The storms and heavy rainfall of the winter 2013/2014 in England and Wales were described by the Met Office as one of, if not the most exceptional period of rainfall in the last 248 years. 1 This prolonged and heavy rainfall resulted in a spate of sinkholes or dolites opening up across Britain, leading to popular media reporting on the ‘Terrifying sinkholes that are opening all over Britain…’2.
The most common way for sinkholes to occur is when a soluble rock, like limestone, is dissolved when exposed to acidic rainfall or groundwater. A more dramatic collapse can happen when the overlying material is more cohesive like clay, which can suddenly ‘drop out’ after a period of dissolution below.3
Natural dissolution is only one way sinkholes are formed and human impact can also play a part. As experienced in Fontwell, West Sussex in 1985,4 burst water mains or leaking drainage pipes can increase subsurface water movement assisting and speeding up dissolution, sometimes leading to the sudden appearance of sinkholes. Mining also has the potential to cause sink holes through the lowering of the water table or by constructing sub surface voids which subsequently collapse. A particularly startling example of a sinkhole formed due to mining was recently seen in Russia at the Solikamsk-2 mine after an accidental release of brine into parts of the mine.5 This caused a rapidly growing dolite at the surface, threatening the mine with closure.
Due to geological conditions, some areas of the UK can be at risk of sinkholes. Regions commonly susceptible to sinkholes are parts of Wales, the Peak District and the Yorkshire Dales where the geology contains Carboniferous limestone. Chalk areas in the Chilterns, Dorset and Hampshire can also be at risk. One of the worst affected areas in the country is the gypsum rich area around Ripon in North Yorkshire.1
Large parts of Ripon are underlain by gypsum -that has dissolved to form an elaborate cave system. Occasionally, further dissolution of the ground can expose the caves, forming a sinkhole at the surface.6 In February 2014 one such sinkhole opened up on the east side of Ripon, causing devastating damage to a house there. The residents had returned home one evening to find the back of their house collapsing, and a huge visible crack which appeared to be widening up to 40cm. After rescuing the dog and shutting off the utilities, surrounding neighbours were quickly evacuated.7 No casualties were caused by this incident however; the damage sustained to the building and on-going ground movement at the site meant that the house had to ultimately be demolished.8
One of the largest and most dramatic urban sinkholes to occur in recent years was in 2010 in Guatemala City. The cavity opened up suddenly, swallowing an entire three story building and causing one fatality.9 This sinkhole was reported to have a diameter of 18m with a depth of 100m, and opened up shortly after Tropical Storm Agatha. Although the cause of the sinkhole is largely unknown, it has been speculated that a combination of burst pipes and an overwhelmed sewage system following the storm could have eroded the underlying unconsolidated pumice, causing the sinkhole to open.10
As unpredictable weather patterns and heavier rainfall are expected to become more common place, it seems likely that we will be seeing more sinkholes in the future. To reduce the risk of sinkholes occurring it is advised that before conducting groundworks, sites are appropriately investigated using geophysical surveys and boreholes so that the developer is aware of exactly what lies beneath.
The British Geological Survey have published the following guidelines for if you do spot any signs of ground movement:
- Ensure that areas and a safety perimeter are cordoned off, and that people are kept away
- Notify the landowner and/or emergency services as appropriate
- If services pipes, e.g. gas or water, are left suspended ensure that infrastructure managers are contacted, e.g. Safety and emergencies (National Grid) and Emergencies (United Utilities)
- Ensure that any triggering processes, e.g. leaking drains, are managed to minimise the potential for ongoing subsidence
- Contact your local council/building control to notify them; find out if there is a history of such features in the area, and request a list of consulting engineers that are able to give appropriate advice on the correct procedures for stabilising them3
References
1. British Geological Survey, 2015, Increased incidence of sinkholes and collapse subsidence features in 2014. [online] Available at: http://www.bgs.ac.uk/caves/sinkholes/feb2014.html [Accessed 9th Feb 2015]
2. Daily Mail, 2014, Terrifying holes that are opening up all over Britain: They’re appearing at FIVE TIMES their normal rate and further bad weather could mean we haven’t seen the last of them. [online] Available at: http://www.dailymail.co.uk/news/article-2560892/Terrifying-holes-opening-Britain-How-sinkholes-appearing-five-times-normal-rate-bad-weather-mean-havent-seen-them.html [Accessed 9th Feb 2015]
3. British Geological Survey, 2015, Sinkholes (or dolines) [online] Available at: http://www.bgs.ac.uk/caves/sinkholes/home.html [Accessed 11th Feb 2015]
4. The Guardian. 2013, What are sinkholes and what causes them? [online] Available at: http://www.theguardian.com/world/2013/mar/04/what-causes-sinkholes-florida-man [Accessed 11th Feb 2015]
5. The Moscow Times, 2014, Giant Sinkhole Still Expanding, Russian Potash Producer Uralkali Says[online] Available at: http://www.themoscowtimes.com/news/article/giant-sinkhole-expanding-by-the-hour-russian-potash-producer-uralkali-says/513127.html [Accessed 11th Feb 2015]
6. British Geological Survey, 2015, Sinkhole at Magdalen’s Close, Ripon, Monday 17 February 2014 [online] Available at: http://www.bgs.ac.uk/research/engineeringGeology/shallowGeohazardsAndRisks/sinkholes/riponFeb2014.html [Accessed 11th Feb 2015]
7. BBC News, 2014, Ripon sinkhole forces three homes to be evacuated [online] Available at: http://www.bbc.co.uk/news/uk-england-york-north-yorkshire-26236667 [Accessed 23rd Feb 2015]
8. ITV News, 2014, House demolished after Ripon sinkhole damage [online] Available at: http://www.itv.com/news/calendar/story/2014-02-17/sinkhole-appears-in-ripon/ [Accessed 23rd Feb 2015]
9. The Week, 2013, 7 terrifying sinkhole disasters [online] Available at: http://theweek.com/articles/466766/7-terrifying-sinkhole-disaster [Accessed 23rd Feb 2015]
10. National Geographic, 2010, Guatemala Sinkhole Created by Humans, Not Nature [online] Available at: http://news.nationalgeographic.com/news/2010/06/100603-science-guatemala-sinkhole-2010-humans-caused/ [Accessed 23rd Feb 2015]
