Beautiful Amsterdam, Built on Poles: Who would pay if it all fell down?
Beautiful Amsterdam / Built on Poles/ Who would pay if it all fell down?
–Old Dutch nursery rhyme translated
Since early times, the houses in Amsterdam have been built on wooden foundation piles[i] that are driven deep into the clay, peat, and water until they reach the first layer of solid sand. Even some of the trees in Vondelpark are supported on wooden piles to keep them from sinking into the marshy ground.
How long can a wooden pile remain in the ground before it has to be replaced?
300 years? 80 years? Until next week? The answer depends on many variables:
Quality of the sand layer
In Amsterdam, the first layer of sand lies approximately 12 meters below the surface. However, the quality of the sand layer is not consistent throughout the city. In some areas, the first sand layer is thinner than normal or missing entirely. As a result, the houses in these areas sink at an abnormal rate, and the foundations have to be replaced faster.
Stability of the water table
The top of the wooden foundation pile should be below the lowest expected water level. If part of the pile reaches above the water, it will be subject to insect attacks and decay. A problem is that the water level under Amsterdam isn’t always stable. For instance, it can drop during long periods of drought. Or a sewer pipe can develop cracks, allowing the ground water to flow into the pipe and be carried away. As a result, the water level drops.
On the other hand, if the water level rises too high, as sometimes occurs after a heavy rain, the Amsterdam cellars and crawl spaces fill up like bathtubs.
Construction projects can also interfere with the water level. An example is the incident on the Vijzelgracht in 2008 during work on the new metro station. A leak in the dam wall at a depth of 60 meters caused water and sand to stream into the pit. Part of the sand layer that was supporting the wooden foundation piles of the nearby houses washed away, causing extensive damage to the structure of the houses.
Type of wood
Spruce is less vulnerable to damage from bacteria than pine. Since around 1970, piles made from pine are prohibited from use. Concrete and steel piles are often used in present day construction.
What are the signs of foundation problems?
The visible signs are slanted floors, cracks in the walls, doors and windows that stick, and broken front steps. The charming crooked roofline of Amsterdam is actually a symptom of unstable foundations.
If a foundation that is in poor condition is not repaired in time, the wooden piles will need to be replaced. No homeowner wants to get this news. A new foundation can easily cost 100,000 euro. Furthermore, the floor may need to be opened up, thereby making the house unlivable for some time.
In recent years, the housing corporations have been replacing the wooden piles under their properties with concrete piles.[ii] For example, Ymere is replacing the decayed pine piles in the Van der Pek neighborhood — a costly investment. However many houses that are at risk are privately owned.
The condition of the old wooden piles will only worsen in the coming years, so the question is: who will pay?
[i] Referred to as poles in the nursery rhyme.
Credits and Sources:
Written by Barbara Austin.
Categorized as Amsterdam.
Originally published at tedx.amsterdam on September 24, 2015.