What is a sinkhole and what are the warning signs of one happening?

 (Mike Stevens/Facebook)
(Mike Stevens/Facebook)

The thought of the ground suddenly collapsing below your feet is the stuff of nightmares, but, unfortunately, not as uncommon as could be thought.

On June 22, a 20-foot wide sinkhole opened up on a quiet residential street in Bexleyheath, swallowing up the entire road - and a passing motorbike.

Emergency services rushed to seal off Martens Avenue after the tarmac caved in at about 9 pm on Tuesday.

It remains unclear what caused the rupture in the road, but drone images show the hole taking up the entire width of the tarmac road that is said to be more than 16 foot deep.

But, what is a sinkhole, and what are the warning signs? Here’s all the information.

What is a sinkhole?

 (ANSA/AFP via Getty Images)
(ANSA/AFP via Getty Images)

A sinkhole is a depression in the ground that has no natural external surface drainage. This means that when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface.

Sinkholes mainly occur in what is known as ‘karst terrain’; areas of land where soluble bedrock (such as limestone or gypsum) can be dissolved by water.

Sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur.

What are the different types of sinkholes?

There are two basic types, those that are created slowly over time (a cover-subsidence sinkhole) and those that appear suddenly (a cover-collapse sinkhole).

It’s usually the latter type that create headlines, but both varieties are formed by the same basic mechanism.

What are the warning signs of a sinkhole happening?

In many cases sinkholes don’t form suddenly — they build up slowly over many months or years.

It’s often the last stage when the formation of the hole or basin is at the very surface, that’s sudden.

Here are signs that might indicate you have a slow-burning sinkhole on your hands:

  • Fresh cracks in the foundations of houses and buildings.

  • Cracks in interior walls.

  • Cracks in the ground outside.

  • Depressions in the ground.

  • Trees or fence posts that tilt or fall.

  • Doors or windows become difficult to open or close.

  • Rapid appearance of a hole in the ground.