Tech Talk - Of wood, coal, the UK and Bangladesh

Ice and snow have returned to the central part of Missouri, so the warm heat from the tile stove is again keeping us comfortable. For many folk, however, this is not an option and they rely on a centralized power station to supply the electricity that is a fundamental part of current Western life. Yet there are moves to use more wood, even there. In an earlier post I had written that Missouri S&T was switching from a coal:wood mix to a geothermal network which, with the use of natural gas, is expected to provide a net saving of about $1 million a year on the fuel bill. Price, while important to a university, is not, however, always the controlling factor when governments get involved.

The rising prices and obscurity of future government policy has stopped progress toward a wood-fired power station in Northumberland. A plan to replace coal with wood at Blyth has reached an impass, with RES ceasing work on the biofuel plant. The $500 million, 100 MW plant had been scheduled to come on line in about two-and-a-half years but has been stopped due to “ongoing uncertainty in UK energy policy.”

On the other hand the largest UK coal-fired power plant, at Drax in Yorkshire, is in process of changing from being a coal-based plant to one that burns wood. But not just any wood, for as David Rose notes the new fuel will be wood pellets, grown and processed in North Carolina and then shipped at an ultimate rate of 7 million tons a year to the UK. The current wholesale market price for power is around $83 per MW/hr relying heavily on coal, but the agreed price for the wood-powered electricity will rise to $174 per MW/hr, higher than that of either onshore wind or the new nuclear power coming on line. (Using $1.66 per English pound). Retail prices are somewhat higher.

Price may not be that critical in the UK, but it remains critical in poorer parts of the world, such as Bangladesh, where the nation needs to infuse power into a country that has, at the moment, only a single power plant. Yet this is not a move without criticism. A recent Op-Ed in the NYT, protested the intent of the government of Bangladesh to begin a program that will develop their coal reserves. The article comes after the government appointed a new minister for Power, Energy and Mineral Resource who has pledged a new coal policy “within the shortest possible time” and it is this (and the existing 2010 policy) which has irritated Joseph Allchin who wrote the opinion.

The major concern at present deals with the Rampal coal plant which will consume some 4.5 million tons of coal a year and generate 1,320 MW of electrical energy. The coal is presently anticipated to come from either Australia, South Africa or Indonesia and is intended to address the acute shortage of power in Bangladesh, with the government aiming to raise power generation from 5,000 MW in 2011, through 7,000 MW in 2013 to 22,000 MW by 2016, that being on its way to a capacity of 39,000 MW by 2030. By 2021 it is anticipated that 14 GW will be generated from coal-fired power, with domestic coal producing 6 GW, and imports powering 8 GW of capacity. The concern comes from the nearness of the coal-fired plant to the Sunderbans mangrove forest, and the threat which this poses. But given that millions of folk live within ten miles of coal-fired power plants around the world (the closest the plant will be) the dangers seem overhyped and unrealistic.

Figure 1. Relative location of the proposed power plant at Rampal and the Sunderbans (Yale)

A second power plant of similar size (1,200 MW) will be built at Matarbari although that will also rely on imported coal, at least initially (sourced from Indonesia, Mozambique, Australia or Canada) and

The government has also a plan to implement three mega coal-fired power plants at Moheshkhali each having capacity to generate 1200MW electricity under private sector or joint venture deals.

. Domestic coal production will require considerable growth in production, given that it was only at around 800,000 tons per year in 2011. The coal coming from the thick seams of the Barapukuria coal deposit has some 200 M tons of reserves, and is being won using longwall top caving, which simplistically involves undercutting the coal thickness with a shearer, and then allowing the overlying coal to fall into the mining opening.

Figure 2. Schematic showing the idea of Longwall top caving, there is a second conveyor at the back of the roof support to carry away the broken coal as it feeds down over the back of the support (University of Wollongong )

Bangladesh has struggled for years with less than half the country having access to electricity and with the rest of the population relying on biomass and waste to provide fuel for heating and cooking. But just to keep up with current demand it must increase natural gas supplies by 35% to overcome current shortages, and thus, to meet the demand for those without power they have chosen to go with the coal-fired option.

It will be interesting to see how the politics of this unfold, given the obvious benefits that will arise as more folk in Bangladesh are provided with electricity, with all the benefits that this entails, and which is being held up by those that one might have thought would have wished to see such progress.

In passing it might be noted that China approved an additional 15 coal mines with a total output of more than 100 million tons last year.

Tech Talk - The evil in the hearts of men.

It was Edmund Burke who said "The only thing necessary for the triumph [of evil] is for good men to do nothing," and with a hat-tip to Bishop Hill who drew my attention to the testimony, I think I had better say something.

Back in the days when I left school the United Kingdom, and most of Europe, was finally emerging from the brutal impacts of the Second World War. That recovery was helped immeasurably by the coal mining industry where thousands of men labored underground across Europe to ensure that adequate power was available to supply the needs of their various countries as industrial capacity was restored. As an eighteen-year old indentured Apprentice I laid on my back in 20-inch high coal to shovel the 15-tons of my ordained shift onto the conveyor, and braced the roof over my head with hand-sawn timber props to hold it for the following week, as we moved the longwall face forward. I was paid just over four pounds a week, but they did allow me, on Wednesdays, to go to the local Technical College in Ashington where I studied from 8 am until 8 pm. In the good weeks I could then go home, but in the alternate weeks I was either on foreshift, which meant that I went to college directly from the mine, or nightshift, when I went from the college to the mine and worked the following shift. I was woken more than once from an exhausted sleep having fallen into the corridor on the bus. Men died in those mines that I worked in to ensure that Britain had the power that it needed to recover and rebuild.

I thus find myself seriously affronted when this industry (and myself), are called evil by some academic from Oxford, no matter how illustrious his qualifications. In his testimony before the House of Lords Committee on “The Economic Impact on UK Energy Policy of Shale Gas and Oil “ last week, Professor Dieter Helm said

Of course one would want to make sure that regulators are on top of any environmental consequences that might flow from drilling, but I find it truly extraordinary that people want to ban fracking in a context where they are not prepared to ban coal mining, and indeed across Europe actually promote coal mining. When one thinks about the relative environmental impacts of the alternatives, coal mining is truly evil in comparison. I find it extraordinary that people are legally allowed to mine coal if you want at the same time to have a blanket ban on shale gas extraction.

It is almost a throw-away line, a necessary genuflection to the politically correct views of the day, a glib reference to transient effects long recognized and ameliorated by the coal industry over the past decades.

In those days of my youth, back in 1962, I walked into Newcastle to take the first bus of the morning from the terminal to the mine some 11 miles away at Seghill. It drove past pit heap after pit heap, and the impacts that Professor Helm is I believe referring to were real, and evident. The mine I worked in was over a hundred years old, and my father had been manager there when my brother was born. You walked stooped for part of the way to the face, since the roof of the passage was low, though the temperatures were pleasant, year round. Coal moved by conveyor and mine car, once hand-loaded from the blasted face, and supplies came in on carts hauled by pit ponies – another unforgettable memory. Hundreds of men worked at each mine, with a typical stint being a 10-yard length along the 200-yard measure of each longwall. We were not evil, the industry was not evil – it was vital and necessary. And it has changed.

Instead of walking there are now mine cars that haul workers to the face, electric trams have replaced ponies, and machines now do the work that muscle did back in those days. Where then is the evil? Is it that we are removing material from the ground? But unless you wish to go back to human densities of 70 people per hundred square miles of the hunter:gatherer era this must happen, for if they aren't grown resources must still be mined from the earth.

Yes there are transient effects, but if instead of dwelling on the ugliness of the excavation on some cold wet day when the shadows are right to emulate some hellish landscape, one were to go back five years after the mine has moved on (presuming that this is some surface site) then those wounds from current mines have gone. Laws require, and company self-interest demands, that land be restored. In parts of the world that can realize unexpected benefits as the land becomes more productive than it was before. You can no longer find much negative evidence of the underground mines in the area I worked in.

But the intent of this post is not to pat myself on the back, nor to run off on a rising rant, but rather to point out a problem which this broadly prevalent and stridently negative attitude is generating. Early this morning I received a call soliciting funds for the Footsteps Program at the University of Leeds, where I was awarded my degrees. But I had to gently break it to the young student calling that I could not participate because the University had done away with the Mining Engineering program through which I obtained my degrees. Further they have turned over the building, donated to the University by the industry and its workers for such studies, to the Art Department.

Leeds is not alone, the Royal School of Mines, merged now into Imperial College, London no longer offers mining courses.. The Welsh School of Mines, now the University of South Wales, no longer offers mining courses – there is but one place in the UK, at what was Cambourne School of Mines but is now the Penryn campus of the University of Exeter, where such a course still exists.

This image of the industry as an evil entity is ironic given, as Professor Helm pointed out:

Practically now, in Europe and the UK, we are switching from gas to coal. We have gone from about 28% of our electricity generated by coal a couple of years ago to about 40% today. Germany is bringing 7 to 8 gigawatts of new coal on to its system. Coal stations are being built across eastern Europe. The coal burn generally has gone up across Europe. Germany has gone from nuclear to coal and from gas to coal. This is a really serious environmental development across Europe.

For many years the rational pursuit of energy policy in many parts of the world was hindered by the demonization of nuclear power. It has taken that industry many years to work through the opprobrium that hindered realistic discussions of costs and benefits, and it is only now that some of the green community are beginning to recognize the irrationality of some of their earlier arguments. Yet the vilification of that industry cost it an entire generation of management and engineers who were persuaded to avoid such a career and who starved the industry of a strong supply of graduates. Now that engineers are needed there not that many with long experience, since the generation running the industry is retiring.

And here we are seeing a growth in the demand for coal, and yet – in part because of the demonization of the industry – there are but limited places where qualified engineers can be found, and at a time when advanced levels of technology will be called for as deposits get leaner, deeper and more difficult to extract there are even fewer places that can carry out the needed research into excavation technology.

Sadly academia and government seem to be unwilling to face the reality of the future that these circumstances will bring. Because the situation cannot be reversed in a year or two, as the knowledge base fades (as it now rapidly is) so it will become harder and harder to meet global needs. But no doubt academia will find a way to blame that on us miners, the few of them that will remain.

Someone told me once – “if you want to find gratitude, you’ll find it in the dictionary, between chump and sucker*.” But, one might have thought that for those who claim to predict the future, perhaps the occasional thought of self-interest might burble its way through, but not, I suppose, at Oxford.

*the statement has been modified for a general audience.“

Working thin coal - the coal plow and Hydrominer

In the last couple of years there has been a growing concern about the amount of coal that remains in the productive reserve for the world. At the same time the incoming British Government, which had been somewhat antagonistic to coal while in opposition, may be giving large coal-fired power plants a reprieve. Questions might therefore arise as to how long this fuel source is going to last, if use increases more than projected. While I am not going to answer that specific question today, I will address a part of the issue. Namely what do you use to mine coal when the seams get too thin for modern equipment?

The lowest coal which I personally have mined was about 1 ft 8 inches high. The low coal was caused by a roll in the middle of the face (the floor got closer to the roof) which was normally about 3 ft high (The Beaumont seam at Seghill Colliery in the spring of 1962). In that height you lie on your back, put a pit prop under your shoulder to give you some leverage, and shovel across your chest. In more advanced mining countries it is unlikely that we will return to such manual labor, which is not very productive. So what do you use?

The most productive machine in most longwall operations is the shearer, which I described in the last tech post. The problem is that it most effectively runs on top of the armored face conveyor (AFC) and the power pack that drives the cutting head and haulage unit takes up quite a bit of space. One idea was to take the shearer off the conveyor and have it slide along the ground on special shoes, with the cutting head mounted ahead of it on the longwall. My father had rather strong opinions on this, since two of the mines he worked with had such machines. Remember that the coal conveyor must snake over behind the machine in order to allow the supports to also advance.

To hold the machine together, the gear boxes at each end and the power pack in the middle, there are through-bolts down the machine. Now it breaks down in low coal. The roof is say 2 ft 6 inches above the floor, the machine is 22 ft long, and the conveyor is 7 inches high. How are you going to take the machine apart to fix it? (The answer involves explosives, and is not a “quick fix.”)*

So if the shearer is not an ideal machine, what is? The answer is known as a coal plow (or Hobel in Germany where they were developed by a company then called Westfalia Lunen.) Very simply in some coal types, particularly those that are brittle, the coal at the front of the face is weakened and cracked by the pressure of the overlying ground. Thus if you take a narrow pick and drag it across the coal it will peel off some coal. Put a number of these picks together and the coal between them will also chip off – perhaps to a depth of a couple of inches. Make the machine move down the face rapidly, with the rams pushing through the AFC to keep the plow pressed against the coal, and you have a simple but effective mining machine.

Plow on face – note the picks in the different elements that can be removed to adjust for varying seam heights. (Shield parts removed to show the plow)

The coal in which this is most effective does not necessarily have to be mined over its full height, since often, when undercut, it will fall under its own weight. Depending on the coal strength, the depth of cut of the machine can be adjusted. As a result the coal produced can be quite large, sometimes bigger than the average size of the fragment coming from a longwall shearer face. It is, however, an "interesting" experience, to see one working under a sandstone roof, where the face rolls a bit.

There were a variety of plows built, depending on coal height, coal type, and the speed at which the plow could be moved down the face. It is a fast moving operation, but one that is not that popular at the moment, since, in most higher coal a shearer may be more effective and produce more coal. However in the years to come the plow may make a comeback.

By then, however, it is possible that the picks on the machine, that generate dust, will be replaced by high pressure (10,000 psi) water jets which cut into the coal perhaps a foot ahead of the machine, and allow mining without the generation of the dust and sparks that make current operations so dangerous. It has been done before, and is a relatively simple technology to adapt to future conditions.

Waterjet plow operating underground in Germany (from Gluckauf)

We called it the Hydrominer, and as I learn how, I’ll put up some video on Youtube of it operating. There are better ways of using the jets than those shown, so that the plow can actually mine coal to a web depth of over 3 ft 9 inches (we have) at shearer speeds.

Incidentally longwall mining was not a part of the Modern Marvels review of Coal Mining, though in Part 3 they do show the development of the continuous miner, and shuttle car. (H/t to Pasttense who gives a list of some Youtube films), though there are a number more than I had thought there would be.

* At least in those days they excavated a chamber ahead of the machine by blasting it out with explosive and removing the coal by hand. This allowed them space to pull off the head, remove the bolts, fix the machine, and put it back together again.

Longwall mining with shearers

The development of longwall coal mining took a significant step forward with the development of the armored face conveyor (AFC) and the self-advancing supports of both chock and shield designs that I described last time. Put together they provide two of the three major parts of a modern longwall. The third, and the topic for today is the mining machine itself.

The longwall panel with shields and a mining machine is at D.

In the evolution of longwall, an undercut beneath the face was initially cut out by a man wielding a pick. By the turn of the 19th century this was starting to be replaced by a machine, much like a giant chain saw, that undercut the face to a depth of around 5 – 7 ft. And, while in earlier times the coal was broken from the solid by hewers that used picks to break out the coal to the free face left by the slot, with machine undercutting the bulk of the coal was broken down by single sticks of explosive set into the coal at about 6-ft intervals along the face.

The AFC, as well as carrying the coal away from the face, had two hard bearing surfaces at the top of each pan, which connected together to provide a path along which a machine might move. But what sort of machine was going to be capable of mining the full face of the coal. There were a number of different designs developed, many of which started with the long cutting chain of the coal cutter, and added other blades to it, in order to fully remove the bulk of the coal. I’ll mention only three of them, in passing.

The first idea was simply to mount a cutting post at the turning wheel of the conveyor, deep in the cut, in order to back cut the coal, and move it out of the web. The machine had a number of teething problems and did not prove very effective in underground trials. It was quickly passed by the Meco-Moore Cutter loader which by 1956 had become one of the most popular integrated mining machines in the United Kingdom. It is important for those who talk about the energy required to mine coal to understand how it worked.

This was still at the time that the roof was supported by manually placed props and bars (which can be seen in the background). However the bottom of the coal was first cut with a cutter bar that was 6 ft long. Concurrently the central part of the seam was cut by a second cutter bar, which cut a slot to a depth of 4 ft 6 inches in the coal. A third slot, at the back of the face, was cut using the triangular shaped cutter bar shown in the illustration. Coal has sensibly no strength in tension, because of the cleats and bedding planes that form within it during the process of forming the coal. Thus the web of coal that has been undercut, mid-cut and back-cut will collapse onto the small cross-conveyor, which carries it over to the main AFC.

As I mentioned, the machine became quite popular, since it both cut the coal, and loaded it onto the conveyor. However the small cross-conveyor needed to move the coal over to the main conveyor was relatively fragile, and frequently broke, dropping production. The scene was therefore ready for two more machines, one of which I will discuss today, and the other (when I talk about mining thinner seams of coal) in a later post.

The new machine was called a shearer. Developed by John Anderton, who worked for the British National Coal Board, the initial concept was brilliantly simple. Take the coal cutting machine that was common in many mines, turn it on its side so that the drive shaft was horizontal, and mount a cutting drum to the drive shaft that used to operate the cutter bar.



The picks on the drum were set on a spiral, so that as the drum turned it would feed to coal over to the conveyor, on which the machine was riding. The shape of the scroll, with and without picks, can be seen from the lower parts of the Anderton Shearer Memorial in St Helens. Lancashire.

(From Lowton Websites) The lower scroll shows how the shape would, as with a wood drill, feed the coal back to the conveyor as the drum rotated.

This proved to be a relatively simple machine, adaptable from existing machines in the mines and became the predominant mining machines for longwall faces. Over time the drum was mounted on a boom, so that it could range up and down to adapt to varying seam conditions, and a second drum, also ranging, was added to many machines, at the other end. In this way higher coal could be mined.


Modern shearer, showing the size, and how it would integrate with the AFC, on which it rides, ahead of the shield supports which protect the miners. (Note the coal face would be where the man is standing).

The machines need many less miners to operate that the fifteen men that would hand load out a face back in the early 1960’s, and now there are automated devices to detect the interface between the coal and the rock, and which can raise and lower the drums to adjust for these geological changes.

Looking down on a model of such a mining operation, with the front canopies of some of the shields removed to show how the conveyor “snakes” over. The operation of the face is as follows:

First the shearer mines off a web of coal that is perhaps 2-ft deep. This is loaded onto the AFC (green) and carried away. The hydraulic rams on the shields then push the conveyor over so that it is beside the face. Then, in turn, each shield lowers, and the ram is reversed, to pull it forward the same 2-ft so that it again covers the working area. It then raises, and resupports the roof, while the support next to it is advanced. In this way the machine continuously slices off the coal as it moves the face forward.

The technology allows high rates of underground production, for example, in May 2009 the Newlands Northern mine in Queensland mined 961,891 t from its longwall, 251,720 t of that in a single week. (And up to 46,000 tons in a day).

For those interested in the technical details:

The Newlands longwall is equipped with Bucyrus EL3000 shearers which have installed power of 1,590 kW and cutting power of 2 x 650 kW.

The shearer employs a jumbotrack 2000 haulage system with haulage power of 2 x 125 kW and is fully automated. The longwall is equipped with 147 two-leg roof supports with a yield load of 1,040 t and a working range of 3 - 5m. The face conveyer is a Bucyrus PF4, 1332mm wide with a 42mm twin inboard chain with 2 x 855 kW CST drives.
The longwall is controlled by Bucyrus PM 4 controllers, with the 400 kW PF4/1532 coal crusher and 400 kW SK11/18 beam stage loader also from Bucyrus, and motors manufactured at ATB Morley’s factory in Yorkshire.

Longwall Mining - using hydraulics to modernize mines

In writing about longwall mining, I have gone into some detail as to how the technology initially developed, in part because of my own interests, but also because some of its features help answer questions about today’s industry and this will provide a background for those answers in future posts.

The change in longwall came about as I went to college. The manual loading of the coal and setting of roof support that I have described in earlier posts, ended as a result of a single technical change. The miners had loaded coal onto a rubber face conveyor, that was advanced by being split into sections, moved forward, and rebuilt every day. This is incredibly inefficient and the innovation was the change to what has become known as an armored face conveyor. Basically this consists of a series of 5-ft long, hinged elements or pans, connected together in such a way that each can move forward and twist slightly, relative to its neighbors. ( In using pictures of existing equipment let me just insert that they illustrate what I am discussing, but that shouldn't let you think that I believe that one particular brand is better than another.) A single element of a relatively modern version looks like this:

View of a 5-ft long element of an armored face conveyor (Joy )

Horizontal link bars fit between elements in the hollows that can be seen at the ends of the piece allowing flexibility in the joints. The face of the conveyor is the wedge shape, that faces the coal face. Coal is loaded onto the conveyor from the mining machine, and any coal left on the floor then rides up the wedge onto the conveyor, as it is hydraulically pushed forward after the machine has passed. (A point I will discuss later). The coal falls into the top trough of the conveyor.

Pin connection between conveyor pans (Bucyrus )

To move it along the conveyor, there are two chains, which connect to lateral steel bars called flights, which slide in the slots on either side of the top trough.

Top view of conveyor showing flights (Joy)

The chains drag the flights down the trough, pulling the coal that falls onto the conveyor with them, and at the end of the conveyor, the bed is raised, so that the coal is dumped onto the belt conveyor that runs in the entry tunnel to the longwall. (For historic reasons this is generally referred to as the Maingate of the longwall.) The empty chain and flights then return along the face in the underside compartment to the conveyor.

The moving parts of the conveyor are thus now the chain and flights, and they are driven by a motor mounted in the maingate of the longwall.

The arrival of this then new tool allowed a number of different changes to occur on the face. The first and initial requirement was for a way to move the conveyor forward after the coal had been mined out it front of it. It was too big and cumbersome to disassemble and reconnect, and flexible enough that it could be pushed over using a series of hydraulic rams, one attached to each of the pans of the conveyor. But what could provide the resistance against which the rams could push?

The only element in the face behind the conveyor are the roof supports. So these also had to be modernized so that they could work in this way.

Initially the supports had been individual elements, two props and an overlying bar, all made out of wood. I had mentioned last time the arrival of hydraulic props, and steel bars. By combining these into a structure, with the props set on a bearing plate, and the steel bar at the top permanently mounted to their upper section, a simple support element was created. This, as we found out, didn’t have much lateral stability, and so they were combined in sets of two. With a hydraulic ram on the front, this could now push the conveyor forward.

Early hydraulic support (note the ram at the bottom left)

The ram would also make it possible, after the props were lowered, to pull the entire unit forward, and then, after raising the props, reset closer to the face. The plates at the back then stopped the rock that was collapsing into the void left behind the chock where the coal had been been, from rolling into the working face. This became known as a chock.

Over time the props became larger, and the canopy which was meant to extend over the conveyor to provide early roof support didn’t work that well. The idea was that it could be slid out, after the machine had mined the coal, until the chock itself could be moved. The problem was that if there was a heavy load applied at the tip it would bend the cantilever so that it could not slide. Then life became a little more difficult. The answer came from Hungary initially. It was known as a shield support.

Early shield design (dimensions in mm)

The design was extremely flexible and able to mine under conditions such as a badly broken roof that would not have been possible with normal supports, since the top and back (canopy) of the shield provided a virtual complete coverage of the roof and back of the working area, and the shields were installed adjacent to one another.

There have been considerable advances in design, but of an evolutionary nature, since this particular breakthrough – which occurred in the 1970’s. But in combination with the use of stronger and more powerful face conveyors these supports were able to make high-speed longwall possible in a wide range of differing conditions.

It is even possible to mine out a thicker seam by taking the lower coal with a mining machine (of which I will write next time) and then letting the upper coal collapse onto the back of the support, from which it can be recovered using a second conveyor. (And not a lot of energy – Charlie).

Top coal recovery – as in the Hunter Valley of Australia (Bucyrus )

Support used for top coal recovery - its a lot more complex than a wooden prop.



As I mentioned above, I’ll talk about the mining machines next time.

Controlling the roof - more on longwall technology

In describing the earliest types of longwall I mentioned that the evolution came from initially putting different mining sections into a line as a way of simplifying the ventilation and the haulage of supplies to the miner, and coal that he had filled into tubs. This is a layout showing one of those early operations, and the roof is temporarily held up either by pit props, typically around 4 – 6 inches in diameter, and cut to length as needed, or small packs. Because the miner was penalized if there was too much stone in the tub (the entire tub would not be counted) he could stack the stone he found in small packs, with the larger stone on the outside and small stuff shoveled within to provide the roof some support, as needed.

Mining occurs in the open spaces, the small squares are the roof supports and the dark line is where the putter would bring the tubs and supplies or leave with the coal.

The first transition was to change the direction of mining so that it wasn’t perpendicular to the goaf (or collapsed waste) but parallel to it, and then to join all the individual stints into a single face. At this point the supports became more organized and regular plans were developed to ensure that the roof was supported. (These were assembled in a book that the mine manager had to certify, with the conditions for each face specified, and any special support locations laid out.) An early version of such a plan showed the plan and spacing of the props.


The face is shown after it has been undercut, but before it has been fired, and the coal blasted down. In addition the back set of supports would be removed, and the conveyor advanced, before the shot went off. After the back row of supports were removed, it would look more like this, and more typically undercutting and inserting the powder would take pace after the move.


The path of the coal cutter can be seen, and thus use of square sets of timber that were used in bad ground to help support the roof long enough to remove the coal. These replaced the stone piles that had been used earlier. Personally I did not work on faces that used them, except when we had a major face collapse, and we had to mine through it – but that’s another story.

Chock location and construction.

Once the back set of props was removed and the conveyor split, moved forward and remade, then the coal was blasted down and then, as each man loaded out his fifteen yards of coal, the new set of supports was installed.


The reason we could get by with using wood was that the process was relatively slow, and we would mine only one cut a day. This gave time for the roof stresses to redistribute, and the collapsed roof to build up and support the overlying strata.

Section through the face (lower left) showing how the roof breaks, bends and ultimately resupports itself.

As mining began to mechanize, one of the first things that changed was the material of the roof supports. The first step was to change the wooden bars on top of the props for steel ones. Sounds like a good idea, right?

Well imagine that you are kneeling on the floor and you have to erect one of these supports. You put the long bar on your shoulder,pick up the axe and a wedge in one hand, the prop in another, and raise your body until the bar is pressed against the roof. Then you put the prop in the middle, and slide the wedge on top, and tap it tight with the axe. With the bar then stabilized, temporarily, you get a second prop and put it at one end and tighten it into place, and then do the other. Then you remove the central prop. With a wooden bar it was relatively easy in heights from 2.5 ft to 5 ft. With the greater weight of the steel it was harder to move, more critical that you be near the middle when you start, and more vital that the first prop was close to the middle. With a wooden bar there was also a little flexibility in driving in the wedge, absolutely none with the steel bar. Oh, and since we were re-using these (pulling them out of the waste as the last row was collapsed) they pretty soon got bent and so had to be straightened a bit before they could work.

But that was the easy step (in retrospect) because the next thing to replace was the wooden prop. Both the wooden prop and bar were easy to move. But to replace the prop the first idea was to use a friction prop. Here’s how it worked:

The prop was made up of a sliding central square section (above the horizontal wedge in the first picture) that slid within a lower section (where the lifting wedge is inserted).

You lifted the central section until it was about right, then tapped the lifting wedge until the prop was tight against the bar. Now you could move to one side and tighten the horizontal wedge that gripped the sliding section and held it in place. Then you removed the lifting wedge. And when you wanted to remove it, all you had to do was tap the horizontal wedge.

It had the advantage that as the roof stress changed so the prop was intended to yield, with the inner section sliding within the friction band, rather than failing as the wooden prop would if the stress got too high. However, you might want to remember that here we were doing this individually in relatively low coal (at least we didn’t have to saw them to length) with a 6-ft steel bar resting on your shoulder while you try and drive these wedges in. It was a learned art, one of the first things to learn was not to hold the prop above the wedge while you tap it out. The prop drops when released and will hit your hand – usually the thumb – and it has the weight of that steel strap on top of it. Funny how you still remember!

They didn’t last long, for a variety of reasons, but were replaced with the forerunner of today’s roof support, the hydraulic prop. The first ones were relatively simple, and heavy, but much easier to use.


Simply you put the pump handle in the socket, and started pumping. This moves fluid from the upper reservoir to the lower, pushing the central cylinder and the top cap up, and against the prop. To release it you put the handle in the link, and pull, and slowly the prop sinks. Much easier to work with.

But that was only the start, and moving these by hand was still slow, and so the transition to full mechanization began. And before long instead of the simple hydraulic prop, the roof was supported by massive, and powerful hydraulic chocks, but this also required other changes to mining practice. For, as it was said at the time, “We cut the coal by machine, we transport the coal by machine, isn’t it stupid to still load it by hand?”

Schematic view along a face showing a more modern roof support.

Pre-mechanized longwall mining

In the last post on this subject, I wrote about how miners were able to remove almost all the coal from a section, either by leaving small remnant pillars or building packs to hold the roof in place, while that coal was removed. By retreating the face back towards the shafts the overlying roof rock was then allowed to collapse into the void left by the coal removal. However, as this process began to evolve the miners noticed a couple of significant things that helped in the understanding of how the roof was responding, and helped to make longwall a safer and more effective method of mining. The first was that at the roof broke behind them, so the rocks would bulk up (they gain about 60% volume as they break and pile). Within a distance of about 2 seam heights, as the roof was converging, without underlying support, it would then meet the broken pile of rock, and thus get some support from this. As a result any support that the miner installed would not need to carry the full weight of the overlying roof to the surface, but only that of a few feet, which needed much less strength.

Thus by about 1870, and possibly in the Lancashire coalfield in the UK, they had modified the process further, and were only supporting the roof around the actual mining operation. How could they get away with this?

There was one other fact that helped make it possible. In some of the earliest tech talks I mentioned that the weight of the overlying ground can be simplified to being around 144 lb./sq ft for every foot of depth – based on the simplifying assumption that a cubic foot of rock weighs 144 lb. Thus converting this to a pressure in lbs/sq inch. (of which there 144 sq ins to a sq ft) this means simplistically that for every foot of depth one goes into the ground, the pressure increases by 1 psi.

Now when you make a hole in the ground, that load, or equivalent rock pressure, has to move somewhere. And it moves just a little so that the weight of the ground over the hole is carried by the rock on either side. However, what happens if this additional load is too high for the rock and it fails?


Well if the rock were just a thin column it would collapse, but if it were thicker, then the weight would just move further into the coal. Now if we came along and moved the coal that had failed, then the hole would just continue to get bigger. But if we leave the coal in place, then the broken coal acts to confine the coal further into the solid. And this confinement gets higher, as the failing pressure continues to move into the wall. And what happens is that this confinement builds up the strength of the coal, so that at some distance into the wall (or face) the coal strength reaches a point that it can carry the weight of the ground above the working area.(For a simple analogy think of a deck of cards, which individually cannot bear weight, but when held together by a rubber band, or a carton, can support quite a bit of weight). (And for those who prefer a more scientific description – the lateral confinement moves the failure from two-dimensions into three, with the minimum principal stress building as one moves into the solid material, and raising the overall failure stress behind it).

This works not only for the coal in pillars, or ahead of the working face of the longwall, but also for the rock that has fallen into the waste and is confined by the rock around each piece allows it to regain some strength, and so collectively the broken rock behind the working face (called the goaf or waste) will continue to compress as the full load comes on it, but will carry the weight of the ground from about twice the seam height, all the way to the surface, and with the other end of the "bridge" as it were resting on the confined coal ahead of the working face.(While the width of this bridge varies with depth, coal and rock strength etc, for an initial estimate you can imagine it as being around 500 ft).

Simplified side view of the coal as the miners removed the coal along the face, moving to the left. They put up wooden supports (three wooden props and a top bar) and let the roof behind the working face that these protected, collapse.

Thus the miner, working at the face, needed only to support only the rock that is up about twice the seam height he was working (in those days women did not do the actual mining). And this could be done with relatively small tree limbs, called props. However, because the rock could break into pieces, the prop support would be distributed, by having a plank, or half split timber, as a bar on top of the prop. Putting one prop at each end thus gave a sort of "goal post" support. Thus, along the face, there would be, at about 4-5 ft intervals, these prop supports holding the roof up.(The coal is made slightly blue in the pictures to give a better contrast - sorry!)

View looking down on the working area from the top of the fallen rock pile. I have erased a small section of the coal to show the position of the cutter bar of the coal-cutter as it is either dragged, or self propels itself along a cable stretched down the working face.

In the initial working of the longwall panel, the coal was undercut by a team of holers, who each cut a slot at the bottom of the seam, to a depth of about 3-ft, and collectively undercut the face over the course of a shift. As the faces grew longer there was a search for a machine that would make that undercut without the intensive manpower. One such tried to mechanize the simple swinging action of the pick.

Early coal cutting machine used at Garth Colliery in Wales in 1863. (National Museum Wales )

The development of the machine, the coal-cutter, dates from around 1876 when a compressed air machine was developed by Francis Lechner, in which picks mounted on a chain, did the cutting of the coal. (The more modern versions of this look like a chain saw on its side). It took a number of years for the machine to evolve into something that was widely accepted, and by that time the company had been taken over by Joseph Jeffrey (a banker) and became Jeffrey Manufacturing Company. (By the time my dad worked for them they had become British Jeffrey Diamond, and they later became part of the Dresser Group). They had spread to Europe by 1905.

And electrically driven machines were developed, which have not changed that much in the intervening years.

Early Coal Cutter (Iron Miners )

With these machines pulled along the face, undercutting the coal, to give a cut depth that was more typically 7-ft deep, the next step was to break down the overlying coal. Sprags (small wooden wedges) were slipped into the slot at intervals, as the cutter passed up the face – usually run by three men. At the same time holes were being drilled along the face, about 6 ft apart, with a stick of dynamite placed in each one.

After the face had been undercut the coal was blasted down between shifts (7.5 hours) then the collier shift would come in and each man would have about 10 yards of face to load the coal from, and to re-support. To get the coal from the face, a rubber conveyor belt was run along the back end of the supports that were in place before the blast, and the coal would normally not break that far from the face. As the miner shoveled he would also put in a new set of timbers, overlapping the old, and supporting the new working area. Typically this would take another seven hours, with an ideal seam height being about 4.5 ft. Above that the coal volume to move was much greater, and below that it got a bit awkward. For example, below 2 ft thick you lie on your back, with a prop under your shoulder and shovel over your head - how would I know? Yes, there was a reason to go to college).

View of the face, after the coal has been loaded out. The rubber coal conveyor between the last two rows of props must now be broken into strips, and moved forward a row, ready for the next cycle. Then the back props and bars are removed. (Saving the front two props and chopping out the back one).

In the third shift, the men would come in and break down and move over the conveyor belt, and then remove the last row of wooden supports, bringing the roof down, beyond the new line of supports.(Smart folk would use a come-along and a chain to pull down the props, young idiots (guess who) would go in with an axe to chop them first). For this was the state of the industry when I went to work in it in 1961. There have been many changes since.

The Earliest Longwall - coal mining before the 1830's

Earlier I have written about the large amount of coal that was often left to hold the roof up while miners excavated the coal from rooms offset from main tunnels, with the rooms themselves being extended to create an intersecting set of passages. But even where the pillars left between the original tunnels are later removed as the mine retreats the working faces back towards the main shafts and exits, a significant amount of coal can be left.

About 250 years ago this was a clear problem in the Shropshire coalfields of the United Kingdom. At that time underground mining was usually carried out by crews of men and boys, where the coal was first removed by undercutting the coal seam manually with a pick, to a depth of about 3 ft. The bulk of the coal was then broken down to this slot and the fragments (ideally about 4-inches in size) were shoveled and hand-loaded into pit tubs, to be hauled away. A good day's work was about 20 tubs, and I have described this method and how it evolved, in earlier posts.

However, even as early as the 17th century (Economic Development of the Coal Industry 1800 – 1914 Brian R. Mitchell p 71) a different method of mining began. At first it was known as Shropshire mining because of where it started, but it later became known as “longwall mining”. The advantages, even then, of the technique were obvious. They included a greater percentage of larger coal (easier to sell), simplicity of working and ventilation, better roof control and a greater production of coal from the workforce, perhaps as much as 30% higher. In particular it was a cheaper method of mining and it allowed a much higher level of production from an area by concentrating the activities of the miners, and focusing the transport.

I just came across the book that appears to have been the first proposal for the more modern version of its use. (The Miners Guide – being a description and illustration of the principal mines of coal and ironstone in the counties of Stafford, Salop, Warrick and Durham”, by Thomas Smith 1836. And so I thought I would begin this short sequence on longwall mining with a description of how the technology first evolved, from that book.


The method was one that gradually evolved from initial headings that were mined by two separate working teams, the first being the holers and the second the brushers..

When the coal was of good quality, and high, then the process was to undercut the coal to a depth of 3-ft, with a man being able to undercut a length of about 22.5 ft a day. He would then cut vertical cuts to the same depth along the edge of the heading, which in the illustration below would be about 30 ft wide – depending on coal and roof quality. There were a group of these men who initially worked the face, and then moved on. They were followed by the brushers, whose job would be to break out the bulk of the coal from the face, and load it into tubs. They would also support the roof with timber props, as this was needed and the coal was removed. This was conventional room and pillar. But it left a lot of coal in the pillars.

Plan view of room and pillar or "on the square" mining.

Initially it was in thinner coal seams that “the long way” was developed as a way of getting almost all the coal out.

First, as with conventional mining, gate roads are dug out to the edge of the property (back in those days this was about 300 to 600 feet) with the direction going down the dip of the seam from the shafts at A and B, which are about 20 ft apart and some 7 ft in diameter. These roads were 6 - 9 ft wide and full seam height. Air passages or thirls were driven between the gate roads to help ventilate them as they were driven (the “a” passages). Cross-connecting tunnels between the gateroads were then driven, near the edge of the property.

In those days it cost around 0.35 to 0.4 English pounds (Ep) per yard, with workers being paid 0.225 Ep per day including candles and drink. The thirl would cost around 0.15 to 0.2 Ep per yard to drive. (An area up to 60-ft in diameter would be left unmined around the shaft area to hold it up).

Once the edge of the property had been reached then a section of the mine, some 90 ft long, would be mined with six miners each taking some 15 ft and holeing the coal. This was undercutting the face, to a depth of 3 ft, over each stint, and it would take a day, with the each miner also cutting a vertical slot at the edge of his section, so that it was held only by the coal at the back of the panel. The sections were mined on either side of the gate roads, moving towards the common middle of the “panels” being mined.

The holers were then finished in that section and moved to a different section, and a second set of miners mined out the rest of the coal, known as brushing the coal to the 3-ft depth. At the same time, since they were removing all the roof support they would put in timber props to hold the roof up, and would also construct small pillars or cogs, that were made from stone, fine coal, and other refuse, when they felt they were needed. In this way the white strip shown in the diagram below, at the back of the mine was extracted first, with the sections progressing first laterally out to the adjacent gate roads, and then back towards the shaft. While it takes 6 men to hole the 90 ft face, it would take only 3 men to brush and cog it. (And they would use small charges of gunpowder to help if the coal was not easily broken out). The difference from conventional room and pillar can be seen in the small size of the cog pillars that were left, as mining progressed. In this case, from one gate road to the next, with the mining face parallel to the gateroads and retreating from one to the next.

Plan view of Shropshire mining, the mining faces were parallel to the gate roads, and the dotted lines show the way that the tracks would be laid to get the tubs in and out.

Wooden tracks were laid along the gate roads, and then bent to pass along behind the face, to allow a horse and boy to collect the tubs as they were loaded, and then to replenish the men with empties. The costs for this method of mining, which was known as broaching, was given as:

(note that there are 12 pennies (d) in a shilling (s) and 20 shillings to an English pound of the period. And an English pound is now worth roughly $1.50). At that time the market for coal was such, that the mine owner would expect to get the following for the coal (with the price based on size).

A profit, at best, of just under 0.10 Ep per day, per working section.

The technique, was still quite dangerous, since the expanse of roof that the miners worked under got larger as the excavation moved away from the gate roads, and the cost of moving rock and dirt into the workings to build the cog pillars would have been significant (as would the time taken to assemble them).


Thus a new method was proposed, and the initial description is as follows:

For getting out the coal by long work, the pits A and B are sunk, as in the other case, at a distance of six or seven yards from each other; and the main gate roads driven to the boundary of the work at C and D, properly thirled with the openings for temporary use. From the ends of the main gate roads branches are cut, at right angles, to E and F, along the boundary line of the proposed area to be cleared, so that the mine may be said to be headed in the form of a Roman T, the roads E C, and D, F presenting the faces of the coal, which are to be worked homewards, or towards the pits. Simultaneously with the traverse gate roads, an air head ef is driven at a distance of three or four yards, with its thirls, which are closed in succession as the work proceeds. . . . . .The necessary roads and heads being completed, the work of getting commences; in order to which, the miners hole one yard under along the entire faces of work EC and DF which may be each from 50 to 100 yards in length according to the extent of the area to be cleared. Cuttings are then made at proper distances, to the height of five or six feet, or to a convenient parting, and the coals are brought down, turned out and drawn away along the gate roads. Cogs or pillars are then constructed of the waste and slack, to support the upper measures.

The holeing and cutting then proceed another yard in width, and then another; still clearing away the coal and supporting the roof with cogs, till the lower measures are drawn out, to the width, along the under face of 8 or 10 yards. By this time the over-hanging measure have, by their gravitating force (sic), sunk and bedded themselves on the cogs, pressing them down to a sort of continuous floor of what is called gob, or compressed and compacted slack. This is assisted by the use, as experience may dictate, of timber, which is taken away when the working of the stage above commences.

This is the first description I have found for what we now call longwall mining. By turning the mining face so that it advanced into the solid and away from the opening left, the overlying roof was able to bridge over the working area. This considerably improved roof control, and made it a much safer method of mining. In presenting the method the author notes that the cost of large coal, using room and pillar mining, which is the top method described, worked out to be around 0.118 Ep per ton mined. When the costs were worked out for the long way, the mined cost was found to be 0.105 Ep per ton, giving 0.013 Ep (3.25d) benefit.

However the increase in the volume of coal produced (and thus the royalty yield per acre) doubled to 2,046 EP per acre.

At the time that the book was written, it was a method just beginning to be developed, and the presentation was as much a proposal as a description of something in place. How it turned into the most productive of underground mining methods, in the course of the following 180 years will take another post or two to describe.

The Mining Disaster in West Virginia

The news of the death of at least 25 coal miners at the Upper Big Branch Mine in West Virginia is a reminder of the human costs that are incurred in the provision of fossil fuels. Although American mines have grown considerably safer over the years, the nature of the work can mean that when there is an explosion in a mine, that there are multiple fatalities because of the layout which is most effective for getting the coal out. It also underlines the higher costs that must be met when mining coal from underground operations, rather than the more visible, and criticized surface mining operations.

In the United States this is the largest mining disaster in more than 25 years at a mine that produced 1.2 million tons of metallurgical coal last year. Because of the pressures on surface mining operations Massey Energy were moving an increasing percentage of their production to the underground. The mine uses longwall techniques as the main means for producing coal and so I thought that it might be helpful if I reposted some of the information on that technique.


Part of the problem in controlling the ignition of gas in such an operation is that the mining machine breaks out the coal in relatively small fragments, by rotating a drum laced with picks against the coal face.

Surface test of a shearer used in longwall (Bureau of Mines Bruceton)

The fine crushing of the coal can lead to the release of methane gas (natural gas) that is found and formed with the coal. Levels of the gas are measured, and controlled by sending enough air down the face to dilute the level of the gas below that at which it is at risk of ignition or explosion. However within the space that the drum is carving out it is not always possible to get that air flow into the area to ensure that dilution is immediate.

At the same time if there are layers of rock within the coal, then the pick can rub against these and generate sparks, and heat up the rock to the point that it becomes hot enough to ignite any methane pockets that have been released. Once that ignition starts the very fine coal dust that is also a part of mining (as the above picture shows) means that this can also ignite, intensifying the resulting explosion. That becomes particularly deadly, given the geometry of the longwall.

And to explain that let me repost something I had written about before.

Back in the mid-1800's underground mining was usually carried out by crews of men and boys, where the coal was first removed by undercutting the coal seam manually with a pick, to a depth of about 3 ft. The bulk of the coal was then broken down to this slot and the fragments (ideally about 4-inches in size) were shoveled and hand-loaded into pit tubs, to be hauled away. A good day's work was about 20 tubs.

As the miners drove the tunnels (also called entries, headings, drifts, drives etc) into the coal they left pillars between the tunnels to hold the roof up. However in about 1870, and possibly in the Lancashire coalfield in the UK, they discovered that if they put these entries together, they could develop a way of getting all the coal out from that section (or panel). How could they get away with this?

Well there are two things that make it possible. Firstly, when you make a hole in the ground, the rock pressure that was applied to the rock (about the same pressure as the depth of the hole) has to move somewhere. And it moves just a little so that the weight of the ground over the hole is carried by the rock on either side. However, what happens if this additional load is too high for the rock and it fails?

Well if the rock were just a thin column it would collapse, but if it were thicker, then the weight would just move further into the coal. Now if we came along and moved the coal that had failed, then the hole would just continue to get bigger. But if we leave the coal in place, then the broken coal acts to confine the coal further into the solid. And this confinement gets higher, as the failing pressure continues to move into the wall. And what happens is that this confinement builds up the strength of the coal, so that at some distance into the wall (or face) the coal strength reaches a point that it can carry the weight of the ground above the working area.(For a simple analogy think of a deck of cards, which individually cannot bear weight, but when held together by a rubber band, or a carton, can support quite a bit of weight).

The second thing to know is that when a layer of rock breaks the rock lumps when piled together occupy more space than the solid rock. As a rule-of-thumb the bulking is about 60%. So that if we let the roof over the working area break and collapse, after we have taken the coal out, then by the time about twice the seam height of rock has collapsed, it has filled the hole where the coal used to be, and reaches up to the solid layers of rock above, to hold them in place. The confinement of the rock around each piece allows it to regain some strength, and so collectively the broken rock behind the working face (called the goaf or waste) will carry the weight of the ground from about twice the seam height, all the way to the surface, and with the other end of the "bridge" as it were resting on the confined coal ahead of the working face.(While the width of this bridge varies with depth, coal and rock strength etc, for an initial estimate you can imagine it as being around 500 ft).


What this means is that the miner, working at the face, needs to support only the rock that is up about twice the seam height above his head (in those days women did not do the actual mining). And this could be done with relatively small tree limbs, called props. However, because the rock could break into pieces, the prop support would be distributed, by having a plank, or half split timber, as a bar on top of the prop. Putting one prop at each end thus gave a sort of "goal post" support. Thus, along the face, there would be, at about 4-5 ft intervals, these prop supports holding the roof up.(The coal is made slightly blue in the pictures to give a better contrast - sorry!)

Now, to get the coal out it was possible to put in mechanical assistance. The first step was to use a machine, rather like a large chain saw, that was pulled along the face, undercutting the coal, to give that first free surface. At the same time holes were drilled along the face, about 6 ft apart, with a stick of dynamite in each one. After the face was cut the coal was blasted down between shifts (7.5 hours) then the collier shift would come in and each man would have about 10 yards of face to load the coal from, and to re-support. To get the coal from the face, a rubber conveyor belt was run along the back end of the supports that were in place before the blast, and the coal would normally not break that far from the face. As the miner shoveled he would also put in a new set of timbers, overlapping the old, and supporting the new working area. Typically this would take another seven hours, with an ideal seam height being about 4.5 ft. Above that the coal volume to move was much greater, and below that it got a bit awkward. For example, below 2 ft thick you lie on your back, with a prop under your shoulder and shovel over your head - how would I know? Yes, there was a reason to go to college).

In the third shift, the men would come in and break down and move over the conveyor belt, and then remove the last row of wooden supports, bringing the roof down, beyond the new line of supports.(Smart folk would use a come-along and a chain to pull down the props, young idiots (guess who) would go in with an axe to chop them first).

The process needed mechanization and this required three different components to work. And these all came together in a period around 1960 - 65. Firstly there was a better way of removing the coal. The machine that was developed initially took the coal cutter power pack, and turned it on its side. By then putting a drum with picks on it, over the shaft to replace the cutter bar, the Anderton shearer was invented (named after its inventor). The drum rotated, and a shaped cover behind it moved the broken coal over onto the second part of the process.



This is a rigid framed conveyor, made up of segments that can move against one-another, and with rigid metal walls. The shearer can ride either on top of, or along side this conveyor, and load the coal onto it. The conveyor then carries the coal to the end of the face, and onto a second conveyor, that carries the coal out of the panel.

It is the third part of the concept that makes the whole system viable, because we now add the powered roof support. These are sets of hydraulic rams that ride on one plate of steel, pressing a second up against the roof. They are connected to the conveyor by a horizontal ram.

The mining process is thus that first the shearer moves down the coal face, grinding off the coal to a depth of around 2 ft. After it passes, the rams on the roof supports, in turn, are released, so that they drop away from roof contact. The horizontal ram is retracted and the support moves forward until it contacts the conveyor. It is then raised, and re-supports the roof. Each support moves forward it turn, so that the miners (which now include women) are always under a roof of steel. After the supports are re-established, the horizontal ram extends, pushing the conveyor over into the open space where the coal has just been mined. The exposed roof rock then collapses into the open space behind the back of the supports.

If one were to look at the operation from above, and with the roof removed, it might look a little like this:



I have taken away some of the canopies of the shields so that you can see the conveyor snake after the shields move forward. The view closer in shows the conveyor and supports better.


Because of the way the roof rock weight distributes, it is usual to drive entries out to the edge of the panel first, and then mine back to the main drive tunnels, rather than mining away from the mains. In part this is to keep the excess weight from acting on the tunnels the miners travel through. Because of the collapse of the roof as the coal is removed the panels usually start at the back of the section (known as a panel) and mine toward the main transport tunnel, with coal and people travelling in roadways on either side of the panel.

Initial stages of a longwall panel development, showing how the access tunnels from the main haulage are located.

Over time the ground movement works to the surface, and the surface of the ground will drop, by some significant percent of the height of the coal removed, since the rock in the waste will crush and consolidate. This is called subsidence, and if the mining is carried out badly, then it can cause significant damage to surface buildings. However, if done properly, it should not.

I will cite two examples of the latter. Firstly in the height of mining and before North Sea Oil and Gas, Britain mined coal at a high rate of production, from where it was found. This included under the city of Coventry, which was at the time home to large manufacturing plants, with precision lathes. By back-filling behind the face props with the blown-in waste from the colliery treatment plant (called stowing the goaf), the waste was filled, and the ground movement minimized to the point that I only heard of one factory being closed for less than a week to realign their lathes.

The second example was in Duisburg in Germany, and a different problem. The town is a port on the Rhine, and over the centuries the river had eaten into the bed, so that the quays were becoming too high above the water for easy loading of the barges, and the town was losing business. They went to the local coal company and asked them to mine out seams under the harbor, and thus to lower the quays to bring them back in reach of the water. The miners complied, and lowered the area by over 11 ft. The area that was mined included highway overpasses, and a Shell oil storage facility in the middle of the river. The story goes that the tank farm manager went to the miners and asked them to tell him when they were going to start, so that he could drain the tanks as a precaution. They pointed out that the farm had actually already been lowered about 3 ft, as I recall the story.

As usual this has been rather a superficial description of a process, but hopefully it gives you more of a sense as to what goes on in a longwall mining operation.

Additional comment : It is possible to mine coal without using picks in a longwall, though it is a technology that has found much greater application in other industries, beyond mining.

Our thoughts and prayers go out to those in West Virginia at this time.