Go back to the blog
04.02.2023, 15:18
Articles
Share with a friend
What are sinkholes?
Sinkholes, also known as dolines, are a depression or hole that forms when the surface collapses due to the movement of soil, rock and other subterranean materials. Sinkholes can range in size from a few feet to hundreds of feet in diameter.
Types of skinholes
Sinkholes can be caused by a few different factors but ultimately it’s a result of the subsurface shifting. The main causes of sinkholes are:
- Dissolution- sinkholes can form as a result of surface dissolution. Limestone is most susceptible to dissolving when it absorbs water and as a consequence, most sinkholes form in this type of rock. The surface will sink into the cracks and cavities.
- Collapse- a collapse sinkhole occurs when an underground cave passage crumbles. This collapse can cause the overlying layers to subside resulting in the surface collapsing altogether. In the UK, these types of sinkholes are usually found in South Wales where layers of sandstone rocks are located above subterranean caves. They are also common in Ripon, South Yorkshire as lots of gypsum (a sulfate mineral) is found here which can easily dissolve.
- Dissolution and collapse- sinkholes also form when both dissolution and collapse occur. If a thin covering of an unsteady, superficial material like sand, clay or soil is covering soluble rocks, it can be washed into cracks and cavities. Over time as the layers of superficial material pour into cracks and fissures, it can slowly create a sinkhole.
- Erosion- Erosion occurs when flowing water wears down a material over time. This can occur in a process known as soil piping which creates big voids within the sediment.
What causes sinkholes?
Sinkholes can form naturally but they can also be triggered by human interference too. Broken drains, water mains, sewage pipes and diverted surface water can erode material below ground like limestone resulting in sinkholes.
Causes of sinkholes include:
- Heavy rain and flooding:
- Water leaks: Leaking pipes, irrigation or burst water mains can result in erosion of subterranean material.
- Construction work: Modifying surface drainage and adding more weight to a surface that is structurally sound.
- Changes in water table level: Underground water is not always a bad thing. In some cases, it actually provides support to a cavity. A drought can cause these cavities to dry out removing the buoyant support which can result in a collapse.
- Mining: This can also cause sinkholes as it can dewater a cavity or it can disturb a clay-filled void causing a collapse. Norwich has had a few sinkholes recently as a result of old mines disturbing stable sediment-filled voids.
North-East England is considered to be the most geohazardous region in the UK because of the multiple sinkholes that occurred there over the years. That is due to the presence of Permian rocks that contain large gypsum deposits; gypsum being highly soluble in water and dissolving more rapid than limestone.
Where are most sinkholes in the UK?
Sinkholes are most common in tropical areas with lots of
limestone deposits as they get much more rainfall here. Dryer places do not have
to worry as much about them.
In the UK, sinkholes are nothing new as we get our fair share of rain, especially in the colder months. They can form anywhere in the country, however, areas with soluble rocks such as chalk, limestone, and gypsum are most likely to get them. Flowing water can dissolve these rocks over time resulting in underground cavities which collapse causing a sinkhole.
The British Geological Survey (BGS) has been monitoring sinkholes in the UK since the 1970s. The number of reported sinkholes in the UK has increased in recent years and this is likely due to a combination of different things. Increased rainfall and activities like mining and construction are the most likely reasons for the sudden upturn in them.
In the UK most sinkholes appear upland in rural locations and are fairly small. As we have already said, sinkholes regularly occur in South Wales and Ripon but they are also common in Mendips, the Peak District, the North Pennines and the Yorkshire Dales.
The North-East of England is most susceptible to sinkholes due to the large deposits of gypsum. This mineral is more soluble than limestone and dissolves quickly. Ripon has had many large sinkholes form and some of them pose a significant risk to buildings and roads in the area.
Currently, the largest sinkhole in the UK is the Souter Hole located in Sunderland, northeast England. It is 40 ft wide and was discovered in 2003. Thankfully it is near a remote coastal area and does not currently pose a risk to anyone.
Are there any sinkholes in London?
Sinkholes occur fairly often in London however most are very
small and will not make the news. Occasionally a larger sinkhole will open. A 20
ft wide sinkhole collapsed on a street in Bexleyheath, South-east
London in June 2022. This was the second large sinkhole the area has had with
another one opening up in 2008.
Construction in London is a common sight and modifying surface drainage and developing new buildings can cause sinkholes to develop.
There are hundreds of sinkholes yet to be discovered in London and the UK. The easiest way to detect a sinkhole is with a GPR (Ground Penetrating Radar) survey.
What are the warning signs of a sinkhole?
Sinkholes usually build up over time so it’s important to
keep an eye out for any signs of one forming. Signs a sinkhole may be forming include:
If you see any of these signs you should get an underground void detection survey. These surveys use GPR (Ground Penetrating Radar) to detect archaeological features without the need for invasive and destructive excavation methods.
info
Has a sinkhole opened up on your site? Do you want to know its extent? Call you on 020 3931 5759 or email us on info@kbsurveyes.co.uk
Sinkhole FAQ
Here are some questions people frequently ask about sinkholes.
Can sinkholes be fixed?
In a lot of cases, a sinkhole can be repaired. Before any work is done to try and fix it, the extent of it should be examined. This can be done using a GPR survey to scan the size of the sinkhole. Finding out the cause of the sinkhole is also important to prevent it from happening again.
How fast do sinkholes collapse?
Sinkholes can collapse at different rates depending on what type of material has moved or eroded and to what extent. It is also affected by what is above such as buildings or roads. Heavier objects on the surface will likely result in the sinkhole collapsing faster. Rain or running water will also speed up the time it takes to collapse.
What to do if a sinkhole appears in the UK?
If you think you have a sinkhole on your property you should get a GPR survey completed to find out if it definitely is one. This will also help you work out the scale of the sinkhole telling you how wide it is and how deep it goes. If it is a sinkhole the area should be evacuated and cordoned off with warning signs so people do not enter the area. There are also various people you should contact if it is on your property such as your insurance provider, Council-Building Control and the British Geological Survey.
How deep do sinkholes go?
Sinkholes can differ in size and depth. Some will only be less than 1 ft but others can be over 100 ft. It depends on the material beneath it and how much has moved or dissolved.
Do sinkholes ever stop growing?
Sinkholes can continue to grow depending on the conditions. If the material underground is quite soluble like limestone it will likely get bigger. If the area is continually impacted by water, this will exacerbate things too.
What is the biggest sinkhole in the world?
The largest sinkhole in the world is Xiaozhai Tiankeng in China. It is over 2000 ft long, 1762 ft wide, and nearly 2200 feet deep. It was discovered in 1994 but is believed to have formed over 100,000 years ago.
See other blog entries
04.11.2022
Articles
