ST Explains: What is a sinkhole and how is it formed?

SINGAPORE - A 48-year-old woman

fell into an 8m-deep sinkhole in Malaysia on the morning of Aug 23.

The Indian national was visiting Malaysia and walking in the Jalan Masjid India area in Kuala Lumpur when a section of the sidewalk suddenly caved in.

Only a pair of slippers believed to belong to the woman has been recovered so far, with search and rescue efforts now hindered by strong water currents running under where the sinkhole appeared.

The Kuala Lumpur police chief, Datuk Rusdi Mohd Isa, said the team will need to proceed cautiously and methodically as any disruption could lead to flooding, and that gas pipes underground are also complicating the rescue efforts.

But what are sinkholes, and how do they form so suddenly? ST explains.

Q: What is a sinkhole?

While the word sinkhole is often used to describe any large hole that opens up in the ground, using it as a catch-all term is technically incorrect.

Strictly speaking, The Geological Society of London says, sinkholes are “a natural surface depression caused by dissolution of soluble rocks at depth”.

According to the British Geological Survey (BGS), this means these typically saucer-shaped hollows are caused by the collapse or removal of an underlying layer of rocks needed to support the surface layer of material.

But the shape, size and depth of a sinkhole can vary greatly depending on the composition of the rocks and different interactions which caused it.

Q: How does a sinkhole form?

Sinkholes are commonly formed when the supporting underlayer of rock is naturally dissolved by acidic rainfall or groundwater circulating through it, causing spaces and caverns to develop underground, according to the US Geological Survey.

Called soluble sinkholes, examples of such dissolvable rocks include chalk, gypsum or limestone. 

“When groundwater seeps into cracks and then pools beneath the surface, it wears away these rocks and leads to the formation of underground caves and other openings. Soil particles fall into these gaps too, enlarging the chasms and facilitating the collection of more water,” National Geographic reported.

According to the BGS, as the water drains from the hollows formed through the fissures or joints of the rock, it continues to dissolve the rock and gradually enlarges the depression on the surface.

The likelihood of sinkholes forming can also be accelerated by drought or heavy rain, as well as human activities such as heavy pumping of groundwater or construction.

The sides of sinkholes can range from gentle slopes to almost vertical edges, and can be shaped similar to saucer-like hollows, cones, cylindrical potholes and shafts.

Q: How long does it take for a sinkhole to form?

The time taken for a sinkhole to form can vary dramatically.

Sinkholes formed on sandier soils where there is an existing void or cave underground tend to form more gradually and can even go undetected for hundreds or thousands of years, according to National Geographic.

“As the soil above transports itself into that cave in the rock, the ground slowly subsides. So, it is not catastrophic. It subsides over time,” the report said, adding that such sinkholes are known as cover-subsidence sinkholes and are the most commonly formed.

But the sinkholes that appear suddenly and make headlines tend to be a type known as cover-collapse sinkholes, which can happen in a matter of hours.

These sinkholes typically occur in clay rocks and form in a similar fashion to their slow-growing counterparts, with loose, unconsolidated soil leaching into a cave underground.

The difference in these cases is that the space in the ground starts to grow upwards, until the thin layer of soil above the void can no longer hold up the land and abruptly collapses, taking with it houses and roads.

However, according to the BGS, sinkholes formed exclusively in this way are quite rare, with many sinkholes forming due to a combination of different processes.

Q: Are sinkholes a natural disaster?

Although sinkholes are generally considered a natural disaster and process, their formation can be greatly exacerbated or triggered by human influences.

According to the BGS, the presence of broken drains, water mains and sewerage pipes, or modified drainage and diverted surface water can wash sediment into underlying materials, causing the ground to sink.

In 2022, tunnelling works by national water agency PUB involving a tunnel boring machine caused a section of Farrer Road to collapse and a sinkhole to form.

The machine works by cutting the earth in front of it and conveying it to the back of the machine, where it is transported to the surface.

The machine was later revealed to be undergoing maintenance when the ground in front of the tunnel face destabilised, causing the sinkhole to form on a slip road leading from Holland Road to Farrer Road.

No one was injured in the incident, but the road was closed to traffic for about a week.

In 2013, a portion of Woodlands Road caved in and resulted in a hole that appeared to be as wide as one lane, causing traffic on both lanes of the road to be blocked.

The affected area was next to where construction work for the Downtown Line 2 was being carried out by a Land Transport Authority (LTA) contractor, and the massive sinkhole was caused by excavation works for the line.

The construction destabilised the soil, which ruptured an underground water pipe, LTA said then.

The soil movement and pipe’s rupture then weakened the ground under the road, causing the sinkhole to open up.

Mr Chong Kee Sen, vice-president of the Institution of Engineers, Singapore, told ST then that a mixture of soft soil and hard rock underneath posed a challenge, as Singapore continued to dig underground to build rail lines and lay pipes.

Reports said boring through soft soil could weaken and move the soil, which may cause the road to sink when it is heavily used.