How are sinkholes formed?

Sinkholes, or dolines, often take thousands of years to form and vary hugely in size.

The deepest is China’s Xiaozhai Tienkeng at 2172 ft (662 m). The Qattara Depression in Egypt is roughly 50 miles (80 km) by 75 miles (121 km) in surface size.

But often sinkholes can be only a few metres in diameter.

They are usually the result of what are known as Karst processes. They happen when a layer of rock underneath the ground is dissolved by acidic water.

Usually this layer is a soluble carbonate rock, such as limestone or its purer form, chalk. Florida is particularly prone to sinkholes as the entire state has limestone underneath it.

Typically rainfall seeps through the soil, absorbing carbon dioxide and reacting with decaying vegetation. As a result, the water that reaches the soluble rock is acidic.

The acidic water causes the erosion of the soluble rock layers beneath the surface – eventually creating cavernous spaces.

The soil or sand over the limestone collapses into a sinkhole when it is no longer supported because of the cavity below. This final collapse of the surface might take anything from a few minutes to several hours.

There are warning signs in urban areas. These include doors and windows failing to close properly, or cracks appearing in the foundations of houses. In some cases ground movement can be detected.

Heavy rainfall or poor drainage systems can trigger a collapse.

But predictions are not easy.

“It can be very difficult to predict collapse because there is very little surface evidence of the features,” says Dr. Vanessa Banks, an expert in shallow geohazards and risk at the British Geological Survey.

Different rock types behave in different ways, she adds.

The timing of a collapse also depends on the nature of the soil or rock at the surface which forms a “bridge” over the growing cavern below.

“Consolidated deposits such as sandstone will bridge voids until their tensional strength is exceeded, when the rock will fail and collapse into the underlying cavity,” says Vanessa Banks.

Certain types of ground – such as gravel and sand – are not fixed in place and so more prone to being washed away.

The erosion may take many years but the collapse may be sudden as it depends on a tipping point determined by the material at the surface, Vanessa Banks says.

Moreover, acidic water varies in its strength – and therefore the rate of erosion – depending on the soil and rock it filters through.

“Water acidity can typically have a pH level of about 6.5 – still drinkable to all intents and purposes – but also be as low as four,” explains Vanessa Banks.

Iron and sand can increase the acidity. So too can sandstone and shale.

Human development can also affect these natural processes.

When people are building a basement, they may need to drain water, explains Vanessa Banks. This drainage of water can destabilize the soil by washing away smaller particles – like sand – that are necessary to keep larger particles together, increasing the chances of collapse.

In January, a sinkhole that swallowed an entire building complex in Guangzhou, China, may have been triggered by the construction of an underground metro line nearby.

Burst water mains or sewage systems also cause many urban sinkholes to happen, regardless of the rock type below.

These cause instability in the surrounding area, often giving the impression of a natural sinkhole, says Vanessa Banks.

Urban development also adds more weight to the surface layer, potentially speeding up the collapse of a sinkhole.

It is crucial to undertake extensive site investigations prior to building work, she says.

Vanessa Banks says that concrete urban development on the whole could in fact be “slowing [sinkholes’ natural] formation by restricting this drainage water from seeping through”.

There are more sinkholes in rural environments, she says, primarily because risky areas are typically avoided by urban developers.

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