Sinkholes and Karstic Risk

Sinkholes received a significant amount of coverage in the media in February 2025 due to the sudden appearance of a sinkhole (approximately 20m long, 5m wide and 5m deep) in Godstone High Street, Surrey which led to the evacuation of 30 households.

What is a Sinkhole?

A “sinkhole” is generally defined as “a saucer-shaped hollow that is the result of some kind of collapse or removal of an underlying layer of rocks that used to support the layer of material at the surface.” (BGS)

How do Sinkholes Form?

Most sinkholes result from natural geological processes, generally involving water, where rock or soil from below the surface layer is removed leaving the surface layer unsupported.

In areas underlain by a carbonate bedrock (chalk/limestone/dolerite), salt deposits (halite) or other soluble rocks such as gypsum, percolating water dissolves the rock over time leaving underground fissures and voids which can suddenly collapse.

What can cause a Sinkhole?

Sinkholes can appear rapidly and without any apparent warning and so it is very important that consideration is given to the potential for sinkholes to form at the planning stage to facilitate a thorough investigation and allow the mitigation of any associated risks.

The gradual process of dissolution can cause a sinkhole to be form at ground level, but due to the long timescales involved these sinkholes may have historically yielded a surface depression which has been subsequently lost through the infilling of surface hollows.

Rapid sinkhole appearance is generally tiggered by one of the following causes:

• Heavy Rain/Flooding – can be responsible for the collapse of cavities which are normally stable, particularly if the cavities are developed within superficial deposits;
• Water Leaks – Leaking drains, burst water mains and irrigation can all trigger the formation of a sinkhole with burst water mains commonly cited as the cause of sinkholes in media reports;
• Building Works – Changes to surface drainage, including the construction of soakaways, or changing the loading of the ground by building or cut/fill works can all trigger sinkhole formation;
• Changes in Water Table – groundwater provides buoyant support to cavities and so lowering of the groundwater level for excavation dewatering or water abstraction can cause these cavities to collapse as the voids empty of water.
• Mining/quarrying – can trigger sinkhole formation through:
  • Dewatering
  • by intercepting clay filled voids which subsequently collapse;
  • the collapse of shallow open tunnels, often triggered by heavy rain or water leaks.

Different types of Sinkhole

There are three main types of sinkhole:

Dissolution sinkholes

Formed by the chemical weathering of the underlying rock where surface water drains through a fissure/joint in the rock which over time gets enlarged with the subsequent hollow being drained through the fissure/joint.  These depressions tend to form gradually;

Collapse Sinkholes

These form where the gradual collapse of an underground void occurs and triggers subsidence at ground level although these sinkholes can form rapidly as a response to the rapid influx of water.  Depending on the depth of the original void, these collapses may or may not show a surface depression;

Suffofusion Sinkholes

These sinkholes form where dissolution causes a depression on the bedrock surface beneath a covering of soil.  The unsupported soil then subsides forming a depression at ground level.  These features tend to form gradually if the covering material is sandy, but can appear suddenly where the material is more cohesive which can allow quite a large cavity to form before collapsing.

Where do Sinkholes Form?

Natural sinkholes most commonly occur in areas underlain by soluble rocks which form karst and which include salt deposits (halite), gypsum, chalk, limestone and dolerite.

More information on where Karst is present can be found in the recently published BGS Karst Report Series.

Sinkholes associated with classic karst landscapes form in areas underlain by Carboiniferous limestone including the Mendips, parts of Wales, Peak district, northern Pennines and Yorkshire Dales.

Areas underlain by the Permian gypsum deposits in northeast England are particularly susceptible to the formation of sinkholes, with the Ripon area being particularly prone to large sinkholes.

Areas underlain by salt deposits are also susceptible although in these areas brine extraction make it difficult to separate natural from manmade sinkholes.

Finally, the Chalk deposits underlying much of the southeast are prone to the formation of sinkholes, with up to 97% of Chalk sinkholes occurring where there is a younger cover deposit over the White Chalk with areas in the vicinity of the Chalk/Palaeogene boundary being of particular concern.