Unsuspectingly, a man drove to work on a scooter in Amersfoort. He sees a puddle of water on the street and decides to avoid it via the sidewalk. Suddenly he disappears up to his chest in the water: he has landed in a sinkhole with scooter and all.
It is likely that sinkholes will become more common in the Netherlands, according to Ramon Hanssen, professor of geodesy and satellite observations at TU Delft. “Human soil activity, for example for the construction of tunnels, is increasing. There are also more and more sewers and water pipes in the ground.” Now several sinkholes or ‘drainholes’ are created every year.
Is it possible to predict where such a bowl-shaped cavity is formed? That depends on how quickly the hole forms underground, Hanssen says. “Sometimes it takes an hour, but it can also take years.” Using satellite images, Hanssen can see that the soil is subsiding in places where a sinkhole may arise. He showed that with an investigation to a sinkhole that was created in 2011 under the ‘t Loon shopping center in Heerlen. First, the soil sank about 3 millimeters per year for 18 years and 15 millimeters per year in recent years before the collapse. “A satellite flies over once a day, so you cannot detect a sinkhole that forms in an hour.”
The Amersfoort sinkhole probably formed too quickly for satellites to detect. A sinkhole starts out as a depression in the bottom that moves to the surface. In Amersfoort such a cavity was created by a burst water pipe: the sand under the street was washed away into the groundwater. Pavements alone could not carry the man with his scooter, so the street suddenly collapsed.
A leaking sewer can similarly cause a sinkhole. That happened in 2019 on the Aluminum Street in Rotterdam. How much water drips (or sprays) from a sewer or pipe and what is built on it determines how quickly the soil collapses. Hanssen: “A concrete parking garage, for example, can hold more than a meadow.”
Limburg marl caves
In Limburg, old mine galleries are underground as a result of coal mining, which sometimes collapse when they are affected by (ground) water. This happened, for example, at the shopping center in Heerlen – which turned out to be built on a mine entrance. Limburg marl caves also sometimes collapse: the lime dissolves in water. In the east of the country, limestone also occurs in the soil. It can take years to resolve.
In the Netherlands, the holes are often small: from about half a meter to a few meters in diameter and a few meters deep. Elsewhere, much larger sinkholes have formed. Like in Florida, where there is a lot of lime in the soil. In 2013, a sleeping man, bed and all, was swallowed by the earth. He was never found. Or in Guatemala City, where in 2010, torrential rain and a leaking sewer left a factory tens of meters deep into the earth. “The impact of a sinkhole is large, but they are rare,” says Hanssen. A detection system must not miss a dangerous sinkhole and must therefore be sharply adjusted. As a result, it will occasionally incorrectly classify a subsidence as a sinkhole.
Hanssen is therefore working on improving and automating his system. For this he uses information about the soil. “If a subsidence occurs in a place where, for example, a mine shaft is located, it is more likely that a sinkhole could form there.” And a lot is known about the soil: “For example, we know exactly where pipes and sewers run.”