Gateshead BGR_Calc Ground Risk Factsheet – Burnt colliery ash (mining waste)

Characteristics

After the second world war there was a shortage of suitable building materials. In coalfield areas there was a surplus of colliery waste that 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 %. 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 raise 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 fusing of large volumes into a brick-like substance and releasing fumes. In 1967, an estimate suggested that out of over 2000 colliery spoil tips owned by the then National Coal Board (NCB) across the country, over 50 % were burning and a further 15 % 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 is associated with 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. When the material is in direct contact with concrete it can cause corrosion and deformation issues with foundations and floor structures.

Occurrence

Red (burnt) shale was used extensively as a sub base material throughout Gateshead in the 1950s – 1970s, particularly on school 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 anthropogenic (originating from human activity) 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 2-3 decades after the end of World War 2 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.

Walk over

Underground fires could cause elevated surface temperatures and unseasonal drying up of the soil leading to distressed vegetation.

Intrusive Site Investigations

The detailed logging of samples from trial pits or boreholes will indicate the likely presence of burnt colliery shale. Testing should include the calorific value and/ or 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 or disposal in a suitable landfill 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% for waste to be accepted at a landfill (Environment Agency 2020).

Document contact

Dr Darren Beriro (darrenb@bgs.ac.uk)