2.7 Geological problems in coal mines
A modern coalface is a very complex operation that represents a large investment in terms of capital, labour and planning. Cutting machines and lengthy conveyors are inflexible and require uniform geological conditions to maximize output. What then are the effects of geological variations on such a mining system?
Geological factors control the selection of working areas. The two principal geological conditions that affect mining operations are, first, the nature of the coal-bearing rocks and lateral variations in rock type. The second concerns geological structure: the dip of the strata and the presence of faults.
Variations in thickness and type of rock may occur both within the seam and in the strata forming its roof and floor. The thickness of a seam has a considerable bearing on the profitability of mining operations. Seams 1-2 m thick are particularly suited to mechanized longwall mining, whereas seams less than a metre thick suffer from the law of diminishing returns, since to obtain the same volume of output from a thin seam as a thick one, a greater area of extraction (and hence more rapid face advance) is required. The presence of layers of mudstone within a seam reduces its value, because the quality and marketable value of coal is determined by its ash content. Such layers may also indicate the beginning of a split within the seam.
Of equal importance in achieving rapid face advance is the nature of the roof and floor strata. Mudstones and siltstones usually provide good working roofs for a coalface, but a soft seatearth below the seam will cause heavy equipment to gouge out several centimetres of floor material, reducing the quality of the coal. To counteract this, coal may have to be left at the bottom of the seam.
The presence of sandstones can also result in operational problems at the coalface. The most serious of these arise where channel-fill deposits are encountered in the mudstones above a coal seam. These structures formed when erosive drainage channels on the original delta plain cut down into the underlying sediments and filled with coarse sand. If such channels cut down into the mudstones above a seam, they usually result in unstable roof conditions, and serious roof falls can occur. Sometimes the channels cut down into the coal seam itself and extreme examples may locally have eroded the seam away completely (Figure 28). Where these washouts occur they will bring a working face to a standstill.
The stripping ratio for the two seams in Figure 28 is less than 7:1 for most of the area to the NW of the fault: the two seams there are economic for surface working and can be considered as reserves. If only the thin seam were present, that would be economic only in the western part of the area. To the SE of the fault, the stripping ratio rises to 23:1, due to the displacement and the angle of dip. This is uneconomic for surface mining and the downfaulted block cannot be considered a reserve for a surface operation. The coal seams in that area would possibly be a prospect for underground mining. However, the features revealed to the west of the fault — washouts, old workings and seam splitting — that pose no great problem for surface mining, would considerably reduce the reserves, were they to be encountered underground. The split might make both branches of the thicker seam too thin to work. Old workings and washouts present hazards from flooding as well as the absence of coal. Large volumes near them would not be workable. Consequently the reserves underground would be considerably reduced.
Whereas the sedimentary setting of a seam and its associated strata determines the profitability of the mining operations, the structural setting imposes physical constraints on the layout of mine workings. Faults form major problems for mechanized faces, as any displacement greater than the seam thickness may bring production to a halt with consequent loss of output (Figure 28). Substantial displacements may also result in the stripping ratio beyond the fault becoming too great for the coal to be economically recoverable by surface mining. Such faults may therefore mark the boundary of a prospect's reserves.
In addition, in some areas of the world igneous rocks intruded into coal-bearing strata will adversely affect the quality of the coal adjacent to the intrusion.