Characteristics
After the Second World War, there was a shortage of suitable building materials. In coalfield areas, there was a surplus of colliery waste, which provided (what appeared to be) a suitable substitute for crushed stone and other fill materials. Older colliery waste tips often contained a significant amount of coal, in some cases up to 10 per cent.
If tipped from overhead buckets, the spoil was not consolidated but instead allowed air to circulate, oxidising any pyrite (iron sulphide). This oxidation caused a temperature rise sufficient for the coal content to start burning. Spontaneous ignition of spoil tips resulted in burnt or partially burnt spoil with changed physical and chemical characteristics. Changes to tip material included the fusion of large volumes into a brick-like substance and the release of fumes.
In 1967, an estimate suggested that, out of more than 2000 colliery spoil tips owned by the then National Coal Board (NCB) across the country, over 50 per cent were burning and a further 15 per cent were burnt out. It is this residual burnt colliery spoil that is known as burnt colliery ash, burnt ash, red ash or red shale (Parry et al., 2019).
Burnt colliery ash can have variable geotechnical properties that can be affected by natural processes such as flooding and freeze–thaw.
Hazardous properties
The main hazard associated with burnt colliery ash is the high calorific value of the residual coal in the waste, potentially resulting in an underground fire and localised land stability issues. It also contains high sulphate levels, which are aggressive to concrete.
Pathway
The main concern is overlying structures catching fire or ground settling and damaging overlying structures. The material can cause corrosion and deformation issues with foundations and floor structures when it is in direct contact with concrete.
Occurrence
Red (burnt) shale was used extensively as a sub-base material throughout Gateshead in the 1950s to the 1970s, particularly in schools and 1960s flats developments. Use ceased when it was found to be unsuitable as a sub-base material due to its associated geotechnical properties (poor freeze–thaw characteristics) and contamination issues.
Natural occurrences
Burnt colliery ash is an artificial deposit.
Site investigation
Desk study
Historical maps and local sources of information, such as local newspapers, will indicate where land was backfilled to level. If made ground is identified and the work took place in the two to three decades after the end of the Second World War, then burnt colliery ash may have been used. Sites that were developed when in public ownership between 1945 and the 1970s may have relied on this alternative to traditional fill materials.
Walkover
Underground fires could cause elevated surface temperatures and unseasonal drying up of the soil, leading to distressed vegetation.
Intrusive site investigation
The detailed logging of samples from trial pits or boreholes will indicate the likely presence of burnt colliery ash. Testing should include calorific value and loss on ignition (LOI) tests.
Foundations
Development cannot proceed safely above an underground fire.
Remediation
Pathway interruption
Sometimes it is possible to smother a fire but often, such fires burn for a long time, spanning months to many years.
Source removal
Excavation of either smouldering or unignited but high calorific-value backfill and pre-treatment to remove combustible material are possible in certain circumstances.
Waste disposal
High calorific-value colliery waste may need pre-treatment before it can be disposed of in a landfill.
Regulatory aspects
There is a cap on the loss on ignition of 10 per cent for waste to be accepted at a landfill (Environment Agency, 2020).
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.)
Environment Agency. 2020. Disposal of waste to landfill. (Bristol, UK: Environment Agency.)
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