Characteristics
Colliery waste includes materials that are removed to gain access to coal, such as:
- topsoil
- overburden
- waste rock
- tailings
Hazardous properties
Heavy metal and polycyclic aromatic hydrocarbon (PAH) contamination is commonly associated with colliery waste. The other major issue is the waste’s high calorific value as a result of high coal content and hence the potential combustibility of the colliery waste.
Colliery waste can have variable geotechnical properties that can be affected by natural processes such as flooding and freeze–thaw.
Pathway
Human exposure to contaminants is possible by ingestion, inhalation and dermal contact. Acid drainage from old workings can affect surface water quality.
Occurrence
Colliery spoil waste has been spread over many areas to backfill old quarries and opencast coal mining sites, and as base for railways and railway embankments. Colliery spoil was often used to make up ground levels for development, such as terraced school playing field sites. Colliery waste was also disposed of ('lost') by spreading it over greenfield sites that have seemingly never been historically developed.
There are currently (December 2020) at least two sites in the borough affected by long-term underground fires.
Natural occurrences
Colliery waste is an artificial deposit.
Site investigation
Desk study
Coal mining activities were generally well documented and detailed plans would have been produced in most cases. County or national archives may hold copies of large-scale plans and sections that would indicate where colliery waste was disposed.
Walkover
The walkover should consider how the surface morphology reflects the underlying material. Poorly compacted waste will have an uneven surface, areas of standing or poorly draining water and be obviously artificial. Standing or slowly flowing surface water may have low (acidic) pH and should not be touched. Evidence of underground heating, such as desiccated soil and burning smells, should be noted.
Intrusive site investigation
Naturally occurring materials often contain contaminants at elevated total concentrations but with very low bioavailability. If generic assessment criteria are expected to be exceeded, then detailed risk assessment may provide evidence to reduce the need for or extent of remediation.
Colliery waste may have high sulphur levels.
The geotechnical properties of the material should be evaluated in response to the proposed development and layout.
Foundations
Uncompacted colliery waste may pose hazards of differential or excessive settlement under load.
Remediation
Pathway interruption
Colliery waste that is geotechnically suitable as a load-bearing horizon can be isolated beneath a suitably thick capping layer.
Source removal
Colliery waste can be excavated, treated and placed under controlled conditions to provide a suitable building platform.
Waste disposal
Colliery waste and soils with a significant coal content that are removed from development sites are mostly hazardous waste due to high loss on ignition (LOI) or total organic carbon (TOC) values associated with the high coal content.
Regulatory aspects
No colliery waste-specific regulations exist.
Colliery spoil tips may be subject to mines and quarries legislation, much of which was introduced in response to the 1966 Aberfan spoil-tip disaster.
Glossary
Bioavailability: the proportion of a chemical released from soil that releases when it enters the human body. It is the fraction of the total concentration that can cause harm to health rather than being excreted.
Loss on ignition (LOI): an analytical geochemistry method used to measure the organic content of soil, determined by the proportion of the soil vaporised when a high and constant heat is applied.
Polycyclic aromatic hydrocarbons: (PAH) organic chemicals commonly found in artificial and some natural soils and rocks, which can be harmful to human health.
Tailings: waste material left over from the processing of valuable mineral deposits, including coal.
References
BGS. 2014. The nature of waste associated with closed mines in England and Wales. British Geological Survey Open Report OR/10/14. (Nottingham, UK: British Geological Survey.)
Parry, D N, The Coal Authority, and CIRIA (editors). 2020. Abandoned Mine Workings Manual. C758D. (London, UK: CIRIA.)
Document contact
Dr Darren Beriro: darrenb@bgs.ac.uk