A sinkhole, also known as a doline, is a depression or hole that forms when the ground surface collapses because soil, rock or other material below has moved. Sinkholes range from about a metre across to tens of metres or more.
Types of sinkhole
All sinkholes result from the ground beneath shifting. The main types are:
- Dissolution. Soluble rock dissolves as water passes through it, and the surface sinks into the cracks and cavities. Limestone is especially prone to this, so many sinkholes form in limestone areas.
- Collapse. An underground cave or cavity gives way, and the layers above subside until the surface collapses. In the UK this happens in South Wales, where sandstone lies above caves.
- Dissolution and collapse. Loose material such as sand, clay or soil over soluble rock is gradually washed into cracks and fissures. Over time this creates a sinkhole.
- Erosion. Flowing water wears material away. A process called soil piping can create large voids within sediment.
What causes sinkholes?
Sinkholes form naturally, but human activity can trigger them too. Common causes include:
- Heavy rain and flooding. More water passing through the ground speeds up erosion of materials such as limestone.
- Water leaks. Leaking drains, sewers and burst water mains can wash material out from below ground.
- Construction work. Changing surface drainage, or adding load to the ground, can set off a collapse.
- Changes in the water table. Groundwater can help support a cavity. In a drought, the cavity can dry out and lose that support, and then collapse.
- Mining. Mining can drain water from cavities or disturb clay-filled voids, causing a collapse. Norwich has had several sinkholes linked to old mine workings.
Where do sinkholes occur in the UK?
Sinkholes can form anywhere in the UK, but they are most likely where the ground contains soluble rocks such as chalk, limestone and gypsum. Water gradually dissolves these rocks, creating cavities that can later collapse.
The British Geological Survey (BGS) has monitored sinkholes in the UK since the 1970s. The number reported has risen in recent years, most likely because of a combination of factors including heavier rainfall, mining and construction.
Most UK sinkholes are fairly small and appear in upland, rural areas. Areas known for them include South Wales, the Mendips, the Peak District, the North Pennines and the Yorkshire Dales.
North-east England is especially susceptible because of Permian rocks containing large gypsum deposits. Gypsum dissolves faster than limestone. Ripon has had many large sinkholes, and some pose a real risk to buildings and roads.
Are there sinkholes in London?
Small sinkholes occur fairly often in London, though most never make the news. Larger ones open occasionally. A sinkhole around 6 metres wide collapsed on a street in Bexleyheath, south-east London, in June 2022, the second large one in the area after another in 2008. Changes to surface drainage and new development can both contribute.
Many more sinkholes in London and across the UK have yet to be found. A ground penetrating radar (GPR) survey is one of the most practical ways to look for them.
Warning signs of a sinkhole
Sinkholes usually develop over time. Signs that one may be forming include:
- new cracks in the foundations of houses and buildings
- cracks appearing in interior walls
- fresh cracks in the ground
- depressions in the ground
- trees or fence posts starting to lean or fall
- doors or windows becoming harder to open or close
- a hole appearing suddenly
If you see any of these signs, an underground void detection survey can show whether voids are present and how far they extend, without invasive digging.
Frequently asked questions
Can sinkholes be fixed?
Often, yes. Before any repair, the extent of the sinkhole should be investigated. A GPR survey can show its size. Finding the cause matters too, so it does not happen again.
How fast do sinkholes collapse?
It depends on what material has moved or eroded, and how much. Load on the surface, such as buildings or roads, can make collapse happen sooner. Rain or running water speeds it up.
What should I do if a sinkhole appears?
Keep people away. Cordon off the area and put up warning signs. A GPR survey can help confirm whether it is a sinkhole and show how wide and deep the affected ground is. If it is on your property, contact your insurer and the council's building control team. You can also report it to the British Geological Survey.
How deep do sinkholes go?
Some are less than 30 centimetres deep. Others go down more than 30 metres. It depends on the ground beneath and how much material has moved or dissolved.
Do sinkholes stop growing?
Not always. If the ground is soluble, such as limestone, or water keeps flowing through it, a sinkhole is likely to keep growing.
GPR depth is limited in wet clay and other conductive ground, so the survey design depends on the site. If you are concerned about a sinkhole or subsidence, our underground void detection service maps voids beneath roads, yards, slabs and open ground. Contact us to discuss your site.