Site Investigation

Underground cavities

The national databases of natural and non-coal mining cavities held by Peter Brett Associates contain thousands of records of underground cavities of natural and man-made origin. It is estimated that at least 32,000 naturally occurring holes and over 15,000 non-coal mines or pits are located around the UK mainland. Natural or man-made cavities caused by historic mining activities or subsidence pose a geotechnical, environmental and hydrogeological risk to existing and proposed development.

The natural cavity database contains records of known dissolution features that have occurred in soluble rocks such as chalk and limestone. The dissolution of these rocks can lead to the formation of caves, karsts and other solution features. Natural cavities can also be formed by slope instability. Sometimes these features can become filled with loose material that can subsequently collapse upon inundation or disturbance, leading to the formation of a ‘sinkhole’.

Water penetration and erosion caused by leaking or faulty pipes or guttering or following periods of heavy rainfall, changes in climatic conditions or changes in the drainage regime during site development can trigger land instability and subsidence. The database records show that Kent, Essex, Berkshire, Buckinghamshire and Hertfordshire have the highest number of natural chalk cavities. Whilst limestone sink holes are most likely to occur in South Wales and the Peak District.

The non-coal mining cavity database records known cavities related to mineral working such as ironstone, gypsum, salt, sandstone, limestone, chalk, flint, metal and clay extraction. This can involve open-cast surface workings (existing or back-filled), large scale horizontal or inclined adits (passage or entrance to an underground mine) or vertical shafts or caves excavated by hand to facilitate small scale extraction, such as ‘deneholes’ in the chalk. The most likely cause of land instability are ‘crown holes’ which appear over mine shafts or surface workings.

Publicly available statistics (published by Dr Clive Edmonds of Peter Brett Associates in the Quarterly Journal of Engineering Geology and Hydrogeology, 2008) record at least 1678 confirmed flint mines and at least 1788 known chalk mines in the UK. More mines are suspected and new ones are discovered each year. The highest recorded number of chalk mine cavities are in Kent, Essex and Hertfordshire and the highest number of flint mine cavities are in Norfolk, Suffolk and West Sussex.

The mines are often very old and can pre-date historical Ordnance Survey maps therefore standard commercial desk study data reports may not highlight the risk. Developers need to be wary and should seek specialist help if their development is located in an area where hazards are likely to exist. Before any development is undertaken it is prudent to review the national databases in order to identify the potential for such features and the implications for the development. A specialist ground investigation may be required to assess ground stability. Remedial works may need to be undertaken in order to make the site developable.

RSA Geotechnics has undertaken numerous ground investigations, hydrogeological assessments and remediation strategies related to natural and mining related cavities.