A coal legacy – protecting the environment
Deep coal mining in the UK might have ceased, but landscapes are still dominated by its legacy. Eric Burgess*, from the Coal Authority, examines how to keep environments safe.
In the UK, the Coal Authority is charged with protecting people and the environment from coal mining heritage, and safeguarding the public and property from the impacts of ground movement, shaft collapse, and gas and water emissions. Much of this work is out of sight, but one aspect that is very much in the public view is the management of colliery spoil tips.
There are over 5,000 coal waste tips in the UK, with over 1,200 in South Wales alone. The Coal Authority manages 40 disused tips and advises local authorities and landowners. In 2016, it was awarded a contract to inspect 28 disused tip and quarry sites for Natural Resources Wales. It has now been awarded a second contract to inspect a further 37 sites.
The Coal Authority is also in the third year of an inspection programme for the Lake District National Park Authority, inspecting a series of tailings dams and tips associated with the former Greenside Lead Mine near Glenridding.
Part of this work includes the authority’s public safety and subsidence team, which has a programme that includes regular walkover inspections, surveys, and the monitoring of groundwater. This is supported by a code of practice.
A need to change
This is a far cry from the past, when excavated waste was simply dumped near to collieries with little structural management.
But, all that changed following the Aberfan tragedy in Wales, when the hazards of coal tips were tragically illustrated in October 1966. A total of 28 adults and 116 children died after one of the spoil heaps at the Merthyr Vale Colliery slipped down the hillside and engulfed the village below.
Other changes in mining legislation – including the EU Mining Waste Directive and the Mines Regulations 2014 – also specify careful management. This means local authorities and landowners who are responsible for tips have to identify all sites that could cause harm.
Although many tips are carefully contoured and enhanced to fit into the immediate landscape, they can still cause problems to the local environment. For example, erosion can result in water and airborne particles polluting the environment, particularly in tips and tailings from former metal mines. This pollution can also end up a long way from the original tip source.
Aside from monitoring, sometimes physical intervention is necessary. This may include tip landscaping and the installation of additional drainage to prevent water contaminants running off site. Drainage routes can be improved and, where necessary, are canalised with their banks being stabilised in an effort to control erosion. They are often planted with vegetation to prevent the generation of dust within local neighbourhoods.
Sites are also inspected for signs of instability and movement, such as slips, erosion, heave, and fissures, as well as the effectiveness of the drainage infrastructure, any evidence of spontaneous combustion, and public safety issues including trip hazards, falls from height and areas of deep water.
However, one of the biggest risks is spontaneous combustion beneath the surface. This issue was highlighted when the Coal Authority was awarded a contract to deal with a burning spoil tip in Midlothian, Scotland. The Kirkhill Tip forms part of the popular Gore Glen Woodland Park, at Gorebridge, and comprises coal waste from the former Arniston Colliery, which closed in 1962.
It caught fire due to spontaneous combustion and was burning both above and below ground. Not surprisingly, it made the area highly unstable and forced the owners, Midlothian Council, to exclude the public from that area of the 108-acre park.
Known as bings in Scotland, the Gorebridge tip overlays several coal seams and lies in a very awkward place between the busy A7 road and the Borders railway line. The burning ground made the area unsafe and resulted in fires, smoke, and falling trees.
In order to assess the extent of the burning, engineers used thermal imaging scanners to determine a general location, and then temperature probes to obtain more specific measurements regarding the depth and spread of the problem.
A number of options were considered but the most viable was mechanical excavation and turning, to cool the burning material.
Full-time site supervision was provided, with work taking eight weeks. Firstly, a haul road was constructed through the trees and around the back of the area on fire – allowing plant access to the specific location.
This was followed by the controlled excavation of the face of the bing, with the removal of the hot material. During excavation, the burning was expected to spread, as the airflow to the material increased, so a project management schedule was put in place.
A 22-tonne long-reach excavator was used to load the material into dump trucks, before it was placed onto a cooling area of wet clay, which was regularly sprayed with water.
The depth of the burning was an unknown factor, so the team had to check the spread of the fire with thermal imaging cameras and temperature probes. Five hotspots were identified, with temperatures of 300°C recorded at 1m below the surface. The hottest temperatures recorded were upward of 400°C.
Excavation went down to a depth of 5m, and due to the amount of material excavated – over 1,000m3 – the entire tip face was regraded to ensure the area is safe for future use.
A 300mm thick layer of clay, which acts as a barrier to help prevent the flow of air into any cavities, was used to minimise the chance of the area heating up again. The haul road, which linked the site compound to the working area, was backfilled with material created by mulching down the felled trees from the area and was fully reinstated to allow public access.
Coal Authority Project Manager Eric Burgess said, ‘We know from experience that over-heating colliery spoil tips are always difficult to deal with, but the work at Gorebridge tip has been a real success story. It might be a former tip, but it has reverted back to its role as a popular and valuable local amenity as a great wildlife habitat that is enjoyed by locals and visitors alike.’
Darren Bryant, Principal Project Manager at the Coal Authroity, added, ‘With the advent of climate change, the management of disused tips is becoming more and more important. Extreme rainfall events can cause significant problems on disused tip sites, and proper inspection and management is essential to ensure that the public and the environment remain protected.’
And so, even though the process of deep coal mining has ended, the safety of local landscapes is as pertinent as ever.
*Eric Burgess is a Principal Project Manager for the Coal Authority. He provides all aspects of commercial advice to other departments across the organisation.