Sinkholes in Ireland
Geology Matters No. 14
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Sinkholes in Ireland
Dr Caoimhe Hickey, Hydrogeologist, Groundwater GSI

What is a sinkhole or doline? A sinkhole (or doline as it is more commonly called outside of North America), is a natural enclosed depression found in karst landscapes. Karst landscapes are formed in rocks that can be dissolved in acidic water. This is usually on carbonate rocks, such as limestones but karst can develop on evaporitic rocks and siliceous rocks such as quartzite. Approximately half of the Republic of Ireland is underlain by Carboniferous Limestone and consequently karst is an important aspect of the Irish landscape.

Dolines are the most common landform in karst areas. They are described as small to medium sized closed depressions, ranging from metres to hundreds of metres in both diameter and depth. Once created dolines function as funnels, allowing the direct transmission of surface water into the underlying karstic bedrock aquifer. They may occur as isolated features or in clusters causing a pock-marked land surface.

How do dolines form? Dolines are formed by two main methods: the slow solutional removal of rock from the surface downward (solution doline), or by the collapse of overlying rock or overlying material into an underground cave or chamber (collapse doline). Most dolines are considered polygenetic in origin and are usually formed from a combination of solution and collapse, however one of these processes usually dominates their appearance, whether it is catastrophic or gradual.

Solution dolines form in places such as vertical or near vertical joint intersections. Surface runoff will focus at these areas of weakness, leading to the solution of the bedrock. Water and solutes will then move downwards through the bedrock openings (such as at joints and bedding planes). The result is a funnel-shaped depression on the surface. Solution dolines are considered to be formed by a gradual process of sagging or settling of the overlying deposits into the hollow left by an area of dissolving rock. Solution dolines are usually characterised by gentle slopes with no obvious rupturing of the soil or surface.

Collapse dolines usually occur very suddenly where the bedrock or subsoil material collapse into an underlying void. Cover collapse dolines (sometimes known as dropout dolines), which are very common in Ireland, occur in karst areas covered by unconsolidated material, such as glacial till. They form by the sudden downward movement of the overburden and usually form in areas where the overburden is somewhat cohesive. They occur in a process called 'piping', where a soil or subsoil arch, which has formed due to removal of material at the bottom of a layer of overburden, suddenly gives way (White, 1988). Although there must be a highly efficient pathway established for sediment transportation in order for the soil arch to form and grow, a large bedrock hollow is not necessary for their development. Cover collapse dolines are characterised by vertical or steep-sided collapses, with a very sharp break in slope and often have stepped sides, where soil is exposed. Over time, however, their slopes may degrade and infilling sediment may build up giving these dolines the morphology of solution dolines (Ford and Williams, 2007).

Solution Doline Formation   Solution Dolines in Co. Roscommon
Solution Doline Formation (Jennings 1985)                                Solution Dolines in Co. Roscommon


The Formation of a Cover Collapse Doline. A) Solutional openings in the bedrock wash material downward, B) A small arch forms in the subsoil where the material is being washed away, C) The void grows in size as more material is being downwashed until it reaches a critical point and starts to rupture, D) The arch suddenly collapses as it can no longer support its own weight, E) Overtime the vertical sides will degrade and the hole will become less deep.

 Collapsed pipe failure

A small recently collapsed pipe failure. Photograph shows caver's ladder going down into opening.

 
 Caver entering collapse feature  

This shows the same collapse feature with a caver going down the ladder inside. Note the void and the neck of subsoil material about half way down.

 Same collapse feature from the base  

This is the same collapse feature from the base of the void in the bedrock. Again, note the subsoil neck about half way up, the void and the small grass opening.

Bedrock collapse dolines are caused by collapse of bedrock into an underlying void. They are commonly due to the collapse of the roof of a cave and can be catastrophic. Although solution is a significant process driving the creation of these dolines, they are principally due to mechanical processes, although the collapse must be preceded by sufficient solution of the bedrock to form a void into which the material can fall (Williams, 2004a).

They can also be caused by lowering of the water table removing buoyant support. They are usually characterised by a high depth to width ratio and vertical bedrock sides, although in time they may degrade and infill. These are less common in Ireland.

If you are worried about sinkholes in Ireland, or have additional information about sinkholes or potential karst collapse features please contact The Groundwater Section at The Geological Survey of Ireland groundwaterinfo@gsi.ie

 Pollelva doline
Pollelva doline in Co. Clare. Photo: Colin Bunce,
Clare Caving Club
 Bedrock collapse
Bedrock collapse Doline Formation (Jennings, 1985)


Dolines near Athenry
Sinkholes or dolines, after heavy rain, near Athenry Co. Galway. Photo by Tobin Consulting Engineers.

What is the likelihood of the ground opening up underneath me?
Sinkhole in Guatemala City
Sinkhole in Guatemala City, May 29th 2010. Luis Echeverria, U.S. EPA

Catastrophic events of buildings falling down into large collapse features, such as the recent sinkhole in Florida, are extremely rare. However, there have been reports of minor collapses of roads and fields in Ireland. These are found in karst areas prone to solution and many landowners fill these in on a regular basis.

 Cow in sinkhole This photo is taken from a field in North Cork. The photograph shows a rescue of a cow that was trapped at the bottom of a cover collapse doline as it suddenly opened up from under her.
Source: O'Brien
There are estimated to be over 6,000 dolines in Ireland. The Geological Survey of Ireland host maps that can help you to determine if karst development in likely in your area. Firstly, the Bedrock Geology Map of Ireland (available in 1:100,000 scale) will show you if you live in a limestone area. There is also a karst landscape database, which will show you some of the recorded landforms in your area including dolines. These datasets are available for viewing on our interactive public viewer site or download from here.

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