Characteristics
A large proportion of Gateshead that developed during the 20th and 21st centuries was built within an area affected by coal mining. The extent of this area is reflected by a 'Development High Risk Area', defined and published by The Coal Authority.
Many parts of Gateshead have only a thin covering of superficial geology (e.g. glacial deposits of clay, sand and gravel as well as river terrace deposits of similar materials) and a shallow (<30 m) depth to the bedrock geology (rockhead). These geological characteristics are often found along with the presence of shallow coal mine workings, which have created a legacy of underground voids and broken ground where coal has been extracted.
In addition to workings, the mining inventory includes old mine entries (both recorded and unrecorded), shafts (vertical) and adits (horizontal). Many of these historical entries have not been remediated, or the nature and degree of remediation is uncertain. There are also areas of former opencast coal mining.
Hazardous properties
Mined-out areas, including those around the Carboniferous Coal Measures strata, present various hazards, such as:
- hazardous ground gas — main gases include:
- methane
- carbon dioxide
- carbon monoxide
- hydrogen sulphide
- potential ground instability in areas of shallow workings and over and around mine entries
- spontaneous combustion of shallow seams or workings that have been backfilled with high coal-content waste materials, which have the potential to naturally combust and ignite
Pathway
Development in locations that fall within The Coal Authority's Development High Risk Area should be evaluated for vulnerability to coal mining-related hazards. Planning applications for most types of development within such locations will need to include a Coal Mining Risk Assessment in the supporting documentation.
Mine-gas pathways may exist as a result of the present condition of the site but they may also be influenced or created by site investigation, remediation and construction activities.
Occurrence
Gateshead is underlain mainly by glacial deposits overlying Westphalian Coal Measures. The Coal Measures comprise a repeating sequence of sandstone, mudstone, siltstone and coal seams, many of which have been mined to various extents and by a variety of methods since the 16th century.
The Coal Authority web viewer shows the extent of the Development High Risk Area in Gateshead. Please note that, whilst this viewer provides a broad indication of the extent of the Development High Risk Area, it should not be relied on for technical purposes. Gateshead Council will be able to confirm if your site lies within the Development High Risk Area.
The Development High Risk Area is based on historical records of coal mine features that have been digitised, but it should also be considered that stability and mine-gas risks can occur outside this area and may be influenced by other factors, such as local geological conditions. The Coal Authority's website provides further information on the types of development and planning application that need to be supported by a Coal Mining Risk Assessment.
Any location defined as a Development High Risk Area contains one or more recorded coal mining-related features that have the potential to cause ground instability or risks of mine gas. Such features include:
- mine entries
- areas of shallow coal workings (recorded and probable)
- recorded coal mining-related hazards
- recorded mine-gas sites
- fissures and breaklines
- previous surface-mining sites
Where a Coal Mining Risk Assessment is required for development within the Development High Risk Area, its purpose is to demonstrate that the site has been, or can be, made safe and stable for the specific development proposed, taking full account of former coal-mining activities. Responsibility for ensuring the safety and stable development rests with the developer and/or landowner.
Localised subsidence of the ground due to coal mining has led to the complete demolition of a small number of properties in the Gateshead area.
Site investigation
Desk study, walkover and intrusive site investigation
The Law Society of England and Wales recommends a CON29M report for sites in Development High Risk Areas. Such a report can be purchased directly from The Coal Authority and a limited number of alternative suppliers. This report will confirm the coal mining features that are recorded as being present on the site.
Please note that, for most forms of development and most types of planning application, The Coal Authority and Gateshead Council will require a Coal Mining Risk Assessment to be submitted with the planning application. The report will help to inform the Coal Mining Risk Assessment, but it is recommended that a variety of other source data is also used.
Comprehensive guidance on abandoned mine working has been produced by CIRIA in the Abandoned Mine Workings Manual (C758D).
Detailed consideration of the contents and purpose of a CON29M report is outlined by The Coal Authority in their Coal Mining Risk Assessment.
Foundations
Foundations may need to be designed to protect properties against subsidence associated with coal mining features including, but not limited to, mine entrances, shallow workings and fissures and break lines.
Remediation
Where the findings of any intrusive site investigations identify that coal mining legacy on the site poses a risk to surface stability or human health from ground gas, a detailed remediation scheme to protect the development and occupants should be submitted to the Local Planning Authority for consideration and approval. In cases where high walls or mine entrances are found, a no-build zone around the legacy feature is likely to be required.
Pathway interruption
A gas membrane is needed to prevent ingress of methane and other hazardous ground gas.
Source removal
- Grouting and/or land stabilisation.
Waste disposal
Not relevant.
Regulatory aspects
Land stability and human health are material considerations under national planning policy. Development proposals falling within the Development High Risk Area will require a Coal Mining Risk Assessment. The Coal Authority is a statutory consultee for planning applications made to Gateshead Council that are within this area.
Glossary
Breakline: a vertical step in the rock or earth, created when underground mining has caused differential settlement at the surface. Breaklines are usually associated with geological features such as fault lines.
Differential settlement: a structural engineering term that indicates the ground and building foundations settle in an uneven fashion, leading to structural damage.
Fissure: a crack or opening in the rock or earth, created by mining in certain circumstances.
Rockhead: the upper surface of bedrock: may be at surface or buried below superficial geology such as tidal flat deposits of sand, gravel, clay and silt.
References
Coal Authority summary of Development High Risk Area
Parry, D, and Chiverrell, C (editors). 2019. Abandoned Mine Workings Manual C758D. (London, UK: CIRIA.)
Document contact
Dr Darren Beriro: darrenb@bgs.ac.uk