatens to form in the soil is immediately filled with the botox of the foundation technology.' Hopefully that rejuvenation treatment will help to leave Amsterdam in its place for a long time…
- Amsterdam has treacherous soil (Our Amsterdam)
- Danger from thawing permafrost (Knowledge link article)
- Background of foundations in Amsterdam (Amsterdam Municipality)