Tech Talk - Coal mining continues to produce

Sadly I was away from home last week to attend a family funeral in Northumberland where, for the past nine generations including mine, our family have been miners. The funeral was for my father’s sister, Linda, who had documented early life in the mining village of Ashington in her books “A Tune for Bears to Dance To,” and “The Pit Village and the Store.” The latter was made into a docu-drama for British Channel Four television. The hotel at which we stayed was next to the Woodhorn Mining Museum which has been built around the colliery offices from the old mine. The rest of the property has been turned into a nature park – with a little twist.

Figure 1. Part of the old mine site, now the Queen Elizabeth II Country Park, with a 40 acre lake. Note the wind turbines in the background – all but one of the 14 were turning during my stay. (Hotel on the right)

The site is also now home to considerable bird life – including over three dozen swans that I counted as I meandered around the lake.

Figure 2. Some of the birds on the lake at Woodhorn

Much of this part of Northumberland has changed considerably since the time – over 50 years ago – that I was an Indentured Apprentice in the National Coal Board working at Seghill Colliery, though on day release once a week to Ashington Technical College. One of the greater changes is illustrated in the background to a painting of my father that my aunt painted.

Figure 3. My Dad as Undermanager at Ashington (note the yard stick, the safety lamp is hidden by the coat). (Linda McCullough-Thew)

The large mounds are the pit heaps which were scattered all around the road as the bus carried me from Newcastle to the pit. They are all gone now, and the land is restored and, as the pictures above testify, now visually contaminated by the latest form of energy generation, though that doesn’t seem to worry the red squirrels and the geese.

Figure 4. Pit heap dominating the miners houses (Sunderland Public Library)

I thought of that as the recent reports on the devastation that mining creates are once again headlining the problems as new and enlarged lignite mines are developing in Europe. The transition to mining lignite, which contains considerable quantities of water and is a geological precursor to the black bituminous and anthracite coals that are preferred, is coming because it is considerably cheaper than alternate sources and nations have it at hand, instead of having to spend currency on importing alternate and increasingly expensive fuels from elsewhere. The reason that lignite is attractive is that the black coal seams that used to be mined in much of Europe have been mined out at currently economic depths, and lignite – even though less energy intense – has become an economically viable alternative.

To mine the surface deposits Europeans rely on the Bucketwheel Excavator (video here) with one machine replacing 40,000 men with picks and shovels (the way I was initially taught to mine). The overlying rock and soil (overburden) is first removed and stored, and then, once the coal has been removed, the land is restored with very stringent requirements for the condition of that restoration, so that in many cases the stone walls around the fields are replaced and the appearance of the land is similar to what was there before.

At present surface mining is becoming the dominant method for coal production. The thick seams in Wyoming and Montana have huge reserves, and the coal is very simple to mine and remove. Once mined it is trucked away from the machines and loaded into rail cars which then carry the coal around the nation. Because this coal has a low sulfur content it has proved competitive even against the more local coals of the East, which must often now be expensively mined from the underground. As the Wall Street Journal recently noted two counties in Wyoming now account for 40% of the US coal mined, while underground mines are closing in Appalachia.

After seeing a drop in coal production of around 9% as coal fired power plants were replaced by natural gas in the 2011 to 2013 time frame, the EIA is now projecting that US coal demand will increase by 3.6% this year, as natural gas prices rise. This will be followed by a 2.5% decline in 2015 as the new EPA regulations bite harder in driving the closure/transition of power plants. However US natural gas prices continue to be much lower than those in most of the rest of the world, and thus, as the WSJ notes , overall coal production in the USA is likely to stabilize around current levels for the next three decades, while domestic demand reduction is offset by increasing demands for coal from other countries which will continue to find it a cheaper alternative.

Much of the alternative replacement fuels for coal (and in some cases nuclear) are presumed to be from the increased levels of shale gas that are being produced in the United States, and which are projected to become domestic sources of fuel in many other countries around the world , including Europe. However while the plans and actions to close coal fired power plants proceed apace the rate and scale at which alternate sources of energy, particularly European shale gas, will appear are much less certain.

And in the interim, as coal mines have found better ways of processing the coal to meet power station demands, the potential for growth still exists, as the recent example in the Illinois Basin shows, where Sunrise Coal are planning to open a new underground mine in Vermillion IL this year, producing around 3 million tons of coal a year. The mine will use room and pillar mining to ensure that there is no surface ground subsidence, which can be a problem in the Illinois Basin.

And those who anticipate that China and India will reduce coal demand in order to overcome the problems that they have with air pollution, should remember that air pollution in the UK was at least as bad in the early 1060’s but by changing the way in which coal was burned the air was cleaned, and Britain continues to rely on coal for a significant portion of its electrical power.

Coal Mining - robbing the pillars

In recent posts I have written about room and pillar mining, where the miners drive tunnels through the relatively horizontal coal seam, until they come to the edge of the property. Depending on the accuracy and honesty of the mine surveys, they then stop. I mention this latter because, on occasion, miners who later worked in an adjacent mine, thinking that they have plenty of room, have worked close to the boundary on the other side, and suddenly, and often tragically, have found that they were too close. Water or gas that had collected in the old workings flooded into the new ones, with usually fatal results. (The Quecreek Mine Rescue was an exception).

Once the mine has reached the boundary, there is still a lot of coal left in the pillars. If the mine tunnels are 15 ft wide, and the pillars are 45 ft wide and the coal is 6 ft thick, then using a rough rule that a cubic yard of coal weighs a ton, gives that the original tonnage between the tunnel center-lines, assuming square pillars, would be 800 tons (20x20x2). The pillar left contains 450 tons, so that the initial extraction only removed 44% of the coal and 56% remains in the pillars. So, providing that the mine does not have major surface construction that would be harmed if the ground subsided, the miner might choose to remove some of that coal, as he retreats back from the boundary. (Although David Kuchta will tell you of times when folk were less fussy about worrying about the houses on the surface).


This practice is known as “pillar robbing,” though to get away from the negative picture that this raises, it may be called “pulling” or “drawing” the pillars. Essentially the miners will start at the boundary and work back towards the shaft, removing coal from the pillars in a systematic pattern as they go. And if you pull out the supports that hold the roof up, then that roof will collapse into the opening beneath it.

This is where there is some skill and forethought required in planning how to mine out the pillars, and to control the way in which the roof breaks. If the pillars are pulled properly, then the coal that is left, because some is, will crush sufficiently slowly that the miners can have enough warning to be out of the way, and it will control the way that the roof breaks.

The process can be illustrated by first showing the layout I used earlier for the development of the mine:

Overview of a working room and pillar section

And then look at a mining plan after the mine has pulled most of the pillars from a section.


With 80% extraction the mine is now only leaving 160 tons of coal behind in the fenders or stumps. Fenders are usually the larger pieces of pillars that are left along the edges of the path the machine must move along, stumps are the residuals in the body of the pillar.

As the miner removes the coal, to keep the operation safer, while the coal is being removed, wooden props used to be installed that would hold the roof in place.


That practice, with the men under unsupported roof is no longer used. Now the coal is extracted using, increasingly, remotely operated mining machines, with the supports more closely located around the mining area and increasingly being hydraulic.

Pillars can either be mined very simply, by cutting into the side of the pillars left in initial mining:

Simple pillar removal sequence SME handbook

The small black circles are where the props (historically wood but now hydraulic props) are located.


Pocket and wing mining – the pockets are removed in sequence, the wings are left to hold up the roof.

In stronger coals or where the roof is better more of the coal can be removed.

Sequence of coal removal (SME Mining Engineering Handbook Vol 2)

The reason that the props are located close together at the edges of the remaining rooms, is that the roof layers above the area will start to break after the coal is removed. By establishing the props along the edge of the pillars the breakup from the previous coal removal will be stopped at the edge of the existing pillars.

The roof breaks into fragments as it falls, and will bulk into the space left by the coal removal. This bulked up rock provides some support to the roof overlying the area where the coal is then removed in the next sequence of mining. And so the process retreats.

It is not as effective as getting all the coal out, and can lead to more difficult problems at the surface than an alternate method of mining called longwall, and we’ll come to that next. But pillar robbing requires a great understanding of the geological conditions before it can be safely carried out. Unfortunately when this is not the case then we get disasters such as that at Crandall Canyon.

Coal mining - Mechanized Room and Pillar Mining

This is the next in the series of Tech Talks on Coal Mining, and I’m now going to move more into more recent mining, rather than dealing with the historic background of the recent past. Specifically I am going to describe how the “Bord and Pillar” mining of the olden days transitioned into the much more mechanized mining method, known as “Room and Pillar” mining – the most popular method of mining in the underground mines of the United States.

To briefly review where we have been, coal has been mined for centuries. It is usually found as a relatively flat layer of soft rock, often under a layer of relatively soft shale, and just above a second layer of shale. These then form the roof and floor of the tunnels, or headings, through which the mine is developed. Because the shale is often softened by water, this can pose longer-term problems. But one can, simply, think of coal as being the layer of cream in the middle of a cake. And one of the tricks is to carve out the cream without the top half of the cake falling on your finger. In earlier times it was, unfortunately too often a method of widen ‘til it falls.

Partial plan of Kehley’s Run Colliery at the time of an underground fire in 1880. The red areas show roof falls. The cracked black are broken pillars, while solid black are pillars with some strength and integrity. (I have cleaned the image a little with Photoshop)

The most common method of mining started with driving tunnels, and then intersecting these with “cross cuts” through the pillars of coal left to hold the roof (the top half of the cake) in place. As I showed in an earlier illustration (above), since it was easier to mine from free faces then the solid, the initial rooms driven were often widened, until the roof fell in. And, over time mining practice came to narrow those initial tunnels to around 10 – 15 ft in width. They were initially smaller, when hand won, but with the advent of more mechanized mining the width grew greater, and required more support in the tunnel itself to hold the roof immediately above.

At the same time, as the mine workings moved away from the shaft (which took coal out of the mine, but also supplied fresh air to it) it became more critical to steer the air entering so that it reached the working areas of the mine, and then could be sent back to the surface. Without that proper circulation of air, the gasses which the coal would give off, including methane (natural gas, carbon dioxide and carbon monoxide) this could well be fateful. And so the miners began to also build temporary or permanent “stoppings” or walls across some of the abandoned tunnels to steer the air currents down the tunnels in and out of the working area. To separate the two flows (in and return) between the main roadways permanent “airlock” types of walls would be built with doors between so that one could cross from one passage to the other without interfering with the passage of the air.

Stoppings and an "air lock"

As the mines developed miners learned, sadly the hard way, that just having one shaft or access into and out of the mine was not safe. A major accident occurred at New Hartley in Northumberland, UK in 1862 where the main beam for the dewatering pump fell into the shaft, blocking it. The 204 men and boys in the mine, virtually the entire working male population of the village, were all killed, It was a result of those deaths that legislation was passed that required that there be two separate ways to get out of a mine. Where the mine is deep underground this means that there are generally two shafts, or more from the workings to the surface. As a small piece of family history my father was manager at the successor to that mine, New Hartley, shortly after I was born. There was also a novel of the disaster, sadly out of print, called “The Darkness in the Morning” by Gordon Parker. It tells of the different pressures on the miners, the management and the owners to survive, and how, in part, some of those decisions led to the disaster.

But air does not move by itself. There has to be some form of power applied to move it, and before there was electricity there was heat. So, if you have two shafts then at one stage, to power this air circulation, they would light a fire at the bottom of a shaft, and this would draw the air to it, and from the mine. It was given the name furnace draft ventilation. In other cases the shaft was divided, with fresh air going down one side, and the heated return air going up the other.

Air shaft (known as an air funnel) from an underground furnace ventilating a mine in 1877.

Once the air was moving into the mine it would toward the workings as Intake air, or fresh air, travelling down the main transport tunnels or drifts to get there. The worked out rooms on either side would be blocked off permanently, until the working area was reached. Here the air had to circulate across the rooms that were being mined, and then, since this air would now contain the gases and dust released from the mining process, it would be moved by temporary stoppings to the Return air passage, and thence out of the mine.

Air path around a working section

Initially men broke the coal from the solid with picks. When they worked this efficiently, a man can be very effective in breaking out the coal (about 4 joules/cc specific energy, for those that are interested.) The machines mine at around 1,000 joules/cc of coal removed). But man is slow, and so the manual pick was replaced with a mechanically swung version. Initially a large number of picks were set into a chain, rather like a large chain saw, and this was used to undercut the coal seam about a hundred years ago. Then holes were drilled into the coal above the slot, filled with a stick of dynamite, and the blast would break the coal into pieces, that the miner could load into tubs.

From this came the idea of putting lots of these chains together to form a belt, wound around a rotating drum, that would grind away coal all across the face, without the need for the explosive. In other words the coal could be won continuously, without stopping to clear the explosive gases, It was found that the picks did not have to be right beside one another but could be spaced, and this was the birth of the modern continuous miner.

Early Joy Continuous Miner (Source Illawarracoal )

If the pick points are spaced around a drum, with a drive motor inside, and laced into a pattern that moves the coal towards the center of the machine, the drive becomes much simpler. The drum itself is mounted above a gathering arm loader apron, so that as the coal falls it is swept into the center of the apron, and falls onto a conveyor that carries the coal through the machine and into a shuttle car that is located behind the machine and can carry the coal from the machine to a nearby conveyor. The machine looks something like this:

Modern continuous miner (Source Mining Weekly )

(The pictures that follow were generated using Strata 3D and portray, a simplified view of the parts of a working section of a relatively modern mine starting with the continuous miner).

Model continuous miner

The continuous miner grinds the coal ahead of it as it moves into the solid carving a tunnel or heading as the coal is removed. You might note that there is a protective cab at the back of the machine. The machine can only go forward until the miner is under the last row of roof support. The machine must then back out of the heading, and move over to a second heading while the section that has just been mined is supported. At the same time the coal is being loaded from the conveyor onto a small shuttle car that will move the coal from the heading to the nearby continuous conveyor belt.

Continuous miner and shuttle car (roofbolter in the top left)

The picture shows the continuous miner starting to drive a heading while loading coal into the shuttle car behind it. (I have not shown the power cables or, people). Normally a section may have two or three of these cars, so that while one is carrying the coal from the continuous miner to the main belt conveyor transfer point, the second can replace it and catch the next batch of coal to come from the machine. In the heading above that being driven there is a machine with two drills on extensible booms. This is the roofbolter.(Note that the seam is about 6 ft thick in the model and I have taken away the roof so that you can get an idea of the layout, which is shown from above. (I’ll discuss how one holds the roof up in a later post.) The shuttle car can brush through the temporary stoppings, which I have shown as plastic hanging curtains.

At the transfer point the cars dump their load onto the protected end of a long conveyor belt that will carry the coal to the shaft, where it is loaded into skips that carry it to the surface. Where the seam of coal comes out to the surface on the side of a hill (outcrops) the conveyor will come out through one of the entry tunnels (portals).

Overview of a working room and pillar section

In the picture you can see that in this district or section of the mine they are driving five headings, with the miner in the second. There are two shuttle cars, one at the machine and one turning to replace the first along the cross-cut. The conveyor belt is in the fourth drift, while in the third, or central heading is the man-trip that carries men and supplies to the section, and the light rail segments that it runs on. (I have not shown the supply piles and other items normally found around this area).

The problem of gas is still present, as it was years ago, and because the coal is ground into smaller pieces (most of it less than an inch in size -while hand-got coal was around four inches), the gas volumes can be somewhat larger than with slower methods. The continuous miner can achieve short-term production rates of around 15 tons a minute, but must stop frequently to change cars, or to move.

The problems of ventilation remain. Large fans at the surface now supply the air to dilute the gas and sweep it away. In the immediate working area these stoppings can be made of cloth or plastic, so that miners can more easily move through them. When the working area moves on the stoppings are made permanent, and the ones shown are made of grey cinderblocks. Doors are placed in these stoppings at regular intervals to allow men and machines to pass, as shown above. Remember that in most of this area, all the miners have to see where they are going are the caplamps that they wear on their helmets, or the floodlights on the machines.

It is still necessary to leave pillars to hold the roof up. Roads, headings and cross-cuts are often the width or some multiple of the width of the mining machine, i.e. generally around 12 - 20 ft. based on the seam depth and the roof rock strength. Pillars may be around 40 ft on each side. It doesn't take long to realize that a lot of coal is being left in the ground.

That will become the subject of later posts. For those who know better, please recognize that this is a very simplified explanation of Room and Pillar mining, and that there will also be a few more details, as this series continues. But it should convey the concept.

Early coal mining machines and their use

This is part of a series of Tech Talks that I write on Sundays. It is meant to provide some background information on the extraction of fossil fuels and metals from underground, both as general education and as a resource when, as with the Gulf oil disaster with the Deepwater Horizon, or with an underground mining disaster such as that at the Upper Big Branch Mine folk need to know what is going on. Thus, for example, at the beginning of the Deepwater Horizon spill, I combined some of these to describe some of the critical parts of creating a well.

Today, in that tradition, I am going to be describing how the extraction of coal progressed from manual mining of the coal, with a pick and shovel, to the next stage which was the early use of compressed air power and led into the mechanized mining that we use today. At the end of the last talk I showed the method by which the pattern of mining evolved, with a miner first undercutting the coal, then cutting vertical slots, and breaking out the ribs between to extract the coal in relatively large pieces.

Face layout at Houghton (A Pitmans Notebook, The Diary of Edward Smith, 1749).


The first step in mechanization was a change from the physical mining of the bulk of the coal, to drilling a hole in the coal face, inserting a stick of explosive, and breaking out the coal more rapidly. The miner then had to do less picking, and more loading of the coal into tubs. Initially the holes were drilled with a manual drill, with the miner bracing the bit with his body, as he turned it into the coal. The bit was held at the right height using the pinned prop that is shown in the picture.

Manual drilling a shot hole, Ashington 1911 (The Miners Anthony Burton)

Drills, over time, changed to first compressed air, and then, more recently to electric drills, though these, though more powerful, were heavier and drilling was done without the aid of the prop.

Power drilling the shot-hole (The Miners, Anthony Burton)

The shotfirer was generally the lowest level of administrator in the mine, and had to be certified, after an examination. The explosive was generally moved around in a wooden box, for protection, and fired in later years, with an electrical detonator, to allow a suitable safe distance. Earlier ignition used a burning fuse (as shown in many movies) and this was more hazardous since you could only get a certain distance away after lighting the fuse. The rod that was used to push the explosive into the hole, and stem the hole (usually with clay or coal dust) to contain the blast, was an initial sign of authority.

Charging a hole with explosive, note the wooden box. (The Miners, Anthony Burton)

Once the coal was broken into pieces, moving it with a shovel became a lot less work, than physically breaking it from the solid. But before the coal could be blasted down, it still required that it was first undercut to give a free surface to break to. And that was still done manually. Until, that is the invention of the cutter bar. Simplistically the first cutters was a series of small picks mounted into what are known as boxes, along a cutting chain that is itself fed around a long bar that is powered (very similar, on a much larger scale, to the bar of a chain saw).

The first of these was developed to help in the driving of the tunnels or rooms, from which the coal was mainly mined.

Picks on a Cutter bar (Colliery Deputy’s Handbook D.G. Maguire)

The initial bar could be swung through a 180 degree arc to undercut the coal at the face of the room, as mining advanced. The machines could cut at different heights above the floor, although the base of the seam was much preferred.

Early Arc wall cutter (Colliery Deputy’s handbook D.G. Maguire)

The earlier machines were manually moved around on rails, which in turn meant that the rails had to keep up with the face. And the early motion was relatively primitive.

With a single swing the cutter undercut the working face.

Two things remained to be mechanized. The first was breaking the coal into small enough pieces that it could be loaded into tubs or onto conveyors. A variety of different ideas were tried over the years. This one used compressed air and a variation on pneumatic pick action to break the coal:

Early percussive mining machine (The Miners, Anthony Burton)

My grandfather, I believe, worked with a variant of this machine.

But the last hold-out was to change from manually loading the coal to mechanized motion. After all If the coal was cut with power, and broken with power it made sense that it should also be loaded by power. Again there were a variety of machines that were tried, but then Joe Joy entered the picture with a machine that led into the tools of modern day mining.



And it is with this image of the Joy Gathering Arm loader (the arms rotate about their pivots to move the coal to the central conveyor) that I will pause, before moving into the modern version of this machine and its offspring. All the components for modern day machines had been invented, it was just a case of putting them together in the right way.