At the end of 2013 and the early months of 2014 the Sinkhole phenomenon hit Britain. Sinkholes started appearing all over the country. Large holes big enough for a house to fall into have suddenly started appearing from nowhere. At the time of writing no houses have actually fallen into sinkholes but cars have. And sinkholes are not shallow ditches. They are deep cavernous pits. Sinkholes have been historically known by a number of names such as, doline, swallets shakeholes, swallow holes and just ‘sinks’. Some sinkholes are caused by something called the ‘karst process’, typically the chemical dissolution of carbonate rocks or ‘suffosion’ processes.
Sinkholes vary widely in size from one to up to 2000 feet deep. Depending on the soil they can appear as soil lined bowls or at the other extreme rock edged chasms. So sinkholes can be formed gradually or appear suddenly.
At the start of 2014 during the severe flooding and after a prolonged period of heavy rain a spate of sinkholes started appearing all over the UK. A 15 foot sinkhole appeared in the M25, a larger 30 foot crater consumed a car in High Wycombe, and homes in Hemel Hempstead had to be evacuated when a 35 foot diameter sinkhole appeared in a street of houses. The Unites States was even worse affected when a 40 foot wide sinkhole swallowed up eight cars which were, at the time, on exhibition at a museum. At the time of writing injuries in the UK have been minimal or non existent but at the rate sinkholes have been appearing a fatality or serious injury seems possible somewhere. But what is a sinkhole and why have they suddenly started appearing so frequently. Essentially a sinkhole is any hole in the ground which occurs owing to drainage or water or erosion. They can vary in size from small and insignificant craters to sinkholes capable of swallowing multi storey blocks of flats or large trees. Often sinkholes are the result of natural processes but sometimes can be set off by human intervention. There are two basic types of sinkhole. Ones which form slowly over time known as ‘cover subsidence sinkholes’, and ones which appear suddenly known as cover collapse ‘sinkholes’. It’s the latter which are most dangerous and receive the publicity.
Sinkholes mainly form in what is referred to as ‘karst terrain’. That’s land where the bedrock is made of soluble material like gypsum or limestone which dissolves easily in water and particularly in water which is slightly acid. Acid rain in vast quantities might well have been the cuplrit in the M25 incident. Following severe flooding where the soil is continuously saturated the risk is at its greatest. ‘Cover subsidence sinkholes’ occur when the ground becomes exposed and has gradually been worn down over time. So there are plenty of warning signals. Observers can see the sinkhole develop over time as the shallow holes develop into sinkhole ponds. Much more dangerous are cover collapse sinkholes where the process develops out of sight. Naturally occurring cracks and small gaps under the surface become large hollows as a consequence of water erosion. The effect is at its worst when the holes become filled with water for weeks at a time. Freezing and thawing can also cause expansion and contraction of the water filling the holes causing the same effect that creates pot holes in the road surface. An undisturbed surface layer of soil however disguises the process until it’s too late and the sinkhole appears suddenly. Eventually the hole reaches such a size that the soil or sediment cover can no longer support its own weight and collapses into a deep hole. If it’s a location where cars are parked regularly the chances are that the collapse will occur whilst a vehicle is parked on it and he car will disappear completely into the sinkhole
Most of the sinkholes which appeared after the severe flooding of 2013/2014 are the result of human activity. But it’s not a lucky coincidence that the holes have been appearing immediately adjacent to houses but with the house itself apparently surviving narrowly by good fortune alone. The sinkholes occur where the rain water is concentrated on a defined patch of surface area, where water run off from roofs lands. So the house itself is safer from being completely engulfed in a sinkhole which might suddenly appear immediately beneath it, than might be apparent. The hole which does suddenly appear is a serious risk to the stability of the house itself or an immediate risk to any car parked on it but there is at least time for the occupants to become aware of the sinkhole long before the risk of the house falling into it becomes significant
Most of the most serious sinkholes which appeared in 2013/14 were in the South East of England. The reason for it is that the ground in the region is naturally comprised of soluble chalk. And the South East has experienced its wettest Winter for decades. The Tames Valley (which is a flood plain) has experienced flooding beyond anything anyone has seen in their lifetimes. And it’s not commonly appreciated that London itself is built on a flood plain with the newer recently built Eastern part of the city actually built on an Estuary. But sinkholes are not quite as dangerous as they look. The tendency even in the worst affected part of the South East has been for the sinkholes to appear a few feet away from locations where they would have caused serious damage and this characteristic allows time for any more serious risks to be dealt with. When the sinkhole appears it grows to its full size almost immediately. But sinkholes are by no means safe and it’s only a matter of time before one causes serious injury. In Guatemala City in 2010 a sinkhole appeared which swallowed up a three story factory killing 15 people. It appears the sinkhole may have been caused by a variety of things including leakage from a local sewerage pipe and vast amounts of water for the Tropical Strom Agatha. In Florida in 2013 a sinkhole suddenly formed within a bungalow and consumed 37 year old Jeffrey Bush, who was sleeping at the time. Mr Bush’s body was never recovered. The deepest known sinkhole was in Xiaozhai tiankeng in China. Tiankeng is the local name for sinkholes and translates as ‘heavenly pit’. The Chinese sinkhole in question, located in the Chongqing region, is 626 metres wide and 662 metres deep. In Croatia a 530 metre deep sinkhole with near vertical walls appeared
Uses for Sinkholes
Sinkholes have been is use for centuries as waste disposal sites, so there’s no reason why more recent ones shouldn’t serve the same purpose. Difficulties however arise with pollution of ground water resources. On the Yucatan Peninsula the Maya civilisation uses sinkholes to deposit their human sacrifices and precious artifacts. Very deep sinkholes can be connected to cave systems and are regarded as challenges by cavers and divers. Sinkholes are also known to form in coral reefs and so called ‘blue holes’ (islands that collapse to great depths) are popular diving spots