Today it happened again, this time in Alkmaar. A woman (79) and a man (83) fell into a sinkhole with a walker and all. It is not the first sinkhole incident in North Holland. How do these holes actually arise? NH looked into it.
“What we in science mean by a sinkhole is caused by the dissolution of limestone underground. Then you get, as it were, a small cave that can collapse,” says geologist Bernd Andeweg. The term ‘sinkhole’ is now also often used for holes that are not caused by the dissolution of limestone. Because in North Holland there is no limestone in the ground, and yet we regularly have to deal with sinkholes.
Sand
The sinkholes we encounter in North Holland are caused by sand that washes away, Andeweg explains. This can happen if, for example, a pipe underground is broken. The sand can wash away there. Which, just like ‘real’ sinkholes, creates a cavity beneath the ground. You then have to wait until the top layer of the soil no longer holds up. Fortunately, sinkholes do not occur often: “There are many risks in life, this is not one you have to constantly worry about,” the geologist puts things into perspective.
To predict
A sinkhole is difficult to predict, Andeweg explains. “You may see tiles placed strangely. But that can have many causes. It does not necessarily have to be a sinkhole.” For example, tree roots can cause tiles to rise or collapse. Nevertheless, it can’t hurt to be extra alert if you see something striking on the sidewalk or road.