Sinkholes can open up in the middle of urban areas
A huge sinkhole swallowed a bus, killing six people in Northwest China this week, sparking an electrical explosion and leaving several missing. Several people disappeared as the sinkhole spread and 16 people were taken to hospital.
Sinkholes are not uncommon in China and experts cite the country's rapid pace of huge construction work as one of the triggers.
Watch the BBC footage of the latest incident below
Dr. Clive Edmonds BSc MSc PhD DIC FGS CGeol EurGeol CSci is a geotechnical specialist and has studied sinkholes for the past 30 years.
"The main trigger for sinkholes is water," says Dr. Edmonds. "In 90% of sinkhole cases, water saturating the ground is the main trigger, known as Karst processes."
"Sinkholes happen when a layer of rock underneath the ground is dissolved by water. In regards to China and their sites, I know there there are some Karst problems with limestone deposits. "Sinkholes depend on geology, some rocks are more soluble than others - salt deposits for instance dissolve more quickly. Gypsum for instance is a very soluble rock. Chalk & limestone deposits may take thousands of years to dissolve but when new cavities occur they can cause spectacular sinkholes."
"Human development can contribute greatly, especially construction work in urban areas where you may have dense development with roadworks, etc, as in the case of China. The surface becomes impermeable with water collecting over time in drains and sewers - water starts leaking into the ground."
"There are warning signs you can spot. First indicators are dips and depression in the ground surfaces, deepening with tiny cracks in pavements or buildings for instance, leading to buildings showing slight movement. "
Tunnelling into water filled cavities or where water bodies can flow catastrophically into an excavation are particularly hazardous.
"In the UK any such instances have been caused by small cracks in the ground leading to the onset of building movement and then a breakdown in the water utilities or leak of a mains water pipe or a sewer eventually draining water into the ground."
In the UK there has been a long history of mining and water abstraction, which in some areas has left a legacy of man-made cavities. The term sinkhole is, in the UK, often extended to cover the collapse of ground into such man-made features.
But predicting a sinkhole collapse isn't easy as there can be very little surface evidence of the features.
Tony Bracegirdle is a senior partner of the Geotechnical Consulting Group.
"There are seldom warnings in urban areas," says Tony. "Although sometimes sinkholes develop slowly to the extent that there is sufficient time to restrict access. Sinkholes tend to focus in specific geological and topographic conditions and so the hazard can be reasonably quantified in areas where there is a history of recurring sinkhole activity"
"The most common response in high-risk locations is to design works that are insensitive to potential sinkholes," says Tony, "to control surface water and to take additional precautions to limit water loss from drains and services."
What can civil engineers do to prevent sinkholes?