The coal is not always found as a single layer, or seam, in fact in most mines there are a number of different layers. At Jacobs Ranch, for example, the coal is found in three mined seams, collectively known as Wyodak. The upper Wyodak is 11 ft thick on average, the Middle Wyodak is 42 ft thick and the Lower Wyodak is 5 ft thick. However you can get some idea of the amount of coal available from this map, produced by the USGS.
Coal Thicknesses in the Powder River Basin (Source USGS) When the coal lies near the surface, it is this thickness of the coal, relative to the thickness of the rock that lies between the coal and the ground surface, that decides whether it is going to be economic to mine at all, and if it is economic whether to remove the rock from over the coal to get it out (hence the strip of strip mining) or whether the coal is better mined from underground. (The name also comes because you work on one strip of land at a time, as you remove the coal sequentially across the property).
This ratio between the thickness of the coal, and that of the overlying rock is known as the stripping ratio, and the economic limit varies with the quality of the coal, and other operational costs. For example a coal seam that was 100 ft underground, and some 6 ft thick, would have a stripping ratio of around 16.7, and there was a time that that would have been about the economic limit. But as the price of coal increases, and earth-moving equipment gets more efficient, that ratio changes.
So what is involved in strip mining (apart from all the permits, surveys etc that make the whole process of installing a mine take a number of years)? The first step is to remove the top soil, and that which lies under it, as either one or two separate lifts. Generally these are scrapped from the surface using specially designed equipment that can remove the more fertile, and underlying layers separately and move them to areas where they can be stored, until the mine has removed the coal, and replaced the overlying rock. At that time the soil is replaced in the same order as it was found. Because a mine is a continuous operation, after the first set of soil is removed and the coal in that segment is also taken out, so the mine will be replacing rock and soil in one part of the mine, as it is removing it in another, so that the storage over time only holds true for a small portion of the overlying material.
Scraper that could be used to remove and replace soil (Source Caterpillar).After the soil has been removed, then there are usually several tens of feet of rock that will lie over the coal. Before the coal can be mined this rock must be broken first, before it can be moved. The fracturing is usually done by drilling large (say 8-inch) diameter holes down through the rock, and then filling them about two-thirds full of an explosive. As a general rule you don't want to fill them all the way, since if you did, then when the explosive went off it would just shoot back out of the hole. The large columns of black smoke you see shooting from such blasts in movies are for effect. A skilled blaster will fire the entire round, and if you were to watch a slow-motion movie, the ground level would rise in a pattern, as the individual rows of charges went off, but there would be almost no gas vented from the holes. To confine the charge, the top part of each hole is filled with what is known as stemming, usually some of the rock particles that were removed from the hole during the drilling operation.
The typical picture of large, uncontrolled blasts that make the popular press are actually quite far from the truth as to what usually happens in this stage. And the fireball from firing a shot in coal is very unusual. (It could come from igniting any gas in the coal, or from burning some of the very fine coal particles that are formed in firing the shot). Where the ground just heaves a little and then settles back is the sign of a good shot, since all the energy has gone into breaking the rock, so that it is then easier to move.
The explosive is fired in rows, and this is to make the explosive work more efficiently. When you "fire" an explosive you are causing the chemicals in the charge to very rapidly turn to gas. At the same time the blast wave from the start of the reaction will have cracked the rock immediately around the drilled hole. Thus as the explosive turns to gas, that gas can penetrate into the cracks around the hole, causing them to grow out into the solid. The gas follows the cracks, and helps them to grow, while, at the same time "lifting" the rock away from the solid as the gas penetrates. At the same time, firing the explosive in a sequence lowers the overall vibration directed into the ground.
If the mine so chooses it may angle the holes that were drilled so that as this gas penetrates under pressure, it will also throw the rock some distance towards the area of the mine that has previously been worked. This is known as blast-casting and is not always needed. However by firing the rows of charges in sequence (using small delays set into the detonators that are connected together to set-off the individual charges) the rock nearest the edge of the last layer of rock removed is broken first. This removes some of the confinement of the next layer. In this fashion and with only millisecond level delays in each row, the entire rock in a strip overlying the coal can be fragmented. (Note that in the video I referenced, the dust is from the rock impact, not the blast.
After the rock is broken in this way, then a dragline bucket will be used to pick up the rock from over the coal, and move it into the space left when the last row of coal was removed. These draglines are the largest of the pieces of mining equipment and drag the bucket up over the rock pile, filling it, so that it can then be moved over.
With a dragline, the machine usually sits on top of the rock, and will lower a bucket that is dragged up the free surface of the blasted rock, until it is full. The dragline then swings its boom, until it is over the strip of land where the last pass of the process had removed the coal, and dumps the rock into that space. By steadily working across the face and back down the area that was blasted, all the coal seam is exposed, and is ready for removal. At the same time, the previous strip of ground is filled back up to about the starting level of the ground.
After the bulk of the rock has been moved off the coal, then the final clearing off is done with smaller shovels that expose the coal. Depending on the coal thickness and strength it can also then either be blasted to break it into smaller pieces, or just shovel loaded into trucks. Typically a shovel can pick up around 100 tons of coal in a single stroke, and can take 3 loads to fill one of the coal trucks, that then carry the coal out of the mine to the surface plant. As with the excavator, the shovel scoops up the broken rock, swings around and dumps it in the cut behind the machine. Note that it is more economic for the rock to be moved only once, and so the width of the strip will be governed by the size of the machine that is used. And a shovel will often only remove rock layers of around 15 - 45 ft high, depending on machine size.
After the coal has been exposed, then, depending on the strength and thickness, it can either be removed without any further process, or it may require some additional explosive fracturing to make it easier to pick up. It depends on the coal. In either case, when it is loose enough, the coal can be picked up by a smaller shovel, and this will usually load the coal into trucks, that will carry it away to the plant where it will be cleaned. At which point you may say, wait a minute, you have just dug a hole 100 ft deep, and have trucks being loaded with coal, but how do they get out? Good point! Generally during the creation of the spoil banks behind the working area, bulldozers will create a ramp that slopes down, from the surface, to the coal level, and this will be kept moving forward as the strip of ground that is being mined moves across the property.
The rock than has been placed into the space where the last strip of coal was removed is initially laid out in ridges, since it was dropped in place by the dragline, which works from fixed positions, and this is generally the view that the general public is presented with, since it leaves the impression of desolation that many of those opposed to mining wish to convey. In fact the operation is quite a bit from being over.
First the ground is leveled, and then the soil is restored, and by law the conditions of this restoration are quite rigorous, so that it follows to as great a degree as possible, to contours that were originally in place. Where necessary additional fertilizer is added to the ground, to re-establish crops and farming conditions. Cattle can then be reintroduced, and wild life return.
This is part of a series of technical posts that I am making to try and explain some of the background to mining of coal, and drilling for oil and natural gas, so that in the debates on some of the issues those discussing the issues have a better understanding of what is going on. A version of this was posted when I was doing the same sort of thing after helping found The Oil Drum, and can be found in the post that I made there on January 29,2006. As with the posts there, this has been simplified to make it fit, so if anyone wants to correct or refine these posts please comment.
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