2.4 Modern mine planning
Once the geological data gathered during the exploration phase has been evaluated, geologists will estimate the quality and quantity of coal present. Coal reserves (in tonnes) are calculated from volume × density (Section 5). The volume of coal is controlled by seam area and seam thickness.
tonnage = seam area × seam thickness × coal density
Seam area is not the same as surface land area, as the coal seam may dip. However, the surface land area will define the land area required for mining. Other considerations are the purity and rank of the coal, the likelihood of encountering any geological problems (which will be discussed a little later), and whether the seam has been worked in the past.
The first decision to be made is whether to open a surface mine, or an underground mine. In surface mines, exposed and/or shallow coal seams are accessed by removal of waste rock or overburden from the surface (Argles, 2005). They are therefore rarely deeper than 100 m below ground level.
What do you think controls the maximum depth to which coal seams can be worked economically using surface mining methods?
There is no profit to be made in removing overburden, so the maximum economic depth for surface mining depends on the value of coal produced compared with the cost of removing the waste material. In addition, surface mining machinery is unable to operate below a certain depth.
The ratio of the amount of overburden to the total amount of workable coal is therefore of critical importance. Within the coal industry, this is known as the stripping ratio (or overburden ratio in Argles, 2005). Stripping ratios can be calculated either in tonnes or in thickness ratios (commonly used in coal operations). The maximum economic stripping ratio for surface mining has steadily increased over the years to around 20:1, helped by improvements in the productivity and life of the plant and equipment.
Figure 13 shows a cross-section through coal-bearing rocks. By counting the squares in the overburden and coal, and by assuming a maximum stripping ratio of 20:1, determine whether the coal seam could be extracted profitably using surface mining techniques.
The coal seam covers 76 mm2 squares, whereas the overburden covers 21 cm2 squares, i.e. (21 × 100) mm2= 2100 mm2 squares.
So, the ratio of overburden: coal is 2100:76, or 27.6:1. This stripping ratio is higher than the 20:1 limit, so this coal seam would have to be extracted by underground mining.
Mines working high-rank coal, which is more valuable, can operate higher stripping ratios than those working lower-rank coal; the surface anthracite workings in south Wales for example, operate a stripping ratio of 35:1. Note also that because coal is a moderately high place-value resource (Sheldon 2005), it is economic for some present-day surface mines in the UK to extract coal at greater depths than underground mines in other parts of the world that are remote from points of sale.
Once it has been decided whether to use either a surface or underground mine, engineers will begin planning the optimal mine layout and the processes that will be used to extract and remove the coal from the coalface. As Figure 14 shows, computers are often used to model the continually changing layout of the mine with time.