Building on a Wet Sponge
Amsterdam was built on a swampy area of the Amstel estuary, regularly flooded, and consisting mostly of peat soil and bogs. This type of soil (thick compacted layers of old plant material conserved while under water) can be compared to a wet sponge — put some weight on it and the water will be forced out while the surface sinks. This is what the early settlers in this area (around 4500 years ago) had to cope with, so very lightweight wooden dwellings were the rule.
Homes were usually constructed on top of small artifical mounds (terpen). But every ten years or so the ground beneath their buildings had sunk so much that they needed to demolish them, add layers of debris on top of the area and rebuild their homes. The Romans may have had a point when around 50 B.C. they labeled the land north of the river Rhine (in the middle of the current Netherlands) as being “uninhabitable and only fit for barbarians”.
Ground Level Worries
The soggy terrain has been a source of worry for centuries. Even though the Romans knew very well how important pile foundations were, this knowledge seems to have been forgotten in the Low Countries during the early Middle Ages. When people started using piles again, they initially had no knowledge of the layers below them, so the piles they used were not nearly long enough to reach the stable sand — they relied on the stickyness of the soil to the piles. Which is one of the reasons why so many older houses in Amsterdam continue to sag.
The added weight of the debris used to raise the terrain just worsened the situation. The more the soil was raised, the heavier it became and the more it sank. And when the City, after a few devastating fires, declared wooden houses illegal and brick houses mandatory in 1669 (actually from 1521 on, but people ignored it mostly), the heavier buildings suffered even more from the sinking terrain. In fact the ground level in 1325 was more than 5 meters (16,5 ft) lower than it is today!
Keeping Dry & Keeping Warm
Polder management (building dikes and pumping out the water) of course aggravated the peat compacting. Around Amsterdam a lot of peat disappeared by subsiding, peat harvesting and erosion of the borders of the newly formed peat lakes. In the polders peat was harvested and dried to serve as fuel for the city during winter and for the growing industries, because wood had gotten quite scarce after the forests had been decimated in the 11th to 13th century.
Extensive peat harvesting created big peat lakes like the Haarlemmermeer (Haarlem Lake), which was in turn poldered in and made into farmland from 1850 on, when steam-powered pumping stations arrived — this is were Schiphol Airport is now. Only very few instances of the original peat landscape remain, like the Meerzicht polder and De Poel, both in the Amsterdamse Bos (Amsterdam Forest).
An Inverted Forest
The thickest forest in the Netherlands lies right below Amsterdam, as the saying goes. The Royal Palace on Dam square alone is held upright with 13,659 piles. Amsterdam’s Central Station was built on 8,687 wooden piles, the Concertgebouw rests on top of 2,186 piles. At one point the Norwegians even used to brag that “Norway carried Amsterdam”. In fact, so many Norwegian and Baltic trees are holding up the old buildings that it can indeed be seen as a thick inverted forest.