What causes sinkholes and where do they occur in the UK?

Areas prone to sinkhole formation occur throughout the UK, although most are relatively small or are in upland rural locations. They include areas underlain by Carboniferous limestones, notably the Mendips, parts of Wales, the Peak District, and the northern Pennines including the Yorkshire Dales.

The Chalk Group is also susceptible, especially where it is covered by younger clay and sand deposits (such as the Clay-with-Flints Formation and other Palaeogene strata), notably in parts of Dorset, Hampshire and the Chilterns.

The most susceptible area in the UK is the Permian gypsum in north-east England, particularly around Ripon and areas underlain by a similar geology. In Ripon, many large sinkholes have developed, some of which have affected property and infrastructure. This is because gypsum is far more soluble than limestone and dissolves more rapidly.

Sinkholes also occur over salt deposits, commonly in areas such as Cheshire where brine has been extracted, making it difficult to separate naturally formed sinkholes from those created artificially.

In Scotland, sinkholes are generally rare except in parts of Assynt that are underlain by limestones of the Cambrian Durness Group.

Several things can trigger sinkholes. Whilst the process of gradual dissolution can cause a sinkhole to form at the surface, other factors, including humans, can induce sinkholes to form.

• Heavy rain or surface flooding can initiate the collapse of normally stable cavities, especially those developed within superficial deposits.
• Leaking drainage pipes, burst water mains, irrigation or even the act of emptying a swimming pool are all documented examples of sinkhole triggers.
• Construction and development modifying surface drainage or altering the loads imposed on the ground without adequate support can cause sinkholes to develop.
• Groundwater abstraction can cause sinkholes by changing the level of the water table. This removes the buoyant support water provides to a cavity. Draining these cavities can cause them to collapse.
• Mining can be a factor in causing sinkholes, either by dewatering and lowering of the water table or by intercepting clay-filled voids, which subsequently collapse. Several sinkholes in Norwich have been caused by old chalk mines intercepting otherwise stable, sediment-filled voids.

Sinkholes form as a consequence of the recharge, through-flow and discharge hydrology of hydrogeological systems associated with soluble rocks and lead to karst (landscapes that result from the dissolution of soluble rocks). Our researchers are interested in a number of aspects of karst, including karst processes, dating, landscape evolution, geoconservation and the implications for applied geology, including groundwater resources and engineering geology.

Sinkholes can occur rapidly and apparently without warning. It is therefore important to:

• consider the potential for sinkholes during planning (hazard susceptibility studies)
• characterise them, so that people can mitigate with a reliable understanding of the likely 3D shape and size
• provide information for emergency response

The hazards associated with sinkholes can be mitigated by:

• appropriate planning
• good site investigation (with geophysics and boreholes)
• appropriate design
• proper maintenance of infrastructure, such as drains and services

Care is required when installing any structures that could affect the local groundwater flow or groundwater levels, including soakaways (sustainable drainage systems or SuDs) and open-loop ground-source heat pumps. In some places, these may be impractical in soluble rocks.