Many neglected and abandoned industrial landscapes are being returned to commercial use. They include Britain’s former coalfields where identifying and systematically removing risks linked to unstable spoil tips, toxic waste, mine gas, contaminated ground water, and surface subsidence is releasing valuable commercial land.
The UK’s five traditional mining regions are the Midlands and Pennines Coal fields – also known as the Derbyshire and Nottingham coal fields; the Durham Coal Field; the Scottish Coal Belt; the Kent Coal Field … and the region I want to focus on here … the South Wales Coal Field.
Coal is extracted using various underground and surface mining systems. In south Wales, most mining was traditionally carried out underground from shafts (mainly vertical), and horizontal adits or drifts.
However, the specific legacy of mining I want to consider now is subsidence and the data-based calculations we make, plus extensive field work we carry out, to prevent damage to existing properties and new developments.
Think of a number …
According to a March 2022 report by The Law Commission for England outlining a new coal tip safety regime for Wales (https://www.lawcom.gov.uk/recommendations-to-improve-safety-of-coal-tips-in-wales/), there are just under 2,500 discarded coal tips of which a “… small minority have the potential to be a danger.” A particular goal is to minimise increasing risks caused by climate change.
However, even when tip spoil has been hauled away for reuse, inherent site hazards can remain and I looked at these in another recent post – ‘Coal fields – from black legacies to green futures’ (Coal fields – from black legacies to green futures (enzygo.com)).
Enzygo in Cardiff
Today, however, as well as looking at the subterranean causes of subsidence, I would also like to explain how Enzygo Geotechnical Ltd.’s ten-strong and expanding team of engineering and environmental professionals – with a combined experience of more than a 100-years – is helping clients to eliminate or remediate coal-related risks that are a serious development barrier.
We will be using this experience, expertise and local knowledge through our newly-established dedicated Cardiff base to work very closely with the Coal authority, clients, and specialist contractors to deliver workable development site solutions for historical coal-related problems found on site.
In a moment, I will give examples of the basic risk calculation we carry out to determine quickly whether individual sites are safe or vulnerable to subsidence damage, how we follow this up with physical on-site investigations, and finally remedial steps that can be taken where real threats are found. But first, it is important to define how subsidence risks are created.
How coal was won
To understand the complex land rehabilitation and restoration problems mining has left, it is important to understand the different ways in which coal is extracted as this is the root of many of today’s risks.
For hundreds of years, south Wales’s ‘black gold’ came from small-scale ‘bell pits’ and surface outcrop workings which started in Roman times and continued until the medieval period and 15th Century when the first adits, drifts and shafts were used. These can still be a danger today.
Coal at this time was used to fuel local industries, and the main colliery sites were opened up in Pembrokeshire and Swansea with transport systems to move coal from the valleys to the coast.
Technology and expansion
It was not until the 16th and 17th centuries, industrial revolution, and invention of machines/steam power and railways, that coal mining really took off in south Wales when charcoal conveniently gave way to coal as a smelting fuel.
The high quality of south Wales’s coal – which contains anthracite prized for burning more efficient, is smokeless, provides a consistent heat, and can be fed easily from gravity hoppers into boilers – was also important.
A place on the global map
By the 1820s, the then small backwater of Cardiff became the world’s second largest coal exporter after Barry Docks. The Earl of Bute’s estate owned both and funded their development via a massive infrastructure investment to connect his coal fields licenced to collieries in the Cardiff Valleys with the new dock facilities, and expanding export markets.
At a mid-1920s peak, there were more than 300 south Wales coal mines; after nationalisation in 1947, this was recorded as 338 coal mines. But for historical reasons this expansion did not continue.
The last pit fell silent in the 1980’s; most were closed between 1921 and 1936. In total, 241 were shut with the loss of 140,000 pitmen’s jobs. Tower colliery – the oldest continuously working in Britain – struggled on until 2008, owned and run as a cooperative and museum by miners.
There are still important exceptions however, including Glyn Neath which excavates 3.5 million tonnes of anthracite annually and has reserves of more than a million tonnes.
Evolution of mining techniques
Early mining after the shallow bell pits initially used a pit and stall system, with coal excavated by hand and workings supported by wooden props that were sometimes replaced by coal pillars. This left un-extractable valuable coal behind, but is the basis of room and pillar mining used widely around the world today. Redundant pillars are often extracted later.
A more efficient development is underground advancing or retreating longwall mining where a mechanical coal cutter, or shearer, runs back and forth between two parallel service roadways – or gates – 250 metre apart. These are driven at right-angles off a main haulage road. Cut coal drops onto a conveyor and is carries out via one gate to be taken by shaft or adit to the surface.
A system of expensive hydraulic props shuffles forward to protect the cutter and miners – leaving the unsupported roof to collapse gradually into the empty mined space, or gob.
Retreat mining has the advantage of leaving unstable mined ground behind. It starts with gates driven first to the furthest extent of the area to be mined. This can total several kilometres, which is one reason why the surface area vulnerable to subsidence can be so extensive. The longwall face shearer starts at this far point and works its way back to the safety of the main haulage road.
Seam must be thick enough to accommodate the cutter and props, and deep enough to avoid future subsidence – or void migration to the surface – as mined out workings collapse. This system was used historically in the Albion and Celynen South collieries near Newport.
Underground mining can present numerous risks, including combustible methane gas, collapse, sealed off exit routes, and flooding. Some mines closed for good after serious accidents. Killan colliery near Swansea was an example where a number of miners lost their lives after being trapped behind flooded and collapsed workings. The mine closed shortly after.
I mention this particular case because Enzygo Geoenvironmental is currently providing a mining assessment of this colliery so the pit head area can be developed safely.
A new south Wales service
As well as Killan, Enzygo Geoenvironmental is currently providing consultancy advice on a number of historical south Wales coal mining schemes in the form of mining desk-based risk assessments, investigations, and remediation solutions appropriate to new developments.
It might be useful if I detail these here.
We begin with a risk assessment. After reviewing all available data sources, and obtaining the coal mining report, we compile a coal mining assessment that analyses all risks associated with former or present coal mining under a site. This interpretation is designed to confirm:
- The presence, or absence, of workings beneath a specific site
- Coal seam names, thicknesses and depths
- The thickness of rock above the coal horizon(s)
- The lateral extent of seams underneath the site
- The presence of any shafts within a 25m radius, or on the site itself
- And make a simple calculation to ascertain whether the site is at risk from any coal seams
This calculation is important.
Broadly speaking, it determines whether you have ‘10 x’ seam thickness of rock materials above the highest worked seam below you to ensure there is no possibility of any collapse at the surface through void migration.
An example might be a seam thickness of 1.0m (generally given in feet) with a solid rock thickness above the seam of 15m. Hence, a rock thickness (15m) multiplied by the seam thickness 1m (15 x 1) is more than the 10 x seam thickness minimum requirement.
If the calculated result is less than 10 x, there is risk that mining could affect the site, and grouting injections may be required to fill seams.
Once this calculation is complete, an investigation is needed to physically confirm the results. This involves applying for a license from the coal authority to undertake coal seam(s) investigation work.
The next step
The next priority is to probe the maximum depth of coal workings and confirm whether seams are actually present, if they have been worked, and the presence of any voids. For shallow workings, given the thickness of seams and superficial materials, the probes will go to a maximum depth of 40m below ground level. However, generally a shallower 30m below ground level is suitable.
In the initial coal mining assessment, we obtain a coal mining report. This may, or may not, identify shafts on the site. If they are present within 20m radius, they too will need to be proved and grouted.
Why is this important? The consequences of not checking are shown in the photographs!
Why shafts matter
If shafts are identified on, or within, the site, generally a 10-25m diameter ‘no-build’ exclusion zone needs to be set and recorded; the shaft’s location should also be noted and logged accurately.
Alternatively, shafts can be infilled with grout – although this can be expensive for large diameters. Other options are for them to be plugged at the rock head with a cement and grout plug, or capped with a reinforced concrete cap.
In some cases, all these measures are needed. Fortunately, in most cases only one or two are required.
I hope this has been a useful introduction. I also need to include a brief note on our parent company.
Enzygo (www.enzygo.com) is an independent planning consultancy and environmental consultancy which provides planning practice guidance (https://www.enzygo.com/planning/, support with environmental permitting regulations (https://www.enzygo.com/permits/), hydrology (https://www.enzygo.com/hydrology/), and landscape management (https://www.enzygo.com/landscape/) as part of a full suite of environmental services.
The work of the company’s teams of multidisciplinary specialists regularly includes ground investigations, flood risk assessments, landscape character assessments, environmental audits, environmental impact assessments (EIA), environmental impact statements, environmental management systems, traffic surveys, noise assessments, plus ecological impact assessments, habitat surveys, tree surveys and tree risk assessments.
If you would like more information, or think that discussing particular problems will be the best next step, then please contact me, Richard Hamilton, at Enzygo Geoenvironmental Ltd. All discussions are confidential.
You can reach me directly via Tel 0788 0197 002, or Richard.firstname.lastname@example.org.
Richard Hamilton, Director – Enzygo Geoenvironmental Ltd.
See the LinkedIn article – https://www.linkedin.com/pulse/de-risking-old-coalfields-new-economic-opportunities-enzygo-limited/