Waterjetting 37d - Underground Drilling with Waterjets

The last post showed some of the experiments that we carried out as we developed a tool for drilling "around corners", demonstrating the ability of a high-pressure water jetting system to turn from a vertical well bore and drill out a lateral well within a turning radius of about 9 inches. This was not our earliest work on drilling, and that earlier effort had been directed to the reaming of geothermal wells.

The earlier project was also funded initially by Sandia Labs, although in this case the objective was slightly different. Overall we set out to show that one could go down a vertical well bore, some 9-inches in diameter, and ream a cavity 6-feet across down in the "hot dry rock" of a geothermal deposit.

When we began we weren't sure that we could drill in stressed rock, or what those effects on drilling rate would be. So we were fortunate to get some help from Doe Run and work in one of their mines, as the following video shows.

Figure 1. Early video on Underground Waterjet Drilling

  It was this series of tests that showed how relatively quiet water jets could be in drilling, unlike the competition at the time. Below the fold I will add the second part of this program, where we demonstrated the technique we planned on using for the reaming program.

Figure 2. Reaming a Geothermal Well Demonstration

Although this program moved on to focus on other aspects of the problem (we also demonstrated that this was practical in cutting granite, which led on to the Stonehenge and Millennium Arch projects) it was not until we were tasked with reaming large scale rocket motors, some years later, that we returned to this program, and developed the tools further.

Waterjetting 22b - Steep seams and shrouds

It was not until we started to move the auger that I described last time, that I realized how heavy and cumbersome it remained. It is true that we could lighten it considerably (and we did in the UK version), but it was still largely a platform mounted device that was relatively easy to move on the surface, but which would be much more difficult to move around in the confined spaces underground.

In this regard it is worth comparing two photographs.

Figure 1. Coal being transported in the pipes at the Hansa hydraulic mine in Germany.

Notice in this first one how the coal is confined, there is no dust, and the tunnel is relatively clean and clear. Contrast this with the typical mining operation where the coal is carried from the working face to at least the main haulage drifts, and often all the way out of the mine using conveyor belts. (The method of transport that would also likely be used in a typical underground auger section).

Figure 2. Conventional belt conveyor carrying coal underground. (Famur )

Note that in the second photo the belt occupies most of the space in the roadway making passage more difficult, and that the coal is openly exposed. The problem that this occasionally generates is that the coal transfers from one belt to another as it moves through the mine by falling off the end of one belt onto the next. This puts dust into the air, and if not properly maintained coal dust can accumulate under the belt and around the support rollers and drives. If not cleaned this can create heat through friction, and can lead to disastrous fires.

Figure 3. Studying a belt fire underground (Office of Mine Safety and Health Research )

Confining the coal, and using the water that mined it as a transport fluid – or at least part of such – has many advantages.

The Russians were aware of this as they developed some of their different mining machines. One of these was a small monitor that could be pushed up a seam, from a lower drift (without the need of a higher one), by adding segments as the unit was jacked forward.

Figure 4. Russian GVD monitor

The monitor could be advanced up the seam, cutting a channel about 3-ft wide, and to the height of the coal. This relatively narrow channel confined the water and the coal produced, so that both ran back down to the drift, where they could be either enclosed in a pipe, or run into an open flume, that would carry the coal away. Once the drive had reached the end of the section, then the two hydraulic cylinders that sat under the monitor could be turned, so that, from a protected section down-slope, the monitor could successively ream out strips on either side of the entry, until the roof collapsed, or the operation holed through to the previous panel.

Figure 5. Schematic showing the sequence of extraction in a) a coal seam with a relatively strong roof b) a seam with a weak roof, where a pillar of coal is left between successive lifts, typically around 30 ft, so as to provide additional support to the roof as the coal is mined out.

Production from these machines averaged about 76 tons/hour from seams that were in the range from 2 to 4 ft thick, this was more than double the production output achieved by more conventional means, with significantly less manpower.

However while there are some conditions where gravity helps to bring the coal and water back together, there are many cases where this is not possible due to other constraints. This is where it becomes necessary to use a shroud to confine the jets, debris and water so that they can be extracted together, often using a vacuum to assist in the process.

Figure 6. Rendition of the combination of three cutting jets rotating around a vacuum tube to slice into material and remove it.

The particular device shown in Figure 6 was developed as a way of remotely slicing into high-level radioactive waste for the Pacific Northwest National Laboratory. The waste is held in underground storage tanks, where the levels are too high for human access. As a result the waste had to be broken into pieces and removed remotely.

The initial problem was that the tanks, though holding perhaps half-a-million gallons of waste, had only small (18-inches or less) ports through which they could be accessed. Thus a relatively small tool was required, yet the mining rate had to exceed 4-cu. ft/minute. The situation required an excavation system on the end of a robotically controlled arm. But the arm would have to be more than 60-ft long (in the end a cousin of the arm used on the Space Shuttle was used). Any mechanical force applied at the end of such an arm would have tremendous leverage on the holding fixture, and a relatively low overall force would have to be found, yet one capable of cutting material perhaps as strong as a weak cement.

The answer came in the form of the device shown in Figure 6. By placing three cutting jets to rotate around a central tube, connected to a vacuum line, one could cut into the material and break out relatively small pieces, which could be aspirated away with the cutting water. (The requirements were that there would be no water left in the tank for any significant amount of time). Because the bottom of the tank was some 40 or 50 ft below ground level a small, high-pressure (10,000 psi) jet pump was developed by Michael Mann, capable of drawing particles of up to an inch in size into the line, and then projecting them up with sufficient energy to carry them out of the tank.

Figure 7. Basic system conceived to mine high-level radioactive waste and remove it from a storage tank.

While this description of the system makes the tool seem to be relatively simple to build and operate there are a number of features to the design that are fairly critical in order for it to work well, and I will cover some of those next time.

Waterjetting 21c - collection in Gilsonite and steep seams

Picking up the debris from a waterjetting operation can be a more expensive part of the process than actually removing the material in the first place. This is particularly true where the material no longer has any inherent value, but is sufficiently toxic that it has to be collected and then properly disposed of. On the other hand, in most aspects of mining, it is the inherent value that is contained in that debris that makes the whole process worthwhile, and in this latter case requires that as much of the material be initially collected as possible. This can be difficult where, for example, gold particles are being produced, since these are heavy and naturally rapidly sink through the water to settle onto the floor where, if this is rough, they can then lie is natural traps that make it hard to re-elevate them and move them to a desired collector.

If one is to continue to move the particles there must be some mechanism that will continue to move the particles to the point that they can be collected. The very condition that makes it difficult to manually mine coal from steeply dipping beds also serves to carry coal and other minerals from those veins, once the material is broken free from the solid.

Consider, for example, the early mining of Gilsonite, which is a naturally occurring hydrocarbon found in thin vertical veins in places such as Utah. It is named after one of its first developers, Samuel H. Gilson.

Figure 1. Pieces of Gilsonite (Metroexpand )

The problem with mining this material is that the dust is very explosive, and remote, safer methods were needed to get men away from the mining process. In 1957 the major American company, American Gilsonite, turned to the use of hydraulic mining. At first the seam was mined using a series of horizontal lifts, with the washed out material and water being collected in drifts that carried it to a pumping station. However, because of the vertical seam, it was easier to used a drill, with lateral jets so that the rotating waterjets mined and flushed the material down into underlying drifts, where it could be collected.

Figure 2. Method for Mining Gilsonite.

By moving the mining tool along the vein the material could be mined out, without miners at the working face, and without the risks of explosion from the conventional mechanical mining methods.

A recent blimp trip over the Bonanza mine shows the results from years of the mining process, as the seam has been removed.

Figure 3. Mined out seam of Gilsonite at Bonanza, Utah. (J.S. and S.W. Aber)

More recently the mine has returned to a mechanical method for mining the material since the customer wanted a dry, rather than a wet product, and, with demand for the material rising, hopefully they will continue to operate safely – the material is now airlifted to the surface. Mining has been restricted to the top 500-ft of the vein.

The hydraulic mining process demonstrated here shows some of the advantages of the use of waterjets in that there are no workers in the mining area, and that the tool is able to remove the mineral without generating the sparks and explosive dust clouds that can be a danger otherwise.

Figure 4. German coal seam side view, showing thickness and angle of slope. (Benedum, W., Harzer, H., and Maurer, H., "The Development and Performance of two Hydromechanical Large Scale workings in the West German Coal Mining Industry," paper J2, Proc. 2nd Int. Symp. Jet Cutting Tech., BHRA.)

Coal mining has some of the same problems, and in Germany the idea was tried (in a slightly modified version) as a way of mining coal. Rather than operate from a drilling rig at the surface, holes were drilled along a very steeply dipping seam. The process was in two parts, evaluating whether it was better to start at the bottom and drill up, or to start at the top and drill down.

Figure 5. Methods tested for bore and mine coal recovery. (Benedum, W., Harzer, H., and Maurer, H., ibid.)

The method where the drill goes down, and then reams back up turns out to be the better approach. The reason for this is that the coal breaks out along natural planes, and these can produce coal pieces that are large enough to block the drilled hole through which all the material must pass, when drilling upwards. This is not a problem when drilling down, since the coal is falling through a much larger space (the mined out volume). This makes it a little more difficult to recover, since it is not confined where it falls into the lower drift. But, by adding a small monitor in the second drift, the broken coal can be lifted back into a slurry that can then be channeled into a flume, and carried away from the working area.

The process was effective, in that it showed that the coal could be mined from one drift to the next, using the method, but it was difficult, because of the varying conditions of the seam over its length, to make the connecting drill holes at an economic speed, and thus to mine the coal at an economic rate.

Figure 6. Looking down to the lower drift from the upper, after mining, with all the coal removed. (Benedum et al ibid)

Note that in the above picture there is little roof collapse into the mined out cavity. This is either beneficial (since it allows all the coal to be removed) or it can be a disadvantage. Other than the safety factor of having a roof hang up until a very large area collapses at one time (where the air blast can do considerable damage), a continual roof collapse as the coal is mined can provide confinement for the water and, hopefully, enough restriction that the water will carry the coal to a collection point.

This was the idea behind the JPL coal mining worm that they proposed some years ago:

Figure 7. The JPL concept for a horizontal jet mining system (Miller, C.G., and Stephens, J.B., Coal Worm: A Remote Coal Extraction Concept, JPL Report 5010-7, December, 22, 1976, Jet Propulsion Laboratory, Pasadena, CA.)

Unfortunately when a coal mine roof collapse occurs the rock breaks into relatively large pieces so that the water can flow away through the pile, without being confined sufficiently to provide the needed motive force to move the coal to the collection point. For that, the system would still need a steeply dipping bed, and I’ll talk more of this and debris collection in future posts.

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.

Highwall Mining of coal

There is an aspect of coal mining that gets relatively little attention, even though it is growing in popularity, It is with this method that I will conclude this series on mining, and its precursor on oilwell production. Most mining can be divided clearly into either surface mining, where the rock over the coal is removed, and the coal taken away before the land is restored, or underground mining, where some of the coal has to be left in place to hold the roof up. But what happens in the middle?

There is an intermediate between the two main methods of mining, when the surface mine has produced coal from a seam that is steadily getting deeper, it reaches a point where stripping the rock from above it is no longer economical. So what to do? And the answer is what is known as Highwall Mining. When the last economic cut has been made with a surface mining operation, the coal seam is still exposed, lying under the rock and overburden in what is known as the Highwall of the mine. So, before the land is reclaimed and that final cut along the face filled back in, sometimes it is economic to look at using different mining machines to mine into that exposed coal face in the Highwall – hence the name.

Coal seams that might be mined this way are not going to be worked to that great a distance into the coal that a full underground operation will develop, rather a machine is mounted at the surface face of the coal, and advances into the coal mining and feeding it back out, so that it can be collected and hauled away.

The early machines that were used for this were often Augers, and in this picture the highwall had three seams of coal in it, (at the current bottom of the wall, and along the two sets of higher augered holes).

Augered seams in a highwall (Bundy Auger)

The coal auger is not dissimilar in shape to the auger used in making large holes in wood, or which might be used to drill large vertical surface holes, except that in this case the holes are drilled horizontally forward into the seam, and the auger might be more than 2-ft in diameter. Once the picks on the cutting head have mined the coal from the solid, then the broken coal is fed back along the scroll of the auger as it rotates, to the surface, where it is loaded, via the conveyor you can see in the above picture, into trucks. The scrolls are clearer in the picture below:

Auger working in Australia

The friction generated as the scroll rubs against the walls of the hole drilled require that it be pushed with increasing force, as the auger head mines deeper into the seam. This ultimately limits production from a single hole, since there is only a certain amount of force that can be applied through the auger string before it starts to deviate from drilling straight and, as a result, the head may begin to penetrate either the floor or roof rock around the coal, which is not to be desired. Back when I did some analysis of these some decades ago that limit was less than a couple of hundred yards.

However as the top picture shows, the holes can be placed fairly close together, though again care has to be maintained that the auger doesn’t punch through into a previous hole, since it is the hole walls that help get the coal back out.

Augered holes showing the narrow pillars between them (Bundy Auger )

The holes, once augered, can be backfilled with mine waste if that is thought viable.

The difficulty in steering the auger head, and thus the limitation on mining depth that this gives, has lead to the development of different pieces of mining equipment that can do the same job, though hopefully more effectively. One of these is the Highwall Miner (2:44 minute video here – the vertical auger is used for bracing posts, before mining begins)

This is a much more sophisticated machine and replaces the simple auger scroll with a miniature version of a continuous miner, which I have previously described being used underground. The resulting support machinery is significantly larger, although the basic functions, mining the coal, and feeding it back out to a cross-conveyor that then loads into trucks, remains the same.

Overview of Mining Machine (Bucyrus )

The mining head can oscillate up and down as it moves forward into the coal, thus mining seams that are larger in size than the machine itself (something that the auger does not do as well).

Highwall mining cutter head (Bucyrus )

The box conveyor sections provide a more robust platform for pushing the head forward into the coal, and contain two screw conveyor segments that carry the coal itself back to the cross conveyor.

Conveyor/push elements for the miner (the scroll feed conveyor segments can be seen inside the central box) (Bucyrus )

A machine of this type can produce between 40,000 and 120,000 tons of coal a month, depending on the conditions in which it is operating. It needs a crew of 3 or 4 folk, and averages 30 – 40 tons per man-hour. Remember that it must reposition at intervals.

Highwall miners of this type can advance much further into the highwall than augers, and machines are available that can mine in as far as 1,600 ft, achieving production levels (in Appalachia) of 300,000 tons a month. There is a video of the Addcarsystem, which is slightly different from the two described above, here).

The Addcar system, which uses an open conveyor concept to bring the coal out.

As I mentioned in an earlier post, this is the last segment of the coal mining series that I have been posting on Sundays. After a short break for Thanksgiving, I hope to begin a new series of Tech Talks, but more focused on nations from which we get our fuel, rather than on just the wells and mines that it comes from.

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.

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 reserve considerations

I am still travelling but have found a book that will help me with the discussion of historic mining that is the usual current fare on Sundays (an autobiography from 1910 that is illustrated). Wanting to include some of this in the next post, I am going to step a little away from the topic this weekend and instead post again an earlier comment on coal reserves. It is also a topic that I will expand on in the future, but here is some background.

So you want to start a coal mine – where to begin? The first thing that you need is some coal, and in most cases today the coal seams that are exposed at the surface are known and owned by somebody else. So you will need to drill some exploratory boreholes down into the earth to find some suitable seams. For the sake of example I am going to use a project in South Africa. Other than having found it on the Web I know nothing about the coal, or the company so please don’t think of this as any endorsement or otherwise of the property.

Quite often new developments are based on where folk have found coal before. If there is a mine then, with deposits such as coal there may well be more coal, out beyond the boundaries of that original property. This is because of the way that coal was formed as vegetation spread over a large swampy area that ran for many miles. Unlike oil, once the vegetation was put in place, and slowly buried, it changed composition in place, and so coal seams may well run for many miles, although they may get different names in different places. Thus, for example the Pittsburgh seam extends over perhaps 8,000 sq. miles, while the Herrin seam in Southern Illinois has about the same range (And this does not include cannel coal, some of which is found in Kentucky, which was moved from the original site by the actions of water).

Because these quasi-horizontal seams were formed over such a large area, and because the stratigraphy (order of the rocks as you take a core down through them) will remain relatively constant, in many cases, it is not necessary to make the exploration holes that close together. Thus in the example above, the boreholes were placed some 4 km apart and the volume of coal inferred from the thickness of the beds found, and assuming that they ran continuously from borehole to borehole.


This is referred to as:

Reconnaissance Resource: is quantified as a minimum one cored borehole with coal quality data per 400 ha (approximately 2km spacing) for multiple seam deposit types, while for thick interbedded seam deposit types a reconnaissance coal deposit is quantified by a minimum one cored borehole with coal quality data per 1,600 ha (approximately 4km spacing).

In this case the property was drilled over an area that required 402 boreholes, and it identified 5 seams of coal that could be produced. However it brings me to the point of this post, which is how much of the coal can be counted and as what type of reserve. This is quite an important distinction, since in the debate that I have had with others in the past, including David Rutledge, the confusions of what has been counted and how it is defined is often overlooked. This is how the information was reported:


So what are the different definitions of the reserves? Isn’t this coal all a reserve, well no, the coal seam is divided into different quality of reserve, depending on how far it is from one of the proving wells. Let’s consider the official definitions:

Points of Observation This is the point where the coal presence has been physically seen either at an outcrop or in the recovered core from a borehole.

Inferred coal reserves are those that can be extrapolated from a Point of Observation but to a distance of no more than 2 km.

Indicated coal reserves are those that can be extrapolated from a Point of Observation, but to a distance of no more than 1 km.

Measured coal reserves are those that can be extrapolated from a Point of Observation, but to a distance of more than 500 m.

So that, when we are assessing the amount of coal that we consider available at a site, if we are conservative, we are only reporting the measured coal reserves as that within 500 m of each of the boreholes, even though the consistency of the seam has seemingly been proven over many kilometers. It is a very conservative system, note that only 17% of the likely coal is considered a measured resource. To make this post more comprehensive, and to include some definitions that I will come back to in later posts, let me now go on to include the ranges of economically recoverable coal (in terms of thickness and depth) that are currently accepted.

In terms of international definition of resource the US Geological Survey has set up some definitions, that have also been adopted by the Federal Government, in their Code of Federal Regulations, which were just revised. The new regulations are a little more inclusive than the older ones (at the USGS site).

(5) Coal reserve base shall be determined using existing published or unpublished information, or any combination thereof, and means the estimated tons of Federal coal in place contained in beds of:
(i) Metallurgical or metallurgical-blend coal 12 inches or more thick; anthracite, semi-anthracite, bituminous, and sub-bituminous coal 28 inches or more thick; and lignite 60 inches or more thick to a depth of 500 feet below the lowest surface elevation on the Federal lease.
(ii) Metallurgical and metallurgical-blend coal 24 inches or more thick; anthracite, semi-anthracite, bituminous and sub-bituminous coal 48 inches or more thick; and lignite 84 inches or more thick occurring from 500 to 3,000 feet below the lowest surface elevation on the Federal lease.
(iii) Any thinner bed of metallurgical, anthracite, semi-anthracite, bituminous, and sub-bituminous coal and lignite at any horizon above 3,000 feet below the lowest surface elevation on the Federal lease, which is currently being mined or for which there is evidence that such coal bed could be mined commercially at this time.
(iv) Any coal at a depth greater than 3,000 feet where mining actually is to occur.
(6) Commercial quantities means 1 percent of the recoverable coal reserves or LMU recoverable coal reserves. . . . .
(19) Logical mining unit (LMU) means an area of land in which the recoverable coal reserves can be developed in an efficient, economical, and orderly manner as a unit with due regard to conservation of recoverable coal reserves and other resources. An LMU may consist of one or more Federal leases and may include intervening or adjacent lands in which the United States does not own the coal. All lands in an LMU shall be under the effective control of a single operator/lessee, be able to be developed and operated as a single operation, and be contiguous.
(20) Logical mining unit (LMU) recoverable coal reserves means the sum of estimated Federal and non-Federal recoverable coal reserves in the LMU.
(21) Maximum economic recovery (MER) means that, based on standard industry operating practices, all profitable portions of a leased Federal coal deposit must be mined. At the times of MER determinations, consideration will be given to: existing proven technology; commercially available and economically feasible equipment; coal quality, quantity, and marketability; safety, exploration, operating, processing, and transportation costs; and compliance with applicable laws and regulations. The requirement of MER does not restrict the authority of the authorized officer to ensure the conservation of the recoverable coal reserves and other resources and to prevent the wasting of coal. . . . . .
(23) Minable reserve base means that portion of the coal reserve base which is commercially minable and includes all coal that will be left, such as in pillars, fenders, or property barriers. Other areas where mining is not permissible (including, but not limited to, areas classified as unsuitable for coal mining operations) shall be excluded from the minable reserve base.
(24) Mine means an underground or surface excavation or series of excavations and the surface or underground support facilities that contribute directly or indirectly to mining, production, preparation, and handling of coal.

It is important to note item 23, since no underground method of mining will remove all the coal from a seam, but will leave significant quantities of the coal in place as pillars to hold the roof up, and thus protect the miners and their equipment from the roof falling in. (In the African example the percentage of coal that might be recovered is about 50% of the reserve volume).

I should be back with some quotes from the book, and discussion of the considerable progress that has been made in mining since the early days, starting back next week